Some of the key questions regarding polymer solar cells are why certain bulkheterojunctions have internal quantum efficiencies higher than 90%, what is the optimal morphology and why fullerenes are by far the best electronic acceptors. We will show that in several highly efficient solar cells there is a three-phase morphology in which there are pure regions of polymer and fullerenes along with a mixed region. We will show that there are energetic offsets that push electrons and holes out of the mixed region and into the pure region. One can control the size and composition of the phases by adjusting factors such as the regioregularity and molecular weight of the polymer, the polymers sidechains, the donor:acceptor ratio, the choice of fullerenes, the use of processing additives and annealing conditions. We will show how one can decide what to adjust to get the ideal morphology and thereby obtain high energy conversion efficiency in solar cells.

Films of the fullerene derivatives [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) have been patterned on silicon nitride membranes using photolithography to study, with X-ray spectromicroscopy, the lateral, solid-state diffusion of fullerene derivatives into conjugated polymer films. After patterning of the fullerene film, a film of conjugated polymer is laminated on top and the structure annealed in order to study lateral intermixing and facilitate measurement of fullerene miscibility. Lateral intermixing of polymer and fullerene readily occurs for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and regiorandom poly(3-hexylthiophene) (RRa-P3HT). A 42 wt.% miscibility of PC₆₁BM in PBTTT is measured, while miscibilities of 20 and 41 wt.% are measured in RRa-P3HT for PC₆₁BM and PC₇₁BM, respectively, thereby demonstrating a significant difference in the miscibilities of these two fullerene derivatives. For regioregular poly(3-hexylthiophene) (RR-P3HT), incomplete lateral intermixing of fullerene and RR-P3HT is observed with PCBM crystallite formation competing with the lateral diffusion of PCBM molecules into the polymer film.

Interfaces are inherent in and essential to organic electronic devices. At every interface, both organic/organic and organic/inorganic, the potential to utilize nanostructuring to control device performance is very high. In this contribution, we examine nanostructuring at the donor/acceptor heterojunction in organic photovoltaics, with the result of modifying efficiency by four orders of magnitude. We show that the length of the exciton dissociating interface can be tuned by changing the substrate temperature for small molecule heterojunction photodiodes based on crystalline diindenoperylene (DIP)/C60 mixtures. Tracking the morphology with AFM, the DIP islands are seen to vary in diameter by one order of magnitude, from 1 micron to ~100nm. Due to this tuneable interface morphology, the performance of such devices can be changed from poor performing planar heterojunctions to higher efficiency ordered nanoscale bulk heterojunction structures. In this way, highly crystalline DIP, tracked by x-ray diffraction and reflectivity studies, can be thought of as a natural “bulk” heterojunction.

All-conjugated block copolymers with p- and n-type blocks represent a promising approach to improving the performance of all-polymer OPVs. Phase separation can be avoided and block copolymer self-assembly may lead to ideal structures for charge dissociation and transport. Here, we report the synthesis, morphology, and performance block copolymers with poly(3-hexyl thiophene) (P3HT) as the p-type block and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2&’&’-diyl) (PFTBT) as the n-type block. The block copolymers are synthesized via a two-step route using Grignard Metathesis Polymerization with LiCl additive for the preparation of P3HT followed by Suzuki Polycondensation for the preparation of the PFTBT. Homopolymer impurities are removed through Soxhlet washing, without the need for column purification or solvent fractionation. The polymer block ratio influences polymer thermal behavior, with P3HT crystallinity observed only in block copolymers with majority P3HT content. Both differential scanning calorimetry and grazing-incidence X-ray analysis is carried out on block copolymer thin films to characterize their crystallinity and microstructure.
Thin film crystallinity and crystalline orientation is largely determined by processing conditions. Out-of-plane pi-pi stacking is achieved with high-temperature solvent casting and thermal annealing below the crystallization temperature of the P3HT block. Conversely, casting at room temperature results in in-plane pi-stacking of the P3HT chains. Long-time solvent annealing results in improved ordering of P3HT crystallites. Under the proper processing conditions and block ratios, near 3% PCE is achieved in a block copolymer OPV. This represents a record for block copolymer or polymer blend OPVs and indicates that block copolymers are promising materials for achieving high-performance OPVs.

Carrier mobility plays an important role in determining the performance of polymer solar cells. Low mobility in conjugated polymer leads to a high series resistance and hence a low fill factor. In addition to carrier mobility, carrier lifetime in the polymer:fullerene blends is another important parameter determining the cell efficiency. In collaboration with John Reynolds at Georgia Tech, we have extensively studied the carrier transport and recombination of several donor-acceptor conjugated polymer systems. We found that while low mobilities in polymers lead to strong charge imbalance resulting in low power conversion efficiencies, high mobilities in some polymers do not always lead to high power conversion efficiencies. In this presentation, we will present our device results based on dioctyldithieno silole (DTS) polymers with a wide-absorption band and excellent transport properties. Even though DTS polymers have a high hole mobility, the power conversion efficiency is limited by its fill factor. In order to study the loss mechanism of the photo-current, we carried out transient photo-voltage (TPV) measurements to probe the bimolecular recombination of photo-carriers under open circuit condition. Based on our photo-CELIV (carrier extraction by linearly increasing voltage) data, we will present a model to explain the loss mechanism in polymer solar cells based on these donor-acceptor polymers.

The morphology of the photo-active layer is a crucial factor in determining the efficiency of organic solar cells. The morphology controls charge generation and charge transport. There are numerous ways to control the morphology, e.g. by addition of a co-solvent or by thermal or solvent annealing. Likewise, there are several methods to study the phase separation by microscopy or diffraction techniques. However, tools for predicting morphologies are virtually absent and very much needed to make further rational progress in this area. We will present a first combined experimental and predictive theoretical study on the morphologies obtained for an alternating diketopyrrolopyrrole-quinquethiophene polymer (PDDP5T) with PCBM and their device performance. When casting mixed films from chloroform we observe the formation of pure PCBM domains, with a size that increases with layer thickness. For thicker films the short-circuit current of the cells decrease and the fill factor increases. This surprising and counterintuitive result can be explained as being the result of a spinodal liquid-liquid phase separation that occurs while drying the films. The dried films consist of a kinetically trapped morphology with a PDPP5T matrix containing large PCBM domains. Dispersed in the PDPP5T matrix we find a residual amount of PCBM with a concentration that decreases with thickness. For thin films the concentration is high enough to create charges, but these are more difficult to extract explaining the higher current but poor fill factor. For thicker films charges are only created at the interface of the PDPP5T matrix and the PCBM domains, but easily transported through the pure domains. This morphology qualitatively describes the J-V characteristics and the field assisted external quantum efficiency measurements performed. We were able to make a first predictive 2D model for the active layer morphology formation based on a square gradient model involving a Flory-Huggins description of the free energy of mixing. Interaction parameters were experimentally determined, retrieved from literature, or estimated. In the model the three relevant components PDPP5T, PCBM and chloroform are considered. When a highly concentrated liquid state is reached upon solvent-quenching, the model predicts that liquid-liquid phase separation occurs, much like we see experimentally, with one droplet phase being rich in PCBM and one continuous phase that is rich in PDPP5T, but also contains some PCBM. In time, the droplets grow larger and the concentration of PCBM inside the matrix is decreasing, in full agreement with the experimental observations. We consider our results as a first successful step towards predicting morphology development and resulting device performance in solution processed organic solar cells.

The improvement of power conversion efficiency (PCE) of organic solar cells often depends on designing new semiconducting materials and controlling the morphologies of the active layer. The mission of device optimization is mainly to achieve a “perfect” morphology, i.e. suitable domain size and domain purity. We find that domain size and domain purity are correlated to the miscibility of the fullerene in the polymer, which can be tuned by symmetrically substituting the side chains of poly[benzo[1,2-b:4,5-b&’]dithiophene-thieno[3,4-b]thiophene] PBDTTT-based polymers. The low crystallinity of those materials reduces the effect of crystallization driven phase separation and consequently highlights miscibility as a key controlling parameter. The results reveal that low miscibility leads to purer domains, which reduces bimolecular recombination and thus improves the fill factor. On the other hand, low miscibility results in relatively large domains. It is known that highly pure domains results in the lack of percolation pathways [1] and large domains decrease the D/A interface area. Both have a negative impact on device performance. Hence, suitable domain purity and domain size are key factors to generate high performance organic solar cells. By measuring the fullerene/polymer miscibility and the relative domain purity and size, the PCE can be predicted. This method offers guidance to design new polymer or fullerene materials for use in high efficiency devices.
[1] J. A. Bartelt, Z. M. Beiley, E. T. Hoke, W. R. Mateker, J. D. Douglas, B. A. Collins, J. R. Tumbleston, K. R. Graham, A. Amassian, H. Ade, J. M. J. Frechet, M. F. Toney, M. D. McGehee, Adv. Energy Mater. 2012. DOI:10.1002/aenm.201200637.

Considerable attention has been devoted to the development of processing protocols involving the use of organic solvents and high temperature annealing procedures to create optimal morphologies of poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC61BM) donor(D)-acceptor(A) materials for high efficiency solar cells.
One common paradigm behind improving solar cell efficiencies involves maximizing the interfacial area between the donor and acceptor components; another is enhancing the purity of the D/A phases. The latter has received little attention due to a lack of appropriate characterization procedures. We show that supercritical CO2 (ScCO2) processing under certain conditions of pressure and temperature leads to increases in short circuit currents, JSC, and the power conversion efficiencies, PCE, beyond that of any of the established high temperature processing protocols for this material system. Energy-filtered transmission electron microscopy (EFTEM) and electron energy loss spectroscopy (EELS) indicate that while the same nano-scale morphologies formed by thermal annealing are achieved using ScCO2, much purer D/A phases are achieved. Photoconductive atomic force microscopy reveals that ScCO2 processing yielded samples with larger active areas than those achieved using any of the conventional heating protocols. We also used an all conjugated copolymer to modify the morphology of the P3HT/indene-C60not; bisadduct (ICBA). The addition of a small weight fraction of block copolymer decreased the D/A domain sizes, and the resulting devices showed higher open-circuit voltage (VOC), JSC, and fill factor (FF), yielding an overall PCE increase of up to 20%. We will discuss details of the morphological and nanoscale photoconductivity characterization of these systems.

Recently, significant efforts have been dedicated to the interface engineering of polymer solar cells. Interface properties are critical to the device performance. Many interfacial materials, such as metal oxides, self-assembled monolayers and conjugated polyelectrolytes (CPEs), have successfully enhanced efficiency of solar cells fabricated through solution processing. More interestingly, several independent groups have found that the device performance could be considerably enhanced by treatment with polar solvents which are commonly used in interface engineering through solution processing in organic photovoltaics before deposition of metal electrodes. However, these interesting phenomena and attractive effects originating from simple solvents treatment are poorly understood.
In this study, we focus on understanding the effect of solvent treatment on the device physics of polymer solar cells. High efficiency polymer solar cells with PCE were achieved by methanol treatment. The effects of methanol treatment on the enhancement of device performance are shown to originate from an increase in built-in voltage across the device due to passivation of surface traps and a correspondingly increase of surface charge density. Combined with decreased series resistance and increased mobilities, accelerated and enlarged charge extraction and reduced charge recombination were obtained. All these effects induce a simultaneous enhancement in the open-circuit voltage, short-circuit current density, and fill factor of device performance after methanol treatment. This study provides deeper insight to understand the mechanism of surface engineering by solution processing which cannot exclude the effect of solvents. Furthermore, this strategy obviates laborious synthesis of interlayer materials and handles with environmental green solvent, which may offer a simple and efficient method to improve PCEs of polymer BHJ solar cells in laboratory study or industry fabrication.

Solution deposition using high boiling point additives provides a simple and widely used fabrication option for improving the power conversion efficiencies of solar cells comprised of conjugated polymer donor/fullerene acceptor blends. Previous examination of the resulting device active layer directly after spin casting have shown that the use of additives induces nucleation of polymeric crystallites within two minutes of deposition as well as promotes further crystallite nucleation over a prolonged period of time. Here, we follow how additives affect the evolution of the bulk heterojunction morphology from the solution state all the way to the solid state. It was recently discovered that by inserting furan moieties in the backbone of the conjugated polymers the use of relatively small solubilizing side chains is enabled because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents.1 These polymers enabled us to tune the side chain functionality and therefore to study specifically the effect of solubility of the polymer in the additive versus the role of the high boiling point of the additive. Using small angle X-ray scattering(SAXS) on the polymer and fullerene components in solution we are able to observe the effect of the additive on the polymer and fullerene solution phase confirmation and aggregation properties. These observations, across this polymer series, has allowed us to propose a mechanism for why solvent additives tend to produce the ideal degree of phase segregation for such a large variety of donor polymer systems. In order to link the solution phase behavior to the final film phase segregation and molecular packing, the active layer was investigated using SAXS and grazing incidence X-ray diffraction (GIXD), respectively on as-deposited films.

Bulk-heterojunction organic solar cells (OSCs) have attracted great attention in recent years, because of their advantages of low cost fabrication, light weight and flexibility. OSCs are usually composed of a blend layer of conjugated polymer donor and soluble C60 derivative PCBM acceptor sandwiched between an ITO positive electrode and a low work-function metal negative electrode. For achieving higher power conversion efficiency of the OSCs, great efforts have been devoted to the design and synthesis of new donor and acceptor photovoltaic materials with broad absorption in visible region, suitable electronic energy levels and high charge carrier mobility. In this presentation, I will talk about our recent progress on the two-dimension-conjugated (2-D-conjugated) organic photovoltaic donor materials, including 2-D-conjugated copolymers and solution-processable 2-D-conjugated organic molecules. Power conversion efficiency of the OSCs based on our new donor materials reached over 7%.

A novel PCBM-based n-type material, [6,6]-phenyl-C61-butyric styryl dendron ester (PCBSD), functionalized with a dendron containing two styryl groups as thermal cross-linkers, has been rationally designed and easily synthesized. In situ cross-linking of PCBSD was carried out by heating at a low temperature of 160 degree for 30 min to generate a robust, adhesive, and solvent-resistant thin film. An inverted solar cell device based on an ITO/ZnO/C-PCBSD/P3HT:ICBA/PEDOT:PSS/Ag configuration not only achieves enhanced device characteristics, with an impressive PCE of 6.2%. Recently, we demonstrated a new active layer configuration where bulk hetero junction of P3HT:ICBA upper layer was deposited on a cross-linkable PCBSD bottom layer with vertically-oriented nanorods to create a well-organized nanostructured interface. The free-standing C-PCBSD nanorods were fabricated utilizing AAO template-assisted strategy. The resultant device (ITO/ZnO/C-PCBSD nanorods/ ICBA:P3HT/ PEDOT:PSS/Ag) achieved an enhanced power conversion efficiency (PCE) of 7.3%, which is the highest value ever reported for inverted solar cells.

The power conversion efficiencies (PCEs) of organic bulk heterojunction (BHJ) solar cells have exceeded 10%. Numerous efforts have been made on chemical modifications of polymeric donors to expand absorption band, increase hole mobility, and fasten the charge separation. In order to fabricate a highly-efficient solar cell, high charge carrier mobility is very important. However, there is a tradeoff between material solubility and charge carrier mobility. Shortening π-π packing distance will increase the charge transport efficiency but meanwhile decrease the solubility significantly, vice versa. Traditional approach to tackle this problem is to screen different alky side chains that are attached to the polymer backbone, to find a balance point where the mobility is maximized while and the material is maintained solution-processable. Such method is tedious and case-dependent. In this context, we developed a general way to outweigh the traditional approach: making a copolymer with strong π-π interaction and good solubility by introducing a non-conjugate polymer as the side chain in a low molar ratio. In this approach, only a low fraction of the side chains of the conjugated polymer were replaced by the polystyrene side chains, and the tight π-π interaction of the polymer backbone was not interfered. As a result, high solubility and hole mobility of the materials were achieved at the same time. The BHJ solar cells based on this copolymer with the polystyrene side chains achieve a high PCE up to 7%.

Conjugated polymers based on Diketopyrrolopyrrole (DPP) units have been successfully applied in efficient single and tandem junction polymer solar cells (PSCs) due to their self-organize to feature high charge carrier mobilities and strong electron-deficient nature of DPP unit to control the optical band gap into the near-IR region of spectrum. However, DPP-based polymers with small energy band gap also suffer from low-lying lowest unoccupied molecular orbital (LUMO) levels, which provide low driving force for efficient exciton dissociation when combining with [6,6]phenyl-C71-butyric acid methyl ester ([70]PCBM). The poor conversion of absorbed photons to the collected charges gives low external quantum efficiencies (EQEs) with ~50% for DPP-based polymer solar cells, which are significantly less than EQE of 65% for other conjugated polymers with high power conversion efficiencies (PCEs). Here, a series of DPP-based conjugated polymers with increased electron-donating groups on the conjugated main chain are designed to tailor the LUMO level to enhance the driving force for exciton dissociation. PSCs based on these polymers as donor materials and [70]PCBM as an acceptor material can give high EQE up to 65% and photocurrent up to 16 mA/cm2. Further tailoring structure of DPP-based conjugated polymers with absorption up to 1000 nm can give high photocurrent up to 18 mA/cm2 and PCE up to 7%. The new DPP-based polymers are also applied in tandem solar cells to give PCE up to 8.9% with broad absorption from 300 nm to 1000 nm.

Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more solar cells with different absorption bands. Here, we present the design, synthesis, and characterization of a series of new LBG polymers based on the benzodithiophene and diketopyrrolopyrrole units specifically for tandem polymer solar cells.
By varying from furan to thiophene then to selenophene moiety that flanked to the diketopyrrolopyrrole unit, the bandgap of the polymers can be tuned from 1.52 to 1.38 eV. By modifying the side chain structures, the charge transport properties and polymer:PCBM thin film morphology are optimized. Single junction solar cells based on the new polymers showed power conversion efficiencies over 7% and photo-response up to 900 nm. When the polymers are applied to tandem devices (P3HT is used as another active material), power conversion efficiencies of 8 - 10% are achieved, which are among the highest efficiencies reported so far.
Due to the strong ability of the new polymers to absorb the NIR light and transmit the visible light, we further replaced the non-transparent metal electrode with a novel transparent silver-nanowire-composite electrode for the devices to make a “visibly-transparent solar cell”. Power conversion efficiencies of 4 - 6 % and average transmittance over 50% in the visible region (400 - 700 nm) have been achieved, demonstrating the great potential of polymer solar cells for real-world applications.

A novel three-step synthetic protocol for CPDT derivatives allowing versatile introduction of different side chains is presented. The dibrominated CPDT precursor was combined with the BenzoThiadiazole-bis(boronate) and the dibrominated dithienyl-ThiazoleThiazole precursor under Suzuki polycondensation conditions affording the corresponding donor acceptor low band gap polymers in good yield and high molecular weight. Depending on the side chains amorphous or semi-crystalline materials were obtained. Fractionation of the polymer in function of molecular weight shows a clear dependence with the performance of said materials in BHJ solar cells. Efficiencies close to 5% were achieved. A specific dependence of efficiency and architecture of the solar cell was observed in which the use of polythiophene polyelectrolyte interphase layers lead to substantial increases of the performance of the solar cells (close to 7% efficiency). A discussion of the interpretation of said effect in function of the properties at the interphase will be presented. Finally procedures are introduced to establish better purification methods which lead to improved electrical properties of the materials used. Finally degradation was studied of the active layer under thermal stress. A remarkable effect was observed of the introduction of multifunctional building blocks in the structure and the behavior or stability of the active layer under thermals stress.

Conjugated molecules, oligomers and polymers with intimately interacting donor-acceptor moieties provide light harvesting and electronic energy levels that can be tuned to optimize their utility in both bulk heterojunction and dye sensitized solar cell applications. In this presentation, we will explore the combination and interaction of a breadth of electron rich and electron poor species. Using dithienogermole and dithienosilole as donors in DA polymers, in conjunction with fullerene blend morphology control and solar cell device architecture, have lead to AM1.5 power conversion efficiencies in excess of 8%. Introduction of aryl end groups enhances the structural order in these systems. Donor-Acceptor-Donor (DAD) triads with internally fused DA cores provide a means of examining the directionality of conjugation in charge injection to metal oxide electrodes. Isoindigo-based polymers using a selection of electron accepting co-monomers, provides n-type doping characteristics, and its use in DAD molecular systems yields bulk heterojunctions with high morphological reproducibility.

Bulk-heterojunction (BHJ) polymer/PCBM solar cells based on low band gap conjugated polymers have attracted increasing attention due to their unique advantages including the ability to be solution processed over large areas, light weight and potentially application in flexible devices. Integration of electron donor-acceptor (D-A) functional units is an effective strategy to construct low band gap conjugated polymers. The band gap, HOMO and LUMO energy levels, carrier mobility and solubility of polymer can be properly tuned by adjustment of different electron donor and acceptor units. Many efforts have been focused on the study of electron donor units until now. However, development of new acceptor units will also be benefit for construction of high efficiency low band gap polymeric materials.
Quinoxalines have been widely recognized as an electron-deficient N-heterocycle and can be used as a good electron acceptor unit. Recently, several quinoxaline-based D-A type photovoltaic polymers with interesting optoelectronic properties have been constructed by copolymerization with different electron donor units. In this article, different aromatic rings (benzene and thiophene) have been fused to the quinoxaline unit to provide new electron acceptor units. D-A low band gap copolymers containing these quinoxaline derivatives as the acceptor units were synthesized by Suzuki polymerization with a well-known donor unit (carbazole). The effects of various acceptor units on photophysical and electrochemical properties and photovoltaic performances of these copolymers were studied in detail. By tuning of the length, and anchoring position of side chains on the quinoxaline derivative with a fused benzene ring, morphology of polymer/PCBM blend film could be efficiently manipulated and a PCE of 3.49% was achieved. Then, the band gaps and HOMO energies could be further decreased by the introduction of chlorine atoms, and a PCE of 4.06% was obtained. Finally, by using the quinoxaline derivative with a fused thiophene ring as the acceptor unit, an optimized PCE up to 5.31% was reached.

Low bandgap (LBG) conjugated polymers are a key component in a variety of polymer-based photovoltaic devices. Design and synthesis of a high performance LBG conjugated polymer has been attracted remarkable attention during the last decade. It has been shown that polymers constructed by selenophene have a relatively lower bandgap than their traditional thiophene counterparts. However, the effects of selenium-substitution on the photovoltaic properties of LBG polymers are not well understood so far. Here, we show that a reduction of the bandgap (Eg) and an enhancement of the carrier transport properties of a thienylbenzodithiophene (BDTT) and diketopyrrolopyrrole (DPP) based LBG polymer (PBDTT-DPP) can be achieved simultaneously by changing the sulfur atoms on the DPP unit into selenium atoms. The newly designed polymer, PBDTT-SeDPP (Eg = 1.38 eV), shows excellent photovoltaic performance in single junction devices with power conversion efficiencies (PCEs) over 7% and photo-response up to 900 nm. Tandem polymer solar cell and visibly-transparent solar cell based on PBDTT-SeDPP showed 9.4% and 4.4% PCEs, which are superior to those of PBDTT-DPP. The results demonstrate the great potential of selenium-substituted LBG polymers for versatile photovoltaic applications.

The morphology of the active layer in OPV devices is known to be critical to the performance of the device. Previously, it has been manipulated using a variety of additives, co-solvents and by thermal annealing. However a significant concern for the long term stability of the device is that changes in the active layer may occur during device operation or storage. Such changes may be due to reordering of the polymer material, or diffusion and aggregation of fullerene derivatives to a more thermodynamically stable state. One attractive solution to this problem could be cross-linking of the active layer to stabilise the morphology.
In this paper we present a series of high performance OPV donor polymers based upon the co-polymerisation of dithienogermole (DTG) with thienopyrroledione (TPD) functionalised with two different cross-linking moieties (alkyl bromide & oxetane). Co-polymers incorporating different amounts of cross-linker TPD were synthesised and their performance investigated. We find that the optimal device performance is observed for co-polymers containing approximately 20% cross-linker TPD. As-cast devices with these polymers exhibit efficiencies up to 7.4% in blends with PC71BM. Various methods of cross-linking were investigated and the effects of cross-linking on device performance and stability will be presented.

Bulk heterojunction (BHJ) solar cells are becoming one of the most promising low-cost solutions to our need for renewable energy sources. In the paper, we report on synthesis of several semiconducting polymers based on ladder-type acenes with the inclusion of fluorene, carbazole or thiophene units. The introduction of heteroatoms (sulfur, nitrogen) in the ladder-type fused-ring system leads to decreased band-gaps of these polymers. The optical absorption spectra for the ladder-type acene based polymers show that they have an improved light absorbing ability with harvested photons from UV to near IR (800 nm). The charge carrier mobilities for the synthesized polymers were investigated. These co-polymers were used as donor materials in both conventional and inverted BHJ solar cells. A high open circuit of 1.07 V was achieved for the ladder-type acene based polymers. For the inverted solar cells, a high efficiency of 6.2 % was kept for more than 40 days so far. Thus this work provides novel strategies for high performance photovoltaic materials in theoretically and experimentally.
References.
1. D. Cai, Q. Zheng, S-C. Chen, Q. Zhang, C-Z. Lu, Y. Sheng, D. Zhu, Z. Yin, and C. Tang, Novel ladder-type heteroheptacene based copolymers for bulk heterojunction solar cells, J. Mater. Chem. 2012, 22, 16032- 16040.
2. Q. Zheng, B. J. Jung, J. Sun and H. E. Katz, Ladder-Type Oligo-p-Phenylene-Containing Copolymers with High Open-Circuit Voltages and Ambient Photovoltaic Activity, J. Am. Chem. Soc. 2010, 132 (15), 5394-5404.
3. J. Huang, Z. Yin and Q. Zheng, Applications of ZnO in organic and hybrid solar cells, Energy Environ. Sci. 2011, 4, 3861-3887.

One of the prime challenges for further advancing the power-conversion efficiency (PCE) of polymer-fullerene organic solar cells lies in developing new materials for photoactive layers and optimizing the device architecture by including suitable interlayers and second absorber layers. For a range of diketopyrrolopyrrole-based small band gap polymers we demonstrate how the molecular weight of semiconducting polymers is a crucial parameter in obtaining high power conversion efficiencies in the range of 6-8% for single junctions, with band gaps down to 1.3 eV. By extending the spectral response to the near infrared region, i.e. to photon energies below 1.5 eV we were able to convert a larger fraction of the solar emission spectrum. We show how the use of interlayers and advanced anti-reflection layers enhance the incoupling of light. When the new semiconductor materials are combined with a wide band gap material it is possible to make very efficient tandem and multi-junction devices. As an example we will present a newly designed, high molecular weight, small band gap semiconducting polymer that provides high photon-to-electron quantum efficiencies down to 1.3 eV and reaches PCE = 7.0% in single junction cells with a fullerene acceptor and PCE = 8.9% in tandem cells, when combined with a wide band gap polymer-fullerene cell. The high efficiency of the tandem cell is achieved by an almost perfect complementarity of the absorption spectra of the two absorber layers that reduces thermalization losses.

Organic solar cells based on oligomer (small molecule) materials have achieved major progress in the last few years and are currently delivering the highest efficiencies of all-solid state organic solar cells, with certified record data slightly beyond 10%. However, for a broad application of organic solar cells, module efficiencies well beyond 10% are needed, which translates in small-are cells well beyond 15%. According to current estimates of organic solar cell efficiencies, this will require tandem solar cell structures which harvest a larger part of the sunlight.
In this talk, I will review our recent work on tandem organic solar cells based on evaporated small molecule structures and the pin-concept including controlled electrical doping. Using these concepts, it is rather straightforward to achieve both optical and electrical optimization of the tandem structures. The recombination contacts can be proven to be almost lossless. By introduction of transparent center contacts, it is possible to characterize both cells better and get insight into the relative working.

The power conversion efficiency (PCE) of the organic photovoltaic (OPV) devices has surpassed the 10 % milestone. The loss-mechanisms in the context of OPV like narrow absorption spectra of the donor materials and thermalization losses could be simultaneously addressed by tandem concept, which involves stacking two or more cells with complementary absorption spectra in series or parallel connection. As predicted previously organic tandem cell has the potential to reach the PCE as high as 15 %, while the PCE of the single cell is restricted to around 10 %.
The intermediate layer consisting of a hole- and an electron transport layer is the heart of tandem cell. Boosting the PCE of the tandem device toward 15 % theoretical efficiency requires the design of efficient and reliable intermediate layers. We have recently found that doped metal-oxides can provide superior performance in a recombination layer over the undoped pendants [1]. To ease the photon management between the subcells, we are employing ternary semiconductor composites. Ternary or multi-component donor-acceptor system was recently suggested to significantly broaden the near IR absorption in organic bulk hetero-junction solar cells. By changing the concentration of the ternary sensitizer, we are able to design efficient sub-cells with distinct absorption profiles, which can be easily integrated and stacked into a tandem cell. In this elegant approach fine tuning of the photons absorbed in each sub cell can be achieved through the varying thicknesses of photoactive layers, and overall > 70% performance enhancement can be observed as compared to the tandem device utilizing mono-component donor material. Moreover, the thickness of the intermediate layer is also tunable to modify the optical electrical field inside the tandem device to further improve the photocurrent as well as the efficiency.
Finally, we demonstrate tandem modules based on this material concept. Laser processing was recently proven to yield solar modules with outstanding high fill factors. Owning to the precise control over the ablation in micrometer range, the tandem modules exhibit a high geometry fill factor of ~90 % [2]. We now combine ternary system-based tandem devices with ultra-short laser patterning to generate 3 and 10 stripes lab-scale tandem devices. We believe that the combination of novel intermediate layer, ternary donor-acceptor system and laser patterning outlined in this contribution shows a clear roadmap towards the large-scale production of the solution-processed organic tandem modules.
[1] Li et al., Advanced Energy Materials, doi: 10.1002/aenm.201200659.
[2] Kubis et al., submitted.

While the efficiency of bulk heterojunction polymer solar cells increases into the 8-9% range, it is still clear that there is much room for improvement in efficiency, lifetime, and cost-effective synthetic and fabrication approaches. We are investigating ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors), which have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density. Recently, using a three-component system, we demonstrated for the first time that the open-circuit voltage of ternary blend solar cells is composition dependent and can be tuned across the full range defined by the corresponding limiting binary blends without negatively impacting the fill factor or the short circuit current. As a result, with judicious choice of components, the attainable product of short circuit current and open circuit voltage (and by extension the efficiency) in a single-layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. We have successfully demonstrated higher efficiencies in ternary blends based on two donor polymers and one fullerene acceptor than could be achieved in either of the limiting binary blends. Efforts toward developing a deeper understanding of the mechanism of operation in these ternary blends will also be discussed.

We report recent progress of ultraflexible organic photovoltaic cells and thin-film transistors that are manufactured on ultrathin plastic film with the thickness of 10 mu;m or less. First, we have demonstrated polymer based photovoltaic devices on plastic foil substrates less than two mu;m thick, with equal power conversion efficiency to their glass-based counterparts. They can reversibly withstand extreme mechanical deformation and have unprecedented solar cell specific weight. Instead of a single bend, we were able to form a random network of folds within the device area. We have also manufactured organic transistors on ultrathin plastic films in order to achieve sharp bending radius less than 50 microns. Bending cycle experiments will be presented to show the mechanical durability. Finally, future prospects and remaining issues of ultrathin organic electronics will be discussed.

Bulk heterojunction polymer solar cells that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based electronics. Despite the envisioned advantages and recent technology advances, so far the performance of polymer solar cells is still inferior to inorganic counterparts in terms of the efficiency and stability. There are many factors limiting the performance of polymer solar cells. Among them, the optical and electronic properties of materials in the active layer, device architecture and elimination of PEDOT:PSS are the most determining factors in the overall performance. In this presentation, I will present how we approach high performance of polymer solar cells. For example, by elimination of PEDOT:PSS, development of novel materials, fabrication of inverted polymer solar cells, we were able to observe over 50 % enhanced efficiency.

A current challenge in organic solar cells is to maximize both light absorption and charge collection. Novel cell designs that simultaneously achieve these are required. Here we demonstrate a nanostructured design consisting of a thin-conformal organic blend coated onto a ZnO nanorod array, incorporating a quasi-conformal top contact. The result is 4 improvemetns in 1 novel design: Short collection distances; Increased absorption path; light confinement; less material used.
The novel architecture shows efficient charge collection due to the thin blend layer, and increased absorption power as a result of enhanced light confinement within each nanorod due to the quasi-conformal thin silver coating. The cells presented herein show a high light absorption and achieve 2-3 times higher current density per unit volume of blend than previously reported nanorod/blend cells using the same materials. Our novel approach can be employed for arbitrary donor-acceptor bulk heterojunctions, and so will enable more efficient nanostructured solar cells than previously demonstrated.

We describe the use of a transparent exciton dissociation layer (EDL) that performs the function of a traditional exciton blocking layer ¹ (EBL), while at the same time using cascade-energy-level-alignment² to provide an additional interface for exciton dissociation, boosting photocurrent generation. Specifically, we introduce N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-2,2'-dimethylbenzidine (α-NPD) as a thin EDL between subphtalocyanine (SubPc) and the anode in an archetypal SubPc/C₆₀ device, providing exciton dissociation interfaces on both sides of the SubPc layer. Overall photocurrent in the device is increased from 3.94 to 4.90 mA/cm², solely due to a 66% increase in SubPc contribution to the external quantum efficiency. Our devices maintain the high open-circuit voltage (V_oc) of the SubPc/C₆₀ junction due to the unusually high built-in field generated by the α-NPD/SubPc heterojunction (which exhibits V_oc = 1.34V on its own, as we demonstrate for the first time³). Our results indicate that exciton dissociation layers are preferable to standard exciton blocking layers for maximizing photocurrent generation in device structures with any number of active layers.
¹P. Peumans, V. Bulovicacute;, and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001)
² G. Zhang, W. Li, B. Chu, L. Chen, F. Yan, J. Zhu, Y. Chen, and C. S. Lee, Appl. Phys. Lett. 94, 143302 (2009)
³A. Barito, M. E. Sykes, D. Bilby, J. Amonoo, P. F. Green, J. Kim, and M. Shtein, Adv. Mater., In Preparation.

The record efficiencies of organic tandem solar cells recently exceeded 10%. The key to further efficiency improvements will be a smart choice of complementary absorbing materials for both subcells within a properly designed device architecture. As the device absorption is ruled by thin film interferences and as the matching of currents in both subcells is of major importance in order to achieve best device performance, the thicknesses of each layer in the device have to be optimized carefully. However, experimentally optimizing numerous functional layers in a tandem device on a sample-by-sample basis is a very time and material consuming task.
In this work we present a facile route to an efficient experimental screening of new materials and layer thickness optimization in solution processable single junction and tandem polymer photovoltaic devices. Therefore, we developed a method of fabricating solar cells with a graded active layer thickness. Spatially resolved mapping of the solar cell short circuit current density for different light absorbing polymers under white light allows for a quick conclusion about the optimum active layer thickness within the device. When plotting the short circuit current density versus the local active layer thickness, the influence of the thin film interference pattern within the device becomes apparent. The measured short circuit current densities are in very good accordance with optoelectronic device simulations for all layer thicknesses.
For single junction devices, this technique allows for a fast material-saving functional layer thickness optimization. In tandem solar cells, this technique is an efficient tool for matching both subcell currents with only little experimental effort and hence building tandem solar cells with high power conversion efficiencies.

In this talk, we will discuss the approach of integrating materials design and interface engineering to significantly improve the performance of organic semiconductors for achieving high-performance and stable polymer photovoltaic cells. The performance of polymer solar cells is strongly dependent on the efficiency of light harvesting, exciton dissociation, charge transport, and charge collection at the metal/organic/metal oxide interfaces. To improve these important parameters, two parallel approaches have been used: 1) developing low band-gap polymers with proper energy levels and morphology, 2) optimizing interfaces between the organic and anode/cathode layers with functional conjugated surfactants and grapheme oxide to tune their energy barriers, and proper optical and device engineering. Molecule engineering approach has also been used to tune charge mobility and morphology of organic semiconductors to achieve significantly improved power conversion efficiency (PCE) of >8%.

Indium-tin-oxide (ITO) is widely used as anode electrode in organic photovoltaics and organic light organic electroluminescent devices. The devices based on a bare ITO, however, exhibited inefficient hole injection due to insufficient high work function. Thus, various surface treatments and chemical modifications of ITO have been attempted to change the work function of ITO in order to reduce the hole injection barrier height. The effect of chemical modifications also can be achieved to increase some parameters such as surface roughness, carrier concentration, mobility, and surface sheet resistance, so that with appropriate surface treatment significant improvement in the organic photovoltaics. In this work, ITO anode electrodes were modified by functional groups with COCl-terminal. The latter was more reactive. After words, the morphologies and physical properties of chemical modified ITO by COCl-terminal functional groups are investigated depending on the immersion time and concentration of the chemical modification using atomic force microscopy (AFM) and four-point method. And we performed to confirm the surface roughness of chemical modified ITO using molecular dynamics simulation.

Polymer solar cells (PSCs) based on blends of conjugated polymers and fullerene derivatives have attracted much attention as a renewable and clean energy sources since they offer the prospects of lower manufacturing costs, lightweight, solution processability, and flexibility. Significant progress has been made in this field, and the power conversion efficiencies (PCEs) of PSCs have reached 7-8%, primarily due to the development of new low-bandgap polymers and better control of the nanoscale morphology of the interpenetrating electron donor/acceptor networks. Efforts toward achieving high-performance photovoltaic devices using a bulk heterojunction mixture have focused on the development of efficient π-conjugated (semiconducting) polymers as photoactive electron donors.
Recently, linearly fused polycyclic aromatic compounds composed of thiophene, such as benzodithiophenes and anthradithiophenes have been studied in this field. In this poster, we report the use of a new naphtha[2,3-b:6,7-d&’]dithiophene (NDT)-based polymer in bulk heterojunction solar cells. We systematically investigated the synthesis, thermal stability, as well as the optical and electrochemical properties of these polymers. Detailed synthetic scheme, optical, electrochemical, and photovoltaic properties of the copolymers will be presented.

Organic solar cells (OSCs) have drawn much attention as a sustainable energy source due to potential advantages such as low fabrication cost, light weight and flexibility. One of the most promising OSC device structure is based upon mixed bulk heterojunction (BHJ) thin film. In this mixed BHJ structure, however, the film morphology of electron donor and acceptor in nanoscale is difficult to control. Furthermore, this nanostructure could be easily changed and unfavorable large phase separation is formed by long-term operation or high temperature conditions. This morphological change drastically lowers device performance.
Extensive research has been done on nanostructure control of domains to overcome the problem. Use of a dyad molecule, i.e. covalently attached donor and acceptor, is one of the promising strategies to prevent the large phase separation and therefore to realize efficient charge separation. We previously disclosed dyad molecules that showed PCE exceeding 1%. [1] Nevertheless, no reports have covered the construction of morphologically stable OSCs using dyad molecules.
In the current contribution, we examine the morphological stability on dyad molecules. We used previously reported highly crystalline oligo(p-pheneylenevinylene) (OPV) [2] as the donor part, and newly introduced C₇₀ fullerene.
The OSC devices based on OPV-C₇₀ dyad showed a broad photocurrent response up to 700 nm was observed due to the absorption of C₇₀ fullerene. This photocurrent increase led to the highest PCE of 1.92% reported to date for dyad-based OSCs. [3] The thermal stability of morphology in the films with OPV-C₇₀ dyad was studied compared with physically mixed BHJ films with OPV and [6,6]-phenyl-C₇₁-butyric acid methyl ester. The devices were thermally annealed at 110 °C for 5 min, 1 day, 2 days and 3 days under N₂ atmosphere. For comparison, devices not subject to thermal annealing were also fabricated. The mixed devices showed a low PCE of 0.65% for the as-cast film, and PCE decreased to 0.23% upon the annealing for 3 days. In contrast, the devices based on OPV-C₇₀ dyad showed high PCE of 1.52% even after 3 days of thermal annealing. Atomic force microscopy images revealed that the mixture BHJ films had the aggregation of PC₇₀BM with the sizes of over 100 nm even in the as-cast film, and the larger aggregations were observed as the annealing time was prolonged. In contrast, such large phase separation was not observed in OPV-C₇₀ dyad films after 3 days of the annealing. These comparisons indicated that the dyad film had a thermally stable morphology. [3]
[1] S. Izawa, K. Hashimoto, K. Tajima, Chem. Commun., 2011, 47, 6365-6367.
[2] T. Nishizawa, K. H. K. Lim, Tajima, K. Hashimoto, J. Am. Chem. Soc., 2009, 131, 2464-2465.
[3] S. Izawa, T. Nishizawa, K. Hashimoto, K. Tajima, Phys. Chem. Chem. Phys., in press.

Plasmonic hybrid materials incorporating metal nanoparticles and semiconducting materials such as conjugated polymers and colloidal quantum dots have attracted intense interests for applications such as optoelectronics and biosensing. Engineering of such hybrids via simple and cost-effective self-assembly methods is sought in order to control the plasmon-exciton interaction between components towards improved performance and depending on the foreseen application.
Here we demonstrate self-assembly of core/shell Au/SiO2 nanoparticles with a cationic conjugated polymer (polythiophene derivative) onto plasmonic hybrids with tunable photoluminescence output. By varying the SiO2 shell thickness in the rage 5-30nm and for Au nanoparticles of 50nm core size, we demonstrate the ability to tune the conjugated polymer photoluminescence from regimes of efficient PL quenching to PL enhancement of up to five fold. We do this by using alternating laser single particle confocal photoluminescence spectroscopy by exciting outside and onto the Au surface plasmon resonance. The utility of the proposed hybrids for optoelectronic applications is demonstrated by coating the Au/SiO2/conjugated polymer plasmonic hybrids with fullerene acceptor molecules for which we observe plasmon assisted enhancement in the charge transfer rate between polymer and fullerene.

We report here the new donor polymers for highly efficient organic solar cells. We have synthesized and characterized a number of narrow band-gap polymers in which conjugated alternating blocks are composed of various electron-rich and electron-deficient moieties. Among these materials, donor polymer-1 in a solid state showed narrow band-gap of 1.58 eV and a high absorption coefficient of approximately 200,000 cm-1 together with a high FET hole mobility of 8.9E-03 cm2/Vs. Solar cell properties of the donor polymer-1 were investigated using [70]PCBM as an electron acceptor in bulk heterojunction (BHJ) type devices with a structure of anti-reflection film / glass substrate / ITO / PEDOT:PSS / donor:acceptor / cathode buffer layer / metal cathode. The photovoltaic device having approximately 300 nm thick BHJ active layer showed a 10.6 % (Jsc = 21.7 mA/cm2, Voc = 0.762 V, FF = 0.641) of power conversion efficiency under an illumination of AM 1.5 G at 100 mW/cm2.

We investigate the driving force required to sustain photoinduced charge transfer (CT) in hybrid bulk heterojunction (BHJ) blends between a conjugated polymer and PbS quantum dots (QDs). We use photoluminescence (PL) quenching and photoinduced absorption (PIA) spectroscopy to probe charge transfer at the polymer/QD interface. We adjust the driving force for charge transfer by altering the QD size, thereby changing the energy level offsets between donor and acceptor materials. We observed that CT can be suppressed by reducing the driving force for hole transfer, and use these data to determine the minimum driving force for charge transfer at the polymer/PbS interface. Finally, we correlate the photoinduced CT properties with device performance data.

Graphene and graphene oxide (GO) have been applied in flexible organic electronic devices with enhanced efficiency of polymeric photovoltaic (OPV) devices. In this work, we demonstrate that a GO buffer layer which was spin-coated on ITO substrate without any further treatment can substantially enhance storage/operation stability of OPV. The power conversion efficiency (PCE) of a standard copper phthalocyanine (CuPc)/ fullerene (C60) based OPV device with a 2 nm GO buffer layer shows about 30% enhancement from 1.5 to 1.9%. More importantly, while the PCE of the standard device drop to 1/1000 of its original value after 60-days of operation-storage cycles; those of GO-buffered device maintained 84% of initial PCE even after 132-days. Atomic force microscopy studies show that CuPc forms larger crystallites on the GO-buffered ITO substrate leading to better optical absorption and thus photon utilization. Stability enhancement is attributed to the diffusion barrier of the GO layer which slow down diffusion of oxygen species from ITO to the active layers.

Carboxylic acid functional group incorporated dye molecule was introduced to modify the surface of ZnO buffer layer for inverted polymer solar cells. The organic dye anchored on the ZnO surface through carboxylate bonding to reduce the shunt path on bare ZnO surface and provides better interfacial contacts and energy level alignments between the ZnO layer and the photoactive layer. This phenomenon consequently leads to highly enhanced photovoltaic parameters (FF, Voc, and Jsc) and power conversion efficiencies (PCEs). The inverted solar cells with the dye modified ZnO layer also show longer device lifetimes compared with their counterparts without the dye layer.

