Despite the recent record power conversion efficiencies of up to 10 % achieved by polymer:fullerene bulk heterojunction solar cells, the fundamental processes leading to photocurrent generation and loss are still intensively studied by many research groups, as a clear picture of the photophysics has not yet been established. The current focus of material development is on donor-acceptor type low-bandgap polymers, since their extended red to near-infrared absorption allows them to harvest a larger fraction of the solar illumination, allowing larger photocurrents to be potentially obtained. Often, record efficiencies are achieved with PC70BM as acceptor, simply due to its intrinsically higher absorbance in the visible compared with PC60BM. In fact, using PC60BM often leads to device performances not superior to those of higher bandgap polymers such as P3HT and PCDTBT, despite the enhanced absorption of lower energy photons by the low-bandgap polymer. This implies that additional loss channels exist in low-bandgap polymer:fullerene blends, which may significantly limit the power conversion efficiency.
In this contribution I will present our recent results on exciton diffusion, charge transfer, charge separation and recombination in low-bandgap polymer:fullerene blends studied by broadband Vis-NIR transient absorption spectroscopy spanning a dynamic range from femto- to milliseconds. More specifically, I will demonstrate that upon excitation of the fullerene, exciton diffusion-limited charge separation can be observed, which extends up to several hundred picoseconds, while excitation of the polymer mostly leads to ultrafast charge separation at the interface. Furthermore, I will show that in several low-bandgap polymers an additional loss channel exists, namely triplet formation by non-geminate recombination of free charges, as has also recently been demonstrated by the Friend group for PCPDTBT:PCBM blends. As an example, I will show our recent spectroscopic results on PSBTBT, the silicon-bridged derivative of PCPDTBT, and furthermore, how soft-modeling of TA data surfaces using multivariate curve resolution (MCR) analysis, can be used to deconvolve the individual spectral components of singlet excitons, free charges and triplet states, including their respective dynamics. Our measurements and data analyses allow us to track the entire cascade of photophysical processes from exciton diffusion and dissociation to free charge formation, and finally to non-geminate recombination and triplet formation in low-bandgap polymer:fullerene blends.

Nanoparticle Organic Memory Field-Effect Transistors (NOMFET) are molecule-based devices that exhibit the main behavior of a biological spiking synapse. This behavior is obtained by virtue of the combination of two properties of the NOMFET: the transconductance gain of the transistor and the memory effect due to the presence of nanoparticles (NPs) which are used as nanoscale capacitors to store the electrical charges, and which are embedded into an organic semiconducting layer [1]. Thus, the transconductance of the transistor can be dynamically tuned by the amount of charge in the NPs.
In this context, we present here a novel method for the elaboration of NOMFET active materials based on the electrochemical deposition of gold NPs grafted with alkanethiol-terminated π-conjugated precursors combining low oxidation potential and high reactivity. The straightforward electropolymerization of these new precursors leads to the formation of a semiconducting network in which the electronic and transport properties and the charging/discharging speed of the gold NPs can be modulated. Such hybrid material could advantageously replace the pentacene layer generally used in NOMFETs. This novel approach is based on previously demonstrated enhancement of charge-tunneling across monolayers of SAMs of alkanethiol-bithiophenic systems on a planar gold surface after electrochemical conversion into more extended conjugated systems [2].
The synthesis of the precursors and nanoparticles will be described and the morphology and electronic properties of the hybrid electropolymerized films will be discussed with regard to the behavior of the resulting NOMFET-devices.
Work supported by European Union - FET project SYMONE (#318597) and ANR project SYNAPTOR (#12BS0301001)
1. F. Alibart, S. Pleutin, D. Guerin, C. Novembre, S. Lenfant, K. Lmimouni, C. Gamrat, D. Vuillaume, Adv. Funct. Mater. 2010, 20, 330-337.
2. M. Occedil;afrain, T. K. Tran, P. Blanchard, S. Lenfant, S. Godey, D. Vuillaume, J. Roncali, Adv. Funct. Mater. 2008, 18, 2163-2171.

Herein, we show the effect of electric field (E-field) on the nanomorphology and properties of air-processed photovoltaic thin films and devices. A Van de Graaff E-field was subjected to wet spin-coated polymer films at different angles: 0, 45 and 90 degrees. We hypothesized that the polymer&’s intrinsic charge is influenced by the field lines creating some preferential orientation and, consequently, improved charge transport. Poly (3-hexylthiophene-2, 5-diyl) (P3HT) thin films were deposited on ITO substrate coated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). These films were characterized by Raman spectroscopy, mobility measurement, steady-state photoluminescence (SS-PL) and conductive atomic force microscope (c-AFM). Raman spectroscopy showed a reduced full width at half maximum (FWHM) of the films subjected to the E-field compared to the control film, which is indicative of an increased polymer crystallization. Moreover, the E-field films showed a red-shift and reduced 0-0/0-1 ratio in the SS-PL curve compared to the reference ones, which is an indicative of an H-aggregation. The P3HT hole mobilities were measured revealing an enhancement of 1.07 x 10-3, 1.32 x 10-3, 7.36 x 10-4 cm2.V-1.s-1 for the 0, 45 and 90 degrees over 5.41 x 10-4 cm2.V-1.s-1 for the reference, respectively. The c-AFM current images of the E-field thin films displayed higher root-mean square values (RMS) compared to the control, especially in the 45 degrees film. Organic photovoltaic solar cells (OPVs) were investigated by light IV, PL-lifetime spectroscopy, mobility and capacitance measurements. P3HT was blended with phenyl-C61-butyric acid methyl ester (PC60BM). After depositing the active layer, the films were exposed to the E-field until they completely dried. The final device&’s structure is ITO/PEDOT:PSS/P3HT:PC60BM/Ca/Al. For the 90 degrees OPV, the power conversion efficiency was low because of fast dry time and demixing between the polymer and fullerenes. Furthermore, the capacitance measurement showed that the 90 degrees diodes have higher doping concentration of one order of magnitude compared to the control device. On the contrary, the 45 degrees solar cells showed efficiencies that are similar or better than the control ones. The mobility of E-field P3HT:PC60BM films showed improved hole-transport compared to the control ones. To generalize our approach, we applied the E-field on solution-processed organic light-emitting diodes (OLED). Wet P3HT or poly(p-phenylene vinylene) (MEH-PPV) were exposed to the E-field. Then, they were thermally evaporated with bathophenanthroline (BPhen), LiF and aluminum. The E-field OLEDs displayed higher electroluminescent efficiencies compared to the control ones. To sum up, we recorded that the Van de Graaff E-field increases the polymer chain π-π stacking, hole mobility, and device performance.

In organic electronic devices like thin film transistors, interfaces between conducting and semiconducting materials play a decisive role. One major field of interest in organic electronics is the effort to minimize barriers for charge carriers at these interfaces to increase the device performance. There are two main approaches: either to influence the morphology of the semiconducting material or adjusting the electronic properties. Since the electronic properties of organic semiconducting materials are difficult to manipulate without influencing their performance a great deal of work is concentrated on adjusting the electronic properties of electrode materials. One promising way to do so is to apply a Self Assembled Monolayer (SAM) with an intrinsic dipole onto the electrode surface altering its surface potential and therefore its macroscopic electronic properties.
The aim of our work is to create compounds which are capable of influencing both morphological and electronic aspects. The design of the synthesized substances is therefore chosen to resemble commonly used organic semiconducting materials and at the same time contain a satisfactory dipole moment in order to sufficiently alter the work function of the electrode.
We recently synthesized new molecules based on these requirements bearing SAM-applicable functionalities.
Studies towards the formation of monolayers using these molecules and their influence on the electronic properties will be shown. Furthermore, preliminary device testing will be presented.

Flexible organic electronic devices (‘plastic electronics&’), such as organic light emitting diodes (OLEDs) and organic solar cells, are very sensitive to oxygen and water vapor, which can quickly degrade these devices. An encapsulation barrier is required, preferably producible by thin film deposition directly onto the sensitive devices, with a water vapor permeation rate (WVTR) of < 1 x 10-5 g/m2.day. For devices made on flexible plastic substrates, the barrier should seal the devices all around. We developed a new organic/inorganic multilayer stack deposited with a single CVD technique.
By combining Hot Wire Chemical Vapor Deposition (HWCVD) of silicon nitride (SiNx) with initiated Chemical Vapor Deposition (iCVD) of poly(glycidyl methacrylate) (PGMA), we developed an “all-hot-wire” deposited thin (400 nm thick), optically transparent encapsulation barrier that can be deposited at temperatures below 100 oC. Using the Ca WVTR test, we achieved a WVTR of 5 x 10-6 g/m2.day at 60 oC and 90% relative humidity, for a simple 3-layer stack. An additional advantage of the deposition of SiNx from silane and ammonia onto the PGMA layer is that the internal adhesion of the successive layers is exceptionally strong, because the PGMA still has functional epoxy rings owing to the non-destructive plasma-free iCVD method. By HR-TEM and XPS we observe an intermediate SiOxNy layer between the polymer and the nitride layer. The interlayer turns out to be highly beneficial for interlayer adhesion and this is probably one of the reasons for the excellent barrier properties of our multilayer.
We present our first results on all-hot-wire direct thin-film encapsulation on working OLED devices. The all-hot-wire method can easily be extended to a roll-to-roll continuous encapsulation technique.

Polarization energy is defined as the energy difference between ionization potentials [or electron affinities] in the gas phase and in the solid state upon addition of a positive [or negative] charge. It is one of the main microscopic parameters governing charge transport in organic semiconductors in general, as well as charge separation at donor/acceptor interfaces in organic solar cells. In this presentation, we discuss how much polarization energy depends on the details of molecular packing. We take pentacene and TIPS-pentacene as representative examples.

The scale up of organic optoelectronic device fabrication requires high deposition throughput and a means of patterning the electronically active layers. This talk will briefly review several approaches to achieve this goal, focusing on vapor jet printing. Material characterization, gas dynamic modeling, and printing experiments are employed to understand the factors controlling deposition rate, pattern resolution, and material utilization efficiency. We also look at how the deposited film morphology (and resulting electronic properties) can be related to process conditions.

Organic semiconductors with tunable optoelectronic properties are of great interest for their applications in high-performance electronic devices, such as organic field-effect transistors (OFET) and organic photovoltaics (OPVs). One of the fundamental aspects related to their optoelectronic properties is the control of molecular packing, long range ordering, as well as donor-acceptor interfaces, all of which are essential for charge separation and charge transport. In this talk I will describe our effort in materials development using building blocks that satisfy bandgap engineering, high absorptivity in the solar spectrum, and strong intermolecular interactions. In particular, new donor units based on coronene derivatives and new acceptor units based on indigo derivatives will be discussed. Their optoelectronic properties and thin film morphologies indicate great promises of such class of building blocks for novel electronic materials.

Molecular engineering is an important tool for developing high-performance electronic materials as small changes in intermolecular interactions can lead to significant variations in solid-state packing and electrical properties. However, it is difficult to explicitly decipher the effect of functionalization due to the paracrystalline nature of most conjugated polymers. Here, single crystals of monodisperse oligoanilines with various aromatic substitutions were synthesized to serve as model systems for elucidating the impact of side-chain functionalization on polyaniline, a benchmark conducting polymer commonly used in organic electrodes and sensors. We found that molecular symmetry is a crucial factor for achieving good solid-state packing, while the nature of the substituents offers control over the packing motif that dictates the efficiency of molecular orbital overlap. The differences in carrier transport properties and electronic structures for these molecules were also investigated. Our findings offer powerful insights to the design rules for engineering a highly efficient polyaniline that can potentially improve the performance of the associated organic electronic devices.

Solution processable polymeric solar cells have drawn a lot of attention due to their potential for low cost roll-to-roll manufacturing. Compared with inorganic semiconductors, polymer semiconductors have relatively low carrier mobilities and therefore carrier transport is considered a limiting factor determining the performance of a polymer solar cell. In addition to carrier mobility, carrier lifetime in the polymer:fullerene blends is another important parameter determining the cell efficiency. We have extensively studied the carrier transport and recombination properties of several donor-acceptor conjugated polymers. We found that while low mobilities in polymers lead to charge imbalance, high mobilities do not always lead to high power conversion efficiencies. In this presentation, we will present our device results based on dioctyldithieno silole (DTS) polymers with a wide-absorption band and excellent transport properties. Even though the DTS polymers have a high hole mobility, the power conversion efficiency of the resulting solar cells is limited by the low fill factor. In order to study the loss mechanism of the photo-current, we carried out transient photo-voltage (TPV) measurements to probe the bimolecular recombination of photo-carriers under open circuit condition. Based on our photo-CELIV (carrier extraction by linearly increasing voltage) data, we found that the carrier recombination rate is strongly related to the energetic disorder of the semiconductor polymers. Finally, we will report on our recent progress of high efficiency polymer solar cells based on a low band-gap polymer having an alternating dithienogermole-thienopyrrolodione repeat unit (PDTG-TPD).

We have demonstrated an increase in internal quantum efficiency (IQE) in planar heterojunction solar cells due to the addition of high permittivity small molecules into a donor film. The low permittivity of organic semiconductors (ε ~ 3) is an inherent limitation for their application in organic photovoltaics because it results in high exciton binding energies. Thus, increasing in the permittivity is a pathway to higher efficiency organic photovoltaics. A model system that we have studied consists of B,O-chelated azadipyrromethene (BO-ADPM), a low bandgap (~1.46 eV) small molecule organic semiconductor, and the high dielectric constant camphoric anhydride (CA) molecule. We have also used this technique of adding CA to other small molecule systems and observed an increase in IQE. To better understand the enhancement in IQE and exciton separation, we have performed an in-depth analysis of the exciton modification mechanism by means of time resolved transient absorption spectroscopy measurements to understand the energy transfer within the donor layer. Additionally, by fabricating donor only photovoltaic devices with hole blocking layers, we have investigated the rate of free carrier generation with the incorporation of CA into the donor film. By understanding the mechanism for greater exciton separation and increased IQE, the technique of increasing permittivity by adding CA could be used to improve the power conversion efficiency of organic photovoltaics.

Measuring the key photo-physical properties of PCxBM molecules and understanding charge transfer dynamics in polymer:PCxBM BHJ systems can better understand current successful devices for better future acceptor materials design. PCBM has already been shown to exhibit both singlet and triplet excitons, which both have the potential to change the performance of OPV devices. In particular, that singlet exciton photo-physics is relevant to the generation of charges from fullerene absorption, conventionally by first passing a hole onto a less electronegative molecule forming a charge transfer (CT) state before completely separating into radical charges. It is important to understand the kinetics of processes after the generation of the singlet exciton in order to predict which mechanisms dominate the route to successful separation. Germinate recombination can occur both in the singlet state and also after it has formed a CT state, which in addition to intersystem crossing (ISC) to triplet states, provide the major loss pathways. While it is not clear if triplets can act as a significant contribution to photocurrent is has already been shown that they play an important role in photostability. [1-5]
In this work we use optical absorption and emission measurements to characterise the photophysical properties of PC60BM and PC70BM excitons. Using femto-second TAS measurements we are able to extend the understanding of formation of charges to the early time-scale both as a neat film and when suspended in an inert polystyrene (PS) matrix.[6]
1. Distler, A., et al., Effect of PCBM on the Photodegradation Kinetics of Polymers for Organic Photovoltaics. Chemistry of Materials, 2012. 24(22): p. 4397-4405.
2. Schlenker, C.W., et al., Polymer Triplet Energy Levels Need Not Limit Photocurrent Collection in Organic Solar Cells. Journal of the American Chemical Society, 2012. 134(48): p. 19661-19668.
3. Cook, S., et al., Singlet exciton transfer and fullerene triplet formation in polymer-fullerene blend films. Applied Physics Letters, 2006. 89(10): p. 101128-3.
4. Benson-Smith, J.J., et al., Charge separation and fullerene triplet formation in blend films of polyfluorene polymers with [6,6]-phenyl C61 butyric acid methyl ester. Dalton Transactions, 2009(45): p. 10000-10005.
5. Ohkita, H., et al., Radical ion pair mediated triplet formation in polymer-fullerene blend films. Chemical Communications, 2006(37): p. 3939-3941.
6. Cook, S., et al., A photophysical study of PCBM thin films. Chemical Physics Letters, 2007. 445(4-6): p. 276-280.

Organic-based solar cells are beginning to emerge with the potential to compete with other low-cost thin film photovoltaic technologies, with efficiencies of 12% recently demonstrated. While recent enhancements can be attributed mostly to novel materials, parallel routes investigating new device architectures and structural control are necessary for further improvements.
Here, I will focus on two aspects of our work to improve our understanding of OPVs as well as their performance. First, I will discuss our recent efforts to template and control film morphology and molecular orientation. These studies allow us to understand the importance of molecular orientation, crystallite size, and crystal phase. We will show templated devices utilizing neat films as well as bulk heterojunctions, with crystallite dimensions spanning from the more standard nano-sized grains to those with highly ordered microcrystalline domains revealing unprecedented thin film exciton diffusion lengths of 100s of nm.
The other aspect concerns routes that allow harvesting a larger portion of the incident solar flux. This includes investigating highly efficient fullerene-free devices as well as the cascade device architecture, an alternative structure to the tandem cell that enables the use of multiple complementary absorbers. Proper optimization to eliminate parasitic quenching effects, increase open-circuit voltage, and utilize Forster resonance energy transfer has enabled us to demonstrate certified state-of-the-art evaporated single cells (whether with or without fullerene) of 8.4% in a fullerene-free device.

Organic photovoltaic (OPV) cells have attracted great interests in recent years due to their potential to offer low cost solar energy conversion. There have been many great advances in developing new materials and new device architectures for OPV applications, and power conversion efficiencies exceeding 10% have now been reported. In this talk, I will present some of our recent work on improving the efficiency of vacuum-deposited small molecule based OPV cells. First, we have synthesized vertically aligned molecular nanorod arrays using the high vacuum process of oblique angle deposition. An interdigitated donor-acceptor heterojunction was subsequently formed by filling the gaps between the donor molecular nanorods with the fullerene acceptor, and such controlled bulk heterojunction exhibited twice the efficiency of a bilayer heterojunction from the same material system. Next, we show that the use of a pentacene polycrystalline template can strongly influence the growth of the infrared-absoring lead phthalocyanine molecules, leading to stronger infrared absorption and enhanced quantum efficiency. In collaboration with Jian Li at Arizona State University, we show that the use of a PEDOT:PSS/tetracene bilayer on the ITO anode also drastically improves the quantum efficiency of zinc phthalocyanine-based OPV devices. Finally, in collaboration with Ronald Castellano at the University of Florida, we have designed a new self-assembly approach to realizing a controlled bulk heterojunction structure by relying on hydrogen-bonding mediated supramolecular assembly. In the preliminary work, we have shown two to three times efficiency improvement for quaterthiophene derivatives compared to control devices.

