Motivated by recent experiments on nanocrystal superstructures, we study theoretically optical and photo-current responses of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), metal nanoparticles (NPs), and photosynthetic molecules. QDs and NWs can be arranged into light-harvesting complexes [1,2]. In these complexes, nanocrystals are coupled via Förster energy transfer (FRET). Consequently, this coupling creates a flow of excitons from QDs to NWs. Excitons harvested in NWs can be ionized and used to create photo-voltage. Using kinetic equations for excitons, we model exciton transport in QD-NW and NP-NW complexes and explain the origin of a blue shift of exciton emission observed in the experiment [3]. Another system of our interest is a complex composed of natural photosynthetic reaction centers, semiconductor QDs, and metal NPs [4,5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance an efficiency of light harvesting in natural photosynthetic systems [6-8]. Potential applications of hybrid exciton-plasmon systems are in photovoltaic devices and sensors. [1] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005). [2] P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007). [4] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007). [5] A. O. Govorov, Adv. Mater., 20, 4330 (2008). [6] S. Mackowski, S. Wörmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Bräuchle, Nano Lett. 8, 558 (2008). [7] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidullin, V. Pachenko, Y. Rakovich, J. F. Donegan, A.B. Rubin, and A.O. Govorov, Angew. Chemie, 49, 7217 (2010). [8] I. Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).

Dye-sensitized solar cells (DSCs) are inexpensive photovoltaic devices that can convert sunlight to electricity with relatively high efficiency. They are favorable alternatives to conventional solid-state solar cells consisting of materials such as Si, CdTe and CuIn_1-xGa_xSe₂. However, their use of organic liquid electrolytes seriously limits long-term performance and durability because of their inevitable problems of high volatility, leakage, and complex chemistry. Despite extensive studies to replace liquid electrolytes, the efficiencies of the resulting DSCs remain modest. Here we demonstrate that the p-type inorganic direct bandgap semiconductor CsSnX₃ (X = halogens or their mixtures) with high-hole-mobility can be solution-processable at room temperature to form all-solid-state DSCs and replace the problematic organic liquid electrolytes. CsSnX₃ compounds are made of inexpensive and earth-abundant elements. The resulting solid-state DSCs consist of CsSnX₃, nanoporous TiO₃ and the Ru dye, and exhibit conversion efficiencies up to ca. 10 per cent. References 1. I. Chung, B.-H. Lee, R. P. H. Chang, M. G. Kanatzidis, Nature2012, 485, 486. 2. I. Chung, J.-H. Song, J. Im, J. Androulakis, C. Malliakas, H. Li, A. J. Freeman, J. T. Kenney, M. G. Kanatzidis, J. Am. Chem. Soc.2012, 134, 8579.

Due to their unique features, semiconductor quantum dots (QDs) are presented as the ultimate frontier as sensitizers for photoelectrochemical solar cells [1],[2]. Up to now, the most interesting results in terms of device performances have been obtained by using polidisperse, in situ generated QDs by means of successive ionic layer absorption and reaction (SILAR) technique [3],[4],[which allows obtaining naked QDs directly grown on the porous structure of the photoanodes, thus guaranteeing an intimate contact between the two interfaces. Moreover, the deposition of networks of QDs presenting absorption features able to collect a wider region of the solar spectrum is easily possible [5]. This study is focused on the application of spray deposition (SD) [6] to the SILAR technique to generate QDs (CdS and PbS) on TiO2 photoanodes. We demonstrate that the use of SD-SILAR systematically results in higher amount of QDs together with smaller nanocrystals as compared with the classical immersion SILAR. Moreover, a reduced amount of chemicals is needed for the preparation of QDs, thus decreasing the environmental impact of the procedure. SD provides for a highly homogeneous coverage of the TiO2 photoanodes for the whole depth of the substrate. Evaluation of the performances of the quantum dot-sensitized solar cells indicates that devices prepared via SD-SILAR present improved functional properties, especially related to photoconversion efficiency and photocurrent density, both of them being almost two-fold the corresponding prepared by immersion SILAR. [1] S. Rühle et al., Chem. Phys. Chem. 2010, 11, 2290 [2] A. Shabaev et al. Nano Lett. 2006, 6, 2856 [3] Y-L. Lee and Y-S. Lo Adv. Func. Mat. 2009, 19 604 [4] H. Lee H et al., Nano Lett. 2009, 9, 4221 [5] A. Bragaet al., J. Phys. Chem. Lett. 2011 2 454 [6] S. Che et al., J. Aer. Sci. 1998, 29 271

By the year 2050, the world&’s energy utilization will have doubled while fossil fuels will be dwindling. Fortunately, if just 0.2% of the earth&’s surface were covered in 10% efficient solar cells by 2050, our energy needs would be met. The only real barrier to such widespread deployment is cost. There are several third generation solar technologies that could make widespread solar cell deployment economically feasible. Amongst those, Dye-Sensitized Solar Cells (DSSCs) show the most promise. With current DSSC efficiencies peaking at over 12.5%, the focus of research needs to shift to mass production. Our research focuses on using inkjet printing to produce the much cheaper DSSCs in a roll-to-roll manufacturing process on flexible substrates with CsSnI3, CsSnI2.95F0.05, and similar molecules as a solid-state electrolyte (SSE) as well as the Z-907 quasi-SSE. Currently, using an aqueous ink containing 10% TiO2 and a liquid I-/I3 electrolyte, we have produced 3.52% efficient DSSCs with a fill factor of 0.668. While this is a good start, there is room for improvement in both the efficiency and the manufacturability of the cells produced with inkjet printing techniques. To further optimize inkjet printed DSSCs, we are investigating TiO2 layer thickness as a function of print speed, drop size, drop spacing, and ink solvent. By maximizing the TiO2 layer thickness the number of printed layers required to produce the ideal DSSC thickness of around 13mu;m can be reduced. This allows cells to be produced with fewer print heads and accompanying lower capital and energy costs. To further reduce cycle times and energy costs, we are investigating the use of fast drying solvents such as low molecular weight alcohols, acetone, and acetonitrile. Additionally, we are optimizing the printability of our ink through the use of various surfactants and viscosifying agents. To ensure the ink will print, a viscosity of at least 7 mPa.s is ideal to ensure the printability of the inks. Currently we are examining glycerol, diethlyene glycol, and polyvinylpyrolidone as viscosifying agents for aqueous inks and polyisobutylene and cyclohexanol for solvent-based inks. In an effort to reduce the variation in efficiency between different cells, improve TiO2 distribution upon drying, and better control film spreading after deposition we are examining the use of Triton 100X, triethanolamine, and Surfynol 465 as surfactants in aqueous inks. To overcome the durability problems associated with I-/I3 while minimizing efficiency loss, we are currently studying the SSEs to investigate potential transport and recombination issues at the P-N junction, ability of the SSE to interact with the dye based on particle size, and the effects of various methods of depositing the SSE on cell efficiency. Also under examination is the effect of various solvents on the SSEs&’ transport characteristics and the potential for inkjet printing.

Efficiency of dye-sensitized solar cells (DSC) reached 11 % (certified efficiency of cells with more than 1 cm2). One of problems remaining is encapsulation of the cell. We focused on cylindrical solar cells because the shape allows easy and perfect encapsulation. In addition, it has been reported that CuInGaSeS (CIGS) type cylindrical solar cell has some advantages over flat type solar cells from the view point of the total amount of solar light harvesting in a day and light weight modules. We have reported cylindrical DSCs with 5.6 % efficiency. The cell was back contact type solar cells which do not need expensive and awkward transparent conductive oxide layered glasses (TCO). The TCO-less structure actually made the fabrication of the cylinder DSC possible. The cell consists of round-shaped glass/TiO2-dye layers fabricated on Ti protected metal mesh (working electrode)/gel electrolyte sheet/Pt-Ti working as an counter electrodes, from the outside to the inside. The fabrication process is precisely explained in the presentation. The photo active area was experimentally measured by a laser beam induced current (LBIC) method. It was found that the area where light does not reach directly also caused photoconversion. Projected photoactive area against the total area was calculated to be 66 %. However, the actual photoacitive area obtained the LBIC method was 93% of the total projected area. This was explained by the fact that the glass wall act as an optical wave guide. The optical wave guide effect was simulated by using ZEMAX software and these results were consistent with experimental results. The optical wave guide effect was largely affected by dielectric constant of electrolyte compositions. Finally, we report comparison of totally generated electricity in a day between cylindrical DSC and flat DSC. The former was 1.3 times larger than the latter which proved the effectiveness of the cylinder type DSC.

Among the other existing photovoltaic technologies, dye-sensitized solar cells (DSSCs) have a large potential for low production cost in the manufacturing of photovoltaic panels, although stability issues need to be addressed before to move towards a mass production. This talk will focus on a full solid device replacing the liquid electrolyte, which is one of the critical sub-systems incorporated in the DSSCs, with solid hole transporting materials, addressing the primary advantage to remove the extrinsic instability due to the presence of a liquid phase. All the materials employed in the device can be processed easily from solution, e.g. using spin-coating, doctor-blade coating and higher speed methods such as roll-to-roll printing. The materials we use include novel carbon-based organic semiconductors with a tendency to self-assemble in supramolecur structures, which allow controlling the morphology in the bulk and at the interfaces. Charge transport and interface charge dynamics will be discussed in detail as well as the origin of performance degradation under stressed solar aging conditions.

Understanding free carrier generation in solid-state dye-sensitized solar cells is crucial for optimizing device performance. Here we present a detailed photophysical study that directly elucidates how the commonly used additive Li-TFSI facilitates electron injection into the TiO₂ conduction band and we also demonstrate that reduction of the photoexcited dye prior to electron injection ("reductive quenching") is an important pathway for charge generation in solid-state cells. By comparing two all-organic sensitizers we find that careful design of the dyes to exploit both mechanisms is of utmost importance for obtaining good power conversion efficiencies in solid-state cells. Furthermore, we show that quantum efficiencies for charge generation close to unity can be achieved in particular by exploiting the reductive quenching pathway, while still maintaining broad photon absorption and a high open-circuit voltage[1]. Although charge generation can be very efficient in these cells, we also identify a significant loss channel: By exploiting the Stark effect on the dye molecules caused by the electric field across the interface, we observe that due to the high dielectric contrast between the organic hole transporter Spiro-MeOTAD and the TiO₂ the image charge of the holes is sufficient to attract some holes back to the interface, where they successively undergo recombination. We have developed a drift-diffusion model to extract the movement of charges close to the interface by monitoring the change of the Stark signal[2]. Our results are of general importance for solid-state dye-sensitized solar cells beyond the particular material system presented in this study, as they help to develop design rules for all-organic dyes in the ongoing quest for higher efficiencies. [1] M. Meister, I. A. Howard, N. Pschirer, R. Sens, I. Bruder, B. Baumeier, D. Andrienko, and F. Laquai, in preparation [2] M. Meister, B. Baumeier, N. Pschirer, R. Sens, I. Bruder, F. Laquai, D. Andrienko, and I. A. Howard, submitted

Organic/inorganic junctions play a central role in several promising technologies, including dye-sensitized solar cells, colloidal quantum dot based devices and spintronics. While many studies have been directed at understanding both inorganic/inorganic and organic/organic junctions, only a few investigations have been undertaken to characterize the fundamental processes that occur at hybrid organic/inorganic heterojunctions.^1,2 Here, we adapt the detailed balanced approach for heterotype organic junctions developed by Giebink, et al.³ to a traditional semiconductor device model. We use this to simulate device characteristics for several planar hybrid structures, and compare the model to that proposed by Forrest, at el.¹ that treats the hybrid structure as an inorganic diode in series with a space-charge-limited organic layer. From this model we infer the processes mediating carrier interactions across the heterointerface, and evaluate the role of Coulombically-bound polaron pairs at a high/low dielectric constant interface. The implications of these results on how to control charge dynamics through choice of materials and nanomorphologies will be considered. 1. S. R. Forrest, M. L. Kaplan, and P. H. Schmidt, J. Appl. Phys. 55, 1492 (1984). 2. H. Mendez, I. Thurzo, and D. R. T. Zahn, Phys. Rev. B 75, 045321 (2007). 3. N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010).

Thin film-photovoltaics, especially organic photovoltaics (OPV) as a low-cost technology require alternative solutions to ITO as transparent electrode. Driven by the desire to manufacture on flexible substrates several alternative technologies and ideas have emerged. Several of these are investigated and integrated into standard OPV cells using our established pin technology with evaporation of small molecules, doped transport layers and bulk-heterojunction geometries. Dielectric/Metal/Dielectric (DMD) layers with ultrathin-metal films are used as transparent top contacts, achieving equal performance as standard devices on ITO (Transmission >85% around 20 #8486;/sq, equal PCE of 2.5%). As bottom electrodes, random metal-nanowire network electrodes, from silver and copper are studied. The nanowire electrodes were deposited by spray or dip coating and achieve highly competitive properties, better than 85% transmission and sheet resistances below 20 #8486;/sq, with copper being slightly worse then silver. In addition, alternative materials like PEDOT:PSS with enhanced conductivity or carbon nanotubes are investigated. Most of these can also be combined with each other for transparent cells, demonstrating the applicability of all technologies to OPV, depending on the properties achieved by the electrode technology and the requirements of the product. All materials and technologies are studied and optimized for application as transparent electrode in OPV and show distinct advantages and drawbacks, which are discussed in a comprehensive summary.

Organic photovoltaic (OPV) cells still show significantly higher conversion losses than most of their inorganic counterparts. The study of the underlying physical limitations to the performance of OPV (and hybrid organic/inorganic) solar cells requires answering a multivariant question: a cell&’s performance, and in particular, its open-circuit voltage (Voc) depend on several inter-dependent parameters, such as the geometry of the junction, morphology and band gaps of the organic materials, the alignment of the donor and the acceptor energy levels, and the intra-gap density of states near the band edges. In order to highlight the contribution of the energy level alignment, we use a heterojunction between a wide band gap inorganic semiconductor, SiC, and various organic semiconductors. This simplified system allows us to manipulate the energetic alignment at the donor\acceptor interface with minimal affect on the other parameters. Furthermore, because of the 3 eV bandgap of SiC, optical absorption of solar photons is mainly limited to the organic semiconductor. Thus, we can modify the inorganic layer and its interface with the organic layer without changing the overall cell&’s absorption and bulk relaxation mechanisms. Thus, we modified the surface of single-crystal 6H-SiC with self-assembled alkyl silane monolayers. When different polarity head groups are attached to the molecules, a dipole is created and the local vacuum level surface is shifted, followed by a relative shift between the conduction band of the SiC and the LUMO of the organic absorber layer. The energy alignment was studied using electron photoemission spectroscopy and contact potential difference measurements. The Voc values of series of solar cell fabricated with different organic dyes all show a common trend of increased Voc as the dipole of the molecular layer becomes more negative. The dipole that is measured at the inorganic surface is, however, not fully manifested in the full device performance. To enhance the effect, we used a binary monolayer of short silanes with the desired head-group and a long alkyl silane. The two-step assembly procedure resulted in a homogenous mixed monolayer, where the long molecules act to protect the shorter dipolar molecules from interacting with the thermally evaporated organic over-layer. The effect of the molecular dipole on the Voc of the series of cells, made with the binary monolayer, was now significantly stronger, which suggests this approach as a viable one for molecular interface engineering of a photovoltaic junction.

In this report, we will present two new D-π-A porphyrin dyes, coded Y350 and Y486. The dyes are designed with a motivation to inhibit the recombination by introducing insulating alkoxyl chains on the dyes and yield higher open-circuit voltage (Voc) in a dye-sensitized solar cell (DSC). The synthetic procedure for the dyes will be described, and the photophysical and electrochemical properties of the dye will be demonstrated. Those results combined with the theoretical calculation enable us to rationalize the structure-property relationship of the porphyrin dyes. Furthermore, we investigate the photovoltaic properties of the dyes in conjunction with a cobalt complex redox electrolyte in a mesoscopic TiO2. The specific structure of the porphyrin Y350 greatly retards the interfacial electron recombination from TiO2 to cobalt electrolyte. The cell shows a very high Voc of 980 mV and a power conversion efficiency of 10.8 percent under simulated air mass 1.5 global sunlight.