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is the oldest [1], the most used [2] and, to date, the most effective acceptor for bulk heterojunction polymer solar cells. The popularity of PCBM, aside from its unique electronic properties, arises from good solubility in many organic solvents and from easy processability. The methyl phenylbutyrate substituent imparts to PCBM - and to its C70 homologue - proper miscibility with poly(3-hexylthiophene) (P3HT), as well as with other donor polymers, able to form the peculiar nanostructured, percolating morphology required by the photoactive film. On the other hand, unfunctionalized C60 and C70, which would be advantageous for their availability and relatively low cost, have been seldom studied, because of their poor solubility in organic solvents that causes aggregation and poor film morphology.
Here, we report bulk heterojunction solar cells based on P3HT:C60 and P3HT:C70 active blends containing a polycondensed aromatic hydrocarbon (PAH) additive that acts as fullerene dispersant and produces smooth active layers free from large crystals. AFM and optical microscopy measurements confirm the solubilization effect. ITO/PEDOT:PSS/P3HT:C60:PAH/Al devices exhibited PCE=1.54% without annealing step under AM1.5G illumination, while ITO/PEDOT:PSS/P3HT:C70:PAH/Al cells efficiencies were as high as 2.5%. To our knowledge, this is one of the highest device efficiencies for photoactive blends containing unfunctionalized fullerenes reported in the literature to date. A detailed computational study showed how the fullerenes form strong intermolecular bonds with the PAH derivative due to π-π interactions. The resulting adduct is characterized by a HOMO level close that of PAH and a LUMO level close to that of fullerene. The theoretical results have been compared with cyclovoltammetric data. UV-visible and PL (photoluminescence) spectroscopy has been used to study the charge or energy transfer processes between the three components of the photoactive blends.
[1] J.C. Hummelen, B.W. Knight, F. Lepeq, F. Wudl, J. Yao, C.L. Wilkins, J. Org. Chem. 60, 532-538 (1995)
[2] M.T. Dang, L Hirsch, G. Wantz, Adv. Mater. 23, 3597-3602 (2011)

The lifetime performance and reliability of photovoltaic (PV) modules are critical factors in their successful deployment. Both organic and inorganic thin film technologies use transparent conducting oxides (TCOs) such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO) as electrodes. These TCOs must retain their functionality over the duration of a PV system&’s lifetime in diverse outdoor environments. Interfaces are frequently an avenue for accelerated degradation in PV systems; this degradation is promoted by exposure to environmental stressors such as irradiance, heat and humidity. Understanding TCO degradation is critical to improving stability and extending the lifetime of PV modules such as organic photovoltaics.
TCOs, including indium tin oxide (ITO), were exposed to damp heat (DH, 85 °C, 85% relative humidity) and modified ASTM G154 cycle 4 hot irradiance (70 °C, 1.55 W/m² @ 340 nm, comparable to 5X Suns UV irradiance) for more than 600 hours. After each exposure a standard cleaning process was used and the TCOs&’ electrical and optical properties and surface energies were determined. Using contact angle measurements with multiple fluids, the total surface energy of ITO decreased from 76 mJ/m² with both DH and hot irradiance exposure.
TCO degradation affects both the bulk material and the interfaces, often causing delamination. The TCO/PV absorber interface can be engineered using interfacial layers (IFLs), tuning the surface free energy to match the next layer in a device, increasing adhesion and controlling the interfacial electrical properties. We used organofunctional silanes, including octa-decyl-trichloro silane (OTS) and 3-aminopropyl-triethoxy silane (APTES) and 3-aminopropyl-dimethyl-ethoxy silane (APDMES), to modify the TCO/PV absorber interface. Historically, silanes have been used as IFLs in other optoelectronic devices (e.g. OLEDs). We characterized the electrical and optical properties and surface energies of these silane mono- and multilayers on ITO films, and compared the results to a standard OPV polymer, PEDOT:PSS. Results demonstrate that varying the functionality of the silane is a simple method of tuning and customizing the surface energy of the TCO, changing the water contact angle from 1° (bare ITO) to 57° (APDMES) to 94° (OTS), without affecting the transparency or conductivity. Degradation data of TCO/silane stacks will be presented.

Organic photovoltaic (OPV) solar cells, which comprise photoactive layers with organic nanoparticle acceptors dispersed in conjugated polymers, have generated great scientific interest. The ease of manufacturing, relative low costs, light weight, flexibility and low operating temperature of OPVs also make them attractive candidates for commercial use. The power conversion efficiency (PCE) is still limited to less than 8-9% in OPVs due, in part, to the recombination losses of excitons. Such losses can be diminished by maximizing the contact surface area between the electron donor and acceptor materials in the photoactive layer. The photoactive layer is most often synthesized through spin coating of a solvent containing nanoparticles dispersed in a polymer. The molecular interactions between the solvent, nanoparticles and polymers during the spin coating process influence the morphology of the photoactive layer and hence determine the PCE. Prediction of optimal synthesis parameters, such as choice of solvent, processing temperature, and concentration of nanoparticles and polymers, requires fundamental understanding of the mechanisms that govern the agglomeration of nanoparticles and polymers in solvents. In the current study we have simulated systems comprising a commonly used organic nanoparticle, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), in various solvents to correlate solvent-nanoparticle interactions with the size of the agglomerate structure of PCBM using molecular dynamics method. Our results demonstrate that PCBMs form large clusters in toluene while they are relatively dispersed in indane. The binding energies between fullerenes, obtained from the potential of mean force (PMF), are 0.85 kcal/mol in toluene and 0.4 kcal/mol in indane indicating relatively enhanced hydrophobic behavior of PCBM in toluene compared to that in indane. Our results further demonstrate that agglomerate size can be tuned by using solvent mixtures. A systematic study of the effect of temperature and concentration of PCBM on agglomerate formation of PCBM shows that the extent of agglomerate formation increases with increasing temperature and concentration in all PCBM-solvent systems that were simulated. We have also studied mixed systems comprising a widely used conjugated polymer poly (3-hexylthiophene) (P3HT) in PCBM and solvents in order to understand the influence of P3HT on PCBM agglomerate formation during synthesis of photoactive layers. Our results determine the nature of association of PCBM with P3HT and diffusion of each species at various concentrations. We have analyzed the agglomerate structures of P3HTs and PCBMs in the solvents and find that they create networks suitable for transporting holes to photoanode and electrons to photocathode respectively. Our results provide fundamental insight that can aid in the selection of favorable solvents for processing PCBM and P3HT for OPV applications.

Transformative device technologies based on new organic electronic materials like OPVs and OLEDs exploit interfaces to control fluxes of charge and/or energy. Inability to design, control and engineer these multifunctional nanoscale interfaces remains the key bottleneck in their widespread use for the next generation organic electronic devices like OLEDs and OPVs. Here we study the effects of inserting an insulating layer at the interface of the well studied exciplex emitting system MTDATA/BPHEN. By incorporating a thin insulating layer we decrease the exciplex recombination rate and decrease the exciplex binding energy. We measure these effects by exciplex emission lifetime measurements and emission spectra as a function of insulator thickness. In addition we also measure the reverse bias dependent photocurrent as a function of insulator thickness across a bilayer device (ITO/MtData/Bphen/LiF/Al). The photocurrent shows a peak ~480 nm that corresponds to the absorption into the exciplex. We find that the peak intensity and area grows with increasing reverse bias indicating that the increase is the photocurrent is directly proportional to the exciplex dissociation. Further increase in the photocurrent yield is observed by incorporating a thin insulator (LiF) at the donor-acceptor interface that suggests an increase in the lifetime of the exciplex. The average exciplex lifetime of ~10 ns using time resolved fluorescence measurements that is expected to increase with the incorporation of an insulating tunnel barrier, consistent with the increase in the exciplex dissociation rate. Finally, a simple transport model will be used to explain the change in the charge transfer and recombination rates.

A novel layer-by-layer method was developed to prepare hybrid nanocomposites. Combinations of SWCNT, CdSe QDot and metalic NPs were prepared in a thin film and transferred to ITO substrate. Typically monolayers of nanoparticles were held between thin layers of semiconducting SWCNTs. The resultant thin films were used in three electrode and two electrode configurations. The devices exhibited enhance psuedocapacitance in the dark. When driven to ~2 V vs SHE the grey films turned red-orange. This electrochromic effect was reversible and switchable. Galvanostatic charge-discharge curves showed increased capacitance near the band edge of the QDots. The films exhibited significant photo-current under mild white light illumination. All of the materials are characterized spectroscopically, electrochemically, and structurally by SEM, TEM, and DLS. Surface area and porosity of the materials are determined by BET and BJH methods. Spectroelectrochemical data will be presented that shows the electronic interaction of the QDot dopants in the SWCNT matrix.

Metal-Organic Frameworks (MOFs) are highly porous supramolecular crystals. The structure and chemistry of these ordered compounds can be tailored to create assemblies with a variety pore architectures, well defined within a functional molecular framework. Control over the composition of this framework further enables tuning of the optical absorption and electronic character of the porous MOF structures. Recognizing the critical, deleterious effects supramolecular disorder can introduce in organic photovoltaics, we seek to take advantage of MOFs as structurally, chemically, and optoelectronically tunable templates to direct organized interactions between donors and acceptors in hybrid organic photovoltaics (HOPV). Guided by periodic Density Functional Theory calculations, we have synthesized and characterized a family of MOFs, designed as platforms to explore the nanoscale ordering and energy-transfer processes of acceptors (e.g., fullerenes) and donors (e.g., oligothiophenes) infiltrated within the ordered porosity of MOF architectures. In addition, we have assembled and tested novel MOF-based photovoltaic devices, evaluating 1) how order imposed by MOF-mediated donor-acceptor interactions affect traditional OPV systems, and 2) the viability of MOFs as optoelectronically-active elements within a new class of HOPV devices. Exploiting these chemically and structurally versatile supramolecular structures represents a promising new approach for next generation photovoltaic development.
Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Two anthracene-based star-shaped conjugated small molecules, HBantHBT, and BantHBT, are used as electron-cascade donor materials by incorporating them into organic photovoltaic cells prepared using a PBTADN:PC71BM blend. The small molecules penetrate the PBTADN:PC71BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short-circuit current (JSC) and fill factor (FF) are obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN:BantHBT:PC71BM blend solar cells is characterized by a J SC of 11.0 mAcm-2, an open circuit voltage (VOC) of 0.91 V, a FF of 56.4%, and a power conversion effi ciency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW-2). The effects of the small molecules on the ternary blend are investigated by comparison with the traditional P3HT:PC61BM system.

There are many parallel methods to increase the power conversion efficiency of bulk heterojunction organic solar cells. One method is to increase the photocurrent by broadening the spectral absorption of the solar cell through the incorporation of additional absorbing materials with non-overlapping absorption spectra in a ternary blend cell.
We have shown that the formation of a ternary blend bulk heterojunction cell made from regioregular poly(3-hexylthiophene), (6,6)-phenyl C60 butyric acid methyl ester (PC60BM), and tetra-t-butyl functionalized silicon naphthalocyanine bis(trihexylsilyloxide) (SiNc) results in increased photocurrent from SiNc absorption as well as increased open-circuit voltage (VOC). This produces an increase in the power conversion efficiency from 4.4% to as high as 5%.
Through external and internal quantum efficiency measurements and experiments with other donor/acceptor combinations, we have found that in addition to the energetic requirements of the SiNc molecule necessary to facilitate efficient charge transfer, the morphology of the ternary blend cell must be such that the majority of SiNc molecules be located within an amorphous mixed phase located between the two relatively pure donor and acceptor phases. This has significant ramifications when considering the optimal structure of the bulk heterojunction device.

Plasmonic metal nanoparticles have been used to enhance the performance of thin-film devices such as organic photovoltaics based on polymer/fullerene blends. We show that silver nanoprisms accumulate long-lived negative charges when they are in contact with a photoexcited bulk heterojunction blend composed of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM). We report both the charge modulation and electroabsorption spectra of silver nanoprisms in solid state devices, and compare these spectra with the photoinduced absorption spectra of P3HT/PCBM blends containing silver nanoprisms. We assign a previously unidentified peak in the photoinduced absorption spectra to the presence of photoinduced electrons on the silver nanoprisms. We show that coating the nanoprisms with a 2.5 nm thick insulating layer can completely inhibit this charging. These results may inform methods for limiting metal-mediated losses in plasmonic solar cells.
References:
1) Abhishek P. Kulkarni, Kevin M. Noone, Keiko Munechika, Samuel R. Guyer, David S. Ginger, Plasmon-Enhanced Charge Carrier Generation in Organic Photovoltaic Films Using Silver Nanoprisms. Nano Lett., 2010, 10, 1501 - 1505.
2) Michael Salvador, Bradley A. MacLeod, Angela Hess, Abhishek P. Kulkarni, Keiko Munechika, Jennifer I. L. Chen and David S. Ginger, Electron Accumulation on Metal Nanoparticles in Plasmon Enhanced Organic Solar Cells, ACS Nano, 2012, DOI: 10.1021/nn303725v.

Recently, many research groups have tried to enhance efficiency of polymer solar cells (PSCs). In particular, interface between anode and semiconducting polymer has been studied as one of candidate for high efficiency and long-term stabilities of PSCs.
The high efficiency can be achieved by increase in workfunction of anode. Therefore, thin film with high workfunction called hole extraction layer has been inserted between anode and active layer. In PSCs, Poly(3,4-ethylenedioxthiophene) /poly(styrenesulfonate) (PEDOT:PSS) is most commonly used HEL due to its solution processability. Although thin layer of PEDOT:PSS is easily fabricable and aligns the workfunction between ITO and active layer, highly acidic property (PH~1) of this material damages indium-tin-oxide and degrade the device performance. Lately, solution processed transition metal oxide such as WO3 or MoO3 is developed to overcome the drawbacks of PEDOT:PSS. However, the interface between ITO and solution processed metal oxide is not studied neither electrically nor chemically.
In this work, we would like to demonstrate the device with solution processed MoO3 (s-MoO3) on O2 plasma modified ITO. It is found that by decreasing the pressure of plasma treatment, the atomic composition of ITO changed to be oxygen rich. This change in chemical composition alters not only workfunction but also electrical conductivity and surface energy of ITO. Regardless of O2 plasma conditions, the energy band alignment between HOMO level of polymer and ITO anode is improved due to increased workfunction of ~5.0 eV. In addition, the polar surface produced by O2 plasma treatment makes it possible for conformal coating of s-MoO3 on ITO. However, the electrical conductivity differ more than four orders of magnitude (0.024 to 4.08 x 102 S/cm). Therefore, the device with 50 mTorr O2 plasma treated ITO shows small PCE value of 1.98 %. However, by reducing amount of oxygen on ITO / MoO3 interface by using 1000 mTorr O2 plasma, the high efficiency of 3.43 % is achieved.

Organic photovoltaics represent an important class of solar technology with potential for cost effective and low energy payback utility. The solution processability of such devices is critical to keep in line with the vision of low cost. The results of solution processed NiO, MoOx and TiOx in standard as well as inverted solar cell structure will be presented. It is found that the metal oxides improve device stability when used to replace the ubiquitous but acidic PEDOT:PSS interfacial layer, possibly due to the suppression of trap states as revealed by IMPS. It is also found that the deep energy structure of MoOx, along with the presence of gap states, enables such n-type semiconductors to act as hole selective layers. Results from ongoing work of TiOx will be presented.

Efficient OPV cells based on state-of-the-art high performance PTB7, thienopyrrolodione dithienogermole (TPD-DTG), and bithiophene imide (BTI) polymers are made in both conventional and inverted architectures. It is observed in all these systems that inverted cells outperform conventional cells in both short circuit current and fill factor. The higher current for inverted device is originated from enhanced light absorption and charge transport. Thus vertical depth profiling by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) on both device architectures is performed and the depth dependent concentration of polymer vs. PCBM is obtained. Optical modeling is investigated and it reveals that the inverted cells geometry is beneficial for better light absorption. The difference in fill factors is investigated by electrical impedance measurement and it reveals lower recombination resistance resistance for inverted than conventional device. In addition, active layer morphology is investigated at both active layer/ZnO and active layer/PEDOT:PSS interfaces.

The influence of the morphology of conjugated polymer (P3HT)/fullerene (PCBM) based blends on the nanoscale charge transport and final device performance was investigated. A combination of nanoscale techniques (transmission electron microscopy and scanning probe microscopy) were used to study the donor-acceptor phase separation and their 3D ordering. Spatially resolved charge transport processes were measured using conducting atomic force microscopy under various optical and electrical bias. A combination of frequency and time domain techniques were used to probe the charge carrier dynamics in sub-microsecond - ms regions. Two sets of blends were prepared with fine phase separation (~ 10 -15 nm) and coarse phase separation (~ 25 - 70 nm). The local electron and hole mobility was estimated from the I-V spectroscopy data measured using current-sensing atomic force microscope tool. The fine phase separated blends showed average hole and electron mobility of 5 (±0.5) x 10^-6 m²/V-s and 1.5 (±1) x 10^-6 m²/V-s respectively, where as for the coarse phase separated blends these values were about 1.4 (±0.5) x 10^-6 m²/V-s and 4.9 (±1) x 10^-8 m²/V-s respectively. The two-orders of difference in the mobility of electrons and holes resulting in unbalanced charge transport in the coarse separated blend. Intensity dependent I-V measurements suggested that strong bimolecular recombination was dominant in coarse phased blends while weaker monomolecular recombination was dominant in fine phased blends. Results suggested that internal structure and morphology of the blends play a crucial role in determining nanoscale charge transport, carrier recombination mechanisms and device performance in bulk heterojunction devices.

Understanding the dominant loss processes in contemporary high-efficiency polymer:fullerene solar cells is critical to advancing this promising technology. We examine a representative high-performance system composed of the copolymer poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b&’]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-C) mixed with PC70BM, which has a reported power conversion efficiency of ~6.5% under simulated AM 1.5G conditions. We reveal the reasons behind the compositional dependence of device performance by probing CT-state emission with electroluminescence and carrier recombination with transient photovoltage and charge extraction. In the optimal composition range of 50-67 wt% PC70BM, we find that the dominant recombination process is of a nongeminate and non-Langevin origin. Unlike previous reports on archetypal polymer:fullerene systems, both the carrier mobility and lifetime significantly increase with increasing PC70BM wt%, showing that these important microscopic properties can be simultaneously enhanced in an impactful manner. Increasing the PC70BM concentration, however, results in less favorable active-layer absorptive properties, yielding an optimal composition at ~60 wt.% PC70BM that is a fine balance between absorptive and electronic losses. Our results suggest that the origin of the favorable changes in charge transport and recombination dynamics with increasing PC70BM content is related to composition-induced changes in the active-layer electronic structure. These findings have broader implications in terms of the understanding and optimization of current-generation polymer:fullerene solar cells.

Organic photovoltaic (OPV) devices have attracted much attention due to their promising properties such as light weight, cheap cost, flexibility and possibility of roll to roll processing. However, the quantum efficiency of OPV is low due to the comparatively low carrier mobility and charge recombination. A thinner active layer can lower the probability of charge recombination. Various methods have been reported to enhance efficiency of OPV, although the more effective methods involve using expensive and complex processes. In this work, we demonstrate a novel and straightforward method to achieve enhancement of EQE by using domain-selective etching (DSE). The PET is composed of crystalline and amorphous domain. By plasma treatment, the amorphous domain of PET could be etched selectively. We observed the various nano and micro-structures such as rod and cone shape using DSE. By controlling etching conditions, we can obtain the scattering enhanced nano or micro structure. With increasing the light scattering, the light diffusion length in OPV is longer. The introduction of DSE increases the light absorbance by scattering resulting in organic solar cells with a 20% increase in photocurrent.

Recently diketopyrrolopyrrole (DPP) based polymers have shown superior performance in transistors and photovoltaic devices due to high ambipolar charge carrier mobility and low band gaps. In this study we have used different additives namely 1-Chloronapthalene(CN), 1,8 Diiodooctane(DIO) and 1,8 Octanedithiol(ODT) in PDPP3T:PCBM blend solution. It was found that inclusion of any additive improves the short circuit current density and power conversion efficiency (2.38 to 4.1%) significantly as observed by other researchers. However we also observed that the phase separation got coarser in the films casted with additives contradictory to previous reports. Films coated without additives showed finer phase separation. AFM phase images on the films showed that the PDPP3T formed short fiber like domains in all cases. But in the case of films casted without additives the fiber like domains consisted of several grains adjacent to each other. Raman spectra on these films revealed high crystallinity in all films and with additives the FWHM of C=C peak reduced from 31 cm-1 to as low as 14 cm-1. Also SCLC hole mobility increased by 4 to 10 times with different additives. Hence in PDPP3T based bulk heteriojunction devices the structural ordering of PDPP3T is of more importance for photogeneration and charge transport than phase separation. In films casted without additives higher grain boundaries and therefore higher defect density hinder the charge transport and increase recombination. Further we will study the crystalline fraction of pristine and blend films when casted with and without additives using XRD. Also transient photovoltage and photocurrent spectroscopy will be performed to calculate charge carrier density, lifetime and mobility.
[1] Bijleveld, J.C., et al., Journal of the American Chemical Society, 2009. 131(46): p. 16616-16617.
[2]Moon, J.S., et al. Nano Letters, 2010. 10(10): p. 4005-4008.

Molecularly doped p-type semiconducting polymer, (PEDOT:PSS) has been studied as the emitter layer in a-Si:H solar cells on transparent oxide coated (TCO) coated glass and stainless steel substrates. The advantages of semiconducting polymer is it&’s high conductivity as well as high transparency, conductivity can be as high as 4000 S/cm. So far, the conductivity of p-type PEDOT:PSS, 30 S/square has been achieved. The optical gap of PEDOT:PSS can be varied depending on the mixing ratio of PEDOT with PSS, this gives flexibility to create fixed band offset at p/i interface, this will certainly led to higher open circuit voltage (V). We fabricated p-type PEDOT:PSS/a-Si:H/n+-a-Si:H/stainless steel or TCO/glass heterojunction solar cells at ambient by solution casting or spin coating technique. In some cases, oxide (SiO<2/Sub>) layer was etched out by hydrofluoric (HF) acid, in some cases it was there as it is. The dark and light J-V curves in two cases are compared, it has been observed that intensity dependence of two type of J-V curves are different, light J-V curves merge for without SiO<2/Sub> layer and light J-V curves appear different, where SiO<2/Sub> layer is there. When the solution casted diode is illuminated through glass/TCO side, we observed, V ~ 720 mV and J ~ 1-2 mA/cm<2/Sup>. The low V is because of large band off-set between PEDOT:PSS and a-Si:H i-layer. The contact layer at p-side was taken directly from PEDOT:PSS. The light J-V characteristics of a-Si:H solar cells shows V ~ 540 mV and Jsc ~ 1 A/cm<2/Sup> for spin-coated diodes. The oxide layer at p/i interface influences photoinduced charge from PEDOT:PSS to i-layer. This type of photoinduced charge transfer process in polymer happens under UV light, however, in our case, we found the effect for the first time under red light. When same diode is illuminated by He-Ne laser through TCO/glass side as well as through polymer (PEDOT:PSS) side, we observed V ~ 585 mV and 715 mV respectively. Higher open-circuit voltage (715 mV) when illuminated through polymer layer is most likely due to photo-induced charge transfer process. We observed diodes fabricated by spin-coating technique, the turn-on voltage in the forward bias is in the range of 1.3 to 1.5 V, some cases, it exceeds 1.5 V. Thus, there is a possibility to have open-circuit voltage higher than 1.5 V in case of a-Si:H solar cells, if emitter layer is p-type semiconducting polymer. We observed leakage current (J) significantly decreases if PEDOT:PSS is spin coated on top of a-Si:H layer, thus, there is possibility to achieve higher fill factor (> 0.72) in a-Si:H solar cells.

1Present address: Silicon Solar, Inc., Fremont, CA 94536.

Organic photovoltaic devices (OPVs) have attracted considerable attentions due to their potentials for flexible, light weight, and low-cost applications of energy devices. Indium tin oxide (ITO) is the most commonly used transparent electrode for OPVs because of its high transparency and high conductivity. However, there are several issues for its application as transparent electrodes in large area, flexible OPVs due to its brittleness, its scarcity of indium, high temperature processing. Recently, metal grids coupled with conducting polymer have been researched and reported on the possible replacement of ITO in optical devices.
Therefore, we have fabricated metal (Au) grids by lift-off method and then spin-coated PEDOT:PTS to construct a metal grid/conducting hybrid transparent electrodes. In addition, high-refractive index metal oxide (TiO₂) was patterned to form underneath metal grids to increase the transmission. The fabricated TiO₂/metal grid/PEDOT:PTS revealed high efficiencies as high as 3.88%, which was similar to the efficiency of 3.9% obtained in ITO-based OPVs. Various types of designed grid patterns were utilized as width and separation with 5mu;m/5mu;m, 5mu;m/11.7mu;m and 5mu;m/45mu;m
hellip;hellip;, respectively. In addition, the thickness of PEDOT:PTS varied from 30nm to 130nm. Furthermore, Self-Assemble Monolayer (SAMs) were used to provide uniform coating of PEDOT:PTS and optimized to reduce the contact resistance between PEDOT:PTS and Au grids. Moreover, the addition of TiO₂ underlayer enhanced the transmission, leading to the further increase of efficiencies of OPVs. As a result of the designing of metal grid structures, the TiO₂/Au/PEDOT:PTS multi-structure electrodes can be used as a replacement of ITO for fabrication of large area, flexible organic solar cells.

A new photovoltaic polymer (6TTF-polymer) containing tetrathiofulvalene (TTF, electron donor) and fullerene (C60, electron acceptor) was successfully synthesized. Electron-rich TTF can work as an electron donor (D) and C60 can act as an electron acceptor (A) so that the chemical structure of 6TTF-polymer is a DA dyads system. The chemical architecture of 6TTF-polymer was determined by utilizing FT IR, 1H and 13C NMR spectroscopes, its thermal behavior was also monitored by DSC. Moreover, the electrochemical properties were investigated by cyclic voltammetry, photoluminescence, and UV-Vis. This work was mainly supported by the Human Resource Training Project for Regional Innovation and the Converging Research Center Program (2012K001428) of Korean government.

Ongoing advances in stretchable electronics necessitate the development of electronic materials which not only meet the performance expectations of conventional materials for rigid substrates, but are also capable of maintaining stable performance under mechanical deformation. Transparent conductors (TCs) as essential constituents of solar cells and light-emitting diodes need to meet stringent performance metrics in terms of sheet resistance and optical transmittance. Despite their exemplary combination of high transmittance and low sheet resistance, indium tin oxide (ITO) TCs fall short of these expectations, due to increasing costs of materials and production process. The need to replace ITO, however, becomes even more crucial for flexible and stretchable electronic applications, since ITO is extremely brittle and virtually incapable of withstanding mechanical deformation. Alternative materials introduced to replace ITO have improved in past few years in terms of sheet resistance and transparency to an extent comparable to that of ITO. They have so far failed, however, in achieving high stretchability while maintaining the sheet resistance. Carbon-based nanostructures including carbon nanotubes and graphene are fairly flexible (withstanding ~20% mechanical strain), yet generally show high sheet resistances. Metallic nanostructures, especially silver nanowires on the other hand, show sheet resistances comparable to that of ITO, however, are less flexible than carbon materials, with typically less than 10% stretchability.
This work presents highly stretchable TCs by depositing or transferring high-transmittance, low-sheet resistance nanostructures onto pre-stretched PDMS substrates. The TCs illustrate a combination of sheet resistance and transmittance comparable to ITO, with stable performance over 100% of mechanical strain. Two separate nanostructured materials are demonstrated to develop separate TCs on pre-stretched substrates. The first TC is based on a conductive polymer (PEDOT:PSS doped with DMSO and a fluorosurfactant) spin-coated on the pre-stretched PDMS substrate. Releasing the pre-strain leads to the formation of surface buckles which provide stable sheet resistance under subsequent mechanical strain, due to unbuckling of the surface waves. The second TC is based on a composite core-shell nanostructure, comprising of polymer electrospun nanofiber cores sputter coated with gold which is then transferred onto a pre-stretched substrate. Releasing the pre-strain in this case leads to the formation of three dimensional curls along the fibers which are untangled over the subsequent stretching, thus providing a stable sheet resistance over more than 100% mechanical strain. SEM and AFM have been used to study the microstructural mechanisms responsible for high stretchability. To show the applicability of such TCs in flexible electronic devices, organic solar cells are fabricated based on both TCs on PDMS substrates.

In this report, we studied the influence of donor buffer layers to planar heterojunction organic photovoltaic (OPV) device. The planar hetero-structure OPV devices were fabricated by thermal deposition with boron subphthalocyanine chloride (SubPc) as donor, C60 as acceptor, and BCP as the cathode buffer layer. Various donor materials, CuPc, NPB, and NPAFN was inserting between indium tin oxide (ITO) anode and SubPc layer as the donor buffer layer with the thickness of 2 nm. While inserting CuPc as the donor buffer, the open-circuit voltage (Voc) and short-circuit current density (Jsc) of the OPV device decrease due to the energy level mismatch of ITO/CuPc/SubPc interface. Contrary, while inserting the ultra thin NPB and NPAFN as the donor buffer, the JSC of the OPV device increase obviously from 5.96 mA/cm^2 to 6.71 mA/cm^2 and 7.02 mA/cm^2. The IPCE spectrum of the device indicated that the enhancement of Jsc is contributed by the photocurrent generation of SubPc layer owing to the better energy level alignment and exciton blocking characteristic of the donor buffers, which prevent the photo generated exciton in SubPc layer quenched by the ITO anode. Furthermore, the Voc of the device also increased from 1.01V to 1.08V by using NPAFN as the donor buffer due to the reduction of the intermolecular interaction of SubPc layer and finally result in the significantly improvement of the device efficiency from 3.93% to 4.77%.

The benzo[1,2-b:4,5-bprime;]dithiophene (BDT) electron donating monomer unit has attracted much interest due to its contribution in copolymers reaching high efficiencies in polymer solar cells up to 7.4%.* Changing the electron withdrawing unit introduces the ability to tune the lowest unoccupied molecular orbital (LUMO) energy level in such a way that the voltage loss created by the offset with the LUMO level of the most commonly used electron acceptor in polymer bulk heterojunction solar cells, PCBM, can be minimized. Moreover, by using two BDT units for one acceptor unit and thereby increasing the electron density in the polymer gives a wide range of polymers which all have a different LUMO-LUMO offset with PCBM. Electron withdrawing monomers used are 5-alkyl-thieno[3,4-c]pyrrole-4,6-dione (TPD), benzo-dithiophene (BT) and the novel cyclopenta[c]thiophene-4,6-dione (CTD) unit.
An extensive photophysical study is done on these polymers, by means of photoluminescence (PL), x-ray diffraction (XRD), and time-resolved microwave conductivity (TRMC). The TRMC signal is indicative of the amount of free charges in the active layer, and this is often observed to be related to the LUMO-LUMO offset. Furthermore, solar cell devices were prepared and show an initial power conversion efficiency of 4.0% for one of the polymers, with an extremely high open circuit voltage of 0.95V relative to its band gap of 1.65 eV. We will also be evaluating the LUMO-LUMO offset on the internal quantum efficiency, and it is expected to go up with larger offset.
*Chen, H.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.
*Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135

We report the catalyst-free growth of gallium nitride (GaN) nanostructures on n-Si (111) substrates using physical vapor deposition (PVD) via thermal evaporation of GaN powder at 1150 °C in the absence of NH3 gas. SEM and EDX analysis indicate that the growth rate of GaN nanostructures varies with deposition time. Photoluminescence spectra showed the suppression of the UV emission and the enhancement of the visible band emission with increasing the deposition time. The fabricated GaN nanostructures exhibited p-type behavior at the GaN/Si interface, which can be related to the diffusion of Ga into the Si substrate. The obtained lowest reflection and highest transmittance over a wide wavelength range (450-750 nm) indicate the high quality of the fabricated GaN films. Hall-effect measurements showed that all fabricated films have p-type behavior with decreasing electron concentration from 1021 to 1012 cm-3 and increasing the electron mobility from 50 to 225 cm2/V s with increasing the growth time. The fabricated solar cell based on the 1h-deposited GaN nanostructures on n-Si (111) substrate showed a well-defined rectifying behavior with a rectification ratio larger than 8.32×103 in dark. Upon illumination (30 mW/cm2), the 1h-deposited heterojunction solar cell device showed a conversion efficiency of 5.78%. The growth of GaN in the absence of NH3 gas has strong effect on the morphological, optical and electrical properties and consequently on the efficiency of the solar cell devices made of such layers.

The mechanisms of broadened spectral response in plants are of great scientific importance. The molecular transitions related to plant optical absorption are characteristically narrow while plant response are broadened and shifted in energy relative to the absorption of isolated Chlorophyll. Elsewhere, these molecular transitions are examined for potential use in photovoltaic cells. However, this application requires a very broad spectral response in order to achieve high efficiency.
In turn solar cells can be a very sensitive probe of the absoption characteristics of molecules capable of transfering charge to a conductive interface. Chlorophyll dye sensitived solar cells were prepared and tested. These photovoltaic devices enabled an investigation of cooperative photon absorption among chlorophyll, various dyes and chromophores.
Forster has shown that the fluorescence of Chlorophyll (more specifically Chloropyll a) is modified and broadened by separate photon absorption (sensitized absorption) events in the nearby accessory pigments. Foster reported that the fluorescence demonstrates that photon energy is transferred from the sensitizing pigments to the chlorophyll as electronic excitation energy. Where the energy contributed by the sensitizer is either consumed in photochemical reactions mediated by an exciton decay associated with chlorophyll or released by fluorescence photon emission.
In our previous Spring 2012 MRS Proceeding we concluded that ERD architecture when combined with a photosensitizer do not appear to have greater absorption in the infrared region of the spectrum than the ERD alone indicating a lack of cooperative absorption. In this work we examine the mechanisms of chromophore-chlorophyll energy exhange and charge transfer mechanisms. It appears that the non-radiative vibrational energy model developed by Foster may not apply to chlorophyll and chormophores - at least in dye-type solar cell enviroments and configurations.

Inversion layer solar cells, based on n-Si and organic conductors such as PEDOT:PSS show promise for reducing the cost of solar electricity by eliminating the need for the energy-intensive dopant diffusion step, required to make a conventional p-n junction, when wafers are typically heated to over 900 °C for hours. However, interface states at the Si surface can enhance recombination, limiting device performance. At sufficiently low interface state density Si type-inversion can be achieved, for example by using Al2O3, layers with fixed charge on n-Si (and SiOx, Si3N4 or HfO2 on p-Si). This type of device takes advantage of charged interface states to generate depletion or inversion in the underlying Si by electrostatic repulsion, if the majority carriers of the Si substrate are of the same sign as the charge at the interface. Inversion becomes possible in this case thanks to the passivating properties of the Si-dielectric interface. However, the oxides are dielectrics and, therefore, ways need to be found to extract the charges without paying a high price in voltage to overcome the resistance of the dielectric film.
We demonstrate a novel solar cell utilizing the high density of negative fixed charge states at the interface between Si and Al2O3 to generate strong inversion in n-Si, where the maximum processing temperature reached is only 425 °C for 30 minutes. Charges are then extracted via an inversion layer (IL), generated at PEDOT:PSS contacts, where the Si surface contacting the PEDOT:PSS is both electrically passivated and inverted by a monolayer of polar molecules with a suitable dipole. While such structures can operate also without the monolayers, i.e., with direct PEDOT-PSS contact onto the Si, the average efficiency of the structure with molecular monolayer (MM) is much better. Also reproducibility was improved by using Al2O3 instead of just PEDOT:PSS. Adding the MM at the contact improved both FF and Isc significantly, and Voc slightly. We attribute this to stronger inversion at the contact with the MM present due to dipole effect, which improves hole transport from the stronger Al2O3 -IL to the MM-IL at the contact (which, ideally, should be matched in strength). Initial results with multi-crystalline n-Si show this approach can be extended to low cost Si, where the presently used high temperature step uses an even higher fraction of the energy, needed to make the cell (estimated at 10% of the total cell fabrication energy or 7% of the panel fabrication energy).

Interfaces, particularly the interface between a Donor (D) and Acceptor (A) material, play an important role in determining the photovoltaic parameters, if a hetero-interface is used in a solar cell. It is mostly agreed in organic PV that the energetics at the interfaces i.e., the ionization potential of (D) and the electron affinity of (A) determine the open-circuit voltage (Voc) and short circuit current density (Jsc) of the solar cell. However, there are other factors that can also play a crucial role in determining the Voc and Jsc of a cell. In order to look for other parameters that could influence solar cell parameters, we fabricated a model hybrid solar cell, where a crystalline wideband gap (~3 eV) inorganic semiconductor, 6-H SiC, is used as acceptor and pentacene (optical gap ~ 1.8 eV) as donor. The interface between SiC and pentacene was modified by introduction of alkyl siloxane monolayers of different chain lengths. No significant change was found in the surface potential, and, thus, in the surface dipole, of the surface that resulted after MM formation, but a gradual change in static water contact angle, was observed. The Voc of the solar cell improves with increasing chain length, even though the Donor and Acceptor entities remain the same. The increase in Voc can be attributed to a combined effect of reduced recombination, due to the increased separation of D and A, and a decrease in the density of gap states. The latter results from an increase in long-range ordering in pentacene, which is due to the change in MMs, as the ordering and surface energy in MMs changes with chain length. The increase in Voc is accompanied by an increase in Jsc. The increment in Jsc reflects this increase in order as trap-assisted recombination at short circuit is suppressed in a more ordered system. This model system illustrates the role of ordering in charge separation and recombination. The above findings may help to find a way to reduce recombination and improve Voc and Jsc of a cell simultaneously.

We demonstrate how two typical donor materials, chloroboron subphthalocyanine (SubPc) and chloroboron subnaphthalocyanine (SubNc), can be used as acceptors in vacuum deposited small-molecule organic solar cells. Currently, fullerene molecules are the most common type of acceptors in organic solar cells, owing this dominant position to their excellent electronic properties. However, the small absorption overlap with the solar spectrum limits the photocurrent generation in fullerene acceptors. Furthermore, the relatively small electrical bandgap of the fullerene acceptor limits open-circuit voltage (V_OC) in these devices. Bi-layer devices with SubPc as donor, and C60 as acceptor, can achieve power conversion efficiencies of 4.0% [1], while 3.5% was obtained with a SubNc donor [2]. In this work, we use SubPc and SubNc as acceptor in combination with an α-sexithiophene (α-6T) donor, and obtain power conversion efficiencies of 4.5% and 5.3%, respectively. The high V_OC of these cells (1.09 V for SubPc/α-6T, and 0.96 V for SubNc/α-6T) results from the energy level alignment between donor and acceptor, while the high short-circuit current (7.9 mA/cm² for SubPc/α-6T, 10.9 mA/cm² for SubNc/α-6T) is related to the strong absorption in the acceptor and the large donor-acceptor interface area resulting from the rough α-6T surface morphology. The performance of these devices exceeds that of previously reported non-fullerene devices, using halogenated subphthalocyanines as acceptor [3]. Furthermore, we fabricated a 3-layer cascade cell where SubPc is used as an ambipolar interlayer between a tetracene (Tc) donor and a C60 acceptor. Notably, the Tc/SubPc and SubPc/C60 heterojunctions operating in series are both able to contribute to the photocurrent. The SubPc interlayer hence acts simultaneously as donor and acceptor. This 3-layer cascade cell architecture produces a photocurrent larger than any of the constituent bi-layer cells, as well as a V_OC greater than the Tc/C60 cell due to reduced recombination losses at the interface. In summary, this work shows that subphthalocyanines are versatile organic semiconductors that allow for demonstrating highly efficient devices, and can be used interchangeably as donor and acceptor, enabling the development of a 3-layer cascade device architecture.
[1] C.-F. Lin, S.-W. Liu, C.-C. Lee, J.-C. Hunag, W.-C. Su, T.-L. Chiu, C.-T. Chen, and J.-H. Lee, Sol. Energy Mater. Sol. Cells 103, 69-75 (2012).
[2] D. Cheyns, B.P. Rand, and P. Heremans, Appl. Phys. Lett. 97, 033301 (2010).
[3] (a) H. Gommans, T. Aernouts, B. Verreet, P. Heremans, A. Medina, C.G. Claessens, and T. Torres, Adv. Funct. Mater. 19, 3435-3439 (2009). (b) P. Sullivan, A. Duraud, I. Hancox, N. Beaumont, G. Mirri, J.H.R. Tucker, R.A. Hatton, M. Shipman, and T.S. Jones, Adv. Energy Mater. 1, 352-355 (2011). (c) B. Verreet, B.P. Rand, D. Cheyns, A. Hadipour, T. Aernouts, P. Heremans, A. Medina, C.G. Claessens, and T. Torres, Adv. Energy Mater. 1, 565-568 (2011).

Organic photovoltaics has attracted much effort and many research groups during the past decade, because of low-cost and easy fabrication techniques. Despite the great progress that has been achieved in increasing the conversion efficiencies of the devices, there are still several problems to be solved to make the solar cells commercially viable, especially for cells based on bulk heterojunctions. One of the crucial parameters affecting the device performance is the mobility of the charge carriers in the active layer of the solar cell.
The purpose of this work is to supply techniques for predicting the order of magnitude of the charge carrier mobilities of bulk heterojunction devices, on the basis of easy-to-perform measurements for experimentalists. A one dimensional model of a bulk heterojunction cell was used, and then simulations were performed in order to obtain the photocurrent as a function of an effective applied voltage. Plotted in a double logarithmic scale, the resulting curves exhibit different signatures depending on the mobilities of the charge carriers. These signatures could be helpful for experimentalists in order to predict an order of magnitude for both the electron mobility and the hole mobility. Thereafter, the predictions could be improved by additional photocurrent measurements for several light intensities.

Increasing energy demands and concerns over global warming have led to a greater focus on the development of renewable energy sources. With the discovery of inexpensive photovoltaic devices, dye-sensitized solar cell (DSSC) technology is being explored extensively to fulfill our green energy demands. In order to improve the cell efficiency, the DSSC components, such as, photosensitizers, photoanodes, redox electrolytes, and counter electrodes are optimized to achieve increased light harvest, enhanced charge transport, and reduced recombination. Among them, the dye is essential in DSSCs for efficient light harvesting and electron generation/transfer. Since their initial report by Grätzel, ruthenium sensitizers have attracted much attention, resulting in the preparation of hundreds of ruthenium complexes with excellent solar-to-electric power-conversion efficiencies. However, to date, most of ruthenium sensitizers for DSSC applications that have been described in the literature are those bearing bipyridine or polypyridine ligands. Herein, we present a new carbene-based ruthenium sensitizer (1), in which one of the traditional bipyridine frameworks has been replaced by the carbene system. A rapid and efficient synthetic method for complex 1 will be reported in the meeting. This carbene-based ruthenium sensitizer with diphenylvinyl-thiophene substituted benzimidazole-pyridine ancillary ligand (1) has been used as a photosensitizer for DSSCs. The influence of the highly conjugated thiophene antenna on the performance of the DSSC anchored with photosensitizer 1 is investigated. It is found that the conjugated thiophene in the ancillary ligand enhances the molar extinction coefficient of π-π* intraligand transition and diminishes the intensity of the lower energy metal-to-ligand charge transfer band of the sensitizer. Correspondingly, the incident photon-to-current conversion efficiency spectrum of the DSSC based on this complex shows a maximum of 62% at 420 nm. Preliminary results show that a DSSC sensitized with highly conjugated ruthenium sensitizer 1 attains a power conversion efficiency of 6.33%, which is lower than that of non-conjugated carbene ruthenium sensitizer, CBTR, anchored cell (8.58%) under same fabrication conditions. The difference in the performance of these sensitizers demonstrates that elongating the conjugated light-harvesting antenna results in the reduction of short-circuit photocurrent density, which might be arisen from the aggregation of dye molecules. In the presence of coabsorbate, deoxycolic acid, the photosensitizer 1 sensitized cell exhibits the enhanced photo-current density and photovoltage and achieves photovoltaic efficiency of 7.90%. Further evaluation of the photophysical and electrochemical properties of this dye is currently underway and will also be reported in the meeting.

In this project, we have explored over a large number of geometry parameters in nanostructured organic photovoltaic (OPV) devices by using a optical-electrical simulation program. We considered two popular polymer:fullerene systems (PTB7:PC[70]BM and P3HT:PC[60]BM). In both systems, we found optical enhancement of absorption in many of the nanostructured devices. The overall efficiencies, however, were not increased in all of those cases. We discuss the tradeoff between light-trapping effects and charge harvesting deterioration induced by nanostructures and significance of nanostructures in OPV devices. A simple nanoimprinting procedure was applied to P3HT:PC[60]BM systems, which demonstrate the tradeoff experimentally.

We investigated Ag nanowire (NW) network embedded ITO films as potential near infrared transparent and flexible electrodes for use in flexible organic solar cells (FOSCs). By embedding Ag NWs network between very thin ITO films using simple a brush painting method, we achieved a flexible ITO/Ag NW/ITO multilayer electrode with a low sheet resistance of 11.58 Ohm/square, a high diffusive transmittance of 84.78 %, as well as superior mechanical flexibility. The effective embedment of the Ag NW network between top and bottom ITO films led to a metallic conductivity, high near infrared transparency, and mechanical durability of the ITO/Ag NW/ITO multilayer. Taken together, these indicate that embedment of the Ag NW network into thin ITO films is a key solution to solve the critical drawbacks associated with brittle ITO electrodes and Ag NW network films with weak adhesion. Furthermore, better performances of FOSCs with ITO/Ag NW/ITO multilayer electrodes than those of the FOSC with a conventional ITO electrode demonstrate that flexible ITO/Ag NW/ITO electrode is a promising alternative to conventional ITO films for high performance FOSCs.

Dye sensitized solar cells (DSSCs) have become a promising photovoltaic technology due to its low-cost and environmental friendliness. One of the key components for a DSSC is the TiO2 photoanode, which is important in photoelectron transport, dye-anchoring, and light trapping/scattering. Therefore, it is important to have a cost-effective fabrication process for photoanodes. What&’s more, the fabrication cost and thermal budget can be significantly lowered if the processing time can be greatly reduced.
In this paper, we demonstrate a rapid sintering process for nanoporous TiO2 photoanodes of DSSCs using atmospheric pressure plasma jets (APPJs). First, the nanoporous TiO2 pastes are deposited onto FTO glass substrates by screen-printing method. After printing, the TiO2 layers are then sintered by APPJs in N2 atmospheres for various durations. From the absorption spectra, the 30-second APPJ-sintered sample presents extra absorption band due to the residue of the organic substances. As the APPJ treatment time reaches 60 sec, the APPJ-sintered TiO2 exhibits nearly identical absorbance spectrum to that of furnace-sintered one. Confirmed by SEM and XRD, there is no significant difference in surface morphology and crystallinity for the APPJ-sintered and furnace-sintered TiO2 nanoporous layers. The cell efficiency with APPJ-sintered TiO2 becomes comparable to that of the reference cell (with a furnace-sintered TiO2 photoanode) as the treatment time reaches 60 sec and beyond. The DSSC with a 30-second APPJ-sintered TiO2 photoanode shows an extremely large interfacial resistance and a shorter electron lifetime, according to the Nyquist plot of the electrochemical impedance spectroscopy. Our experimental results demonstrate that a 60 sec APPJ-sintering process can completely replace the conventional 510oC×15 min furnace-sintering for TiO2 photoanodes of DSSCs.