In an organic photovoltaic cell (OPV), optically generated excitons must diffuse to a dissociating electron donor-acceptor (D-A) interface in order to be dissociated into mobile charge carriers. In simple planar heterojunction OPVs consisting of a D-A bilayer, active layer thickness and power conversion efficiency is often limited by the unfavorable trade-off between the exciton diffusion length (LD) and optical absorption length. This bottleneck can be circumvented through the use of bulk heterojunction architectures that increase the area of the dissociating D-A interface. This talk will instead focus on alternative device architectures aimed at directly addressing the deficiencies in excitonic energy transfer that limit LD. Specifically we present an approach to engineer LD by optimizing the intermolecular separation in thin film and consequently, the photophysical parameters responsible for energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host, we optimize the degree of interaction between donor molecules and observe a ~50% increase in LD. Interestingly, we find that dilution of the active material offers the ability to engineer energy transfer not only in the bulk, but also across interfaces. This is possible as dilution of one layer adjacent to a neat layer can create an imbalance of origin and destination sites for an exciton at an interface. Using a kinetic Monte Carlo formalism, we show that an imbalance of sites at interfaces can bias exciton motion toward the D-A interface, creating one-way gates for excitons. In this talk, we will discuss the potential to engineer LD by tailoring intermolecular interactions in both neat and dilute film, and also will discuss the potential importance of passive exciton gating in the design of OPVs for enhanced exciton harvesting.

The photo conversion efficiency of organic solar cells (OSCs) has seen a tremendous growth rate over the last decade. This progress is based on new materials tailored for OSC applications as well as on advanced skills in device engineering. But many of the models and characterization techniques applied in the field of organic electronics are restricted to small clusters of molecules or model systems as individual interfaces only, thus lacking prediction when it comes to full devices. With scanning Kelvin probe microscopy (SKPM) accompanied by device simulations we try to access the physics of entire OSC devices and bridge the gap between the molecular and the macroscopic understanding. On this road, the nature of SKPM as a surface characterization method and of OSCs as horizontally layered devices poses an experimental challenge. In our measurements we gain access to the OSC cross sections by milling trenches into the devices with a focused ion beam (FIB).
Our scanning probe station is placed within the vacuum of a scanning electron microscopy (SEM)/FIB cross beam system. Under observation with the SEM we place the cantilever right at the FIB prepared cross section and investigate the potential distribution of the solar cell in situ with SKPM.
The SKPM measurements are backed by IV characterization and device simulations. Varying the parameter of the active layer/contact interface, we simulate their impact on the potential distribution and the device characteristics of the OSC. Experimentally, we translate the varying interface parameter by systematic contact modulation on F4ZnPc/C60 OSCs. In IV measurements and cross sectional SKPM measurements we check the validity of the applied models and identify loss mechanisms and their localization in the solar cell device.

Organic solar cells present unique properties, such as an inherent visible semi-transparency of the active material, that make them an ideal technology for integration in urban areas. However, a standard organic solar cell device is made opaque to obtain an optimal performance; the back metal electrode must be sufficiently thick to effectively reflect back to the active material non-absorbed photons in the first pass. To obtain a semi-transparent device this electrode has to be made highly transparent implying that the cell performance drops to just above 60% of the reference opaque cell.
In the current paper we present two approaches using non-ordered one-dimensional photonic crystals to limit such loss in performance. We fabricated 30% visible transparency cells with a power conversion efficiency (PCE) equivalent to 72% the PCE of the opaque cell and 20% visible transparency cells with a PCE equivalent to 92% the PCE of the corresponding opaque cell. Both type of cells were fabricated using PTB7:PC71BM blend as active material. In one case we fabricated direct semi-transparent cells, which exhibited 30% visible light transmission and 5.6% power conversion efficiency. Non-periodic photonic crystals were used to trap near infrared and near ultraviolet photons and enhance light harvesting.1 In the other case, we enclosed the active material in between two metal electrodes forming an optical cavity designed to optimize photon trapping inside the cell. At the same time such electrodes were kept sufficiently thin to ensure a visible transparency higher than 20%. To increase near IR light trapping, while maintaining transparency in the visible, an anti-reflection coating was deposited on top of the front metal contact while a non-periodic multi-layer was inserted in between the back metal contact and the substrate. The cavity configuration was designed specifically for the cell architecture used and we achieved semi-transparent cells with 5.3% PCE, corresponding to 90% the PCE of the opaque cell.2
To conclude, in organic transparent cells light harvesting diminishes because the reflectivity of the top layer is reduced and the device loses its capacity for photon trapping. We opened a path for an effective light harvesting recovery when thinning down the top electrode. We demonstrate that the photon management provided by non-periodic multilayers is sufficient to bring back the device light harvesting capacity very close to that of the opaque cell.
1 Rafael Betancur, Pablo Romero-Gomez, Alberto Martinez-Otero, Xavier Elias, Marc Maymo
and Jordi Martorell, doi:10.1038/nphoton.2013.276
2 Francesco Pastorelli, Pablo Romero Gomez, Rafael Betancur, Alberto Martinez-Otero, Nicolas Bonod and Jordi Martorell, paper submitted.

We experimentally probe the neutral, anion radical, and dianion as well as the photo excited states of indenofluorenes and similar molecules and interpret the data with the assistance of calculations. Implications for the applications of indenofluorenes as electron accepting organic materials are discussed.

Organic nanowires consisting of π-conjugated building blocks are model systems for the study of charge transport. By drawing inspiration from DNA, we have developed a novel class of nanowires in which organic semiconductor building blocks are assembled in predetermined arrangements along a DNA-like backbone. Our approach is advantageous because it furnishes one-dimensional nanowires with precisely controlled length, geometry, and sequence context, while greatly simplifying purification and handling. We have self-assembled monolayers of our nanowires at gold surfaces and investigated their properties with electrochemical techniques. Our findings hold significance not only for fundamentally understanding charge transport phenomena but also for the development of new types of biological and molecular electronic devices.

We developed a wireless and flexible sensor system with organic transistors for biomedical application. When the electrodes directly contact with a wet human skin, electrostatic discharge are generated and damage a device. To utilize the organic sensor sheet for biomedical application, we integrated the organic electrostatic discharge (ESD) protection circuits with organic schottky diodes. An ESD protection circuits with organic schottky diodes using copper phthalocyanine (CuPc) achieves 2kV ESD tolerance.
An organic circuit (ESD protection circuit and sensor system) was fabricated directly on a 12.5 mu;m-thick flexible Polyimide substrate. The ESD protection circuit was fabricated using an organic schottky diode. The sensor system was fabricated ring oscillator circuit with a pseudo-CMOS layout comprising four p-channel organic transistors. The flexible, p-type channel was formed using dinaphtho[2,3-b:2prime;,3prime;-f]thieno[3,2-b]thiophene (DNTT), and gate dielectrics were formed with 70-nm-thick parylene by chemical vapor deposition (CVD) process. 1-mu;m-thick parylene layer was deposited on the organic circuits to serve as a passivation layer against oxygen diffusion, humidity, and mechanical attrition. DNTT transistors showed mobility exceeding 1 cm²/Vs at 8 V.
The power is wirelessly transmitted between coils using the magnetic resonance at 13.56MHz. In this study, we use the magnetic resonance to increase the distance between flexible coils, instead of the conventional electromagnetic induction. This sensor system can detect the resistance change between two electrodes by measuring the frequency of RC ring oscillator.
Although circuit design and system configuration of proposed device will be presented in 2014 IEEE International Solid-State Circuits conference by our group, we will present materials, device processes and electrical characteristics of organic schottky diode and organic transistor at MRS, for the first time.

Material quantitative structure-property relationship (MQSPR) modeling is a technique to find the multivariate functions that relate sets of calculable or easily observable quantities called descriptors to known molecule or material properties. Two major components of such technique are descriptor design and statistical method. While sophisticated machine learning methods with validation techniques have been widely developed and used in MQSPR-like studies, the descriptor system used for polymers and other materials are usually either purely based on composition or directly borrowing those used in drug discovery, which, however, in nature limits the domain of application of the models. Due to the uncertainty of polymer single chain molecular weight and the asymptotic behavior of many properties against the chain length, descriptors designed for small molecules should be modified so as to directly represent the situation for the infinite chain. In the current study, commonly used 2D descriptors are modified into Infinite Chain Descriptor (ICD) and used for both dielectric and thermomechanical properties of polymers.
The algorithms for 2D descriptors calculation have been modified so that the descriptor values will be insensitive to the representation of polymers (by repeating units) and still “keep the spirit” of the original descriptors.
Basically three kinds of treatments have been used to convert those descriptors into the infinite chain version:
a. Separate side chains from backbones.
b. Redefine the connection condition for connecting atoms (head and tail) in descriptor algorithm.
c. After redefining the connection condition, normalize the descriptor value by either the total number of bonding or atoms (depend on the meaning), with prefix “polyb”, or the repeating unit displacement (the “distance” from head to tail), with prefix “polyq”. The latter can be seen as the measurement of the linear density of corresponding properties.
228 infinite chain descriptors were developed and used in support vector regression with RBF kernel on dielectric constant, band gap and also glass transition temperature. Data for band gap is from DFT calculation while dielectric constant and glass transition temperature are from Bicerano[1]. All the models have cross-validation and true test r^2 above 0.85.
The infinite chain descriptor system was successfully used in the MQSPR models on predicting several important properties. And the accurate prediction of band gap and dielectric constants can be very useful for related application and material discovery. This modeling system eliminates the bias induced by using the traditional descriptors directly on repeating units as small molecules and avoids the extra work on the determination of the convergence length of oligomers.
Reference:
[1] J. Bicerano, Prediction of Polymer Properties, 2nd ed., Marcel Dekker, Inc., New York, 1996.

Continuing improvement in organic photovoltaic (OPV) device efficiencies demands greater understanding of carrier transport and recombination within bulk heterojunction (BHJ) materials. We employ a lateral electrode geometry to study carrier transport and recombination in BHJ. In this geometry, two asymmetrical electrodes (Al, Au) are patterned in-plane on a glass substrate. BHJ material is deposited on top of the electrodes, defining the channel length. This arrangement decouples absorption of light in a BHJ from carrier transport between the electrodes, unlike in a vertical OPV structure where the two are linked. Such a structure also permits spatial probing of the active area by electrical or optical means. Theoretical simulations show that under certain bias conditions carrier transport becomes space-charge limited (SCL). Imbalance between charge extraction and transport near the electrodes manifests in space-charge regions and significant generated photocurrent contribution. A central recombination zone forms between the SCL zones. Since the carriers generated there are unable to reach the electrodes and recombine, contribution to photocurrent is negligible. We have investigated this behavior in P3HT:PCBM, PSBTBT:PCBM, and PSBTzTz:PCBM BHJ blends. These choices include blends exhibiting Langevin recombination as well as non-Langevin type recombination. PSBTzTz, synthesized via the Stille coupling of two monomers, is a promising BHJ material which displays non-Langevin recombination. This polymer allows investigation of non-Langevin behavior by varying the linkage between the two monomer units. SCL formation was optically confirmed by scanning confocal probe microscopy (SCPM). In this experiment, the electrodes are biased for charge extraction and a laser pulse locally excites charges within the BHJ generating photocurrent. This method allows for direct mapping of photocurrent distribution in the BHJ, enabling differentiation of the recombination zone from space-charge zones. Fitting the SCL photocurrent peaks to Gaussian decay convoluted with laser beam width gives carrier drift lengths. In-situ potentiometry measurements with sub-micron voltage probes in the BHJ channel give access to the potential distribution within the BHJ under different applied biases. Potentiometry confirms presence of a recombination zone and SCL zones. After analysis, mobility of the slower carrier in the BHJ is extracted, yielding the mobility ratio, the recombination coefficient, and carrier concentrations. Lateral geometry devices were fabricated and sealed in nitrogen atmosphere. We will report results on combined SCPM and in-situ potentiometry performed on the same BHJ blend and in the same device structure. From such measurements, a wealth of information about material parameters can be extracted.

We designed and synthesized a novel triazine-based material functionalized by four pyridine groups 2-phenyl-4, 6-bis (3,5-di-3-pyridylphenyl) triazine (B3PyPTZ) for organic light-emitting devices (OLED&’s). The molecule has been found to have a deep lowest unoccupied molecular orbital (LUMO) (Ea=3.0eV) and a highest occupied molecular orbital (HOMO) (Ip=6.8eV) energy level compared to conventional electron transport material, 2,2', 2"-(1,3,5-benzinetriyl)-tris (1-phenyl-1-H-benzimidazole) (TPBi) (Ea=2.8eV, Ip=6.3eV). The thermal properties of B3PyPTZ are also investigated. We employed B3PyPTZ on OLED&’s as a electron transport layer (ETL) for fac-tris (2-phenylpyridine) iridium (III) [Ir(ppy)3], a well-known green phosphorescent OLED&’s emitter. The device structure is [ITO (130nm)/ poly(arylene ether ketone)-containg triphenylamine (TPAPEK): 4-isopropyl-4&’-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (PPBI) (20 nm)/TAPC (30 nm)/Ir(ppy)3 8 wt% doped CBP (10 nm)/ ETL (50 nm)/LiF (0.5 nm)/Al (100 nm)] (ETL=B3PyPTZ, TPBi). By using B3PyPTZ as an ETL, the device showed tremendously low operation voltage of 2.0V at 1cd m^-2. This result is 0.7 V lower than that of the device using TPBi. And also, driving voltage of the device was 2.3V at 100cd m^-2 with keeping up a high power efficiency of 96lm W^-1 and a high external quantum efficiencies of 20% at the same time. Our devices with B3PyPTZ showed the best performance so far as ETL with 1,3,5-triazine core.

Intermolecular photoinduced electron transfer (PeT) has found a wide range of photoelectronic utility. One of the most notable examples includes the natural photosynthesis, where PeT between chlorophyll and quinone triggers photon-to-chemical energy conversion. We observed that phosphorescent Ir(III) complexes exhibited efficient PeT to trigger a cascade of catalytic intermolecular electron transfer among electrochemically labile molecules. To establish the photoelectronic utility of the PeT, a series of cyclometalated Ir(III) complexes were prepared and evaluated for photoelectrocatalytic conversion of dithienylethene (DTE) compounds. Selective photoexcitation of the Ir(III) complexes facilitated ultrafast PeT from DTE at rate constants of ca. 10^9 M-1 cm-1. The oxidative PeT initiated electrocatalytic, chromic conversion of DTE, yielding one order of magnitude enhancement in quantum yields relative to direct photochromic conversion. We performed mechanistic investigations into PeT using a variety of techniques, including transient absorption spectroscopy and photoluminescence spectroscopy, stopped-flow UV-vis-NIR absorption spectroscopy, electrochemical and EPR measurements, and computational investigations based on time-dependent density functional theory. The studies allowed for assessments of the influence of molecular factors, such as redox potential and reorganization energy, on PeT. Our results revealed that PeT accompanied back electron transfer (BeT), and the kinetics of BeT was crucial to the performance of the overall electron transfer. We proved that our system was uniquely characterized with possessing bidirectional electron transfer of PeT and BeT, and that the former and the latter occurred in the Marcus-normal region and the Marcus-inverted region, respectively. In addition, our results strongly indicated that BeT regenerated an excited-state Ir(III) complex that greatly boosted net current efficiencies. These establishments provided novel guidance to optimization of PeT, and further enabled high efficiency molecular chromism. Furthermore, our novel discovery suggests high propensity for intermolecular PeT in the binary emitting layer of typical organic light emitting devices comprising a phosphorescent Ir(III) complex and a host material. It is thus envisioned that kinetic controls of PeT and BeT will prevent electrochemical formation of byproducts in the emitting layer under continuous electrical operation of devices.

Currently, one of the major challenges in the microelectronic field is to find an economically viable lithographic technique that could address sub 10 nm pattern size. Interesting candidates are self-assembled block copolymers. The main challenge in this area is to reduce the block copolymer chain length in order to bring the pitch below 15 nm, without losing the network self-organization. Indeed, in order to conserve the segregation and decrease significantly the degree of polymerization, it is necessary to increase the incompatibility between the two polymers. This conclusion generates the ongoing development of the so-called high-Chi copolymers. Our strategy is to increase this incompatibility by clicking a hydrophobic coil block (polystyrene) with a hydrophilic rod block (maltoheptaose) to create a hydrophilic/hydrophobic rod-coil diblock copolymer.
We have already showed the capability of this high-Chi copolymer to organize on a sub 10 nm patterns.1-3 However, the high organization of this copolymer has been obtained only in the case of a solvent vapor annealing process (SVA).4 The thermal annealing doesn&’t allow a full control of the BCP film nano-organization and in this work we will show how the SVA overcomes this constraint that permit a better control of the self-assembly.
We show that the first step of the organization is not a homogeneous mixture, as in the case of PS-PMMA, PS-PEO, etc hellip;but a heterogeneous solution composed of 7 nm core-shell aggregates with an internal sugar core and a PS shell. We assume that under the SVA, these aggregates are a full-fledged step of the self-assembled organization. To explain this phenomenon we propose a mechanism of growth, coalescence and hexagonal organization of these small aggregates controlled by the proportion of water in the THF/water solvent vapor. THF is a good solvent for the polystyrene block and water for the sugar block. The film organization has been characterized by SAXS, AFM and GISAXS. The QCM-D that measures the total quantity of solvent absorbed in the film during the SVA shows that the total quantity of solvent absorbed is closely related to the quantity of water in the vapor. We propose that the establishment of the cylindrical hexagonal organization from the aggregates core-shell structure is the result of a competition between the interfacial tension developed by the “rod” maltoheptaose absorbing the water, and the tension developed by the “coil” polystyrene absorbing THF.
1- Giacomelli, C., Schmidt, V., Aissou, K. and Borsali, Langmuir 2010, 26, 15734-15744
2-K. Aissou, I. Otsuka, C. Rochas, S. Fort, S. Halila, & R. Borsali Langmuir, 2011, 27(7), 4098-4103
3-Cushen, JD; Otsuka, I; Bates, CM; Halila, S; Fort, S; Rochas, C; Easley, JA; Rausch, EL; Thio, A; Borsali, R; Willson, CG; Ellison, CJ ACS NANO, 2012, 6, 4 ,3424-3433
4-Otsuka, I ; Tallegas, S; Sakai, Y; Rochas, C; Halila, S; Fort, S; Bsiesy, A; Baron, T; Borsali, R Nanoscale 2013, 5, 7, 2637-2641

A novel hexa-peri-hexabenzocoronene (HBC) derivative with six covalently tethered alkyl-trizaole-quadra-3-hexylthiophene units is synthesized via click chemistry and shown to self-assemble into a hexagonal columnar liquid crystalline (LC) phase. The material can be easily introduced into standard ITO/glass LC cells by capillary filling in the isotropic phase. Compared to other HBC-based LC materials, the mesogen shows unprecedented responses to applied electric fields with thickness of several microns in the liquid crystal phase, resulting in uniform homeotropic or parallel alignment depending upon the electrode structure. Furthermore, the columnar orientation, once developed by an electric field, can be maintained even after removal of the electric field unless the material is heated above the clearing temperature.