Aerogels are attractive structures due to their high surface area, high porosity and particle interconnectivity. These structures are made via a sol-gel synthesis. The gel is dried supercritically to avoid the collapse of the structure as the solvent is withdrawn. Previous work has employed aerogels as an inert support for a thin TiO2 film. We have synthesized a conducting aerogel of fluorine-doped tin oxide (FTO). The 3D structure is intended to transport electrons faster than the traditional TiO2 film. TiO2 is deposited over the aerogel by atomic layer deposition (ALD) forming a favorable interface with dye molecules and protecting the conductive FTO from recombination with redox species in the electrolyte. We will report on characterization of the FTO layer and sintering temperatures. Additionally, iodide/triiodide and ferrocene/ferrocenium will be used as electrolytes in the FTO-TiO2, FTO-only and TiO2-only films to study photovoltaic cell performance.

We have designed and synthesized four new promising D-π-A conjugated organic sensitizers (RPT 9, NAT-440, NAT 622 and SFO-346) with a 4H-pyran-4-ylidene as a donor, a thiophene ring in the bridge and 2 cyanoacrilic acid as acceptor. The choice of this donor moiety relies on its proaromatic character, that is expected to improve the charge transfer process through the gain in aromaticity experienced by the donor fragment. The deliberate use of proaromatic donors for solar cells (Dye sensitized solar cells and organic solar cells) has not yet been explored, although the photovoltaic properties of some benzothiazolylidene merocyanines and other pyran derivatives have been recently reported. The aim of this work is the study of the effect of the side-chain modification of these dyes on the photovoltaic properties of complete devices. Thus, we have modified the donor and the thiophene groups in these dyes. The donor group has been exchanged between a phenyl and a tert-butyl group, anchored to the positions 2 and 6 of the 4H pyran-4-ylidene-unit, RPT-9 and SFO 346 dyes respectively. The thiophene group have been modified by the addition of two bulky hexyl groups, the NAT 440 and NAT-622 dyes respectively. The dyes can be easily synthesized in moderate yields by well-known organic reactions including Wittig-Horner, Knoevenagel and formylation reactions.

Dye-sentisized solar cells (DSCs) have attracted much attention as one of the most promising next-generation photovoltaics of low-cost fabrication. A crucial issue in this field is the development of the excellent organic dyes to achieve higher light-to-electricity energy conversion efficiency. Although many organic dyes have been developed so far, most of them have been limited to the compounds, in which the anchoring group having strong electron accepting ability, such as cyanoacrylic acid, is just introduced to the known organic dye skeleton. The development of organic dyes based on a new molecular design concept is necessary for the dramatic improvement of efficiency. We here disclose our new molecular design concept for the organic dyes in DSC, in which an intramolecular boron-nitrogen (B-N) coordination bond formation is utilized as a key scaffold for the π-electron accepting unit. In this system, we envisioned several effects; 1) the B-N coordination would not only constrain the π-frameworks in a planar fashion leading to effective π-conjugation, 2) but also enhance the electron accepting ability by lowering the LUMO level. 3) Fine-tuning of electron accepting ability is possible by the electronic effect of the substituents on the boron center. These effects would have many advantages in the development of DSC dyes that need improvement of their optical absorption properties with fine-tuning of their electronic structure. As the model compounds of this molecular design concept, we designed and synthesized a series of boryl-substituted thienylthiazole derivatives and demonstrated their photophysical and electrochemical properties as well as their performance as DSCs

Hybrid organic-inorganic photovoltaics (HOPVs) based on a planar heterojunction combine a solution processed, stable inorganic acceptor layer with a highly absorbing organic donor material. In HOPVs, transition metal oxides (TMOs) such as titanium oxide (TiO_x) and zinc oxide (ZnO) can act as an electron acceptor material to replace the more commonly used fullerene based acceptors. The use of a solution derived TMO layer offers the long term potential of lower manufacturing costs, better device stability and allows the morphology of the material to be tuned from a dense material to a nanoridged structure. The field of HOPVs has predominately focused on the use of polymers as the donor material. In planar architectures these devices result in a J_SC commonly below 1 mA cm^-2 due to their low exciton diffusion lengths (typically < 10 nm). Here, we demonstrate the use of the small molecule organic semiconductor, boron subphthalocyanine chloride (SubPc) as a promising new donor material for the fabrication for inverted HOPV devices utilising TMOs as the electron acceptor. SubPc offers well controlled film growth using organic molecular beam deposition, a longer diffusion length and improved light harvesting at longer wavelengths compared to P3HT. The optimised TMO/SubPc hybrid devices give a relatively high photocurrent > 1.60 mA cm^-2 and an external quantum efficiency (EQE) peaking at 22 % which leads to a power conversion efficiency of greater than 0.5 % under AM1.5 solar illumination. Sufficient exciton dissociation at the hybrid interface was confirmed by comparing the EQE of TMOs/SubPc devices to standard SubPc/C₆₀ devices. Both devices show a similar contribution from the SubPc, with the organic donor being the only current contributor in the hybrid devices, due to the high optical band gap of the TMOs used. The effect of inserting a molybdenum oxide (MoO_x) optical spacer layer between the SubPc and the aluminium electrode is also demonstrated, with optical modelling used to maximise photon absorption within the device. This clearly shows the potential of this new small molecule based hybrid interface which is suitable for TiOx and ZnO based HOPV devices.

Solid-state dye-sensitized solar cells (DSC) are promising next generation solar cells, yet they are limited by their ability to only absorb a small amount of the solar spectrum. Broad spectrum dyes are being developed to tackle this issue; however these tend to have weak extinction coefficients. Solid-state DSCs, which are thinner than liquid DSCs due to issues with the infiltration of the hole conductor in thick films, require strongly absorbing dyes in order to absorb enough light to be efficient. Organic dyes have higher extinction coefficients and their absorption spectrum can easily be tuned making them interesting alternatives to conventional metal-based dyes. However, organic dyes have been designed to reduce charge recombination with TiO2 electrons and little has been done to improve hole injection into the hole transporter. We present here a strong, broad absorbing donor-acceptor-donor based oligothiophene containing a barbiturate group, termed TTB, as a sensitizer in a solid-state DSC with 2,2prime;7,7prime;-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9prime;-spirobifluorene Spiro-MeOTAD or poly(3-hexylthiophene) P3HT as the hole conductor. The thiophene units have alkyl side chains making it a potentially good compatibilizer with P3HT. These devices show a broad spectral response extending from 400 nm to 800 nm with external quantum efficiencies reaching 30 % and an open-circuit voltage of 650 mV. Ultrafast transient absorption spectroscopy is performed on this sensitizing oligomer to determine its interaction with the titania and hole transporter, along with charge generation times and loss mechanisms within these DSCs. These results show rapid hole transfer from TTB to the hole-transporting material occurring within 50 ps, demonstrating this oligothiophene is a good compatibilizer with P3HT making it a potential sensitizing dye for dye-sensitized solar cell applications.

The performance of solar energy conversion devices employing mesoscopic junctions depends critically on their nanostructure [1,2]. Here we describe our latest efforts to improve the photon harvesting and the charge carrier collection transport in these mesoscopic solar energy conversion systems. For dye sensitized solar cells, only triodide/iodide based redox electrolytes had attained power conversion efficiencies (PCEs) over 10%. However recent studies combine Co(II/III) complexes with organic donor -acceptor dyes [3,4]. Porphyrin-sensitized systems of this type have now attained PECs of 12.3 % [5]. At the same time we have witnessed several breakthroughs in the development of new light harvesters, in particular semiconductor quantum dots and perovskite nanoparticles. These latest advances forebode well for realizing mesoscopic solar cells with even higher performance in the near future. Literature: 1. Grätzel, M. Nature 2001, 414, 338#61485;344 2. Grätzel, M. Acc. Chem. Res. 2009. 42, 1781-1798. 3. Feldt S. M.; Gibson E. A.; Gabrielsson, E. ; Sun L.; Boschloo, G.; Hagfeldt, A. J. Am. Chem. Soc. 2010, 132, 16714-16724. 4. Yum J-H, Baranoff,E ; Kessler F; Moehl T; Shahzada A; Bessho T, Machioro M; Ghadin E; Moser J-E; Yi C; Nazeeruddin Md-K; Grätzel M. Nature Comm. 2012,3, 1655/1-1655/8. 5. Yella A.; Lee H.-W.; Tsao H. N.; Yi C.;Kumar Chandiran A., Nazeeruddin Md. K.. W-G Diau W- G I E, Yeh, C.-Y. Zakeeruddin S. M.; Grätzel M. Science 2011, 334, 629 - 634.

Dye-sensitized solar cells (DSSCs) are a promising third generation photovoltaic technology that uses relatively inexpensive materials. In DSSCs, photons are absorbed by a ruthenium-based dye with a bandgap of ~1.6eV meaning most of the solar spectrum&’s energy is lost due to incomplete absorption. This makes DSSCs ideal candidates for upconversion, a process that converts two, or more, lower energy photons into a single higher energy photon. NaYF₄ doped with Er³⁺ and Yb³⁺ ions is one of the most efficient upconversion materials known to convert infrared light to visible light. In this work, NaYF₄:Er³⁺,Yb³⁺ is synthesized using a hydrothermal method, with sodium citrate as a crystal modifier to control particle morphology. By optimizing the particle morphology and annealing conditions, we have recently observed marked improvement of green light output, where the dye is best able to absorb. The upconversion process is highly sensitive to particle size and shape, which is controlled by the concentration of sodium citrate and synthesis temperature and time. Annealing the powder at elevated temperatures following hydrothermal treatment also drastically increases the upconversion fluorescence intensity. The purpose of the present work is to exploit the increased upconversion response for spectral engineering of DSSCs. For integration into DSSCs, the upconversion powder is made into a paste and applied as an external rear layer acting simultaneously as reflector to emit upconverted light back into the cell. DSSC integration by employing the NaYF₄:Er³⁺,Yb³⁺ as an internal upconverting/scattering layer is also investigated. The upconverting material is made into a paste and applied as an additional layer in contact with the nanostructured titania. A full host of photovoltaic metrics are employed to evaluate the impact of the additional upconversion layer on device performance including efficiency measurements under simulated solar light, external quantum efficiency across the visible and infrared spectrum, and electrochemical impedance spectroscopy.

We present photoelectrochemistry and solar cell performance of ITO/TiO2 electrodes sensitized with two (4, 4prime;-difluoro-4-bora-3a, 4a-diaza-s-indacene) BODIPY dyes. BODIPY-1 bears two carboxylic acid groups at its 2, 6 positions and BODIPY-2 is modified with two cyanoacetic acid groups at its 2, 6 positions. The photophysical and photoelectrochemical properties of modified electrodes are studied by steady-state and transient absorption spectra, fluorescence, photocurrent action spectroscopy and temporal photoresponse signals. BODIPY-2 shows a better photostability and higher photocurrent gain due to the quality formation of monolayer on TiO2 surface to allow efficient charge injection as evidence by ultrafast spectroscopy study and photoelectrochemical results. The efficient photocurrent generation of BODIPY-2 cell is also due to the efficient redox reaction with hole transport media I-/I3- in comparison with BODIPY-1.

Molecular photovoltaics based on dye-sensitized solar cells (DSC) have gained significant attention due to the ease of device fabrication and low material cost. Recently, DSC reached a record power conversion efficiency of 12.3% at AM 1.5 G sun conditions using a novel phorphyrin dye sensitizer and single electron redox mediator based on Co(bipyridine) shuttles.[1] The one electron outer sphere redox systems are known for fast electron transfer rates which enhances the electron recombination, affecting the achievable power conversion efficiencies. To circumvent this issue, recently we developed an ultrathin Ga2O3 electron tunnelling layer by atomic layer deposition technique (ALD) to arrest the recombination, leading to a new record open-circuit potential of 1.1V. This was achieved with simultaneous increase in the photo-current density of the DSC. [2] To realize the ALD technology beyond the tunnelling layers, in the current work, we will introduce a new concept of step-wise injection wherein we deposit an oxide ALD layer based on In2O3 which has a conduction band(CB) position lying between the CB of titania and LUMO (or π* orbitals) of the dye sensitizer. With the intermediate layer, the excited electrons in the dye are injected into the TiO2 by taking a step at the bottom of the CB of the In2O3. After injection, the electrons are trapped inside the conduction band of titanium oxide and cannot recombine easily due to the thermodynamic barrier that exists between the TiO2-electrolyte interface. In this work, we will show that the intermediate-band barrier layer enhanced the open-circuit potential of the dye-sensitized solar cell by blocking the electron recombination. The evolution of the recombination rate as the function of In2O3 thickness was investigated using transient photovoltage decay measurements. A systematic study has been done to find an optimum thickness of the ALD In2O3 over layer where a balance of blocking and injection properties are achieved to improve the conversion efficiency of the solar cell. We will also show that the photo injected electrons do not take a path, to the external contact, along the In2O3 layer by studying the similar layers on the insulating silica mesoporous substrates. At the end, we compare the effectiveness of step-wise injection layer with a tunnelling layer in blocking the electron recombination. [1]. Yella. A. et al. Science 2011, 334, 629. [2]. A. K. Chandiran, et al. Nano Letters 2012. in press.

Dye sensitized solar cells (DSSC) have recently reached power conversion efficiencies (PCE) >12%, making them quite attractive for application in low cost solar energy technology.1 Although ruthenium based sensitizing dyes are commonly used, metal-free sensitizing dyes are advantageous due to their higher molar absorption coefficients, ease of chemical synthesis and modification, lower cost, lower environmental impact, and increased performance in solid-state DSSCs (ssDSSCs).2,3 Typically, metal-free sensitizers consist of three moieties: an electron donor (D); an electron-rich conjugated bridging group (π); and an electron acceptor (A) which also serves to chemically bind the dye to the titania surface. These dyes are commonly referred to as D-π-A dyes and typically have a broad visible-light absorption spectrum and the ability to separate charges due to their photoinduced intramolecular charge transfer (PICT) properties.2 To date, the world record sensitizing dye, Y123, has a maximum absorption at 532 nm and a PCE of 7.2%.4,5 Moving forward, overall efficiency gains can be realized in two ways: 1) by lowering the bandgap of the sensitizing dye in order to red-shift the absorption and thereby harvest more solar photons; or 2) molecular-engineering of the dye-sensitizer to provide a more effective recombination barrier between the hole-transport material and titania surface in order to achieve a higher open-circuit voltage. In this presentation we will address the second method through the synthesis of a series of dyes in which the number and position of alkyl chains are methodically varied on the sensitizing-dye in order to block recombination and improve subsequent device performance of ssDSSCs. A fluorene π-group is utilized to allow for the addition of an extra pair of alkyl chains along the π backbone while a donor moiety similar to that of Y123 is employed to add additional chains on the donor. Using this strategy we introduce dyes that to our knowledge have the best recombination properties of any highly performing dyes in ssDSSCs resulting in PCE values near 7%. References 1. Cao, Y. M.; Bai, Y.; Yu, Q. J.; Cheng, Y. M.; Liu, S.; Shi, D.; Gao, F. F.; Wang, P. J. Phys. Chem. C 2009, 113 (15), 6290-6297. 2. Chen, R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007, 19, 4007-4015. 3. Chen C.; Hsu, Y.; Chou, H.; Thomas, K.R.J.; Lin, J.T.; Hsu, C.P. Chem. Eur. J. 2010, 16, 3184-3193. 4. Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H., Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. ChemSusChem. 2011, 4, 591-594. 5. Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.-L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. J. Am. Chem. Soc. 2011, 133, 18042-18045.

Fullerene systems have to date dominated the acceptor landscape in solution processed and indeed vacuum evaporated organic solar cells. Notably, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) generates respectable open circuit voltages with low optical gap polymers in high efficiency bulk heterojunctions. The accepted "historical wisdom" is that photoinduced electron transfer (PET) via photoexcitation of the polymeric donor and its subsequent oxidation at the acceptor-donor interface is the main mechanism for current generation. We term this the Channel I process. More recently, the role of acceptor photoexcitation followed by photoinduced hole transfer (PHT) has been recognised as a potentially valuable (and maybe even dominant) current generation pathway (Channel II) [1, 2]. Hence, the concept of designing acceptor-donor pairs with complementary absorption to maximise spectral coverage is a valid and potentially powerful strategy for enhancing photocurrent generation without compromising open circuit voltage. Motivated by this possibility, and also by a desire to understand and quantify Channel II, we have created a family of model non-fullerene small molecule acceptors based upon the benzothiadiazole (BTD) motif variously functionalized with, for example fluorene and dithienosilole [2]. We have engineered and manipulated the optical gaps, electron affinities, solubility and solid-state morphology of these acceptor molecules in order to create complementary absorbers with standard donor polymers such as poly(3-n-hexylthiophene) (P3HT), generating respectable device efficiencies of order 1-2% and clearly demonstrating Channel II photocurrent. In this presentation, we will summarise the current state of non-fullerene acceptors and discuss the design considerations for creating complementary absorbing systems. In addition, the essential transport physics controlling photocurrent extraction in these BTD molecules will be detailed. [1] A. A. Bakulin, J. C. Hummelen, M. S. Pshenichnikov, P. H. M. van Loosdrecht, Adv. Funct. Mater. 2010, 20, 1653. [2] P. E. Schwenn, K. Gui, A. M. Nardes, K. Krueger, K. H. Lee, K. Mutkins, H. Rubinstein-Dunlop, P. E. Shaw, N. Kopidakis, P. L. Burn, P. Meredith, Adv. Energy Mater. 2011, 1, 73.