An increase in molecular weight of the polymer generally impedes solubility in common solvents and may cause an influence on optoelectronic properties as well. On the other hand, they are expected to increase charge carrier mobilities and therefore give rise to better photovoltaic performances of bulk heterojunction solar cells. In this work we use copolymers based on 2,1,3-benzothiadiazole, thiophene and thieno[3,2-b]thiophene units of various fractions differing in molecular weights almost by a factor of 4 with a fullerene based acceptor material Indene-C₆₀ Bisadduct (IC[60]BA) to fabricate bulk heterojunction solar cells. We investigate the influence of post-deposition annealing temperatures and polymer:fullerene ratios on the final cell performances. The use of IC[60]BA as an acceptor was expected to mainly increase the open circuit voltage due to its higher lying LUMO level. Additionally, charge carrier mobilities were probed using bottom contact organic field-effect transistors. As expected, higher molecular weights resulted in an increase of the hole field-effect mobility (up to 7x10^-3 cm²V^-1s^-1). Consequently the power conversion efficiencies of bulk heterojunction solar cells increased with the copolymer molecular weight except for the highest molecular weight polymer for which poor film qualities were obtained. A power conversion efficiency of 2.4% with an open circuit voltage of 0.82V was reached in a standard device configuration after post-deposition thermal annealing.

Two benzo[1,2-b:4,5-bprime;]dithiophene (BDT) derivatives with conjugated substituents, triisopropylsilylethynyl (TIPS) and 4-octylphenylethynyl (OP) groups, were synthesized as weak donor (WD) units and copolymerized with two acceptor units, 4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole (BT) as strong acceptor (SA) and 4,4'-diundecyl-2,2'-bithiazole (BTZ) as weak acceptor (WA), to afford four new copolymers, PTBDT-BT (WD-SA), PTBDT-BTZ (WD-WA), POPEBDT-BT (WD-SA), POPEBDT-BTZ (WD-WA). All polymers exhibited HOMO energy levels that were deeper than -5.4 eV due to the conjugated substituents. Small band gaps were successfully achieved for PTBDT-BT (1.67 eV) and POPEBDT-BT (1.67 eV) and were attributable to the strong intramolecular charge transfer within the D-A alternating structure. The resultant photovoltaic performances showed high open-circuit voltages (Voc) ranging from 0.73 V to 0.92 V, whereas the power conversion efficiencies (PCE) depended strongly on the blend morphologies. The polymer solar cell based on the blend of PTBDT-BT and PC71BM gave the best photovoltaic performance among the series, with a high Voc of 0.81 V and a PCE of 4.61%.

We have investigated the characteristics of transparent and flexible PEDOT:PSS electrodes passivated by thin InZnSnO (IZTO) film for flexible organic solar cells (FOSCs). Using damage-free linear facing target sputtering (LFTS), an IZTO passivation layer was deposited on the gravure printed PEDOT:PSS electrode without causing plasma damage. The thickness of the IZTO passivation layer was found to critically affect electrical and optical properties of the IZTO/PEDOT multilayer because the continuity and morphology of the thin IZTO passivation layer were greatly influenced by thickness. At optimized IZTO passivation thickness of 20 nm, the IZTO/PEDOT:PSS multilayer electrode exhibited decreased sheet resistance of 353.6 Ohm/square and optical transmittance of 83.09 % without flexibility degradation. The FOSC with an optimized IZTO/PEDOT:PSS electrode showed better performances than the FOSC with PEDOT:PSS electrodes due to the effect of a stable IZTO passivation layer. This indicates that IZTO passivation on the PEDOT:PSS electrode using a LFTS is one of a key solution for improving the properties and stability of flexible PEDOT:PSS electrodes for high performance FOSCs.

We developed highly transparent and flexible poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag nanowire (NW)/PEDOT:PSS (PAP) multilayer electrodes fabricated by simple brush-painting for highly flexible organic solar cells (FOSCs). Combining flexibility of PEDOT:PSS and low resistivity of Ag NW, we fabricated high performance PAP multilayer electrodes with a low sheet resistance of 60.27 Ohm/square and high diffusive transmittance of 83.41% as well as mechanical flexibility. Embedment of Ag NW network into the conductive PEDOT:PSS layer led to metallic resistivity and high diffusive transmittance desirable electrodes for FOSCs. In addition, PAP electrode showed constant resistance during outer and inner bending test due to mechanical flexibility of both PEDOT:PSS and Ag NW network. Identical current density-voltage of the FOSCs with brush painted PAP electrode to those of the FOSC with a conventional ITO electrode demonstrate that simple brush painted PAP multilayer is a promising alternative to ITO film for cost-efficient FOSCs. Furthermore, brush-painting is a simple and low-cost coating process for fabricating paintable FOSCs.

We report on the characteristics of GeO2-doped In2O3 (IGO) films for use as transparent electrodes in organic solar cells (OSCs). The electrical, optical, and structural properties of IGO electrodes were investigated as a function of RF power and post-annealing conditions. At optimized conditions, the IGO electrode exhibited a low sheet resistance of 14.0 Ohm/square, a high optical transmittance of 86.9%, a root mean square roughness of 1.27 nm and a work function of 5.2 eV. In particular, the IGO film showed higher optical transmittance in the near-infrared region due to a lower free carrier concentration and higher mobility than conventional ITO electrodes. The higher Lewis acid strength of the Ge4+ (3.06) dopant, compared to that of a Sn4+ (1.62) dopant, led to higher mobility of the IGO films. In addition, we observed that the strongly oriented (222) grains in the IGO films enhanced carrier mobility and relaxation time. Furthermore, a bulk heterojunction OSC with the optimized IGO anode exhibited a good cell performance with a fill factor of 67.38%, a short circuit current of 8.438 mA/cm2, an open circuit voltage of 0.606 V, and a power conversion efficiency of
3.443%, which are comparable to OSCs with ITO anodes.

We report on all brush painted flexible organic solar cells using Ag nanowire (NW) network electrodes. By simple brush painting of Ag NWs on a PET substrate, we achieved transparent Ag NW network electrodes with a low sheet resistance of 38.7Omega;/square and a high diffusive transmittance of 87.62% as well as superior mechanical flexibility. After brush painting of a flexible and transparent Ag network electrode, we fabricated all brush-painted flexible organic solar cells with structure of Ag NW (bottom)/PEDOT:PSS/P3HT:PCBM/ZnO/Ag NW (Top) on the Ag NW/PET sample to test the feasibility of brush painted Ag NW electrode and simple brush painting process. The all brushed organic solar cells exhibited a cell-performance with a fill factor of 49.2 %, a short circuit current of 7.27mA/cm2, an open circuit voltage of 0.57 V, and a power conversion efficiency of 2.06 %, even though it was fabricated without any vacuum process. Comparable performance of all brush painted organic solar cells to reference organic solar cells with evaporated Ca:Al cathode and sputtered ITO anode demonstrate that simple brush-painting method is a low-cost and simple coating process to fabricate solution-based simple transparent electrodes and cost-efficient organic solar cells.

We investigated the mechanical integrity and flexibility of the transparent Ag nanowire (NW) network electrode coated on colorless polyimide (CPI) substrates using a lab-made bending test apparatus involving detailed outer/inner bending, twisting, and stretching for flexible organic solar cells. At the optimized conditions with a coverage rate of 42.25 %, the Ag NW network electrode showed a sheet resistance of 21.76 Ohm/square and an optical transmittance of 84.84 %, which are better than the values obtained from a conventional flexible amorphous ITO film. Notably, the Ag NW electrode had a constant resistance change ( Δ R/R0) within an outer and inner bending radius of 5 mm. The twisting test also revealed that the resistance of the Ag NW network electrode remained constant below a twisting angle of 35 degree due to the superior flexibility of the Ag NW network. Furthermore, the stretched Ag NW electrode showed a fairly constant resistance change ( Δ R/R0) up to 4 %, which is more stable than the resistance change of a flexible ITO electrode. Flexible organic solar cells were fabricated on flexible Ag NW network electrodes and peeled off the supporting glass substrates. The power conversion efficiency of the flexible solar cells was measured to be 2.62%, demonstrating the possibility of Ag NW networks as flexible transparent conducting electrodes for flexible organic solar cells.

We investigated flexible Ag nanowire (NW) network electrodes fabricated by simple brush-painting for cost-efficient FOSCs. By direct brush-painting Ag NWs on a PET substrate, we achieved Ag NW network electrodes with a low sheet resistance of 38.7 Ohm/square and a high diffusive transmittance of 87.62 % as well as superior mechanical flexibility. The electrical, optical, and mechanical properties of the brush-painted Ag NW electrodes were investigated as a function of the number of repeated brush-painting cycles. Identical performances of the FOSC with the brush painted Ag NW network electrode to those of the FOSC with a conventional ITO electrode demonstrate that the brush-painted Ag NW electrode is a promising alternative to ITO films for cost-efficient FOSCs. Furthermore, simple brush-painting is a low-cost and simple coating process to fabricate solution-based simple transparent electrodes.

We report on transparent amorphous In-Si-O (ISO) electrodes for flexible organic solar cells (FOSCs). The effective Si doping above critical RF doping power into the In2O3 matrix led to a completely amorphous structure as well as low sheet resistance of 51.91 Ohm/square and high near infrared optical transmittance of 81.51%(550nm), and which are desirable electrode characteristics for FOSCs. In addition, ISO film showed outstanding flexibility in outer and inner bending tests due to stable amorphous structure of the ISO films. Based on analysis of x-ray absorption spectroscopy, and high resolution transmission microscope examination, detailed microstructure and electronic structure of ISO film was investigated. Furthermore, FOSCs with amorphous ISO anode showed a open circuit voltage (0.578 V), short circuit current (7.641 mA/cm2), fill factor (62.96 %) and power conversion efficiency (2.78%), indicating that ISO is a promising amorphous transparent electrode for FOSCs

The development of barrier films is a critical technology for the reliability of a range of electronic devices including organic photovoltaics, organic light emitting diodes (LEDs), medical device implants, thin-film photovoltaics, and corrosion protection for heat exchangers. Alumina barrier films deposited through atomic layer deposition (ALD) are known to suffer from process-induced defects that inhibit their stability in aqueous and hygroscopic environments. We explore the aqueous stability of ALD deposited alumina barrier films capped with zirconium oxide (ZrOx) and titanium oxide (TiOx) thin films and their composites deposited through e-beam or ALD deposition. The performance of the barriers was evaluated by measuring their ability to protect zinc oxide thin film sensors immersed in aqueous solutions. X-ray photoelectron spectroscopy was used to determine the composition of the barrier films with the various capping layers. The characteristic photoluminescence (PL) peak for the ZnO sensor was used as a marker to study changes in peak intensity due to degradation of the barrier films as a function of immersion time. No change in ZnO PL spectra for TiOx capped films was observed when immersed in water for 10 days. These results show that metal oxide capping layers such as TiOx capping layers can offer superior protection to minimize the degradation of ALD alumina barrier films in aqueous environments. Such composite laminates will enable a wider range of applications for ALD deposited alumina barrier films where stability to aqueous environments is a critical prerequisite to effective functioning of the device.

The performance of organic solar cells has improved continuously over the last decade, with power conversion efficiency reaching almost 10%. However, in order to render organic photovoltaics a realistic alternative for future energy applications, the fabrication has to be made compatible with large-area, high-throughput, inexpensive industrial manufacturing processes. Currently, one of the most promising approaches to meet those requirements is solution printing of organic solar cells.
In this contribution, we present our recent progress in solution-printed organic bulk-heterojunction (BHJ) solar cell blends. The morphology and molecular packing of printed polymer-fullerene blend films were studied using grazing incident x-ray diffraction (GIXD), optical microscopy and absorption spectroscopy. We demonstrate that the film properties are highly dependent on the printing conditions (such as printing speed, ink viscosity, and solvent), and can be controlled by tuning those parameters.

Synthesis of Ultrasmall Sized PbS NQDs Achieving Highest VOC
in QDs Schottky Junction Solar Cell
Hyekyoung Choi, Jun Kwan Kim, Jung Hoon Song , and Sohee Jeong*
Nano-Mechanical Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon, 305-343, Republic of Korea. Fax: +82-42-8687883 E-mail address: [email protected]
We investigate a new synthetic route for ultrasmall sized PbS nanocrystal quantum dots (NQDs) in diameters ranging between 1.5 and 2.9 nm (2.58 - 1.5 eV in first exciton energy) for the first time by adjusting growth temperature and growth time. In this region, the Stokes shift increases as decreasing size, which is testimony to the highly quantum confinement effect of ultra-small sized PbS NQDs. We fabricate self-assembled films of PbS NQDs to find out the electrical properties by using spin-coating method and replacing oleic acid to short ligands with 1,2-ethandithiol (EDT) in the course. We achieve the highest open-circuit voltage ever with 0.7 V in Schottky junction solar cell made in a glass/ITO/PbS/LiF/Al structure using PbS NQDs with first exciton energy 2.58 eV. We also studied solar cell parameters with open circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and power conversion efficiency(eta;) as size of PbS NQDs changes. The results will be beneficial to approach multi-junction solar cell.

In polymer solar cells consisting of active layer sandwiched between a transparent anode and a low work function metal cathode, direct electrical contacts between the interfaces of active layer and electrodes lead to recombination of carriers and current leakage. Although conventional water-soluble poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely used as an HTL for more efficient hole collection, several groups have tried to replace PEDOT:PSS due to the several problems including high acidity, hygroscopic properties, and inhomogeneous electrical properties, resulting in poor long-term stability. In this study, as an alternative HTL for the replacement of PEDOT:PSS, we demonstrate moderately reduced graphene Oxide (GO) films fabricated by a simple and fast thermal treatment of solution processed GO layers. Effect of conditions of thermal treatment on the cell-performances was investigated and stability of PSCs with moderately reduced GO was compared to conventional systems including PEDOT:PSS HTL. Also, we fabricated GO based HTL by spray pyrolysis, which is very compatible with roll-to-roll based manufacturing lines and effective to realize large area devices. Structural properties of GO based HTLs prepared by spray pyrolysis at various temperatures were characterized and performance of PSCs were investigated.

Highly conducting transparent electrodes form an important part of any organic solar cell and may be a limiting factor in terms of cost, flexibility or performance of the device. Indium tin oxide (ITO) has been the most commonly used transparent electrode for organic solar cells but it cracks upon bending and is relatively expensive, which limits its viability in flexible devices or commercial applications. It is therefore imperative to identify alternative electrode materials which are inexpensive, solution processable and suitable for roll-to-roll processing.

We have investigated solution processed silver nanowires (AgNWs) as a possible alternative to ITO. AgNWs of high aspect ratio form sparse continuous networks when deposited in thin films, allowing them to form robust highly conductive electrodes with high transparency and low cost due to the high void content. A typical spin coated AgNW film displays sheet resistance of 15 #8486;/sq at 93% transmittance, which is comparable to device-grade ITO (17 #8486;/sq at 97% transmittance).
Inverted solar cells on glass substrates were fabricated on ITO (17 #8486;/sq) or AgNWs (15 #8486;/sq) by spin coating a layer of ZnO sol gel, followed by P3HT:PCBM, PEDOT:PSS and evaporating 100 nm of Ag. Different conditions for each layer were tested, including Zn concentration in the sol gel and the thickness of the active layer and the PEDOT:PSS for optimisation of the AgNW-based device. ITO and AgNW-based devices had typical efficiencies of 3.7 and 3.4% respectively. Preliminary results obtained thus far show AgNWs to be a promising material for replacing ITO.

We presented a solid state gel electrolyte as an alternative to the liquid electrolytes used in dye sensitized solar cells (DSSCs) because the use of liquid electrolyte raises significant technological problems associated with device sealing, long-term stability and corrosive I2. The first solid state gel electrolyte was prepared by using the dimehtylsulfoxide (DMSO) as the solvent, doped polypyrrole (PPy) as the hole-conducting polymer. Then, we introduced block copolymer of poly(vinylalcohol-co-ethylene) (PVA-EL) with low transition temperature into a DMSO-PPy system to change liquid electrolyte into the gel electrolyte. Vinylalcohol (VA) homopolymer can dissolve well in the DMSO electrolyte. The high solubility is attributed to the effect of significant interaction between DMSO and VA. This interaction originates in the OH group of VA and the moiety of the DMSO molecule. Therefore, the high solubility of the PVA-EL in the electrolyte is attributed to the effect of the VA segments, while the EL segments contributes to the following gelation process through the interaction and cross-linking among the EL chains. We have changed EL composition to investigate gelation process. The results show that the diffusivity and conductivity significantly were changed during the gelation period with increasing the ratio of EL segments in the copolymer.

The efficient conversion of photons into free charge carriers in blends of conjugated polymers and fullerene derivatives is very promising for the development of bulk heterojunction solar cells. The power conversion efficiency of BHJ solar cells has significantly improved from 2.5% to more than 9.5% over the last decade. A possible way to improve the efficiency is by a reduction of the driving force (ΔGET) involved in the electron transfer from the excited, conjugated polymer to the LUMO of the fullerene. However, until now, it is unclear to what extent ΔGET affects the yield of mobile charge carrier generation. In this work the time-resolved microwave photoconductance technique (TRMC) is employed to investigate the effect of ΔGET on the quantum yield (phi;) for charge carrier generation. By utilizing various conjugated polymers with different LUMO levels in combination with different fullerene derivatives as an electron acceptor, ΔGET was systematically varied. The value of ΔGET is given by: ΔGET = ES1* - ECT , where, ES1* is the energy of the singlet excited state of the polymer found from the onset in the optical absorption spectrum. ECT is the energy of the charge transfer state deduced by determining the onset of the response in a photoconductance action spectrum. The onset corresponds to the minimum energy required to promote an electron from the HOMO of the polymer directly to the LUMO of the electron acceptor yielding mobile charges. Interestingly, we find that phi; determined by TRMC measurements, is not affected by the ΔGET despite changing the ΔGET from 0.15 eV to 0.83 eV. According to the Marcus theory, the electron transfer rate (KET) initially increases as ΔGET becomes larger. The maximum KET is obtained if ΔGET equals the total reorganization energy, while the KET becomes again smaller if ΔGET gets more negative. However, our findings indicate that changing ΔGET does not affect the yield of charges. This can only be explained by assuming that KET is for all studied blend systems is large in relation to other decay processes. This work clearly demonstrates that developing low band gap polymers to extend the spectral response to NIR part and/or to improve the open circuit voltage is a viable route to further enhance the efficiency of polymer solar cells.

Cost reduction is considered as the next important milestone for organic photovoltaics (OPVs). The application of printing technology as a fabrication tool for organic photovoltaics is critical for the future technological progress of OPVs - enhancing the potential of the use of these novel materials for future light-activated plastic power sources. Most attempts to process highly efficient OPVs have focused on traditional coating techniques. The first highly efficient organic solar cell by inkjet printing was reported by adjusting the chemical properties of a poly(3-hexylthiophene) polymer donor [1] and by using a novel inkjet solvent mixture [2]. Based on this work, control over the nano-morphology of poly(3-hexylthiophene):fullerene blends during the inkjet-printing process was achieved, which yielded a power conversion efficiency of 3.5 % [1]. Other recent papers have also reported PCEs for inkjet printed RR-P3HT:PCBM OPVs exceeding 3 % by using additives and combinations of different solvents [3-4]. Despite the recent progress in the field of inkjet printed OPVs, the influence of processing parameters on device PCE has not been investigated in details.
In this poster, details of the inkjet printing processing conditions of the P3HT:PCBM active layers such as solution viscosity, substrate temperature, drop-spacing and droplet height are going to be presented [5]. The choice of the above parameters is critical for achieving high power conversion efficiency inkjet printed OPVs. By using materials and a piezoelectric inkjet printer which are commercially available the optimized printing process can produce OPVs with power conversion efficiency in the range of 3 % - a PCE achieved under similar experimental conditions for optimized reference OPVs fabricated with conventional spin coating and doctor blading processing methods [5].
Acknowledgements: This work was co-funded by the EU Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Strategic Infrastructure Project ΝEpsi;A ΥΠΟΔΟΜΗ/ΣTau;ΡATau;Η/0308/06).
[1] C.N.Hoth, P.Schilinsky, S.A.Choulis, C.J.Brabec, NanoLetters, 2008, 8, 2806.
[2] C.N.Hoth, P.Schilinsky, S.A.Choulis, C.J.Brabec, Adv.Mater., 2007, 19, 3973.
[3] S. H.Eom, H. Park, S.H. Mujawar, S. C. Yoon, S.S. Kim, S.I. Na, S.J. Kang, D. Khim, D.Y. Kim, S.H. Lee, Org. Electron., 2010, 11, 1516.
[4] A. Lange, M. Wegener, C. Boeffel, B. Fischer, A. Wedel, D. Neher, Sol. Energ. Mater. Sol. Cells, 2010, 94, 1816.
[5] M. Neophytou, W. Cambarau, F. Hermerschmidt, C. Waldauf, C. Christodoulou, R. Pacios, S.A. Choulis, Microelectron. Eng., 2012, 95, 102.

The steeping energy costs with concerns over the global climate change and demand for clean energy technologies has spurred great deal of interest in organic photovoltaic (OPV) research to capture abundant solar energy to generate cheaper electricity. The conjugated polymers are one excellent and viable candidate due to their potential low-cost, flexibility, light-weight, colorful and it can be solution processable and printed like inks. Individualization and debundling of pristine SWCNT and MWCNT were investigated in an N-N dimethyl tetraformamide solvent by a combination of ultrasonication and centrifugation. The wt% (mg) of pristine CNTs loading was optimized in respect to quantity of solvent (ml), additionally the choice of solvent, ultracentrifugation speed and ultrasonication time was also essential parameters to determine the good individualization of pristine CNTs. Then BHJ solar cell devices were modified by spin casting pristine CNTs at an incorporating stacking thin layer (~15nm) at different positions in the solar cells for efficiency evaluation. Comparisons of the devices made with well-known documented method of functionalized CNTs (acid treated) has been carried out and found that stacking of pristine CNTs between the PEDOT:PSS and P3HT:PCBM layer demonstrate a significantly enhanced efficiency of 2.65% (JSC of 11mA/cm2, VOC of 0.58, FF of 42) from the normal BHJ of 1.51% ((JSC of 6.68mA/cm2, VOC of 0.60, FF of 37). However, functionalized CNTs with acid treated shows degrading efficiency (0.25%) which can be attributed to degradation of corrugated tubular side walls leading to potential loss of optoelectric properties. The enhanced efficiency of devices with pristine SWCNTs can be conjectured due to better opto-electrical properties and undamaged tubes. The microstructures of the heterojunction active layer were examined by using AFM, TEM, UV-Vis spectra, IV curve and EQE techniques.

Photovoltaic technology is being widely recognized as a viable choice since it directly converted solar energy to electrical energy. Among many cost effective options, solution processable hybrid solar cells holds the promise of utilizing combined advantage of the unique properties of inorganic nanoparticles with the film forming properties of polymers. The thin organic/inorganic (P3HT/CdSe) hybrid materials in which both components are photovoltaically active and can be processed from solution at room temperature, enabling the manufacturing of large area, flexible, and lightweight devices. We synthesized quantum dots with average diameter of 7nm, with wurtzite crystal structure. The UV-Vis absorption spectra show an excitonic peak at 650 nm and photoluminescence spectra at 660 nm. The synthesized quantum dots were successively ligand exchanged by pyridine and tert butyl thiol to remove the TOPO ligands on quantum dot surface and then hybrid solar cells were fabricated with P3HT/CdSe as an active layer in chlororoform as a solvent on ITO glass. The maximum solar cell conversion efficiency of 1.54% was observed with maximum Jsc of 4.2 mA/cm-2 in the ligand exchanged samples compared to as prepared CdSe quantum dots where just 0.12% with Jsc of 0.98 mA/cm-2 was observed. The increase in solar cell efficiency was attributed to the better ligand exchanged and additional treatment with tert butyl thiol at ambient temperature. Such an exchange of organic ligands by successive ligand exchanger will open new domain for hybrid solar cell research. The morphology of QDs and microstructures of the heterojunction active layer (P3HT/CdSe) were examined by using TEM, XRD, UV-Vis spectra, PL, IV curve and EQE techniques.

Comparison between spin coating and wire bar coating:
Various organic photovoltaics (OPV) fabrication techniques have been developed in the past few years for laboratory and industry purposes.
Spin coating has been found to induce prominent OPV device performance in laboratory owing to the good control of film homogeneity. However, the main disadvantage of spin coating is that it is not suitable for industrial commercialization which requires the production of cost effective large scale device due to the very low transfer yield (more than 90% material spin coated is lost in the process). Wire bar coating, with the transfer yield well above 80%, has the potential to overcome these disadvantages while maintaining the device performance.
Other methods such as slot die coating flexo and reverse gravure are commercially available, although more expensive to run and technically challenging to mimic in laboratory environment.
With the power conversion efficiency (PCE) of OPV reaching about 10% in laboratory (a threshold for industrial commercialization), the wire bar coating technique has drawn increasing interest for potential large scale, low cost industrial level products.
Currently, the performance of wire bar coated devices have been systematically studied in our group with varying parameters such as solvents, temperature, coating speed and so on, and we have reached an encouraging PCE for wire bar coated cells, comparable to those fabricated via spin coating technique.
Morphological Stability:
While an optimized morphology is essential for superior device performance, its stability is another important issue that remains to be solved for future industrialization, since unstable morphology leads to the fast degradation of OPV devices leading to a short device lifetime.
Variation of conditions such as climate, moisture, pH and so forth could modify OPV structure resulting in an alteration in their efficiency. Therefore, morphology stability is vital for the OPV device to have a constant performance over a long lifetime.
Currently various morphological stability studies have been carried out in our group including thermal and environmental degradation techniques as well as their impact on device performance. Besides, a number of parameters namely, different donor and acceptor, solvent selection and additives have been examined in terms of their impact on morphological and device stability.

We have investigated the effect of oligothiophene units on the performance of photovoltaic devices based on three co-polymers with alternating units of fluorene and thiophene whereas we have increased the number of oligothiophene units along the co-polymer backbone. These materials named as LaPPS 23 (fluorene-thiophene), LaPPS 43 (fluorene-bithiophene) and LaPPS 45 (fluorene-terthiophene) were prepared by Suzuki coupling reaction and used as active layer in bilayer photovoltaic devices with C60. The devices were fabricated as follows: poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) film with thickness of 40 nm was spin-coated at 6000 rpm onto the cleaned fluorine tin oxide (FTO) patterned glass substrate and annealed at 100 °C by 15 min in vacuum. The co-polymer film was spin-coated from chloroform solutions with concentrations of 3 mgmL-1 and as cast and annealed films at 70 °C, 100 °C and 200 °C were produced. Then, 30 nm of C60 and 100 nm of Al were thermally evaporated through a shadow mask. Co-polymers film morphology was probed by atomic force microscopy (AFM). The bilayer devices were characterized by their spectral response (Incident Photon to Current Efficiency - IPCE) and by J-V curves measured under solar simulation using air mass (AM1.5) filter with a power illumination of 100 mWcm-2. Density functional theory was used to perform analysis of parallel or alternate configurations of the thiophene moieties. For LaPPS 43 there is an increase in six orders of magnitude in the hole&’s mobility upon annealing at 200 °C, resulting in the best device efficiency of 2.79 %. In this case, the thermal annealing induces a higher degree of planarity of polymer&’s backbone which maximizes the intermolecular π-π stacking. Except for the annealing at 200 °C, LaPPS 45 presented the highest efficiencies when compared to LaPPS 23 and LaPPS 43.

Recently, lanthanide complexes have been intensively studied as materials for light-emitting devices [1]. Polymer composites, in which conductive polymers used as electron donors and complexes of lanthanides as acceptors, arouse the major interest. The uniform distribution of molecules in the polymer matrix without crystalline inclusions can be obtained due to the specific structure of some liquid crystalline lanthanide complexes. It leads to a significant increase of the device&’s luminescence efficiency. Another advantage of glassy lanthanide compounds is the possibility of creating composites with a large percentage of the acceptor and the most completed energy transfer from the polymer to the complex. This abstract describes mesogenic adducts of tris(β-diketonate) europium with Lewis bases, abbreviated as Eu(DK)3L, where DK - B-diketonate, L- Lewas basis.
Composite materials based on conjugated polymers PVK, PFO, PF_DMB etc. with different percentage of complexes, by spin coating are obtained. It was found that a full energy transfer is achieved when the content of the complexes are 50-60% as evidenced by the absence of the polymer emission.
Acknowledgment: This work was supported by the Russian Foundation for Basic Researches project 11-03-00597-a
1. A. Eliseeva and J.-C. Bunzli, Lanthanide luminescence for functional materials and bio-sciences Chem.Soc.Rev., 2010, 39, 189-227

Poly(3-hexylthiophene)/Titania heterojunction has been widely studied in the field of hybrid solar cells. To improve the conversion efficiency in this heterojunction, titania (TiO2) is sensitized with dyes to shift its absorption edge to the visible range or well its surface is modified with organic components to increase the electron transport. On the another hand, copper sulfide (CuxS), non-toxic semiconductor, has functioned as an electron acceptor in the bulk organic solar cells based on Poly(3-hexylthiophene) (P3HT), increasing the photocurrent density of devices. In this work, we propose the use of copper sulfide as charge transport mediator in the TiO2/P3HT heterojunction. Copper sulfide nanocrystals of about 14 to 17 nm of length and 6 to 7 nm of width where synthesized at 230, 240 and 260 C under argon atmosphere. Chalcocite phase (CuxS; x=2) was obtained at 230 and 240 C and, a mixture of chalcocite-djurleite (CuxS; x=1.94) phases at 260 C. According to the UV-Vis results this nanocrystals absorb in the range of 400 to 700 nm. This nanocrystals were mixed with P3HT in order to form a CuxS:P3HT bulk heterojunction and they were casted onto surface of titania thin films. P3HT:CuxS bulk heterojunction presented a resistance of about 108 Omega;/sq and its optical absorption in the visible range was improved. ITO/TiO2/P3HT:CuxS/Au hybrid solar cells were prepared and electrically characterized. The influence of the CuxS nanocrystals in the electrical I-V behavior was studied and analyzed.

In the present work we study the performance of bulk heterojunctionPoly(3-hexyl thiophene)(P3HT):PCBM solar cells that incorporate octahedral Au nanoparticles (NPs) in the PEDOT:PSS layer or position Au nanoislands on the top of the active layer and underneath of the cathode in the device. Incorporating Au NPs to the PEDOT:PSS layer enhaces the performance of the power conversion efficiency of the P3HT:PCBM cell to 4.3% from 4.0% for the control device, the near-field enhancement arising from the excitation of the localized surface plasmon resonance of Au NPs along the PEDOT:PSS/active layer interface. Alternatively, placing the Au nanoisland on the top of the P3HT:PCBM active layer and underneath the cathode resulted in apower conversion efficiency of 4.6% for the device; backward scattering by Au nanoisland increases theoptical path in the active layer, thereby enhancing the degree of light absorption. When both of Au NPs and Au nanoislands, adual metallic nanostructure, were incorporated in the P3HT:PCBM device, a higher power conversion efficiency(PCE) of 4.8% was obtained. This approach indicates the potential of using Au NPs and Au nanoisland for high efficiency plasmonic enhanced polymer solar cell applications.

Sustainable and cost-effective expansion of organic photovoltaics will require the replacement of indium tin oxide, currently the leading transparent electrode material, with an earth-abundant and mechanically stable alternative. Networks of single-walled carbon nanotubes have great potential to fill this role: in addition to their exceptional electronic and optical properties, carbon nanotubes can be deposited from inks by a number of scalable and low-temperature coating processes. The conductivity of a carbon nanotube network is limited by the contact resistance at tube-tube junctions. Transparent electrode performance can therefore be enhanced by maximizing nanotube length and by suppressing aggregation, both of which minimize the density of junctions in the network. However, the typical ink fabrication method, dispersing carbon nanotubes in water with the aid of sonication, is known to damage and shorten the nanotubes.
In an alternative fabrication route, the nanotubes are first chemically reduced in the presence of alkali metal and liquid ammonia, then, the resulting nanotubide salt dissolves spontaneously in a polar solvent without the need for sonication [1]. This process produces solutions of individual, unbroken nanotubes which, when deposited on a substrate, form significantly higher performing transparent conductive films than those cast from aqueous dispersion. For example, films fabricated using this reductive dissolution method have achieved a sheet resistance of 250 Omega;/square with transmittance of 92% at 550 nm wavelength, compared to 76% transmittance for a 250 Omega;/square film from the aqueous dispersion. The promise of these new highly conductive films as transparent electrodes has been demonstrated by their successful incorporation into efficient, flexible organic photovoltaic devices.
[1] S. Fogden, K.-C. Kim, C. Ma, K. Ligsay, and G. McFarlane. Scalable single walled carbon nanotube separation: from process to product. NSTI-Nanotech 1, 163-166 (2011).

The solid-state dye-sensitized solar cell made by spiro-MeOTAD already reached a power conversion efficiency of 4% in general and a record efficiency of 7.2% from Grätzel&’s group. However it is still below a desired level for manufacturing usage. The charge recombination is what has prohibited advancement of ss-DSSC over the past decade. People have already found that additives like 4-tert-butylpyridine (4-tBP) can suppress the recombination process and improve greatly the fill factor and the open circuit voltage of ss-DSSC. Here we design and synthesize a new additive incorporating pyridine and triphenylamine unit. We use this new material instead of 4-tBP in the application of ss-DSSC and study the charge recombination.

In the roadmap of the development of BHJ solar cells, significant efforts have been devoted to develop various polymeric donors. For acceptors, however, the species are rather limited, with fullerene derivatives as the most investigated and the most successful architectures. Soluble non-fullerene electron acceptors in solution processed BHJ photovoltaic cells received considerable attention recently, because their energy levels can be significantly different from those of current fullerene derivatives, and their capacity of effective derivatization and functionalization is larger than the fullerenes. Hence, developing non-fullerene acceptors will enrich the diversity of acceptors, match the present high performance donors, and ultimately lead to higher PCEs by proper selection and combination of donors and acceptors.
We developed a novel organic n-type small molecule based on fluoranthene-fused imide as the electron acceptor in the BHJ photovoltaic cells to replace [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The PCE as 1.86% was achieved by using Th-CN as electron acceptor and P3HT as donor with the weight ratio as 1:2 in the regular solar cell configuration.We further modify the structures of the acceptors via tuning the substituents from thiophenyl group to other aryl groups to seek higher performance and to understand the relationship between the acceptors and the device efficiency. We also find the effective improvement of the device stability through modulation of the devices structure from regular configuration to the inverted one . By tuning bulky substitution groups and optimizing the process of the fabrication of the devices, all devices using these acceptors show the excellent the PCE ( > 2.15%), and the highest one is up to 3.70%. To the best of our knowledge, it is the highest PCE based on the non-fullerene acceptors in solution processed BHJ solar cell. Besides getting high PCE, we propose a clear structure-property relationship in these non-fullerene acceptors which is quite different with other proposal in the previous reports.

Considerable progress has been made in the exploration of organic semiconductors as active elements in electronic and optoelectronic devices such as field-effect transistors (FETs) and organic photovoltaics (OPV). Although polymer-based bulk heterojunction solar cell exhibited significant progress in device performances recently, small molecule-based solar cell is also highlighted owing to their intrinsic advantages such as high purity and reproducibility of the internal morphology. Among many conjugated semiconducting molecules, more updated research results about π-extended linear-conjugated molecules have been suggested in many literatures recently. In this work, we demonstrate the synthesis of anthracene-based X-shaped conjugated molecules for use as soluble p-type organic semiconductors in FETs and OPV. We also investigated the carrier mobility of the new molecules in FETs and applicability to OPV by steering the molecular structures and tuning the molecular energy levels.

Cyclopentadithiophene (CPDT) units, which is the fused aromatic rings, can make the polymer backbone more rigid and coplanar, which undergo long conjugation lengths, narrow band gaps, and strong intermolecular π-π interactions to develop the PCE. Especially, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b&’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), with low-bandgap, include CPDP unit as the electron-rich moiety and shows 6.0% PCE which results in many investigation of the derivatives of CPDT unit in OPVs. We reported novel conjugated donor-acceptor conjugated polymers containing phenanthro[9,10-c][1,2,5]thiadiazole. These donor-acceptor conjugated polymers, PCPDTPTp and PCPDTPTm were synthesized by palladium catalyzed Stille coupling reaction of cyclopenta[def]phenanthrene and phenanthro[9,10-c][1,2,5]thiadiazole. All of polymers have good solubility in common types of organic solvents. The spectra of the solid films show absorption bands with maximum peaks at 450, 550nm and the absorption onsets at about 540, 580 nm, corresponding to band gaps of 2.30 , 2.13 eV, respectively. Under white light illumination (AM1.5G, 100mW/cm2), The device with polymer:PC71BM blend demonstrated an open-circuit voltage (VOC) of 0.97, 0.79 V, a short-circuit current (JSC) of 5.5, 4.0 mA/cm2, and a fill factor (FF) of 0.33, 0.32 leading to the power conversion efficiency of 1.80, 1.00%, respectively.

A photochemically stable linear copolymer, poly(oxyethylene)-amide-imide (POEM), was used to hybridize a 1-methyl-3-propyl imidazolium iodide (PMII) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4)-based ionic liquid (IL) electrolyte to prepare a new polymer-gelled ionic liquid (PG-IL) electrolyte for quasi-solid-state dye-sensitized solar cells (QSS-DSSC). We demonstrate that it is possible to gel this binary IL mixture with an amphiphilic organic gelator at concentrations as low as 5 wt%. The PG-IL electrolyte exhibits high conductivity, long-term stability, and thermoplastic behaviour, namely, with both gel and sol states; thus can be reversibly interconverted with a change in temperature, which enables the PG-IL electrolyte to effectively fill the porous TiO2 film while avoiding problems of solvent leakage. The POE segments in the copolymer structure may solvate ionic pairs in the electrolyte and consequently enhance the mobility of the counter I- ions. This PG-IL electrolyte became fluid liquid at an elevated temperature and showed high ionic conductivity and long-term stability. The synthesized PG-IL electrolyte overcomes problems of leakage out with cells based on IL electrolytes, and should help in accelerating the widespread use of QSS-DSSC.
The QSS-DSSC behaved as a quasi-solid state cell which demonstrated a high conversion efficiency of 6.28%, superior to the room temperature ionic liquid (RTIL) electrolyte cell of 5.37% under 100 mWcm-2 irradiation. Moreover, the cell had exhibited a long-term stability over 1,000 h test without losing its efficiency. The finding of the novel copolymer for gelling ionic liquid has advanced the development of high efficient QSS-DSSC.

In this work, iodide/triiodide (I-/I3-) redox reaction on the Pt surface is investigated by density functional theory (DFT) simulation and further characterized by electrochemical analysis. In the DFT calculation, I3 was considered to be the key functional groups in obtaining the adsorption energy (Eads) on Pt(111) 4×4 slab by using Pt32 supercell to model the I3-Pt(111) 4×4 slab. On the Pt(111) 4×4 slab, Eads of the I3-Pt(111) 4×4 slab is -1.86 eV. I3 with linear structure is easy to form stable molecules on the Pt(111) 4×4 slab. The difference between the charge distribution of Pt atom and I atom was studied by contour map. The electron donation plays an important factor in determining the Eads of I3 molecule being adsorbed on the surface of Pt nanoparticles (PtNPs). The more negative the Eads value, the stronger the adsorption. Thus, the more negative Eads implies the stronger adsorption strength of the iodine on a given Pt(111) 4×4 slab. The DFT computation reveals that the Eads for I3 on Pt surface would be affected by the charge distribution between the I atom and Pt atom. We also observed that the Eads of I3- on the PtNPs was strongly influenced by the sizes of PtNPs, which can be estimated through CO stripping voltammetry, and the corresponding binding energies were obtained through XPS. In order to further understand the effect of Eads for reducing the I3- on PtNPs surface, PtNPs with various sizes were used as catalysts for further study by electroanalytical methods. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization plots were performed to understand the reaction kinetics and electrocatalytic ability of PtNPs with various sizes (2~6 nm). The intrinsic heterogeneous rate constant (k0) and the effective electroactive surface area (Ae) of PtNPs with various sizes were determined by using a rotating disk electrode (RDE). The results indicate that the electrocatalytic ability is influenced by the values of Eads for I3- on the PtNPs surface, which can be further confirmed by Arrhenius plots. Based on the DFT computation and electrochemical analysis, the mechanism and related factor involving in the I-/I3- redox reaction on the Pt surface were proposed in detail. A better understanding of this interaction would facilitate the design of better dye-sensitized solar cells (DSSCs) and other applications in electrochemical fields.

Small-molecule organic photovoltaic cells (OPVs) have the potential to be a low-cost, flexible power conversion solution to many energy problems [1]. These OPVs take advantage of an extremely thin active layer which enables this flexibility and reduces material volume. However, it is this thin quality that calls for improved power conversion efficiency compared to traditional silicon solar cells. Thin films suffer from reduced optical path lengths, which hinder light absorption and hence, power conversion efficiency. Many designs have been proposed to improve light absorption [2]-[6]. A novel light-trapping substrate geometry for OPVs is presented which is based on a conformally-coated, subwavelength-textured substrate design which is intended to substantially increase optical path lengths. The subwavelength nature of these Nanocones/Nanowedges decouples the light propagation from the exciton diffusion path. This is an optimized situation for efficient charge transfer. Enhanced power absorption into the OPV active layer has been demonstrated via numerical computation methods, including COMSOL FEM and Lumerical FDTD. The challenge to fabricate a working device by using nanoimprinting to create the structures in a conductive polymer will be presented, where the nanoimprinting process is optimized to maintain good electrical properties of the patterned conductive film. We will also present an alternative approach that utilizes a conformal coating of the organic conductor PEDOT onto the pre-patterned nanostructures. Uniform and conformal PEDOT coverage over the nanoscale features was achieved using an all-dry deposition process [7].
***
[1] P.Peumans, A.Yakimov, S.R.Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693, Apr.2003.
[2] V.Karagodsky, F.Sedgwick, C.J.Hasnain, “New Physics of Subwavelength High Contrast Gratings,” Quantum Electronics and Laser Science Conference Technical Digest Optical Society of America, 2011.
[3] S.Mallick, N.P.Sergeant, M.Agrawal, J. Lee, P.Peumans, “Coherent light trapping in thin-film photovoltaics,” Materials Research Society Bulletin, vol. 36, no. 06, pp. 453 - 460, 2011.
[4] K.Nalwa, J.M.Park, K.M.Ho, S.Chaudhary, “On realizing higher efficiency polymer solar cells using a textured substrate platform,” Advanced Materials, vol. 23, pp. 112-116, 2011.
[5] A. Raman, Z. Yu, S. Fan, “Dielectric nanostructures for broadband light trapping in organic solar cells,” Optics Express, vol. 19, pp. 19015-19026, 2011.
[6] S.B.Rim, S.Zhao, S.R.Scully, M.D. McGehe, P. Peumans, “An effective light trapping configuration for thin-film solar cells,” Applied Physics Letters, vol. 91, pp. 243501, 2007.
[7] Im, SG, Gleason,KK.“Conformal coverage of poly(3,4-ethylenedioxythiophene) films with tunable nanoporosity via oxidative chemical vapor deposition.”ACS Nano. vol.2(9), pp. 1959-67, 2008.

Organic photovoltaic (OPV) devices, through the use of conjugated organic polymers and dye molecules, harvest solar energy for conversion to electricity or clean fuels like hydrogen. Through the process of photosynthesis, natural systems direct sunlight to drive reactions where the energy is eventually stored in chemical bonds. In an attempt to imitate this phenomenon, we focus on the development of porphyrin-based dyes, observed in nature as chlorophyll analogues, as the primary light harvester in OPV devices. Modification of substituents (such as aminophenyl, tolyl, carbomethoxyphenyl, and nitrophenyl) attached at the porphyrin&’s meso carbon allows us to adjust their absorbance as well as the HOMO/LUMO energy levels of these compounds. As a result of their proven capacity as light absorbing agents, tetraphenylporphyrins are optimal for incorporation into new porphyrin dyad compounds. These porphyrin derivatives can be linked at the meso position using a range of electron rich materials including thiophene, bithiophene and dithienosilole (DTS) to form a tetraphenylporphyrin dyad with enhanced electrochemical and spectroscopic properties. This introduction of bridging linkers allows us to further tune their absorbance and redox potentials. By studying these new tetraphenylporphyrin dyads in thin films with a traditional fullerene acceptor (PCBM - [6,6]-phenyl-C61-butyric acid methyl ester), we attempt to gain further insight into the photochemical properties of these light-absorbing materials.

The electron accepting isoindigo unit is an attractive synthon due to its simple yet versatile synthesis leading to monomers for Stille, Suzuki or direct heteroarylation polymerizations. In terms of their properties, isoindigo-based polymers exhibit broad absorption spectra (400 to 800 nm), high extinction coefficients (12,200 to 40,500 M^-1.cm^-1), and appropriate energy levels, in particular low HOMO levels (usually around -5.7 eV in donor-acceptor copolymers), leading to air-stable materials. Herein, the synthesis and device performance of novel n- and p-type isoindigo-based conjugated polymers designed for solar cell applications is reported.
Three n-type polymers based on isoindigo have been synthesized to harvest energy from low energy photons. The homopolymer of isoindigo P(iI), poly(isoindigo-alt-2,1,3-benzothiadiazole) P(iI-BTD), and poly(isoindigo-alt-thieno[3,4-c]pyrrole-4,6-dione) P(iI-TPD) exhibit absorption onset at 730 nm, 700 nm, and 720 nm respectively. The LUMO levels of P(iI), P(iI-BTD) and P(iI-TPD) are -3.84 eV, -3.90 eV, and -4.08 eV, which are suitable for exciton dissociation at the interface with P3HT. The electron mobility of these polymers and their efficiency in P3HT:n-type polyisoindigo devices is being studied. To date, P3HT:P(iI) blends have efficiencies of 0.5%.[1] P-type polyisoindigos are also studied to understand the influence of torsion and packing on the optical and transport properties of donor-isoindigo polymers. Poly(dithieno[3,2-b:2prime;,3prime;-d]silole-alt-isoindigo) P(DTS-iI) [2] and poly(terthiophene-alt-isoindigo) P(T3-iI) were synthesized. Their energy gap is 1.6 eV and their LUMO levels are -3.95 eV and -3.89 eV respectively. In inverted architecture devices, P(T3-iI):PC₇₁BM blends reach 6.6 % efficiency, showing great promise for isoindigo materials in organic solar cells.
[1] Stalder, R.; Mei, J.; Subbiah, J.; Grand, C.M.; Estrada, L.A.; So, F.; Reynolds, J.R. Macromolecules2011, 44, 6303.
[2] Stalder, R.; Mei, J.; Grand, C.M.; Subbiah, J.; So, F.; Reynolds, J.R. Polymer Chemistry2012, 3, 89.