Electrospray deposition is an emerging manufacturing method for large-area organic electronic device fabrication in ambient conditions. As opposed to spraying methods using pressure or vibrations to atomize liquids or solutions, electrospray uses electro-hydrodynamic atomization to produce a highly uniform spray with droplet size control from 10 nm to 100 µm. Choice of processing method influences film morphology, e.g. wet coating methods of poly(3-hexlythiophene) (P3HT) result in polymer chains oriented preferentially with high-conductivity π-π stacking parallel to the substrate plane to promote in-plane charge carrier mobility. However, for devices with charge transport direction perpendicular to the substrate plane, such as solar cells utilizing P3HT as light absorber and electron donor, charge collection can be impeded by the in-plane preference of the anisotropic charge carrier mobility. Previous studies attempt to address this problem through control of polymer chain orientation by nano-imprint lithography on spin-cast films or by casting polymer within the confines of large-area nano-grating templates [1,2].
In this study, P3HT is deposited by electrospray using partial drying to achieve non-spherical particle shapes with sub-100 nm lengths. In contrast to films formed by wet processing, electrospray deposited P3HT exhibits π-π stacking aligned normal to the substrate plane, i.e. rotated 90 degrees relative to the typical orientation for spin-cast films. We characterize the chain orientation using grazing incidence X-ray diffraction to measure the influence of electrospray conditions such as droplet size, solution concentration and drying rate. The findings are explained by considering the nanoscale size of drying droplets prior to polymer entanglement, which exhibit a template-free confinement effect on polymer organization via the substantial polymer-air interfacial surface energy.
To correlate relationships between film morphology and charge transport, we measure transverse current—voltage characteristics of prototype devices incorporating electrospray deposited semiconducting polymer layers. In addition to capital cost and material savings inherent to electrospray deposition, a detailed understanding of the benefits to layer morphology and electrical properties will inform development of low-cost nano-electronic manufacturing processes.
[1] H. Hlaing, X. Lu, T. Hofmann, K. G. Yager, C. T. Black, and B. M. Ocko, ACS Nano 5, 7532 (2011).
[2] D. E. Johnston, K. G. Yager, C.-Y. Nam, B. M. Ocko, and C. T. Black, Nano Lett. 12, 4181 (2012).

We have investigated how to modulate the electrical property of a metal-semiconductor junction by alkanethiolate self-assembled monolayers (SAMs). Traditional approaches to attach organic monolayers on oxide-free silicon single crystal include the reaction of n-alkenes and their derivatives with hydrogen-terminated silicon (H-Siequiv;) under UV exposure or high temperature. It is impossible to change the electrical signal of an Hg || organic monolayer-Siequiv; junction (a diode) once the densely packed and robust monolayer has formed on the silicon surface via Si-C bond. Different from forming covalent bond with silicon, we have been interested in the modulation of the monolayer structure upon forming rather mobile SAMs on mercury. The mobility of Hg electrode provides us a way to tune the junction from a diode to a resistor or vice verse. By simply enlarging the volume or pressing down the mercury drop, we have discovered that the current-voltage response of these molecular junctions change remarkably, i.e., from Ohmic to rectifying, and more importantly, such switching behaviour is reversible and reproducible. Further evaluation of the ideality factor and barrier height proves that we can indeed mechanically switch on and off the molecular diode. This is an unprecedented example of switching the junction without any complicated modifications and offers an approach to controlling the conversion between diode and resistor behaviour in electrical devices.

Polyaniline-Zinc oxide nano-composite material was prepared by chemical polymerization of aniline with ZnO nano-particles doping. Surface Pressure-Area (π-A) isotherms of Polyaniline (PANi) and Polyaniline-Zinc oxide nano-composite shows phase transformations of monolayer during compression process. Multiple isotherms indicate that the monolayer of the composite material can retain its configuration during compression-expansion cycles. The structural, topographical and electrical properties of these deposited Langmuir Blodgett films were studied and characterized by UV-Visible spectroscopy (UV), Atomic force microscopy (AFM), Conductive Atomic force microscopy (C-AFM) respectively. For detailed investigations of the LB film properties, Conductive AFM is used to measure the I-V relationship of a surface of Langmuir Blodgett (LB) films of Polyaniline (Emeraldine Base) and Polyaniline-Zinc oxide nano-composite. The contact size of the AFM cantilever tip can be as small as a few nanometers, so, the local variation of the electrical property, which is unseen in the macroscopic level, can be observed by the I-V curve. A current ranging up to 3 nA and 20 nA have been observed in the case of PANi and PANi-ZnO nano-composite LB film, respectively. Conductive data images of the ITO substrate, PANi and PANi-ZnO nano-composite LB film on the ITO substrate obtained with an applied bias voltage of 4V showed that the distribution of current on the whole surface is almost uniform and very less inhomogeneities have been observed in the surface conductance of the PANi and PANi-ZnO nano-composite LB film.

A new donor-acceptor structured conjugated polymer (PDODTBI) with trifluoromethylated benzimidazole and benzo[1,2-b;3,4-b']dithiophene (BDT) unit have been designed and synthesized using Stille coupling polymerization reaction. The polymer is highly soluble in common organic solvents such as chloroform, tetrahydrofuron and chlorobenzene with good film forming properties. The structure of the polymer was elucidated by 1H NMR and FTIR spectroscopic techniques. The introduction of a trifluoromethyl group at 4-position of the benzimidazole unit has significantly altered the optical and electrochemical properties of polymer. The polymer film exhibited broad absorption band in the range of 400-680 nm. The optical band gap of the polymer film estimated from the absorption band edge and is found to be ~1.88 eV. Polymer exhibited deeper HOMO (-5.06 eV) and the LUMO (-3.65 eV) energy levels. Bulk heterojunction (BHJ) solar cell device with PDODTBI as a donor and PC61BM as an acceptor were evaluated.

Polymer-based electronic devices including organic field-effect transistors (OFET) and organic photovoltaics (OPV) have gained significant research interest due to their potentially low cost, light-weight, large area and flexible nature. In order to improve device performance for these polymeric materials, processes must be developed to enhance the degree of intermolecular ordering between chains in order to facilitate hopping of charges through the material. We have studied the commercially available poly(3-hexylthiophene) (P3HT) as a model π-conjugated polymer and have investigated the emergence of a lyotropic liquid crystalline phase that could serve as a means of intermolecular ordering in solution and as a potential precursor to well-ordered thin films of P3HT for device applications. Through various processing techniques including the use of mixed solvent systems, slow solution cooling and aging, we have observed capillary-flow-induced alignment of P3HT aggregates in solution. Using polarized optical microscopy, we have observed near monodomain-like behavior and other birefringent textures indicating the presence of a liquid crystalline phase under certain solution conditions. Raman spectroscopy performed on capillaries of these solutions also reveals anisotropic properties in the aligned P3HT. UV-Vis spectroscopy was employed to understand the intermolecular interactions that give rise to alignment, and dynamic light scattering was used to shed light on the dimensions of P3HT aggregates. To characterize macromolecular ordering and charge transport in the solid state, films were created using techniques analogous to those mentioned above and characterized with atomic force microscopy and testing of OFET electrical properties.

Light managementin a photovoltaic device is much about efficient light harvesting by increasing internal reflection and absorption. Structured architectures aids in enhancing internal reflection hence higher light absorption.Various techniques have been employed to fabricate these structures. In this work, organic photoactive materials were electrosprayed, utilizing a high voltage electric field to charge a droplet suspended through a nozzle, at various solution concentrations. Various process parameters like applied voltage, flow rate, tip to collector distance were optimized to obtain hierarchical structures. Morphological and optical properties of these optimized structures were analyzed to get interesting pattern with enhancement in surface area and increase in light absorption. These structures were further evaluated in photovoltaic device architecture.

Confinement has a strong effect on the crystallization of poly(3-hexylthiophene) (P3HT) thin films [1,2]. The ability to control the crystallinity in thin confined films is paramount for next generation of organic electronic devices [3]. Here, we study the structure of the thin P3HT films confined between two solid and smooth walls.
The films were characterized by 2D synchrotron grazing incidence X-ray diffraction (2D GIXD) at different angles of beam incidence, and by atomic force microscopy (AFM). We observe a significant enhancement in the film crystallinity compared to a traditionally spun and annealed thin film.
We present a depth-resolved analysis of the distribution of crystallites in the films, and discuss the effect of confinement on the crystalline structure as a function of the depth probed by the beam from the top surface of the film to the substrate. We show that the distribution of crystallites is different in the confined films than in the annealed films, with the high density of crystallites present in the topmost layer of the confined films.
[1] Scott Himmelberger, Javier Dacuña, Jonathan Rivnay, Leslie H. Jimison, Thomas McCarthy-Ward, Martin Heeney, Iain McCulloch, Michael F. Toney, and Alberto Salleo, Adv. Funct. Mater. Vol. 23, issue 16, p. 2091-2098.
[2] Kline R.J., McGehee M.D., Toney M.F., Nature Materials 2006, 5, 222-228.
[3] Duc T. Duong, Michael F. Toney, and Alberto Salleo, Physical Review B 86, 205205 (2012).

Here we present a study of the efficiency of cascade of excitation energy transfer in blends of p-type organic semiconductors with spectrally offset bandgaps. Single, binary, and ternary solutions of the following p-type organic semiconductors are prepared in chloroform: poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), poly(3-hexylthiophene) (P3HT), and 2,3,9,10,16,17,23,24-Octakis(octyloxy)-29H,31H-phthalocyanine, and subsequently deposited as sub-55 nm thin films via spin coating. Spectroscopic analysis shows absorption band full-width-at-half-maximum (FWHM) values increase from between 60 and 160 nm for the individual materials to greater than 450 nm for the composite thin film ternary blend. Additionally, photoluminescence measurements show significant quenching of excitations on the larger band-gap semiconductor (donor) in the presence of the lower band-gap semiconductor (acceptor). When the highest-occupied molecular orbital (HOMO) energy of the donor is less than that of the acceptor, resonant excitation energy transfer is observed with efficiencies approaching 100%. However, when the HOMO energy of donor semiconductor is greater than that of the acceptor, a highly efficient photoinduced charge transfer process occurs. Grazing incidence wide angle X-ray scattering illustrate the change in molecular packing and in the degree of molecular order and crystallinity in the blends compared to the neat materials. P3HT and the phthalocyanine derivative exhibit poor molecular blending and the extended separation of the P3HT chains from the phthalocyanine derivative molecules leads to a reduced charge transfer efficiency. All other cases show enhanced blending and packing, which correlate to closer donor/acceptor distances, and, thus ultra-high energy/charge transfer efficiency. This study suggests that further addition of an n-type organic semiconductor to the ternary blend could be used as a quaternary blended organic photovoltaic active layer with enhanced broadband solar energy conversion.

Pigments and dyes such as diketopyrrolopyrrole (DPP) and indigo are very popular building blocks for the preparation of organic semiconductors because they are relatively inexpensive to prepare and stable under device operation conditions. Recently, anthanthrone, or VAT Orange 3, attracted a lot of attention for organic electronics owing to its large planar conjugated backbone that provide it with excellent optical and electronic properties and pi-stacking interactions in the solid state. From the low-cost, commercially available 4,10-dibromoanthanthrone, we have been able to synthesize a large variety of small molecules and polymers with a broad range of optical and electronic properties. Using few synthetic steps, p-type, n-type and ambipolar materials have been obtained. Also, anthanthrone derivatives that absorb light in the entire visible spectrum can be prepared. The outstanding versatility of the 4,10-dibromoanthanthrone comes from the possibility of functionalizing this unit through the 4,10 and 6,12 axis, enabling the modulation of the frontier orbitals energy levels.
Here, we present the synthesis, properties and device testing of new materials based on the anthanthrone unit. Both small molecules and polymers will be presented. The best candidates have been tested in bulk hetero-junction solar cells (BHJSCs) and field-effect transistors (OFETs) and the results we obtained are presented.

We characterized the electrical properties of field-effect transistor (FET) and nonvolatile memory based on the low band gap small molecule, Si1TDPP-EE-C6.The small molecule consisted of electron-rich units and electron-deficient units. The as-spun Si1TDPP-EE-C6FET device exhibited ambipolar transport properties with a hole mobility of 7.3 × 10-5 cm2/(Vs) and an electron mobility of 1.6 × 10-5 cm2/(Vs).Thermal annealing at 110 °C leads to a dramatic increase in the carrier mobility, i.e. hole and electron mobilities of 3.7 × 10-3 and 5.1 × 10-4 cm2/(Vs),respectively. The change in crystalline structure and surface morphology were investigated by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM), which indicated that thermal annealing improved the film crystallinity and promoted the formation of a longer-range lamellar structure. Moreover, nonvolatile memory based on Si1TDPP-EE-C6 was successfully demonstrated by the incorporation of Au NPs as charge trapping sites at the interface between SiO2 and cross-linked poly(4-vinylphenol) dielectrics. The device exhibited reliable non-volatile memory characteristics, which include a wide memory window of 98 V, a high on/off-current ratio of 103, and good electrical reliability

Doped conducting polymers are often used as injection and transport layers in organic electronic devices. The electrical stability of these materials is vital to the stability of the devices. In this work, we study the electrical stability of sulfonated poly(thiophene-3-[2-(2- methoxyethoxy)ethoxy]-2,5-diyl) (P3MEETS), which is a potential replacement material for the conventional poly(ethylenedioxythiophene): poly(styrene sulfonate) (PEDT: PSSH) that is almost universally used as hole transport and injection layers. We show that P3MEETS has greater resistance to conductivity fading under high bias than PEDT: PSSH. This has been traced to a superior redox stability of P3MEETS, and its superior resistance to injection (or bias)-induced dedoping, using in-situ microRaman spectroelectrochemical techniques and microRaman mapping. We will also describe the mechanism for this enhanced stability which reveals a new design rule for stable doped organic polymers.

Conjugated, semi-crystalline polymers, like poly(3-hexylthiophene) (P3HT), have gained great attention for organic electronic devices combining impressive optoelectronic properties with access to low-cost solution processing. Although it is well known that the morphology is of great importance in organic solar cells and field-effect transistors, the complexity and small feature size of the microstructures resulting from high nucleation densities make an understanding of the structure-function relationship often difficult. It is therefore of great interest to control the molecular orientation over macroscopic distances in semicrystalline polymer thin films and then to study charge transport along specific crystallographic directions.
Within the talk different methods to induce crystalline order over large areas will be presented. These include swelling and deswelling in defined solvent vapors of good solvents to reduce nucleation densities, crystallization under confinement and mechanical rubbing. Regarding polymer materials we are currently extending our studies from p-type semiconductors such as P3HT[1] and (poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b&’)dithiophen]-2,6-diyl-alt-(2,1,3-benzo-thiadiazol)-4,7-diyl}) (PCPDTBT) [2] to n-type polymers such as poly{[N,N&’-bis(2-octyldodecyl)-1,4,5,8-naphthalene-dicarboximide-2,6-diyl]-alt-5,50-(2,2&’-bithiophene)} (PNDI2OD-2T) [3]. Charge transport measurements of highly ordered and oriented films both on p-type and n-type polymers strongly indicate anisotropic charge transport with the highest mobility pointing in the direction of the polymer chain.

[1] E. Crossland, K. Tremel, F. Fischer, K. Rahimi, G. Reiter, U. Steiner, S. Ludwigs, Adv. Mater. 2012, 24, 838.
[2] F. S. U. Fischer, K. Tremel, A.-K. Saur, S. Link, N. Kayunkid, M. Brinkmann, D. Herrero-Carvajal, J. T. Loacute;pez Navarrete, M. C. Ruiz Delgado, S. Ludwigs, Macromolecules 2013, 46, 4924.
[3] K. Tremel, F.S.U. Fischer, N. Kayunkid, R. DiPietro, R. Tkachov, A. Kiriy, D. Neher, S. Ludwigs, M.Brinkmann, Charge Transport Anisotropy in Highly Oriented Thin Films of the Acceptor Polymer P(NDI2OD-T2), Manuscript submitted.

In this work we present the development and synthesis of a new class of N-heterocyclic ligands based on a [2.2]paracyclophane core structure and the pursuing synthesis to the corresponding mu;-dichloro and neutral heteroleptic iridium(III) complexes. In the course of our work a set of four different ligands was prepared and characterized. The combination of these materials with three different saturation ligands resulted in nine successfully prepared iridium(III) complexes.

As the complexes were initially designed as potential emitters for organic light emitting diodes (OLEDs), we had a special interest in which way the integration of a [2.2]paracyclophane into the structure of a ligand might affect the optical properties of the resulting complex compared to its 2-phenylpyridine analogon. Especially because the cyclophane with its stacked benzene rings features a unique aromatic system due to their overlapping π-orbitals. Besides the optical properties, the [2.2]paracyclophanes might as well improve the film forming properties of the emitter because of its sterical demand and isomers.

The photophysical properties of the obtained materials were investigated and characterized. When irradiated with UV-light the obtained complexes exhibit emission maxima ranging from 539 to 610 nm with a full width at half maximum varying from 60 to 80 nm. Based on CIE-coordinates the color impression of the obtained materials was determined. They generally feature saturated colors ranging from slightly greenish up to an intense red/orange.
A selection of complexes was used to prepare solution processed single layer thin-film devices, which were optically characterized regarding their quantum efficiencies as well as their emission spectra. Additionally corresponding complexes have been investigated within an OLED.

A comparison between the newly developed emitters and the literature known 2-phenylpydiridine derivatives reveals that the emission maxima generally show a significant bathochromic shift (Δlambda; > 40 nm). Given the fact that a bathochromic shift of the iridium(III) complex emission is caused by a narrower HOMO-LUMO gap, the assumption is justified that a [2.2]paracyclophane moiety acts as a stronger electron donor compared to 2-phenylpyridene based ligand systems.

In conclusion we will report the developed synthetic pathways to obtain this new class of ligands in iridium(III) chemistry along with its challenges. We will present the key features of those systems including the optical and physical characterization as well as an outlook for potential applications.

Indium-tin oxide (ITO) is extensively used in optoelectronic applications, especially organic photovoltaics (OPVs). The high market demand and rare indium metal resources are affecting the ITO cost. Economically, instead of trashing old ITO-based solar panels, reusing these substrates can reduce the OPVs fabrication cost by 90%. Herein, we studied the effect of reusing old ITO substrates, which have a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film, on the photovoltaic characteristics of OPVs. Up to thirty months old OPV layers were removed from the ITO substrates by ultrasonication in chloroform and de-ionized water. Then, the new and old ITOs were investigated by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), four-point probe, conductive atomic force microscope (c-AFM), vis-UV-NIR absorption and photovoltaic characteristics. The XPS measurements on both new and old ITOs revealed the same surface composition. On the other hand, XRD analysis showed a lattice constant reduction of the old ITO substrates compared to the new ones, which is attributed to an Indium content reduction in the ITO matrix. The conductivity of the fresh and old ITO thin films was tested. The four-probe measurement showed a conductivity change within -10% to 5% with respect to the fresh substrates, which is still in the same range specified by the manufacturer. The c-AFM emphasized the four-point probe results in which the old substrates displayed darker current map with lowered root-mean square (RMS) values compared to the fresh ones. The old ITOs exhibited higher transmission of ~ 10% and ~ 3 % in the visible and infra-red regions, respectively. This transmission gain restors the conductivity loss to yield a short circuit current (Jsc) enhancement of ~ 4%. The new-made OPVs were prepared on both the old and fresh ITO glass slides by sandwiching poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:Phenyl C70 butyric acid methyl ester (PTB7:PC70BM) active layer in-between PEDOT:PSS and Ca/Al. A 9.24% power conversion efficiency (PCE) was achieved on old ITO substrates compared to 8.72% on the new ones. Several patches and different polymer systems were tested on old ITOs, and they displayed the same current density and PCE enhancement.