Fullerenes are currently the most popular electron acceptor materials used in organic photovoltaics (OPV) due to their superior properties, such as good electron conductivity and sufficient charge separation at the donor/acceptor interface. However, low absorptivity in the visible spectrum region is a significant drawback of fullerenes. In this study, we design a Zinc Dipyrrin derivative (ZIPYCl) that absorbs strongly in the visible region (450 600 nm, optical density is sevenfold higher than C60 film) and can transfer energy to C60 in the thin films. Application of ZIPYCl as an energy sensitizer in OPV devices has shown to improve photoresponse of C60 up to 35% without changing other device characteristics such as open circuit voltage and fill factor. While searching for a new electron acceptor remains challenging, our sensitization approach allows improving absorption of the electron acceptor layer and utilizing advanced properties of C60 in OPV devices.

Cyanine dyes exhibit extraordinary high extinction coefficients. For most cyanine dyes, saturated absorption can be achieved with a film thickness of about 35 nm within their respective absorption ranges. On the other hand, the aggregates formed within cyanine films are very favorable for the diffusion of excitons, resulting in an average diffusion length of between 30 to 40 nm. The combination of these two advantages makes cyanine dyes an excellent material class for organic photovoltaics. Here we report an inverted cyanine organic solar cell with planar heterojunction geometry. The device was consisted of an ITO glass as the cathode, a chemically converted graphene oxide layer as the electron transporting layer, a C60 layer of 70 nm thick as the electron accepter, a cyaine film of 35 nm thick as the electron donor, a MoOx layer of 10 nm thick as the hole transporting layer, and a silver anode of 70 nm thick. The graphene oxide and cyanine layers were spin-coated, while the C60, MoOx and silver anode layers were formed by thermal sublimation. The graphene layer established a nice ohmic contact between ITO and C60. The device showed very high rectification ratio. Under simulated AM 1.5 solar spectrum (100 mW/cm2), the short-circuit current was 9.2 mA/cm2, the open-circuit voltage was 0.76 V, and the fill factor reached 0.73. The power conversion efficiency was calculated to be 5.1%.

Efficiencies of organic solar cells have surpassed 10%, yet the vast majority of these solar cells use fullerene derivatives as the electron acceptor. However, devices containing fullerene acceptors are energetically limited to open-circuit voltages of 1.0V or less, thus limiting their potential use as the high-voltage device in a tandem architecture. This is in addition to other drawbacks such as high cost of synthesis, purification and functionalization and relatively poor light absorption. In our recent publications, a new material, HPI-BT, has recently achieved as high as 3.4% efficiency, a record for organic solar cells using poly(3-hexylthiophene), P3HT, as donor material. Through investigation of the dependence of quantum efficiency on applied electric field and light intensity in the most efficient devices, we have determined that the fundamental loss mechanism in these devices is recombination of geminate charge pairs before they have reached a charge-separated state. In addition to P3HT, we have investigated these effects using additional donor materials. In donor materials with ionization potentials more than 0.1 eV larger than P3HT, energy is transferred from the charge transfer state to the excited state on the donor polymer, where it can no longer be effectively separated. Also, devices with PDHTT, a polymer with an ionization potential higher than P3HT by 0.1 eV, and HPI-BT demonstrate a high open-circuit voltage of 1.1V with efficiencies up to 3.4%.

The organic bulk-heterojunction solar cell has a large area donor-acceptor (D-A) interfaces owing to the micro-segregated mixture of donor and acceptor materials. Such a larger D-A interfaces are essential to enhance the dissociation of the photo-generated excitons. However, carrier conduction pathways for photo-dissociated electrons and holes are not sophisticated because of the random orientation and aggregation of donor and acceptor molecules. For further improvement of carrier collection efficiency, we need to enhance the charge carrier transport properties in the D/A mixed thin film: this improvement can be achieved by introduction of molecular order to the film. A few strategies can be considered for the introduction of such the ordered structure. Here, we are focusing on the liquid crystalline material which exhibits self-organization in molecular aggregates. In this study, we have investigated the idea described above with small molecular liquid crystalline pyrrolopyrrole derivative as a donor and C61PCBM as an acceptor. From XRD measurement, we found that molecular orientation of the liquid crystalline pyrrolopyrrole could be oriented parallel to the substrate in the films when the thermal annealing was carried out, which enhanced its self-organization. The resulting molecular orientation appeared in both one-dimensional ordered liquid crystalline material (nematic) and two- dimensional ordered one (smectic A). However, favorable micro-phase segregation was achieved only in the latter case. As a result, power conversion efficiency was improved from 0.028 to 1.1% owing to the improvement of photo-carrier generation and their transport. This difference of self-organization behavior in the nematic and smectic liquid crystals seems to be attributed to the degree of molecular interaction in the materials. These results indicate that such self-organizing ability in liquid crystals is effective to realize the ordered structure in the D/A mixed thin films.

Recent progress in molecular organic photovoltaics (OPVs) revealed the realization of 10% power conversion efficiency (PCE) for single-junction cells, which put them in direct competition with amorphous silicon PVs. Incorporation of plasmonic nanostructures in these thin-film devices for light trapping offers an attractive solution to realize ultrahigh-efficiency OPVs with PCE>>10% . In this talk, we review recent progress on plasmonic-enhanced OPV devices using metallic nanoparticles, and one-dimensional (1-D) and two-dimensional (2-D) patterned periodic nanostructures. A discussion will be given on the benefits of using various plasmonic nanostructures for broad band, polarization insensitive and angular independent absorption enhancement, and their integration with one or two electrode(s) of an OPV device.

We demonstrate a tandem organic photovoltaic cell incorporating a combination of sub-cells, one based on solution- and the other on vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride (SubPc):C70 graded heterojunction subcell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer heterojunction subcell resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends strongly on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with unannealed cells. The incorporation of C70 as the acceptor leads to an increase of short-circuit current in each subcell by at least 30 % compared to analogous cells using C60. The optimized tandem cell&’s power conversion efficiency is 6.5 ± 0.1 % and an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. To our knowledge, this is the highest efficiency reported in the scientific literature for a small molecule based tandem cell. We will discuss optimization of this device which employs the unusual combination of solution and vapor deposition, and will consider routes to achieving efficiencies of 8% by the methods and materials described.

Bilayer small molecule organic photovoltaic devices typically have significant current collection losses that occur between the generation of excitons and collection of current. Typically, the external quantum efficiency is 50-75% of the flux absorbed in the donor and acceptor layers; this loss is often associated with an exciton diffusion length that is shorter than the layer thickness. Here, we present results of asymmetric functionalized squaraine1, 2 (DPASQ)/C₆₀ cells. The as-cast cells have results typical of bilayer cells with a peak EQE of ~40% and a short circuit current of J_SC= 4.4 mAcm^-2, open circuit voltage V_OC=1.0 V, fill factor FF=0.75 and a power conversion efficiency of PCE=3.4%. When the bilayer cells are solvent vapor annealed, the average EQE increases to asymp;50% from lambda;asymp;350 nm to lambda;asymp;600 nm with a peak EQEasymp;60%, values that are approximately equal to the absorbed photon flux. From this we infer that nearly all absorbed photons in the active materials are collected at the contacts. These cells have J_SC= 7.1 mAcm^-2, V_OC=0.86 V, FF=0.75 and PCE=4.6%. We associate the unusually high FF and EQE with improved bulk and interface morphologies resulting from solvent vapor annealing.

The use of self-assembling characteristics is one of the most potential candidates for the realization of a prevailing solar cell. We have demonstrated a high-efficient bulk-heterojunction solar cell based on liquid crystalline phthalocyanine, 1,4,8,11,15,18,22,25-octaoctylphthalocyanines (C6PcH2), exhibiting a high carrier drift mobility in excess of 1 cm2/Vs. The device can be fabricated through a spin-coating process from the blend solution of C6PcH2 and 1-(3-methoxy-carbonyl)- propyl-1-1-phenyl-(6,6)C61 (PCBM). For the formation of the optimally phase-separated nano-structure for efficient carrier generation and transportation, the mesogenic properties should play an important role. Solar cells have demonstrated a high external quantum efficiency (EQE) above 60% in the Q-band absorption region of C6PcH2, a high open circuit voltage (Voc) above 0.8V and a high energy conversion efficiency of 3.8%. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) (P3HT), and a high Voc of 1.27 V has been achieved. C6PcH2 is also available as a dopant for conventional organic thin-film solar cells with a bulk hetero-junction active layer composed of P3HT and PCBM. The improvement of long-wavelength sensitivity in P3HT:PCBM bulk hetero-junction solar cells (EQE>40% in the wavelength range of 650-750nm) has been achieved by doping C6PcH2. The molecular alignment has also been investigated in this mesogen. The coexistence of different molecular packing of C6PcH2 with different electronic states has been observed in a planarly aligned sandwich cell.

Recent progress in small donor molecule BHJ OPVs suggests the potential to overcome the many of the synthetic issues inherent with polymeric donors. The molecular designs follow similar principles to polymer donors in which the “push-pull” concepts are applied to lower the band gap.1 Thus, proximate electron donating and electron withdrawing units are incorporated in alternating positions to extend the electronic conjugation. A common “pulling” element is the electron deficient thiophene-capped diketopyrrolopyrrole (TDPP) unit, which has successfully been used in both polymeric and small molecular systems.2,3 The common “pushing” element benzo[1,2-b:4,5-b&’]dithiophene (BDT) unit has also been used in both polymers4 and small molecules5. Unfortunately, for the majority of OPVs based on small molecule donors, low FFs are observed, and further mechanistic understanding is needed to enhance their efficiencies. We analyze the performance of bulk-heterojunction solar cells fabricated from solution processed small molecule donors based on the TDPP moiety. By using impedance spectroscopy on completed solar cells we are able to separate the electrical processes taking place in a working cell. In particular, we are able to separate recombination processes6 from transport of carriers7 and analyze the interfacial properties8. Each of these factors play an important role towards the low FFs observed. In this paper we will present results on how high FFs can be achieved by optimising each of these parameters independently. 1. P. L. T. Boudreault, A. Najari and M. Leclerc, Chemistry of Materials, 2011, 23, 456-469. 2. F. Silvestri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani and T. J. Marks, Journal of the American Chemical Society, 2008, 130, 17640. 3. S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, Journal of the American Chemical Society, 2011, 133, 8142-8145. 4. H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nature Photonics, 2009, 3, 649-653. 5. Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian and Y. Chen, Advanced Materials, 2011, 23, 5387-5391. 6. P. P. Boix, A. Guerrero, L. F. Marchesi, G. Garcia-Belmonte and J. Bisquert, Advanced Energy Materials, 2011, 1, 1073-1078. 7. G. Garcia-Belmonte, A. Munar, E. M. Barea, J. Bisquert, I. Ugarte and R. Pacios, Organic Electronics, 2008, 9, 847-851. 8. A. Guerrero, L. F. Marchesi, P. P. Boix, S. Ruiz-Raga, T. Ripolles-Sanchis, G. Garcia-Belmonte and J. Bisquert, ACS Nano, 2012, 6, 3453-3460.

Organic solar cells have recently achieved much progress and have broken the 10% efficiency barrier. However, for a broad market application, a further significant increase of efficiency seems necessary. Although most research has been performed on polymer solar cells, small molecule cells have recently gained increased attention: Due to the possibility to easily deposit multilayer structures e.g. for tandem cells and the easier purification of the materials, small molecule solar cells are a promising materials choice. However, the control of the key element of efficient organic solar cells, the donor-acceptor bulk heterojunction, is more difficult in small molecule solar cells compared to polymer cells since there are fewer handles to control the morphology, in particular when vacuum deposition is used. In this talk, I will discuss recent results showing that the bulk heterojunction morphology sensitively depends on the molecular structure. By using various structural methods, we could show that the morphology of the donor and acceptor phases vary in a subtle manner e.g. with temperature. Due to a better understanding of these effects and the basic physical effects in the structures, significantly higher efficiencies are in reach. The major obstacles are to find materials with better mobilities in the bulk heterojunction, allowing higher active layer thickness, and better infrared absorbers.

Over the past several years significant advances have been made concerning our understanding of the growth of crystalline small molecule organic thin films consisting of a single component. An important challenge in organic electronics, photonics and photovoltaics is to develop and improve methods to integrate both p-type and n-type small molecule organic semiconductors into the same device microstructure. Thus, developing an understanding of the molecular scale events that lead to heterojunction formation is essential in these systems consisting of multiple components. To this end, we report on our examinations of the nucleation, growth, and dynamics of adsorption of a n-type organic semiconductor, N,N'-ditpentylperlyene-3,4,9,10-tetracarboxylic diimide (PTCDI-C5), on SiO2 surfaces modified by self-assembled monolayers (SAMs) and on pre-deposited layers of pentacene (a p-type semiconductor) using supersonic molecular beam techniques, in situ synchrotron x-ray scattering and ex situ atomic force microscopy. From real-time x-ray scattering we find that PTCDI-C5 exhibits prolonged layer-by-layer growth for approximately the first 10 to 15 monolayers (MLs) of deposition on all SAMs examined, as well as on pentacene surfaces. Concerning the kinetics of growth we find that the adsorption probability of PTCDI-C5 on itself is similar to that observed on two SAMs that possess aromatic endgroups, but it differs significantly to that observed on a relatively short, methyl-terminated SAM and bare SiO2. These differences could reflect mechanisms such as direct molecular insertion of PTCDI-C5 into either the existing PTCDI-C5 film, or the longer chain SAMs with aromatic endgroups. Concerning growth in the submonolayer regime, we find that nucleation is homogeneous, and that the absolute density of islands depends on the nature of the surface, while the relative change of the island density with increasing growth rate is essentially independent of the underlying SAM. Finally, we will discuss our recent results concerning the growth of heterostructures composed of a few to several monolayer stacks of PTCDI-C13 and pentacene. In this work we find that PTCDI-C5 grows in a smooth layer-by-layer fashion on pentacene, but the opposite is not true—pentacene grows in a purely 3D mode when deposited on PTCDI-C5. We will discuss the implications of this observation concerning the growth of organic heterostructures for applications in electronics, photonics and photovoltaics. Additionally, we have performed real time in situ grazing incidence wide angle x-ray scattering (GIWAXS) experiments to probe the evolution of the in-plane structure of PTCDI-C5 films on SAM surfaces during growth.

The LUMO level (the lowest unoccupied molecular orbital-derived levels in solid; unoccupied states) of organic semiconductors is crucial to the charge separation, electron transport, and electron correction in organic photovoltaic cells. In principle, the LUMO levels can best examined by inverse photoemission spectroscopy (IPES), which is a complimentary of photoemission spectroscopy (PES). In the previous IPES, vacuum ultraviolet (VUV; hnu;asymp; 10 eV) photons has been detected following the injection of electrons with energies of 5 - 20 eV into solid materials. The high energy electrons can cause damage on the organic samples. Also, the energy resolution is limited to about 0.5 eV due to the difficulty of detecting VUV photons. Surprisingly, such instruments have been used without any fundamental improvement since the late 1970s. Recently we have developed the IPES in the near ultraviolet (NUV) range. Detection of NUV photons allows us to use high resolution optical bandpass filters that improve the energy resolution to 0.27 eV, which is better than that of the commonly used apparatus by a factor of two. By detecting NUV light, measurements can be made with electrons having a kinetic energy less than 4 eV, reducing the damage to the organic samples by a factor of at least 1/100. This technique is especially suitable for examining the conduction levels of organic semiconductors because of low sample damage and high resolution. This new method has been applied to the several acceptor materials most frequently studied for organic photovoltaic cells, including C60, C70, phenyl-C61-butyric acid methyl ester (PC61BM), and phenyl-C71-butyric acid methyl ester (PC71BM). The determined electron affinities range between 3.6 and 3.9 eV. The values are consistent with those predicted from a series of open circuit voltages in the photovoltaic cells with the same donar material. In the presentation, the values will be compared with the previously reported values measured by the conventional VUV-IPES, the HOMO energy (from PES) and the optical gap energy, and the reduction potentials in the solution, demonstrating the advantages of the present method.