Silver nanowire arrays, which have excellent optoelectrical properties and low price, are promising substitute for the traditional indium tin oxide thin films as a top electrode for solar cells. Furthermore, the flexibility of the structure and the low processing temperature makes them especially suitable for flexible organic devices.
In this poster, we numerically investigate the optical and electronic properties of ordered one-dimensional silver nanowire arrays over the wavelength range from 280 nm to 1000 nm. We calculated the transmission and sheet resistance of different silver nanowire array geometries and represent the results with comprehensive contour plots. The physics of the optical behaviors of silver nanowire arrays are revealed by the study of the transmission contour plots, angle dependency contour plots and electric field profiles. Rich optical resonances are supported by the geometry, which makes possible to control the optical performance by sophisticated tuning of the structure.
We compare the optoelectronic performance of silver nanowire arrays with other transparent conductor materials using different figure of merits. A better performance is demonstrated with ordered silver nanowires than other nanomaterials, such as carbon nanotubes, graphene, and random metal nanowire networks.

Despite the technological progress observed in Bulk-Heterojunction Organic Photovoltaics (BHJ-OPV), mainly with the appearance of new donor-polymers, a conclusive model for its photocurrent (Jph) is not established. We developed a model to explain the photocurrent curve in of a BHJ-OPV in which the active layer is a blend composed by a regio-regular poly-3-hexylthiophene (rr-P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM), in a 1:1 molar ratio. The model was motivated by the coincidence between the evolution of the short-circuit current and the mobility with the temperature. The mobility were measured by the Photo-CELIV technique. Jph vs V0-V curves are then modeled using the continuity equation in the steady state, taking into account the contribution of generation and recombination phenomena, which involve kinetics slower than the Charge-Transfer mechanism (CT). Experimental curves (that presented efficiencies up to 4.5%) were fitted using the following parameters: probability of dissociation (P), the carrier lifetime, and the carrier generation rate (G). Fittings for different temperatures reinforced the consistence of the model and provided important physical parameters of charge transport phenomena of the active photovoltaic layer.

Organic photovoltaics, (OPVs) are a promising class of solar cells that offer a viable cleaner, cheaper, flexible, and mass producible technology than silicon or other inorganic solar cells. However, OPVs suffer from poor power conversion efficiencies due to inadequate absorption characteristics, poor charge separation and collection, limited reproducibility, and severe time dependent performance degradation.
We will demonstrate the synthesis of nanoporous metal (Au, Ag, Cu) electrodes. These electrodes allow enhanced light absorption and the porous metal also serves as the controlled BHJ . These novel electrode materials circumvent the current BJH issues that are highly dependent on the appropriate mixing of electron donor and acceptor materials and thus have poor reproducibility. The pore-size of these films can be tuned to match the exciton diffusion length. (5-20 nm) We have optimized the nanopore size by (a) modifying the alloy composition (b) thermal annealing, and (c) choice of the etchant. The BHJ will be fabricated by functionalizing PCBM to the nanoporous metal electrode followed by infiltration of the donor polymer. This new metal organic based architechture for creating controlled BHJs is expected to dramatically increase the overall device external quantum efficiency and also the power conversion efficiency.

With increased demand for manufacturing printed device, such as printed organic photovoltaic (OPV) devices, new types of transparent and conductive electrodes are being developed. Many have studied to replace expensive transparent conductive indium tin oxide (ITO) with printable conductive materials. The high cost and the brittleness of the ITO limits the advantages of organic photovoltaic (OPV) such as flexibility. Various types of transparent and conductive electrodes have been used in optoelectronics, such as single-wall carbon nanotubes, graphenes, metal nano-wires, and hybrids of these. Among these, silver nanowire (Ag NW) show optoelectronic performances close to that of ITO [1, 2] and are regarded as good candidate to replace ITO [3] for OPV applications. Nevertheless, Ag NWs cannot be used alone for ITO replacement. Mixture with other conducting polymer such as poly(3,4-ethylenedioxy thiophene)-polystyrene sulphonic acid (PEDOT:PSS) can be utilized successfully.
Most of PEDOT:PSS solution used for OPV application are acidic and their pH scale is around 2. In this work, mixture of Ag NW and PEDOT:PSS with different pH level were utilized. With introduction of Ag NW into PEDOT:PSS, the sheet resistance is significantly reduced and the effect of this mixture was characterized in OPV devices. For every pH levels, Ag NWs were mixed with PEDOT:PSS and several process conditions were modified to find best optoelectronic properties. Several samples were prepared to test for OPV applications and showed promising results.
[1] S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N. Nirmalraj, W. J. Blau, J. J. Boland, J. N. Coleman, ACS Nano 2009, 3, 1767.
[2] D. Azulai, T. Belenkova, H. Gilon, Z. Barkay, G. Markovich, Nano lett. 2009, 9, 4246.
[3] A. Kumar, C. Zhou, ACS Nano 2010, 4, 11.

Due to low manufacturing cost, organic photovoltaic devices (OPV) are currently attracting much interest as a promising clean-energy source. OPV devices by printing or coating method are thought to be inexpensive but the materials related it such as transparent conductive indium tin oxide (ITO) substrate is remained persistently expensive. Replacing ITO with various materials has been tried out for OPV applications [1, 2] showing favorable performance compared to that of ITO. Achieving highly transparent conductive electrode are one of the challenging tasks towards inexpensive, high yield printed OPV. Especially, organic photovoltaic with flexibility and possibility of roll-to-roll production requires printed electrodes. To be utilized as transparent electrode for OPVs, Ag NWs [3, 4] have been studied because of their intriguing electrical properties. Meanwhile, there are still several problematic issues to solve for electrode application in organics-based photovoltaic devices.
In this work, the effect of Ag NWs layer and conjugated polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer on the cell performances in ITO-free OPVs based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 were examined. With introduction of Ag NWs layer, the transparency and conductivity can be comparable to ITO showing good suitability for OPV cell fabrication. Several sets of samples were prepared with various additives in those sets of samples. These are mixtures of Ag NWs and PEDOT, 2 sets of multi-layer structure with Ag NWs and PEDOT. These combinations of Ag nanowire and PEDOT:PSS have proven to be an effective way as a low-cost and easy process for printed conductive transparent substrates. Introducing these electrodes in our OPV cells, the optoelectronic properties such as short-circuit current improved with optimized printing process conditions. Utilizing these results, equal or even better efficiencies for large area printed OPV devices can be reached.
[1] J. Y. Lee, S. T. Connor, Y. Cui, and P. Peumans, Nano Lett. vol. 8, 689-692 (2008).
[2] J. Zou, H.-L. Yip, S. K. Hau, and A. K.-Y. Jen, Appl. Phys. Lett. 96 203301 (2010).
[3] Liangbing Hu, H. S. Kim, J. Y. Lee, Peter Peumans, Yi. Cui. ACS NANO. 2010. Vol. 4. No. 5. 2955-2963
[4] Liu and Yu. Nanoscale research letters. 2011. 6:75

Corroles are tetrapyrrolic macrocycles that have recently received attention as a light-harvesting material for nanoparticulate-based photovoltaics. The corrole structure is unique due to its ability to host high valent metals in their center, and their high-fluorescence quantum yield which is often much greater than similar tetrapyrrolic ring systems. A silicon containing 5,10,15-Tris(4-phenyl)corrole containing either nitrophenyl or pentafluorophenyl substituents were synthesized using a lithiated intermediate followed by the introduction of dichloromethylsilane. The trilithiated corrole intermediate shows a red shift in UV-Vis prior to the introduction of silicon while a dramatic increase in the fluorescence was observed for the Si(CH3)-corrole derivative. Spectroscopic studies have suggested that these Si-corrole compounds provide better visible light spectral coverage with an increase in absorbance from 650 - 750 nm. The HOMO / LUMO energy levels of the corrole show good overlap with common electron acceptors such as PCBM that could eventually be used with Si-corroles in an organic solar cell. Presented is the synthesis, electrochemical, structural, and photochemical characterization of these new corrole materials.

Organic photovoltaics (OPVs) have attracted much attention as a promising alternative to silicon based solar cells due to their advantages in terms of applying low-cost, large-area, and flexible substrates as well as the possibility of using roll-to-roll fabrication processes. In bulk-heterojuction (BHJ) OPVs, an important factor in determining the device performance is the charge extraction from the active layer to the electrodes. The exciton blocking layer (EBL) between inorganic electrode and acceptor materials is a one solution to improve the device performance. The transport mechanism of BCP in the device performance, however, has not been clearly understood because of the complicated properties such as the interfacial dipole and gap states formed by metal-BCP complex.
We investigated the mechanism for the formation of gap states in electron transporting BCP used in P3HT:PCBM based solar cell. The P3HT polymer and PCBM diffused out and mixed with BCP layer during deposition of BCP. As the deposition rate is low and the deposition temperature is higher than room temperature, there is an enough time and sufficient thermal energy for diffusion of P3HT. The diffusion of P3HT into BCP layer induced N-S bonding, meaning the chemical reaction between BCP and P3HT. This chemical reaction might induce the gap states, enabling electron transport through these gap states. The gap states formed by the intermixing of P3HT and BCP layer play an important role in electron transport properties at P3HT:PCBM/BCP interface. We suggest the novel origin of BCP gap states in P3HT:PCBM based-OPVs.

Since the first report of a dye-sensitized solar cell (DSSC) by Grätzel, they have been seen as a promising solution for many impending energy and environmental problems, due to their inexpensive fabrication, eco-friendly characteristics, and reasonable efficiency ( > 11%).1 However, the cost of the FTO-glass is estimated to be about half of the module cost.2
In order to decrease the module cost, we previously reported a DSSC that only use one transparent conductive oxide (TCO) substrate.3 However, if the TCO substrates are unnecessary in the DSSC structure, the practicality of the DSSC would be greatly increased.
In the present study, in addition to the previous reports about DSSC that used a metal substrate,3 we report a cost-effective TCO-less structure for DSSCs.4 Instead of the TCO substrates, the newly proposed cell uses only two metal foils - a kind of TCO-less DSSC. J-V characteristics and electrochemical impedance spectra substantiated the dependence of overall performance on the perforation structure. Furthermore, using IMPS/IMVS, the reason for the performance difference between conventional and newly proposed structures would be verified.
Reference
1. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737.
2. J. M. Kroon, et al., Prog. Photovoltaics, 2007, 15, 1.
3. H. -G. Yun, et al, Adv. Ene. Mat., 2011, 1, 337
4. H. -G. Yun, et al, Phy. Chem. Chem. Phy., 2012, 14, 6448

We report a novel concept enabling the generation of an embedded passivation layer along the 3-dimensionally intertwined interfaces between the nanoscopic domains of P3HT and PCBM in the polymer bulk hetero-junction solar cells. We used an asymmetric block copolymer of a P3HT block and a short tail. The tail blocks are segregated in the interfacial region of nanoscopic donor/acceptor domains and thermally transformed to a layer containing silica-like residues of sub-nanometer thickness. The additional interfacial thin layer generated in this way was shown to act perfectly as a block layer effectively depressing the recombination of charge carriers, affording 50 % enhancement of the power conversion efficiency over he devices without the copolymer. (Acknowledgement: This work was supported by the Basic Science Research Program (2010-0000282 and 2011-0027629) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) and the Program for Integrated Molecular System (PIMS) at GIST, Korea.)

Organic/inorganic multi-layer thin films have proven to be effective barriers for protecting organic electronic devices from exposure to oxygen and water vapor in a variety of environmental conditions. With the rise in flexible organic device applications and devices intended for use in outdoor conditions, there exists a high potential for mechanical failure of such barrier films due to loss of adhesion/cohesion when exposed to harsh environmental operating conditions. While most research has focused on the barrier performance of these multilayers, there is very little information about their mechanical performance and reliability. In this work, we measured the interfacial fracture toughness of a model organic/inorganic multilayer barrier made from SiNx and PMMA. Measurements performed using four point bend (FPB) tests showed the interfacial fracture toughness of the SiNx/PMMA interface to be 4.8 J/m2. Using a range of chemical modifications to the interface including oxygen plasma treatment and phosphonic acid coatings resulted in an increase of Gc from 4.8 J/m2 to 10.03 J/m2. Additionally, it was found that Gc was dependent on the thickness of the PMMA layer used in the barrier film. Contact angle measurements were utilized to determine the surface energies of individual surfaces after each modification and X-ray photoelectron spectroscopy (XPS) was used as a post mechanical testing to verify the chemical components of delaminated surfaces. Overall, it was found that interfacial fracture toughness scales with increasing polar component of the surface energy as well as with increasing thickness of the PMMA layer. The results of this study will help inform the choice of barrier layers not only with respect to their barrier performance but also with regard to the reliability of their performance under extended operating conditions.

The development of inverted organic photovoltaics has led to devices with improved environmental stability due to the remove of the highly reactive low work function electrodes from the device structure. This is accomplished by treating ITO with molecular (e.g., phosphonic acids) or inorganic layers (e.g., Al2O3, ZnO) to create electron selective contacts, allow more stable materials such as Ag to be used as the hole selective electrode. In general, these devices are fabricated in a laminate structure with multiple interfaces where the potential loss in adhesion will result in device failure. However, the mechanical reliability of the interfaces and active layers in inverted organic photovoltaics employing bulk heterojunction architectures is not well understood.
In this work, we will present mechanical adhesion data for inverted bulk heterojunction solar cells made using PBDTTTminus;C:PCBM (1:1.5 wt%) as the active layer. These solar cells produced power conversion efficiencies above 6%. The interfaces in the solar cells were varied using ZnO/ITO and PEIE/ITO (polyethylenimine ethoxylated (PEIE)) as electron selective contacts and MoOx/Ag, and PEDOT:PSS CPP/Ag as hole selective contacts. The data show that the interface between the PEDOT:PSS CPP and PBDTTT-C:PCBM was the weakest of all with an interfacial adhesion on the order of 0.13 J/m2. The location of the failures was determined using X-ray photoelectron spectroscopy and AFM scanning. The use of short plasma treatment prior to the deposition of the PEDOT:PSS CPP did little to improve the overall adhesion strength. By using MoOx/Ag, the strength improved to 0.42 J/m2. The failure occurred partially adhesively and cohesively at this interface showing that we are near the interfacial strength of the active layer material. The use of ZnO and PEIE remained stronger than all other contacts used in the architecture. Additional analysis of methods to improve the adhesion strength and the impact of the ratio of donor/acceptor materials on the cohesive strength of the active layer will be presented.

For the enhancement of efficiency in tandem organic photovoltaic (OPV), charge generation layer with metal dot sandwiched by n-type and p-type metal oxide structure was formed by the surface energy mediated inkjet printing. The surface energy of n-type semiconducting oxide (e.q. Zinc Oxide thin film; ZnO) layer was controlled by the self-assembled monolayer (SAM) with octadecyltrichlorosilane (OTS) for inhibition of well-spreading characteristics of hydrophilic Ag ink. Not only by controlling of surface energy but also by changing jetting temperature, dot per inch (DPI) for metal dot could be adjusted. To observe electrical properties of oxide/metal dot/oxide structure, we fabricated ZnO/Ag dot/tungsten oxide (WO3) with typical thickness of 30nm/(50~100)nm/30nm, respectively. N-type ZnO layer was formed by spin-coating of zinc precursor (sol-gel process) while WO3, as p-type semiconductor, was thermally deposited on top of the printed Ag dot. Transmittance of ink-jet printing with silver dot patterns (20um diameter with 70um pitch) resulted in the range of 90~92%. When the pitch decreased from 70um to 30um, transparency was significantly decreased, while the vertical conductive of oxide/Ag dot/oxide composite charge generation layer was correspondingly improved. Considering the transparency and conductivity of oxide/Ag dot/oxide structure, more than 20% improved performance can be expected compared with conventional oxide/Ag thin film (5-10nm)/oxide structure. Our modified charge generation layer with inkjet printed silver dot patterns was applied to the fabrication of tandem organic solar cell, yielding an improved power conversion efficiency compared to the conventional tandem device with charge generation layer with metallic thin film.

The dye-sensitized solar cells (DSSC) are a technological and economical alternative compared to conventional pn junction solar cells. The DSSC is composed of a counter electrodes (SnO2: F), coated by a porous nanocrystalline film of n-ZnO, to which, dye photosynthetic N-719 molecules are attached, an electrolyte (AN-50) containing a reduction-oxidation couple, finally an counter-electrode (SnO2: F) coated by a thin film of platinum. The highest efficient dyes for DSSC cells are synthesized based on Ruthenium polypyridyl complexes, due to adsorption coefficient in the entire visible range and the efficient injection of electrons into the conduction band of TiO2. However, the ruthenium polypyridyl complex containing a heavy metal of high cost and also complicated synthetic routes and low yields. Moreover, natural dyes in addition to its availability are cost-effective, non-toxic and biodegradable materials, and can be extracted by simple procedures. In this paper we report the synthesis of natural dyes, from the stems of mangrove and tinto trees as well as walnut shell. For extraction of the dye was first necessary dehydrating materials by thermogravimetric method, extraction is performed using ethanol, water and sodium hydroxide. The dyes were characterized using visible infrared and ultraviolet spectroscopy. The analysis of the infrared spectrum shows an intense and broad band and stretching of the OH bond in 3393, 3442 and 3390 cm-1 for the mangrove tree, tinto tree and walnut shell, respectively. In 1051, 1123 and 1050 cm-1, there was a very strong absorption due to the stretching vibration of CO group, for the mangrove tree, tinto tree and walnut shell, respectively. The results of the U-Vis spectroscopy show that the increased absorption in the visible region is provided by dyes of the walnut shell.

Transparent and electrically conductive coating films have a variety of fast-growing applications ranging from window glass to flat-panel displays. These mainly include semi-conductive metal oxides such as indium tin oxide (ITO), antimony tin oxide (ATO) and polymers such as poly(3,4-ethylenedioxythiophene) doped and stabilized with poly(styrenesulfonate) (PEDOT/PSS). Furthermore, conductive polymer of PEDOT:PSS is one of the candidates for IR shielding materials. It has many merits compared to other conducting polymers such as a high transparency in the visible range, outstanding thermal stability and it was possible to process in aqueous solution. In recent years, many attempts have been done to increase the conductivity of PEDOT:PSS films by mixture with organic solvents or the addition of small amount of different additives. Especially, there has been an increasing interest in the conducting polymers using single-wall carbon nanotubes (SWNT) and poly(ethylene glycol)(PEG) as alternatives to ITO and ATO in recent years. Carbon nanotube (CNT) are a good conductive dopant for conducting polymer and these CNT-based technologies offer conducting substrates having a broad range of conductivity, excellent transparency, neutral color tone, good adhesion, abrasion resistance, and flexibility as well as the reliability of processing and patterning. PEG was reported that the conductivity of PEDOT:PSS film can be enhanced by more than an order of magnitude by the addition of PEG. In the other hand, metal oxides such as tungsten trioxide (WO3) and molybdenum trioxides (MoO3) exhibit not only electrical conductivity but also transparent property in the visible and NIR lights. Particularly, these materials have better optical and electrical properties as changing doping ions such as Li+, Na+, Cs+.
In this paper, we investigated the conductivity enhancement and optical properties on alternative material to ITO and conducting polymers, PEDOT:PSS composites thin films with the Metal Oxide based nanoparticle as additives and the mechanism of conductivity enhancement in the composite films will also be studied. The synthesized size of metal oxide particles was observed average below 100 nm by PSA spectrometer. Transparent PEDOT:PSS/ metal oxide nanocomposite films exhibited the excellent conductivity and high transparent in the visible range as well as the stronger NIR shielding ability compared to the pure PEDOT:PSS films in the NIR range. Finally, our study suggests that transparent and electrically conductive coating films were expected the many commercial application fields because of the photovoltaics, smart windows, flat-panel displays, light-emitting diodes, touch screens, electromagnetic shielding, et.

Organic based materials, including polymers, for photovoltaic application is a promising direction in research as well as in industry. It attracts attention due to possibility to obtain low-cost easy-processable solar cells with relatively high performances. Important advantage of semiconducting polymers is their ability to be easily modified to fulfil specific requirements using commercially available compounds and well-known chemical tools.
By using approach of designing semiconductors with donor-acceptor alternating units, it is possible to obtain effective low band-gap organic semiconducting materials. Recently, the research group synthesised so called PPBzT2-EHbeta polymer which consists of an alternation of alkylthiophene and thieno[3,2-b]thiophene electron donor units and benzothiadiazole electron accepting units.
Structure-property relationship between solubilising side chain of PPBzT2-EHβ and photovoltaic properties has been studied. Investigation revealed that alkyl chains in beta-position (fourth position of thiophene rings) lead to better pi-electron delocalisation due to enchanced planarity, because such position avoids rotational hinderance. Also, ramified alkyl side chains were found to be more appropriate for giving the compound better solubility. Additionally, change from linear alkyl chain to ethylhexyl branched solubilising side chain affected interaction between fullerene and polymer in bulk heterojunction devices.
According to computational study (Density Functional Theory method on Spartan 10 software) copolymer keeping the same chemical architecture but replacing benzothiadiazole by a pyridinothiadiazole (PTD) as central unit may have lower band-gap due to the higher electron affinity of PTD unit. In this work, we report the synthesis of three new copolymers based on this chemical structure as well as their physical and optoelectronical properties. In addition, we will present their performances as active materials in both, organic field-effect transistors (OFET) and organic solar cells (OPV)
Ref.: Laure Biniek, Sadiara Fall, Christos L. Chochos, Nicolas Leclerc, Patrick Lévecirc;que,
Thomas Heiser, Organic Electronics 13 (2012) 114-120.

The morphology of bulk-heterojunctions (BHJ) is critically important in achieving optimum photovoltaic properties of fullerene loaded conjugated polymer system. The primary objective of the study is to investigate the influence of processing conditions on the phase morphology of bulk heterojunction solar cell. A novel fullerene derivative (N- (3-methoxy propyl)-2- carboxy ethyl -5- (4-cyano phenyl) fulleropyrrolidine) [NCPF] was synthesized and characterized by 1H NMR, 13C NMR, MALDI-TOF, UV-VIS, Cyclic Voltammetry, and TGA. The solubility and electronic properties of NCPF was found to be similar to that of PCBM. Morphology of as cast and post annealed thin film casted from NCPF/P3HT (1:1) blend prepared by mixing solution of NCPF and P3HT in 1,2 dichlorobenzene at room temperature without preheating, shows large aggregates. In contrast, films casted from blend prepared by mixing of preheated NCPF and P3HT in 1,2 dichlorobenzene followed by cooling to room temperature showed homogenous dispersion of NCPF in P3HT matrix, and noteworthy improvement in interfacial contact was observed when films were prepared at different spin rate. Furthermore, the phase morphology of thin film casted from preheated NCPF/P3HT blend solution revealed significant crystallization of P3HT in the as cast and post annealed films. Our results suggest that the protocol used in device fabrication could be critical in achieving desired morphology. Future studies will investigate the power conversion efficiency (PCE) of devices formulated with NCPF and P3HT.
Financial Support: US Department of Energy

Low hole mobility and high recombination rates severely limit the incident photon-to-current collection efficiency (IPCE) of organic photovoltaics, thus preventing practical, widespread adoption of inexpensive organic-based solar cells. In this study, we use first-principles density functional theory computations to design a novel hybrid organic-inorganic photovoltaic material to ameliorate these issues. We demonstrate that one can create a potentially self-assembling periodic crystal with parallel electron and hole conducting channels composed of two-dimensional atomically-thick transition metal oxide sheets separated by nanoscale regions of a photoactive organic material. Furthermore, we show that one can functionalize the organic component to provide a built-in electric field across each of the nanometer-thick photoactive regions, thereby enabling efficient exciton separation orthogonal to the light absorption direction. By shortening the distance the holes and electrons have to travel in the lower mobility organic material as well driving exciton separation, this approach could lead to significant improvements in the IPCE of organic-based photovoltaics.

Organic photovoltaic devices have gained a broad interest in the last few years due to their potential for large-area, low-cost solar cells. Various material concepts have been employed using small molecules, conjugated polymers, and combinations of inorganic and organic materials as the active layer. Although significant progress has been made, the performances obtained from polymer solar cells (PSCs) are still not satisfactory for commercialization. As a result of intensive investigations by many researchers, it is well known that by collaboratively lowering the HOMO energy level and band-gap, both high short circuit current (Jsc) and open circuit voltage (Voc) can be achieved thus rendering high power conversion efficiency (PCE) upon thorough device and materials optimization. We reported the synthesis of novel polymeric semiconductors Polymer1 for the fabrication of soluble polymer solar cells, these polymers with high yield were synthesized by Stille coupling reactions. The polymers showed broad absorption peaks, Polymer1 showed at 660 nm in solution and at 670 nm in film. The optical band gap for the polymers in film was calculated by UV edges. The electrochemical properties for the polymers were investigated by CV. The polymers showed HOMO energy level were -5.39 eV. Copolymers have lower the band gap, enhance the interchain packing, and improve the charge mobility of the resulting polymer. Novel Copolymers exhibited considerably better photovoltaic performance with a power conversion efficiency (PCE) of 5.00% when compared with the alkoxy polymer, which gave a PCE of 1.91%.

Colloidal quantum-dots (CQD) have attracted much attention due to their distinctive optical properties such as wide spectral responses and tunable absorption spectra with simple size control. These properties, together with the advantage of solution processing and superior robustness to organic materials, have motivated the recent investigation of CQD-based solar cells, which have seen rapid growth in power conversion efficiency in just the last five years, to a current record of around 7%. However, in order to continue to push the efficiencies higher, a better understanding of the charge transport phenomena in quantum-dot films is needed. While experimental efforts to design more favorable device architectures have been explored, including hybrid nanowire/quantum-dot structures and tandem cells with multiple size quantum-dots, optimization based on computational approaches can accelerate the pace of such designs, since detailed information such as optical generation rate, electric field distribution and current matching in complex structures cannot be obtained easily from experiments. In addition, the transport properties of quantum-dot films, such as dot-to-dot hopping rates, effects of energy-level distributions and the role of ligands, can be understood in a systematic manner computationally.
In this work, we use computational approaches based on a hopping transport model to simulate the charge transport in quantum dot film as a function of energy levels and density of states of individual quantum dots, distance between the dots, and the effects of external electric fields. Based on these transport properties, we study a range of different device geometries for CQD solar cells using finite element methods. Our calculations show the effect of modification in the interfaces between quantum-dot films and charge transport layers, and suggest improved device designs for tandem solar cells with higher efficiencies.

A new method for trapping light in bulk-heterojunction (BHJ) solar cells is the incorporation of metallic nanostructures that support surface plasmons onto the electrodes of a device. A nanostructured metallic electrode can couple both photons and surface plasmons supported at the metal/semiconductor interface, where light may be converted into photocarriers. Light energy not absorbed during the first pass through the active layer may be redirected into guided surface plasmon modes, increasing the electric field intensity on regions surrounding the nanostructure. When applied to organic photovoltaics, this technique may allow considerable shrinkage of active layer thickness, while keeping effective optical absorption length constant.
However, for effective carrier collection the workfunction and electrode type (anode or cathode) must be carefully chosen. Here we present theoretical calculations of open circuit voltage (Voc) for both conventional and inverted BHJ photovoltaic device formats for a variety of metal and metal oxide-on-metal electrodes that may support surface plasmons. The results establish which metals demonstrate the largest Voc in both configurations. For inverted devices the contacts that optimize Voc are copper (Cu), copper (II) oxide (CuO), and gold (Au), whereas silver (Ag) and aluminum (Al) are optimal for the conventional configuration.
In addition to this theoretical work, arrays of metal nanoparticles have been fabricated by thermally evaporating metal through nanoporous alumina masks to form nanostructured optically-active electrodes. Preliminary bright-field and dark-field spectroscopy measurements have been carried out on the arrays with and without the presence of a thin (<150 nm) film of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM), to determine the enhancement in absorption and scattering of the solar spectrum. These measurements showed that uncoated silver nanostructures on a planar silver film enhanced extinction (sum of incident light absorption and scattering) by a factor of 2.3 relative to a planar silver film. In addition, dark-field scattering was enhanced by a factor of 0.7. Dark-field spectroscopic measurements of the silver nanoparticles on a planar silver film coated with P3HT:PCBM showed dark-field scattering enhancement of up to 7.5 relative to a planar coated silver film. Ultimately, these measurements will establish which nanostructured material results in the largest absorption enhancement. Furthermore, surface conductivity of nanostructured silver and copper electrodes before and after partial electrode oxidation will be determined using a four-point probe current-voltage measurement system. This will determine their potential for use as “optically-active” metal electrodes in bulk-heterojunction solar cells. Successful developments in optically-active electrodes have the potential to push organic photovoltaics closer to competitive power conversion efficiencies.

In bulk heterojunction (BHJ) organic solar cells, morphology controls as well as the HOMO and the LUMO energy level alignments between donor and acceptor materials turned out crucial factors in obtaining maximum power conversion efficiency. In this perspective, systematic replacement of heteroatoms in conjugated backbone, for example, replacing sulfur either by selenium or by tellurium, would be the good way to change blend film morphologies while maintaining or fine-tuning energy levels.
Despite abundant research on thiophene-based conjugated materials, the inefficiency of standard Stille and Suzuki coupling reactions with tellurophenes has hampered studies of tellurophene-based materials. Here, we develop efficient palladium-catalyzed arylation reaction of functionalized tellurophene and demonstrate solar cells based on tellurium-containing low bandgap conjugated materials.

In the field of organic electronics, printing is a desired means for large scale organic electronic device fabrication. Photoelectron spectroscopy (PES) is a powerful method for characterization of chemical and electronic surface and interface properties, but is constricted to UHV-integrated step by step interface preparation. We extend PES to solution processed organic semiconductors by integrating ultraclean wet deposition via an ultrasonic nebulizer unit. To meet the requirements of contamination free preparation the deposition is performed at normal pressure in an Ar 6.0 purged glass cell that is directly attached to the UHV cluster-tool of the analytic competence center at InnovationLab Heidelberg. The suitability of this setup for solution based film preparation for PES was established by comparing spectra of PVD deposited α,omega;-Dihexylsexithiophene (DH6T) to drop-casted films of the same molecule yielding excellent accordance.
The nebulizer method was applied to dilute solutions of Phenyl-C61-butyric acid methyl ester (PCBM) in Chlorobenzene and Copper phthalocyanine-3,4prime;,4Prime;,4Prime;prime;-tetrasulfonic acid tetrasodium salt (CuPc-TS) in water to investigate their interface behavior to ozone-treated ITO surfaces. To test consistency, both molecules were additionally deposited via drop-casting. The resulting spectra show perfect agreement thus allowing the conclusion, that the molecules remain intact during nebulization. Repeated nebulizer deposition increased step by step the molecule emission intensity. The nominal layer thicknesses derived from the substrate emission attenuation during XPS measurements is 10 to 20 #8491; in the case of PCBM and approximately 2 #8491; for CuPc-TS both on ozone-treated ITO. Substrate coverage and homogeneity of the films was investigated in addition using optical microscopy. The electronic states lineup is derived including the systematic work function decrease. Thus the nebulizer method allows deriving interface band diagrams for electronic devices deposited from inks.

A new class of alternating π-conjugated copolymers incorporating novel cyclopenta[c]thiophene-4,6-dione (CTD) based-acceptors with benzodithiophene (BDT) or bis-benzodithiophene (BBDT) donor units have been synthesized and explored in bulk heterojunction solar cells. The synthesis of CTD is conducive to attaching a wide range of substituents in its final step that ultimately allows the degree of intramolecular charge transfer between CTD and the donor moiety of the copolymer to be readily tailored. For example, substitution with fluorine, hydrogen, or methyl groups on CTD results in copolymers with BBDT having optical band gaps ranging between 1.4 and 1.9 eV, while HOMO values remain relatively constant at -5.4 eV. This feature not only allows fine tuning of the LUMO values to promote efficient photo-induced charge transfer to the fullerene, but it also offers a unique opportunity to easily tailor the solubility and the self-assembly of the absorber in the active layer of the OPV cell. The latter was demonstrated by attaching several aryl substituents onto CTD for morphological studies. Free carrier generation and decay dynamics in the film blends were correlated with electronic, optoelectronic, and structural data. Furthermore, solar cell devices based on these CTD-containing copolymers possessed open circuit voltages in excess of 0.94 V and achieved initial power conversion efficiencies of 4%.

Optical and electronic interactions between organic conjugated polymer materials and metal nanostructures that support surface plasmons (i.e., plasmonic nanostructures) have important implications for organic optoelectronics. To optimize surface plasmon-conjugated polymer interactions for improved active layer light-harvesting, light-trapping, internal quantum efficiency or spontaneous emission rate, the plasmonic nanostructures must be designed to minimize exciton quenching and light absorption in the metal. Additionally, the placement of plasmonic nanostructures with respect to the active organic layer in optoelectronic devices must be carefully chosen to preserve or enhance the electrical characteristics of the device such as charge carrier collection or injection.
This presentation will discuss theoretical and experimental studies of a variety of metal nanostructure/conjugated polymer configurations that identify routes to optimized surface plasmon/conjugated polymer interactions. The emission properties of metal-polymer nanoheterostructures consisting of gold nanorod antennas (nanoantennas) coupled to sub-50-nm poly(3-hexylthiophene) (P3HT) layers will be presented [1]. The radiative decay rate (Tr) and modified quantum efficiency (QE) of P3HT integrated into three different nanoantenna configurations: a monomer nanoantenna; a dimer nanoantenna; and a monopole nanoantenna; will be compared in both theory and experiment for nanoantenna resonance wavelengths overlapping with the emission band of P3HT. Tr and QE enhancements of up to 29 and 12, respectively, have been determined from experiments using optimized metal-polymer nanoheterostructure configurations.
In more recent work, we determine theoretically the degree of light absorption in plasmonic nanostructures with respect to thin (< 50 nm) conjugated polymer layers. In particular, we focus on two systems where: (1) vertically-oriented gold nanorod arrays are placed directly on top of ultra-thin polythiophene films [2]; and (2) arrays of vertically-oriented plasmonic nanorods are integrated directly onto the metal electrode of a bulk-heterojunction organic photovoltaic device. In the first configuration, for gold nanorods with diameters of 60 nm and aspect ratio ranging from 0.5 to 2, we find that more than 50 % of incident light in the 400 - 800 nm wavelength range is absorbed by the plasmonic nanorod array compared with an underlying 20-nm-thick polythiophene film. For the plasmonic nanorod electrode, high-albedo, large-diameter silver nanorods can improve photocurrent generation while small-diameter nanorods degrade device performance [3]. In summary, this work seeks to elucidate the effectiveness of plasmonics when applied to organic optoelectronic materials and devices.
[1] D. M. O'Carroll, et al., Adv. Mater. 24, OP136 (2012).
[2] B. Yu, S. Goodman, A. Abdelaziz, D. M. O&’Carroll, Appl. Phys. Lett. 101, 151106 (2012).
[3] C. Petoukhoff, D. Vijapurapu, D. O&’Carroll, submitted.

The development of small molecule light-absorbers for organic solar cells is an area of great interest providing new molecular systems that can be more readily synthesized with greater ease than their polymer counterparts. We report the synthesis of two (4-carbomethoxyphenyl)porphyrin macrocycles coupled together with varying thiophene moieties specifically, thiophene, bithiophene, thienothiophene, and a dithienosilole (DTS) unit, for potential use as the light-absorbing donor material in a bulk heterojunction organic solar cell. These dyad systems have been designed to function as an ACCEPTOR-DONOR-ACCEPTOR light-harvesting p-type material for a bulk heterojunction solar cell. The electron-rich thiophene linkers serve as the intramolecular donor portion of the molecule while the tetraphenylporphyrins serve as intramolecular acceptors. We observe that thiophene, bithiophene, and thienothiophene dyads establish electronic communication between the two porphyrin moieties through the extended conjugated backbone. Based on DFT calculations, the DTS unit segregates the HOMO and LUMO levels within the molecule, with the HOMO residing on the DTS linker and LUMO on the porphyrins. We present spectroscopic, electrochemical, and computational investigations of these materials.

Recent experiments show that the photocurrent quantum efficiency of organic photovoltaic devices can be improved by inserting a thin insulating tunnel barrier between the donor and acceptor layers. Excitons created by photon absorption generate photocurrent if they dissociate into positive and negative charge carriers in the donor and acceptor layers, respectively, and these charge carriers are collected by the contacts. Competing with these processes is the formation of a charge transfer state and its recombination. A thin insulating layer between the donor and acceptor materials suppresses the charge transfer state recombination and enhances its dissociation. We study theoretically the dependence of critical microscopic processes on the thickness and electronic structure of the insulating material. By including the dependences of transfer and recombination rates on the tunnel barrier characteristics into a numerical device model, results consistent with the experimental observations for the photocurrent quantum efficiency are obtained. We also show that the incorporation of a tunnel barrier can increase the open-circuit voltage of the device, and its power efficiency. The numerical results are understood by properly estimating and combining the rates of different processes. The device performance can be tuned separately by designing the acceptor/donor interface and selecting the most suitable tunnel barrier material. Our conclusions provide guidance for device designs of organic photovoltaic cells that exploit this control over the interface processes.

Metal-Organic Frameworks (MOFs) are nanoporous materials with tunable pore sizes that can accommodate and stabilize small molecules. Because of their long-range order and well-understood pore environment, the nano-confinement of p/n-type materials within MOFs offers a new methodology for creating uniform phase-segregated donor-acceptor interfaces. Phase segregation and the photo-physical effects of confining α,omega;-Dihexylsexithiophene (DH-6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in several MOFs and the potential role of the MOF in creating a nano-heterojunction for organic photovoltaics are discussed. We demonstrate infiltration of both molecules into MOF pores and use luminescence and absorption spectroscopies to characterize the MOF-guest energy transfer processes. Comparison with density functional theory allows us to determine the energetics and band alignment within the MOF. The results demonstrate the utility of MOFs as scaffolds for sub-nanoscale ordering of donor and acceptor species within a highly uniform environment, allowing both the interaction and separation distance to be much more controlled than in the classical bulk heterojunction.

Short diffusion lengths (~10nm) in organic semiconductors are the primary factor limiting active layer thicknesses in organic photovoltaics (OPVs), yielding a competition between absorption and exciton transport. We find that exciton lifetime can be enhanced through the Purcell effect, decreasing homogeneous recombination losses and significantly improving exciton diffusion lengths. We use high quantum yield “model” Lorentzian absorbers with realistic exciton diffusion lengths to probe the interplay between device geometry and absorber properties. Through combinatorial device simulations, we demonstrate over 40% enhancement in lifetime, resulting in ~20% enhancement in photocurrent using planar microcavity devices, and discuss the impact of quantum yield, Stokes shift, and absorber wavelength on the effects of near-field interference and diffusion length within OPVs. To date, most organic molecules used for PV have extremely low QY. Our results suggest a new paradigm in molecular design for OPV applications.

We have studied the crystallinity of thin poly-3hexathiophene (P3HT) films confined between two surfaces using X-ray diffraction, Uv-vis spectroscopy and polarized microscopy. The liquid films are annealed in a controlled way in a home built chamber containing solvent vapor atmosphere. Various processing conditions and surfaces in contact with the liquid film are studied. We varied the time of annealing, the nature of the contacting surface (smooth vs patterned) and we compared the results to spun films and films annealed in solvent vapor but without confining surface. We find a very different crystallization behavior in thin films which have been confined between smooth surfaces, and an increased crystalllinity in very thin layers compared to thicker ones. For patterned layers we also show that edge on lamellae grow only in thin residual layers but not inside the patterns.

Conventional solar cells become dramatically less efficient when the sun is not shining directly on the surface, and quoted efficiencies of 15-19% are only achieved for sunlight within a narrow range of acceptance angles. Sunlight beyond this range will suffer from diffraction, and the reduced flux of solar energy causes a drop in internal cell efficiencies. Consequently, sub-optimal operating efficiencies occur during the morning and evening hours, as well as winter days, when sunlight is outside of this acceptable range. Hence, suitable materials must be developed to enhance light collection within a wider angular range.
Our group previously reported an optochemical organization route to 3-D optical and microstructural lattices that combines the spontaneity of self-organization to the precision and directionality of lithography. This method exploits the inherent instability of a broad, uniform beam of white light propagating in a photopolymer and its consequent division into identical filaments of light. By imposing spatially controlled noise on the light beam, the self-organizing filaments can be coaxed into 2-D and 3-D lattices. Unlike any other known self-organized or lithographically constructed structure, these lattices comprise functional, multimode and multi-wavelength cylindrical waveguides.

The objective of the work presented here is to apply multidirectional waveguide lattices (MWLs) as wide-angled, light-capturing coatings that increase the intensity of light that is incident on optical devices including photovoltaic (PV) cells. Our approach is strongly motivated by the fact that even small increments of energy conversion efficiency (<1%) of PV modules are critical in the field. Furthermore, wide-angled, light-capture could eliminate (expensive) mechanized rotation of solar panels to track the Sun&’s diurnal trajectory. We employ optochemical organization to prepare elastomer films comprising up to 5 intersecting arrays of waveguide lattices, which are oriented over a large range of angles (± 60°) with respect to the surface normal. When integrated into solar cell devices, the MWLs capture light seamlessly over a large range of angles and deliver intensity to the photovoltaic module and in this way, could increase conversion efficiencies.

Pyrite FeS₂ is an earth-abundant and environmentally benign semiconductor with low procurement costs, a large optical absorption coefficient (>10⁵ cm^-1), and adequate carrier mobility for photovoltaic (PV) applications. Here, we report the synthesis of pyrite nanocrystals (NCs) using the hot injection method and the mixing of these NCs with poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-c61-butyric acid methyl ester (PCBM) to form ternary hybrid bulk heterojunction (BHJ) solar cells in an inverted architecture. Cyclic voltammetry measurements were used to estimate the valence band energy (E_VB) and the conduction band energy (E_CB) of the NCs and the values of -5.6 eV and -3.9 eV were obtained for E_VB and E_CB, respectively. This E_VB agrees with literature values while the E_CB was higher than expected yielding an increased band gap (E_g) of ~1.7 eV. UV-Vis-NIR absorption measurements also indicated an elevated E_g at ~1.4 eV. The chemical states of Fe and S species were probed with X-ray photoelectron spectroscopy (XPS) and a much greater presence of Fe(II)-O species existed in the pyrite NCs synthesized with TOPO additives, which could potentially cause the increased E_g without introducing trap states, according to recent density functional theory (DFT) calculations. The pyrite FeS₂ NC concentration was varied systematically from 0 to ~ 4 wt% in the active layer of the inverted ternary solar cells and three distinct performance regimes were observed that appear linked to microstructure transitions, as shown by atomic force microscopy (AFM). By adding FeS₂ NCs up to ~0.5 wt% in the films, photocurrent enhancements of up to 20% were consistently obtained but were accompanied by decreased fill factors (FF) resulting in an unchanged overall power conversion efficiency (PCE) compared to the control binary device with P3HT:PCBM. Interestingly, after ageing with intermittent exposure to air for 73 days, these inverted devices with NCs showed a large improvement in FF and a PCE enhancement from 2.0% for the control P3HT:PCBM device up to 2.6% for the ternary BHJ device with 0.5 wt% NCs. The inverted configuration of the ternary hybrid solar cells with pyrite NCs clearly improved device stability and lifetime and helped reduce leakage current and prevent shorting. The photoluminescence (PL), time-resolved PL and external quantum efficiency (EQE) were measured for active layer thin film and devices. The possible charge generation, separation and transport mechanism will be discussed. The efficiency enhancement brought on by FeS₂ NCs in this inverted design offers a promising architecture for future FeS₂-based devices. Furthermore, the NC characterizations presented here provide insight to advance the development of FeS₂ as a cheap, non-toxic PV material.

Herein, we report on the latest results of our research on merocyanine (MC) based small-molecule organic solar cells (SM-OSC). We present results on tandem solar cells with complementary absorbing subcells in series connection, containing red and blue dyes, respectively. Due to the versatility of MCs, all possible combinations of solution- (SOL) and vacuum-processed (VAC) active layers can be studied. Therefore, tandem solar cells with VAC/VAC, SOL/SOL, SOL/VAC and VAC/SOL active layer combinations are fabricated and characterized. The results are compared to optical simulations and the respective single-junction solar cells. In the SOL-devices the influence of the casting from solvent mixtures is investigated in detail.

Crystalline poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) ultrathin films have been found to enhance the power conversion efficiency of bulk heterojunction organic photovoltaic devices when inserted at the cathode interface by inducing a large built-in electric field in the active layer [1], and to increase the open circuit voltage of bilayer organic photovoltaic devices when inserted between polymer donor and fullerene acceptor by tuning their relative lowest unoccupied molecular orbital (LUMO) offset [2]. However the previously reported method to deposit ultrathin crystalline P(VDF-TrFE) films by Langmuir Blodgett coating is not compatible with the process of many recently developed low bandgap polymers due to the high temperature thermal annealing needed to convert P(VDF-TrFE) into ferroelectric phase. Here we report the pre-formation of P(VDF-TrFE) ferroelectric nanocrystals (NCs) with variable size down to 60 nm by a simple hydrophobic interaction induced polymer self-organizing method for the first time. The formation of P(VDF-TrFE) NCs solution enables the deposition of the NCs onto an organic semiconductor with a scalable solution process. The P(VDF-TrFE) nanostructures were converted into ferroelectric phase in solution, and thus avoid the high temperature annealing process during OPV device fabrication. The pre-formed P(VDF-TrFE) NCs with tunable size also enable precise control of their morphology for a maximized OPV efficiency. The application of synthesized P(VDF-TrFE) NCs as cathode interfacial layer was demonstrated to enhance the PCE of poly[N-9prime;-hepta-decanyl-2,7-carbazole-alt-5,5-(4prime;,7prime;-di-2-thienyl-2prime;,1prime;,3prime;-enzothiadiazole)] (PCDTBT) based OPVs to 6.5% which is 25% higher than the optimized efficiency of the devices with regular low work function metal of calcium as the cathode.[3]
[1] Y. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang and J. Huang, Nature Materials 2011, 10, 296-302
[2] B. Yang, Y. Yuan, P. Sharma, S. Poddar, R. Korlacki, S. Ducharme, A. Gruverman, R. Saraf, J. Huang, Adv. Mater. 2012, 24, 1455-1460
[3] Z. Xiao, Q. Dong, Y. Yuan, S. Li, W. Tian and J. Huang , Submitted

Small molecular organic photovoltaics have seen a steady increase in power conversion efficiency in the past few years with the development of new materials and device designs but ultimately are still limited by their low exciton diffusion lengths, low mobilities and incomplete absorption of sunlight. Device architectures such as those using anode interfacial layers, bulk heterojunctions, and planar-mixed heterojunctions alleviate some of these issues but still struggle to achieve efficiencies as high as those typically seen in inorganic photovoltaics or even polymer based photovoltaics. We present here the development of a highly efficient small molecule organic photovoltaic device using polyacene materials, such as anthracene, tetracene, and pentacene, as an interfacial layer between the ITO anode and the organic donor material. Using organic interfacial layers has been shown to increase the crystallinity in the donor film resulting in an enhancement of over 80 percent in the donor contribution to the external quantum efficiency. Thus, the active layers can be kept thinner and still absorb the majority of the light. Using a well-known system of ZnPc as the donor material and C60 as the acceptor material, we fabricated planar-mixed heterojunctions with a tetracene anode interfacial layer. As a result we are able to fabricate devices with high JSC while maintaining a high FF and achieving power conversion efficiencies well over 5%. Furthermore, through the application of advanced light trapping techniques, a tandem device architecture, or novel small molecular donor and acceptor materials, even higher efficiencies are possible.