Despite extensive research on using graphene as transparent electrodes, its application to organic electronic devices has been rather limited. In the most commonly used fabrication technique [1], graphene-polymer stack floating on water is transferred onto a target substrate, and the polymer layer is subsequently removed by an organic solvent. As a result, in most previous applications, graphene has been deposited on a substrate before organic layer depositions, and it has been used mostly as a bottom electrode. Furthermore, when one works with wet chemicals that are known not to damage a given organic material [2], patterning of the graphene on the organic layer is likely to involve chemical and/or physical processes that may adversely affect the electrical and/or optical properties of the underlying organic layer.
Here, we describe a completely dry technique capable of transfer printing a graphene monolayer onto an organic layer. In this process, a CVD-grown graphene monolayer is first transferred onto a PDMS stamp by scooping a graphene-support layer stack floating on a liquid with the PDMS stamp, following the conventional wet-transfer process [1] except two important differences. First, we decrease the surface tension of the liquid by mixing ethanol and water. This enhances the wetting of the hydrophobic PDMS surface by the liquid, which is crucial to obtain a graphene monolayer with a low defect density on the PDMS stamp. Second, as a support layer deposited on graphene on a copper foil, we use a thin film of gold as opposed to commonly used polymers, such as PMMA and polystyrene. This allows us to use an aqueous chemical to remove the support layer, and hence prevents the swelling of PDMS, which would severely degrade the quality of the graphene monolayer on the PDMS stamp.
We discuss the effects of the surface tension of the liquid as well as the thickness of the gold support layer on the quality of a graphene monolayer transferred on an organic material commonly used in organic electronic devices, such as PEDOT:PSS and ZnPc. Furthermore, we discuss applications of our technique to organic and inorganic devices where graphene layers patterned by tranfer printing are used as transparent electrodes.
[1] X. S. Li, Y. W. Zhu, W. W. Cai, M. Borysiak, B. Y. Han, D. Chen, R. D. Piner, L. Colombo, R. S. Ruoff, Nano Letters 9, 4359-4363 (2009)
[2] Jie Song, Fong-Yu Kam, Rui-Qi Png, Wei-Ling Seah, Jing-Mei Zhuo, Geok-Kieng Lim, Peter K. H. Ho, Lay-Lay Chua, Nature Nanotechnology 8, 356-362 (2013)

Charge carrier mobility is still the most challenging issue that should be overcome to realize everyday-organic electronics in near future. We development new materials for high hole mobility. In this presentation, we will report various DPP-based and NDI-based materials for high hole mobility and electron mobility, respectively. Especilly, introduction a smart side chain engineering will be discussed for high mobillity

In this contribution we demonstrate that the change of molecular architecture represents a straightforward tool to tune material properties of conjugated polymers like absorption behavior, energy levels or charge transport. Linear and branched polythiophenes bearing an α-β-conjugated terthiophene (3T) as main building unit are compared to conventional linear poly(3-hexylthiophene) (P3HT) as reference system.
Branched 3D-polythiophenes are obtained by polymerizing an AB2-type terthiophene monomer using different polymerization techniques[1,2]. The strong influence of the branched α-β-conjugation on the optoelectronic properties, e.g. reduced HOMO and LUMO levels and isotropic charge transport is shown in comparison to the purely α-conjugated P3HT. While the branched polymer structure itself enables solubility in organic solvents without the need of solubilizing alkyl side chains, we could recently show that the post-functionalization of the inherently large number of polymer end groups with ionic moieties provides solubility in alternative non-toxic solvents like water or ionic liquids[3]. The branched character further opens up the perspective towards branched push-pull low band gap copolymers by adding comonomers and hence the combination of architectural and molecular design[4].
Implementing the branched terthiophene building unit in linear 2D-architectures we could prepare novel linear side chain π-extended polythiophenes with β-conjugated alkylthiophene side groups attached to the α-conjugated thiophene backbones[5]. The polymers exhibit a combination of the good properties of branched (e.g. lowering of HOMO/LUMO levels) and linear polythiophenes (e.g. high charge carrier mobilitiy) and showed first promising results in combination with P(NDI2OD-T2) in polymer-polymer bulk heterojunction solar cells.
References:
[1] H. S. Mangold, T. V Richter, S. Link, U. Würfel, S. Ludwigs, J. Phys. Chem. B 2012, 116, 154-159.
[2] M. Scheuble, M. Goll, A. Ruff, T. V. Richter, S. Ludwigs in preparation.
[3] T. V. Richter, C. Bühler, S. Ludwigs, J. Am. Chem. Soc. 2012, 134, 43-46.
[4] S. Link, M. Scheuble, M. Goll, E. Muks, A. Ruff, A. Hoffmann, T. V. Richter, J. T. L. Navarrete, M. C. Ruiz-Delgado, S. Ludwigs, submitted.
[5] T. V Richter, C. H. Braun, S. Link, M. Scheuble, E. J. W. Crossland, F. Stelzl, U. Würfel, S. Ludwigs, Macromolecules 2012, 45, 5782-5788.

The development of soluble high-mobility organic semiconductor materials is crucial to elevate performances of organic field-effect transistors (OFETs) toward creation of printed electronics. Recently, thiophene-based compounds are gaining popularity for solution-processable high-mobility organic semiconductors reported for OFETs with mobility of more than 1 cm2/Vs, highlighting large electronic population in sulfur atoms. Interestingly, furan, an oxygen-containing congener, also has been of particular interest because of its densely packed nature, which originates from the smaller element size. In this work, we report the oxygen-incorporated congeners, dinaphtho[2,3-b:2',3'-d]furan derivatives (DNF-Vs), as new entity of organic semiconducting materials.
We developed a facile and cost-effective synthetic method utilizing dehydration reaction as a key reaction to construct DNF-V framework. By use of this methodology, hexyl and decyl group-substituted DNF-V at 3,9- and 2,10-positions (hereinafter referred to as ‘Cn-DNF-VW&’ and ‘Cn-DNF-VV&’) were synthesized to unveil the effect of the length and the position of the alkyl chain on the physicochemical properties. It is found that the shorter alkyl chain substituted DNF-V derivatives possess higher solubility in common organic solvents as well as higher thermal stability in crystal phase. Unlike thiophene counterparts, DNF-V derivatives exhibit high quantum efficiency with deep blue emission. X-ray single crystal analysis clarified that both DNF-V itself and alkylated DNF-V derivatives stack into a herringbone packing manner. In these structures, each molecule interacts with adjacent molecules with either slight or no displacement in the direction of molecular long axis, which is favourable for effective orbital overlap.
Finally, we evaluated the intrinsic charge carrier mobility of Cnnot;-DNF-VW and Cn -DNF-VV in the form of crystalline films, which were prepared by solution-crystallizing ‘edge-casting&’ methods originally developed by our group. The hole mobilities of C10-DNF-VW and C10-DNF-VV FETs were up to 1.1 and 1.3 cm2/Vs, respectively.

Polypyrrole (PPy) structures, compared to their non-structured counterparts, are appreciated for their high electrical conductivity (tens of S cm-1) and specific surface. So far, one dimensional (1-D) structures of PPy, namely nanowires, nanorods and nanotubes, are intensively studied. They are usually prepared by hard-template, soft-template or non-templated methods and are applicable in many areas like chemical sensors, supercapacitors, biological substrates, packaging technology and many others. On the other side, these 1-D polymer structures usually suffer from cross-linking and from this reason are also hardly processable.
This contribution describes a new type of polypyrrole structure prepared using a structure guiding agent - Sunset Yellow FCF. Sunset Yellow FCF belongs to a class of lyotropic chromatic liquid crystals (LC). The structures of synthesized PPy have uniform three dimensional (3-D) "mace head" shapes with diameter around 5 µm. There was studied the influence of: (i) different synthesis procedures, (ii) LC concentration (from 5 mM to 50 mM) and (iii) temperature on volume yield and shape perfection of the polymer product. Electrical conductivity of obtained 3-D structures (3 S cm-1) was discussed and compared with that of non-structured PPy (2.8 × 10-2 S cm-1) and 1-D structured PPy (60 S cm-1). Morphology and composition of the prepared structures were studied by SEM and FTIR respectively. Finally the supposed mechanisms of 3-D structures formation are also discussed.

There are a number of organics or polymers, which can be used for optoelectronic applications; especially, organic electronics have taken a great attention to develop flexible electronics, including OLED and photovoltaic devices. Soft lithography has been widely used to fabricate electronic circuit integrations based on soft elements such as organic and polymeric printing materials. Soft lithography employs a silicon rubber (polydimethylsiloxane, PDMS) as a stamping material to directly assemble and print small patterns onto a variety of substrates. This technology can also replicate and then transfer electronic patterns from the original masters onto flexible substrates for plastic electronics at a low cost. However, commercially available silicon elastomers often results in collapse and mergence due to its low mechanical strength, especially in the nano-scale regimes. Since the resolution of soft lithography technique relies on the elastomeric stamps, the limitation has motivated us to develop a new version of photocured silicon elastomers. The new stamping material is based on a photocurable silicon rubber prepolymer, which was designed to satisfy a diverse set of our multiple demands in soft lithographic purposes, such as adjusted physical stiffness and elastomeric properties, low linear polymerization shrinkage, photocurability, and freedom from stress. It is a hybrid rubber based on of organic/inorganic components and also designed for an advanced semiconductor industry. We then carried out a soft lithography using the new stamps. It results in enhanced mechanical performances, especially in our nanofabrication purposes. We also demonstrate ‘elastomeric photopatterns&’ at the micro-scales using the modified silicon rubber prepolymer for a photocurable integration, which would be free from thermal shrinkages to enhance the resolutions.

We herein report on unique optoelectronic characteristics of tailor-made donor (D) - acceptor (A) charge-transfer (CT) cocrystals comprising rationally designed distyrylbenzene (DSB) and dicyanodistyrylbenzene (DCS) based binary molecular systems. As demonstrated in our previous report, the isometrically designed 4M-DSB (D) and CN-TFPA (A) established regular dense-packed one-dimensional mixed-stacked CT co-crystalline state to show peculiar ambipolar transporting nature (p-/n-channel mobility up to 6.7×10^-3 cm²V^-1s^-1 and 6.7×10^-2 cm²V^-1s^-1, respectively) due to the balanced transfer integrals for hole and electron by superexchange rule.^1,2 Moreover, due to the remarkable luminescence properties of both components, highly luminescent and red-shifted emission originating from intermolecular CT states were observed; which would pave the way to advanced optoelectronic application. As an extended study of this multicomponent system, herein we designed a series of modular D/A systems aiming at both fine-tuned electronic states and deliberately varied stacking structures. By the series of spectroscopic and structural analyses supported by quantum-chemical calculations, we could rationalize and correlate their charge carrier transporting characters and luminescence properties. Finally, potential optoelectronic applications of these D/A systems will be briefly discussed.
References
1. Park, S. K.; Varghese, S; Kim, J. H.; Yoon, S.-J.; Kwon, O. K.; An, B.-K.; Gierschner, J.; Park, S. Y. J. Am. Chem. Soc. 2013, 135, 4757.
2. Zhu, L.; Yi, Y.; Li, Y.; Kim, E.-G.; Coropceanu, V.; Bredas, J.-L. J. Am. Chem. Soc. 2012, 134, 2340.

Volatile organic compounds especially halogenated hydrocarbons are widely used as solvents in many industries. Nevertheless, the toxicity of these compounds can have an adverse effect on the breathing and when coming in contact with skin. So, effective vapor sensing systems are required to identify these hydrocarbons. In this work, composites of polydiphenylamine (D-PDPA) and zeolite Y with H+ as the cation (Y-H+) have been fabricated to be used as a sensing material towards non-halogenated and halogenated solvents (hexane, dichloromethane, 1, 2-dichloroethane, chloroform). The composites can discriminate a non-halogenated solvent from halogenated solvents. They possess maximum electrical conductivity sensitivity values towards dichloromethane, but the composites do not respond to hexane. Generally, the sensitivity of the composites increases with increasing zeolite content and vapor concentration. To enhance the sensing properties of the composites, zeolite Y was modified by the dealumination process. The resultant dealuminated zeolite Y shows a higher sensitivity when exposed to those solvents than the pristine zeolite. An acid treatment time of 12 h provides the highest sensitivity. The optimal dealuminated zeolite content in the composites is 30 vol.%. The interaction between the polymer and the vapor is irreversible as confirmed by EFM technique. A mechanism for the interaction between the composites and the solvents is proposed.

One dimensional structures of conductive polymer polypyrrole possess remarkable properties, in particular high specific surface area and electrical conductivity. The present contribution deals with synthesis of polypyrrole nanotubes by oxidation of pyrrole in the presence of methyl orange as a structure-guiding agent. It is a very effective and simple method to achieve high yield of nanostructured material homogeneously distributed throughout the volume of native liquor.
Methyl orange belongs to a class of chromophores called azo dyes. Under appropriate conditions methyl orange can polymerize and create one dimensional structure. According to one hypothesis these methyl orange templates play a key role in the formation of structured polypyrrole. However, it is also possible that the growth of structured polypyrrole starts from small nuclei formed by reaction of methyl orange with oxidizing agent. This contribution brings better understanding of processes taking place during the synthesis of polypyrrole nanotubes.
Polypyrrole was prepared by oxidation of pyrrole monomer with iron (III) chloride in the presence of methyl orange. Two reaction schemes with reversed order of addition of oxidant and monomer were tested. It was found that the morphology of the produced polymer is dependent on the reaction scheme. Formation of polypyrrole nanotubes with circular or rectangular profiles was observed. The diameter of polypyrrole nanotubes obtained from SEM micrographs by image processing ranged from tens to hundreds of nanometres, electrical conductivity measured by Van der Pauw method reached values up to 70 S/cm. Fourier transform infrared spectroscopy was used to assess the molecular structure of polypyrrole and to detect residual methyl orange in the samples.

Over the past 10 years, extensive research has been conducted about organic electronics, particularly organic solar cells (OSCs) and organic thin film transistors (OTFTs), due to their light-weight, cheap, flexible properties. To attain the high device performance, research on new materials for photoactive layers and semiconducting layers have received a lot of attention. Conventional polythiophene derivatives such as regioregular poly(3-hexyl thiophene) (rr-P3HT) have shown reasonable high power conversion efficiencies (PCE) up to 5% because of their relatively high short circuit current (Jsc) and fill factor (FF). Although rr-P3HT is a promising candidate as an electron donor for bulk heterojunction (BHJ) OSCs, PCE of rr-P3HT has been limited by low open circuit voltage (Voc) and limited photon absorption at the long wavelength region. Our approach to achieve better performance is to increase Voc by introducing phenanthrene-based polymers as donor materials for OPVs. Phenanthrene is one kind of the polyaromactic compounds and they have chance to achieve higher Voc due to their deeper HOMO level than that of thiophene derivatives. Moreover, their rigid planar structure has potential for high mobilities. However, many researchers have reported that phenanthrene homopolymers showed large energy bandgap (Eg) properties as high as 3 eV, resulting in reduced Jsc value. To solve this problem, donor-acceptor alternating copolymer (push-pull structure) concept was introduced. We used a diketopyrrolopyrrole (DPP) unit as an electron accepting moiety for donor-acceptor type. In this presentation, we will report the material synthesis, characterization, and device performance of these novel low band gap polymers.

Poly(vinyl triazole) (PVT) is a promising material as dielectrics for organic field effect transistors (OFETs) [1], because of their high dielectric constant, good solubility in polar solvents and excellent film formation properties. A detailed study is necessary to elucidate the effect of moleular weight on the OFET device performances based on PVT dielectric.
In this work, poly(1-vinyl-1,2,4-triazole) polymers were synthesized by controlled radical polymerization based on a RAFT process (RAFT: Reversible Addition Fragmentation Chain Transfer) [2]. Different molecular weight PVT polymers were characterized by size exclusion chromatography (SEC), proton nuclear magnetic resonance spectroscopy (1H-NMR), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dielectric properties of different moleculer weight PVTs on organic field effect transistors (OFETs) were investigated using the bottom gate/top contact device configuration. Two types of devices were prepared using poly(3-hexylthiophene) (P3HT) as p-channel and [6,6]-phenyl-C61-butyric acid methyl ester (C60PCBM) as n-channel semiconductor layers. All types of devices using water soluble PVTs as dielectrics show low threshold voltages, high on/off ratios and low hysteresis transfer characteristics. We have identified the effect of the molecular weight on the device performances. Our experiments demonstrate that poly(1-vinyl-1,2,4-triazole) is a stable dielectric supporting both n and p channel solution processed OFET devices.

[1] Abbas, M.; Cakmak, G.; Tekin, N.; Kara, A.; Guney, H. Y.; Arici, E.; Sariciftci, N. S. Organic Electronics 2011, 12, 497-503.
[2] Mori, H.; Ishikawa, K.; Abiko, Y.; Maki, Y.; Onuma, A.; Morishima, M. Macromolecular Chemistry and Physics 2012, 213,1803-1814.

Phosphorescent organic lighting-emitting diodes (PhOLEDs) which utilize triplet excitons to achieve superior performance have attracted a great deal of attention due to their potential applications in full-color flat-panel displays and lighting sources. Recently, solution-processed PhOLEDs provide an economically attrative alternative to vacuum-deposition. Soluable type PhOLEDs using small molecules and conjugated polymers as host materials have been reported. However, it is often difficult to control molecular weight, end groups and chemical defects in the polymer backbone, which could result in exciton quenching. In contrast to polymer counterparts, solution processible small-molecule host materials are advantageous as they can conventionally synthesized and purified. A good host material for solution processed PhOLEDs should have the following features: high triplet energy, good charge carrier transport properties, high solubility and film-forming ability. In addition, suitable highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels matching those of the adjacent layers are required to reduce the hole/electron injection barriers.
In this wok, we synthesized a series of novel carbazole derivatives 3,3'',6,6''-tetra-tert-butyl-9'-(9,9-diphenyl-9H-fluoren-2-yl)-9'H-9,3':6',9''-tercarbazole (2DPF-TCz), 9'-(9,9'-spirobi[fluoren]-2-yl)-3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole (2SBF-TCz), 9',9''''-(9,9-diphenyl-9H-fluorene-2,7-diyl)bis(3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole) (27DPF-TCz), and 2,7-bis(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)-9,9'-spirobi[fluorene] (27SBF-TCz), using of a fluorene monomer as the rigid core with carbazole containing groups linked to the core through the 2 and (or) 7 positions. The compounds exhibit high glass-transition temperatures (231-310 °C) and high triplet energy levels (2.61-2.73 eV). Atomic force microscopy measurements indicate that high-quality amorphous films of the hosts can be prepared by spin-coating. Additionally, they also exhibit appropriate HOMO energy levels (-5.36 to -5.23 eV), resulting in improved hole-injection property. Additionally, tert-butyl groups units are introduced into the molecular structure to ensure good solubility to form high-quality films. Solution-processed blue phosphorescent organic light-emitting devices containing FIrpic dopant exhibits the maximum efficiency of 21.2 cd A-1. Moreover, the performance of Ir(mppy)3 based green-emitting devices shows the maximum efficiency of 34.8 cd A-1. Remarkably, the performances of these novel host materials based devices are far superior to those of the corresponding mCP based devices, which is outstanding for a solution-processed blue and green PhOLED.