Organic photovoltaics hold promise as a renewable energy source due to their low material and processing costs. The most implemented device framework, a bulk heterojunction (BHJ), consists of a blend of a light harvesting p-type and an n-type electron accepting compound. To date, the most investigated p-type organic semiconductor materials have been conjugated polymers, however they can suffer from pitfalls such as labor intense purification methods and batch to batch variation in device performance. A number of small molecule p-type organic semiconductors have recently been designed that absorb a large portion of the solar spectrum and have ideal energy levels for efficient charge transfer with PCBM. A recent solution processable small molecule has achieved device efficiencies up to 6.7% when a high boiling solvent additive is used. Structural evolution due to the use of different solvent additive amounts are correlated to device performance by examination via GIWAXS, GISAXS, and EF-TEM.

Solution processed small molecule bulk heterojunction solar cells (SMBHSCs) with power conversion efficiencies (PCE) of 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, the fill factor (FF) of SMBHSCs remains an area for improvement as many of the most efficient systems exhibit FFs below 50% while efficient polymer systems have FFs exceeding 65%. A deeper understanding of the field-dependent recombination and charge transport processes that limit the FF of SMBHSCs could lead to significant improvements in PCE. The nature of the field-dependent recombination losses that determine the FF of 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 play a role depending on materials and device processing conditions. Initial studies of SMBHSCs have concluded both geminate and nongeminate recombination also influence the FF of small molecule systems, however, it has yet to be seen if this is true for all SMBHSCs. In this work, we report on the use of light intensity measurements and a novel differential resistance analysis technique to probe the voltage dependent recombination mechanisms of SMBHSCs with diketopyrrolopyrrole (DPP) based donor molecules blended with [6,6] phenyl-C71-butyric acid methyl ester (PC71BM). By extracting resistances from impedance measurements, we demonstrate that the field-dependent recombination losses in the DPP based systems studied occur within 1-10µs - a timescale comparable to what has been reported for polymer based systems dominated by bimolecular recombination. Light intensity measurements provide further evidence that these field-dependent recombination losses are primarily bimolecular with only negligible losses from trap-assisted and geminate mechanisms. These results suggest that future material design should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to bimolecular recombination.

It has recently been shown that the fundamental physical origins of the photogeneration process can be simply understood in the context of the kinetics of polaron-pair dissociation and recombination at donor-acceptor heterojunctions[1, 2]. This analysis has given rise to the derivation of an ideal diode equation that simply describes the dark current and photoresponse characteristics of organic photovoltaics. In the context of this framework, we discuss methods of controlling the nanostructure of organic thin films that follow the design principles inferred from the ideal diode theory. Nanostructure control demands that the properties of the donor-acceptor interface and that of the bulk thin films be independently optimized, whereby there needs to be disorder at the interface to reduce the polaron-pair dissociation rate and order in the film bulk to reduce series resistance. We show that this morphological control using both solution and vapor phase deposition technologies can create nanostructures with solar conversion efficiencies of nearly 6% based on squaraine donors and electron conducting buffer layers. Indeed, using a combination of solution and vapor deposition of layers, tandem cells employing functionalized squaraines [3] have achieved efficiencies of >6.5% in our lab. As in all “ideal theories”, deviations often point to interesting and important physical phenomena. We find that photoconductivity of the layers plays an important role that results in significant deviations from theory in the 4th quadrant of the current-voltage characteristics. We will discuss this and other aspects of the photon-to-electron conversion process that lead from nanostructural control to high efficiency organic solar cells. 1. N. C. Giebink, B. E. Lassiter, G. P. Wiederrecht, M. R. Wasielewski and S. R. Forrest, Phys. Rev. B 82, 155306 (2010). 2. N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski and S. R. Forrest, Phys. Rev. B 82, 155305 (2010). 3. G. Wei, X. Xiao, S. Wang, K. Sun, K. J. Bergemann, M. E. Thompson and S. R. Forrest, ACS Nano 6, 972 (2012).

Solution processing of blends of organic semiconductors is one of the main technologies currently being researched for organic solar cells. Organic semiconductor cores are modified with side groups to increase solubility and allow mixing of the electron-donating and electron-accepting molecules in solution before they are processed into their functional morphology in the thin film state. It is of interest to establish the chain of relationships between the molecular structure of the semiconductors, the resulting morphology of the thin film, and the resulting impact on device performance. Towards this goal, we have studied a set of prototype, electron-donating, small molecules based on the benzothiadiazole-dithienopyrrole (BTD-DTP) core, mixed with the PCBM fullerene derivative. The BTD-DTP molecules possess equivalent molecular opto-electronic properties, but differ in either having solubilizing side chains (type I) or not (type II), allowing us to focus on the effects of the molecular structure on morphology and solar cell performance. The side chains (type I BTD-DTP) lead to an increase in the degree of crystallinity for the small molecule, and this correlates with a reduction in miscibility with PCBM. In contrast, without side chains (type II), the material shows a lower degree of crystallinity and forms a vitreous state upon mixing with PCBM. Solar cell device studies, covering a range of processing conditions, showed three distinct performance states could be obtained from the BTD-DTP:PCBM blends. The crystalline type I molecules are less affected by processing conditions compared to the type II, which demonstrated distinct high and low photocurrent performance states. Our work points towards the need to encompass the full derivation of OPV morphology, from molecular to thin film structure, as well as demonstrating the influence of side chains in small molecules on intermixing and morphology.

To realize commercially attractive organic photovoltaic cells (OPVs) modules, increasing cell area while maintaining high yield and performance is of paramount importance. The total thickness of the organic layers sandwiched between thick conductors of OPV cells is typically ~100 nm. Particulates on the indium-tin-oxide (ITO)-coated glass can result in electrical shorts between the electrodes that reduce yield, especially in large-area cells. Increasing the thickness of the deposited layers can partially mitigate this yield loss1, but this approach can result in reduced efficiency. Here, we demonstrate large-area OPVs based on boron-subphthalocyanine chloride (SubPc)/C60 and solution-processed 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine (DPSQ)/C60 heterojunctions on substrates previously “snow cleaned” in a jet of supercritical CO2. Carbon dioxide snow cleaning relies on the expansion of either liquid or gaseous CO2 as it emerges from an orifice. The resulting stream of supercritical CO2 physically removes particles both through impact by the CO2 particles, and by dissolution of contaminants.2,3 Snow cleaning reduces particulates on the ITO-coated glass substrates, thereby reducing device shorts, and hence improving yield. The standard deviation in efficiency for a population of nineteen, 1.44 cm2 SubPc/C60 devices is le;4.0% of the average, which predominately comes from variations in fill factor caused by varying dark current and series resistance. By using a subelectrode structure, we obtain a power conversion efficiency of 2.21±0.05% for 6.25 cm2 SubPc/C60 devices, while that of a 0.00785 cm2 device is 2.69±0.03%, with the loss due to series resistance of the ITO. References: 1. P. Peumans and S.R. Forrest, Applied Physics Letters 79, 126 (2001). 2. W.V. Brandt, SPIE 4562, 600 (2002). 3. J. King and L. Williams, Current Opinion in Solid State and Materials Science 7, 413 (2003).

During the last few years, organic solar cells have seen a fast progress both in efficiency and lifetime. Efficiencies well above 10% have been demonstrated and certified independently for different approaches of OPV. Heliatek pioneers the vacuum-based sublimation deposition of oligomers. In April 2012, Heliatek reported 10.7% cell efficiency, which was confirmed independently by SGS Germany. We will report on latest developments on oligomer-based, vacuum deposited organic solar cells developed at Heliatek. Starting with chemical synthesis, material testing and standard test cells, we will report about Heliatek&’s progress in OPV cell efficiency and lifetime. Next, we will report progress in the field of scaling, i.e. the transition from cells to lab-scale modules, and beyond: We will report about the latest progress in the first vacuum based roll-to-roll OPV fabrication world-wide. A significant prerequisite for commercial success of OPV is its real-life behavior, both with respect to light harvesting factor and device lifetime. This will allow OPV distinguishing from conventional inorganic PV technologies by more than just cost-roadmap arguments. We will demonstrate the peculiar advantages of OPV with respect to light harvesting, e.g. under low-light conditions, non-optimum light incidence angle and under hot climate.

Organic solar cells offer the possibility to develop cost-effective solar electricity using simple low-environmental impact processes and may on the longer term, provide a possible complement or alternative to silicon technology. Intensive multi-disciplinary research efforts developed in the past decade have generated impressive progress and conversion efficiencies exceeding 8.00 % have been reported for single-junction devices fabricated by solution-process or vacuum deposition In this context, BHJ cells based soluble low band gap conjugated polymers as donor material have attracted considerable interest motivated by a combination of high efficiency and simplicity of fabrication. However, the variability of the structural regularity, molecular weight and chain-length distribution associated with the polydispersity of polymers, as well as the presence of eventual residual end-group can render problematic the reproducibility of the composition and purity of the donor material and hence of the performances of the resulting devices. A few years ago we have proposed an alternative approach based on the use of soluble conjugated molecular chromophores as donor materials in BHJ. In fact molecular donors present several specific advantages in terms of unequivocal monodisperse chemical structure, reproducibilty of synthesis and purification and analysis of structure-properties relationships. In this presentation, the design of various classes of molecular donors for organic solar cells will be discussed on the basis of the analysis of structure-properties relationships with an emphasis on the control of the light-harvesting properties, energy levels of the frontier orbitals, and hole mobility

A study of morphological effects in molecular-doped organic charge transport layers is presented. Electrochemical doping can significantly enhance the performance of organic devices. As e.g. demonstrated by Walzer et. al. [1] efficient small molecule solar cells can be realized by employing p-i-n structures for which doped transport layers are essential. Compared to inorganics, carrier-doping in organics tends to be less efficient, which is expressed in the high doping concentrations which are often employed. Examples for such systems are P3HT doped with F₄-TCNQ [2] or small molecule materials like CBP doped with the transition metal oxides WoO₃ or MoO₃ [3]. Until now the physical origin of this low doping efficiency is not fully understood. A possible explanation would be clustering of the dopant molecules, opposing the common assumption of evenly dispersed dopant molecules. For our studies we used MoO₃ as dopant, which is capable of doping organic materials with very deep lying HOMO levels as e.g. CBP (4,4&’-Bis(N-carbazolyl)-1,1&’-biphenyl). With bright-field TEM as well as TEM-spectroscopy in different energy regimes we could show that in co-evaporated MoO₃-doped CBP thin films agglomeration of the dopant molecules occurs. Moreover we could show that it is possible to control the amount of agglomeration and therefore the doping efficiency by changing the substrate temperature during the evaporation. A sample evaporated onto a cooled substrate led to more evenly dispersed dopant molecules than for a sample deposited at room temperature. This result shows that the formation of MoO₃ agglomerations might be diffusion driven. To gain information on the 3-dimensional morphology of the dopant agglomerations we performed TEM tomography. A 3-dimensional reconstruction of the acquired tilt-series revealed that MoO₃ forms filament-like structures preferentially oriented perpendicular to the substrate. Combining our results with FTIR-measurements, which indicate charge carrier localization onto single molecules, we can model the charge transport to occur at the interface of the MoO₃ filaments and the organic matrix and expect an anisotropic charge transport. Indeed, electrical measurements indicate that an anisotropic charge transport occurs in our doped systems. Since similar effects are also likely to take place in other co-evaporated material systems, not only electronic structure but also morphological issues should be considered, when discussing the fundamental issues of electrochemical doping in organic semiconductors, especially concerning the doping efficiency of certain donor or acceptor materials. [1] Walzer et al., Chem. Rev. 107, 1233 (2007) [2] Pingel et al., Appl. Phys. Lett. 100, 143303 (2012) [3] Hamwi et al., Appl. Phys. Lett. 94, 253307 (2009)

Small molecules can play several roles in an organic photovoltaic (OPV) or hybrid devices, acting variously as light absorbers, electron acceptors, or interfacial layers. For each role, the desired electronic properties are distinct, e.g., a hole-blocking layer should have different electronic properties than a hole-transport material. Thus to create novel small molecule devices, it would be useful to be able to design materials to have specific electronic properties. One approach would be to discover design rules that allow one to accurately predict properties of a potential material directly. Another approach would be to predict the properties of a large number of candidates and then search for materials with the desired properties. We have taken the latter approach, using density functional theory (DFT) and time-dependent DFT to predict the electronic structure and properties of a large set (> 20,000) of small molecules. The different molecular structures in this set were created, in silico, by combining several fused-ring monomers into a single molecule capped on one or both ends by any of several end groups. Within the set of molecules studied, we have identified sub-sets that have desirable properties for active layer materials, as well as other sub-sets with energy levels appropriate to either hole transport or electron transport layers. We will present an overview of the results for the large set of candidate small molecules, with comparisons to experiment where applicable. We also will discuss the chemical implications that several trends and regularities observed in the dataset have for molecular design of small molecule systems.

The development of small molecule light-absorbers for organic solar cells is an area of great interest providing new molecular systems to reproducibly synthesize with greater ease than their polymer counterparts. We report the synthesis of a meso-substituted tetraphenylporphyrin dyad for use as the light-absorbing donor material in a bulk heterojunction organic solar cell. The dyad has been constructed by coupling two porphyrin macrocycles together with a conjugated dibenzoacetylene dithienosilole (DTS) unit at the beta-pyrrole porphyrin positions. DFT calculations have shown the highest occupied molecular orbital (HOMO) to be located on the center DTS unit and the lowest unoccupied molecular orbital (LUMO) to be located on the porphyrin capping units. The porphyrin dyads serve as an ACCEPTOR-DONOR-ACCEPTOR system to facilitate intramolecular charge transfer upon excitation. The porphyrin dyad&’s LUMO level exhibits sufficient energy difference between the LUMO of the fullerene acceptor PCBM, allowing for efficient exciton dissociation at the BHJ interface. Variation of the phenyl groups around the porphyrin macrocycle, specifically, changing from electron-donating to electron-withdrawing substituents, allows for a lowering of the HOMO-LUMO levels and an increase in the open-circuit voltage (Voc). This increase in Voc not;can potentially lead to higher overall photoconversion efficiencies in a bulk heterojunction organic solar cell.