In bulk-heterojunction organic solar cells (BHJ-OSC) usually both phases, i.e., donor and acceptor, are in contact with one or both electrodes. For this reason the efficiency can be reduced drastically by surface recombination at the electrode(s) if one or even both contacts are not sufficiently selective. In this case electrons and holes flow towards the same electrode interface where they are lost by recombination. It is often a complex task to find appropriate electrode materials which ensure a good selectivity, especially in non-standard device configurations such as e.g. ITO-free cell architectures.
When a BHJ-OSC shows a poor current-voltage characteristics it is often not clear by which process the performance of the solar cell is limited, i.e., if it is due to a poor exciton dissociation rate, an enhanced recombination in the bulk due to a non-favourable morphology of the photoactive layer or if surface recombination reduces the efficiency of the device.
To overcome this problem we applied electroluminescence measurements (EL) and modelling to identify the impact of (electrode) surface recombination that occurs if one (or even both) contacts are not sufficiently selective.
In the experiments cells with the same photoactive layer but with different electrodes are compared with each other. In the electroluminescence experiment the cell is put under forward bias voltage in the dark. The corresponding current is made up of different contributions:
i) electrons injected through the electron contact into the acceptor phase and holes injected through the hole contact into the donor phase recombine non-radiatively in the photoactive layer.
ii) the same as in i) but the recombination being radiative
iii) electrons injected through the electron contact into the acceptor phase or holes injected through the hole contact into the donor phase move through the whole photoactive layer and recombine non-radiatively at the other contact, i.e., surface recombination
In an EL experiment, only the charge carriers in ii) contribute to the luminescence signal which is recorded with a camera.
One advantage of EL is that by adjusting the current it can be made sure that exactly the same number of charges flow through the different solar cells per time. We found that if cells with the same photoactive layer but different top electrodes show different open circuit voltages (Voc) the one with the lowest Voc also shows the smallest EL signal.
The results are discussed with the help of a model which is capable to reproduce the experimental findings and which can even explain why small EL signals are not only related to lower values of Voc but also to lower values of the forward current.

We have investigated the counter intuitive phenomenon of inserting a metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission, x-ray photoemission, and inverse photoemission spectroscopy (UPS, XPS and IPES). We observed that metal oxides, such as MoO3 and WO3, substantially increase the work function when deposited on indium-tin-oxide (ITO). The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer, therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band bending region that results in a drift electric field that encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We have also investigated the effects of ambient or O2 exposure of MoO3. We observed that while most of the electronic energy levels of the oxide remained largely intact, the work function reduction was significant. This opens a way for optimal energy level alignment by modifying the work function through exposure. Furthermore, we observed that the work function reduction by exposure could be reversed by proper annealing of the sample in vacuum. The investigations therefore point to manipulate the interface electronic structure and charge injection/extraction by thin metal oxide films.

Organic photovoltaic cells (OPVCs) based on the heterojunction concept are most promising for realizing low-cost solar cells, and energy conversion efficiencies over 10% were achieved. However, fundamental issues concerning the origin of the open circuit voltage (VOC) remain. Controversial discussions in this context concern the dependence of VOC on the energy offset between the highest occupied molecular orbital (HOMO) level of the donor material and the lowest unoccupied molecular orbital (LUMO) level of the acceptor material. Often electrical device parameters are compared with literature values for the HOMO and LUMO levels of the separate donor and acceptor materials assuming vacuum level alignment at the interface. This assumption has yet to be ascertained, which is particularly challenging for polymer heterojunctions due to difficulties preparing defined interfaces from solution processing. Therefore we used insolubilized polymer films to investigate the energy level alignment in two bi-layer OPVCs comprising the donor poly(3-hexylthiophene) (P3HT) and the acceptors 1-(3-methoxycarbonyl)propyl-1-phenyl[6.6]C61 (PCBM) and poly(9,9'dialklylfluorene-alt- 4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT). Ultraviolet photoelectron spectroscopy (UPS) revealed that notable interface dipoles occur at all interfaces across the OPVC structure for both material combinations, highlighting that vacuum level alignment cannot reliably be used to estimate the electronic properties for device design. Particularly the effective electrode work function (after contact formation with the organic material) differs significantly from those of the pristine materials. Chemical reactions between poly(ethylenedioxythiophene):poly(styrenesulfonate) and P3HT on the one hand, and Ca (Sm) and PFTBTT (PCBM) on the other hand, are identified as cause for the interface dipoles. The vacuum level shift between donor and acceptor is related to mutual energy level pinning at gap states. In addition, we find that equilibrium between the two electrodes can no longer be sustained upon white light illumination during UPS measurements. Whereas the electrostatic potential distribution across the layer stack in the dark is comparable to short circuit conditions in the device; the situation during illumination is comparable to open circuit conditions, because negative charges are collected at the metal clusters (that exist in the early stage of cathode formation) due to exciton dissociation at the heterojunction [1].

[1] J. Frisch, M. Schubert, E. Preis, J. P. Rabe, D. Neher, U. Scherf, and N. Koch, J. Mater. Chem., 2012, 22, 4418

While much effort has been put into the development of new donor materials for organic photovoltaics, less effort has been focused on the development of alternative acceptor materials. Here we present a new promising class of acceptors and show very importantly how the frontier energy levels of these truxenone-based materials can be effectively tuned through chemical modifications.[1] Interestingly, the LUMO can be adjusted through chemical modifications on the truxene core, while the HOMO independently can be adjusted through chemical modifications on the truxenone periphery. The promise of this class of compounds as acceptor materials is illustrated by the fabrication of efficient bilayer solar cells with subphthalocyanine and zinc phthalocyanine donors and bulk-heterojunction solar cells with polymeric donors such as poly(3-hexylthiophene).
[1] C. B. Nielsen, E. Voroshazi, S. Holliday, K. Cnops, B. P. Rand, I. McCulloch, J. Mater. Chem. A, 2013, DOI: 10.1039/c2ta00548d.

One-dimensional semiconductor nanostructures represent an attractive class of nanomaterials for solar cell devices because they provide a direct path for charge transport. Here we demonstrate the feasibility of lateral organic solar cells with laterally patterned back contact electrodes and active layer consisting of self-assembled poly (3-hexylthiophene) (P3HT) nanowires and PCBM blend. With an incorporation of preformed P3HT nanowires, the charge carrier mobility and photocurrent generation were enhanced in the longitudinal direction, which resulted in improved power-conversion efficiencies. Moreover, for a high power-source application, multiple devices connected in parallel and serial were demonstrated by designing electrode patterns. For the application to the power source of flexible optoelectronic device, lateral solar cells on flexible substrates were demonstrated.

Organic photovoltaic (OPV) cells hold great promise as an alternative solar cell technology; however their development is hampered by limitations of materials used in the devices. Continued progress in the area of OPV will require the design and development of new active materials, such as electron acceptors. Nearly all high efficiency OPV devices employ C₆₀ or C₇₀ as the principal electron acceptor. The use of fullerenes for the acceptor severely limits the number of donor materials that can be used in OPVs. There is a pressing need to identify new classes of acceptor materials with tailorable electron affinities that can be used in place of fullerenes.
The process of virtual screening has become the central paradigm for industrial drug discovery. Virtual screening involves the automated generation of structural libraries, the analysis of each structure using an automated simulation workflow and then the filtering of large chemical structure databases to identify lead systems. Transferring this paradigm to challenges in materials science is now possible due to advances in the speed of computational resources and the efficiency and stability of chemical simulation packages. State-of-the-art software tools have been developed to efficiently explore the chemical design space to identify new chemical solutions for problems such as electron acceptors for use in OPV cells. In this presentation, virtual screening for OPV acceptors based on intrinsic quantum mechanical properties is illustrated, and synthetic efforts to realize these new materials is discussed.

A surprisingly facile and unprecedented hextuple Friedel-Crafts carbamylation is demonstrated in the synthesis of a new class of multivalent monomer and electroactive materials -- decacyclene trianhydride and triimides. The decacyclene triimides bear three electron-withdrawing functionalities and are modeled after perylene and naphthalene diimides that have been extensively studied as n-type organic semiconductors. Optical and electrochemical data demonstrate the significant electronic interaction of the three imide groups with the decacyclene core. Owing to the extended π-surface of the triimides, these novel polycyclic hydrocarbons exhibit a propensity to self-assemble and crystalline columns or ultra-long fibers are obtained by solely modulating the alkyl substituents. The self-assembly and the high electron affinities together render the triimides promising for organic semiconducting devices such as field-effect transistors and photovoltaic cells. Initial results in bulk heterojunction solar cells demonstrate power conversion efficiencies comparable to the best non-fullerene acceptors.

The maximum solar-to-electric power conversion efficiency of a conventional solar cell is determined by the Shockley-Queisser limit of ~31%. One viable approach to exceed this limit is to create two or more electron-hole pairs from the absorption of one photon in a process called signlet fission or multiple exciton generation. Singlet fission has attracted renewed interest because of the great potential of designing molecules for optimal fission yields. We illustrate how singlet fission can occur in organic semiconductors due to a many electron quantum coherent process and how to efficiently extract two electrons from the quantum coherence [Science 2011, 334, 1541-1545; Nature Chem. 2012, 4, 840-845.]. Using model organic semiconductor interfaces, we demonstrate the competitive processes of charge and energy transfer from the singlet, the triplets, and the multi-exciton states [J. Am. Chem. Soc. 2012, 134, in press]. These discoveries are allowing us to formulate design principles for the implementation of singlet fission in solar energy conversion.

Interface physics is at the heart of photovoltaics, one of the most important challenges of this century. At the interface photons get absorbed and charge separation takes place. Common knowledge suggests that following absorption, excess energy with respect to the optical gap is dissipated, competing with interfacial charge separation in the first tens of fs. The accepted photophysical scenario suggests that at the donor/acceptor interface both bound interfacial charge transfer states (CTS) and free polarons are generated in <100 fs. However, the actual charge generation mechanism on such a short timescale is still highly debated. Here, we monitor for the first time the exciton dissociation process in an efficient low-band gap polymer: fullerene (PCPDTBT:PCBM) blend combining ultrafast transient absorption spectroscopy with sub-15 fs time resolution with quantum chemical calculations [1]. We find that exciton dissociation leads to both bounded interfacial CTS and free polarons within a time scale of 20-50 fs, with a branching ratio that depends on the excess energy.
In particular, upon pumping the first excited singlet state, the resonant lower-energy CT1 state and polarons are populated. However, in the first 500 fs, the CT1 does not evolve, indicating that it does not contribute to the early-time charge dissociation yield. On the other hand, by exciting the more energetic singlet states, a fast conversion to the hot CTS* manifold opens up a new channel for effective charge dissociation occurring with a time constant of 22 fs. Such CTS* undergo further separation into polarons within 150 fs while partially thermalizing within the CT manifold, but before complete relaxation in the CT1 state. We relate this higher efficiency to the nature of the CTS* involved in the process, since they are more delocalized and preferentially dissociate into free polarons. Thanks to stronger coupling between high energy singlets states and CTS*, we demonstrate the opening of additional paths for charge generation that would otherwise be quenched by internal conversion to the lowest lying states. Our results demonstrate that providing a large amount of excess energy, the higher lying polymer singlet states dissociate before internal conversion, ultimately leading to a higher fraction of polarons. The higher charge generation yield is supported by measuring the external quantum efficiency in a PCPDTBT: PCBM prototypical device. The spectrum, indeed, reveals a higher yield of charge separation in the spectral region corresponding to photon energy above the band-gap, thus suggesting that hot dissociation is a strategic option to enhance the photovoltaic conversion.
[1] G. Grancini, M. Maiuri, D. Fazzi, A. Petrozza, H-J. Egelhaaf, D. Brida, G. Cerullo and G. Lanzani, "Hot Exciton Dissociation in Polymer Solar Cells", Nature Mater., accepted.

Previous attempts to determine the energy level alignments in polymer BHJ systems preliminary relied on measurements of individual pristine materials by electrochemical methods. The effective bandgap (Eeff), ED-HOMO-EA-LUMO, is typically determined by the values of the HOMO energy level of the donor and the LUMO energy level of the acceptor measured separately. Therefore, the impact of polymer-fullerene interactions has been ignored which makes it difficult to directly correlate the photovoltaic characteristics with the actual energy level alignment in BHJ PV cells.
Using charge modulated electroabsorption spectroscopy (CMEAS), we directly determine the energy level alignment in a polymer:fullerene bulk-hetrojunction photovoltaic cell. Upon photo-excitation at energies below the bandgap, organic molecules are excited to the higher energy manifolds of the charge transfer (CT) states through which exciton dissociation occurs subsequently at the polymer-fullerene interface. Charges from these dissociated excitons are able to couple with the modulation electric field and introduce subtle changes in optical absorption in the sub-bandgap region. The minimum required energy for such photo-induced charge transfer is defined as the effective bandgap. In addition, based on the CMEAS results, we directly prove that photo-generated excitons can be dissociated without a positive lowest unoccupied molecular orbital (LUMO) offset in a polymer:fullerene bulk heterojunction cell.

The efficient conversion of photons into free charge carriers in blends of conjugated polymers and fullerene derivatives is promising for the development of bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of BHJ solar cells has significantly improved from 2.5% to more than 9.5% over the last decade. This enhancement can be partly attributed to the use of low band-gap conjugated polymers to extend the spectral absorption into the NIR region and to the advance of the nanomorphology of the BHJ to improve the charge generation and transport. Further enhancement in the PCE can be achieved by minimizing geminate and bimolecular (or non-geminate) recombination of charge carriers.
In this work bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative (PC71BM) is studied by two complementary techniques. TRMC (time resolved microwave conductance) monitors the mobility and decay of photo-generated mobile charge carriers locally on a timescale of nanoseconds, while using photo-CELIV (charge extraction by linearly increasing voltage) the same parameters are obtained on a macroscopic scale and on tens of microseconds. Interestingly, despite these significant differences in the length and time scales, both techniques show a reduced bimolecular (non-Langevin) recombination with a prefactor zeta; close to 0.05. The prefactor zeta; can be described by the ratio between the rate constants for recombination of an electron hole encounter complex back to the ground state (kr) and for dissociation forming mobile charges again (kd). Most importantly by combining the results from both techniques we deduce that for TQ1:PC71BM blends the zeta; value is independent of temperature. On comparing TRMC data with electroluminescence measurements it can be concluded that the encounter and the CT complex have very similar energetic properties. Furthermore, the zeta; value for annealed P3HT:PC61BM is approximately 10-4 i.e. two orders smaller than found for TQ1:PC71BM. This large difference may be attributed to the extent of charge delocalization of opposite charges in an encounter complex. The enhanced delocalization for holes in annealed crystalline P3HT as compared to TQ1 is expected to reduce the binding energy between the charges, enhancing the possibility for a complex to dissociate. Therefore control of the nanomorphology of a blend layer, in order to enhance the organization of the conjugated polymer along with the fullerene domains seems like a prerequisite to achieve reduced bimolecular recombination.

Interfaces between electron donating (D) and electron accepting (A) materials have the ability to generate free charge carriers upon illumination. In order to use such interfaces for the construction of efficient organic solar cells, a high yield for this process, combined with a minimum of energy losses is required. In this work we use electroluminescence (EL) experiments on fully operating devices to identify the lowest energy emissive excited state, which is a vibrationally relaxed interfacial charge transfer state (CT0). In order to investigate the role of CT0 in the free charge carrier generation process, we compare the quantum yields for selective excitation of CT0 and the higher energy excited states. We show that such a comparison is possible by performing a detailed analysis of EL and external quantum efficiency spectra, measured by sensitive techniques in the low energy spectral region. For a series of polymer:fullerene, small molecule:C60 and polymer:polymer based photovoltaic devices with varying photovoltaic performance, we find a quantum yield which is quite independent on whether or not states with higher energy than CT0 are excited. This indicates the majority of excitations under solar irradiation decays to CT0 prior to the formation of fully separated free charge carriers. The results imply that excess energy is not needed to obtain a high yield of free charge carriers from interfacial CT states.

The charge transfer state (CTS) is an intermediate excited state at the interface between two intermixed materials, in general a polymer (donor) and a fullerene derivative (acceptor). In this state the exciton couple, generated after photo excitation, is weakly coulombically bounded: the electron has been already promoted to the next acceptor molecule but is still correlated to the parent hole in the donor.
The CTS can be depicted as an intermediate step between the photogenerated excitons and the generation of free charges. Therefore a better understanding of the CTS dynamics in polymer-fullerene blends is fundamental due their relevance in the organic solar cell working mechanism [1].
Evidences of the presence of CTS in polymer-polymer blends have been provided combining optical absorption, steady-state photoluminescence (PL) spectroscopy and time resolved measurements [2-4].
However, to fully understand the nature of this state its spatial localization in the polymer-fullerene blends has to be studied.
In this work, by coupling a confocal laser scanning microscopy (CLSM) to spatially resolved photoluminescence signals, we select and excite different regions of the sample, correlating local sample composition to the photoluminescence spectrum. In this way we are able to probe spectroscopically the presence of the CTS, providing at the same time, spatial map of the CTS in the blend. Our experiments point out that the CTSs are mainly localized in the regions where the two materials are more intimately mixed.
[1] Markus C. Scharber, Christoph Lungenschmied, Hans-Joachim Egelhaaf, Gebhard Matt, Mateusz Bednorz, Thomas Fromherz, Jia Gao, Dorota Jarzab and Maria A. Loi ”Charge transfer excitons in low band gap polymer based solar cells and the role of processing additives” Energy Environmental Science 4, 5077 (2011)]. [2] Arne C. Morteani, Richard H. Friend and Carlos Silva “Exciton trapping at heterojunctions in polymer blends” The Journal of Chemical Physics 122, 244906-244906-7 (2005). [3] Maria A. Loi, Stefano Toffanin, Michele Muccini, Michael Forster, Ulrich Scherf, and Markus C. Scharber “Charge Transfer Excitons in Bulk Heterojunctions of a Polyfluorene Copolymer and a Fullerene Derivative” Advanced Functional Materials 17, 2111-2116 (2007). [4] Claudia Piliego e Maria A. Loi “Charge transfer state in highly efficient polymer-fullerene bulk heterojunction solar cells” Journal of Materials Chemistry 22, 4141 (2012).

Impedance spectroscopy is an effective, nondestructive tool for evaluating organic photovoltaics. Many parameters of interest can be extracted including shunt and series resistance, carrier concentration, effective carrier lifetime, and mobility. These parameters are heavily influenced by processing conditions and device structure. We report on the use of impedance spectroscopy to quantify the effect of processing on these important device metrics. Interestingly, we find that faster carrier (electron) mobility can vary over the range of 2 x 10^-3 to 1 x 10^-2 cm²/Vs by changing the spinning recipe. Recombination is heavily influenced by contacts, and in fact the order of recombination can be varied from 1.1 to 1.8 by changing the backside cathode from a more electron selective to a less selective material. Impedance Spectroscopy allows us to measure the effective recombination lifetime near the maximum power point under illumination. We find effective lifetimes of 1 x 10^-4 s for 3% efficiency cells vs 3 x10 ^-6 s for a 1.8% efficiency cell, highlighting the importance on the suppression of recombination for increased efficiency. Devices made via a slowly dried films exhibit repressed recombination compared to quickly dried films. The active layer capacitance is strongly dependent on device processing, with lower efficiency devices displaying a decreasing capacitance in moderate forward bias. This effect is reduced in higher efficiency cells, suggesting that density of states and trap profiles can be optimized through the fabrication recipe.
Measurements are taken across a bias range of -1 to 1 volt with a 25 mV ac signal, and with illumination intensities spanning .001 to 3 suns, in order to test under conditions which are most relevant to real device operation. Impedance spectra are analyzed through the use of a 5 element compact model based upon the work of Bisquert et al.[1,2] We report an array of device metrics measured via impedance spectroscopy including shunt resistance, effective carrier lifetime, mobility, and capacitance for P3HT:PCBM devices with efficiencies of 3.5% to <1%, fabricated via several common recipes, in an effort to elucidate the varied and complex interplay between processing and device physics, and the overall effect on solar cell efficiency.
[1] Fabregat-Santaigo, F., Garcia-Belmonte, G., Mora-Sero, I., and Bisquert, J. Phys. Chem. Chem. Phys., 2011, 13, 9083-9118
[2]Garcia-Belmonte, G.,Boix, P.P., Bisquert, J., Sessolo, M., and Bolink, H.J. Solar Energy Materials & Solar Cells 94(2010)366-375

Carbon allotropes possess unique and interesting physical, chemical, and electronic properties that make them attractive for next-generation electronic devices and solar cells. Here, we describe our efforts into the fabrication of the first reported all-carbon solar cell in which all components (the anode, active layer, and cathode) are carbon based. We have optimized the active layer in these devices, which is composed of a bilayer of regioregular poly(3-dodecylthiophene) sorted semiconducting single-walled carbon nanotubes and fullerenes. The optimized devices with an indium tin oxide anode and metallic cathode has a maximum power conversion efficiency of 0.46% under AM1.5 Sun illumination. We have also evaluated the use of carbon based electrodes using a graphene based anode and an n-type doped conducting carbon nanotube cathode. Using these carbon based electrodes along with the bilayer active layer, we have demonstrated an all-carbon solar cell. Finally, we discuss the challenges facing the development of efficient all-carbon solar cells.

The development of high performance electrodes is of great importance for efficient, low-cost organic optoelectronic devices such as organic solar cells and organic light-emitting diodes (OLEDs). Doped ZnO thin films are regarded as promising alternative electrodes to replace conventional indium tin oxide (ITO) electrode due to their high conductivity and transmittance, low-cost, non-toxicity, and ease of doping. Although their performance has already reached that of ITO, applications in organic devices are not as efficient as other alternative electrodes due to their chemical instability and low work function. Organic devices with ZnO electrodes typically suffer from limited hole injection/extraction due to the low work function of ZnO which results in an energy barrier formed at the interface of the ZnO and hole transport layers. Furthermore, ZnO films deteriorate significantly, if they are coated with acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), typically used as a hole transport layer in organic devices.
In this work, we demonstrate high performance ITO-free small molecule organic solar cells and OLEDs based on optimized ZnO electrodes with alternative non-metallic co-dopants. By co-doping of hydrogen (H) and fluorine (F), ZnO thin films show greatly improved optical properties in terms of lower absorption loss, higher transmittance, and a lower refractive index compared to the conventional ITO electrode. The transmittance of the optimized ZnO and ITO is 83.3 and 87.2 %, respectively, in the visible range. The average absorption coefficient of the optimized ZnO film is 703 cm^-1 which is much lower than that of ITO (1724 cm^-1). In addition, the lower refractive index of the optimized ZnO film (n~1.8, at 500 nm) compared to ITO (n~1.94, at 500 nm) is expected to reduce the total internal reflection at the interfaces of electrode in organic devices.
The application of optimized ZnO in organic solar cells and OLEDs yields significantly improved efficiencies of devices in comparison to ITO-based devices. The efficiency of organic solar cells (4.4 %) is greatly improved by a factor of 1.2 and the external quantum efficiency of OLEDs (12.5 %) is improved by a factor of 1.37 compared to ITO-based devices. The enhancement factor is among the highest values reported on ITO-free organic solar cells. The high transparency and the reduced total internal reflection at the interfaces of electrodes significantly improve light in- and out-couplings of organic solar cells and OLEDs, respectively. Furthermore, the molecular doping performed in this work strongly reduces energy barriers at the interfaces of electrodes and organic layers, which allows to avoid the degradation under the acidic PEDOT:PSS layer. We believe that optimized ZnO films with non-metallic co-dopants are a promising electrode for low-cost and high performance organic solar cells and OLEDs.

The traditional materials for creating transparent conducting films consist of transparent, conductive metal oxides (TCOs). Despite having high electrical conductivity and low optical absorption, TCOs are limited by expensive instrumentation for processing, native defects, diminishing resources, and their brittle nature makes them unsuitable for applications demanding mechanical flexibility. This contribution will discuss an alternative route towards flexible transparent conductive materials based on 1-dimensional randomly networked structures. Research on film coatings from networks of silver nanowires is fast gaining ground, as they are amenable to both easy processing and scaling.
We report here on our studies on the transmittance and electrical conductivity of silver nanowire network structures. Ag nanowire films that were spun-cast from solution showed bulk-like electrical conductivity (~2-50 Omega;/sq) while being highly transparent (~70-90%). In networked structures such as these and also of materials like carbon nanotubes, however, percolation effects limit the simultaneous increase in conductivity as well as transmittance, i.e. a reduction in the thickness/density of the films/conducting units (nanowires or nanotubes), causes a dramatic increase in the sheet resistance, effecting a marked deviation from bulk-like behavior. Meanwhile, at higher nanowire concentrations aggregation effects hamper film reproducibility and attainable transmittance. The presence of a thin native poly-vinylpyrrolidine (PVP) coating from solution synthesis also limits conduction by impeding the effective contact between nanowires. Through our study we intend to propose a method to reduce the percolation threshold significantly, by dispersing the Ag nanowires in a conducting medium, like that of a conducting polymer like PEDOT:PSS. The role of the polymer would be to stabilize and uniformly disperse the nanowires in the matrix as well as present an alternate path for conduction as the percolation limit for the network is reached. Preliminary results of spun-cast films of composites of these nanowire networks with PEDOT:PSS show similar transmittances and conductivities (~10-120 Omega;/sq) at significantly lower Ag nanowire network densities, indicative of a lower percolation threshold. Through analysis of microstructural characteristics of these films, a quantitative correlation (using accepted models) of variation in density of nanowires to improvements in conductivity at little expense of transmittance will be presented. The advantages of using such a composite structure in reducing the percolation threshold (especially in applications like organic photovoltaics where such composites can function both as a transparent conductor and a hole transport layer, owing to the use of PEDOT:PSS as the dispersing medium) will also be discussed. Additionally, investigations of surface treatments of the nanowires as a means to improve nanowire-nanowire contact will also be reported.

Efficiency enhancement in plasmonic bulk heterojunction (PCDTBT:PCBM) organic solar cells (OSCs) is demonstrated with the integration of large area periodic Ag nano-triangle (NT) arrays (that were fabricated using the cost-effective, high throughput nanosphere lithography technique) in the OSC device. The improvements to the power conversion efficiency (from 4.24% to 4.52%) and to the short circuit current density (by ~12%) are attributed to an increase in exciton generation induced by the strong local E-field and the scattering generated by the localized surface plasmon resonance of the hexagonal NT arrays. These findings are validated by a range of steady state and transient optical spectroscopy and correlated with device performance data. Importantly, our work demonstrates the feasibility of integrating a simple cost-effective, tailorable and scalable nanofabrication technique with existing OSC fabrication processes.

The generation of free charge carriers and their extraction to the electrodes are key processes in organic solar cells. There is an ongoing debate on how the electric field across the active layer of such devices affects the efficiency of these processes in competition to geminate and non-geminate recombination.
We have used time-delayed extraction experiments to quantify generation and extraction of charge in various polymer-based solar cells. We find that for P3HT:PCBM, the efficiency for free carrier generation is not affected by the electric field in both poorly-performing as-prepared blends as well as in efficient annealed blends, meaning that the solar cell properties of this material combination are entirely determined by the field-driven sweep-out of carriers in competition with non-geminate recombination [1]. Similar conclusions are drawn for polymer-polymer blends based on P3HT with N2200. These blends were shown to exhibit exceptionally high fill factors [2]. In contrast, blends of the low bandgap polymer PCPDTBT with PCBM exhibit a pronounced field-dependence of charge generation, suggesting efficient geminate recombination [3]. We show that the efficiency of geminate and non-geminate recombination in these blends is correlated to the blend morphology, and that both decay channels are strongly reduced in blends with extensive interchain order [4].
[1] J. Kniepert, M. Schubert, J.C. Blakesley, D. Neher, Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments, J. Phys. Chem. Lett. 2011, 2, 700.
[2] M. Schubert, D. Dolfen, J. Frisch, S. Roland, R. Steyrleuthner, B. Stiller, Z. Chen, U. Scherf, N. Koch, A. Facchetti, D. Neher, Influence of aggregation on the performance of all-polymer solar cells containing low-bandgap naphthalenediimide-copolymers, Adv. Energy Mater. 2012, 2, 369.
[3] S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J.C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf., D. Neher, On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: influence of solvent additives, J. Phys. Chem. Lett. 2012, 3, 640.
[4] S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kuipiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, D. Neher, Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells, J. Am. Chem. Soc. 2012, 134, 14932.

Fullerenes are well-known to undergo Diels-Alder reactions with dienes and we have made use of this reaction to position electron releasing groups such that they interact with the fullerene electronic system. As is typical in fullerene chemistry, multiple addition reactions are possible and we find that the electron affinity decreases as measured by cyclic voltammetry. Specially, for the range of substituents we observe shifts in the reduction potential of the mono-addition product to 110-130 mV more negative of C60 with the first addition and 280-350 mV for the double-addition product. The electron affinities are lower than the well-known fullerene, PCBM, which is used in solar cells. The offset in the electron affinity is shown to produce larger open circuit voltages in solar cells with poly(3-hexyl)thiophene.

Dye-sensitized solar cell (DSSC) has been regarded as one of the promising candidates for the third generation photovoltaic in virtue of their low manufacture cost and impressive conversion efficiency. At present, DSSCs based on the ruthenium sensitizers have achieved over 11 % conversion efficiency and the heat stability of DSSC was also demonstrated. However, to become a commercialize product, lots of problems need to be solved. In this talk, I will present the development of some key components for DSSC in our laboratory. Our goal focuses on increasing the conversion efficiency through designed ruthenium based dyes and reduced the fabrication cost via changing the components of counter electrode and fabrication process. In this talk I will also present the work of new founded laboratory (supported by National Science Council, Taiwan, ROC.) which focuses on the research and development of Organic Photovoltaic specially in DSSCs and polymer solar cells.

We have succeeded in developing a new small molecule as donor material of organic photovolataic cells (OPVs), which have high power conversion efficiency (PCE) of 6.5% with single cell strucutre and that of 8.2% with tandem cell strucuture, respectively.
For improving a photo-conversion efficiency, the process of light absorption by the donor material plays an important role. The transition dipole of the molecule in thin film is crucial for the absorbance of light. Therefore, we have developed a new small molecule which has high absorbance in thin film by lying-down orientation with respect to the substrate.
At first the anisotropy of the vacuum deposited films were detected using variable angle spectroscopic ellipsometry (VASE). The orientation order parameters which have been obtained by the VASE suggested that the molecules have lying-down orientation with respect to the substrate. As the results of the horizontal molecular orientation, the light absorption in thin film was stronger than that when the molecules have no specific orientations.
Next, OPVs were fabricated by the vacuum deposition with a three layer p-i-n structure. The PCE was increased from 4.4% to 6.5% by increasing the substrate temperature from RT to 50 degrees centigrade, respectively.
Small molecules based OPVs typically use a simple thermal evaporation approach for fabrication, therefore the realization of tandem structures is relatively easier than that by all solution processes of polymer tandem cells. The tandem OPVs of conventional structure were comprised of small molecules and low band gap polymers (Sumitomo Chemical Company, Limited) sub-cells which have different absorption range. The intermediate layers consisted of doped organic semiconducting materials and a thin metal.
Finally, we also have tried to fabricate tandem OPVs with inverted structure using AZO as a cathode buffer layer and doping techniques for the intermediate layers. The tandem cell achieved a Voc of 1.49 V, a Jsc of 7.67 mAcm-2 and a FF of 71%, resulting in a PCE of 8.2%. The efficiency of 8.2% is the highest value that has ever reported for organic tandem devices based on both small molecules and conjugated polymers.
This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). We would like to thank Sumitomo Chemical Company, Limited for providing us low band gap polymers.

Fullerenes have been the dominant acceptors in bulk-heterojunction organic photovoltaic (OPV) devices for almost two decades. In contrast, very few n-type small molecules or polymers giving comparable performance have been reported in the literature. Here we report a new family of small molecule n-type semiconductors which can be used as drop-in replacements for fullerenes. To date, over 20 examples of this family of semiconductors have been prepared. The ease of synthesis and purification of this family of compounds is exemplified by the scale up of the initial compound, F8IDT, to 20 gram quantities. Solution processed BHJ OPVs prepared using these compounds as the acceptor and P3HT as the donor give power conversion efficiencies in excess of 2% with open circuit voltages approaching 1V. This presentation reports on the synthesis, purification, electronic and optical properties and device performance of this new family of n-type semiconductors.

Photovoltaic performance of hybrid solar cells can be improved by introducing single-walled carbon nanotubes (SWCNTs) as an electron donor; SWCNTs increase the hole mobility by providing ballistic pathways for the hole transport1. In order to show those positive characteristics, SWCNTs should be semiconducting and well dispersed. However, as-synthesized SWCNTs are bundled due to the strong van der Waals forces. Hence, achieving well dispersed suspension of SWCNTs in water or organic solvents without physical or chemical modification of pristine SWCNTs that may affect their properties is challenging but important for various applications.
The current study describes the optimization of concentration of surfactants (SDBS: Sodium dodecyl benzenesulphonate and DOC: Sodium deoxycholate) to achieve the best dispersion of CoMoCAT-SWCNTs (7, 6) in aqueous medium for photovoltaic applications. By using UV-vis-NIR, SEM and AFM, the dispersion was characterized to quantify optimum dispersion condition. UV-vis-NIR absorption spectra of solution containing dispersed SWCNT show well-resolved absorption peaks corresponding to semiconducting SWCNT (Es11, Es22 and Es33 electronic transition) and metallic SWCNT (Em11). By analyzing the peak corresponding to Es11 electronic transition of semiconducting nanotubes, the resonant ratio (peak height to peak width) was calculated. It was found that the optimum surfactant concentration to achieve best dispersion for 0.25 mg/ml of SWCNTs is 9-10 mg/ml (SDBS) and 8-9 mg/ml (DOC). SWCNTs on glass and silica were prepared using SWCNTs-dispersed solutions with optimal concentration of surfactant being characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). According to the AFM analysis, the diameter of the SWCNTs lies between 0.8 and 1.2 nm. This is in good agreement with the chirality of the nanotubes (7, 6). Moreover, AFM images confirm well-dispersed nanotubes along with some amount of surfactant particles. The presence of surfactants implies that some nanotubes may be covered with surfactants, which may be detrimental to solar cell performance in terms of the hole transport limiting. The effect of surfactant on the SWCNT-P3HT junction is studied by using Kelvin probe force microscopy (KPFM).
The advantage of using organic solvents (1,2-Dichloroethane, Dichlorobenzene, Toluene) over surfactant-aided dispersion in aqueous medium is discussed. It is found that organic solvents provide pure individual SWCNTs3 after drying, which may possibly improve the hole transport and device performance.
References
1. Liu, L.; Li, G. Appl. Phys. Lett. 96, 083302 (2010).
2. Mallajosyula, A.T.; Iyer, S. S. K.; Mazhari, B. J. Appl. Phys. 108, 094902 (2010).
3. Cheng, Q.; Debnath, S.; Byrne, H. J. phys. stat. sol.(b) 245, No. 10, 1947-1950 (2008).
4. Blanch, A. J.; Lenehan, C. E.; Quinton, J. S. J. Phys. Chem. B 114, 9805-9811(2010).

Organic photovoltaics (OPVs) have gained widespread attention as viable alternatives to more traditional silicon-based solar cells because they have the potential to be cheaper and lighter-weight. One significant drawback limiting the efficiency of OPVs, however, is their inability to absorb all of the photons available in the solar spectrum. We are focusing on the development of dyes that exhibit strong absorption maxima in the visible region of the solar spectrum and serve as favorable energetic complements to the commonly-used C60 acceptor. One class of dyes that meet these criteria are boron-dipyrromethenes (BODIPYs). In addition to having strong absorption maxima in the visible region, the photophysical properties of BODIPY dyes are also very susceptible to synthetic modification, allowing for tuning of their physical properties. Such synthetic modifications have yielded a variety of interesting donor candidates suitable for use in organic devices. The device characteristics are being examined for Lamellar OPVs using these materials as donors.

In organic photovoltaic heterostructures, an electrochemical potential difference, approximated by the offset between the lowest unoccupied molecular energy (LUMO) levels of the donor and acceptor materials, is necessary to drive charge separation of the photogenerated exciton. However, the offset present in typical devices greatly exceeds the energy required to fully separate charges leading to a large overpotential and significant reduction of device inefficiency. In order to quantify the effect of electrochemical potential difference on the exciton dissociation rate, we present results of photophysical measurements of a low bandgap copolymer mixed with a series of fullerene based acceptor materials in a bulk heterojunction geometry. By systematically tuning the absolute energy levels of the polymeric donor and molecular acceptor, as well as the relative offset between them, we are able to determine the smallest necessary voltage loss required to sustain a high short circuit current. We have found that a molecular donor-acceptor framework (Marcus theory) provides a quantitative description of the charge transfer rates in our system and describes the scaling of the photognerated current with LUMO-LUMO offset between the donors and acceptors. We conclude that there are considerable efficiency gains to be had in high performing polymer devices by recovery of up to 200 mV of additional voltage without significant modification of the current level.

Geminate recombination of positive and negative charge carriers was observed in THF. Pulse radiolysis method facilitated rapidly creation of solvated protons (RH2+) and anions (Mbull;-) in THF. Annihilation of anion within THF is due to proton transfer (PT) reaction of ion pair, (Mbull;-,RH2+). The data indicate the two-steps nature of the PT reaction: consisting of diffusion to bring the Mbull;- and RH2+ together under influence of the Coulomb potential followed by PT within the ion pairs. With large driving force (ΔG°=-0.3 to -1.6 eV) for PT reaction, some of ion pairs react rapidly (kMPT > 109 s-1) but others react more slowly. For most of 17 molecules studied the amplitude of the long-lived homogenous (free) ion yield is constant at 0.356±0.03 of the total regardless of kMPT, indicating no geminate ion escaping to become free ions. For anions of oligo(9,9-dihexyl)fluorenes, Fnbull;- (n=2-4), do some escape to become free ions. The observations of free ion yield illustrate principles of escape of Coulomb-bound ion pairs that the more delocalized charge distribution results greater free ion yield.

Solution processed small molecule bulk heterojunction solar cells (SMBHSCs) with power conversion efficiencies (PCE) exceeding 7% have recently been reported. This achievement demonstrates that SMBHSCs fabricated from blends of small molecule donors and fullerene acceptors are a viable alternative to polymer:fullerene based systems. Advantages of using small molecules as donors include the ease of synthesis and purification. Moreover, in contrast to polymers, conjugated small molecules do not suffer from broad molecular weight distributions or batch to batch variations. However, to date there have been few fundamental investigations into the nature of recombination and charge transport processes in SMBHSCs and how these relate to the nanoscale morphology. A deeper understanding of the processes that limit even the most efficient SMBHSCs may be essential to continuing the advancement of PCE.
Recombination losses and charge transport in polymer:fullerene based solar cells has been the subject of much research. There is evidence that field-dependent exciton dissociation (geminate recombination) and both bimolecular and trap-assisted (nongeminate recombination) mechanisms may all limit PCE depending on materials and device processing conditions. Initial studies with the first generation of SMBHSCs suggested that both geminate and nongeminate recombination processes significantly reduce the efficiency of small molecule based systems as well. It remains to be seen if the new class of high efficiency SMBHSCs systems are limited by the same mechanisms. Furthermore, the relation between nano-scale morphology and recombination in SMBHSCs has never been thoroughly investigated.
In this work, we report on the use of time delayed collection field measurements to probe the voltage dependent recombination mechanisms of state of the art SMBHSCs with PCEs up to 7%. By directly studying the influence of applied bias both during and after photogeneration, we are able to determine the nature of voltage dependent losses in solar cell devices. We observe that both geminate and nongeminate losses can be mitigated by choice of device processing conditions. Coupling these insights with a detailed investigation of the morphology using transmission electron microscopy and x-ray scattering techniques, provides a clear picture of the relation between recombination, transport, and nanoscale morphology. These results demonstrate that precise control of the crystalline domains within the active layer is essential to maximizing generation efficiency and enabling the sweep out of photogenerated charges before they are lost to recombination.

Solid-state dye-sensitized solar cells are a promising device concept for low-cost photovoltaics. However, their performance is limited by the electronic properties of the hole-transport material, typically an organic semiconductor. Although vastly improved performances have been measured using ionic “additives” in the organic semiconductor, the role of the additives in the device remains unclear. In this work we study the transport and interfacial charge dynamics of the device to elucidate the function of the additives on these processes. We show that the efficacy of ionic additives lies in their ability to dope the hole-transporter and also tune the surface potential of the electron-transporting metal oxide. Finally, we use the insight gained from this study to improve device performance as well as device stability under stressed solar aging conditions.

Recently organic solar cells are the new renewable source of energy which has infinite potentials. Their advantages include flexible device and low temperature process and are cost effective. Currently many researches have been done on bulk heterojunction structures based on blends of fullerene acceptor and polymer donor and the system has reported the highest efficiencies. However in the fullerene and polymer blend, due to comparatively low charge carrier mobility and short exciton diffusion length, the solar energy conversion efficiencies of the devices are limited. Current density is increased through the scattering effect that depends on the roughness of the optically active layer which can be tailored by annealing temperature or using metal oxide. We report an improved organic solar cell performance obtained by focusing on the effects of metal oxide nanoparticle concentration in the buffer layer. PEDOT:PSS (Poly(3,4 ethylenedioxy-thiophene) poly(styrenesulfonate)) used as a buffer layer was mixed with Fe2O3 (iron oxide) nanoparticles (NPs). The optical and electrical properties of the device were analyzed as a function of Fe2O3 nanoparticle concentrations in the PEDOT:PSS buffer layer. We studied the effects of Fe2O3 NPs (50 nm) by changing its concentration from 0.01wt% to 1.0wt%. Increasing concentrations of NPs showed good light absorption ability. The solar cells with a bare active layer showed a power conversion efficiency (PCE) of 2.21 % while with Fe2O3 NPs at 0.5wt% concentration, showed higher performance resulting in a PCE of 3.13 % under AM 1.5G illumination. The photovoltaic parameters like fill factor and open circuit voltage increased significantly with an overall decrease in series resistance when the Fe2O3 NPs concentration increased from 0.01 to 1.0wt%.

Polymer thin films are usually fabricated using solvent-based thin-film deposition technologies (e.g., spin coating, drop casting). Depending on the specifics of the polymer blend and processing conditions, different morphologies are typically formed in the thin film. It is of paramount importance to understand how phase separation is initiated and how it evolves to form the final morphology. Such an understanding will enable rational design of thin films with desired structure and thus tailored properties.
We investigate the competing effect of kinetics and thermodynamics on morphology evolution in a typical thin film organic system using an experimentally validated computational framework. A high-throughput computational analysis by varying processing parameters (evaporation rates, blend ratio) is performed to observe morphology evolution. We identify four mechanisms through which the morphology evolves and phase-separation is initiated -- (i) from the top surface, (ii) homogeneously along the thickness, (iii) from the bottom surface, and (iv) initiation from middle zone. Each of these mechanisms have been reported in earlier experimental studies. We unravel the origin of this behavior by identifying processing conditions necessary for each mechanism to operate. Specifically, we utilize a powerful mathematical technique called linear stability analysis to identify which mechanism of phase-separation is chosen for a given processing condition. This allows us to construct a phase-diagram of the processing condition phase-space. We further elucidate the spatial and temporal heterogeneity of these conditions and the interplay between the kinetics and thermodynamics for these four mechanisms. Finally, we utilize this analysis on a fruit-fly system of F8BT:PFB:xylene and methodically investigate phase space of evaporation rate and blend ratio.

The ability of metal oxides to selectively collect and block charge makes them ideal candidates for recombination layers in organic tandem solar cells. The tunnel junction layer, which electrically connects cells in the multijunction stack, fulfills multiple design parameters: (1) optical transparency, (2) “metallic” electrical contact of the quasi-Fermi levels of one cell to the next, and (3) efficient charge recombination of cell current to prevent voltage and power loss. Leakage between the mechanically connected front and back cells (i.e. inefficient recombination at the tunnel junction) results in a reduction in potential, and hence, power conversion efficiency. This study utilizes doped metal oxides to tune the work function of tunnel junction layers for charge selectivity and efficient recombination at the interlayer, toward optimized multijunction organic photovoltaics. We demonstrate that interface dipole modifiers can be used at the interface of the recombination layer to modulate interfacial band alignment for optimized recombination and act as a passivation layer for reduced light soaking when using metal oxides. Tunnel junctions are demonstrated in test device structures and full multijunction device structures with PCDTBT:PC70BM BHJ active layer.