It is well-known that benzophenone possess high intersystem crossing (ISC) efficiency and small singlet-triplet exchange energy (ΔEST). However, benzophenone itself is difficult to apply in organic light-emitting diodes (OLEDs) because of its intrinsic low photoluminescence efficiency (PLQE) at room temperature and instability in redox reactions. In this study, we designed new benzophenone-based butterfly-shaped molecules to achieve high thermally activated delayed fluorescence (TADF) efficiency. To obtain a high efficiency of TADF, rather small ΔEST between S1 and triplet excited (T1) levels should be necessary for light-emitting materials which can be attained by small orbital overlapping between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Based on this conception, we demonstrated TADF-OLEDs employing the newly designed benzophenone derivatives (4,4&’-bis{3-(9H-carbazol-9-yl)}methanone (Cz2BP), 4,4&’-bis{3-(9H-carbazol-9-yl)-9H-carbazol-9-yl}methanone (CC2BP), 4,4&’-bis{4-(10H-phenoxazine-10-yl)}methanone (Px2BP), 1,4-bis{4-(10H-phenoxazine-10-yl)benzoyl}benzene (pPx2BP2), and 1,3-bis{4-(10H-phenoxazine-10-yl)benzoyl}benzene (mPx2PB2) can achieve high external electroluminescence quantum efficiencies from deep-blue to red emission. Our molecular design for the benzophenone derivatives allows spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to exceptionally small energy difference between S1 and T1 (under 0.1 eV) using the TD-DFT/B3LYP/6-31G(d,p) method. In addition, a high electrochemical stability of benzophenone derivatives were confirmed by cyclic voltammetry (CV). Photoluminescence quantum yields of co-deposited films of host : benzophenone derivatives were found to be 35.9-72.7%. Moreover, the transient PL properties of host : benzophenone derivatives co-deposited film have been further studied over a wide range of temperature from 150 to 300K and confirmed the temperature dependent fluorescence intensity which promoted by strong thermal energy in high temperature. TADF-OLEDs using benzophenone derivatives as emitting materials showed relatively high external quantum efficiencies ranging from 14.7 to 4.2% in a whole visible light region.

Engineering the morphological and electronic structures of supramolecular assemblies based on π-conjugated molecules is important for the creation of high-performance organic electronics devices. Herein, we present π-extended heteroacene derivatives that act as high-mobility p-type organic semiconductors. These molecules self-organize into well-ordered crystalline microstructures, and show excellent electrical properties with high charge carrier mobilities under ambient conditions. The integration of the multiple heteroatoms into the molecular scaffold causes the molecules to adopt highly compressed packing structure in the self-organized microstructures via strong intermolecular interactions, and thereby provides efficient charge transport pathways.

Organic semiconductors have received huge attention for their mechanical stability, earth abundant nature, synthetic molecular tunability, and low processing cost. Especially, the organic field-effect transistors (OFETs) have been intensively studied to replace a-Si and to enable unprecedented large-area and flexible electronics. Unfortunately, the low performance of OFETs in early stage hindered the realization of organic electronics in our daily life. The significant improvement of performance and stability has been achieved with new types of materials, which now surpass conventional a-Si and compete with carbon materials and nanomaterials. However, the relatively poor understanding of the origin of high performance in newly developed organic semiconductor requires detailed investigation of its structure-property relationship for further progress. To understand the effect of the molecular packing on charge transport, we have studied derivatives of high hole mobility organic semiconductor, [1]benzothieno[3,2-b][1]benzothiophene (BTBT).

To investigate whether charges are localized onto individual molecules or exhibit extended-state band conduction in high performance n type semiconductor, quantitative measurements of the temperature and lateral field dependent performance of 6, 13- bis ((triisopropylsilyl) ethynyl) - 5,7,12,14-tetraazapentacene (TIPS-TAP) in field effect transistors (FETs) were presented here. Thin films with saturation mobility close to or larger than 1cm2/Vs at room temperature were made by thermal deposition and by simply drop-casting solution onto AlOy/TiOx dielectrics. The dielectrics were pretreated with self-assembled monolayers (SAMs) of a newly- developed phosphonic acid [1], leading to enhanced surface energy and resulting fine-grained needle-like crystals with uniform grain direction in hundreds of micrometer scale from solution-cast method. Devices with electron transport direction “along the grain boundaries (GBs)” and “across the GBs” were created in those solution-cast films, by making the channel parallel and perpendicular to that of the grains, respectively. The resulting Arrhenius-type thermal activation behavior for all the devices supported the hopping transport mechanism, and the variable range hopping model (VRH) provides the best interpretation of the data. There was weak field-dependence for the “along GBs” devices, while unexpected Poole-Frenkel dependence in quite low electric field was observed in the “across the GBs” and “thermal-deposited” devices, demonstrating detailed effects caused by grain boundaries. This work revealed thermal-activated and field-assisted hopping transport for a high-performance n-type semiconductor, and suggested well-controlled device architecture for investigation of the disorder-induced effects in polycrystalline thin films.
References:
[1] D. Liu, X. Xu, Y. Su, Z. He, J. Xu, Q. Miao, Angew. Chem. Int. Ed. 2013, 52, 6222 -6227.

We investigated the properties of a dielectric/metal/dielectric multilayer structured transparent electrode for organic light-emitting diodes (OLEDs). The sol-gel processed HfO2 thin film, annealed at 250 °C, had high refractive index value of n > 2.0 and smooth film morphology. The Au metal layer sandwiched between sol-gel processed HfO2 and MoO3 dielectric layers. Using theoretical calculation the optimum thickness of metal and HfO2 layer were identified. The calculated electroluminescence spectra were consistent with the experimental results, validating the simulation results. Due to the multiple reflection and interferences in dielectric materials with high refractive index the outcoupling efficiency was enhanced. For the OLED device with HfO2/Au/MoO3 transparent electrode, the external quantum efficiency increased about 15 % and the luminance at 100 mA/cm2 increased from 29,000 to 34,000 cd/m2, compared with the device having indium-tin-oxide (ITO) electrode. The emission pattern dependency on viewing angle was close to Lambertian.

Discrete π-conjugated molecular systems display opto-electronic properties similar to their polymer counterparts, but have more well-defined structures, are monodisperse, and allow for a more modular synthetic methodology. On the other hand, polymers tend to have better mechanical properties, and are thus more amenable toward processing techniques commonly used for various thin-film based electronic devices. At the same time, polymers do suffer from disadvantages relating to polydispersity, batch-to-batch variability and less-defined structures, which prevent a straightforward correlation of structure to properties. One strategy to bridge these two disparate domains is to take advantage of the modular nature of discrete conjugated oligomer synthesis to functionalize synthons such that copolymers of both flexible alkyl spacers and rod-like discrete conjugated oligomers in the main chain are obtained. We show three distinct routes to attain this: 1) photopolymerizable acrylate-functionalized (telechelic) conjugated oligomers, 2) thermally polymerizable alcohol-functionalized conjugated oligomers, and 3) Suzuki and Stille polymerizable aliphatic chain-functionalized conjugated oligomers. The opto-electronic properties of these polymers are tuned by the use of electron-rich heterocycles including thiophenes and dioxythiophenes, along with electron-poor heterocycles including 2,1,3-benzothiadiazole and isoindigo. Specific functionalities can be introduced to create potential for supramolecular ordering, such as formation of insoluble cross-linked networks of π-stacked chains, or from potential mesomorphic character induced from phase separation of aliphatic and aromatic segments. These polymers show electrochromic behavior, and also show promise for use as active layers in organic field-effect transistors and solar cells. Comparisons with their monomeric discrete conjugated oligomer counterparts are also presented to better understand structure-property relationships.

The LED has received much attention for use in lighting and display fields due to its low energy consumption, long lifetime and durability. Among components of LED, the LED encapsulant plays an important role for obtaining high luminescence efficiency. The luminescence efficiency of LED can be further increased by using high refractive index encapsulant promoting light extraction for the bottom LED chip. The stability against heat and yellowing during the operation is another important requirement for LED encapsulant.
In this study, we report a sol-gel siloxane nanohybrid material (hybrimer) with a high refractive index and excellent stability against heat and yellowing which can be used as a high-performance LED encapsulant. Hybrimer is fabricated by a platinum (Pt)-catalyzed thermal hydrosilylation reaction of two different phenyl-containing oligosiloxane resins functionalized with vinyl and hydrogen groups, respectively. These vinyl-oligosiloxane and hydrogen-oligosiloxane resins are synthesized by hydrolytic sol-gel condensation. The resulting hybrimer exhibits a high refractive index (1.57 at 633nm), high optical transparency (~90% at 450nm), and excellent long-term thermal stability. The refractive index of hybrimer is further increased by introducing sol-gel derived heterometallic zirconium oxide in the oligosiloxane resins, which additionally allows the reduction of curing process temperature and the amount of Pt catalyst used. Finally, we demonstrate a LED chip encapsulated with the hybrimer encapsulant.

We fabricated electrical bistable devices based on spherical Ag2S using spin coating method. An electrical hysteresis loop and the negative differential resistance (NDR) effect were observed on cyclically sweeping the electrical bias. The ON/OFF current ratio can reach maximum level of 104 and is related to the mass ratio of Ag2S to PVK and the amplitude of the sweeping voltage. Further studies about the NDR effects reveal that there is a proportional relationship between the erasing voltage (VNDR) and the charging energy of the device, which suggests the transition between the two states is attributed to the charge trapping/detrapping process in the Ag2S nanocrystals. Theoretical conducting models also have been introduced to illustrate the charge transport mechanism.

Yang-Doo Kim1, Joong-Yeon Cho1, Ju-Hyeon Shin1, Hak-Jong Choi1, and Heon Lee1 *
1Department of Materials Science and Engineering, Korea University, South Korea
Effective light out-coupling enhancement of organic light-emitting diodes (OLEDs) is one of the most important elements for enhancing the efficiency of OLEDs. Optical functional layer such as light scattering and anti-reflection layers have often been used for the high effective absorption of light. However, light emission layers and TCO on substrate with sharply rough surfaces can easily be degraded by surface protrusions, resulting in degradation of the overall performance of solar. Especially for OLEDs, optical functional structures often caused defects in the light emission layer during its deposition process.
Fabrication of substrate with optical functional surface structures without degrading the electrical properties of light emission layer and TCO is thus important for improving the conversion efficiency of OLEDs. However, the fabrication of optically functional substrate without sharp surface structure is extremely difficult.
In this paper, the functional pattern on glass substrate was fabricated using direct printing technique for increasing the light scattering. The pattern was fabricated with hemisphere shaped structures in order to preventing degradation of thin active layer of OLEDs. An In-doped SnO (ITO) layer followed by an electron transfer layer. light emission layer and hole transfer layer were deposited on the patterned glass substrates. Efficiency of OLEDs on patterned glass was increased 40% without degradation of active layer, compared with flat glass substrate.

Hybrid materials have drawn attention of scientist in the last decades especially because of its wide range of applications[1-3]. The use of polymers and inorganic materials has given opportunity for the development of different electro-chemical devices with applications in gas and humidity sensors, biosensors, electronic devices, optical devices, batteries and biological tissue engineering. Mixed Polyaniline/titanium dioxide (PANI/TiO2) films is a candidate for sensors applications. The material can be fabricated using different techniques, among which we focused on electrospraying (ES). ES is a technique which uses high voltages (in the range of kV) for film production. It consists of a glass syringe with metal needle and a metal plate that works as substrate holder. A solution is inserted in the syringe and a spray is formed caused by the voltage between the metal needle and the plate. PANI was already obtained by chemical synthesis [2,3]. In our study PANI/TiO2 mixed compound was obtained by the following recipe: an amount of TiO2 was dissolved in 20 mL of 1M HCl by sonication process (10 min). The mass of TiO2 oxide varied from 50mg to 1000mg. Dodecyl Benzene Sulphonic Acid-DBSA (1.50g) was also dissolved in 20 mL of 1M HCl and added to the previous solution. After, 1 mL of aniline was added in this solution in ice bath at 0°C at constant stirring. Ammonium persulfate-APS (1.2g) was dissolved in 40 mL of 1 M HCl and finally added to the previous solution by slow dripping. The reaction occurred at 0 °C by 24 h and, finally, the result was washed with water and acetone for several times and dried at room temperature. PANI/TiO2 films were prepared on ITO-PET (tin dioxide doped with indium on poly(ethylene terephthalate)) and FTO (tin dioxide doped with fluorine) as substrates by dissolving 50 mg in 2.5 mL of chloroform (used as carrying solvent). The chloroform enabled spray formation by ES technique. The stability of the film was tested in buffers with acid and alkaline pHs with and without introduction of Nafion® (sulfonated tetrafluoroethylenenafion). ES voltages experimented ranged from 5kV to 15kV, corresponding to electric fields from 1.5 kV/cm to 4.5kV/cm. According to the structural characterizations, TiO2 crystallites were scattered in the polymer chains. This structural organization improved sensor responsivity, as measured by the extended gate field effect transistor (EGFET) configuration. Values ranging from 20 up to 70mV/pH were obtained, with an anti-correlated dependence with the ES voltage. Results will be discussed focusing also on the optimization of the device.
[1] P. Li, Y.Li, B. Yinga, M.Yanga. Sensors and Actuators B 141, 390-395 (2009).
[2] Q. Yub, M.Wanga, H. Chena, Z. Daib. Materials Chemistry and Physics 129, 666- 672 (2011)
[3] S. Neubert, D. Pliszkaa, V. Thavasia, E. Wintermantel, S. Ramakrishna. Materials Science and Engineering B 176, 640-646(2011).

The development of diindenothiophenes and diindenothieno[3,2-b]thiophenes as fully conjugated, electron accepting scaffolds for organic electronics is described. Quinoidal oligothiophenes are an important class of materials for organic electronics due to their amphoteric redox properties and favorable solid-state packing. The reported compounds feature quinoidal (thieno)thiophene cores and a variety of electron-deficient groups attached via indene fragments. Cyclic voltammetry, UV-vis absorption and solid-state packing will be discussed.

In semitransparent organic photovoltaic (OPV) devices a high-conductivity PEDOT:PSS layer is often used as a transparent electrode to collect photogenerated holes. Unfortunately, in order to minimize resistive losses in the PEDOT:PSS layer it must be thick, which diminishes the amount of light transmitted through the device. Films of silver nanowires have shown much higher conductivity and transparency than PEDOT:PSS, but these films are mechanically weak making them unsuitable for use alone. Commonly, the nanowires are imbedded in a nonconductive, but mechanically robust, polymer matrix. Here, we investigate PEDOT:PSS/silver nanowire composite films coated from a single solution. Coating the materials from a single solution diminishes the number of processing steps and eliminates the need for a nonconducitve polymer matrix. We find that the blend films have lower resistance than pure PEDOT:PSS films with the same average visible transmittance. It is also found that the addition large weight fractions of the nanowires to PEDOT:PSS results in only a modest decrease in the work function. When the composite films are incorporated into semitransparent OPV devices there are increases in open-circuit voltage, short-circuit current density, and fill factor, which result in a significant increase in power conversion efficiency compared to OPV devices with PEDOT:PSS electrodes.

The rapid development of AC-Field -Induced Polymer Electroluminescent Lighting (FIPEL) is reviewed in the context of basic physical mechanisms, functional polymer and device fabrication process which demonstrate FIPEL as a bright, efficient alternative. FIPEL generates light-coupling by field-excitons that are confined a recombination region defined by a sandwich of insulating materials. In this review, we focus on internal physical mechanism concerning the impact factor on dopant/host band alignment exciton formation, nano-antennae effects and role of SWNTs playing in exciton enhancement. High efficient FIPEL also be discussed through considering frequency dependence characteristic and phase angle. Besides, various high performance polymers including dielectric material, emitting material and electron/hole generation material are discussed in this work.

We report newly synthesized polymeric semiconductors Polymer 1 & Polymer 2 the enhancement in field-effect mobility achieved through donor-acceptor copolymer concept. Novel electron acceptor based copolymers were designed by stille coupling reaction. New electron acceptor unit has strong electron accepting property lead to low band gap, two polymers showed strong absorption. Polymer 1 and Polymer 2 has narrow band gap. The high thermal stability of polymers were determined by thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) studies. Newly synthesized polymers ware observed that field-effect mobility of the organic thin film transistor device.

A high-performance conjugated polymer for use as the active layer of n-channel organic field-effect transistors (OFETs) is reported. The solution-processable n-channel polymer is systematically designed and synthesized with an alternating structure of long alkyl substituted acceptor and donor units.
The material has a well-controlled molecular structure with an extended π-conjugated backbone, with no increase in the LUMO level, achieving a high mobility and highly ambient stable n-type OFET. The top-gate, bottom contact device shows remarkably high electron charge-carrier. Moreover, new polymer OFETs exhibit excellent air and operation stability. Such high device performance is attributed to improved π-π intermolecular interactions owing to the extended π-conjugation, apart from the improved crystallinity and highly interdigitated lamellar structure caused by the extended π-π backbone and long alkyl groups.

We have been intensively involved in developing highly crystalline n-channel organic semiconductors based on the push-pull π-conjugated frameworks. Recently, we reported that a specific dicyanodistyrylbenzene (DCS) derivative with multiple electron-withdrawing units, HEX-4-TFPTA, exhibited an excellent n-type field-effect electron mobility as high as 2.14 cm² V^-1s^-1.^[1] On the basis of our previous observation that a stabilized energy level and outstanding 2D molecular tacking inherent to the DCS backbone favorably enhance the electrical transport, herein we designed and synthesized a new DCS derivative, (2E,2'E)-3,3'-(2,5-dimethoxy-1,4-phenylene)bis(2-(5-(4-(trifluoromethyl)phenyl)thiophen-2-yl)acrylonitrile) (ME-4-TFPTA), which maintains sufficiently low energy level but secures good solubility by eliminating interdigitation units at the central position of Hex-4-TFPTA. Solution-grown millimeter sized 2D single crystalline ribbon of ME-4-TFPTA showed clear correlation between the crystal structure and the electrical characteristic. Compared to the smaller mobility toward herringbone direction (the short axis of the crystal: electron mobility up to 1.26 cm²V^-1s^-1), that of π-stacking direction (the long axis of the crystal) exhibited much higher value (electron mobility up to 7.82 cm²V^-1s^-1), which is among the highest values reported for n-type organic field-effect transistors.
Reference
[1] S. W. Yun, J. H. Kim, S. Shin, H. Yang, B.-K. An, L. Yang, S. Y. Park, Advanced Materials 2012, 24, 911.