In organic electronics, the ability to support both ionic and electronic conduction is regarded as one of the possibilities to go beyond traditional device architectures and achieve exciting and new functionalities. Examples in which such mixed systems are employed include electrocromic devices, artificial muscles, biosensors and light-emitting electrochemical cells. In the latter, the incorporation of mobile ions inside a single layer active material allows one to mimic the more traditional p-i-n light emitting diode by the application of an external electric field. Upon application of the electric field, the mobile anions (or cations respectively) migrate to the cathode (anode) facilitating efficient charge injection. In this way efficient electroluminescence can be achieved using a single active layer with air stable electrodes, greatly simplifying production processes. Where the incorporation of ionic conductivity in light emitting devices is a relatively often pursued approach, remarkably this is not the case for photovoltaic devices. Currently the best performing solid state organic photovoltaic device utilizes the same p-i-n geometry as the light emitting devices described above. In analogy to the light emitting electrochemical cell, the possibility of achieving such device architectures by the movement of ions offers many advantages. We will present normal and inverted solution processed bi-layer solar cells using ionic absorbers such as cationic cyanine dyes as the electron donor and a functionalized fullerene derivative as the electron acceptor (ref 1-4). Cyanine dyes are interesting candidates for use in solar cells because of their very high absorption coefficients allowing a large number of photons to be absorbed in extremely thin layers. The cells exhibit high open circuit voltages of 1 volt and efficiencies in excess of 3 % are obtained for these simple bi-layer devices. Additionally, we demonstrate the large effect that cyanine dye counter ions can have on the energetic of the solar cells, illustrated by (for instance) changing the output voltage of a cell in situ from 0.35V to 0.74 volts. 1. B. Fan, R. Hany, J. Moser and F. Nuesch, Org. Electr., 2008,9, 85-90. 2. X. Ma, J. Hua, W. Wu, Y. Jin, F. Meng, W. Zhan and H. Tian,Tetrahedron, 2008, 64, 345-350. 3. M. Lenes and H. J. Bolink, ACS Appl. Mater. & Inter., 2010,2, 3664. 4. O.Malinkiewicz , M.Lenes , H.Brine and Henk J. Bolink, RSC Adv., 2012,2, 3335-3339

The high demand for low-cost, renewable energy has spurred strong developments in the research of organic photovoltaics. To date, the highest efficiency of 8.6% has been reported for a polymer tandem solar cell.[1] However, small molecule (SM) solar cells have recently gained increased interest, reaching efficiencies around 6%, in tandem as well as single-junction structures. [2] In the past, we successfully introduced merocyanine (MC) dyes in vacuum-processed singlejunction solar cells, reporting efficiencies beyond 6%, while maintaining an extremely simple single-junction device setup consisting of only two organic layers. Remarkable open-circuit voltages up to 1.1V and high short-circuit current densities beyond 11 mA cm-2 were achieved. [3] Unfortunately, the MC solar cells still suffer from rather poor fill factors (FF) below 0.5, mainly due limited charge transport processes and recombination losses. Charge-extraction-by-alinearly- increasing-voltage (CELIV) measurements are carried out in order to study various charge carrier processes and recombination losses in the device. For further improvement of the charge transport and the charge collection at the electrodes, a pdoped hole transport layer and an n-doped electron transport layer are introduced to MC solar cells. In the past, the introduction of the pin-concept led to highly promising results.[4] Implementing doped charge transport layers in simple-structured MC solar cells will improve the FF and thus the overall device performance, challenging the - to date - most efficient SM- as well as polymer-based solar cells. [1] L. Dou, Y. Yang et al. Nat. Photonics, DOI: 10.1038/NPHOTON.2011.356. [2] (a) L.-Y. Lin, K.-T. Wong et al., J. Am. Chem. Soc. 133, 15822-15825, 2011., (b) M. Riede, K. Leo et al., Adv. Funct. Mater. 21, 3019-3028, 2011. [3] V. Steinmann, K. Meerholz et al., Adv. Energy Mater. 1, 888-893, 2011. [4] (a) D. Gebeyehu, K. Leo et al., Sol. Energy Mater. Sol. Cells 79, 81-92, 2003., (b) B. Maennig, K. Leo et al., Appl. Phys. A 79, 1-14, 2004

Solution-processed organic materials hold great promise as inexpensive alternatives to silicon in devices such as field-effect transistors and organic photovoltaics (OPVs). A relatively large amount of research in photovoltaics has focused on donor polymers and small molecules with much less attention given to small-molecule acceptors beyond those based on fullerenes, with PC₆₁BM and PC₇₁BM receiving the most attention in the literature. However, significant progress has also been made in developing alternative small-molecule acceptors. Recently, a functionalized perylene diimide was developed for solar cells reaching 3.88% power conversion efficiency (PCE)¹. One goal of our research group is to develop efficient non-fullerene acceptors for use in OPVs. Acenes are widely investigated as materials in organic electronic devices with pentacene and its derivatives among top performers in transistors. Their use as acceptors in OPVs has also been demonstrated with several derivatives including a cyano-functionalized pentacene achieving a 1.29% PCE². Single-crystal x-ray data showed this derivative crystallizes in a “sandwich herringbone” motif, and this was believed to be an important parameter for this class of compounds. Subsequent analysis of crystal data from a number of derivatives has now revealed a more general parameter which correlates to higher PCE: PCEs are generally higher among materials in which two crystal unit cell axes approached the same length. As synthetic precursors of acenes, acene quinones are generally more stable and have also been shown to function as n-type materials in solar cells³. As a material easily produced in large scale from cheap reagents, dioxolane pentacene quinone serves as a basic framework from which several derivatives have been synthesized and explored as acceptor materials. By carefully choosing specific functional groups, such as various trialkylsilylethynyl substituents, the molecule&’s dimensions, and, subsequently, the crystal unit cell axes are altered so two lengths become nearly equal. While solubility of acene quinones can be somewhat low and make solution-processing difficult, this functionalization strategy can be used to increase solubility, modify crystal packing, and tune the LUMO energy level. Together, optimizing these parameters make this scaffold more suitable as a small-molecule non-fullerene n-type material for use in OPVs with increased PCEs. 1. Mikroyannidis, J.; Suresh, P.; Sharma, G. Synth. Met., 160, 932, 2010. 2. Shu, Y.; Lim, Y.-F.; Li, Z.; Purushothaman, B., Hallani, R.; Kim, J.; Parkin, S.; Malliaras, G.; Anthony, J. Chem. Sci., 2, 363, 2011. 3. Deng, X.; Zheng, L.; Yang, C.; Li, Y.; Yu, G.; Cao, Y. J. Phys. Chem. B., 108, 3451, 2004.

Solution processable conjugated small molecules are attracting more and more interest in organic solar cell applications. Comparing to their polymeric counterparts, these small molecules have several advantages such as relatively simple synthesis and purification process, mono-dispersity and thus well defined molecule structures without batch-to-batch variations, no end group contaminants, and generally high charge carrier mobility. Push-pull chromophores involving electron-rich and electron-deficient groups have been widely investigated for small molecule organic semiconductors. Here, we report a novel class of acceptor-donor-acceptor (A-D-A) molecules based on oligothiophenes and heterocyclic linkages and their applications in solution processed solar cells. All these molecules have strong visible light absorbance between 400 and 700 nm and bandgap ranging from around 1.7 to 1.8 eV. The preliminary photovoltaic investigation has lead to highest efficiency of 4.75% in the best small molecule/PCBM system. High open circuit voltage (Voc) of 0.97 V and a short circuit current density (Jsc) of 7.37 mA/cm2 have been achieved. The system particularly shows a very notable fill-factor (FF) of 66.4%, which is among the highest in solution processed small molecule solar cells reported. The effects of electron-rich and electron-deficient linkages in the conjugation backbone on the performance of small molecules and their application in solar cells will be discussed.

The open-circuit voltage V_OC of bulk heterojunction small molecular organic solar cells is investigated varying temperature and illumination intensity. The open-circuit voltage is limited by the effective gap E_g^eff, that is the difference between the electron affinity of the acceptor and ionization potential of the donor material. Characterizing the open-circuit voltage as a function of illumination intensity and temperature V_OC(I, T), an upper limit V₀ can be determined by extrapolating the open-circuit voltage to 0 K. The extrapolation to V₀ is valid if V_OC(T) is linear and the value of the extrapolation is independent of the illumination intensity. In a p-i-n type solar cell, the bulk heterojunction is sandwiched between p- and n-doped transport layers. The effect of the hole transport material on V_OC(I, T) is investigated with a series of materials with varying ionization potentials. The influence of this variation on V₀ is found to be negligible. Furthermore, the donor-acceptor mixing ratio is found to have no measurable influence on V₀. The quantitative relation between V₀ and the effective donor-acceptor gap E_g^eff is investigated for various donor-acceptor material systems, with E_g^eff values in the range from 1.0 eV to 1.5 eV. We find that the relation between V₀ and E_g^eff is linear with slope 1, plus a constant and material independent offset. This can be modeled on different levels of detail, both by a generation-recombination approach and an equivalent circuit model, which agrees well with our experimental findings. This indicates that the determination of V₀ is a possibility to quantitatively compare the E_g^eff of different material systems. As a consequence, calibration of this method with one well characterized material system allows for the determination of the ionization potential of a donor material or the electron affinity of an acceptor material.

Pentacene is a prototypical organic semiconductor with optoelectronic and photovoltaic applications. It is known that the lowest-energy singlet excitation has a Stokes shift between absorption and emission of about 0.19 eV, but the deformation associated with this self-trapped exciton remains unknown. Understanding its nature is important for singlet fission and photodegradation processes. We begin with a calculation of the optical properties via the first-principles GW/Bethe-Salpeter (BSE) theory [ML Tiago, JE Northrup, and SG Louie, Phys. Rev. B 67, 115212 (2003); S Sharifzadeh, A Biller, L Kronik, and JB Neaton, Phys. Rev. B 85, 125307 (2012)]. We then study the self-trapping phenomenon via our reformulation of the Bethe-Salpeter excited-state forces approximation of Ismail-Beigi and Louie [Phys. Rev. Lett. 90, 076401 (2003)], which can describe the structural relaxation after optical excitation. Whether excitons in pentacene have charge-transfer character has been controversial in electro-absorption experiments. We use the same BSE analytic derivatives approach to calculate the changes in excitation energies due to an applied electric field to understand this experimental controversy.

A widely explored issue in the field of organic photovoltaics is the tradeoff between long-wavelength absorption and high open-circuit voltage. In this study, we use the small molecule donor species lead phthalocyanine (PbPc) and boron subthalocyanine (SubPc) in the front and back cells, respectively, of a tandem device to achieve photovoltaic response past 900 nm and an open-circuit voltage above 1.5V. PbPc can be grown on a thin Pentacene template to enhance near-infrared absorption while SubPc&’s deep HOMO energy leads to open-circuit voltages (VOC) greater than 1V in single heterojunction devices with acceptor C60, making these donors an ideal pair for a tandem device. Devices are fabricated in a vacuum chamber with base pressure 3x10-6 Torr and have the structure Glass/ITO/Front cell/Charge recombination zone (CRZ)/Back cell/Aluminum where the CRZ consists of 10 nm BPhen/0.5 nm Ag/10 nm MoO3. We have demonstrated tandem devices with a short-circuit current density (JSC) of 3.4 mA/cm2, a fill factor (FF) of 0.55, a VOC of 1.55V and a power conversion efficiency (PCE) of 2.9%, which is greater than that of each individual subcell [JSC = (7.2 ± 0.2) mA/cm2, FF = (0.47 ± .01), VOC = 0.49V and PCE = (1.7 ± 0.1)% for PbPc/C60 planar heterojunction (PHJ) devices and JSC = (4.6 ± 0.1) mA/cm2, FF = (0.54 ± .01), VOC = 1.14V and PCE = (2.8 ± 0.1)% for SubPc/C60 PHJ devices]. Significant spectral mismatch is present in single cells due to the incomplete spectral coverage of the individual PbPc/C60 and SubPc/C60 absorption profiles, which can drastically affect the accuracy of tandem device characterization. By optically and electrically biasing the individual cells in the device we can independently observe the quantum efficiency of each subcell for precise calibration and performance optimization. We observe that the PbPc front cell is actually current-limiting in our best tandem device at present, suggesting that optical management can significantly improve device performance. We use transfer matrix calculations to simulate the optical response of the tandem stack to determine a 200 nm transparent and conductive BPhen:Cs/Ag/MoO3 CRZ is optimal for maximizing near-IR response of the front PbPc cell. These devices have been fabricated to produce a tandem device with a lower JSC, where the EQE-integrated JSCs of the front and back cells are 4.8 and 2.2 mA/cm2, respectively, showing that the long wavelength-absorbing front cell is no longer current-limiting [full device: JSC = (2.4 ± 0.1) mA/cm2, FF = 0.59, VOC = 1.55V, PCE = 2.2%]. This study has the potential to demonstrate very high efficiency devices while also exploring the importance of accurate measurement and characterization methods. Significant improvements in tandem device performance are expected with the incorporation of (a) a SubPc:C60 bulk heterojunction as the back cell and (b) introducing the wide bandgap, high open-circuit voltage materials DBP and DIP as back cell donor species.

Singlet exciton fission splits one singlet exciton into two triplet excitons. Because it should double the photocurrent per photon, fission is a promising approach to surpassing the single junction efficiency limit in solar cells. To date, however, the highest efficiency obtained from the process is only 145%, and it has been realized in photodetectors, not solar cells [1]. To increase the efficiency of fission, we analyze the loss processes using magnetic field dependent photocurrent spectroscopy. We find that organic solar cells and photodetectors featuring singlet exciton fission materials are subject to two particular loss processes: singlet exciton dissociation at the donor-acceptor junction, and triplet-doublet annihilation. When devices are affected by singlet dissociation prior to fission, an external magnetic field changes the photocurrent by reducing the singlet fission rate relative to the rate of singlet dissociation into charge. We determine that the high field asymptotic value of the change in photocurrent is correlated the fission yield. The triplet yield closely approaches 200% for pentacene layers that are thicker than ~ 5 nm. Triplet-doublet annihilation is another particular concern for fission in solar cells because the triplets are frequently formed close together, and if one is dissociated, that charge can annihilate the other exciton. This loss process can be alleviated by sweeping out carriers in a carefully designed solar cell. We summarize this understanding using a pentacene PV with a poly(3-hexythiophene-2,5-diyl) exciton blocking layer. The internal quantum efficiency (IQE) of pentacene is approximately 200%, and the IQE of P3HT also exceeds 100%, due to sensitization by pentacene. [1] J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Applied Physics Letters, vol. 95, p. 033301, 2009.

My talk will focus on charge recombination losses organic solar cells and their impact upon device photovoltaic performance. My talk will compare the behaviour of small molecule solar cells, including devices fabricated by both solution and vacuum processes, with that of polymer / fullerene devices. Small molecule devices studied will include both bilayer and bulk heterojunction devices. Experimentally my talk will be based around transient optical and optoelectronic studies of the yields, lifetimes and densities of dissociated charges, focusing in particular upon quantification of the impact of geminate and non-geminate recombination losses upon device fill factor and open circuit voltage. Issues which I will address will include photoactive layer charge density, electrode charge, molecular structure and crystallinity, film structure, interfacial energetics, and macroscopic electric fields.

Singlet exciton fission is a process known to occur in a range of polyacene materials whereby one optically generated singlet exciton splits into two lower-energy triplet excitons. By generating two excitons from a single photon, singlet fission is capable of pushing the efficiency of solar cells past the Shockley-Queisser limit, but only if two conditions are met: the absorption of the large-energy-gap singlet fission donor material must be complemented by the absorption of a lower-energy-gap acceptor material, and the dissociation of triplet excitons at the donor-acceptor interface must occur with a high quantum yield. Here, we analyze the performance of singlet fission-based solar cells fabricated with two different donor materials (pentacene and 6,13-diphenylpentacene) and three different classes of acceptor materials (fullerenes, perylene diimides, and lead chalcogenide quantum dots (QDs)), constituting donor-acceptor pairs with charge transfer (CT) state energies spanning a range of nearly 1 eV. By fabricating pentacene/QD photovoltaic devices with a range of QD energies, we demonstrate that pentacene contributes to the photocurrent through dissociation of triplet CT states at the pentacene/QD interface, rather than through singlet CT state dissociation or triplet energy transfer. Through measurements of the external and internal quantum efficiency and the magnetic field dependence of the photocurrent, we also show that across the different donor-acceptor pairs tested, the ability of singlet fission to contribute to the photocurrent scales inversely with the CT state binding energy of the donor-acceptor pair. Lead chalcogenide QDs are identified as potentially ideal acceptor materials for singlet fission-enhanced donors, as their high dielectric constant leads to a small exciton binding energy and their low bandgap enables absorption into the near-infrared beyond the typical range of the organic donors.

In contrast to conventional (inorganic) semiconductor photovoltaic cells, the working mechanism of their organic counterparts is markedly different, most importantly due to the excitonic nature of photo-excitations in organic semiconductors. Using them as photovoltaic materials thus requires finding ways for an efficient dissociation of excitons with binding energies of the order of 0.5 eV into free charge carriers that can deliver an electrical current into the external circuit. In this context, the well-established donor-acceptor (D/A) concept enabling photo-induced charge transfer between two partners with suitable energy level alignment has proven extremely successful. Nevertheless, the introduction of such a hetero-junction is accompanied with additional energy losses as compared to an inorganic homo-junction cell, owing to the presence of a charge-transfer (CT) state at the D/A interface. Based on the principle of detailed balance we have developed a modified Shockley-Queisser theory including the effects of interfacial CT states that allows for a quantitative assessment of the thermodynamic efficiency limits of molecular D/A solar cells. Key parameters, apart from the optical gap of the absorber material, entering into the model are the energy and relative absorption strength of the CT state. We demonstrate how the open-circuit voltage and thus the power conversion efficiency are affected by different parameter values. Furthermore, we show that temperature dependent device characteristics can serve to determine the CT energy, and thus the upper limit of Voc for a given D/A combination, as well as to quantify non-radiative recombination losses. The model is applied to diindenoperylene (DIP)-based photovoltaic devices, where open-circuit voltages between 0.9 and 1.4V, depending on the partner, have recently been reported.