In recent years, organic photovoltaics (OPVs) have enjoyed considerable attention due to the promise of lightweight, low materials cost energy harvesting with flexible form factors, and potential for solution-based roll-to-roll formulation making them particularly attractive for large scale production. While functional devices are typically less than 400 nm thick, OPVs are subject to multiple degradation pathways when exposed to ambient air, necessitating thick encapsulation layers to mitigate atmosphere diffusion. Consequently, commercially available, free-standing OPV panels feature thick epoxy encapsulating layers, bringing the total panel thickness to several hundred microns. Furthermore, the epoxy encapsulating layers and various other protective layers far out-weigh the actual device, comprising 95 % of the total device weight.
In an effort to reduce extraneous weight, next-generation device architectures focus on “painting” multi-layered OPV devices directly onto desired surfaces. Such an approach promises the versatility to directly deposit OPVs onto both optically transparent and opaque surfaces, eventually leading to solar cells that are structurally incorporated onto surfaces rather than laminated as panels thereby eliminating the need for multiple epoxy layers, free standing cell support, and backsheets. However, key to this application is appropriate selection of a transparent electrode and encapsulating layer. To date, most OPV studies have utilized ITO electrodes as the transparent electrode, which is convenient for lab-scale fabrication, but rather impractical when intended for larger scale production. Instead, metallic grids such as those of silver have been used at flexible, transparent electrodes in large OPV panels.
This work explores the substitution of a modified, highly conductive grade of PEDOT:PSS (PH1000) as the top transparent electrode. Although PH1000 has been employed for complex, multi-layered OPV devices, our simple device architecture (metal electrode, P3HT:PCBM, PH1000) allows direct exploration of the effects of substituting PH1000 for ITO without convoluting its effects with multiple interfacial regions and blended layers. A snapshot of device parameters is presented where attained fill factors and efficiencies are 59 % and 2.2 % respectively, indicating that employing PH1000 provides comparable device performance to ITO-based OPVs. Furthermore, we demonstrate this method as a truly substrate-insensitive approach to device manufacturing by directly fabricating OPVs on substrates with varying degrees of hydrophobicity and surface roughness.
(1) Kippelen, B., et al. Organic Electronics, 12, 827-831 (2011)

Small molecule absorbers have recently been shown to perform comparably high, relative to state-of-the-art polymers, in solution-processed bulk heterojunction solar cells. Small molecules can be more easily synthesized and offer monodispersity, which make them more attractive for large-scale production of solution processed solar cells. We examined a variety of interlayers between the hole-collecting electrode (indium tin oxide, ITO) and the bulk heterojunction layer of 7,7prime;-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-bprime;]dithiophene-2,6-diyl)bis(6-fluoro-4-(5prime;-hexyl-[2,2prime;-bithiophen]-5-yl)benzo[c][1,2,5] thiadiazole), p-DTS(FBTTh2)2, and [6,6]-phenyl C71 butyric acid methyl-ester, PC71BM. We compare differences in efficiency between devices with MoO3, NiO, and poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PEDT:PSS, interlayers, both with and without surface modifications as well as the differences in energetic alignment between adjacent layers using photoelectron spectroscopy.

Thermophotovoltaic (TPV) devices convert radiative heat, at wavelengths that are below the bandgap wavelength of a PV diode, into electricity. In this work, we experimentally investigated a one-dimensional photonic crystal (1DPhC) as a high-temperature spectrally selective emitter paired with a GaSb (0.72 eV) PV. The multilayer Si/SiO2 structure of the 1DPhC improves the spectral matching between the emissivity of the emitter and the quantum efficiency of the PV. In a planar TPV layout, we studied the voltage-current (IV) characteristics of the PV diode, including photocurrent and maximum power generated, as a function of the emitter temperature (up to 1200 K) and the gap between the emitter and the PV. Theoretical predictions match the results within experimental uncertainty. In addition, we demonstrated TPV (thermal-to-electrical) efficiencies of 3.4 % at 1200 K and predict, based on detailed numerical models, peak efficiencies as high as 10 % for this 1DPhC/GaSb configuration at higher temperatures. Although un-optimized, these efficiencies represent more than a two-fold improvement over a blackbody/GaSb TPV system without any spectral control. This work experimentally demonstrates the importance of spectral control in thermophotovoltaic energy conversion.

Using novel polymer (polythienothiophene-co-benzodithiophenes 7 F-20) as donor and phenyl-C71-butyric acid methyl ester as acceptor of bulk heterojunction, inverted organic photovoltaics (OPV) were fabricated. Wet-chemically prepared ZnO and poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) were used as buffer layers. Particularly, for PEDOT:PSS deposition, no annealing step was employed. This inverted OPV showed a power conversion efficiency (PCE) of ~7.0 %, which is comparable to the hitherto reported highest efficiency of the inverted OPV with vacuum-deposited MoO3 as hole-collecting buffer layers without plasmonic enhancements. Incorporation of Au nanoparticles in PEDOT:PSS was used for plasmonic enhancement of the electromagnetic field, whereas ZnO and TiO2 thin layers were deposited on ZnO using atomic layer deposition for quenching electron-hole recombination at surface defects of ZnO ripples. These additional treatments could be used for improving the performance of OPV, which ultimately resulted in a PCE of 7.9 %.

ecause there are growing demands in the fields of security, image recognition, and medical applications. A Si diode device is one of NIR optical sensors. However, they cover broad areas from visible to IR. Our goal is to develop optical sensors sensing only NIR. We focused on organic dyes, especially, macrocyclic compounds which have a sharp absorption peak at NIR region. Various phthalocyanine derivatives were synthesized. Finally, Pc-1 was selected a dye as the light harvesting layer. Pc-1 has optical absorption at around 900 nm. For sensing device applications, photoconductive film with low leakage current under dark condition is necessary. The high ratio of ON/OFF current is also desirable. In this experiment, the sensor consists of Au/Mo2O3/Pc-1/ZnO/ATO/ITO/Glass layers. Mo2O3 and ZnO acts as an electron blocking layer and hole blocking layer, respectively. Photons are absorbed by Pc-1 and excitons are diffused to the interface between a Pc-1 layer and a ZnO layer, followed by charge separation at the interface. Pc-1 and ZnO have p and n type characteristics, respectively, which help them charge-separated. After first stage optimization, we achieved the following performances at 900nm, leak current under dark condition at 0 V: 5 x 10-16 A/µm2; Current under irradiation/current under dark: 1 x 106. The maximum sensitivity was 0.043 A/W for 4 mm2. The NIR sensor has sensitivity in the area from 560 nm to 950 nm. It was proved that the Pc-1is a candidate for a dyes of NIR optical sensors.

Organic photovoltaics (OPVs) are promising cost-effective and lightweight alternatives to traditional silicon solar cells. The organic materials used in OPVs can be subdivided into polymer or small molecule based solar cells. Though polymers tend to have higher power conversion efficiencies in OPVs, small molecule dyes are advantageous because of their well-defined molecular structure and synthetic reproducibility. Boron dipyrromethene (BODIPY) dyes are a versatile class of highly absorbing materials suitable for OPVs. They have already been used as electron donors in OPVs, showing promising performance. By tuning the chemical structures, it is possible to shift the HOMO and LUMO energies of these materials for use as acceptors as well as donors. BODIPY acceptors offer an attractive alternative to traditionally used fullerene acceptors because they are cheaper to make and their absorption maxima can be tuned to complement the absorption spectra of the donors. Here we report the synthesis and characterization of BODIPY dyes that have been prepared for use as electron donors or acceptors in OPVs. Lamellar bilayer devices with BODIPY dyes were fabricated and tested. The resulting device performance and film morphologies will be discussed.

Interface property is one of the important issues in optimizing the performance of hybrid polymer/metal-oxide solar cells. To obtain an interface of good charge transport quality, we choose conjugated small molecule, 2-naphthalenethiol (2-NT), to manipulate the ZnO-nanorod array surface before contacting with a polymer blend (poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM)) in an inverted solar cell configuration. This conductive molecule demonstrates a promising potential in improving the charge transport properties at the organic-metal oxide interface as well as in the polymer bulk heterojunction. As a result, there is a substantial improvement in photocurrent, open circuit voltage, and fill factor leading to double the power conversion efficiency of the unmodified device from 1.86% to 3.71% in ITO/ZnO-nanorod/P3HT:PCBM/Ag based configuration.

The dye-sensitized solar cell (DSC) imitates nature&’s photosynthesis by the principle of a light-absorbing dye chemically adsorbed on mesoscopic TiO₂. The charge separation of the photoexcited electron occurs at the interface between the dye and mesoporous semiconducting TiO₂ and charge is transported in the TiO₂. A redox couple I^- /I₃^- electrolyte reduces the oxidized dye molecule and is reduced back at the catalytic Pt counter electrode [1].
The present work investigates the UV stability of the DSC by parametrical investigation of material influence on UV stability. UV illumination has been observed to cause degradation of the DSC as UV illumination of TiO₂ causes photo catalysis if it is not perfectly covered by the dye. It is believed that if impurities, such as H₂O, OH^- and O₂ [2, 3, 4] are present in the DSC, the UV-generated electron hole in the TiO₂ will be reduced by impurities instead of I^- whereas the generated conduction band electron will react with I₃^- to form I^-. This leads to I₃^- depletion under UV illumination as long as impurities are present in the cell and thus decrease in electrical performance of the DSC.
A reproducible cell design was employed. A study on the DSC cell was carried out with intermediate electrical characterization by Cyclic Voltammetry (CV) and Electrical Impedance Spectroscopy (EIS) to map the influence of UV illumination as function of the material parameters:
- Electrolyte iodine concentration
- Electrolyte additives (MgI₂, LiI)
- H₂O concentration
- TiO₂ (layer thickness, transparent/scattering)
- With/without UV foil
- Cell plate distance
The results show that H₂O content has detrimental influence on the DSC stability during UV illumination. A higher concentration of I₃^- can buffer the reaction with impurities, so long-term stability is achieved. The possible regeneration of I₃^- by application of an external negative bias is to be explored and the results are to be presented with guidelines on how to make UV-tolerant and long-term stable dye-sensitized solar cells.
[1] B. O'Regan, M. Grätzel, Nature 335 (1991) 737-740.
[2] A. Hinsch et. al. Prog. PhotoVoltaics 9 ( 2001) 425-438.
[3] H. G. Agrell et. al. Solar Energy 75, issue 2 (2003) 169-180.
[4] G.E. Tulloch et. al. Photovoltaics for the 21th II: Proceedings of the International Symposium. By R. D. McConnell, V. K. Kapur, Electrochemical Society. Energy Technology Division (2001) 153-160.

Poor adhesion significantly affects the processing yield and long-term reliability of organic photovoltaics. Our research has shown that the PEDOT:PSS/P3HT:PCBM is generally the weakest interface in inverted polymer solar cells. Adhesion properties are affected by the bulk heterojunction (BHJ) layer composition and the deposition method. In addition, the power of the oxygen plasma pre-treatment on the BHJ layer prior to PEDOT:PSS deposition, the organic layer thicknesses and the replacement of PEDOT:PSS by various metal oxide hole transport layers are important.
Here, we focus on how the annealing temperature and time increases the adhesive properties in inverted polymer solar cells. We report how the BHJ morphology and interfacial chemistry is affected by annealing above and below the glass transition temperature and before or after electrode deposition. We show how the morphological reorganization and the interfacial chemistry changes are correlated with the increased fracture resistance and device efficiency. Using X-ray absorption spectroscopy (XAS) we were able to study in detail the molecular composition on each side of the failure path. Without annealing, or with annealing below 85C devices failed at the interface of the PEDOT:PSS and P3HT:PCBM layers, consistent with previous findings for inverted OPVs. In contrast, annealing at higher temperatures results in a cohesive failure path within the BHJ layer, where the initially weak interface was reinforced by the formation of a strong P3HT+:PSS- interface layer. Surprisingly, we also found that during post-deposition annealing the morphological reorganization is constraint by the Ag electrode and top glass substrate, whereas pre-electrode deposition annealing results in substantial PCBM segregation and clustering. These findings and their implications on device reliability will be discussed along with the effects of environmental factors such as humidity and solar irradiation.

A broad range of emerging solar technologies consist of inexpensive materials and show great potential for improvement.
Active layers often need to be as thin as possible in order to maintain high charge collection efficiencies at low costs. Going thinner also saves material expenses for conventional crystalline silicon solar cells.
However, this comes with a fall-off in light absorbance. The ideal device is both physically thin and optically thick at the same time.
Plasmonic metallic nanostructures show great promise to be a major route for effective light trapping by utilizing surface resonance plasmon modes. They exhibit strong local field enhancement as well as geometrical cross sections which are many times larger than the original object. The spectral behaviour and intensity of the plasmonic coupling is highly sensitive to the size, geometry, metal type and surrounding medium which make plasmonic systems highly tuneable.
We successfully model the electromagnetic field of metallic core-shell nanoparticles embedded in an effective medium providing a realistic environment for a multilayer solar cell architecture.
These results are used for device fabrication with Au-core silica-shell nanoparticles integrated into the mesporous titania structure of a solid-state sensitized solar cell. We use a perovskite absorber and spiro-OMeTAD as a hole transporter.
At optimal gold core size we achieve a 20% improvement of power conversion efficiency by optimising the nanoparticle incorporation.
Our findings show great potential for plasmonic enhancement in conventional solid-state thin-film solar cells and provide a basis for more complex nanostructures in the future.

We developed polymer organic solar cell by using simple method of blending two light absorbing donor materials in active layer. 2,4-Bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] (SQ) with a long wavelength absorption was added to poly-(3hexylthiophene)(P3HT): [6,6]-phenyl-C61-butyric acid methyl ester(PCBM) to improve the absorption range of the active layer. The light absorption of P3HT:PCBM was expanded up to 750 nm and the absorption between 630 nm and 750 nm was increased according to SQ content. The SQ additive did not affect the crystallinity of P3HT donor material and maintained the fill factor of P3HT:PCBM at 5% SQ content open circuit voltage of P3HT:PCBM was changed by 0.02 V because of deep HOMO level of SQ. Therefore, the SQ additive enhanced mainly high short circuit current (Jsc) and the power conversion efficiency of P3HT:PCBM device was improved by more than 30% compared to control device.

Organic semiconductors (OSCs) are widely used in many applications (e.g., solar cells, light emitting diodes, transistors). OSCs often form disordered films (weak lattice forces), which lead to a large number of defect states within the energy band gap. These defects trap free carriers, thereby degrading mobility and enhancing carrier recombination [1]. Thus, understanding the origin of defect states and its electrical characterization (in terms of energetic position, density and distribution) are important for development of any viable technology based on OSCs. State of the art defect characterisation techniques (e.g., impedance spectroscopy) are tailored for relatively high mobility inorganic materials. These techniques do not translate easily to OSCs due to their low mobility values and high band gaps. For example, poor mobility suppresses the dielectric relaxation frequency beyond the shallow defect levels [2] and the high band gap makes the demarcation frequency of deep defects much lower than the typical frequency range of most measurement instruments. Thus, the defect levels in OSCs are not well characterized [3].
In this work we present a new defect characterization technique well-suited for a wide range of low mobility OSCs. The characterization method relates the capacitance collapse in forward bias C-V as a signature of defect levels in low-mobility OSCs. A theoretical framework has been developed that extends the classic Mott-Schottkey analysis in the presence of traps; it is then applied to the defects in a widely used polymer semiconductor, poly(3-hexylthiophene) (P3HT). The device structure used was ITO/PEDOT:PSS/P3HT/Al.. We find that P3HT not only contains shallow defects (usually called dopants) but also has deep level (closer to mid gap) states. The defect density is approximately 1016 cm-3, as reported in other studies [1-4]. We find that these defects act as recombination centres, which is the main reason of poor fill-factor in the organic solar cells.
References:
1. Nicolai et al., Nat. Mater., 2012(11).
2. J.V. Li et al., Organic Electronics, 2011(12).
3. Carr et al. APL, 2012(100).
4. Guerrero et al., ACS Nano, 2012(6).

A hybrid polymer solar cell with the active layer of poly(3-hexylthiophene) (P3HT) and the inorganic scaffold composed of the ordered TiO2 nanorod (NR) array and ZnO nanoparticles (NPs) is fabricated in this work. The ZnO NP/P3HT hybrid is formed by the infiltration of diethylzinc/P3HT solution into the interstices of the TiO2 NR array followed by heat treatment. TEM characteristics reveal that the ZnO NPs are well-distributed in the P3HT matrix and the interstices of the TiO2 NR array are occupied with the ZnO NP/P3HT hybrid. Both Jsc and Voc are significantly enhanced in the TiO2 NR/ZnO NP-P3HT hybrid solar cell compared to the TiO2 NR/P3HT and ZnO NP-P3HT hybrid solar cells. Time-resolved photoluminescence (TRPL) spectroscopy and electrochemical impedance spectroscopy (EIS) are employed to investigate the photocarrier dynamics of the hybrid solar cells. The results show that the increase of the Jsc is mainly due to an appropriately interfacial area constructed in the cell by the inorganic scaffold of TiO2 NR array/ZnO NPs. Moreover, the elevated conduction band edge and the elongated electron lifetime in the inorganic scaffold result in the improvement of Voc in the TiO2 NR/ZnO NP-P3HT hybrid solar cell. The efficiency of the TiO2 NR/ZnO NP-P3HT hybrid solar cell is therefore significantly higher than those in the TiO2 NR/ P3HT and ZnO NP-P3HT hybrid solar cells. A remarkable efficiency of 2.46% is achieved in the TiO2 NR/ZnO NP-P3HT hybrid solar cell.

Fullerene and its derivatives have been used almost exclusively for electron acceptors in organic photovoltaics (OPVs). However, low absorption coefficients of fullerene derivatives in visible region limit photocurrent generation. Here we report a new non-fullerene acceptor applicable to OPVs. This acceptor (E1), derived from s-indacene-1,3,5,7(2H,6H)-tetraone, exhibited strong absorption bands in visible region. From the cyclic voltammetric and electronic absorption profiles of E1, we determined the HOMO and LUMO levels to be 6.1 and 3.4 eV, respectively. By combining E1 with poly(3-hexylthiophene) (P3HT) as an electron donor, we fabricated bulk heterojunction OPV devices with a structure of ITO/PEDOT:PSS/P3HT:E1(weight ratio of 1:2)/Ca/Al. Typically, a CHCl₃ solution of a mixture of P3HT and E1 was spin-coated onto PEDOT:PSS-covered ITO. The resulting film was loaded into a vacuum chamber and was subjected to vacuum-deposition of Ca and Al vapors.
The performance of the OPV devices thus obtained was evaluated under a N₂ atmosphere using a solar simulator with an AM1.5 filter at 100 mW cm^-2. For example, a device with a 196 nm-thick active layer, upon irradiation at 25 °C, showed current density (J_sc) of 0.30 mA cm^-2, open circuit voltage (V_oc) of 0.90 V, and fill factor (FF) of 0.35, thereby giving a power conversion efficiency (eta;) of 0.096 %. Interestingly, thermal annealing of the active layer, e.g., at 150 °C for 10 min, increased the overall performance (eta; = 0.29 %) as a result of enhanced J_sc (0.82 mA cm^-2) and FF (0.40). On the other hand, V_oc (0.87 V) was hardly changed. Powder X-ray diffractometry and fluorescence spectroscopy of a blend film indicated the occurrence of a phase separation between P3HT and E1 upon thermal annealing. We assume that the phase separation most likely leads to the formation of better carrier-transport pathways, resulting in the enhancements of J_sc and FF. We found that the bulk heterojunction OPV device with P3HT:E1 active layer, when the thickness was optimized, showed the maximum eta; of 0.80 %.

Organic photovoltaic devices (OPVs) which are composed of conjugated polymers and fullerene blends provide an attractive approach for renewable energy applications. In this work, eifficient phase separated organic solar cells based on mixed solutions of 1,2 o-dichlorobenzene (DCB) and dichloromethane (DCM) were fabricated. With donor material of PCDTBT ((poly [N-9prime;-heptadecanyl-2,7-carbazole-alt-5,5-(4,7prime;-di-2-thienyl-2prime;,1prime;,3prime;-benzothiadiazole)) and acceptor material of PC71BM ( [6,6]-phenyl-C71-butyric acid methyl ester), as well as the cell structure of ITO/PEDOT:PSS/PCDTBT:PC71BM/LiF/Al, the effect of PCDTBT:PCBM active layer phase separation on the blend ratio of DCB:DCM was meticulously examined. The morphology and phase of the active layer were investigated by atomic force microscopy, scanning electron microscopy, and x-ray diffractometery, respectively. By carefully adjusting the DCB:DCM blend ratio, the ohm contacts between PCDTBT and the anode, as well as between PC71BM and the cathode were established. The current-voltage characerstics comfirm that the cell resistivty was greatly reduced due to the improved eletron and hole transport properties. Our finding paves the way to fabricate an efficient BHJ solar cells with a facile spin coating method.

In our recent work we adapted dedicated atomistic forcefields for P3HT and PCBM from literature based on benchmarking against Density Functional Theory calculations and experimental data.[1] Here we report on a coarse-grained version of this forcefield that allows for simulations of larger P3HT:PCBM heterojunction models over extended periods and its application to study the morphology evolution of P3HT:PCBM bilayers.
Derivation of coarse-grained forcefield involved fitting against experimental density and melting temperature for van der Waals parameters and fitting against the atomistic forcefield for Coulomb, angle bend, torsion and bond stretch parameters. The ability of the coarse-grained forcefield to reproduce experimental melting temperatures, crystal structure incl. side chain orientation and radial distribution functions from the atomistic model demonstrates that coarse-graining has not compromised the reliability of morphology simulation despite a 20-fold reduction in computational effort. Together with the commonly observed acceleration of dynamics by coarse-graining, a 200 times longer period can now be monitored for the same system size.
Using the coarse-grained forcefield, we study bilayer structures with different crystallinity and orientations of the P3HT molecules at 450K to emulate experimental annealing conditions. Orientational order of PCBM is not maintained at 450K; hence different crystal orientations of PCBM were not considered. The lowest interface energy is observed for PCBM:P3HT(amorphous). Among the PCBM:P3HT(crystalline) interfaces, the edge-on orientation exhibits about 30% lower interface energy than end-on or face-on configurations, but still 10% higher than the interface with amorphous P3HT. Lower interface energy also corresponds to rougher interfaces for all investigated cases.
Simulated energy barriers for the diffusion in the different simulated P3HT/PCBM interface region suggest that the energy barrier for PCBM intercalation is lower for amorphous P3HT. To study low energy diffusion pathways for PCBM inside P3HT, we monitor energy variations as a probe molecule is moved within P3HT. The gap between terminal repeating units is found to constitute such a low energy path. PCBM will thus show higher mobility in low molecular weight crystalline P3HT, where gaps between terminal repeating units are more densely distributed. In P3HT:PCBM solar cell made by pseudo-bilayer approach, PCBM would mainly form continuous paths along such “grain boundaries” of P3HT rather than being trapped within P3HT regions, which could help explain their superior reproducibility and performance.[2]

We studied the exciton quenching characteristics in a planar heterojunction organic photovoltaic (OPV) device with boron subphthalocyanine chloride (SubPc) and C60 as donor and acceptor materials, respectively. With the insertion of a 3-nm buffer layer, N,N-dicarbazolyl-3,5-benzene (mCP), between indium tin oxide (ITO) anode and SubPC donor layer, the power efficiency of the OPV device increased from 3.96% to 5.08%. From the time-resolved photoluminescence (TRPL) experiments of ITO/mCP/SubPC thin films by 400 nm fs laser pulse excitation and streak camera detection, a bi-exponential characteristic of fluorescence decay was observed. In all samples, a small portion of the molecules exhibited a decay time-constant of 1.7 ns, consistent to the results of unaggregated SubPC in solution. In contrast, the fast decay is strongly sample dependent, and one can see a sudden jump of fluorescence time-constant from 52 to 185 ps when the 3-nm mCP buffer layer is added. That was clear evidence that 3-nm mCP can block the excitons and prevent the exciton quenching by ITO anode. When further increasing the mCP layer to 20 nm, the fluorescence time-constant increased to 266 ps, but the device efficiency decreases because of the increase of serial resistance. We attribute this to a competition between the mCP blocking the excitons and blocking charge carriers, and find that 3 nm thick mCP is optimal for device function.

It is well known that the molecular weight of a polymer can significantly affect its physical, thermomechanical, and electrical properties. For semiconducting polymers, such as poly-(3-hexylthiophene-2,5-diyl) (P3HT), which are used as active layer materials in bulk heterojunction (BHJ) solar cells, the molecular weight has been correlated to carrier field-effect mobilities, surface morphology and gelation rates in solution. This has important implications for internal device efficiency, large-scale manufacturing and future applications of polymer-based solar cells. In this work, we show that the molecular weight of P3HT in solar cells can significantly change internal cohesion which affects the mechanical reliability of the solar cell both during processing and in-service. Cohesive values ranged from ~0.5 J/m2 to ~20J/m2, following the general trend of greater cohesion with increasing molecular weight. We attribute the increase in cohesion to greater degree of plasticity which allows for greater energy dissipation during de-cohesion. Analysis of the resulting cohesive surfaces using XPS indicates mechanical failure occurs within the BHJ layer for all devices measured. We also probe the rate of cohesive failure within the solar cells under varying temperatures, relative humidity and the presence of oxygen. Finally, we correlate internal device efficiency with mechanical cohesion to assess the device physics pertinent to optimizing device reliability. Our research aims to elucidate the fundamental parameters which can affect the mechanical stability of BHJ solar cells which will ultimately aid in the development and realization of organic electronics with greater reliability.

The new epoch of building hybrid devices from organic and inorganic moieties offers tremendous possibilities as replacements in emerging technologies ranging from electronics, thermoelectrics, sensors, and photovoltaics. Hybrid solar cells, in particular, marry the processability and tunability of organic and polymeric systems with the robustness and superior electronic properties of solid-state inorganic materials. The integration of highly-ordered, anisotropic, nanostructured semiconducting materials within bulk-heterojunction solar cells represents a vital goal for the development of next-generation photovoltaics. Such integration is imperative to realize breakthroughs in improvements in photocurrent generation and light-harvesting efficiencies. To realize the full potential of hybrid photovoltaics, however, a number of formidable challenges need to be addressed, including the ability to control the assembly of organic and inorganic components; the ability to tune organic-inorganic interfaces, and the ability to guide the direction of charge transfer. This talk focuses on a slice of this problem by engineering nanostructured inorganic semiconducting materials with tunable dimensions, morphologies, and photochemical properties to be assembled with organic semiconductors in hybrid solar cells. Critical to the controlled growth of inorganic semiconductor nanowire arrays is the deposition of a sputtered seed layer. This seed layer, for example, promotes the subsequent hydrothermal growth of vertically-oriented, one-dimensional zinc oxide (ZnO) nanostructures onto which poly-(3-hexylthiophene), P3HT, is subsequently deposited to complete the photoactive layer of our hybrid solar cells. With this seed layer, our devices exhibit higher external quantum efficiencies and photovoltaic conversion efficiencies (PCE) compared to those reported in the past; devices with PCEs as high as 1.6 % are routinely obtained. Likewise, we report strong correlations between device short-circuit current density (Jsc), open-circuit voltage (Voc), and PCEs with surface monolayer functionalization, where self-assembled monolayers are absorbed onto ZnO nanowire arrays using tethered-by-aggregation-and-growth (T-BAG) prior to polymer infiltration. Self-assembled monolayer adsorption induces interfacial dipole, which in turn alters device performance. Further, statistical and surface analyses have demonstrated the successful adsorption of phosphonic acid derivatives on the ZnO surfaces which contributes to the passivation of interfacial traps.

Tandem polymer solar cells can employ multiple conjugated polymers working efficiently in different region of the solar spectrum, thereby more effectively harvesting the solar energy than the single junction cells. However, the concomitant issues with tandem cells such as device complexity and increased cost of fabrication significantly impair the commercial viability of this technology. Here we demonstrate a conceptually new device configuration, parallel-like bulk heterojunction (PBHJ) solar cell. The proof-of-concept PBHJ solar cells are fabricated from one single blend of two donor polymers with phenyl-C61-butyric acid methyl ester (PCBM), which maintains the simple device configuration and low cost processing of single junction BHJ cells while inherits the major benefit of incorporating multiple polymers of tandem cells. In this PBHJ, free charge carriers travel through their corresponding donor polymer linked channels and fullerene enriched domain to the electrodes, equivalent to a parallel-like connection. The short circuit current (Jsc) of the PBHJ solar cell is nearly identical to the sum of individual Jsc of each single “sub-cell”, while the open circuit voltage (Voc) is in between of them, which is a clear indication of a parallel-like connection of two “sub-cells”. Preliminary optimization of PBHJ devices leads up to 40% improvement in Jsc and 30% in overall efficiency when compared with these of single BHJ devices.

A series of solution processable phenyl substituted diketopyrrolopyrrole small molecules has been used as a model system to investigate the dependence of exciton diffusion length on chemical structure and processing conditions. Exciton diffusion lengths were obtained by measuring the photoluminescence decay of blend films composed of dilute concentrations of [6,6]-phenyl-C61-butyric acid methyl ester. Photoluminescence decay for pristine and blend films where used to calculate quenching efficiencies which were inputted into a Monte Carlo simulation software to extract the exciton diffusion length. In the compounds studied here, it is found that the exciton diffusion length increases with decreasing conjugation length and increasing molecular bulkiness. It is also shown that annealing does not result in a general increase or decrease in exciton diffusion length. Rather, the optical and physical properties of a material dictate how annealing impacts the exciton lifetime, diffusion coefficient, and therefore exciton diffusion length.

Sequential bilayer solution-processing to form a nanostrutured planar heterojunction (PHJ) structure provides a new and simple way to exploit an advantageous combination of photoconversion layer in organic photovoltaics (OPVs). We demonstrate here high-performance quasi-bilayer OPVs with a power conversion efficiency (PCE) of 5% based on nanostructure in the bilayer film by using an isoindigo donor polymer (PII2T-C10C8) and [6,6]-phenyl-C[71]-butyric acid methyl ester (PC71BM). To accomplish that, we systematically controlled the swelling and interdiffusion processes of PII2T-C10C8 donor layer as a function of the type of solvents to dissolve PC71BM to allow efficient vertical nanostructures. We found that a distinguishable quasi-bilayer nanostructure with an intermixed heterojunction part on the nanoscale and a PC71BM-rich top layer of 16nm is derived from orthodichlorobenzene (ODCB) induced swelling, interdiffusion, and reorganization of PII2T-C10C8 chains with PC71BM molecules. This might be governed by recrystallization during solvent evaporation, giving rise to well-ordered nanofibrills in PHJ films, which enables enhanced interfacial area comparable to BHJ system as well as pure interfaces with the electrodes. Furthermore, we revealed that sequential processing of PC71BM derived from ODCB induces a change of molecular orientation from predominantly edge-on to isotropic orientation, even with partially face-on in PII2T-C10C8 bottom layer, allowing efficient charge transport in the out-of-plane direction.
We believe this strategy of using quasi-bilayer nanostructure can be extended and generalized to other conjugated donor polymer systems.

Organic tandem solar cells have attracted much attention in recent years as the efficiencies of tandem devices can exceed the efficiencies which are possible with single junction solar cells. Tandem solar cells offer the possibility to harvest a broader part of the solar spectrum which already led to record efficiencies above 10% prepared by organic small molecules but also with polymer/fullerene composites.
In this contribution we present our results on nanocomposite tandem solar cells, where the active layers consist of mixtures of semiconductor nanostructures and a semiconducting conjugated polymer. In contrast to PCBM/polymer solar cells it is, therefore, not only possible to tune the absorption properties by those of the polymer but also by the choice of the inorganic semiconductor.
We realized such hybrid tandem solar cells following a recently developed in-situ preparation route using tailored metal xanthate precursors to guarantee high solubility in organic solvents and to form the pure metal sulfides at low temperatures. Thus, a common solution of these metal xanthates with the conjugated polymer is coated directly on the transparent electrodes (e.g. ITO, ITO/PEDOT:PSS) and subsequently converted to the corresponding metal sulfides within the polymer matrix using temperatures below 200 °C. Therefore, no capping ligand is needed to stabilize the nanoparticles during the formation. Using this approach various metal sulfides are accessible (CdS, CuInS2, PbS, Bi2S3).
In a first step, we show, that it is possible to prepare nanocomposite tandem solar cells by doubling our benchmark system [1] - CIS (copper indium sulfide) / PSiF-DBT (Poly[ [9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl) - and Ag as recombination layer. The first layer can withstand a second thermal treatment during the preparation of the upper layer. This first tandem solar cell already shows almost the doubled VOC but the ISC remained lower, due to the overlap in absorption. Scanning electron microscopy investigations on a cross section of such a tandem cell show two homogeneous and smooth active layers both with a thickness in the range of 70 nm.
In the second step we combined a CIS/PSiF-DBT layer with complementary absorbing layers e.g. CdS/P3HT, CIS/F8T2 but also PCBM/PCPDTBT.
Up to now the prepared nanocomposite tandem solar cells do not exceed 2% power conversion efficiencies, however, we think that this value can be significantly improved in the near future by precisely optimizing the recombination layer to the used inorganic/organic material blends.
[1] T.Rath et al. Adv.Energy.Mater. 2011, 1, 1046-1050.

Organic thin film solar cells have large potential as renewable energy source due to resource friendly production and low material consumption.
In the optimized design of organic solar cells, the compromise between absorption in the active layer, determining the short circuit current, and the transport properties of the used materials, determining the Fill Factor, is a practical limit.
Plasmonic metal nano-structures with sub wavelength sizes can be used to enhance the absorption within the solar cells' active layers, increasing the short circuit current while keeping the thickness of the device low and ensuring good charge carrier collection at the electrodes.
We investigate this strategy in the framework of evaporated small molecule solar cells in the p-i-n concept, incorporating self assembled silver particles inside doped transport layers.
In these devices we can control the thickness of every layer, the positioning of the nano-particles and the prevention of recombination at the silver particles.
The plasmonic devices show increased photo-current for wavelengths between 400 nm and 1100 nm, while maintaining open circuit voltage and Fill Factor of the reference devices without plasmonic structures. Thus, such plasmonic structures can be used to enhance existing device structures.
Furthermore, the increased absorption can be used to decrease the absorber layer thickness - e.g. down to about 25 nm in a ZnPc:C60 cell - in an optimized solar cell design with high short circuit current densities and increased Fill Factors.
The possibility of using such thin absorber layers can pave the way for using new material classes as active materials whose transport limitations otherwise would not make them suitable for organic solar cells.

Hybrid solar cells are an emerging solar cell technology with a great potential for cheap fabrication. They usually consist of a nanostructured junction of inorganic and organic semiconductors and therefore combine cheap and abundant organic materials with the advantages of inorganic materials in terms of stability and charge transport. For devices based on blends of inorganic nanoparticles and organic polymers, power conversion efficiencies exceeding 3 % have been obtained and there is a large potential for further efficiency improvements. In order to achieve these, a detailed understanding of the working mechanism of hybrid solar cells is of crucial importance.
We have used femtosecond transient absorption spectroscopy to study the details of charge generation in polymer/metal sulphide blends and to identify loss processes in hybrid solar cells. We show that charge generation can either occur via electron transfer from the polymer to the metal sulphide or via hole transfer from the metal sulphide to the polymer by selectively exciting each material. The time constants of these two charge transfer processes were measured for different material combinations and the charge generation efficiencies were determined.

High performance semi-transparent cells with assisted infrared light trapping using a photonic nano-layer architecture
The semi-transparency of the active material in organic photovoltaic (OPV) cells has been recognized as an important material characteristic to widen the scope of applications for such type of cells. Recent progress in the field has shown that high transparency can be reached when using near infrared light sensitive photovoltaic (PV) polymers [1]. However, the performance of opaque PV devices using such type of polymers is significantly lower than OPV cells using bulk hetero-junctions of the best performing polymers, which, in general, have the peak of maximum absorption closer to the red region of the spectrum. For instance, with PTB7:PC71BM as active material one can fabricate opaque direct cells with efficiencies higher than 8% [2].
To make highly transparent OPV cells in the visible using PTB7:PC71BM as active material, we reduced the thickness of the top Ag electrode down to 10 nm. This implied a significant loss in the light harvesting at all wavelengths, including those at the IR and UV which are not visible to the human eye. The outcome was that the power conversion efficiency (PCE) of the semi-transparent cell dropped down to 50% that of the opaque cell. To recover for conversion to electricity the lost infrared and ultraviolet photons, we designed a photonic nano-layer structure that we deposited on top of the thin Ag electrode. Such photonic multi-layer architecture that combined low and high index of refraction dielectric materials was numerically designed ad hoc to achieve the highest reflectivity at the infrared and UV wavelengths while maintaining a good transparency in the visible. For the semi-transparent cell incorporating the photonic nano-layer structure, the PCE measured was 5.2% which corresponded to 71% the efficiency of the opaque cell. On the other hand, the transmission for visible wavelengths averaged close to 30%.
In summary, combing high performance PV polymers with a photonic nano-layer structure designed ad hoc we were able to fabricate semi-transparent cells with a PCE above 5.2%. The transparency of the device was measured to be above 30% for the most part of the visible spectrum. Breaking the 5% efficiency barrier constitutes a major step towards the development of a semi-transparent PV technology.
1. Chun-Chao Chen et al., ACS Nano 6, 7185 (2012).
2. Alberto Martinez-Otero et al., submitted to Advanced Optical Materials (2012).

The organic/inorganic hybrid solar cell can combine the advantages of the organic and inorganic materials. In organic solar cell, only few choices for acceptor give a large limit to the small band gap donor materials. Designing new types of acceptors for organic solar cell is a big challenge because of the requirement of energy level, mobility and the ability of accepting electrons. The inorganic materials has good electron mobility and varies of inorganic materials provide many choices of different energy levels. Therefore, using inorganic materials as acceptor in organic solar cell might be a good approach to solve the limitation of organic acceptor. The high carrier mobility and absorption coefficient of GaAs indicate it might be a good candidate for new acceptor in hybrid solar cell application. We performed systematic studied on plannar GaAs/polymer hybrid solar cell with different active polymers. The thickness of the active polymer layer is found to be important. Thermal annealing for the polymer and surface passivation for GaAs are found to be necessary to get good device performance. The energy levels of polymers are systematically tuned to study the real function of active polymer in GaAs based hybrid solar cell. The external quantum efficiency results suggest the polymer contributes little to photocurrent.

We report on our recent progress in fabricating single-layer organic solar cells using hydrogen-bonded pigments as the active material. The donor-acceptor heterojunction architecture has emerged in the past decade as the only successful design principle for organic solar cells. We have found that many H-bonded pigments provide photocurrents in the milliampere range in the absence of a donor-acceptor interface. This corresponds to quantum efficiencies in the range of 10-20%, orders of magnitude higher than what is typically found for neat organic films. Herein we discuss spectroscopic evidence clarifying the intrinsic charge generation mechanism in H-bonded pigment films, which apparently is based on the efficient generation of excimeric species that easily polarize into free carriers. Secondly, we report the photovoltaic performance of single-layer diodes with H-bonded pigments. Diverting from the well-established donor-acceptor concept frequently used in organic solar cells by applying only one absorber layer is potentially highly-advantageous: In addition facile fabrication of single-layer devices, the theoretical power conversion efficiency limit can be higher compared to conventional donor-acceptor type organic solar cells due to elimination of polarization losses.

Polymer solar cells offer a promising approach for a low-cost and flexible photovoltaic technology with certified efficiencies exceeding 10 %. Before widespread commercialization, large area production and stability issues have to be solved. For the reliable large area production with high yield and low shunts, thick, stable, robust and printable buffer layers are a prerequisite. In previous reports, we have reported on low temperature, solution processed metal oxide (MeO) interface layers that fulfill these requirements. On the n-type side we focused our work on ZnO, TiOX and Al-doped ZnO (AZO) [1,2] material systems and on the p-type side on MoO3 and WO3 [3,4,5] to avoid the use of the commonly employed PEDOT:PSS. PEDOT:PSS is well known for the device stability issues caused by its hygroscopic and acidic nature.
We selected ZnO/AZO and WO3 from nanoparticular dispersions as the prototype materials for the further investigations. We employ these MeO interfaces in normal and inverted architecture solar cells (ITO/WO3/Active Layer/ZnO or AZO/Ag vs. ITO/ZnO or AZO/Active Layer/WO3/Ag). The MeO layers on top of the active layer serve different functions. The key ones are (i) to work as an electronic interface layer to enable the use of stable, high-workfunction Ag as both, anode and, more importantly, cathode, as well as (ii) to prevent shunts through the active layer when applying the top electrode (e.g. during Ag evaporation). Typically, solution processed MeOs require an evaporated Al electrode for highest performance [6]. In our investigations, we demonstrated p- and n-type MeOs which both work with Ag electrodes. This is an essential step towards all solution processing of organic solar cells and shows the possibility to change the polarity of the device by only switching the respective MeOs.
Furthermore we investigate and discuss the impact of switching the MeO interlayers on active layer microstructure of different donor/acceptor systems and discuss the resulting device performance and lifetime. Previous studies have shown that environmental stability is a function of the interface robustness. Due to their excellent inherent intrinsic stability, MeOs provide the unique opportunity to investigate interface failure processes unaffected by electrode degradation.
[1] H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, C. J. Brabec, Sol. Energy Mater. Sol. Cells 95 (2011) 2194.
[2] T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov, C. J. Brabec, Org. Electon. 12 (2011) 1539.
[3] T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik and C. J. Brabec, Appl. Phys. Lett. 98 (2011) 253308.
[4] T. Stubhan, N. Li, N. A. Luechinger, S. C. Halim, G. J. Matt and C. J. Brabec, Adv. Energy Mater., 2012, DOI: 10.1002/aenm.201200330.
[5] N. Li, T. Stubhan, N. A. Luechinger, S. C. Halim, G. J. Matt, T. Ameri and C. J. Brabec, Org. Electron. 13 (2012) 2479.
[6] H.-L. Yip, S. K. Hau, N. S. Baek, H. Ma and A. K.-Y. Jen, Adv. Mater. 20 (2008) 2376.

Interfacial contact layers improve the performance of organic photovoltaic (OPVs) devices, by extracting photogenerated carriers at separate contacts from the bulk-heterojunction (BHJ) active layers and reducing the recombination of charge-separated carriers, a major loss mechanism affecting the performance of (OPVs) devices. However, the criteria of a good ICL material are still not explicitly established partially due to the lack of a straightforward technique to correlate the ICL materials properties and carrier recombination in devices. We recently showed that white-light biased external quantum efficiency (EQE) measurements enable quantitative determination of the recombination parameter alpha as a function of background DC light intensity and AC probe light wavelength. In addition, we found strong correlation between alpha measured using this method and fill factor for poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ devices. Here, we fabricate OPV devices with P3HT and acceptors (e.g. PCBM or indene-C60 bisadduct (ICBA)). ICBA has a shallower lowest unoccupied molecular orbital (LUMO) than PCBM and exhibits a higher open circuit voltage (Voc) in BHJ devices, but the performance may be further improved by inserting a lower work function electron selective layer (ESL). To study this, we will characterize the complete electronic structure of PCBM and ICBA using Kelvin Prove, UV-Vis and Photoelectron Spectroscopy in Air (PESA). By employing ESLs with different work functions in the P3HT:ICBA devices, such as polyethylenimine ethoxylated (PEIE), ZnO sol-gel films, ZnO polymer composites and amine-functionalized ZnO nanoparticles, we evaluate the correlation among ESL work function, device characteristics, and bimolecular recombination determined using white-light bias EQE measurements. We will examine the intensity and wavelength dependence of alpha, which reveals the recombination mechanism(s). For example, carrier loss at low illumination intensity arises from charge trapping, while that at high illumination intensity signals bimolecular recombination and/or the existence of a space-charge region. By comparing wavelength dependence of alpha and generation profiles simulated using TMM, we will be able to elucidate the physical origin of the recombination. Combining device characterization with white-light biased EQE measurements on devices with ESLs of different work functions enables us to elucidate the role of ESL in improving device performance.

We investigate solution-processed tetra-n-alkyl ammonium bromides (TAABs) as electron extraction layers (EELs) in bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The EELs of TAABs lead to simultaneous increases in open-circuit voltage (VOC), short-circuit current, and fill factor, enhancing the power conversion efficiency of BHJ solar cells from 2.38% to 4.02-4.19%. It is interesting that the same increase in VOC about 0.14 V is obtained for devices with Al, Ag, and Au cathodes by using the EEL of TOAB. The TOAB molecules self-assembled into the lamellar structure stacked upright atop P3HT:PCBM, corroborated by synchrotron X-ray diffraction. The surface morphologies of TAABs atop P3HT:PCBM, explored by atomic force microscopy, showed that the thin layers of TAABs facilitating electron extraction in the regions besides large island clusters. The underlying mechanism is inferred that the TAAB molecules introduced anisotropic dipoles significantly shifting the vacuum level at the P3HT:PCBM/metal interface. Our results provide a simple method to fabricate high performance BHJ solar cells.