Three new benzothieno[3,2-b]thiophene (BTT) derivatives, which were end-functionalized with phenyl (BTT-P), benzothiophenyl (BTT-BT), and benzothieno[3,2-b]thiophenyl groups (BBTT), were synthesized and characterized in organic thin-film transistors(OTFTs). A new and improved synthetic method for BTTs was developed, which enabled the efficient realization of new BTT-based semiconductors. The crystal structure of BBTT was determined by single-crystal X-ray diffraction. Within this family, BBTT, which had the largest conjugation of the BTT derivatives in this study, exhibited the highest p-channel characteristic, with a carrier mobility as high as 0.22 cm2V-1s-1 and a current on/off ratio of 1107, as well as good ambient stability for bottom-contact/bottomgate OTFT devices. The device characteristics were correlated with the film morphologies and microstructures of the corresponding compounds

We have studied stable and efficient doping behavior of reduced graphene oxide (RGO) by constructing self-assembled monolayers (SAMs) on the surface of RGO electrodes. Considering electron-withdrawing and -donating properties of functional groups, two kinds of alkylsilanes were employed for p- and n-type doping of RGOs. Various thin film characteristics of SAM-functionalized RGOs such as surface energy, sheet resistance, charge transport behavior and work function (WF) were thoroughly investigated. Especially, the WFs of RGO electrodes could be tuned from 4.31 eV to 5.54 eV, the exceedingly wide WF range for doped graphene. In an effort to elucidate the doping mechanism of SAMs on RGOs, the electron-donating and -withdrawing properties of the anchored chemicals, as well as directions of generated dipole moments and acidic properties of unreacted free silanol groups were considered together. These tunable electrical properties of SAM-functionalized RGOs have optoelectronic applicability to organic-semiconductor/graphene-electrode device platforms, which are believed as an outstanding candidate toward the next-generation soft electronics. To this end, WF-tuned RGO electrodes via SAMs were demonstrated as source and drain (S/D) electrodes of bottom-contact organic thin film transistors. The energetic matching between the RGO S/D electrodes and organic semiconductors could reduce a charge injection barrier and produce significantly improved device performance.

The use of block co-polymers as active components in biosensors and other nanoscale devices has been an area of intense focus in the last decade or so. One way of achieving this is the functionalization of the polymers with appropriate ligands that are capable of binding to high affinity receptors (FcεRI) on the surface of mast cells or Immunoglobulin E (IgE) for instance. Consequently, we have synthesized block co-polymers of 3-decylthiophene with 2-hydroxy ethyl methacrylate via atom transfer radical polymerization, ATRP. The macroinitiator, poly (3-decylthiophene) was first made by Grignard metathesis reaction followed by a series of end group modification to yield the bromoester end-capped polymer. ATRP of 2-hydroxy ethyl methacrylate was carried out in dimethylformamide with CuBr/PMDETA (catalyst/ligand) duo to yield the co-polymers which were then functionalized with 2,4 dinitrophenyl (DNP)-ε-amino-n-caproic acid. Characterization of the co-polymers was done by 13C and 1H NMR, FTIR, UV-Vis and Raman spectroscopy. Thermal analysis was done by differential scanning calorimetry. Electrical behavior was studied via cyclic voltammetry. The copolymer solution in chloroform was drop-cast on to silicon substrates to form films whose morphology was studied using electron and scanning probe microscopy. Biocompatibility studies, done through incubation in fluorescently tagged IgE indicated that there was preferential binding of the protein to the DNP groups, as observed under the fluorescence microscope. The results obtained so far, strongly indicate that these block co-polymers have potential use in biosensors, with the incorporation of specific functional groups (ligands) that antibodies recognize and adhere to.

Three benzo[d,dprime;]thieno[3,2-b;4,5-bprime;]dithiophene (BTDT) derivatives, end-functionalized with benzothiophenyl (BT-BTDT; 2), benzothieno[3,2-b]thiophenyl (BTT-BTDT; 3), and benzo[d,dprime;]thieno[3,2-b;4,5-bprime;]dithiophenyl (BBTDT; 4), were prepared for bottom-contact/bottom-gate organic thin-film transistors (OTFTs). An improved one-pot [2 + 1 + 1] synthetic method of BTDT with improved synthetic yield was achieved, which enabled the efficient realization of new BTDT-based semiconductors. All of the BTDT compounds exhibited high performance p-channel characteristics with carrier mobilities as high as 0.34 cm2/(V s) and a current on/off ratio of 1 × 107, as well as enhanced ambient stability. The device characteristics have been correlated with the film morphologies and microstructures of the corresponding compounds.

Hole transporting layer (HTL) plays an important role in OLED device performance because its injecting and transporting properties affect the efficiency and stability of OLEDs. Here we report on OLED devices fabricated using PLEXCORE® HTL, which is a multi-component arylamine derivative cross-linkable hole transporting material made by Plextronics Inc. This new HTL can be crosslinked upon heat treatment. Its HOMO energy is 5.4 eV which is a good match to the commonly used HTLs. AFM results show the solution processed PLEXCORE®HTL has the same roughness as the commonly used thermal evaporated 4,4prime;-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB). Further direct comparisons were made in terms of the materials mobility, device performance and operating lifetime time of phosphorescent OELDs with an orange emitter Iridium(III) tris(2-phenylquinoline) (Ir(2-phq)3). While the mobility of the new HTL is one order lower than NPB, the OLED devices with PLEXCORE®HTL give a higher efficiency (18 cd/A at 1000 cd/m2) and a smaller roll-off than the NPB devices. More remarkably, a longer lifetime was obtained from PLEXCORE®HTL OLEDs in an accelerated lifetime test with an initial luminance at 8000 cd/m2. PL of OLEDs stack was studied in order to explain degradation of HTL with different fabrication processes.

Thin film transistor (TFT) is a kind of field-effect transistor (FET) and it works as a switching part in liquid crystal display (LCD). Moreover, this TFT is also used in next-generation display panel consisted of AMOLED (Active Matrix Organic light-emitting diode). According to the smart phone and tablet market using AMOLED grow rapidly, various researches are in progress actively to improve the performance of TFT.
In this study, we used nano patterned graphene on channel layer in TFT. Graphene has superior electric mobility and excellent hardness, so electrical conductivity is maintained despite of stretching and folding. Moreover, graphene has high transparence so it can be used on flexible and transparent devices.
To fabricate flexible and transparent TFT using nano patterned graphene, we prepared graphene on a PMMA substrate. Then, SiO2 as a dielectric layer and ITO as a gate layer were deposited sequentially on the graphene/PMMA substrate. And PI solution was coated on the stacked structure (ITO/SiO2/graphene/PMMA) to use as a flexible and transparent substrate of TFT. After curing PI solution, PMMA was removed using acetone. Through this process, free-standing structures -graphene(channel)/ SiO2(dielectric)/ ITO(gate)/ PI(substrate)- were manufactured for TFT.
Next, sub-50nm sized line patterns were formed on graphene layer as a etch mask. These patterns were formed using methacryloxypropyl terminated polydimethylsiloxane (M-PDMS) resin. M-PDMS resin has superior etch resistance against to O2 plasma used for etching graphene. After etching the graphene layer using M-PDMS patterns, ITO patterns were fabricated as a source/drain electrode finally. These ITO patterns were fabricated using sputter with shadow mask.
Electric properties of manufactured flexible and transparent TFT were measured using a 4200-SCS semiconductor characterization system, made by Keithley. Also, optical properties were measured using a V650 Spectro-photometer, made by JASCO. Moreover, a series of bending test was performed to confirm the stability of fabricated graphene patterns.
Keywords: Graphene, Thin Film Transistor, Flexibility, Transparence, Nano structures

In recent years, there has been tremendous interest in highly conjugated polycyclic hydrocarbons, such as the higher acenes, because of their fascinating optical and electronic properties. Although pentacene and its derivatives have been utilized in device applications such as field effect transistors, photovoltaics, and light emitting diodes, these molecules are susceptible to oxidative and photolytic degradation; therefore, alternative, acene-like topologies have been explored. Recently, our group has been investigating isomers of indenofluorenes as potential n-type materials due to their ability to reversibly accept up to two electrons. Despite the body of research into this promising scaffold, little has been done investigating the incorporation of heteroatoms into these structures. The inclusion of heteroatoms allows for the tuning of both electronic and physical properties. Thieno-fusion, for example, onto an acene skeleton such as pentacene has shown high mobility, improved stability as well as ease of functionalization. Herein we report the synthesis, as well as optical and electrochemical characterization of indacenedithiophene. Additionally, we explore how functionalizing the indacenedithiophene core with electron-withdrawing substituents at the 2 and 8 positions affects both the electronic and solid state properties of these molecules.

We fabricate Au nanoparticle(NP)-embedded organic light-emitting devices (OLEDs) using a dry, room-temperature aerosol technique, where Au NPs are placed at a specific plane of a hole transfer layer. In this technique, charged Au NPs are generated by a spark discharge generator (SDG), transported to an electrostatic precipitator, and deposited onto an electrically biased substrate. The cross-sectional transmission electron microscope images of Au NP-embedded OLED show that aerosol-deposited Au NPs are well formed without penetration into an underlying organic layer. This is in contrast to commonly employed methods to incorporate metal NPs in an organic layer, such as vacuum thermal evaporation (VTE) or spin coating of a polymer solution blended with pre-synthesized NPs, where the position of NPs is not fully controlled. In addition, unlike VTE where metal atoms boiled off from a source impinge on an organic layer with a high velocity, the impinging velocity of Au NPs in the aerosol deposition process, which is primarily determined by the velocity of the carrier gas, is estimated to be 0.4 m s-1 only. As a result, possible damage to the underlying organic layer is minimized in the aerosol deposition of Au NPs.
We fabricate bilayer OLEDs consisting of Alq3 and NPB, where the vertical (perpendicular to the substrate surface) position of Au NPs are varied. Owing to the precise control of the Au NP position enabled by the aerosol technique, we are able to find an optimum position of the Au NPs within the NPB layer, in which case the external quantum efficiency is 38 % larger than the control device. Our electrical and optical characterizations show that the enhanced device performance is due to the increased electron-hole recombination probability arising from the electrostatic effects of holes trapped in the Au NPs.

The exciton is a critical part of each of the processes leading to photocurrents in Organic PhotoVoltaics (OPVs), and being able to control the location, lifetime and energy of the exciton is essential to achieving high efficiency. We have investigated methods for tuning exciton energies and controlling their migration paths, both intramolecularly and within a thin film. I will discuss our most recent work with both organic dyes, such as squaraines and dipyrrins for OPVs. This involves a careful materials design study that leads to both low energy absorption (into the nearIR) and the efficient use of multiple absorbers to efficiently harvest photons through the entire visible spectrum. In particular, we have used careful control of exciton and carrier energies to design and implement sensitizers that give fullerene films efficient light collection throughout most of the visible spectrum. I will also discuss a new approach to designing materials for OPVs that involves symmetry breaking charge transfer. These materials are symmetric molecules that spontaneously form an intramolecular charge transfer complex, with nearly complete one electron transfer form one part of the molecule to another. This intramolecular CT state readily forms a charge separated state at the D/A interface of the OPV. We have explored these materials as both donors and acceptors in OPVs and found that they give good performance and relatively high Voc.

Small molecule organic semiconductors have attracted increasing interest for preparing organic solar cells (OSCs) due to the advantages of its well-defined structures, facile synthesis and purification, and generally high charge carrier mobility. Enormous progress has been made in the past few years to improve the PCE of the solution-processed devices from lower than 2% (before 2008) to higher than 7%, with a recent best PCE of 9.02%, making solution processed small molecule OSCs a very fast growing field. Push-pull chromophores involving electron-rich and electron-deficient groups have been widely investigated for small molecule organic semiconductors. Here, we report on the high performance single junction and double junction OSCs based on a conjugated small molecule (SMPV1). Using the solution spin-coating fabrication process, the certified power conversion efficiency (PCE) of 8.02% from single junction device was obtained by using the following: a blend of SMPV1 and PC71BM as the active layer, and polydimethylsiloxane (PDMS) as an additive, combined with a high open-circuit voltage of 0.94 V, a short circuit current density of 12.17 mA cm-2, and a notable fill factor of 70.4%. A homo-tandem solar cell based on SMPV1 was constructed with a novel solution processed thermal annealing-free interlayer (or tunnel junction), demonstrating an unprecedented PCE of 10.1%. These results strongly suggest solution-processed small molecular materials are excellent candidates for organic solar cells.

Current state-of-the-art research on organic photovoltaics (OPVs) have achieved solar power conversion efficiencies (PCEs) in the range of 9-11% based on conjugated polymers or small molecules via solution processing methods. These achievements owe to the development of donor materials blended with electron deficient molecules, primarily fullerenes, to form the active layer for light absorption and photocurrent generation in the device. Fullerene derivatives show good electron affinities that are crucial for exciton dissociation and have good electron mobilities for transport of the electrons to the corresponding electrode. However, the fullerene derivatives have limited absorption in the visible region and fixed LUMO levels (limit on Voc), as well as being very expensive which accounts for a significant fraction of the total OPV cost.
In this contribution, we detail our latest results based on a series of small molecule electron acceptor materials to serve as low cost alternatives for fullerene derivatives. For the design of these molecules, we pursued a B-D-A-D-B motif to achieve larger absorption in the visible spectrum with tunable LUMO energy levels. The central A electron deficient aromatic unit is benzothiadiazole that is in conjugation with donor units (designated as D) such as thiophene or benzene. The flanking B units are based on alkylated cyanoester and rhodanine derivatives to yield solubility and fine tuning the LUMO energy levels. As such, we achieved absorption onsets varied from 580 nm to 710 nm in the visible region while LUMO energy levels were between -3.85 eV to -4.10 eV.
While theoretical calculations confirmed planarity for these small molecules, UV-VIS spectroscopy showed extensive Π-stacking in the thin films. Our current studies are targeted on the preparation of bulk heterojunction (BHJ) and bilayer solar cells prepared from solution processed rr-P3HT and other polymers as donor materials and the aforementioned small molecules as electron acceptors.

Bulk-heterojuction (BHJ) solar cells with bicontinuous interpenetrating network of organic donor (D) and acceptor (A) offer promising merits such as low cost, lightweight and flexibility and have been considered as potential contender for novel energy market. The power conversion efficiencies (PCEs) of BHJ solar cells based on conjugated polymer donors have been exceeding to 10 %. Recently, molecular donors with well-defined dimensions are being studied intensively, leading to small molecule BHJ solar cells which exhibit comparable PCEs ~7 %. Although there are much attractive properties along with molecular donors, it is generally pretty difficult to obtain film with good wettability from molecular donors when compared to analogous polymer systems. Moreover, the molecular films are so thin that it is not possible to achieve sufficiently high optical densities upon solar spectrum. To circumvent these problems, a novel method of active layer control by incorporating a small addition of an inert high molecular weight polymer to molecular D/A system was investigated. It is found that one can obtain increase in film thickness without sacrificing desirable morphology of phase separation and structural order. Specifically, high molecular weight polystyrene (PS, Mn=20,000,000) is incorporated to a p-DTS(FBTTh2)2:PC70BM blend on an order of 1-5 % by weight relative to the BHJ components. It shows that the device performance is extremely beneficial with a PCE increasing from 7.0 % to 8.2 %. Furthermore, different techniques such as grazing incidence wide-angle X-ray scattering (GIWAXS), dynamic secondary ion mass spectroscopy (DSIMS), cross-sectional transmission electron microscopy (TEM) and bright-field TEM were employed to illustrate the film composition and morphology. It reveals that the critical accumulation of insulating PS within the interior of film and away from the charge-collecting electrodes.

Here we present our recent work on the development of new electron-deficient structures that can be utilised not only as a component in push-pull type donor materials for organic photovoltaics, but also as the central component of non-fullerene electron acceptors for organic photovoltaics. We will discuss how the development of larger fused structural motifs comprising several well-studied units such as thiadiazole and pyrrolodione affects the electron-accepting properties and how these novel materials can be incorporated into promising donor and acceptor materials for organic solar cells.

The process of singlet exciton fission provides a potential route to overcome the Shockley-Queisser limit on the efficiency of solar cells, by allowing two charge pairs to be collected following absorption of a high-energy photon. I will show how this process can be exploited in devices comprising pentacene as the singlet-fission material and PbSe nanoparticles as a low-energy absorber. I will then focus on recent results investigating the recombination of triplet excitons generated by singlet fission in the solution-processable molecule TIPS-tetracene. Using the technique of optically detected magnetic resonance (ODMR), we find negative and positive features associated respectively with geminate and bimolecular recombination of triplet excitons, showing that both these processes may compete with charge generation. From the shapes of the triplet powder pattern resonances we demonstrate that bimolecular recombination occurs preferentially in ordered regions of the films where the triplet excitons are more mobile.

Over recent years, solution-processed molecular dopants have received much attention due to their potential for the realization of stable and controllable doped transport layers for both p- and n-type materials. Such doped layers are indispensable for a variety of applications including organic light-emitting diodes (OLED), organic photovoltaics (OPV) and transparent conducting electrodes. Yet despite the increasing availability of p- and n-type materials from organic synthesis, the solution doping process and overall distribution of dopants within thin films are poorly understood.
To this end, we investigate the relationship between solution-state doping and the corresponding phase separation between dopant and host molecules in thin film. Here we focus specifically on the p-type doping of a prototypical, high performance semiconducting polymer poly(3-hexylthiophene) (P3HT) using F4TCNQ (7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane). Using conducting-AFM (c-AFM), we are able to directly map dopant sites in blended films and observe the dopant-polymer phase separation as a function of doping concentration and solution temperature. Our data, firstly, confirm the existence of the “weak” and “strong” doping regimes and the resultant phase separation previously reported. In the “weak doping” regime, F4TCNQ remains as a neutral species in solution and becomes homogeneously dispersed among amorphous domains in thin films. In the “strong doping” regime, F4TCNQ molecules bind to the polymer backbone in solution, which leads to the formation of new crystalline domains in the solid state. Furthermore, we find that it is possible to tune the doping strength by controlling the solution of the spin casting solutions. A 2% doped film spin casted at 30oC, for instance, exhibits a doping efficiency of nearly one order of magnitude higher than a film spin casted at 80oC. The observed change in doping efficiency also resulted in a drastic change in phase separation in thin film. To our knowledge, this is the first report to successfully image doped sites in F4TCNQ:P3HT thin films and substantiates c-AFM as a powerful tool for characterizing doped polymer systems.

Organic materials have enabled large scale semiconductor production on flexible substrates, but are often more mechanically fragile than their inorganic counterparts with a higher tendency for adhesive and cohesive failure. We use a thin-film adhesion technique to quantify the impact of various processing, film structure and environmental variables on the cohesion and adhesive properties of organic semiconductor materials and their interfaces.
Specifically, we will compare solution processing (spin, spray and slot die coating) with thermal evaporation, and demonstrate that overall solution processing leads to improved adhesion. We also show the importance of various film structure parameters, such as the polymer layer thickness, composition and molecular weight. We discuss how to tune key interfacial and film parameters, such as interface chemistry, bonding and morphology, by thermal annealing to improve the adhesion. For example, the P3HT:PCBM/PEDOT:PSS interface in an inverted polymer solar cell has an adhesive value of only ~1.5 to 2 J/m2, and can be significantly increased by post electrode deposition thermal annealing time and temperature. Using near edge X-ray absorption fine structure (NEXAFS), we precisely quantified the interfacial composition at the delaminated surfaces and correlated the increase in adhesion to changes in the interfacial structure. The over 50% increase in adhesion is caused by the development and expansion of an intermixed layer of P3HT:PCBM and PEDOT:PSS at already 45C annealing. However, thermal annealing before electrode deposition and above the crystallization temperature of PCBM (120C) should be avoided to ensure device reliability. At these conditions, micrometer sized PCBM aggregates form that not only weaken the P3HT:PCBM but also decrease the device efficiency. The structural and chemical reorganizations are correlated with glass temperature and crystallization temperature of the materials used in the structure and thus the conclusion can be generalized to other materials systems. Understanding the interlayer adhesion and developing strategies to improve the adhesion of organic semiconductors is essential to improve the overall mechanical integrity and yield general guidelines for the design and processing of reliable organic electronic devices.