Phthalocyanines (Pc) are an archetypal material class for incorporation in organic photovoltaic cells (OPV). The key benefits are their high absorption coefficients and polymorphism that enable tuning of thin film optical and electrical characteristics. Incorporation of different metal ions in the Pc ring changes their functional properties: small metal ions lead to planar Pcs, while large metal ions, metal-oxygens and metal-halides disrupt planarity. In both cases, thin solid films of these molecules can have different molecular packings depending on the growth conditions, which have an effect on the opto-electronic properties and the anisotropy thereof. Here, we demonstrate recent progress in controlling the growth of Pcs to reach a favorable absorption spectrum and electronic landscape. Several parameters influence this growth, and here we focus on 1) the interaction with the underlying layer, 2) substrate temperature during deposition, and 3) solvent annealing after deposition. An example of the first case is the use of copper-iodide (CuI) to influence the crystal orientation of the Pc layer. Experiments with zinc Pc (ZnPc) / fullerene (C60) planar heterojunction cells show a 90% increase in OPV efficiency, mostly attributed to an increased photocurrent. In-situ thermal annealing of the substrate during growth increases the diffusivity of the adsorbed molecules, which results in a higher packing order. Using this technique, devices based on chloro-aluminum Pc (ClAlPc) are enhanced by 25%. The last discussed option is solvent annealing of a pure layer. Using titanyl Pc (TiOPc), the absorption spectrum edge can be shifted from 700 nm to almost 1000 nm by inducing a phase transition. Curiously, the packing order of a pure Pc layer has an influence on the growth of a blended Pc:C60 layer on top of it. In certain cases, the pristine, donor layer templates the blend deposited on top. Here, we present templated Pc:C60 blends on top of CuI templated Pcs. The largest influence is observed by using ZnPc. Proper thickness control of the templating layer in combination with a heated substrate during the blend deposition resulted in efficiencies of 4%, a very high value for this well-studied material combination. Similar observations were made for lead Pc (PbPc), TiOPc and ClAlPc, as proven by XRD and ellipsometry measurements. For PbPc, devices covering the wavelength range from 595 to 950 nm (external quantum efficiencies > 35%) are made, with efficiencies above 3%, opening the opportunity for broadband absorbing tandem devices.

Cyanine dyes are successfully applied in dye sensitized solar cells; they are well known as sensitizers in photographic films and are used in optical data storage media. Still, reports on all-organic thin-film cyanine photovoltaic devices are scarce. This is most surprising as the extraordinary high extinction coefficients of cyanine dyes allow for the manufacturing of efficient bilayer devices. The ease of synthesizing dyes that only absorb in the NIR wavelength range makes them suitable for transparent solar cells, as we will show in recent results on NIR-absorbing cyanine-C60 bilayer cells. Though, the focus will be on some peculiarities of cyanine dyes that make them most attractive for advanced solar cell concepts: firstly, cyanine dyes are ionic dyes, and ion movement allows for the formation of so-called dynamic heterojunctions. Secondly, cyanine dyes are known to assemble into J-aggregates with a delocalization of the excited state far beyond the usual exciton diffusion length found in organic semi-conductors. In the last part we discuss the controlled formation of bulk heterojunction morphologies in dye- PCBM blend films.

Many technological applications indispensable in our daily lives rely on carbon. By altering the periodic binding motifs in networks of sp3, sp2, and sp-hybridized carbon atoms, researchers have produced a wide palette of carbon allotropes. Over the past two decades the physico-chemical properties of low-dimensional nanocarbons including fullerenes (0D), carbon nanotubes (1D), and, most recently, graphene (2D) have been explored systematically. We expect that many of the strategies that have recently been exploited and established in the context of 1D nanocarbons can be applied to the chemistry of 2D nanocarbons, especially single layer graphene. Two-dimensional nanocarbons are currently attracting extensive attention due to their striking mechanical, optical, and electrical features. Nanocarbons a single atom-thick are gapless semiconductors and exhibit electron mobilities reaching values of up to 15000 cm2 V-1 s-1 at room temperature. Researchers have made rapid progress in the covalent and / or non-covalent functionalization of single layer graphene with photo- and or redox active building blocks. In this contribution, we summarize our work on the integration of photo- and / or redox active building blocks, including oligomers, molecules, and particulates onto graphenoid materials to yield multifunctional electron donor-acceptor conjugates and hybrids. Intriguingly, we produce graphene in the form of single layer, bilayer, and multilayer graphene through the exfoliation of graphite by surface active agents. The exfoliation occurs through π-π, hydrophobic, van der Waals, electrostatic, and charge transfer interactions, and the surface active agents also serve as versatile anchor groups. We studied the electronic interactions in terms of photo- and / or redox activity in depth by steady-state and time-resolved spectroscopy. Finally, we present examples of proof-of-principle solar energy conversion devices.

If the active layer components are properly organized at molecular length scales there is no fundamental reason why small molecular organic solar cells (OPVs) cannot compete with polymer/small molecule OPVs in terms of both efficiencies and lifetimes. It is challenging, however, to design both donors and acceptors with appropriate visible and near-IR absorbance, frontier orbital energies, and thin film morphologies and molecular architectures which optimize photocurrent formation and high Voc, and efficient charge collection. This talk will focus on recent areas of interest for us: i) the use of solvent-annealed and textured phthalocyanine donor layers based on chloro-indium phthalocyanine (ClInPc) and titanyl phthalocyanine (TiOPc), where photocurrent yields can be greatly enhanced by increasing the interfacial contact area between the Pc and the C60 acceptor, but where the surface composition of the hole-harvesting contact plays a critical role in device performance; ii) the extrapolation of these studies to liquid crystalline versions of these Pcs, making solution processing of fully mixed small molecule OPVs possible; iii) the performance of high Voc OPVs using new fluorinated sub-phthalocyanines (F5-subPc) which demonstrate that the subPc can act both as a donor toward C60 (F5-subPc/C60 heterojunctions) and as an acceptor toward donors like pentacene (PEN/F5-subPc heterojunctions), with Voc approaching 1 volt in both cases.

A donor-acceptor bilayer forms the active core of an organic planar heterojunction solar cell. However, for optimal device performance, the use of additional transport layers is indispensable. For evaporated planar heterojunctions, phenanthroline-based materials like bathocuproine (BCP) are a very popular choice as a buffer layer between acceptor and cathode. [1] It has been shown previously that BCP fulfills multiple roles: it blocks excitons, protects the active layers during metal evaporation and aids electron extraction. [2] Here, we show that various donor/C₆₀ configurations present higher fill factors (FF) and short-circuit currents (J_sc) when the cathode has an additional 3,4,9,10- perylenetetracarboxylic-bisbenzimidazole (PTCBI) layer in between BCP and the cathode metal. Voltage dependent external quantum efficiency (EQE) of a zinc phthalocyanine (ZnPc)/C₆₀ /BCP/Ag device reveals that at high inverse voltages, the C₆₀ signal increases compared to short-circuit conditions, while the ZnPc signal is unaffected. [3] In the structure ZnPc/C₆₀/BCP/PTCBI/Ag, this C₆₀ current is already collected at lower voltages, resulting in both higher J_sc and FF. A model is proposed to explain this phenomenon. This advanced BCP/PTCBI/Ag cathode configuration is next applied with the donor diindenoperylene (DIP). [4] The reference device DIP/C₆₀/BCP/Ag has relatively high open-circuit voltages (V_oc) of 0.91 V and FF = 62%. Most of the current in this device (J_sc=4.5 mA/cm²) is generated by C₆₀, as DIP has a relatively weak absorption coefficient. Using the double blocking layer BCP/PTCBI boosts J_sc to 5.66 mA/cm² and FF to 69%, resulting in eta;_P = 3.6%, which is an almost 50% increase. According to optical simulations, more than 80% of the current in this device originates from C₆₀ absorption. This leads us to replace C₆₀ in the device with C₇₀, which is known to have a stronger absorption profile, especially in the wavelength range between 450 and 700 nm. This increases J_sc to 8.9 mA/cm², with EQE peaking at 63% around 500 nm. As the C₇₀ based devices reach similarly high FF = 69% and V_oc = 0.92 V, this solar cell achieves an unprecedented 5.7% planar heterojunction efficiency. [1] Peumans et al., Appl. Phys. Lett.2001, 79, 126 [2] Gommans et al., Adv. Funct. Mater.2008, 18, 3686 [3] Jeong et al., Adv Funct. Mater.2012, DOI: 10.1002/adfm.201200069 [4] Wagner et al., J. Appl. Phys.2012, 111, 054509

Asymmetric tri-valent and tetravalent phthalocyanines (ClInPc, TiOPc) have excellent near-IR photoresponse, the extent of which is controlled through solvent annealing of these donor layers. OPVs have shown VOC asymp; 0.8 volts (ClInPc) and efficiencies exceeding 4%, when blended, co-deposited, or layered with an electron acceptor like C60. The strong internal dipole moment for these donor materials plays a critical role in the interaction with the hole-harvesting contact. We show here that direct interaction between the dipolar donor molecule and a hole-harvesting contact like ITO is essential to create sufficient ordering in the first 1-2 monolayers of donor dye deposited so that current-voltage parameters are optimized. UV-photoemission spectroscopies, during the formation of the initial ITO/Pc heterojunction, show significant and abrupt changes in both local vacuum level and ionization potential at the completion of the first monolayer. These shifts are a result of a strong interaction between the donor and ITO which leads to efficient hole-harvesting in the completed OPV. Once this dipolar interaction is disrupted, either by insufficient activation of the oxide, or by chemical modification with a dipolar phosphonic acid, which prevents this interaction, OPV efficiency is compromised through increases in series resistance, and significant decreases in fill-factor associated with enhanced recombination. The extension of these studies from planar heterojunction to bulk-heterojunction platforms obtained through systematic variations in co-deposition and solvent annealing parameters, where nanometer-scale control is shown over the degree of D/A intermixing and the near-IR photoresponse, where interactions with the oxide surface are even more significant.

We demonstrate increased photocurrent quantum efficiency at zero and small forward bias in a model donor/acceptor (tetracene/C60) photodiode by incorporating an insulating tunnel barrier between the tetracene and C60 layers. Photodiode efficiency is the result of the interplay of a number of processes with reaction rates which add to or subtract from the overall device efficiency. The positive rates are those of exciton dissociation and charge separation, the negative rates include exciton and charge transfer complex recombination. We show that by introducing a thin insulating layer between the donor and acceptor layers in a photodiode, we can modify the exciton dissociation and charge transfer complex recombination rates and improve device performance.

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 and mobilities. 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 a thin organic interfacial layer followed by a planar-mixed heterojunction. For optimal utilization of the light in the planar-mixed architecture, the thickness of neat films must be kept within their exciton diffusion lengths and the thickness of the mixed layer is constrained by the ability to maintain continuous percolating pathways of phase-separated donor and acceptor materials. Using a tetracene interfacial layer between the anode and the donor material has proven to have the effect of increasing the crystallinity in the donor film resulting in a large enhancement in the quantum efficiency of the neat film through enhanced charge collection and absorption. Thus, the thickness of the mixed layer 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 JSC of over 12mA/cm2 and a VOC of 0.66V while maintaining a high FF of 0.66 and power conversion efficiencies over 5.2%.

Conjugated polymer/fullerene-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices show promise to be lightweight, flexible, cost-effective alternatives to traditional inorganic solar cells. However, the efficiencies and lifetimes of BHJ-OPVs are unable to compete with current photovoltaic technologies, with a maximum recorded efficiency of 10% and typical lifetimes less than 1 year (cf. inorganic solar cells with maximum efficiency of 34.1% and lifetime of 25+ years) [1]. It has been shown that the lifetime of OPVs can be improved by using the device in an inverted configuration (ie. metal anode and transparent conducting oxide cathode). Similarly, it has also been proposed that the efficiency of OPVs can be improved by utilizing plasmonic nanostructures to enhance the optical electric field within the photoactive layer [2]. Here, we propose the use of plasmonic nanoantennas integrated into high work function metals as optically-active anodes in inverted OPV devices. We have calculated the performance parameters - the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the fill factor - for inverted OPVs containing optically-active anodes and have compared the results to 1) planar inverted OPVs (which lack the nanostructures), 2) conventional OPVs containing optically-active cathodes, and 3) planar conventional OPVs. The optical electric field was calculated using finite-difference time domain (FDTD) simulations for both nanostructured and planar devices and verified analytically using transfer matrix (TM) methods for planar devices. We have fabricated an array of optically active electrodes to be used in the various device configurations by thermally evaporating metal through nanoporous alumina membranes. The optical properties of the bare nanostructures and the photoactive layer on the metal electrode (both planar and nanostructured) have been characterized using UV-VIS reflectance spectroscopy with an integrating sphere attachment and dark field imaging spectroscopy, and the nanostructures have been imaged using scanning electron microscopy (SEM). Preliminary calculations in devices containing the optically-active electrodes show an enhancement in the photocurrent of 1.5, resulting from an increased red-edge absorption in the photoactive layer, leading to an improvement in the overall efficiency of the device. [1] Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 39). Prog. Photovolt: Res. Appl. 2012, 20, 12-20. [2] Morfa, A. J.; Rowlen, K. L.; Reilly III, T. H.; Romero, M. J.; van de Lagemaat, J. Plasmon-Enhanced Solar Energy Conversion in Organic Bulk Heterojunction Photovoltaics. Appl. Phys. Lett. 2008, 92, 013504.

Highly flexible organic photovoltaic (OPV) cells are fabricated on zinc indium tin oxide (ZITO) as transparent conducting electrode. Amorphous ZITO films are deposited on Arylitetrade; substrate via pulsed laser deposition (PLD). Excellent optical transmittance for visible spectrum is achieved. The ZITO films demonstrate a resistivity of ~20Omega;/sq, comparable to the commercially available indium tin oxide (ITO) films. Only small degradation in conductivity is observed after controlled bending test. Using a novel high efficiency bithiophene imide (BTI) polymer, the resulting pristine OPV devices demonstrate similar power conversion efficiencies (PCE) to devices using commercially available ITO on glass as substrate. Furthermore, good device PCEs are maintained after bending test.

Silicon-organic heterojunction (SOH) solar cells represent a new class of photovoltaic device., in which light is absorbed in crystalline silicon, but without a p-n junction formation or high temperature (> 100°C) processing. A thin organic layer with a high LUMO serves to block majority carrier electron current from the silicon to the anode (for low dark current and high VOC). The work function of a transparent conductive anode provides the build-in field in the silicon to collect photo-generated minority carriers [1]. For low-cost production, screen-printing of silver contacts is typically used in silicon cells, but this requires a sintering process at around 300-600°C, which is too high to be compatible with organics. In this work we present (i) a transfer printing method to create patterned metallization on the device with maximum temperature of 80°C, and (ii) show experimentally that the blocking of majority carriers is so efficient that the remaining dark current consists predominately of minority carrier injection into the silicon, and (iii) demonstrate an efficient hybrid photovoltaic device with power conversion efficiency of 10.5% and fill factor reaching 85% under AM 1.5 illumination, (with all processing below 90°C). In this work three different approaches towards the anode structure, which consists of doped PEDOT as a transparent conductor followed by a fingered metallization to extract the photocurrent are used. First, a traditional electrode formed via shadow mask on PEDOT is used as the baseline device but is not scalable to large area. Second, Silver nanowire dispersions were also deposited on the devices to enhance lateral conductivity, improving fill factors greatly. Finally, a printed metallization scheme was developed in order to allow for future large-scale, low temperature processing of these devices. Devices with printed metal electrodes show minimal differences from those with thermal evaporation. To verify the effectiveness of the organic electron-blocking layer, the predominant mechanism of the resulting dark current mechanism was verified by a transient measurement of the stored minority carriers in the silicon. This showed that the current from majority carrier electrons was so effectively blocked that the remaining current was due to the injection of holes from the anode into the n-type silicon as minority carriers, independent of the type of anode. [1] S. Avasthi et al., Adv. Mater. Vol. 23 Iss. 48 p.5762-5766 (2010)

Remarkable progress happened in organic photovoltaic during the last years. Efficiencies beyond 10% were reported, as well as lifetimes beyond 10 years. With the first applications coming to market, the costs of OPV become more and more decisive. Large area coating and printing are certainly the technologies with the lowest production related costs. Large area printing appears to be more attractive for module production, but suffers from low viscous inks and the drawback of contact based printing principles. Large area coating is very attractive in terms of homogenous coating at high speed and is compatible to the low viscous inks. However, the drawback of coating is the very limited down-web resolution and the non- existing cross-web resolution. Thin-film photovoltaic modules require the sectioning into multiple cells which are connected in series. Three scribing patterns P1, P2 and P3 (Fig. 1) are typically required to form a monolithically interconnected module. In this contribution we discuss the combination of laser patterning and slot-die coating as a high speed, high precision production method for organic photovoltaic. A state of the art high power, high repetition rate femtosecond lasers with micro joule pulses is utilized for this study. The unique advantage of material processing with sub-picosecond lasers is efficient, fast and localized energy deposition, which leads to high ablation efficiency and accuracy in nearly all kinds of solid materials. The most sensitive materials in an organic solar cell stack are the semiconductor layer and the electrodes. We determined the ablation threshold of the single materials, and demonstrate that the individual ablation threshold for the single layers is sufficiently different to allow patterning with a single lasing wavelength and at highest speed. Functional modules are demonstrated for the normal as well as inverted architecture. We further demonstrate patterning with 10 micron resolution, and discuss whether today's electrode materials are sufficiently conductive for geometrical fill factors beyond 90%.