High-conductivity films of the transparent conductive polymer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) or PEDOT:PSS are desired for applications in many optoelectronic devices such as liquid crystal displays (LCDs), light emitting diodes (LEDs), lasers, detectors, and solar cells as an alternative to Indium Tin Oxide (ITO) films. ITO offers high conductivity and transparency but high material cost, concern about Indium supply, lack of flexibility, and expensive deposition processes have forced research into alternatives. In this work, we present (i) the demonstration of directly coating silver nanowire (AgNW) meshes onto PEDOT films with low (10 Omega;/square) sheet resistance, (ii) demonstrate use of UV wavelength lasers to locally fuse nanowire junctions in the AgNW/PEDOT films, and (iii) fabrication of a functional solar cell using a AgNW/PEDOT film demonstrating 10.6% efficiency and 85.5% Fill Factor (FF).
Silver nanowire meshes and conductive polymers have attracted interest as an alternative to ITO [1-4]. PEDOT:PSS films, while commonly used as the anode or hole-injection material in organic solar cells, do not offer the same level of conductivity as ITO films. As such, combining conductive AgNW meshes with PEDOT:PSS has attracted interest [5-7]. These studies have not demonstrated the coating of AgNWs directly onto a PEDOT:PSS film, however. PEDOT:PSS and AgNW networks were applied via sequential spin coating and evaluated with TLM structures. The PEDOT:PSS 80nm thick films have sheet resistance of 500 Omega;/sq. as coated. Application of AgNWs in methanol solution via spin coating can lower the sheet resistance up to an order of magnitude (10-200 Omega;/sq.). The conduction and transmission properties of the combined film are dominated by the AgNW properties and these films can offer 20 Omega;/sq. and 68% transmission at 600nm wavelength. To achieve lower resistances laser processing of the AgNW films was used to fuse nanowire junctions [8]. A silicon heterojunction solar cell was fabricated by coating PEDOT:PSS and AgNWs onto n-type silicon. The resulting device demonstrated 10.6% efficiency, Isc of 19.9mA/cm2 and FF of 85.5%, as compared to the control without AgNW 9.11%, 24.90ma/cm2 and 59.0%. We anticipate higher Isc and efficiency for the AgNW device with an optimized metal scheme to take advantage of the lower sheet resistance.
This work was supported by the Princeton Center for Complex Materials NSF MRSEC grant DMR-0819860 and the Department of Energy DE-EE0005315
1 De, at al. ACS Nano Vol. 3 No. 7 1767-1774 (2009)
2 Hu, et al. ACS Nano Vol. 4 No. 5 2955-2963 (2010)
3 Yu, et al. Adv. Mater 23, 664-668 (2011)
4 Xia, et al. J. Mater. Chem. 21, 4927 (2011)
5 Wu, et al. ACS Nano Lett. 10, 4242-4248 (2010)
6 Zhu, et al. ACS Nano Vol. 5 No. 12 9877-9882 (2011)
7 Leem, et al. Adv. Mater. 23, 4371-4375 (2011)
8 Spechler, et al. Appl. Phys. A 108:25-28 (2012)

All-solid-state DSSC such as those feasible by the utilization of organic hole conductors or polymer electrolytes have the potential to further reduce the production cost of this promising solar technology [1] by allowing utilization of flexible, low-cost plastic substrates manufactured by a roll-to-roll coating procedure [2]. Currently, the highest solar energy conversion efficiencies are obtained with corrosive liquid electrolytes such as I-/I3- redox couples resulting in challenging sealing of the cells and strict requirements for the substrate material. Although, some very promising results have been recently demonstrated with perovskite electrolytes, organic hole conductors and polymer electrolytes, required for the synthesis of flexible cells, still result in considerably lower performances. This is due to the difficult penetration of such materials in the nanostructured TiO2 film that limit its maximal thickness and thus light absorption efficiency. Recently, highly porous films have showing the potential to improve the penetration of liquid electrolytes in thick working electrodes up to 200 mm [3]. Here, we present a novel semiconductor film morphology and composition that enables the synthesis of thick and thus significantly more efficient working electrodes for all-solid-state DSSC. Structural-functional correlations of the nanoparticle semiconductor films are presented as a function of their main morphological properties focusing on the maximization of their solar energy conversion efficiency and future application on flexible substrates.

[1] M. Gratzel, Accounts Chem. Res. 2009, 42, 1788.
[2] I. Chung, B. Lee, J. Q. He, R. P. H. Chang, M. G. Kanatzidis, Nature 2012, 485, 486.
[3] A. Tricoli, A. S. Wallerand, M. Righettoni, J. Mater. Chem. 2012, 22, 14254.

Absorption of UV photons by PbSe quantum dots in the size range of 2 - 10 nm have been shown to generate multiple excitons. However, dissociation of the excitons to enhance device currents is controlled by multiple factors that include the nature of the QD interface. Presence of surfactants between the QD and the surrounding matrix hinder charge transport. Ligand transfer reduces this barrier but still affects the exciton dissociation. We have developed a laser-assisted spray (LAS) growth process to deposit uniform coatings of surfactant-free PbSe QDs on a substrate. TEM analysis showed the QDs to be in intimate contact with other QDs as well as the background material. QDs also retain their single crystal nature, and optical absorption spectroscopy confirmed quantum confinement. I-V characteristics show orders of magnitude increase in charge transport properties in surfactant-free QD films in comparison to QDs with surfactants. For the investigation of multiple exciton generation-dissociation in these surfactant-free QDs, the hybrid structure ITO/PEDOT:PSS/PbSe QD/PCBM/Al was fabricated on an ITO coated glass substrate. The device was reverse biased with an external voltage in order to enhance the exciton dissociation and photocurrent for incident light from a wavelength tunable source in the range of 532 nm - 1.06 mu;m was measured. For QDs with average band gap of 0.7 eV this wavelength range corresponds to a transition from a single exciton generation to 3 exciton generation. Enhancement in device current during this transition indicates effective multiexciton dissociation. Comparison of wavelength dependent current for devices with surfactant-free QDs produced by LAS process and QDs with surfactants will be presented.

Interfacial adhesive and cohesive cracking of photovoltaic (PV) backsheets and encapsulants exposed to hostile application environments are not well understood. In particular, the effects of ultraviolet (UV) light and the environment on the debond kinetics of the highly strained material at a propagating crack tip remains unexplored. The damage of solar environments on PV module materials has traditionally been studied by exposing them to "stress parameters" such as elevated temperatures, moist environments and large doses of UV light. After exposure, degradation in the material is quantified by measuring changes in selected properties such as color, mechanical strength and chemical composition. Although this approach can capture the synergistic effects of multiple environmental "stress parameters", the mechanical stress and its importance in determining the kinetics of interface adhesive and cohesive cracking is rarely controlled but critically important. The understanding obtained from improved control of multiple "stress parameters" forms the basis for life-time prediction and the design of accelerated tests.
Using a recently developed quantitative debond characterization method with in-situ UV, we report debond growth rates of ethylene-co-vinyl acetate (EVA) encapsulants and laminated tedlar-polyester-EVA (TPE) backsheets of solar modules in terms of the applied mechanical loads. We employ a load relaxation technique that allows debonding rates as low as 1 A/sec to be accurately quantified and related to the rupture of molecular bonds at the crack tip. The synergistic effect of moisture, temperature and UV exposure on debond growth acceleration is demonstrated. To elucidate the degradation processes leading to envrionmental debonding, the kinetics of the debond growth process are interpreted using atomistic and viscoelastic fracture mechanics models, which we modified to include the contribution of UV light. The molecular bond rupture processes investigated underlie the principal causes of degradation in PV materials exposed to terrestrial environments and can be exploited to acquire, not only a fundamental understanding of damage formation and progression, but also to develop accelerated testing techniques and make long term reliability predictions.

Lead sulfide (PbS) nanocrystal quantum dots (NQDs) are promising materials for various optoelectronic devices, especially solar cells, because of their tunability of the optical band-gap controlled by adjusting the diameter of NQDs and the high quantum confinement effect due to its large Bohr exciton radius (20 nm). To exploit these desirable properties, many research groups have intensively studied to apply for the photovoltaic devices. Understanding the charge transport mechanisms in NODs solids should take precedence prior to the fabrication of solar cells. The carrier transport mechanisms of NQDs solids have been extensively studied based on the current-voltage (I-V) characteristics of field-effect transistors (FET). However, it is a noteworthy that FET is the device operated over 1 V, whereas solar cell is operated under 1 V. Furthermore, charge carriers in FET device should explore over thousands nanometer of channel length (i.e. between source and drain electrode), whereas they travel below hundreds nanometer of current path (i.e. between top and bottom electrode) in solar cell device.
In this presentation, the active layer thickness for full depletion in Schottky junctions was estimated based on the parameters as a function of PbS QDS-thickness. The experiment results were verified by fitting an appropriate photoconduction model. To investigate the transport properties of charge carriers in real solar cell devices, we performed dark I-V measurements of Schottky devices at temperatures ranging from 15 to 300 K. From the temperature dependent I-V measurements, we resolved two governing transport mechanisms.
This work was supported by the Global Frontier R&D Program by the Center for Multiscale Energy Systems funded by the National Research Foundation under the Ministry of Education, Science, and Technology, the Industrial Core Technology Development Program funded by the Ministry of Knowledge Economy (No. 10035274).

Organic solar cells have improved in the last few years and recently crossed the 10% efficiency barrier [1]. At this stage these high efficiencies are only achieved for small active areas (asymp;1 cm^2) fabricated with processes like spin-coating or thermal evaporation. For organic solar cells to become commercially viable the efficiencies need to be transferred to large area devices (> 25 cm^2) and mass production compatible production processes like printing or coating. The results reported for large area solar cells utilizing printing and coating techniques [2] are still far from what has been reported for small area devices.
In this report we describe monolithic, organic bulk heterojunction solar cells with an active area of 25 cm^2 where the photoactive layer, namely PCDTBT:PC70BM is fabricated by blade coating instead of spin coating. Blade coating has been used as a model process for roll-to-roll processes like slot-die coating. The devices with the structure ITO/PEDOT:PSS/PCDTBT:PC70BM/Sm/Al achieve a power conversion efficiency of 1.9%, which is higher than the efficiency of the equivalent spin coated devices. We further show that the efficiency is mainly limited by the low conductivity of the ITO/PEDOT:PSS electrode. The low conductivity leads to a reduced fill factor and current collection due to the higher series resistance of the cell. Another important factor determining the series resistance is the layout of the electrodes. An unoptimized geometry can decrease the efficiency by 20% due to limitations in current collection [3].
[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 40), Progress in Photovoltaics: Research and Applications, 20 (2012) 606-614.
[2] M. Manceau, D. Angmo, M. Joslash;rgensen, F.C. Krebs, ITO-free flexible polymer solar cells: From small model devices to roll-to-roll processed large modules, Organic Electronics, 12 (2011) 566-574.
[3] H. Jin, A. Pivrikas, K.H. Lee, M. Aljada, M. Hambsch, P.L. Burn, P. Meredith, Factors Influencing the Efficiency of Current Collection in Large Area, Monolithic Organic Solar Cells, Advanced Energy Materials, (2012) doi: 10.1002/aenm.201200254.

Molecular layer modification of metal oxides and metals has proven to be a very useful tactic in improving the charge transport properties of inorganic/organic interfaces in organic electronic devices. Such modifications have been employed to improve organic layer morphology near the interface, reduce energy barriers to charge transport at contacts, and improve open circuit voltages in hybrid metal oxide/organic photovoltaic (PV) devices. Zinc oxide (ZnO) is a high bandgap metal oxide often employed as an electron-selective contact in organic PV and light emitting diode devices and as an acceptor in hybrid PV devices. While interfaces containing ZnO have benefitted from monolayer modification, ZnO itself has displayed weak resistance to etching by protic modifiers such as carboxylic acids and phosphonic acids. This has necessitated the use of highly kinetic deposition processes (i.e. spin coating) that do not necessarily allow for optimal monolayer formation or control. Such interfaces would benefit from a similar metal oxide with improved etch resistance. We demonstrate that alloying of ZnO with magnesium oxide (MgO) to form Zn_1-xMg_xO alloys dramatically improves etch resistance without significantly deviating from the electronic properties of ZnO. To illustrate this, we have treated Zn_1-xMg_xO thin films of varying Mg concentrations (0 < x < 0.3) with the prototypical protic modifier benzoic acid (BA), while monitoring film thickness and UV absorption to determine the etch rate. We found that films containing 10% Mg showed an etch rate frac14; of that of ZnO, with further rate reductions as Mg content increased. Infrared absorption measurements showed that benzoic acid is present on the surface after soaking and rinsing, confirming layer formation and providing insights into the bonding mechanism. Zn_1-xMg_xO also shows enhanced resistance to etching by propiolic acid. The improved etch resistance of Zn_1-xMg_xO alloys allows for longer deposition times with protic modifiers compared to ZnO. This allows the attachment chemistry to approach equilibrium more closely than kinetic depositions resulting in a more controllable process with the potential for greater molecular density, improved monolayers, and enhanced control of surface properties. This study is currently being extended to other protic modifiers such as other carboxylic acids, phosphonic acids, and thiols. This work is supported by the NSF through DMR-0907409 and the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) through DMR-0820518.

Here in this study, we introduce a novel active layer structure of organic photovoltaics, what we called the inter-diffused ordered bulk heterojunction (IDOBHJ). The morphology of the IDOBH hybridizes those of the ordered bulk heterojunction (OBHJ) which possesses efficient charge transport pathways, and the disordered bulk heterojunction (DBHJ) which is beneficial for exciton separation. We calculated the power conversion efficiency of the IDOBHJ based on the Monte Carlo method. It was shown clearly that the performance of the IDOBHJ is better than those of OBHJ and DBHJ. We also confirmed this with experimental measurement on the power conversion efficiency of the IDOBHJ. Monte Calro method combined with first reaction method was used to simulate transports of excitons and charges. Also, the light absorption was simulated by solving the Maxwell&’s equations. The IDOBHJ shows around 14% higher power conversion efficiency than that of DBHJ which shows the optimum performance. This outperformance of IDOBHJ comes from 2% higher short circuit current density and 14% higher fill factor than those of DBHJ. We also experimentally demonstrated the superiority of the IDOBHJ. In our case, the active layer is composed of P3HT/C60. The P3HT was patterned by the nanoimprint technique. The IDOBHJ morphology was constructed by depositing C60 on the P3HT followed by the thermal annealing process. The IDOBHJ shows the PCE of 2.48% which is the twice of the PCE of pristine OBHJ. Our finding should serve as a useful tool to increase the power conversion efficiency by engineering active layer morphologies.

Transparent conductive oxides (TCO), especially ITO has been widely used in thin film optoelectronic devices. But there are technical and economic limitations for a large scale manufacturing of flexible devices. Recently, a triple layer electrode consisting of a thin metal layer and two embedding transparent metal oxide layers has been actively researched to replace ITO because the triple layer (Metal oxide/Metal/Metal oxide) can produce transparent, low resistant and low temperature processing.
We have developed a MoO₃/Au/MoO₃ (MAM) tri-layer structure as a transparent, low-resistance anode for use in organic solar cells. Transmittance was maximized at 82% using symmetric bottom and top MoO₃ layers (each of thickness 30 nm) either side of a 12 nm Au layer. Low sheet resistance (7.4 ohm per square) was resulted. The series resistance and optical transmission of devices employing these structures as anodes were tailored by varying the thickness of the top MoO₃ layer. The MAM tri-layer with 12 nm Au showed the highest figure of merit (18.08×10^-3 #8486;^-1) indicating that MoO₃ (30 nm)/Au(12 nm)/MoO₃(30 nm) was the optimal MAM structure. Dissolution of the top MoO₃ layer in PEDOT:PSS have to be addressed. A self-assembled monolayer of 16-phosphonohexadecanoic acid was deposited on the MoO₃ prior to spin-coating the PEDOT:PSS. Cells fabricated on PEDOT:PSS/SAM/MAM multilayer electrodes showed a power conversion efficiency of 2.33%, comparable to that of ITO-based organic solar cells. The PEDOT:PSS/SAM/MAM electrode was shown to be a promising replacement of ITO for use in low-cost optoelectronic devices.

An in-depth investigation of an inkjet-printed Ag nanoparticle grid combined with PEDOT:PSS of different conductivities as an alternative to an indium tin oxide (ITO)-based transparent anode for organic photovoltaic (OPV) applications will be presented. Based on the proposed current collecting grid a record power conversion efficiency (PCE) of 2.5 % for ITO-free organic solar cells has been achieved.
ITO is one of the most expensive components of the OPV devices and the need for inexpensive alternative transparent electrodes is imminent. A promising alternative to this is the integration of PEDOT:PSS, a widely-studied transparent conductive buffer layer, with inkjet printed Ag-nanoparticle grid lines. Using this method, ITO-free P3HT:PCBM based OPVs presenting limited PCE to 1.47 % have been up to now reported in the literature.[1]
During the presentation the design Ag-grid requirements and inkjet printing processing challenges will be described in details as well as our future concepts towards fully sustainable inexpensively produced OPVs. ITO transparent electrode was efficiently replaced by inkjet printed Ag-nanoparticle current collecting grid lines embedded in low cost-conductivity PEDOT:PSS. Morphological images draw attention to the high importance of sintering process and are closely correlated to overall OPV performance. This performance was further enhanced by establishing a mild thermal sintering process achieving not only high grid line conductivity but desirable printed line shape as well, by avoiding the “coffee ring” effect [2]. Printed grid shape and pattern was then optimized in such a way that it ensures efficient photogenerated current collection. Furthermore performed measurements revealed higher light transmittance in almost all the visible range for the printed grid based devices when compared to different thicknesses of sputtered ITO. To summarise the presentation will focus on our experimental results to achieve a record PCE of 2.5 % for ITO-free OPVs using the combination of PEDOT:PSS/inkjet printed nanoparticles based current collecting grids as well as future trials for even higher PCE values [3].
Acknowledgements: This work was co-funded by the EU Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Strategic Infrastructure Project ΝEpsi;A ΥΠΟΔΟΜΗ/ΣTau;ΡATau;Η/0308/06).
[1] Y. Galagan et al, Current Collecting Grids for ITO-Free Solar Cells, Adv. Energy Mater. 2012, 2, 103.
[2] M. Neophytou et al., Highly efficient indium tin oxide-free organic photovoltaics using inkjet-printed silver nanoparticle current collecting grids, Appl. Phys. Lett. 2012, 101, in print.
[3] M. Neophytou et al, 2.5 % Efficient ITO-Free Inkjet Printed P3HT:PCBM based OPVs, to be submitted, Advanced Functional Materials (2012).

Alternative energy technologies must become more cost effective to achieve grid parity with fossil fuels. Dye sensitized solar cells (DSSCs) are an innovative third generation photovoltaic technology, which is demonstrating tremendous potential to become a revolutionary technology due to recent breakthroughs in cost of fabrication. The study here focused on quality improvement measures undertaken to improve fabrication of DSSCs and increase process efficiency and effectiveness. Several quality improvement methods were implemented to optimize the seven step individual DSSC fabrication processes. Lean Manufacturing&’s 5S method successfully increased efficiency in all of the processes and Six Sigma&’s DMAIC methodology was used to identify and eliminate each of the root causes of defects in the critical titanium dioxide deposition process. These optimizations resulted with the following significant improvements in the production process: 1. fabrication time of the DSSCs was reduced by 54 %; 2. fabrication procedures were improved to the extent that all critical defects in the process were eliminated; 3. the quantity of functioning DSSCs fabricated was increased from 17 % to 90 %.

For last decades, silicon solar cell has been researched to improve power-conversion-efficiency (PCE) through surface texturing, anti-reflection coating, selective emitter, back contact cell, and metallization. The silicon solar cell fundamentally absorbs the incident light of wavelength between 300 and 1100 nm. In particular, the reflectance of the surface of silicon-solar-cell exhibited very high in UV wavelength region (250 ~ 425 nm), indicating that the absorption amount of silicon solar cells is almost nothing in the UV wavelength region. Thus, it is necessary to implement a new concept in silicon solar cells to enhance more light absorption in the UV wavelength region. Our proposed idea is to introduce the energy-down-conversion through core-shell-structure quantum-dots, e.g. CdSe core-shell coated by ZnS capping layer, in silicon solar cells to enhance the light absorption amount. The concept of energy-down-conversion follows the absorption of UV light through core-shell quantum dots, the emission of green or red light through core-shell, and the absorption of green or red light into silicon solar cells. The core-shell quantum dots were synthesized by hot injection method and coated on silicon nitride layer by spin-coater after firing process. The dependency of molecular weight of core-shell quantum dot on the light absorption enhancement via EDC was investigated. The optimized molecular weight of core-shell quantum will be correlated with the maximum photo-voltaic performance. In our study, we investigated the evidence of the energy-down-conversion via core-shell quantum dots (ZnS/CdSe) and the effect of weight percent of core-shell quantum dots on power-conversion-efficiency of silicon solar cell. We confirm UV absorption range of synthesized core-shell quantum dots by UV-visible absorption and photoluminescence spectroscopy. Also, we confirm the reflectance of the silicon solar cell in UV wavelength region decreases 20% resulting from the optimized molecular weight of core-shell quantum on silicon nitride layer. Thus, it results in the increase of 35 % in the external quantum efficiency (EQE) of the silicon solar cell in UV wavelength. Finally, the power-conversion-efficiency has been increased 0.9% compared to that of reference without core-shell quantum dots. * This work was financially supported by the grant from the “Next-generation Substrate technology for high performance semiconductor devices (No. KI002083)” of the Ministry of Knowledge Economy (MKE), the Brain Korea 21 Project in 2012 and SiSoC (Silicon Solar Consortium), an Industry/University Cooperative Research Center under NSF Grant.

The total thickness of the active components of organic electronic devices, including organic solar cells, is typically less than 500 nm. For this reason, the mechanical properties of weight and flexibility are almost entirely determined by the device substrate. Here we demonstrate polymer based photovoltaic devices on plastic foil substrates less than two micron thick, with equal power conversion efficiency to their glass-based counterparts. They can reversibly withstand extreme mechanical deformation and have unprecedented solar cell specific weight. Instead of a single bend, we form a random network of folds within the device area. The extreme flexibility of the device allows them to function as stretchable solar cells when adhered to a prestretched elastomeric support. These ultrathin organic solar cells are over ten times thinner, lighter, and more flexible than any other solar cell of any technology to date.

Polymer solar cell (PSC) has attracted considerable attention over last two decades featuring light weight, low cost, solution processibility, flexibility, semi-transparency, etc. Significant breakthrough of Efficiency of 9.2% and >11.4% has been reported in academic journal and announced by company respectively in 2012. There is no ending quest for developing novel materials in PSC applications. In this presentation, we&’ll present a newly developed conjugated polymer: poly(cyclopentadithiophene-alt-isoindigo) in applications of both single junction solar cells and tandem solar cells with promising efficiency.
Isoindigo is a green renewable raw material and available from plant. We will first demonstrate a series of soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as donor unit and isoindigo (I) as acceptor unit, decorated with two kinds of side chains, octyl (8) and 2-ethylhexyl (e). They are denoted as PCeI8, PC8Ie, PC8I8 and PCeIe respectively. The molecular structure effect of the side chain on the optical, electrochemical and photovoltaic properties have been quantitatively investigated and will be demonstrate systematically. The photovoltaic characteristics of open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) are critically dependent on the different side chains, leading to the variations of 0.69~0.82 volts, 8~13 mA/cm2, 0.43~0.56 and 3.5~5.6 % respectively. The PCE of 5.6% has been achieved based on the PCeIe/PC71BM bulk heterojunction in inverted device structure. (Voc=0.82 volts, Jsc= 12 mA/cm2, FF=0.56)
The PCeIe conjugated polymer is of interest in tandem cell application because of its absorption band. It reveals an extended absorption onset and maximum of 855 nm and 767 nm respectively with band gap around 1.4 eV. These values result in the small overlapped absorption range with that of the front cell materials such as P3HT or PCDTBT. As compared to the PBDTT-DPP which was adopted by Yang&’s group reporting the highest efficiency of 8.3%,[1] the absorption band of PCeIe is very similar to that of PBDTT-DPP. However, owing to the deeper highest occupied molecular orbital (HOMO) level of PCeIe, the PCeIe/PC71BM exhibits higher Voc of 0.82 volts than that of PBDTT-DPP of 0.74 volts. As a result, the adoption of PCeIe as the rear cell material shows great potential in further improving the PCE higher than 8.6%. Up to date, the best tandem solar cells using PCeIe as the rear cell material shows PCE of 7.0 % with Voc=1.63 volts, Jsc=7.7 mA/cm2 and FF = 0.56. As we expect the Voc is higher by 0.7 volts than the reported value of 1.56 volts. With further device optimization, particularly the inter-connection layer, we expect the PCE of PCeIe based tandem cell would be significantly improved.
[1] Dou, et al. Nature Photonics, 6, 180.

Great advances have been made in the development of conjugated polyelectrolytes (CPEs), which provide tunable properties including water solubility and processability, main-chain exciton and charge transport, variable energy light absorption and fluorescence, non-covalent interactions, and formation of tertiary structures via self-assembly.[1] These characteristics allow CPEs to be considered for use in various optoelectronic applications, including photovoltaics. Our ability to control a desired characteristic, however, is limited by the current knowledge available in terms of structure-property relationships (e.g. optical, mechanical, and electronic properties).
In this talk, we will discuss our recent efforts toward understanding various aspects of CPEs at the molecular and polymeric level that determine the feasibility of our systems to work as light harvesting components of photovoltaic systems. From the creation of single molecules to oligomers and, eventually, polymers, the variability with which we approach the design of our systems will be discussed, along with their electronic properties and photophysical aspects. We will illustrate examples of molecular and polymeric systems based on benzotriazole (BTz) and benzothiadiazole (BTD), and we will show how, through chemical manipulation, we are able to combine physical properties of both acceptors for the preparation of low energy light-absorbing systems bearing carboxylate groups.[2] These studies led to the further exploration of fused D-A molecules that enable good charge separation which can be tuned by changing the flanking position of the π-electron donors. The results obtained from the fused D-A systems motivated the preparation of polymers as extended chromophores, which effectively delocalize the generated cation after electron injection into a semiconducting oxide. This property makes these materials potentially useful in photoelectrochemical cells. Finally, we will overview the utilization of hybrid systems based on ruthenium-based electrolytes pending from rod-like conjugated polymers and highlight the interplay between frontier orbital energies of the chromophoric units and the resulting kinetic events ensuing photoexcitation.[3]
[1] Jiang, H.; Taranekar, P.; Reynolds, J. R.; Schanze, K. S. Angew. Chem. Int. Ed. 2009, 48, 4300.
[2] Patel, D. G.; Feng, F.; Ohnishi, Y.-Y.; Abboud, K. A.; Hirata, S.; Schanze, K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2012, 134, 2599.
[3] Wang, L.; Puodziukynaite, E.; Vary, R. P.; Grumstrup, E. M.; Walczak, R. M.; Zolotarskaya, O. Y.; Schanze, K. S.; Reynolds, J. R.; Papanikolas, J. M. J. Phys. Chem. Lett. 2012, 3, 2453.

The organic donor-acceptor blend P3HT:PCBM is used in both organic photovoltaics (OPV) and photodetectors (OPD) where the conjugated polymer poly(3-hexylthiopene) acts as the electron donor (hole transporter) while the soluble fullerene derivative PCBM is the electron acceptor. We investigate the influence of the addition of CuInS2 nanocrystals (NCs) on the optoelectronic properties of this blend. CuInS2 NCs have been synthesized by reaction of indium acetate and cupper iodide in dodecanethiol (DDT), acting as the solvent, sulfur source and surface ligand. OPV and OPD devices have been fabricated by spin-coating a mixture of P3HT, PCBM and NCs from dichlorobenzene solution on ITO-coated substrates covered with a PEDOT: PSS layer. Top Al contacts have been evaporated under vacuum. The NCs used in the devices were capped either by the synthesis ligand DDT or by the shorter molecule ethylhexanethiol (EHT) introduced through ligand exchange.
The surface ligands play a decisive role in the device characteristics. In OPD configuration (under reverse bias) the best photoresponse was obtained with the initial surface ligands whereas the use of NCs capped with EHT led to a strong increase of the dark current. Addition of NCs with DDT to the organic blend resulted in a significant increase of the external quantum efficiency (EQE). Values up to 55% were obtained in the range of 450-550 nm under -1V bias, while the P3HT:PCBM device without NCs yielded a maximum of 30% in the same conditions.
In case of OPV configuration, adding CuInS2 NCs capped with EHT permitted to increase the efficiency of the solar cell from 0.8% for pure P3HT:PCBM to 1.6% (active area of 0.07 cm2), while NCs capped with initial DDT ligands resulted in only 0.24%. The efficiency increase when using CuInS2-EHT was mainly due to the improvement of the short-circuit current density, which was two times higher than in the case of P3HT:PCBM. At the same time a strong increase of EQE of around 30% was observed. The ligand exchange influences the morphology of the hybrid thin film as revealed by cross-section SEM imaging: initial DDT ligands lead to a homogeneous dispersion of the NCs in the organic matrix, while EHT induces a segregation of the NCs towards the top of the film. These morphology changes strongly influence the charge extraction in the different device configurations. We will correlate the observed device characteristics with the charge carriers mobility in the various devices, obtained by CELIV (current extraction by linear increasing voltage) measurements.

One of the essential aspects in designing efficient and stable organic optoelectronic devices is the engineering of interface carrier transport layers between the active layer and electrodes. [1-7] Here, we propose a self-assembly and solution-processed method to fabricate an electron transport layer by titanium oxide nanoparticles (TiO2 NPs). The method can simultaneously achieve the film features of good uniformity, homogeneity, and electron transport properties as well as capable for large-area applications. Distinguished from conventional spin-coating process, our approach imposes good control on the evaporation rate of the solvent, which allows TiO2 NPs to undergo a slow self-assembly process. Investigation of surface morphology by scanning electron microscope (SEM) and atomic force microscopy (AFM) indicates that a TiO2 films with ordered and uniform TiO2 nanocrystals are formed during the self-assembly process. Device studies with TiO2 as electron transport layers in inverted polymer solar cells (PSCs) demonstrate that the self-assembly approach can lead to enhanced electron collection compared with films deposited from spin-coating (average PCE improved from 3.65% to 4.03%). To demonstrate that the self-assembly method are capable of large-area applications, we make self-assembled TiO2 films with size of 4”×4” on ITO glass substrates. We utilize different regions of the film to fabricate PSC devices. These self-assembled devices are found to have similar improvement in device performance as well, and have good uniformity (average PCE of 3.91±0.1%). The method introduced here is efficient, applicable to large area, and easily controlled and implemented, therefore can be advantageous for low-cost and high-throughput applications.
[1] J. Y. Kim, S. H. Kim, H. H. Lee, K. Lee, W. Ma, X. Gong, A. J. Heeger,. Advanced Materials, 2006, 18, 572.
[2] J. S. Huang, C. Y. Chou, M. Y. Liu, K. H. Tsai, W. H. Lin, C. F. Lin, Org. Electron., 2009, 10 1060.
[3] J. Meyer, R. Khalandovsky, P. Görrn, A. Kahn, Adv. Mater., 2011, 23, 70.
[4] M.H. Park, J. H. Li, A. Kumar, G. Li, Y. Yang, Adv. Funct. Mater., 2009, 19 1241.
[5] S. K. Hau, H. L. Yip, N.S. Baek, J. Zou, K. O'Malley, A.K.Y. Jen, Appl. Phys. Lett., 2008, 92, 253301.
[6] C. D. Wang, W.C. H. Choy, Sol. Energy Mater. Sol. Cells, 2011, 95, 904.
[7] D. Zhang, W.C.H. Choy, F. X. Xie, X. Li, Org. Electron., 2012, 13, 2042.

The formation of excitons with high binding energies, rather than free carriers, upon light absorption is a major barrier to high efficiencies in organic solar cells (OSCs) relative to inorganic solar cells. Due to their low dielectric constant, organic semiconductors have high exciton binding energies as compared to conventional inorganic semiconductors, which results in low exciton dissociation and power conversion efficiencies. In order for excitons to dissociate, the lowest unoccupied molecular orbital (LUMO) offset between the donor and acceptor materials must be greater than the exciton binding energy; this often necessarily large LUMO offset results in significant losses in OSCs. We have developed a simple method to control the dielectric constant of B,O-chelated azadipyrromethene (BO-ADPM) donor films by blending BO-ADPM with high permittivity camphoric anhydride (CA) to form BO-ADPM:CA donor films. The dielectric constant of the donor film can be controlled via the ratio of BO-ADPM to CA in the film. Through spectroscopic methods, we demonstrated that the increased dielectric constant in the donor film reduced the exciton binding energy. To observe the effects of higher dielectric constant donor films in OSCs, we fabricated planar heterojunction solar cells with BO-ADPM:CA as the donor and fullerene C60 as the acceptor. We observed that as the permittivity increased and the exciton binding energy was reduced, the internal quantum efficiency of the device increased in the BO-ADPM active region of the spectrum. Our findings suggest organic semiconductors with high permittivity have great potential in improving the power conversion efficiency of organic solar cells.

In order to surpass the Shockley-Queisser limit for single junction photovoltaic cells, novel types of physical phenomena are being considered. In particular, multiple exciton generation and singlet fission are processes that can transform one high energy photon into two or more excitons. If these excitons can be efficiently converted into electron-hole pairs, then solar energy conversion efficiencies could be increased by up to 30%. For organic materials, where singlet fission creates a pair of triplet excitons, most schemes concentrate on the direct ionization of the triplets by electron acceptors like fullerenes or inorganic nanoparticles. Our group has taken a different approach by using singlet fission materials as sensitizers to create excitons in a neighboring inorganic semiconductor through interfacial energy transfer. This mechanism avoids injecting charges into the organic layer, but requires triplet Frenkel excitons produced by singlet fission to transfer their energy to Wannier excitons residing in the silicon. In order to assess the feasibility of this approach, we deposit thin films of tetracene, a high efficiency singlet fission material, on clean 100 silicon surfaces, with an intermediate spacer layer of variable thickness. We measure the lifetimes of both singlet and triplet excitons using time-resolved photoluminescence. We find evidence for strong singlet quenching by the silicon via a through-space interaction that extends over nanometer lengthscales. There is also evidence for quenching of the triplet excitons via a contact mechanism. Measurements are preformed as a function of both tetracene and spacer layer thickness. We conclude that it is possible to transfer energy from excitons created in the organic layer to the silicon substrate, but further work on engineering the interface between the two materials will be necessary to enhance efficiencies.

Three important aspects for organic photovoltaics (OPV) to be a commercially viable technology are: Rate of Return, Life Cycle Payback, and Levelized Cost of Energy. Life Cycle Payback contributes to a reasonable return on investment, both monetarily and in energy-payback terms. As such, any commercial OPV product will likely need to be based on the inverted architecture, in order to provide sufficient lifetimes for favorable Life Cycle Payback. In the inverted OPV architecture, the hole-transport layer (HTL) material, often PEDOT:PSS, must be on top of the bulk heterojunction (BHJ) in order to alter the work function of the silver top electrode to allow efficient transport of holes, and minimize migration of electrons, into the silver. Coating of high surface tension aqueous PEDOT:PSS dispersions on top of the relatively low surface energy BHJ is problematic, however. We explore a number of ways to either increase the surface energy of the prototypical P3HT:PCBM BHJ film or decrease the surface tension of the PEDOT:PSS dispersion in order to allow reproducible coating of the HTL material on top of the BHJ, and examine the effect of these methods on the OPV device performance.

Quantum dot solar cells have attracted significant attention due to the quantum confinement effect characterizing the quantum dots, which may push their theoretical thermodynamics efficiency up to 44% . PbS quantum dots sensitized solar cells with multiple exciton generation (MEG) effects have already been confirmed by Parkinson&’s group . Moreover, the tunable bandgap of quantum dots and their high extinction coefficient make it be a promising sensitizer for dye sensitized solar cells. The efficiency of PbS quantum dots sensitized solar cell is still low, which is ascribed to serious charge recombination at the interface between quantum dot and polysulfide electrolyte . Appropriate passivation reduces charges recombination and increases cells efficiencies , . In this study, we employed PbS/CdS (core/shell) quantum dots on a mesoporous TiO2 surface. Compared with the one using PbS, the PbS/CdS sensitized solar cells show highly enhanced efficiencies. The CdS shell plays an important role to prevent carrier recombination, which reduces cell efficiency. The charge recombination resistance obtained from electrochemical impedance measurement indicates an electron lifetime 1-2 order of magnitude higher for PbS/CdS than for PbS. Moreover, ZnS was further deposited on PbS/CdS sensitized solar cells as a barrier layer by successive ionic layer adsorption and reaction (SILAR) method. After covered with thin ZnS passivation layers, cells&’ efficiencies got 30 times higher than the one using PbS as absorber.

ZnO is a versatile cathode buffer layer for organic photovoltaics (OPV) due to its remarkable optical and electronic properties. Here we demonstrate a low temperature synthesis through zinc acetate gel decomposition. We find that with this method, low temperature annealing at 300 Celsius actually leads to higher device performance than using higher temperatures, where higher annealing temperatures have been anticipated to result in improved crystallinity, electron mobility and thus better cell performance. This reduced temperature optimum is found to stem from the ZnO film surface quality which affects the contact electrical resistance and the modification of the ITO work function which influences the open circuit voltage and leakage current.

Nanoparticle organic photovoltaics (NP-OPVs) are an emerging area of research which offer the prospect of water-processability as well as the ability to control semi-conducting polymer and blend morphology on the nano-scale. In order to realize the potential for nano-scale control of these films we must first develop an understanding of the factors which affect morphology in these devices. We report a morphological study of the poly(3-hexylthiophene) (P3HT) / phenyl C61 butyric acid methyl ester (PCBM) system in nanoparticle organic thin films. The morphology and chemical composition of P3HT:PCBM nanoparticles has been investigated using scanning transmission x-ray microscopy (STXM) for a range of P3HT molecular weights in both the unannealed and annealed states. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to support the STXM data. We find the common morphology of these P3HT:PCBM nanoparticles to be core-shell, with a PCBM-rich core and P3HT-rich shell region for the range of P3HT molecular weights tested. When a thermal annealing treatment (140°C for 4 minutes) is applied to these nanoparticles and nanoparticle films we find that the morphological changes which occur depend highly upon the polymer molecular weight. Nanoparticles containing low molecular weight P3HT begin to undergo gross phase segregation. At higher P3HT molecular weight this gross phase segregation is not evident; however there is a more subtle change in morphology with the nanoparticle P3HT shells joining to form a continuous P3HT network. This study is key to the creation of effective nanoparticle organic thin film morphology for device applications and acts as a stepping stone in the optimization of annealing conditions for such devices. Furthermore, it highlights the dramatic effect that molecular weight and post-film deposition treatments have on final film morphology.

Organic photovoltaic cells are advantageous over conventional inorganic solar cells in the aspects of light-weight, flexibility, and low-cost fabrication. Besides the photoelectric conversion efficiency, cell stability is another important issue for practical applications. Encapsulations with low permeation rates have been achieved by alternating depositions of multilayer organic-inorganic thin films. However, this method requires complex deposition architecture and long processing time that could obstacle the practical integration to photovoltaic devices. In this study, we demonstrate an efficient single-layer encapsulation technique for organic photovoltaic cells.
The single-layer hybrid encapsulation thin film is deposited from a gas mixture of hexamethyldisiloxane and oxygen by plasma-enhanced chemical vapor deposition at room temperature. An average optical transmission in the visible light regime of > 90% and a water vapor transmission rate of 3.6×10^-6 g/m²-day is obtained for a 1.5 mu;m-thick layer. Inverted-type organic photovoltaic is used to evaluate the effectiveness of this encapsulation thin film. Efficiency decay is not observed in the cell coated with this encapsulation layer after 3000-hour exposure to the air; on the contrary, the un-encapsulated counterpart degrades rapidly and completely fails after 120-hour exposure. The result shows that this single-layer hybrid encapsulation can greatly improve the lifetime of organic photovoltaic cells.

We fabricated the hierarchical TiO2 electrodes via bottom-up growth. We have implemented a hierarchical growth strategy in which two stages of controlled growth yielded first macroscale TiO2particles, followed by mesoscale TiO2 particles. The hierarchical electrodes were then applied as an electrode for quasi-solid-state dye solar cells. We investigated the effect of TiO2 particles as a nano-filler in polymer gel electrolyte (PGE) based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The energy conversion efficiency of the cell using PGE was 5.07%. When TiO2 particles were introduced, the efficiency increases with increasing content of TiO2 particle up to 5.67%. Especially, TiO2 nano-particles were proved to be an efficient nano-filler in enhancing the current density in accordance with the increase of ion diffusivity, the rate of charge transport and the decrease of the resistance in TiO2/electrolyte interfaces. The EIS analysis was achieved to investigate the interfacial resistance in quasi-solid-state cells. The performance of the hierarchical electrodes was compared to the conventional mesoporous electrodes. The photo-current density was much more increased when used the hierarchical electrodes from 13.74 to 15.41mA/cm2 (increased about 12.15%) compared to the NP-TiO2 electrodes from 9.31 to 9.36mA/cm2. The size of the pores in each electrode was influence on the infiltration of the PGE into electrodes, leading to the difference of the increase in photo-current density.

Organic materials have attracted much attention due to their potential applications in organic electronics. In this regard, charge-transporting properties of organic thin films have found to be important in organic transistors, whereas excellent charge separation and charge-transporting properties of organic thin films are essential for organic photovoltaics. In this talk I will highlight some of our recent examples of novel organic materials including conjugated polymers and carbon nanostructures for bulk heterojunction solar cells and related systems. In particular, the first application of separated fullerene bisadduct regioisomers to polymer solar cells will be presented. 1) J. Am. Chem. Soc. 2009, 131, 3198; 2) J. Phys. Chem. C (Feature Article) 2009, 113, 9029; 3) J. Phys. Chem. Lett. (Perspective) 2010, 1, 1020; 4) Adv. Mater. 2010, 22, 1767; 5) J. Am. Chem. Soc. 2011, 133, 7684; 6) Angew. Chem. Int. Ed. 2011, 50, 4615; 7) J. Phys. Chem. C 2012, 116, 1256; 8) J. Phys. Chem. C 2012, 116, 17414; 9) J. Phys. Chem. Lett. 2012, 3, 478; 10) Chem. Commun. (Feature Article) 2012, 48, 4032; 11) Chem. Commun. 2012, 48, 8550.

Dye-sensitized solar cell has a unique property such as transparent and colorful characteristics,. compared to other type of solar cells. Recently, plastic substrate based DSSCs are able to retain the transparency like glass based ones, in addition to this advantages, and hence it is promising as the light weight, flexibility, and roll-to-roll process. However, conventional high temperature fabrication technology for glass based DSSCs, cannot be applied to flexible devices because polymer substrates cannot withstand the heat more than 150 oC. Therefore, low temperature fabrication process, without using a polymer binder or thermal sintering, was required to fabricate necked TiO2. Besides, the photovoltaic performance, good mechanical property is also an important factor as a requirement for the flexible solar cells. In order to address these issues, we developed polymer/TiO2 nanocomposite electrodes, which can be applicable to plastic DSSCs. Poly(methyl methacrylate) (PMMA) were coated on the dye-sensitized TiO2 photoelectrode by spin-coating method to construct hybrid polymer- TiO2 electrode. The concept of composite electrode takes an advantage of utilizing elastic properties of polymers, such as good impact strength. We will report photovoltaic performance and mechanical properties of these plastic DSSCs

Bipolar organic devices, such as solar cells and light emitting diodes, are particularly susceptible to atmospheric gases and need ultra-hermetic protection. In rigid devices, plate glass in conjunction with desiccants and edge seals provides this protection. Future flexible devices will need flexible (yet optically clear) permeation barriers. For this purpose effective inorganic/polymeric multilayers and inorganic-polymeric hybrid monolayers have been developed. Studies of water permeation into devices encapsulated with such barriers reveal three mechanisms of intrusion: (i) through the bulk of the barrier, (ii) along defects that penetrate the barrier, and (iii) along interfaces that the barrier forms with the substrate or other device materials. We have been developing and evaluating new barrier materials. Their categorization by quality relies on the unambiguous identification of the contribution from just bulk diffusion. Therefore we have been measuring the diffusion coefficient of water in barrier layers, using two techniques that are sensitive only to bulk diffusion. One technique is the dependence of mechanical stress in the barrier layer on the exposure time to water. The other technique is the equivalent dependence of the electrical capacitance of the barrier layer, when placed as the dielectric between two parallel-plate electrodes.
Samples are made by depositing (silicon dioxide - silicon) hybrid layers by plasma-enhanced oxidation of hexamethyldisiloxane vapor. Layer thicknesses range from 100 nm to 2 micrometers. Silicon wafer substrates are used for the stress measurements, and chromium-coated glass slides for the capacitance measurements. The capacitance samples also receive Cr top electrodes. The two principal characterization techniques are measurements of the radius of curvature of the layer-on-Si samples, and of the capacitance at 1 MHz of the capacitor samples. These measurements are first made on the as-prepared samples, which then are boiled in water for times of up to several days. Permeation of water into the layers produces curvature in the layer-on-silicon samples, and raises the capacitance of the capacitors. The curvature change reflects increasing compressive stress in the layer. Analyzing layer stress and capacitance in function of both, layer thickness and exposure time to boiling water, shows that water sets up a complementary error function diffusion profile in the layers. For the samples tested in this series we extract a coefficient of diffusion for water at 100C of 5x10^-15 cm2/s. We estimate that the two techniques can be used to measure diffusion coefficients that are even lower, by about two orders of magnitude.

Perylene diimide (PDI) and perylene monoimide (PMI) belongs to the class of n-type semiconductors with high molar extinction coefficient and high electron mobility. Significant research efforts have been focused on the modification of perylene substituted molecules to improve their optoelectronic and charge transport properties. Design and synthesis of advanced materials using perylene chromophore have been an active area of research in the past few years. Unique light-harvesting and redox properties on conjunction with high thermal stabilities of perylene dyes offer potential applications in organic photovoltaics. A series of multi donar substituted perylene dyes were designed and synthesized. The photophysical, electrochemical and theoretical calculations of the synthesized compounds were investigated. The structure property relationships were studied to show the effect of position and number of substituents on perylene core unit. All molecules showed a broad absorption up to NIR region. Corresponding anhydrides of perylene substituted molecules were used for fabrication of dye-sensitized solar cells.
Acknowlegement: The authors acknowledge technical and funding support from National University of Singapore (NUS) and AK thanks NUSNNI-NanoCore for a PhD scholarship.
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Thin-film organic solar cells (OSCs) have gained intensively interests for the next generation of solar cells. However, thickness of the active layer in OSCs is typically a few hundred nanometers or less due to their short diffusion length of exciton, which sets a fundamental limitation on light absorption. Consequently, improvement in light trapping is very important for their practical utilization in photovoltaics. Photonic crystal structure, due to its strong modification of electromagnetic distribution, has attracted lots of attention and can be used to increase solar cell efficiency based on different physics mechanisms[1-4]. Recently, OSCs with a PC active layer have been studied theoretically[5,6] and experimentally[7]. But we notice that column shaped photoactive layer has relatively low absorption rate due to its abrupt change of optical impedance. Motivated by this issue, we present designs of OSCs incorporating periodically arranged gradient type active layer, in which normal incident light couples adiabatically with laterally scattered light and thus higher absorption performance can be achieved. These designs can enhance light harvesting by the patterned organic materials themselves (i.e. self-enhanced active layer design) to avoid degrading electrical performances of OSCs in contrast to introducing inorganic concentrators into OSC active layers such as silicon and metallic nanostructures. Geometry of the OSC is fully optimized by rigorously solving Maxwell&’s equations with fast and efficient scattering matrix method[8]. Optical absorption is accessed by a volume integral of the active layer excluding the metallic absorption. Our numerical results show that the OSC with a self-enhanced active layer, compared with the conventional planar active layer configuration, has broadband (400nm-800nm) and wide-angle range absorption enhancement due to better geometric impedance matching and prolonged optical path[9]. This work provides a theoretical foundation and engineering reference for high performance OSC designs.
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For rather a long time, there has been increased interest in making polymer/inorganic composite materials. The composite materials synthesized through blending, interfacial interaction, in situ polymerization and other methods which can offer us different and optimized properties in thermal, mechanical, biomedical, and energy application. Here we present a “grafting-from” method to grow polymer hole conductors on the silica nanoparticles by surface-initiated nitroxide-mediate polymerization ((SI-NMP). We apply this novel composite material in the application of solid-state dye-sensitized solar cell (ss-DSSC) with a dual function: transporting holes effectively and providing light scattering in potential.