Luminescence imaging has evolved to a versatile characterization method for studying the laterally resolved behavior of polymer solar cells. Especially in degradation studies the use of luminescence imaging is beneficial as it is non-invasive and offers short measurement times [1]. Depending on the excitation mechanism, i.e. either electrical or optical excitation, separate characterization of the electrical contacts and the active layer is feasible [2]. However, except for the correction of the active area [2] in degradation studies the data analysis so far is mainly qualitative, i.e. interpretation of the measured luminescence image by comparison with other techniques [3]. In a first step we introduced a quantitative analysis for calculating the current-voltage characteristics of the active layer as well as for determination of the involved resistances in an equivalent circuit network model [4]. However, this analysis was restricted to laterally homogeneous solar cells. In this work we present a quantitative analysis of electroluminescence images of laterally inhomogeneous polymer solar cells. By decoupling the local equivalent circuit parameters within an iteration procedure this analysis allows calculation of the local current flow through and the local voltage applied to the active layer. Furthermore quantitative images of the local series resistance and the saturation current-density are achieved. Applied to polymer solar cells based on an anthracene containing PPE-PPV alternating copolymer (poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)) [5, 6] blended with PCBM (phenyl-C61-butyric-acid-methyl-ester) the local saturation current-density contrast was found to correlate perfectly with the strong lateral phase separation occurring in these devices. Further analysis of the lateral difference in the saturation current-densities delivers information on the thermal activation of charge carriers at the donor/acceptor-interface and in the phase separated bulk.
Keywords: Polymer, Solar Cell, Luminescence, Imaging, Aggregation
References:
[1] M. Seeland, R. Rösch and H. Hoppe, Imaging Techniques for Studying OPV Stability and Degradation, in Stability and Degradation of Organic and Polymer Solar Cells, edited by F. C. Krebs (John Wiley & Sons, 2012).
[2] M. Seeland, R. Roesch and H. Hoppe, J. Appl. Phys. 109 (6) (2011).
[3] R. Rösch, D. M. Tanenbaum, M. Jorgensen, et al., Energy Environ. Sci. 5 (4), 6521-6540 (2012).
[4] M. Seeland, R. Rösch and H. Hoppe, J. Appl. Phys. 111 (2), 024505 (2012).
[5] D. A. M. Egbe, S. Turk, S. Rathgeber, et al., Macromolecules 43 (3), 1261-1269 (2010).
[6] C. Kastner, S. Rathgeber, D. A. M. Egbe, et al., Journal of Materials Chemistry A 1 (12), 3961-3969 (2013).

Advances in controlled synthesis, processing, and tuning of the properties of organic conjugated polymers have enabled significantly enhanced performance of organic electronic devices. Our laboratory employs a molecular engineering approach to develop processible and high charge carrier mobility polymers for enhancing power conversion efficiency of solar cells. We have also developed several innovative strategies to modify the interface of bulk-heterojunction devices and create new device architectures to fully explore their potential for solar window applications.
The performance of polymer solar cells is strongly dependent on their efficiency in harvesting light, exciton dissociation, charge transport, and charge collection at the metal/organic/metal oxide interfaces. In this talk, the integrated approach of combining material design, interface, and device engineering to significantly improve the performance of polymer photovoltaic cells will be discussed. Specific emphasis will be placed on the low band-gap polymers development with low reorganizational energy and proper energy levels, optimized morphology of bulk-heterojunction layer, and minimized interfacial energy barriers with functional surfactants and graphene oxide. At the end, several new device architectures and processing strategies will be discussed to explore the full promise of polymer solar cells.

PBnDT-FTAZ, a “weak donor-strong acceptor” polymer that combines benzodithiophene (BnDT) and fluorinated benzotriazole (FTAZ), has received significant amount of attention because it is a peculiar material with many outstanding features that need to be further explained. For example, with a medium band gap of 2.0 V, PBnDT-FTAZ can still generate over 7% power conversion efficiency with a noticeably high fill factor over 70% in its BHJ cells with a relatively thick film (200 - 300 nm).
In this talk, I will discuss our ongoing efforts to further understand fundamental issues concerning polymer solar cells through tuning and optimizing PBnDT-FTAZ. First, we discovered that all these interesting properties shown by PBnDT-FTAZ can only be obtained with polymers of a quite narrow range of molecular weight. Second, molecular engineering of PBnDT-FTAZ by substituting alkylthiol and selenophene in PBnDT-FTAZ give mixed results: Voc seems fixed around 0.8 V; however, Jsc can be further increased.

Narrow band-gap conjugated small molecules and polymers have both been drawing significant attentions in application as organic semiconductors, especially in organic solar cells. An inevitable fact is that, intrinsic advantages and disadvantages of these two classes of materials relevant to solution-processable photovoltaics are often contrary. In particular, small molecules possess monodispersity in chemical structures, higher crystallinity, and ready purification through chromatography, while on the other hand, poorer film formation properties and lower thermal stability in the solid state, as compared with their polymeric counterparts. More importantly, solar cell devices based on small molecular materials are extremely sensitive towards chemical impurities, of which polymers are relatively more tolerant. However, the polydisperse nature of polymers significantly complicates the interpretation of solid state structure, resulting in poor practices towards satisfactory device application.
Considering the fact that polymers and small molecules provide complementary material properties, we have designed molecular frameworks with successive extension of donor-acceptor characteristics. The dimension of such molecular structures lie in right between small molecules and polymers, which offers a useful bridge for studying their structure-property correlation while transitioning from shorter to longer molecular lengths. Molecular materials synthesized by following these frameworks are able to adopt some of the most important merits from both polymers (e.g., film quality and thermal robustness) and small molecules (e.g., structural monodispersity and crystallinity). A synergistic beneficial effect on device performance has also emerged upon elongation of molecular structures while in conjugation with fluorine substitution. Fabrication of solar cell devices with these materials does not require delicate control over solvent additives or post-deposition treatment, yet reaching power conversion efficiencies close to the highest reported value obtained for discrete molecules to date (6~7%). More importantly, while integrated in field effect transistors, the extension of molecular length leads to device thermal stability beyond 200 °C, which can be further improved to exceeding 300 °C by simple structural modification.

Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more solar cells with different absorption bands. However, for polymer solar cells, the performance of tandem devices lags behind single-layer solar cells mainly due to the lack of a suitable low-bandgap (LBG) polymer. Here, we present the design, synthesis, and characterization of a series of new LBG polymers specifically for tandem polymer solar cells.
To achieve a small optical bandgap, strong donor-acceptor strategies were used to construct the plolymer backbone. To fine-tune the molecular weight, solubility, solid-state packing and thin film morphology, different alkyl side chains were attached to the main chain. By developing new building blocks and optimize the polymerization and purification conditions, LBG polymers with excellent photovoltaic performance were obtained. In conventional single junction devices, they show power conversion efficiencies reaching 8%. To have a better understanding of the efficiency enhancement, several characterization techniques such as X-ray diffraction, transmission electron microscopy and atomic force microscopy were used to investigate the thin film morphology; photo-CELIV and space charge limit current techniques were used to study the charge transport properties of the new polymers and the blend films. It was found that high molecular weight polymers provided better thin film morphology and charge transport properties. When the polymers were applied to tandem devices (P3HT is used as another active material), power conversion efficiencies of over 10% are achieved, which are the highest efficiencies reported so far.

Polymer-based photovoltaic devices provide an opportunity to generate energy from light at a low cost. Great efforts have been made in designing polymers with different bandgaps and several state-of-the-art polymers of bandgap around 1.4-1.8 eV have been developed successfully. However, compared to the theoretical limit of each given bandgap, there is still a plenty of room for improvement. Instead of spending more effort on bandgap engineering, herein we turn to focus on improving an existing polymer through side-chain optimization, which has not drawn much attention so far. Through a triple component random copolymerization approach, the amount of triethylene glycol (TEG) side-chain, which can be regarded as a stack-inducing agent, can be introduced precisely into a given polymer backbone. TEG side-chains result in a more favorable morphology in a polymer:PC71BM blend. Based on low-bandgap PBDTT-DPP (PBD) photovoltaic polymer system, this methodology can bring an overall 10% improvement in power conversion efficiency (PCE). A cell constructed by newly synthesized PBDTEG10 shows both higher JSC and FF% than its PBD analogue with VOC nearly maintained, resulting in 7.0% PCE. We demonstrate that the current methodology provides an easy and effective way for fine-tuning the polymers&’ aggregation properties as well as their solar cell performance when blended with PC71BM. We further anticipate this approach can be applied to other polymer systems in the future.

Bulk heterojunction (BHJ) organic photovoltaic (OPV) is regarded as one of the most promising alternatives to inorganic-based solar cells. Here, we report high performance inverted OPV with exceptionally thick BHJ film by using alcohol-soluble thermal cross-linkable conjugated polyfluorene as an electron extraction layer (EEL), high mobility donor polymer, PBDT-DTNT, mixed with PC71BM. An efficiency of 8.62 % is observed from the inverted OPV with BHJ thickness of 280 nm. Remarkably, an efficiency of 7.20 % is observed from the inverted OPV with BHJ thickness greater than 1000 nm. Such high efficiency from thick BHJ composite film is attributed to the high hole mobility of PBDT-DTNT from PBDT-DTNT:PC71BM BHJ composite, which is characterized by the space-charge-limited-current method and GISAXS and GIWAXS measurement. The high efficiency from the inverted OPV with such thick BHJ composite film certainly opens a door for manufacturing OPV products by low-cost high throughput roll-to-roll processing.

Among Organic Electronics, solution-processable π-conjugated polymers are proving particularly promising in bulk-heterojunction (BHJ) solar cells with fullerene acceptors such as PCBM. In the past few years, we have found that varying the size and branching of solubilizing side-chains in π-conjugated polymers impacts their self-assembling properties in thin-films. Beyond film-forming properties, nanoscale ordering in the active layer governs material and device performance. For example, in poly(benzo[1,2-b:4,5-b&’]dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD), TPD substituents of various size and branching impart distinct molecular packing distances (i.e., π-π stacking and lamellar spacing), varying degrees of nanostructural order in thin films, and preferential backbone orientation relative to the device substrate. These structural variations have been found to correlate with solar cell performance, with power conversion efficiencies ranging from 4% to 8.5%. In parallel, discrete backbone modifications in PBDTTPD can impact polymer backbone conformations and self-assembly, and various ring-substituents directly appended to the π-conjugated backbone can induce morphological effects and may also affect polymer performance in BHJ solar cells with PCBM. Our recent developments emphasize how systematic structure-property relationship studies impact the design of efficient polymer donors for BHJ solar cell applications.

The relationships between molecular structure, solid state order, charge transport, and photovoltaic performance of solution-processed perylenediimides (PDIs) are developed to inform their incorporation into organic photovoltaic devices (OPVs). Four PDIs with varying peri and bay substituents are tested in solution-processed bilayer OPVs and field-effect transistors and characterized by grazing-incidence wide-angle X-ray scattering, atomic force microscopy, and UV-vis spectroscopy of thin films, and by density functional theory. For PDIs with bulky bay substituents, photovoltaic performance is limited by poor charge carrier mobility, likely stemming from inhibited molecular packing and, in turn, a lack of solid state order. PDIs with fewer or less disruptive bay substituents exhibit solid state order leading to higher charge carrier mobilities and AM1.5 G power conversion efficiencies (PCEs) up to 2.0 % in solution-processed bilayer OPV devices with benzoporphyrin as the donor material.[1] This work demonstrates the potential of PDIs as electron acceptors in OPVs and highlights the importance of molecular geometry in the design of new materials in order to realize non-fullerene acceptors with mobilities approaching those of high performance fullerene derivatives.
PDIs have been used in OPVs for decades, yet most reports of PDIs report very low efficiencies within 1 % PCE. Our work suggests that the low performance of PDIs in bulk heterojunction (BHJ) OPVs has, to a large extent, been due to poor nanoscale morphology of the particular donor-acceptor thin films explored. This work then underlines the need to pair promising PDIs with a number of donor materials to realize high performance non-fullerene BHJ OPVs. As an example of this strategy, the facile screening of PDIs in bilayer OPVs in the work presented here enabled the incorporation of PDIs in solution-processed BHJ OPVs with the molecular donor p-DTS(FBTTh2)2, which achieved a PCE of 3.0 %, one of the highest performing non-fullerene OPVs to date.[2]
[1] M. Guide, S. Pla, A. Sharenko, P. Zalar, F. Fernández-Lázaro, Á. Sastre-Santos, and T.-Q. Nguyen, Phys. Chem. Chem. Phys., 2013, 15, 18894-18899.
[2] A. Sharenko, C. M. Proctor, T. S. van der Poll, Z. B. Henson, T.-Q. Nguyen, and G. C. Bazan, Adv. Mater., 2013, DOI: 10.1002/adma.201301167.

Conjugated polyelectrolytes (CPEs) are defined by a backbone with a delocalized electronic structure and pendant groups bearing ionic functionalities. These materials combine the optoelectronic properties of organic semiconductors with the ability of polyelectrolytes to have their function determined by electrostatic forces. Increased solubility in highly polar organic solvents, or water, allows CPE deposition atop typical non polar layers, such as those commonly found in bulk heterojunction (BHJ) solar cells or the electroluminescent layer in polymer light emitting diodes (PLEDs).
The ionic component in CPEs has been shown to play an important in the overall function at device interfaces. Two limiting mechanisms have been observed. The first concerns the formation of self-assembled dipole layers that can influence the effective work function of adjacent electrodes. Here, special considerations need to be taken to overcome the coulombic repulsion between dipoles. In a second situation, the ion in the CPE layers can migrate under the application of an external bias and thereby redistribute the electric field within the device. Steep gradients, akin to those in the electrical double layer, are formed that decrease barriers to charge injection.
More recently the ions on the side chains have been used to modify the electronic properties of the backbone. For example, n-type transport in FET configuration can be obtained for cationic CPEs with a narrow bandgap optical absorption. In the case of their anionic counterparts, one finds that p-doping can be readily achieved, even by simple treatment with mild acids. The resulting materials can function readily as replacements for widely used PEDOT:PSS in the fabrication of organic photovoltaic devices.

We present the effects of organic-organic and organic-inorganic interfaces on increasing the efficiency and stability of inverted polymer solar cells where CsI or CsCl interlayers, fabricated by spin-coating their aqueous solutions, serve as buffer interface layers at the ITO cathode. The significant effect of a solution-processed hole and exciton blocking BPhen layer interfacing the halide on efficiency and stability is discussed. The power conversion efficiency (PCE) of unencapsulated cells with the halide/BPhen layers decreased by ~2% after storage in air for 2 weeks; it decreased by <20% after 2 months, while the PCE of such cells devoid of the BPhen layer decreased by ~60% after 2 weeks and the cells were non-performing after 2 months. The effect of BPhen in increasing the long-term stability following exposure to high relative humidity is also described, as well as the enhanced absorption of the active layer using a thin layer of polystyrene nanobeads (100 nm in diameter) on the blank side of the cells&’ substrate.

Organic photovoltaics have been emerging as interesting candidates for sustainable energy. Especially appealing to this technology are desirable properties such as simple preparation, novel aesthetical possibilities, semi-transparency, reduced weight and mechanical flexibility, and better performance in diffuse light conditions. The main focus in the field of polymer-based solar cells has always been on improving photovoltaic performance. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivatives have been developed and blended in bulk heterojunction active layers. Nonetheless, further optimization of the device architecture is also of major importance. In previous work, we have shown that the introduction of an imidazolium-substituted conjugated polyelectrolyte between the active layer and the top electrode resulted in a significant enhancement of the I-V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output.[1] In the presented work, we report on a wide range of ionic (block co)polymer materials as possible candidates for (n-type) interfacial layers, exceeding the previously reported power conversion efficiencies. Through the use of various characterization techniques, deeper fundamental insights are conceived on the working principles of these interlayers.
[1] Kesters J.; Ghoos T.; Penxten H.; Drijkoningen J.; Vangerven T.; Lyons D.M.; Verreet B.; Aernouts T.; Lutsen L.; Vanderzande D.; Manca J.; Maes W., Adv. Energy Mater., 2013, 9, 1180.

In organic electronic devices, charge injection at the contacts is crucial for high electrical performance. Most devices require at least one electrode with a sufficiently low work function (WF) to be able to inject into or collect charge carriers from an electron transporting layer. Low-WF electrodes which meet this requirement are available; however, they are chemically very reactive and oxidize in ambient atmosphere. A smart way to overcome this problem is the use of self-assembled monolayers (SAMs) to tune the work function of the electrode and the wettability. As solution processibility is one major advantage of organic semiconductors, an important aspect of modified electrodes is their wetting behavior.
We have studied the properties of SAM-modified substrates via photoelectron spectroscopy (XPS/UPS), infrared spectroscopy (IR), ambient Kelvin probe (KP) and contact angle (CA) measurements. We have used different SAMs prepared from novel, tailor made precursor molecules. These molecules combine anchor groups with strongly dipolar head groups. Thus, the SAMs can change the work function of various metal substrates and thereby e.g. enhance electron injection in organic devices.
SAMs were prepared on thin Au films from oxygen-free ethanol solutions. The immersion time was varied and for each sample XPS/UPS, IR, KP and CA measurements were performed. In this way we were able to optimize the SAM processing and thus the surface characteristics.
We prepared organic field-effect transistors (OFETs) with SAM treated electrodes using AcitveInk N2200® as an air-stable n-type organic semiconductor and compare the obtained device performance to devices without SAM treatment. In a top-gate, bottom-electrode OFETs Au is used for drain and source contacts. The relative improvement of the device characteristics determined from transfer measurements allow ranking of the new SAM compounds.