The quest for an efficient and inexpensive way of converting solar energy into electricity has drawn many researchers&’ attention to Organic Photovoltaic Cells (OPV) during recent years.¹ To date, the most successful solution processable OPV devices are achieved with Polymer-Fullerene blends, consisting of an electron-donating (p-type) conjugated polymer and an electron-accepting (n-type) fullerene, such as PCBM, as active layer composite. Power conversion efficiencies exceeding 9% were demonstrated recently.² However, for commercialization of this technology, further improvements not only concerning efficiency, but also lifetime and stability, are needed. Hence, new organic materials which can fulfill all necessary parameters for high performance OPV, such as light harvesting, charge transfer, charge transport and thermal, chemical and photostability are required.³ Perylene bisimides (PBI) and PBI related derivatives are a relevant class of n-type semiconductors due to their relatively high electron affinity and strong visible light absorption, combined with good photochemical and thermal stability and low material costs.⁴ Even though it was shown that PBIs can be better electron acceptors than PCBM with respect to charge photogeneration,⁵ their performance in solar cells is still significantly lower. This is mainly attributed to charge recombination in domains of strongly aggregated PBIs. To optimize these devices, structural order of PBI thin films and morphology in corresponding blends with p-type materials need to be controlled. Herein, three different N-substituted PBIs are compared. All three compounds comprise an unsymmetrical substitution pattern, which reduces aggregation in thin films significantly, compared to the symmetrical analogue. The nature of the substituents was varied from hydrophobic alkyl chains to hydrophilic ethylene oxide chains to allow for tuning self-assembly properties of the compounds. Aggregation and crystallinity in thin films were studied by polarized optical microscopy, UV/vis spectroscopy, X-ray diffraction (XRD) and Atomic force microscopy (AFM). A correlation of structure/morphology and charge transport properties, measured by the Space-Charge Limited Current method (SCLC), is given. XRD experiments give evidence for a highly ordered and oriented packing of the PBIs after an annealing step. Electron mobilities as high as 7×10^-3cm²V^-1s^-1 were measured. REFERENCES: ¹ J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, Science2007, (317), 222. ² http://www.polyera.com. ³ C. Li, H. Wonneberger; Adv. Mater.2012, (24), 613. ⁴ A. Wicklein, A. Lang, M. Muth and M. Thelakkat, J. Am. Chem. Soc.2009, (131), 14442. ⁵ S. Shoaee, Z. An, X. Zhang, S. Barlow, S.R. Marder, W. Duffy, M. Heeney, I. McCulloch, J. R. Durrant, Chem. Commun.2009, 5445.

Spectrum modification through bi-molecular upconversion, with a liquid phase upconverter, has been shown to enhance the external quantum efficiency (EQE) of partially transparent amorphous silicon solar cells under moderate concentration, Cheng et al. [1]. Sensitizer and emitter molecules work together to combine the energy of two below bandgap photons to produce a single photon with sufficient energy to be absorbed by the solar cell. The process exploits long lived triplet states and the spin conservation inherent in triplet-triplet annihilation to combine the energy of these below band-gap photon pairs. The result is prompt fluorescence from an emitter molecule, which produces a photon possessing significantly higher energy than either of the incident photons. The efficiency of bi-molecular upconversion depends strongly on the concentration of excited triplet states, as the rate of annihilation (k_Annihilationprop;[triplets]²) must be sufficiently rapid such that the rates of phosphorescence and non-radiative decay(k_Phos,NR prop;[triplets]) become insignificant, Cheng et al. [2]. The concentration of triplet states can be increased by providing more intense incident light or by concentrating the active molecules spatially. This spatial concentration has a limit when in solution as the molecules are able to aggregate or otherwise precipitate. This stalls the upconversion process by reducing the degree of mixing in the solution. The formation of a well mixed solid would reduce the average empty volume around active molecules by a factor on the order of 10³, increasing the potential density of excited states by a correspondingly significant amount. Therefore the fabrication of a well mixed thin film is a clear route to producing an efficient solid state upconverter. In this study, a number of thin films containing upconverting molecules were fabricated through spin casting, wire bar coating and drop casting. The ability the films to perform upconversion was recorded by observing time resolved phosphorescence and delayed fluorescence from each film under pulsed laser excitation. Initial experiments with drop casting and spin coating of a mixture of the upconverting molecules found that, on their own, the small molecules films did not produce any measurable upconverted light. It was found that the addition of a small amount, up to 10% by weight, of the optically and electrically inert polymer poly(methyl methacrylate) (PMMA) increased the number of upconverted photons such that up to 20% of photons emitted from the films were created through upconversion, the remaining 80% being produced by phosphorescence. Kinetic data from these experiments are presented and compared to a detailed rate model allowing insight into the changes in the energy transfer processes. Steps towards further efficiency gains are also discussed. [1] Cheng et al. Energy Environ. Sci., 2012, 5, 6953-6959 [2] Cheng et al. Phys. Chem. Chem. Phys., 2010, 12, 66-71

The charge transfer state (CTS) at the donor/acceptor interface in organic solar cells plays a crucial role as an intermediate state in charge separation. In order to minimize energetic losses between photon absorption and free charge carriers, it is important to understand the need for excess energy between the exciton and the CTS. This work aims to discuss more in detail the function of energy offsets between donor and acceptor materials in driving charge separation. Toward this goal, we have studied the photovoltaic properties of archetypal planar heterojunctions based upon phthalocyanines as donors and C60 or a perylene derivative as acceptors by means of a highly sensitive technique called fourier transform photocurrent spectroscopy (FTPS). FTPS is an ultrasensitive technique able to measure the external quantum efficiency (EQE) over up to 9 orders of magnitude.[1] We demonstrate the ability to detect in the EQE spectra the photocurrent from direct charge transfer state excitation despite the intrinsically small interface area for bilayer systems. We evaluate the energy of the CTS, and discuss the implications of triplet vs. singlet excitons in these systems by showing that triplet and singlet excitons from the phthalocyanine donors are able to dissociate with equal efficiency, even though the driving force is 0.5 eV less,[2,3] and in fact the driving force for triplet exciton dissociation is only about twice the background thermal energy. Therefore, hot charge transfer states are not required, and efficient exciton dissociation is driven by an internal electric field at the heterojunction from either an interface dipole or beneficial polarization effects. 1) Vandewal, K. et al Thin Solid Films 2008, 516, 7135. 2) Zahn, D. et al Chem. Phys. 2006, 325, 99. 3) Kahn, A. et al J. Polym. Sci. Part B: Polymer Physics 2003, 41, 2529.

The energy-level alignment between the organic layer and metal oxide surface often dictates the performance of OPVs. The approximate hole-/electron-collection barrier heights cannot be determined from the molecular IPs and EAs as interfacial dipoles shift their relative positions at interfaces. Here, we have used perylene-based dyes as prototypical electron acceptors, and zinc phthalocyanine as a prototypical donor to evaluate the extent of charge transfer on a non-polar ZnO(10-10) surface. We first considered the neat (10-10) non-polar surface. We observe a substantial interaction between the zinc atoms on the surface and the carbonyl oxygens of the adsorbed 3,4,9,10-perylene-tetracarboxylic acid dianhydride and diimide (PTCDA, PTCDI). This leads to a slight tilt in the molecules at the interface, but only small changes in the electronic structure. There is overall little modification of the molecular and oxide energy levels, with the energy level alignment indicating that there is an unoccupied state (approximately the molecular LUMOs) around mid-gap. The resulting geometry has a molecular dipole moment pointing toward the surface and leads to a consequent decrease in the work function. In the case of zinc phthalocyanine, there is little interaction between the adsorbate and the surface. The energy level alignment between the oxide and the molecule indicates that there is an occupied state (the molecular HOMO) at mid-gap. We then investigated the edge-on and co-facial configurations where these molecules were electronically coupled to each other as well as to the surface. Next, we considered the presence of defects in the metal oxide. An oxygen vacancy (V_O) on the surface has been observed in experimental STM images. Our calculations show that the surface then has an occupied level approximately 0.5 eV above the valence band and a corresponding shift in the Fermi level. Based on the energy-level alignment in our system, this leads to occupied defect states above the perylene LUMO. Our calculations indicate that there is electron transfer from the surface to the molecule. We have finally considered cases where the organic layer is covalently linked to the zinc oxide surface. The resulting changes in the work function and the implications for solar cells will be discussed.

Although there is potential for high external quantum efficiencies in singlet fission organic solar cells, the experimental external quantum efficiencies have yet to exceed 100%. Due to the limited number of materials that exhibit singlet fission, it is difficult to find one that has high absorption, exciton diffusion, charge separation, charge collection, and efficient singlet fission to achieve the theoretical external quantum efficiencies of over 100%. One way to ease the design constraints for singlet fission based organic solar cells is to orthogonalize the process of singlet fission from absorption, exciton diffusion, charge separation, and charge collection by inserting a thin film of singlet fission material at the donor/acceptor interface. Singlets generated in the donor diffuse and energy transfer to the singlet fission sensitizer where they undergo fission. In this device structure, the donor can be chosen for high absorption, exciton diffusion, and charge collection, and the singlet fission sensitizer can be chosen for high singlet fission efficiency. Here, we report the concept of singlet fission sensitization using the singlet generating donor TPD and rubrene as the singlet fission sensitizer. Singlet fission sensitization is shown in the optical domain via changes in fluorescence versus magnetic field in a thin film of neat TPD, rubrene, and a co-doped film. The concept is demonstrated in organic solar cells comprised of TPD and rubrene donors where we show the effect of sensitization by changes in photocurrent versus magnetic field and a doubling of the external quantum efficiency of TPD with singlet fission sensitization.

Currently, C60 is the most widely used acceptor for small-molecule organic photovoltaic cells due to its high mobility and long exciton diffusion length; however, C60 suffers from a low absorption coefficient that limits the external quantum efficiency, and therefore the short-circuit current, of these devices. Compared with C60, C70 has a much stronger absorption between wavelengths of lambda;= 450 nm and 600 nm, while its other physical properties are similar, particularly the lowest unoccupied molecular orbital energy that ultimately determines the open circuit voltage of the device.[1] In this talk, we describe an efficient small-molecule OPV based on a SubPc:C70 graded heterojunction inspired by the recent work on SubPc:C60 graded heterojunctions.[2] Compared to structures employing C60, the EQE spectrum is significantly enhanced across the visible spectrum, resulting in a short-circuit current of (9.1±0.1) mA/cm2, while the open circuit voltage (1.04±0.01 V) and the fill factor (0.46±0.01) are almost identical. The SubPc:C70 graded heterojunction cells exhibit a power conversion efficiency of (4.3±0.1)% at one sun, AM 1.5G illumination (spectrally corrected), approximately 20% higher than comparable C60 structures using grown in our lab; to our knowledge this is the highest efficiency achieved for SubPc-based OPVs. Optimization on the SubPc:C70 graded heterojunction can further improve the device performance. In this talk, we will also discuss functionalized squaraine donor based OPV devices improved by substituting C70 for the C60 acceptor. Reference: [1] S. Pfuetzner, J. Meiss, A. Petrich, M. Riede and K. Leo, Appl. Phys. Lett. 94 (22) (2009). [2] R. Pandey and R. J. Holmes, Adv. Mater. 22 (46), 5301-5305 (2010).

Short-wavelength photoexcitation of organic donor-acceptor materials results in a significantly higher polaron-pair yield than absorption of long-wavelength photons close to the optical bandgap. This is a further strong indication that extraction of weakly-bound polaron pairs might be a more promising way for efficient solar cells than extraction of strongly bound excitons. A lower required driving force for their separation can be beneficial for an increase in open-circuit voltage and power-conversion efficiency. Recently, we demonstrated that the branching ratio of polaron-pairs and excitons upon photoexcitation in neat films is up to three times higher in donor-acceptor polymers than in wide spread homopolymers (1). It can be influenced significantly by changing the chemical composition and structure of the material. Our recent results proof that this effect is not only present for excitations in the low-energy absorption band, but is even more pronounced at shorter wavelengths. This finding is of great importance with regard to the usage of such materials in efficient organic solar cells covering a large range of the solar spectrum. In our experiments we compare polymers and oligomers of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl] ] (PCPDTBT) and very similar materials at different excitation wavelengths, ranging from the near-infrared through the visible up to the ultra-violet spectral region. Ultrafast spectroscopic measurements clearly show continuously reduced excitonic and increased polaronic features for shorter excitation wavelengths directly after photoexcitation. This effect is observable in all studied donor-acceptor polymers and even much more pronounced in short oligomers. This and further experimental and theoretical facts lead us to the conclusion, that on-chain localization and wavefunction confinement of the photo-excited states play a key-role for efficient polaron-pair formation and charge separation. (1) R. Tautz, E. Da Como, T. Limmer, J. Feldmann et al., Nature Communications, accepted for publication (2012)

In an organic solar cell an exciton, electron-hole pair, is formed when the donor or acceptor absorbs a photon. Generated excitons diffuse toward donor-acceptor interfaces where they are disassociated into free electrons and holes, a crucial step in the operation of organic solar cells. An important limitation in this process is the short exciton diffusion length of organic semiconductors, which is on the order of 3-10 nm. If the donor and the acceptor domains are large, there is a high probability for excitons to relax back down to the ground state before reaching a donor-acceptor interface. Thus, it is important to know exciton diffusion lengths and to understand the influence of chemical structure. Several methods have been reported to measure exciton diffusion lengths by different groups on various materials, but it is unclear which method is more reliable. To resolve this issue, we selected test small molecule materials based on the diketopyrrolopyrrole (DPP) core, and measure exciton diffusion using four different methods and compare results obtained from these methods. The impact of chemical structure on the exciton diffusion length is also probed for a series of solution processed small molecules used as donors in bulk heterojunction solar cells.

Singlet exciton-fission is the organic equivalent of multiple exciton generation in semiconductors. A high energy photon is absorbed as a singlet exciton and rapidly converted into a pair of lower energy triplet excitons. In combination with a low bandgap absorber, solar cells can be designed in such a way that they are not limited by the Shockley-Queisser limit. We present the first solar cell of this kind using pentacene as the material that undergoes efficient exciton fission and PbS/PbSe nanocrystals as the infrared absorber. We have already demonstrated competitive power conversion efficiencies. To rationally design such a solar cell, the triplet energy in pentacene needs to be known. However, the triplet state in pentacene is non-luminescent and hence inaccessible to direct observation. We have developed an in-situ technique to measure non-luminescent excited states using a size series of PbSe nanocrystals in solar cells and we find the triplet energy to be at least 0.85eV, close to half the pentacene band gap. This indicates surprisingly low energy loss in device operation.

Improved materials design and the examination of the underlying physical processes involved in photocurrent generation have led to enhanced power conversion efficiencies for organic solar cells. A key bottleneck, yet critical requirement, that remains is a thorough understanding of the combined processes of exciton dissociation, charge separation, and charge recombination that occur at the interface between the electron-rich (donor) and electron-poor (acceptor) materials. Here we will discuss the development of computational models of molecule:molecule interfaces through the use of classical molecular mechanics and dynamics approaches in combination with quantum mechanics. Our goal is to provide a molecular-scale description of the interfacial morphology in order to assess the evolution of the key electronic states involved in charge separation and recombination. These models serve as an initial step toward the development of integrated, multiscale modeling approaches that will be used to enhance our understanding of organic solar cells.

We examine the performance of organic photovoltaic cells (OPVs) containing the electron donor-acceptor (D-A) pairing of boron subphthalocyanine chloride (SubPc) and C₇₀. As an acceptor, C₇₀ is an attractive alternative to C₆₀ due to its red-shifted absorption edge and large extinction coefficient, both of which permit increased photocurrent generation from the acceptor. In uniformly mixed OPVs based on SubPc:C₇₀, peak power conversion efficiency is realized in devices containing 80 wt.% C₇₀. A further enhancement in device performance is realized through the use of a continuously graded D-A heterojunction. Optimized graded heterojunction OPVs show a power conversion efficiency of eta;_P = (5.4±0.2)% under simulated AM1.5G solar illumination at 100 mW/cm², significantly larger than the value of eta;_P = (4.7±0.2)% obtained for OPVs based on uniform mixtures of SubPc:C₇₀. This improvement comes from an enhanced short-circuit current density in the graded heterojunction OPV due to increases in both the absorption and charge collection efficiencies. The charge collection efficiency is probed directly using measurements of the external quantum efficiency under reverse bias. The efficiency of the SubPc:C₇₀ graded heterojunction OPV is found to be significantly larger than the value of eta;_P = (4.5±0.1)% realized using C₆₀ due to a substantial increase in the short-circuit current density when using C₇₀ as an acceptor.