We synthesized highly stable ultrasmall PbS NQDs, of which diameter is down to 1.5 nm, and found a transition in the air-stability as the NQD size increases. X-ray photoemission spectroscopy and density functional theory reveal that the stability transition is closely associated with the shape transition driven by steric hinderance and thus size-dependent surface energy of oleic terminated Pb-rich surfaces.

Recently, organic photovoltaic (OPV) cells have attracted attention because of low production cost via simple PV cell structure and flexibility for mobile application. However, OPV cells have not demonstrated sufficient power conversion efficiency (PCE) for replacing another type of PV cell such as Si, thin, III-V PV cell etc. Thus, many researchers have put their efforts to enhance the PCE of OPV. In our study, we investigated the surface plasmon resonance on the light absorption PCE enhancement, depending on the surface morphology of Ag nano-particles. In this experiment, the cells were composed vertically 150-nm transparent electrode (ITO:10 Omega;/cm2), Ag nano-particles, PEDOT-PSS(20-nm), P3HT:PCBM blend layer (2wt%), 6-nm BCP(exciton and hole blocking layer), and 80-nm cathode electrode (Al). Ag nano-particle size and density were controlled by Ag thin film thickness. Ag film were thermally evaporated and cells were annealed at 140oC for 10 min for PEDOT:PSS and at 125oC for 10 min for P3HT:PCBM blend. As a result, morphology trend of the Ag nano-particle changes from spherical to oblate shape form, size of the Ag nano-particles increased when the Ag layer thickness increases. The aspect ratio (height/diameter) of Ag nano-particles decreased with increasing Ag film thickness during evaporation. The light absorption was maximal enhanced at a specific size of Ag nano-particle; I.E 32% at 28 nm approximately in size. In particular, through external-quantum-efficiency analysis, the light absorption enhancement induced by surface Plasmon resonance was found at approximately 340 and 480 nm in wavelength. We calculated optical loss obtained from transmittance and reflectivity. The optical loss for the polymer solar-cell implemented with Ag nano-particles rapidly decreased up to approximately 38 nm when the Ag nano-particles size increases. Then, it slightly increased with the Ag nano-particles size. The light absorption enhancement rapidly increased with the Ag nano-particles size up to ~ 28 nm, and then slightly decreased. The photo-voltaic performance for the polymer-cell solar-cell implemented with Ag nano-particles was well correlated with the enhanced light absorption. At the optimized shape, size and density of Ag nano-particles, the power-conversion-efficiency was enhanced by 32% for polymer solar-cell. The detail mechanism about the light absorption enhancement induced by surface Plasmon resonance of Ag nano-particles will be discussed. * This work was supported by energy R&D program (20093021010010) under the Korea Ministry of Knowledge Economy (MKE) and the Brain Korea 21 Project in 2012

Polymer-nanocrystals (NCs) hybrid solar cells have attracted attention due to their solution based low cost fabrication and ability to control the bandgap and shape. Typical polymer-NCs solar cells have the bulk hetrojunction (BHJ) structure of polymer-NC layer sandwiched between an ITO anode and metal cathode. It has been reported that tetrapods can provide improved electron extraction pathways compared with nanorods and quantum dots in photovoltaic devices, due to the 3-dimensioal structure of tetrapods. However, the surfactant on the surface of NCs, such as oleic acid, causes poor performance of solar cells due to their long insulating alkyl chains. Replacing the long alkyl chain with shorter one deteriorates the colloidal stability of NCs, resulting poor BHJ morphology and reliability.
Here, we report novel inverted-structured solar cells where the active layer of P3HT:CdSe tetrapods BHJ structure was fabricated with the two-step process. First, porous CdSe tetrapod film was formed by spin-casting on top of the ZnO layer which was coated on the ITO/Glass. The surface of the CdSe tetrapod film was replaced with ligands such as hexylamine or pyridine by additional spin-casting. Then the BHJ structure of P3HT:CdSe tetrapods was constructed by spin-casting P3HT solution on the CdSe tetrapod film. We have investigated the P3HT:CdSe tetrapod film morphology and device characteristics for various sizes of CdSe tetrapods.

Organic solar cells show great promise in providing the necessary balance of low-cost and efficiency needed to make affordable solar energy. However most of the issues about low performance in both lifetime and efficiency are still need to be thoroughly addressed at the intrinsic length scale of the opto-electrical processes involved in the generation of free charge. Moreover, a complete understanding of the whole process requires measuring and correlating several complementary physical properties. For example, how the morphology or the molecular order are related to the induced photocurrent and how these relations change due to the degradation of the material. We have developed two different experimental approaches to achieve a simultaneously bi-tridimensional mapping of several physical mesoscopic properties of OPV film. In the first configuration we measured in parallel hyper-spectral Tip Enhanced Raman Spectroscopy, local photo current and topography. This enables us to correlate topography, local chemistry and local photo current generation simultaneously. We observed a strong correlation between the photocurrent and the intensity of the Raman signal. However the opto-electrical spatial resolution of this technique, around 100 nm, is not sufficient for several specific studies. So, in the second approach we employ optical antenna, fabricated into the bottom electrode of the PV device to excite organic PV material locally (near field optical spot < 15nm in diameter) while the photocurrent is measured with a conductive Atomic Force Probe. This idea has only been made feasible recently with the concept of optical antennae allows to characterized the PV materials at their critical scale below the 10 nm.

The charge-generating mechanism in bulk heterojunction, organic photovoltaic devices is dominated by exciton dissociation at the interface formed between the donor and acceptor species. The mechanism assumes that the donor is the dominant light absorber in the system, with diffusion of the exciton to the interface limited by its lifetime and the diffusion length. But is this the only route to generating charges in these systems?
This presentation will examine an alternative energy transfer mechanism in a bilayer of poly(p-phenylenevinylene) and C60, using flash photolysis, time-resolved microwave conductivity (fp-TRMC) to probe the efficiency of charge generation and recombination. Using additional interfacial layers at the interface between the polymer and fullerene, the role of long-range energy (exciton) transfer will be examined in detail.

My lecture will focus on stability and thermal processing of the photoactive layer of bulk heterojunction organic solar cells. Comparative studies will be presented of different materials systems, with the aim of materials design guidelines for enhanced photoactive layer stability. My lecture will include consideration of the role of triplet states in causing singlet oxygen mediated photodegradation, including the parameters determining triplet generation, and correlations with singlet oxygen yields and materials photodegradation rates. I will go on to consider the role of polymer polarons in inducing material degradation. I will then go on to consider the morphological evolution of photoactive layers under thermal annealing conditions, and strategies to address this both to enhance film stability and as a processing aid to enhance device efficiencies. Finally, I will present some recent results on different topic - the potential of piezoelectric effects to enhance the efficiency of hybrid metal oxide / polymer solar cells.

Organic photovoltaics (OPV) is a promising candidate technology for the low-cost fabrication of modules to harvest solar energy. Although OPV technology has significantly matured over the past few years, there remain many challenges in OPV manufacturing, from materials selection, to device design, to design and control of the fabrication process. Structure-property-performance relationships for polymer-fullerene blend OPV devices are still underdeveloped, and relationships based on one system are not necessarily transferrable to new, higher-performance systems. This talk will describe our efforts to develop measurements that support OPV manufacturing. An important issue in materials selection is the use of OPV systems that are more manufacturable because they perform well even when the polymer-fullerene blend is coated at thicknesses greater than 100 nm. Some systems that work well in thicker films appear to have slowed bimolecular recombination, but the structural origin of this feature is unclear. I will discuss our efforts to identify it using a suite of comprehensive structural characterization techniques. Solution formulation and the design of the coating process could also benefit from more structural characterization - during the film solidification process itself. Using a blade coating process as a prototype for slot-die coating, we have developed several techniques to observe the structure of OPV films in-situ as they dry. We use these techniques to identify the mechanisms by which different additives to polymer-fullerene blends influence the structure of the final films. Using in-situ techniques provides far more information about the solidification process than can be obtained by measuring already-dried films, providing a valuable tool to guide the selection of formulation and processing parameters.

In this presentation, we will present the recent research progress on plasmonic organic photovoltaic devices(OPVs) in our group. There are two general approaches to utilize the surface plasmon to enhance the efficiency of the organic photovoltaic.
Firstly, we incorporated gold nano-particles (NPs) into (a) the interface buffer layers (e.g. PEDOT:PSS layer), and (b) the photo-active layer. By adjusting the shape and size, we demonstrated that the plasmon resonance of gold NPs can be tuned to match with the absorption spectrum of various polymers. This concept has been proven to work well in polymers with different bandgaps. The Au NPs embedded PEDOT:PSS layer was incorporated in the tandem device as the interconnecting layer to improve the device performance.
Secondly, we studied the surface plasmonic effects of large-area metallic grating on patterned active layer. About 10% of short current density improvement is obtained, and PCE achieves 7.73% for the plasmonic inverted solar cells with the low-bandgap polymer as the active layer. An observable improvement in PCE is mainly ascribed by the surface plasmonic and scattering effects due to the electrode (Ag) grating.
Finally, by working together with Prof. W. Choy of Hong Kong University and Prof. J. Hou of the Institute of Chemistry of Chinese Academy of Science, we combined the above two strategies together and studied the dual plasmonic effect in the polymer solar cells
In addition to the plasmonic effect, recent progress at UCLA PV program will also be summarized.

Bulk hetero-junction organic photo-voltaic (OPV) solar cells are limited in thickness to 100-150 nm to achieve acceptable performance. At these thicknesses the light absorption in OPV films is incomplete. For typical P3HT:PCBM films, optical photons are poorly absorbed at both long wavelengths (above 600 nm) and short wavelengths (below 480 nm). Hence approaches are necessary to increase optical absorption in both these wavelength regimes. Recent approaches have utilized plasmonic light enhancements in OPVs with metal nano-particle arrays. We theoretically investigate an alternate approach for enhancing light absorption in thin OPV films using periodically textured photonic crystal substrates-a method that recently showed spectacular absorption enhancements in thin film silicon solar cells, both in simulation[1] and experiment[2].
Our approach uses the scattering matrix method where Maxwell&’s equations are rigorously solved vectorially in Fourier space for both polarizations of the incident wave[1]. We find conformal OPV solar cells on periodically textured substrates of nano-cone arrays to demonstrate strong diffraction of light leading to waveguided modes within the OPV layers, as well as plasmonic light concentration at the periodically textured metal cathode. Each layer of the structure conforms to the substrate texture. For a 100 nm P3HT:PCBM layer we find the enhancement factor of optical absorption and photo-current to exceed 45%, and 55% respectively for optimized structures. Optimized values of the pitch are between 600-700nm. Moreover, the absorption for common thicknesses of the OPV layers has approached the Lambertian limit over the entire wavelength range of absorption. We discuss the sensitivity to the texture height and propects for achieving such high light absorption in OPVs.
[1] R. Biswas, C. Xu, Optics Express 19, A664-A672 (2011).
[2] J. Bhattacharya et al, Appl. Phys. Lett. 99, 131114 (2011).

When using low-bandgap polymers efficiencies above 7% have been reached when electrically optimized devices, chemically tuned polymers or changes in the device architecture have been implemented. In the current work we show that the optical material constants for the charge blocking layers play a key role in order to achieve the highest power conversion efficiency (PCE) for the device. We demonstrate that by using as hole blocking layer (HBL) a material with a vanishing extinction coefficient such as bathocuproine (BCP), light harvesting in a direct PTB7:PC71BM cell can be largely optimized resulting in efficiencies of 8.1%.[1] The observed increase in PCE was 10% relative to the reported PCEs for the same type of cells that use as HBL a material with a large extinction coefficient such as Ca.
For the study, we used the high efficiency benzodithiophene derivative PTB7 as the low-bandgap donor material. An average PCE of 7.4% has been reported with the direct ITO/PEDOT:PSS/PTB7:PC71BM/Ca/Al architecture.[2] We performed a numerical calculation of the optimal light harvesting which demonstrated that a reduced extinction coefficient for the HBL is essential to achieve an optimal light absorption by the active layer. In accordance, we replaced the Ca/Al back metal electrode by a few nanometer thick BCP layer and a thick Ag layer. Simultaneously, to achieve an optimal optical interference, the thicknesses of the BCP, PEDOT and the active layer were adjusted. As a result, a 7% increase in short circuit current (Jsc) was obtained. In addition, we observed that when using the BCP/Ag electrode, the photovoltaic parameters of the cell not directly related to photon harvesting also improved, obtaining a 1% increase for the open circuit voltage (Voc) and a 2% increase for the fill factor (FF). This optical enhancement can be generalized to other type of low bandgap polymers. When using PBDTTT-C as the donor material instead of PTB7 we measured a 8% increase in the Jsc when we replaced the Ca for BCP.
In conclusion, we demonstrated that an optimization of the optical properties is essential to achieve high performance thin film OPV devices. By taking advantage of such optical optimization and the improvement in the electrical properties, we were able to fabricate cells based on PTB7 with PCEs of 8.1% and cells based on PBDTTT-C with PCEs of 7.5%. With respect to the same direct cells using the typical Ca/Al electrode, this amounted to an improvement in the PCE of 10% and 19%, respectively.
[1] A. Martinez-Otero et al. Advanced Optical Materials, submitted
[2] Y. Liang et al. Adv. Mater. 2010, 22, E135

Polymer-fullerene-based bulk heterojunction (BHJ) solar cells have many advantages including low-cost, low-temperature fabrication, semi-transparency, and mechanical flexibility [1,2]. However, there is a mismatch between optical absorption length and charge transport scale [3,4]. These factors lead to recombination losses, higher series resistances and lower fill factors. Attempts to optimize both the optical and electrical properties of the photoactive layer in organic solar cells (OSCs) inevitably result in a demand to develop a device architecture that can enable efficient optical absorption in films thinner than optical absorption length [5,6,7]. Here, we report the use of multiple metal nanostructures to achieve the broad light absorption enhancement, increased short-circuit circuit (Jsc) and improved fill factor (FF) simultaneously in bulk-heterojunction OSCs. Depending on metallic nanostructures, OSC structures and polymers, the power conversion efficiency (PCE) can be increased by ~32% [8] and reach 8.79% [9]. The metal nanostructures are metal nanomaterials (such as nanoparticles, nanoprisms, etc) and nanogratings. The changes of the optical (e.g. broadband absorption enhancement) and electrical properties (e.g. Jsc and FF) by using the multiple metal nanostructures in different layers of OSCs will be discussed. Meanwhile, electrical and optical models have been developed for further understanding the changes by incorporating the metal nanostructures into polymer layers. As a consequence, the introduction of multiple nanostructures into the multilayered OSCs can improve both optical properties and electrical properties of OSCs and PCE for practical applications in photovoltaics.
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The luminescent solar concentrator (LSC) promises to decrease the installed cost of solar energy through seamless building integration [1]. The LSC, a semi-transparent glass or plastic waveguide with embedded lumophores, concentrates sunlight by frequency downconversion; the lumophores absorb diffuse incident sunlight and luminesce at a Stokes-shifted wavelength. Approximately 74% of the luminescence is emitted into modes that can be guided by total internal reflection (TIR) to the waveguide edges, upon which small-area photovoltaic (PV) cells can be fastened [2]. Despite the LSC's simplicity, the concept has not been commercialized due to low efficiencies. Extensive theoretical and experimental work has identified reabsorption of luminescence and subsequent re-emission into non-waveguided modes as the performance bottleneck [3-4] .
We present simulations and initial fabrication results of a novel LSC design that seeks to circumvent the efficiency bottleneck via photonic engineering. In this design, a two-dimensional slab photonic crystal (PC) is evanescently coupled to TIR modes in the underlying glass substrate by a low refractive index spacer layer. The patterning of the luminescent layer into a PC affords direct manipulation of the spatial and spectral distributions of luminescence [5-6], while the 2D PC-LSC design leverages potentially scalable top-down manufacturing methods based on solution processing and nanoimprint lithography [7].
First-principles simulations explain how the 2D PC-LSC should be engineered to maximize light emission into TIR modes while minimizing re-emission losses. By solving for the eigenmodes and computing the local photonic density of states [8], we find a partial photonic bandgap that can be used to suppress emission in the region of overlap between lumophore absorption and emission spectra. Additionally, we predict narrowband enhancement of emission into specific TIR modes by up to a factor of two as compared to previous photonic LSC designs [9]. Finite-difference time-domain simulations [10] indicate that up to 10% more luminescence over a broad spectral range are emitted into TIR modes as compared to the standard LSC design. Finally, rigorous coupled-wave analysis [11] of the 2D PC-LSC&’s transmission and reflection spectra demonstrate control of reabsorption and diffraction of luminescence by structure geometry and materials selection.
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[10] A. F. Oskooi et al, Comp. Phys. Comm. 181, 687 (2010).
[11] B. Dhoedt, Ph.D. thesis, Universitet Gent, 1995.

Dye sensitized solar cells (DSSCs) are currently being explored as a cheaper alternative to the more common silicon (Si) solar cell technology. In addition to the cost advantages, DSSCs have good performance in low light conditions and are not sensitive to varying angles of incident light like traditional Si cells. However, they also have lower efficiencies, with recent studies showing efficiencies of around 12%. There are several components of the DSSC that have the potential to improve the efficiency such as material used for the anode, cathode, and electrolyte, as well as the fabrication procedure itself. This research work presented in this paper focuses on the effects of using nanoclay material as a gelator in the electrolyte of the DSSC. The data showed that the quasi-solid cells are more stable than their liquid electrolyte counterparts, and achieved equal or better I-V characteristics.
One of the major challenges facing DSSCs is loss of the liquid electrolyte, through evaporation or leakage, which lowers stability and leads to increased degradation. Current research with solid-state and quasi-solid DSSCs has shown success regarding a reduction of electrolyte loss, but at a cost of lower conversion efficiency output. The quasi-solid cells were fabricated with a gel electrolyte that was prepared by adding 7 wt.% of Nanoclay, Nanomer® (1.31PS, montmorillonite clay surface modified with 15-35% octadecylamine and 0.5-5 wt.% aminopropyltriethoxysilane, Aldrich) to the iodide/triiodide liquid electrolyte, (Iodolyte R-50, Solaronix). Various gel concentrations were tested in order to find the optimal ratio of nanoclay to liquid. The gel electrolyte made with 7 wt. % nanoclay was more viscous, but still thin enough to allow injection with a standard syringe. Batches of cells were fabricated with both liquid and gel electrolyte and were evaluated at STC conditions (25°C, 100mW/cm2) over time. The gel cells achieved efficiencies as high as 9.18% compared to the 9.65% achieved by the liquid cells. After 10 days, the liquid cell decreased to 1.75%, less than 20% of its maximum efficiency. By contrast, the gel cell efficiency increased for two weeks, and did not decrease to 20% of maximum efficiency until 45 days. After several measurements, the liquid cells showed visible signs of leakage through the sealant, whereas the gel cells did not. This resistance to leakage likely contributed to the improved performance of the quasi-solid cells over time, and is a significant advantage over liquid electrolyte DSSCs.

Dye-sensitized solar cells (DSSCs) consist of three distinct parts, namely the light-absorbing organic sensitizer, the electron donor and the electron acceptor, the properties of which can be tailored individually. The positions of the frontier orbitals of the dye with respect to the valence band maximum (VBM) of the donor and the conduction band minimum (CBM) of the acceptor, are crucial for the overall energy conversion efficiency of the solar cell. These energy levels were determined by combining element-specific x-ray absorption spectroscopy (XAS) with photoelectron spectroscopy, optical spectroscopy, and cyclic voltammetry. On the donor side, we explored the possibility of replacing corrosive liquid electrolytes by an inert, heavily p-doped diamond film as electron donor [1]. The ruthenium-based dye molecule Ru(tpy)2 was attached covalently to the diamond surface [2]. Bulk-sensitive fluorescence yield data revealed boron-induced acceptor states in the band gap of diamond, while surface-sensitive electron yield and photoemission measurements revealed the positions of the frontier orbitals. On the absorber side, various porphyrin and phthalocyanine molecules were investigated. Particular attention was drawn to the changes induced in the frontier orbitals of the dye by the attachment of axial and peripheral ligands to the central ring [3]. Density functional theory and atomic multiplet calculations were employed for the determination of the frontier orbitals and their energies. Going one step further, molecules with the donor-π-acceptor architecture were investigated [4]. The bond selectivity of XAS made it possible to identify molecular orbitals characteristic of each of the three constituents in zinc porphyrins with amine groups as electron donors and carboxyl linkers for attachment to oxide electron acceptors. Increasing the number of carboxyl linkers led to a substantial increase in the molecular coverage. The implications of these results on the systematic synthesis of materials for DSSCs will be discussed.
[1] I. Zegkinoglou, P. L. Cook, P. S. Johnson, W. Yang, J. Guo, C. Rogero, R. E. Ruther,
M. L. Rigsby, J. E. Ortega, R. J. Hamers, F. J. Himpsel, J. Phys. Chem. C 116, 13877-
13883 (2012).
[2] R. E. Ruther, M. L. Rigsby, J. B. Gerken, S. R. Hogendoorn, E. C. Landis, S. S. Stahl, R. J. Hamers, J. Am. Chem. Soc. 133, 5692minus;5694 (2011).
[3] D. F. Pickup, I. Zegkinoglou, J. M. García-Lastra, P. L. Cook, P. S. Johnson, R. González-Moreno, C. Rogero, F. de Groot, A. Rubio, G. de la Torre, J. E. Ortega, F. J. Himpsel, in preparation
[4] I. Zegkinoglou, M. E. Ragoussi, P. S. Johnson, D. F. Pickup, J. E. Ortega, G. de la Torre, F. J. Himpsel, in preparation

Solid-state dye-sensitized solar cells (ssDSSC) can be prepared using a mesoporous metal oxide sensitized with a visible absorbing dye and infiltrated with a solid organic hole-transport material. These offer an attractive alternative to liquid-electrolyte DSSCs with great potential for long-term stability and ease of manufacturing. Power-conversion efficiencies have reached over 7% [1] and can rise further with developments in materials and processing. We will present the synthesis and study of new hole-transport materials, based on triaryl amines, and new dye molecules, based on conjugated organic molecules, to addrress current limitations in the efficiency, cost and stability of ssDSSC. We will present the design, synthesis and characterisation of new materials leading to efficiency data competitive with that achieved using current champion materials in the field.
[1] Julian Burschka, Amalie Dualeh, Florian Kessler, Etienne Baranoff, Ngoc-Le Cevey-Ha, Chenyi Yi,
Mohammad K. Nazeeruddin, Michael Gratzel, J. Am. Chem. Soc., 2011, 133,18042

Series of organic dyes containing a fluorene moiety in the spacer have been designed, synthesized, and characterized for the application in DSSC. These D-π-A dipolar compounds contain an arylamine or cycloalkylamine as the electron donor, a 2-acrylic acid as the electron acceptor, and a conjugated spacer between the donor and the acceptor. We designed variety of organic dyes with fluorine moiety as well as different Donor structures . Typical dye-sensitized solar cells (DSSCs) were fabricated with a liquid electrolyte composed of iodide reagent in polar solvent at 1 cm2 size. The device performance data were obtained under AM 1.5 illumination. and the efficiency of organic dyes exhibited good performance (eta;), which reached at 87 % with respect to that of the standard cell from N719-based device fabricated under the same conditions.
Beside the organic DSSC dyes development, We also pay an attention in the improvement of Ruthenium dyes with high photoelectric conversion efficiency (eta;),too ! The innovative molecular structures of our new Ruthenium dyes were investigated by changing the cations species , and the performance results of these Novel Ruthenium dyes also shown have higher efficiencies compared to the reference dye, N719 and Z907 , respectively .

The path to successful commercialization of the nanostructured photovoltaic devices necessitates that we identify new technology markets, which will provide distinct advantage to these new photovoltaic technologies over the established silicon and thin film solar cells. The power conversion efficiencies of select nanostructured photovoltaics under solar illumination are already sufficiently high for many applications. However, the operating lifetimes of these new devices have not been proven to extend beyond a few years, identifying the studies of device aging as the paramount target in future development of these structures. Through examples of recent technical advancements in both molecular organic and colloidal quantum dot photovoltaic technologies, the talk will examine the electronic and excitonic phenomena that determine the photovoltaic device operation. From these findings the talk will infer the challenges that face commercialization of the nanostructured photovoltaic devices, outlining the case for the next steps that will be needed to broadly deploy these technologies.

Organic photovoltaic technology (OPV) offers the promise of lightweight, flexible and inexpensive panels with applications in a variety of areas, including off-grid deployment, mobile power sources and transparent photovoltaics. This presentation will review recent progress made at Solarmer Energy towards efficient, scalable and durable OPV panels. Developments in material structure and device architecture for bulk heterjunction (BHJ) cells allowed our team and its collaborators to set a new certified world record at 9.31% power conversion efficiency (PCE) for polymer/fullerene single junction photovoltaics. Combining these advances with our expertise in printed electronics, Solarmer has produced modules with 3% PCE using all roll-to-roll processes at the pilot scale. Lifetime measurements have placed T80 for our most efficient cells at 1000 hours using ISOS D2 standards. Continued development in active layer composition, device engineering and manufacturing should allow Solarmer to begin small-scale production in 2013.

In order to achieve reasonable efficiencies, large area organic photovoltaic cells (OPVs) require highly conductive current colleting grids, which decrease resistive losses. These can be produced by a variety of deposition techniques, one of which is printing of conductive inks and pastes and subsequent sintering. This approach is especially interesting for high volume and large area fabrication, because it is roll-to-roll (R2R) compatible and cost-efficient and thus allows high-throughput production. After deposition and sintering of the metallic grid, the next processing step usually involves a full-area coating with a very thin functional layer. In order for the final OPV device to work properly, a complete and fully continuous coverage is indispensible, since any local defect will compromise its performance. Metal structures printed on a flat substrate will, however, inevitably result in a surface topology which is much higher than the thickness of the functional layer itself (typically a few microns vs. less than 100 nm). It is therefore evident, that embedding of the metal in the underlying layer will significantly ease the coating process and is very likely to result in higher quality functional layers. In this contribution, we demonstrate a novel approach for the production of highly conductive current collecting grids embedded in plastic foils. It is based on the R2R transfer of printed and sintered patterns from a donor substrate with low adhesion into an uncured resist coated on the surface of an acceptor substrate. During the transfer process, the resist is cured and the metal lines are detached from the donor substrate. Via this route, functional foils can be produced containing metal structures of up to 3 µm height, but with a surface topology of less than 100 nm. Preliminary experiments demonstrated that the integration of such structures into printed solar cells produces functioning devices, albeit at current state with rather limited efficiencies.

In this work we present an ultrasonic spray coating technique which is capable of being scaled up for large area device fabrication and roll-to-roll processing. This technique was used in ambient conditions to deposit a photoactive blend consisting of carbazole and benzothiadiazole based low energy gap copolymers and the fullerene derivative [6,6]-Phenyl-C71-butyric acid methyl ester (PC70BM). Using a range of substrate-nozzle distances, casting solvents and substrate temperatures we have been able to control film thickness and drying kinetics.
Films were characterized using interferometry, and AFM, and show that spray-cast films have a comparable surface to spin-cast films. Neutron reflectivity measurements showed that vertical stratification occurred during film drying, with PC70BM concentration reducing towards the underlying PEDOT:PSS interface.
The carbazole-based co-polymers PCDTBT, poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis-(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (PCDTBT-8) and poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(2,20-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (PCDT2BT-8) were blended with PC70BM. High power conversion efficiencies of 4.3%, 4.5% and 4.6 % were obtained for solar cells made with PCDTBT, PCDTBT-8 and PCDT2BT-8 respectively. The efficiency of spray-cast solar cells based the PCDT2BT-8:PC70BM blend increases to 5.0% upon thermal annealing at 80oC.
We have explored the scale-up of photovoltaic devices using spray-coating, and have explored film deposition and device studies on a range of different substrates. These include a 36 x 36 array of 9mm2 pixels, which we compare with a single pixel of 900mm2. We use such techniques to comment on the yield and efficiency of device scale-up.

There has been considerable progress in the development of conjugated small molecules and polymers for applications as blends with solution-processable fullerenes in bulk heterojunction organic solar cells.
In this abstract we report the progress in the development of substituted hexabenzocorononene derivatives and a variety of conjugated polymers for printing on polymer substrates. The synthetic protocols call for the scale-up of materials in quantities that can be delivered on a roll to roll printer.
A report will be given of progress in the continuous flow synthesis of conjugated polymeric materials using Suzuki and Stille cross coupling processes as well as the Gilch polymerization. Controlled polymerizations can also be realized.
It is now possible to prepare sufficient amounts of active layer materials in a day to print 1500 metres of a 10 cm wide film of ITO-coated polyethylene terephthalate. Progress in preparing fully printed poly(3-hexylthiophene)/fullerene bulk heterojunction solar cells will be described.

We present thermally evaporated dielectric/metal/dielectric (DMD) multilayer structures as transparent top electrodes for inverted bulk heterojunction small molecule organic solar cells. The starting point of our investigations is a molybdenum oxide and silver stack. It is shown that MoO3 between the organic hole transport layer and the metal electrode improves charge carrier extraction and sufficiently prevents metal diffusion into the organic. Sandwiching the metal layer with oxide enables higher transmission through the metal electrode, enhances the light incoupling into the solar cell and acts as passivation of the electrode against most of the degradation processes resulting in an significant improvement of the organic solar cell lifetime (a nip reference cell using ZnPc/C60 as absorber materials yields a T80 of approx. 4200h [1]).
In such DMD systems, metal thin films grown on dielectric material by thermal evaporation tend to form isolated nuclei and islands for the deposition of the first few nanometers, leading to a non-conductive layer. Controlling the growth of ultra thin Ag on molybdenum oxide is crucial, since island growth structures cause high sheet resistances and surface plasmon resonances (SPR) which significantly reduce transmission. Varying seed layers like calcium, aluminum, or gold are successfully introduced to optimize the silver growth resulting in a very smooth, closed, and almost transparent but highly conductive Ag layer even at 5nm thickness. Our optimized electrode shows a sheet resistance of RS = 17Omega;/sq and 90% peak transmission at 600nm wavelength (mean transmission between 450nm and 800nm Tvis = 87.5%) without any substrate correction. Such flexible and “in line” producible top electrodes exhibit superior performance compared to common ITO, which is difficult to apply as top electrode.
Top illuminated small molecule organic solar cells with a transparent MoOx/Au/Ag/MoOx multilayer top electrode and a F4-ZnPc/C60 bulk heterojunction as photoactive layer on opaque substrates are fabricated showing a power conversion efficiency of 4.7% under standard AM1.5G spectrum which is even higher than the 4.6% achieved with an similar but bottom illuminated solar cell based on glass and ITO.
[1] S. Schubert, M. Hermenau, J. Meiss, L. Müller-Meskamp, K. Leo, Adv. Funct. Mater. 2012, in press, DOI: 10.1002/adfm.201201592.

In this work we present ultrasonic spray coating; a deposition technique compatible with roll-to-roll processing that allows us to print organic layers that achieve performances comparable to devices that have been spin coated and for certain materials exceeding that of spin coating. The main focus of this work is in using this technique to deposit multiple layers of an organic photovoltaic device sequentially. We are able to show that using a solution processable form of Molybdenum Oxide called Ammonium Molybdate Tetrahydrate as the hole extraction layer and a PCDTBT:PC70BM active layer that devices that have had multiple layers deposited sequentially can achieve efficiencies exceeding 4.5%.
By using a combination of several techniques such as Ultraviolet Photoelectron Spectroscopy, X-ray Photoelectron Spectroscopy, Spectroscopic Ellipsometry and Atomic Force Microscopy we are able to show how different processing technique affect the deposited layers. Device results show that annealing of the molybdenum oxide layers causes an increase within device efficiency. The measurement techniques used on the molybdenum oxide layers show that this annealing step does not induce changes within the electronic or chemical structure of the molybdenum oxide layers. The only change that is associated with the heating process is a change within the thickness of the layer due to the driving out of solvents. This thickness dependence upon performance for Ammonium Molybdate tetrahydrate films is in line with previously reported work for spin coated hole extraction layers. Therefore it is possible to deposit these layers using low temperature processing techniques in order to be compatible with flexible substrates and inverted device architectures.

I will update on recent progress in optoelectronic devices based on colloidal quantum dots. In particular I will discuss advances in synthesis; post-synthesis surface modification; film-forming strategies; doping; and device architectures that leverage the resultant materials. I will discuss applications in light sensors and in solar cells.

Colloidal nanocrystals are highly suitable for applications in solar cells and photodetectors. The major difficulty in obtaining high sensitivity is achieving sufficient charge carrier transport in the nanocrystal films, which is hampered by organic ligand shell of the nanocrystals. Improvements can be obtained by replacing or removing the ligands, resulting in a decreased ambient stability of the nanocrystal films, or by blending them with charge accepting and transporting organic species such as the fullerene derivative PCBM. By blending PbS nanocrystals with PCBM high specific detectivity is obtained up to the infrared spectral region. [1] Expanding this concept to ternary blends containing in addition the conjugated polymer P3HT even allowed the demonstration of an infrared camera. [2] In the photodiodes forming the camera the PbS nanocrystals act as the photosensitizer, whereas the P3HT (PCBM) act as hole (electron) acceptor and transporter. This interpretation is confirmed by transport measurements in transistor configuration as well as by transient optical spectroscopy. [3]
While the combination of inorganic colloidal nanocrystals with PCBM shows up the high sensitivity of such hybrid photodetectors, their application is restricted to the high potential toxicity of the inorganic colloidal nanocrystals, containing elements such as Pb or Cd. Thus we have developed as an alternative to the inorganic materials organic colloidal nanocrystals from nontoxic pigments. In particular we use quinacridone because of its well-known light-fastness and stability up to high temperatures. The colloidal synthesis of this material results in nanocrystals with two different polymorphs, depending on the chosen reaction temperature. Nanocrystals with different sizes and shapes are obtained, including rods, platelets and hierarchical nanocrystal arrangements with shapes similar to “european chestnuts” or “tee-flowers”. While the low temperature alpha form of the quinacridone nanocrystals absorbs at wavelengths up to 600 nm and emits at in the green the emission of the gamma polymorph nanocrystals is found 100 nm shifted to the red. The difference in the band gap of these polymorphs causes a completely different behaviour in blends of the nanocrystals with PCBM. Alpha type nanocrystals blended with PCBM show evidence for an electron transfer which is absent for the gamma type. Nevertheless, for both materials blended with PCBM photosensitivity is observed in the visible, which is by far superior to that of pure PCBM films.
[1] K. Szendrei et al., Adv. Mat. 21, 683 (2009)
[2] T. Rauch, et al., Nature Photonics 3, 332 (2009)
[3] D. Jarzab, et al., Adv. Funct. Mat. 21, 1988 (2011)

New semiconductor materials, which can be manufactured by low cost, solution-based processes, hold the promise for next generation electronic and opto-electronic device applications. In particular, colloidally synthesized semiconductor nanocrystals (NCs) are of interest in this field, due to their excellent optical properties and the tunability of key properties, such as the bandgap. Although a variety of NC based devices with relevant performances have been demonstrated, e.g. FETs with charge carrier mobilities of 30 cm²/(Vs)[1] and solar cells with power conversion efficiencies of 7.4%[2], performance must be increased for commercial application.
It is well established in traditional semiconductor research that device performance is strongly limited by the presence of electronic states within the bandgap [3]. These states act as charge carrier traps and - even in very small concentrations (<< 1ppm) - can alter virtually all electrical properties of the material, such as mobility, carrier recombination rates, and the Fermi energy. While the significance of trap states in NC based electronics is gaining recognition, systematic and quantitative understanding is still lacking.
In this work, we demonstrate how deep level transient spectroscopy (DLTS) can be used to quantify traps in NC solids. In the past, DLTS has been used very successfully to characterize the density of traps N_T, the trap energy E_T, and the capture cross section σ_T in conventional and organic SCs. We show that a DLTS technique based on the measurement of current transients (also known as Q-DLTS) is advantageous for NC solids, because it does not require an accurate understanding of the space charge region as the more common capacitance transient DLTS.
We fabricate Schottky-diodes using PbS NCs and ethanedithiol (EDT) - one of the best characterized NC materials for solar cell applications - and perform DLTS measurements to identify an abundant trap with E_T = 0.40eV, N_T = 1.7x10¹⁷cm^-3 and σ_T = 8.2x10^-15 cm². These measurements are in excellent agreement with previous explanations of device performances [2] and imply that, in such a system, the formation of the space charge region at the NC solid-metal interface is dominated by the properties of the traps.
[1] D. S. Chung et al., Nano Letters 12, 1813-20 (2012).
[2] A. H. Ip et al., Nature Nanotechnology (2012).
[3] C. T. Sah, Solid-State Electron. 19, 975-990 (1973).

The size-dependent tunability of the electronic energy levels of colloidal quantum dots (CQD) offers convenient components for solution-processed, flexible, resource-saving and compact electronic devices. While the focus of these materials research for solar cell application has shown promising prospects to achieve high power conversion efficiency, there is still lack of understanding on the fundamentals of the charge carrier transport in the assembly of these materials. As consequence, there is much less focus on the use of these kinds of materials for transistor applications. One of the reasons is that the charge carrier mobility values in thin films fabricated with CQD are generally rather low. So far, most efforts in increasing charge carrier mobility in CQD assembly have been done only by optimizing the formation of the nanocrystal films as well as chemical post-treatments on the films to change the doping levels or filling charge carrier traps. Less attention has been paid on the physics of the device. Therefore, the optimization of the device structure that can promote high carrier mobilities is highly important.
Here we present strategies, which allow us to increase carrier mobility values in CQD transistors by five orders of magnitudes, surpassing 1 cm^2/V.s, with high on/off ratio value. Firstly, a systematic ligand variation of the CQD was done to select which ligand type gives the most balanced ambipolar carrier transports, besides this, variation in the carrier mobilities was also observed depending on the ligand used. Secondly, because transistors are interfacial-based devices, we focused our attention on the modifications of the interfaces that are crucial for both charge carrier injection into the films as well as the interfacial field-effect charge carrier transport.
Finally, the use of advanced ion-gel gate allows very high carrier density accumulation, where nearly complete trap filling is achieved by electrostatic doping. Therefore, the intrinsic transport properties of the CQD can be accessed and exploited. The combination of these techniques improved the charge carrier mobility from 10^-5 cm^2/V.s to values above 1 cm^2/V.s with significant enhancement of the on/off ratio for more than two orders of magnitude. The improvement of the charge carrier mobilities are important for the utilization of CQD, not only for solar cells, but also to open up new direction of applications in electronics and optoelectronics.

We developed a novel technique to fabricate all-inorganic bulk heterojunction solar cell devices between PbS and macroporous TiO2 which show enhanced performance when compared to conventional planar junction devices. In our approach, we use polystyrene-spheres to fabricate a porous TiO2 film via sintering which can be easily infiltrated with PbS QDs. The pore dimension is engineered to be on the same scale as the depletion width such that the maximum amount of charge can be extracted by the built-in field. Electron microscopy confirms that good pore filling with PbS QDs is achieved. The bulk heterojunction device showed improved short circuit current and fill factor. Under AM1.5 illumination the power conversion efficiency of the best planar junction device was 4.4% while the best bulk heterojunction devices achieved 5.7%. This enhancement is shown to be due to increased absorption and improved electronic properties.

High external quantum efficiencies have been obtained in the visible region in lead chalcogenide CQD photovoltaics. However, the corresponding efficiencies in the near-infrared lag behind because the thickness of CQD photovoltaic layers is limited by short carrier diffusion lengths. Time resolved infrared (TRIR) spectroscopy in conjunction with electrical characterization methods are used to demonstrate twenty-fold enhancement of the mobility-lifetime products of minority carriers in PbS CQD films passivated with various ligands. Mid-infrared electronic transitions measured by TRIR spectroscopy provide information about charge trap densities and energetic distributions. Direct observation of the vibrational features of ligands attached to surface trap states provide unique insights into the nature of charge traps and helps define pathways for their elimination. These findings demonstrate that TRIR spectroscopy provides a means to directly probe the electronic properties of charge trap states and the underlying ligand-nanocrystal interactions that give rise of those states.

Organic Photovoltaics (OPV) off a potential low cost route to solar energy production. Unlike other solar technologies the primary absorber layer, the bulk heterojunction, is constantly evolving with new materials over time. This includes small molecules, polymers and fullerenes in the roles of donors and acceptors. As the homo/lumo move for both the donor and acceptor the contacts will need to be reoptimized to match the new energy levels and surface chemistry. We will present current approaches to producing hole and electron selective contacts using HTL and ETL layers respectively. We will present data on both inorganic oxide based layers and small molecule interface layers the can independently or in tandem be tuned to optimize the interfacial chemistry and charge transfer. We will discuss a potential model for the systematic optimization of these layers in devices with high performance polymers and small molecules. Charge selective contacts have a potentially broader application in OLEDs and solid state PV devices we will discuss this potential extension of the work presented here.

Solution-processable organic and hybrid photovoltaic devices have the potential to compete with conventional thin-film technologies on both cost and efficiency. However, their performance is usually limited by the energetic cost of separating or transferring the bound charges from the absorber following photoexcitation. A possible route to overcoming this problem is to find a solution-processable thin-film absorber which can fulfil the multiple roles of light absorption, charge separation and charge transport in a single material.
One interesting family of solution-processable materials suitable for optoelectronic applications are the inorganic-organic perovskites. Although these materials have historically received little attention for photovoltaic applications, they have recently been shown to rival the state-of-the-art for solution-processable semiconductors when used in solar cells [1]. Here we present solid-state photovoltaic devices based on the organometal mixed halide absorber, CH3NH3PbI2Cl. This perovskite can be processed from solution in air and harvests light from the visible out to the near-infrared. Analogous to sensitized solar cells, high performance is achieved by depositing a thin layer of the absorber on a mesoporous scaffold. Remarkably, it is found that when the scaffold is insulating, charge carriers are transported within the absorber to carrier selective contacts, minimising losses encountered at charge separating/transferring interfaces. This provides open-circuit voltages of 1.1 V from a band-gap of 1.55 eV.
In high-efficiency sensitized solar cells, the scaffold requires sintering up to 500°C which increases fabrication costs and renders stacked multijunction cells and plastic substrates prohibitive. We show that since the scaffold has no electronic function, it can be fabricated without exceeding processing temperatures of 150°C. Based on our process we can demonstrate low temperature devices with efficiencies approaching 10% which are among the highest reported for low-temperature solution-processed systems. This work paves the way for high-performance single-junction, multijunction and flexible perovskite solar cells.
[1] Lee et al., Science, DOI: 10.1126/science.1228604 (2012)

Mesoporous solar cells using (CH3NH3)PbI3 and (CH3NH3)PbI2Cl as absorbers present stable high power conversion efficiencies. Recent reports of solid state solar cells based on these organometal halide perovskites, which reach efficiencies higher than 10%, open a new field of study. For the first time since the report of dye-sensitized solar cells (DSC) in 1991, solid state devices present power conversion efficiency similar to the highest one obtained with standard liquid DSCs. Additionally, with these new materials it is possible to build new device architectures, where the perovskite can act as a light absorber as well as an electron conductor or hole conductor. The organic hybrid perovskites crystal structure alternates stacking sheets of organic components, which facilitates their functionalization, and inorganic ones, which at the same time, keep a high extinction coefficient. That, altogether with the solution processability, makes them suitable for photovoltaic applications. Here, we present a complete study of the material where we analyze its structural characteristics as well as its physical properties that can be useful for the photovoltaic performance. We also investigate different possibilities to take advantage of these unique features in new device architectures. A deep electrical and optical characterization of the different fabricated structures reveals their working mechanisms that determine the cell behavior. At the same time, this analysis also facilitates a model which points the limiting factors that need to be addressed in order to improve their performance.

It is estimated that for photovoltaics to reach grid parity around the planet, they must be made with costs under $0.50 /Wp, and in order to keep installation costs down, must also achieve power conversion efficiencies above 20%. Recently we proposed a novel solar cell architecture,[1] a hybrid tandem photovoltaic (HTPV), in which the top cell is an emerging photovoltaic technology that can be deposited at low temperatures and with rapid throughput, such as an organic or dye-sensitized cell, and the bottom cell is one of a variety of traditional inorganic photovoltaics, such as silicon or CIGS. The advantage of HTPV over traditional tandem cells is that the top cell may be printed on existing inorganic cells at or near room temperature with little additional cost; the cost of the organic layers in a bulk heterojunction solar cell, for example, is estimated to be as low as $10 m^-2.[2] Low temperature processability is critical because most inorganic photovoltaic technologies have a highly optimized thermal processing flow, and adding a top cell at high temperatures can damage the layers already there. Our efforts in modeling have already demonstrated that it should be possible to add an organic photovoltaic on top of moderately efficient silicon and CIGS bottom cells to improve their efficiencies to over 20%. In this work, we demonstrate a prototype HTPV device, which consists of an organic photovoltaic that is mechanically stacked on a CIGS bottom cell. This 4-terminal architecture allows for the independent optimization of the top and bottom cells, and eliminates the challenges associated with the interface of the two subcells. A 4-terminal architecture also removes the current matching constraint of traditional tandem photovoltaics, allowing the top cell to add power to the bottom, even if it is not an equal partner in absorbing the solar spectrum. To make high performing organic top cells with two transparent electrodes, we use meshes of silver nanowires embedded in a less conductive material to fill the gaps in the mesh. We present our strategy for fabricating these electrodes in a device architecture that replaces traditional electron and hole collecting electrodes with transparent metal oxides. Our architecture is designed to minimize the parasitic absorption and reflection of infrared light in the top cell, a challenge that is unique to tandem solar cells, and is critical for achieving high tandem efficiencies. Lastly, we present an analysis of the factors limiting the efficiencies of state-of-the-art HTPV devices, and discuss the developments in organic photovoltaic technology that will be necessary to push HTPV efficiencies above 20%.
[1] Z.M. Beiley, M.D. McGehee, Energy & Environmental Science 2012, 5, 9173.
[2] B. Azzopardi, et al., Energy & Environmental Science 2011, 4, 3741.