Organic solar cells offer significant challenges for commercialization and promising pathways for development of fundamental understanding; the process of charge photogeneration is still not fully described, and the selection of unique materials with highly interesting optical and electrical properties is crucial. We focus on a class of near-infra-red absorbing small molecule donor materials called squaraines; with high extinction coefficients (ε>10⁵ L mol^-1 cm^-1) at 700-800 nm, relatively simple synthesis, high solubility, high photostability, and propensity for self-assembly our materials show great promise for incorporation into industrial-scale organic solar cells. Our published power conversion efficiencies of 2.4% in a bulk heterojunction device with phenyl-C61-butyric acid methyl ester [PCBM] as the acceptor material indicate that the materials can be utilized in devices; the high short circuit current (~14 mA cm^-2) and open circuit voltage (500-600 mV) combined with the low fill factor (30-33%) show that there is still significant room for improvement if the morphology of the materials in the active layer device architecture can be better understood. We demonstrate the formation of both H- and J-aggregates in the squaraines using linear absorption measurements, and characterize these aggregates with absorption, thin-film X-ray diffraction, atomic force microscopy, and time-resolved microwave conductivity. The presence of these self-assembling aggregates impacts the homogeneous domain&’s ability to transport charge, as well as the efficacy of charge photogeneration at the bulk heterojunction interface. The excitons that are formed on each type of aggregate are distinct both optically and electrically, and we use both the solid state analysis of thin films as well as device measurements to describe the impact the aggregate has on the exciton and the possible effects it may have on the charge transfer state and the charge photogeneration process.

We present photoluminescence (PL) quenching and ultrafast interfacial charge transfer dynamics studies of poly (3-hexylthiophene-2,5-diyl) (P3HT) thin films doped with monodisperse TiO₂ nanoparticles. Significant PL quenching is obtained when the P3HT films contain up to 80% TiO₂ nanoparticles in weight due to charge transfer. Polaron absorption after the photoinduced charge separation in the presence of 80% TiO₂ is significantly enhanced with longer-lived lifetimes in contrast to neat P3HT films. The absorption of polarons created at the P3HT/TiO₂ interface is found to exhibit a linear dependence on the P3HT/TiO₂ interfacial area per unit volume. Surprisingly, PL is enhanced for films containing 20% TiO₂ as compared to neat P3HT films, which is attributed to disordering of polymer chain and increase in film roughness as shown by steady state PL and electrogenerated chemiluminescence (ECL) studies. ECL studies show a decrease in the ECL turn-on potential of P3HT/TiO₂ composites proportional to their TiO₂ content, corresponding to the ECL yield increases, indicating the formation of an effective interpenetrating network of TiO₂ and disordering of polymer packing which allows the ECL coreactant to transport through the film for efficient ECL generation.

Organic solar cells have recently passed the 10% efficiency barrier, which is a milestone in the road towards their commercialization.(1, 2) Like their fully organic counterparts, hybrid solar cells use a donor polymer to absorb light, while the organic acceptor (mostly fullerene) is typically replaced by a metal oxide. The latter can be synthesized in various nano-patterns prior to the deposition of the donor polymer, which finally results in a rigid heterojunction, thus giving hybrid solar cells the edge regarding morphological stability.(3, 4) Unfortunately, their efficiency is still falling behind on fully organic solar cells, despite the favorable properties of typical acceptor metal oxides as compared to fullerene derivatives.(5, 6) Several reasons have been postulated for this, with the poor compatibility between polymer and metal oxide being designated as the biggest culprit, both in terms of morphology and associated electronic properties.(7, 8) This contribution presents systematic strategies to improve the efficiency of hybrid solar cells. After substantiating the physical equivalence between organic and hybrid solar cells (and thus the rationale that they should be able to perform comparably), different aspects of the hybrid devices will be highlighted with a keen eye on optimization, both from a fundamental as well as an engineering point-of-view. Taking into account geometric and optical considerations together with charge transport properties and electronic structure, for both ingredients (metal oxide, polymer) several tactics for efficiency enhancement will be discussed that might assist in raising the efficiency of hybrid solar cells to the level of organic solar cells or beyond. (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovoltaics Res. Appl. 2012, 20, 12 (2) Soslash;ndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Mater. Today 2012, 15, 36 (3) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26 (4) Djurisic, A. B.; Chen, X.; Leung, Y. H.; Man Ching Ng, A. J. Mater. Chem. 2012, 22, 6526 (5) Baeten, L.; Conings, B.; Boyen, H.-G.; D&’haen, J.; Hardy, A.; D'olieslaeger, M.; Manca, J. V.; Van Bael, M. K. Adv. Mater. 2011, 23, 2802 (6) Huang, J.; Yin, Z. G.; Zheng, Q. D. Energy Environ. Sci. 2011, 4, 3861 (7) Conings, B.; Baeten, L.; Boyen, H.-G.; Spoltore, D.; D&’haen, J.; Grieten, L.; Wagner, P.; Van Bael, M. K.; Manca, J. V. J. Phys. Chem. C 2011, 115, 16695 (8) Park, B.; Lee, J.-H.; Chang, M.; Reichmanis, E. J. Phys. Chem. C 2012, 116, 4252

The energy cost associated with separating tightly-bound excitons, photo-generated electron-hole pairs, and extracting free charges from highly disordered low mobility networks represent fundamental losses for the two most prominent low cost photovoltaic technologies: organic and dye-sensitized solar cells. Here, we present a low cost solution-processable solar cell based on a mesoporous superstructured absorber with intense visible-to-near-IR absorptivity. This meso-superstructured solar cell (MSSC) exhibits exceptionally few fundamental energy losses illustrated by generating open-circuit photovoltages of over 1.1 V, despite the relatively narrow absorber band gap of 1.55 eV. With a power conversion efficiency of 10.9% in a single junction device under simulated full sunlight, this solid-state photovoltaic concept is close to competitive with established thin-film solar technologies. It has imminent scope to deliver a nanotechnology solution for large scale solar power generation at the lowest cost.

The tunable energy levels and high carrier mobility of silicon quantum dots (SiQD) complement the strong visible range absorption of conjugated polymers and so offer a promising hybrid solar cell paradigm. However, the efficiencies of these composites are typically lower than those of their purely organic counterparts [1,2]. A lack of understanding as to why this is so, and how it can be remedied, has motivated our theoretical study of a representative SiQD/P3HT system. First principles computational tools were used to implement a new, non-adiabatic phonon assisted quantum tunneling model in order to quantify the charge separation and transport dynamics. The predicted charge separation rate is in good agreement with that measured experimentally [3], and it is much higher than the photoluminescence rate of the P3HT. This implies that excitons should be able to efficiently dissociate into free carriers. In addition, the charge hopping proceeds much more quickly than interfacial charge recombination, so a considerable fraction of free electrons and holes should be able to diffuse to the electrode based on the predicted carrier mobilities. In fact, the results indicate that P3HT/SiQD blends have the potential to out perform the widely used, purely organic, P3HT/PCBM architecture. We conclude that the low efficiency of current P3HT/SiQD solar cells originates from the high density of defects and a distribution of SiQD sizes. The influence of dangling bonds, oxidation, passivation and doping on the charge dynamics will be discussed to provide insights for improving the performance of these hybrid materials. This work is supported by the NSF through the Renewable Energy Materials Research Science and Engineering Center (REMRSEC). References [1] S. Niesar, et al., Green 1, 339, 2011. [2] C. Liu et al., Nano Lett. 9, 449, 2009. [3] D. Herrmann et al., J. Am. Chem. Soc. 133, 18220, 2011.

It has been known for over half a century that the efficiency of even ideal single-junction photovoltaic cells that absorb all the incoming solar radiation is limited to 31% (for one sun illumination) by intrinsic losses such as band edge thermalization and radiative recombination. Approaches to exceed this intrinsic thermodynamic limit, named after Shockley and Queisser, included using concentrated sunlight, engineering multiple-junction and intermediate-band solar cells, exploiting mechanisms of electronic up- and down-conversion of photons, thermophotovoltaic, and thermophotonic. The resulting limiting efficiency can e.g. reach 72% in the case of 36-junction solar cell under illumination by a 1000-times concentrated sunlight. We will present a fundamentally new approach to exceeding the Shockley-Queisser limit, which is based on the mechanism of thermal upconversion of photons. We will demonstrate that the limiting intrinsic efficiency of an ideal single-junction solar cell can be increased up to 72% under illumination by an unconcentrated sunlight. A more detailed analysis of non-ideal solar cells that allows for up to 20% of absorption/re-emission losses yields limiting efficiency values exceeding 45%. Finally, we will discuss practical device realizations of the proposed theoretical scheme. Acknowledgment: This work is supported as part of the ‘Solid State Solar-Thermal Energy Conversion Center (S3TEC)&’, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.

Single-walled carbon nanotubes (SWNTs) possess outstanding electrical and thermal conductivities and enormous tensile strength, which makes them a versatile material for many applications. Recently, hybrid photovoltaic based on Carbon nanotube (CNT)/n-type crystalline silicon (Si) has been reported [1]. A major benefit of such an approach is the combination of charge separation at the CNT/Si interface along with charge transport and collection, which eliminating metal wiring from the device surface that usually blocks a portion of incident light. It also avoids the high-temperature diffusion and element doping steps in traditional Si cell fabrication, leading to a reduced production cost. We present a comparative study on various SWNT/Si hybrid solar cells with SWNT TCEs prepared by three different preparation methods, Mayer rod coating [2], spray as well as a newly developed super-acid sliding cast method. Films by super-acid sliding method demonstrate optoelectronic performance, morphology (smoothness and alignment), mechanical properties that are advantageous to other methods. By applying simple/cost-effective post-treatments to as-fabricated cells including HF treatment and various p-type doping, we have greatly and uniformly improved the photovoltaic efficiency irrespective of SWNT preparations methods; A maximum efficiency of 10.5 % is achieved under AM 1.5G illumination. We systematically compare the photovoltaic performance of each solar cell before/after the treatments and between SWNT preparation methods, and discuss various factors contributing to the cell efficiencies. 1. A. Y. Cao, Y. Jia, J. Q. Wei, K. L. Wang, Q. K. Shu, X. C. Gui, Y. Q. Zhu, D. M. Zhuang, G. Zhang, B. B. Ma, L. D. Wang, W. J. Liu, Z. C. Wang, J. B. Luo and D. Wu, Adv. Mater., 20, 4594 (2008). 2. X. Li, F. Gittleson, M. Carmo, R. C. Sekol and A. D. Taylor, Acs Nano, 6, 1347 (2012).

Roll-to-roll process has been widely used in the printing industry at high process speeds and recently adopted by organic solar cell manufacturers to demonstrate flexible, large-area, low-cost organic solar modules. The so-called stripe geometry is widely used in roll-to-roll process in order to minimize the resistive power loss in large-area solar modules. However, it has its limitations since the area used by the cell-to-cell connections reduces the overall active area of the module, leading to a significant reduction of the power conversion efficiency in modules compared to small-area cells. Recently, we demonstrated improved performance for large-area organic solar cells and modules by integrating metal grids directly with transparent electrodes. However, we found that it was difficult to coat the solution-based organic materials uniformly on top of the substrate due to the thick metal grids. In this talk, we will report on a new approach to implement a transparent flexible substrate with an embedded thick metal grid. Because the thick metal grid was embedded in the flexible substrate, we were able to spin coat thin organic materials uniformly on top of the large-area substrate where thick metal grids were embedded. For the same reason, organic materials can be coated on this metal-grid-embedded flexible substrate using a roll-to-roll process. Furthermore, the metal lines can be made narrow, reducing the shadowing loss to only 5.5%, and thicker, resulting in lower resistive losses in large-area devices with higher currents. To make an embedded metal grid on an SU8 thin film, we deposited 200 nm copper on a glass substrate as a seed layer. On top of this seed layer, mold structures were created using SU8 followed by electroplating Cu with a thickness of up to 15 µm. Another layer of SU8 was applied on top of the entire surface. The fabricated SU8 film with embedded metal grids was peeled off from the glass substrate. After removing the seed layer, a high conductivity PEDOT:PSS (PH1000) with 5% DMSO was spin-coated followed by polyethylenimine ethoxylated (PEIE) spin-coating. The active layer of poly(3-hexylthiophene) (P3HT): Indene-C60 Bis-Adduct (ICBA) (1:1, weight ratio) was spin-coated followed by deposition of MoO3 and Ag under vacuum. The completed device had an active area of 9.3 cm2 and a structure of SU8/embedded-metal-grid/PEDOT:PSS(PH1000)/PEIE/P3HT:ICBA/MoO3/Ag. The device exhibited a fill factor of 0.6, a short-circuit current density of 5.8 mA/cm2, and an open-circuit voltage of 0.81 V, yielding a power conversion efficiency of 2.8% under 100 mW/cm2 air mass 1.5G illumination.

With the recent advancements in the field of thin film solar cells, numerous light-trapping paradigms have been developed to help mitigate the low absorptance rates inherent in having thin photovoltaic active layers. This report presents experimental and simulated data on a particular type of plasmonic concentrator designed for photovoltaic applications, known as a nano-hole array (NHA). When employed in a solar cell, thousands of shallow sub-wavelengths holes are milled on the surface of the back metal contact. These holes serve to collect the incident light and redirect it along the dielectric / metal interface in the form of surface plasmon polaritons (SPPs). Absorptance in the dielectric material can then be enhanced through two distinct mechanisms: (i) diffractive scattering at each hole location generates SPP modes that propagate parallel to the interface, thus increasing the scale of interaction length between light and matter; (ii) constructive interference can occur between SPP modes excited by neighboring holes, potentially leading to increased local electromagnetic fields. Despite preliminary research on nano-hole arrays for photovoltaic applications, a systematic understanding of the role of structural geometry and SPP interference effects in plasmonically-enhanced absorption paradigms is still lacking. To this end, a certain class of two-dimensional array will be investigated: quasi-periodic (QP). QP arrays lack the long-range translational symmetry typical of periodic arrays, but can have very high degrees of local order and rotational symmetry, which is completely absent in random arrays. Most importantly, QP arrays can pack many more holes into a plane than any periodic (e.g. square, triangular, etc.) array can. A 24-nm-thick layer of the organic bulk heterojunction P3HT:PCBM was deposited on silver substrates milled with various periodic, QP and random arrays of nano-holes; their optical properties were studied by illuminating the sample with broad-band, randomly polarized light. Absorption was seen to increase across the entire lambda; = 400 nm to 800 nm spectrum; the exact nature of the enhancement depends on the type of pattern used (identified by its order of local rotational symmetry) and the average hole-hole spacing. When expressed in terms of short-circuit current density (Jsc), it was observed that all the devices with NHA&’s milled on them had higher Jsc values that devices without an NHA, with the biggest improvement being a 56% relative increase. The purpose of this talk is to expound on these and other results which show the dependence of absorptance enhancement on rotational symmetry and hole-hole spacing. Additionally, the underlying physics of SPP&’s (generation, propagation, interference, and absorption) will be discussed; finally, current-voltage data from an operating solar cell will be presented.

The domain size of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within the bulk heterojunction of organic photovoltaics (OPVs) is of great importance to the performance and lifetime of the device. This study focuses on the use of polymer-tethered fullerenes (PTFs) as an additive to reduce the initial domain size of PCBM as well as to mitigate the growth of these domains over time that typically results in decreased device efficiency. Films were prepared from a 1:1 mixture of PCBM:P3HT on silicon wafers to approximate a bulk heterojunction system. PTFs were added in concentrations ranging from 0 to 20 percent. The samples were then annealed at 150°C for varying times ranging from 5 to 60 mins and examined by SEM to determine the average PCBM domain sizes for each sample. As expected, the control sample containing no additive showed significant growth in the PCBM domains as the sample was annealed with a final average domain size of 290 square microns. However, addition of as little as 5% PTFs produced average PCBM domain sizes after 60 minutes of annealing of 28 square microns. Addition of 20% PTF resulted in an average PCBM domain size of 2 square microns. Based on these encouraging results, devices containing these architectures are being prepared.