Electropolymerized polythiophenes offer additional advantages to solution processable thiophenes because insoluble thiophenes can be deposited electrochemically which are otherwise not processable wet-chemically. Here in this work, a new method for electropolymerization of polythiophenes as anode buffer layer (ABL) in combination with a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer is presented. The ABL is used in a bulk-heterojunction solar cell with poly(3-hexylthiophene-2,5-diyl)- [6,6]-phenyl-C61-butyric acid methyl ester (P3HT-PCBM) as active material. Our approach is superior to a similar approach known from literature [1]. It is shown that a stack of PEDOT:PSS and electropolymerized polythiophene (ePT) as anode buffer layer gives a significant enhancement in short circuit current and hence better power conversion efficiency.
Thiophenes are grown electrochemically with a standard three electrode system on transparent indium tin oxide (ITO) contacts. These ePT layers are are characterized by UV-vis measurement and in Schottky diode configuration between ITO and copper electrodes. For growth, the new time dependent cyclic potential polymerization method and the constant potential electropolymerization method are applied and compared. Atomic force microscope (AFM) topography reveals that a uniform homogeneous film of ePT is obtained by the time dependent cyclic potential growth method. A 10-15nm layer of ePT was grown on ITO substrates with PEDOT:PSS for each method. As an active layer a bulk-heterojunction of P3HT-PCBM was used. Thermally deposited aluminum was used as cathode. These devices were compared with a standard cell with a bare PEDOT:PSS layer. The constant potential ePT device showed an enhancement of more than 10% in short circuit current against the standard cell. In the case of cyclic potential ePT device, even more enhancement, about 20%, in short circuit current was observed. Improvements by the ePT layer are attributed to a good band alignment for the HOMO level and a LUMO offset of 0.2eV which gives electron blocking characteristics. In summary, the cyclic potential method results in a better quality of the ePT layer with good homogeneity and area coverage leading to a further improvement in device performance.
[1] Wei, H. Y., Huang, J. H., Ho, K. C., & Chu, C. W. (2010). ACS Applied Materials & Interfaces, 2(5), 1281-1285

The observation of external quantum yields exceeding 100% in the visible spectrum represents a significant advance in the application of singlet fission to solar cells. One key opportunity is to follow a decades-old proposal of Dexter and partner singlet exciton fission with a silicon solar cell1. In theory, singlet fission materials such as tetracene or rubrene could be integrated with high quality silicon cells to double the photocurrent from high-energy solar photons ( < 550 nm), ultimately boosting the efficiency of the silicon cell to 30% or more. The remaining concern is how to get the energy from the triplet excitons (energy ~ 1.1eV) from tetracene into silicon (bandgap 1.1eV). The challenge is that the triplet is a dark state, and its radiative transitions are spin-forbidden. The triplet could tunnel into silicon through a very thin interfacial layer. Or, longer range energy transfer might be possible if the triplet is coupled to an emissive material.
Coupling triplet excitons to quantum dots looks especially promising. The µs-scale triplet lifetime and µm-scale diffusion length of triplet excitons in tetracene ensures a large, mobile triplet population available for transfer to the quantum dot. Indeed, here, we report recent experiments on energy transfer from low energy triplet excitons in tetracene to emissive materials including quantum dots. Successful triplet transfer generates additional infrared emission from the quantum dots that is detectable via the excitation-wavelength dependence of the photoluminescence. Additionally, the fission rate can be modulated by an applied magnetic field, exhibiting the characteristic dependence of triplet generation after singlet exciton fission in tetracene.
Reference: 1. Dexter, D. L. J. Lumines. 18-9, 779-784, doi:10.1016/0022-2313(79)90235-7 (1979).

The ultrafast carrier dynamics in antradithiophene derivatives are investigated using femtosecond transient absorption spectroscopy and photoluminescence spectroscopy. A novel antradithiophene derivative, 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT), was used in this study. The diF-TES ADT films are prepared by zone-casting method to form well-aligned nano-ribbons on the spectrosil-B substrate. At room temperature, we observe the decay of singlet excitons is associated with the growth of triplet excitons within 200 picoseconds time-scale after the photoexcitation. We attribute such ultrafast decay of singlet exciton to the formation of triplet excitons via the singlet exciton fission to generate two triplet excitons from a single absorbed photon, which reduces the photoluminescence quantum yield. Furthermore, by blending the diF-TES ADT with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to form bulk-heterojunctions, the triplet exciton lifetime is reduced and we observed the formation of charge-transfer state. Such triplet exciton generation yield is reduced substantially at low temperature. As the substrate temperature is reduced to 10K, we observe the superradiance of the singlet exciton emission, which is substantiated by a nearly linear increase of the PL quantum yield. Our results suggest that antradithiophene are potential materials to be implemented in the organic photovoltaics to increase the photocurrent generation via the singlet exciton fission.

The coupling of surface plasmons and excitons in organic materials can improve the performance of organic optoelectronic devices. The surface plasmon resonance effect of carbon dot-supported silver nanoparticles allows significant additional light absorption, leading to remarkably enhanced power conversion efficiency (PCE) of 8.31% and an internal quantum efficiency of 99% in polymer solar cells (PSCs) compared with control devices. Furthermore, we have demonstrated high-performance PSCs using the plasmonic effect of multi-positional silica-coated silver nanoparticles. The location of the nanoparticles is critical for increasing light absorption and scattering via enhanced electric field distribution. The device incorporating silica-coated silver nanoparticles between the hole transport layer and the active layer achieves a PCE of 8.92% with an external quantum efficiency of 81.5%.

In organic solar cells (OSCs) where the thickness of the active material is, typically, 100 nm, several approaches have been implemented to increase light absorption by increasing the effective light path. Using the light scattered by plasmonic nanoparticles an enhancement in the light absorption was recently demonstrated and single junction OSCs with a power conversion efficiency (PCE) above 8% were obtained [1]. More recently, a disordered multilayer architecture with properties similar to those from one-dimensional photonic crystals has been used to enhance near infrared and near UV light absorption in semi-transparent OSCs which exhibited a PCE above 5% [2]. In other thin film cells, whispering gallery (WG) resonant modes from a low quality-factor spherical nanoshell structure were used to facilitate the coupling of light and substantially enhance the light path in the active material [3]
In the current work, we propose the use of WG coupling but in a drastically different configuration based on implementing an organic cell on the back side of an array of parallel fibers. Such fibers serve as the substrate for an OSC device. The different layers of the cell architecture are on the back side of the array relative to the incoming sunlight incident on the front side of the array. Light is coupled into the low quality factor WG resonant modes of the fibers. Such coupling becomes more effective as the angle for the incident light relative to the normal of the fiber array increases up to 55 deg. In any case, light absorption at the active layer is at all angles larger for the fiber array organic cell (FAOC) when compared to the standard planar configuration. We show that the EQE for the FAOC is largely improved at almost the entire absorption wavelength range for the active material while, at the same time, the short circuit current can be increased up to 30% relative to the planar cell.
In conclusion, we demonstrated that an array of fibers can be used as substrate to significantly improve light harvesting capacity for OSC when compared to the standard planar device. The incident light is coupled to some low-Q WG modes of the fibers. Such coupling is effective for a broad wavelength range as required when converting light into electricity from wideband sources. In the event that the longitudinal axis of the FAOC would be oriented in the meridian direction, the angular increase in PCE would provide, over the course of one year, an energy harvesting capacity 17% times larger than the one from a mechanically fixed cell. Such an increase in energy harvesting capacity is very close to the one obtained from one-axis sun tracking systems.
[1] L. Lu, Z. Luo, T. Xu, L. Yu, Nano Lett. 2013, 13, 59.
[2] R. Betancur, P. Romero-Gomez, A. Martinez-Otero, X. Elias, M. Maymoacute;, J. Martorell, Nature Photonics, 2013 [online DOI: 10.1038/nphoton.2013.276]
[3] Y. Yao, J. Yao, V. K. Narasimhan, Z. Ruan, C. Xie, S. Fan, Y. Cui, Nature Communications, 2012, 3, 664

While impressive advances have been achieved with organic bulk heterojunction solar cells, a deterministic control of their nanoscale morphology has been elusive so far. It would be a major breakthrough to be able to create interpenetrating networks of electron donor and acceptor phases with appropriate dimensions for maximum collection yields.
We report a significant step towards this goal on the basis of crystalline covalent organic frameworks (COFs). Rational design of their molecular building blocks enables the formation of highly porous frameworks with defined pore size and tailored optoelectronic properties. We have investigated semiconducting thiophene-based COFs and succeeded in incorporating electron acceptor molecules into their porous channels. Spectroscopic results demonstrate light-induced charge transfer from the COF donor network to the pore-located acceptor phase. We moreover succeeded in the preparation of high-quality oriented COF thin films and were able to create the first working photovoltaic devices through infiltration of the COF network with fullerene derivatives.[1]
Here we will present new results on COF-based photovoltaic devices, spectroscopic investigations of the light induced charge transfer, and strategies to extend the light harvesting capabilities.
[1] M. Dogru, M. Handloser, F. Auras, T. Kunz, D. Medina, A. Hartschuh, P. Knochel, T. Bein, Angew. Chem. Int. Ed. 2013, 52, 2920-2924.

Microstructure in organic semiconductor thin films critically determines the charge carrier mobility (mu;FET) in organic field-effect transistors (OFETs). In the case of bottom-contact OFETs, the surface characteristics of both gate dielectric and source/drain (S/D) electrodes can govern the mesoscale and/or nanoscale ordering of the semiconductor assembled on them. Here, we present various approaches for controlling the growth of organic semiconductors on the gate dielectric and S/D electrodes to achieve high performance OFETs. Firstly, we demonstrate that polymer residues remaining on the surface of graphene electrodes induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene FETs using S/D monolayer graphene electrodes with polymer residues showed a high mu;FET of 1.2 cm2/Vs. Secondly, we found that the grain size of a ordered self-assembled monolayer (SAM), which were constructed on the gate-dielectric surface, considerably affect the microstructure of a pentacene thin film. The grain boundaries of an underlying SAM were found to act as structural defects in the context of the packing and rearrangement of pentacene molecules during pentacene growth. As a result, the deposited pentacene films on the SAM with lower density of grain boundaries showed better OFET performance. Thirdly, we report a macroporous high-quality pentacene thin film assisted by a sequential deposition of pentacene and small-molecular insulating layers. We found that the underlying soft insulating small molecules with low glass transition temperature and strong intermolecular interaction with pentacene activated the 2-dimensional growth of pentacene, thus resulted in the structural homogeneity with far less grain boundaries of the pentacene layers. Furthermore, this single crystal-like pentacene thin films yielded considerably high pentacene mu;FET, up to 6.3 cm2/Vs.

Organic field-effect transistors (OFETs) have been extensively investigated motivated by their potential application to flexible and printed electronics. Organic semiconductor materials used in OFETs are generally categorized in two classes; small molecules and polymers. While high-mobility exceeding 10 cm²/Vs has been reported in OFETs based on small-molecular semiconductor single crystals, where charge transport efficiency is maximized owing to their perfectly periodic structures, mobility is generally lower in polymer semiconductors because of their poor crystallinity. Band calculation, however, predicts the effective mass of carriers can be even lighter than that in the highest-mobility small molecular semiconductors by an order of magnitude in the direction of a π-conjugated backbone. This fact suggests that there is a room for further improvement in mobility by forming a well oriented structure of the polymers. In this report, we introduce a new method of fabricating highly aligned polymer semiconductor films by mechanical compression of the polymers spread on ionic liquids (ILs).
Solution of a polymer semiconductor, poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno(3,2-b)thiophene (PB16TTT), was dropped on the surface of IL (emim-TFSI) which is filled in a 1 cm x 1 cm trough and kept at the temperature of 120 °C. Immediately after the droplet reaches the IL, a thin film of the polymer started to spread to cover the surface of the IL. Subsequently, the thin film was mechanically compressed by a blade to align the polymer chain uniaxially. The compressed films were transferred on substrates for further characterization. Because of high-thermal stability of the IL, the process temperature can be elevated up to ~350°C where flexibility of the chain of common polymer semiconductors is well enhanced, which is advantageous to increase the degree of ordering in present compression method. We have confirmed the versatility of the method by applying this technique to typical crystalline polymer semiconductors such as poly(3-hexylthiophene) (P3HT) or poly(2,7-bis(3-alkylthiophen-2-yl)naphtho[1,2-b:5,6-b&’]dithiophene) (PNDTBT).
High degree of polymer chain ordering was actually clarified in the present thin films. The dichroic ratio reached 7.9 which is the highest value among reported uniaxially aligned PBTTT thin films. By XRD measurement, the polymer chain aligns perpendicularly to the compression direction. The FET mobility as high as 0.7 cm²/Vs was achieved when the transport direction is parallel to the π-conjugated backbone, which is significantly higher than the mobility in randomly oriented thin film fabricated by spin-coating fabricated as a reference.
In conclusion, we have demonstrated a novel method to form oriented polymer thin film. High degree of chain orientation was achieved by mechanical compression of the polymer on the surface of hot IL. This method provides a versatile fabrication route of high-mobility polymer TFTs.

Recently, self-assembled fullerene crystals are attracting great interest due to their peculiar self-assembly behavior as well as outstanding optoelectronic properties. However, position control of the self-assembled crystal is still a challenging issue despite the fact that they are essentially placed on a solid substrate for optoelectronic device application. Here, we present an efficient method to obtain array form of fullerene crystals by solution phase crystallization. Furthermore, the fullerene crystals can be selectively wired to each other by electron beam irradiation followed by proper solvent treatment. The interconnected fullerene crystals become electrically conducting upon the exposure to alkali metal vapor.

Molecular packing in crystals is a crucial factor for determining the mobility of organic semiconductors. A simple indicative parameter is the degree of overlap between adjacent molecules, or transfer integral, since the mobility is proportional to the square of transfer integral according to the hopping model based on the Marcus theory. Recently, however, more and more studies report the band-like transport properties of high-mobility organic semiconductors, which are supported by experiments such as Hall measurement and the temperature dependence of mobility. This means that the transport mechanism of high-mobility organic semiconductors with mobility > 1 cm2/Vs can not be described by the incoherent hopping model. In the band model, full band calculation is required for more reliable prediction of transport properties such as effective mass and its anisotropy.
Here we explore the variation in effective mass and mobility for more than 20 kinds of high-mobility materials including our V-, W- and N-shaped molecules [1-3]. The effective mass was calculated by the band calculation while the mobility was measured by single-crystalline field-effect transistors. A clear inverse relationship was observed between the two physical parameters, which indicates that the measured mobility should be determined by the coherent band mechanism and not by extrinsic factors such as grain boundaries, lattice defects or impurities. We also show that the variation in mobility largely deviates from the prediction by the hopping model because it neglects the sign of transfer integrals. More detailed analysis will be presented in order to discuss the importance of full band calculation in understanding the mobility of high-performance organic semiconductors.
[1] T. Okamoto et al., Adv. Mater., published online (2013).
[2] C. Mitsui et al., Chem. Mater. 25, 3952-3956 (2013).
[3] J. Soeda et al., Appl. Phys. Express 6, 076503 (2013).

We show that diffusion of triplet excitons in high-quality triplet organic semiconductors can occur over macroscopic distances of up to a few micrometers, which accounts for surface generated photoconductivity in these materials [1]. Indeed, dissociation of these excitons at the surface of the crystals was found to be the main source of photocurrent in one of the best organic semiconductors - rubrene. In addition, clear evidence of the interaction of diffusive triplet excitons with each other and with mobile charge carriers is observed in photoconductivity measurements. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and multiparticle interactions of these triplets involving charge carriers and other excitons are important in governing the optical and photoconductive properties of these materials.
[1]. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, V. Podzorov, Nature Materials 9, 938-943 (2010).

We discovered that the molecular packing of some organic semiconductors can be tuned through solution processing to give rise to meta-stable molecular packing, i.e. strained organic semiconductor. This results in the change in the electronic properties of these organic semiconductors. In some cases, the charge carrier mobility can significantly increase. In this talk, I will discuss our understanding on how they are formed, how the molecular structures influence their formation and the impact on charge transport.

Ambipolar molecular materials recently synthesized and studied in the laboratories of the authors are reviewed. Carbazolyl-containing glass-forming stable free radicals have appeared to be effective ambipolar organic semiconductors, with electron mobilities approaching 10^(-2) cm^(2)/Vs and hole mobilities reaching 10^(-3) cm^(2)/Vs at high electric fields in air [1]. Carbazole trimers with different linking topology represent another group of glass-forming ambipolar molecular materials [2]. 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was found to show very high electron mobility ( 2X10^(-3) cm^(2)/Vs at an electric field of 1.8X10^(5) V/cm) and reasonably high hole mobility of 8X10^(-4) cm^(2)/Vs. Since 3,6-di(9-carbazolyl)carbazoles exhibit high triplet energy and ambipolar charge transport, they were successfully used for the development of single layer electrophosphorescent devices with external quantum efficiencies of 8-9% for blue and 13-14% for green and 10% for white devices. Ambipolar charge transport was also observed in carbazolyl substituted perylene bisimides under ambient conditions [3]. Depending on the linking topolygy of carbazolyl groups either hole transport or electron transport is more efficient. Time-of-flight mobilities in these materials reach 10^(-3) cm^(2)/Vs at high electric fields for both holes and electrons. Molecular glasses of the recently synthesized conjugated derivatives of triphenylamine and 1,8-naphthalimide were also found to be capable of transporting both holes and electrons in air.
[1] Castellanos, S.; Gaidelis, V.; Jankauskas, V.; Grazulevicius, J.V.; Brillas, E.; Lopez-Calahorra, F.; Julia, L., Velasco, D. Chem. Commun., 2010, 46, 5130.
[2] Tomkeviciene, A.; Grazulevicius, J. V., Kazlauskas, K., Gruodis, A., Jursenas, S., Ke, T.-H., Wu, C.-C. J. Phys. Chem. C. 2011, 115, 4887.
[3] Reghu, R. R.; Bisoyi, H. K.; Grazulevicius, J. V.; Anjukandi, P.; Gaidelis, V.; Jankauskas, V. J. Mater. Chem. 2011, 21, 7811.

Organic semiconductors are useful materials for thin film electronic devices, such as field-effect transistors and solar cells. Because microstructure and molecular packing strongly influence charge transport in organic materials, considerable research effort is devoted to materials processing. Control of nucleation and growth in thin films is critical to the performance of crystalline molecular semiconductors in electronic devices. It is therefore of interest to investigate how molecular species can be used as additives to control growth and transport in crystalline organic semiconductors.
We have found that structurally similar pentacene derivatives will form single crystals that are solid solutions (alloys). We present single crystal structures which demonstrate that blending two structurally similar materials perturbs the packing very little. X-ray scattering on solution-cast alloyed films reveals a structure that is similar to what is observed in the mixed crystals, as well as additional thin film phases. Optical spectroscopy reveals charge transfer excitation in the mixed films, consistent with the electronic levels of the two derivatives, which are offset similar to donor-acceptor pairs for efficient organic solar cells. These results suggest pathways for controlling nucleation in organic thin films and incorporation of molecular species to introduce dopants or intentional traps without significant perturbation of a molecular crystal lattice.

Fully conjugated diblock copolymers (CBCPs), comprised of p- and n-type blocks, form intriguing materials alternatives to polymer-small molecule blends for their control of mesoscopic order in low-cost organic semiconductor devices. In both bulk heterojunction (BHJ) photovoltaics, consisting of a phase-separated interpenetrating network with high donor-acceptor interfacial area, and ambipolar transistors, the transport of charge carriers through continuous p- and n-type paths in thin films is a controlling factor in device performance. In this work, the ideal morphology of CBCP thin films for this carrier transport is displayed. GIWAXS, AFM, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and resonant soft X-ray scattering (RSoXS) are used to probe the structure of films of CBCPs with a p-type poly(3-hexylthiophene) (P3HT) block and an n-type poly(diketopyrrolopyrrole-terthiophene) (DPP) block. Thermal annealing in the P3HT melt after casting causes these CBCP films to form ordered domains on the scale of the exciton diffusion length, with ~50 nm in-plane lamellar spacings, as confirmed with GISAXS and RSoXS. GIWAXS diffraction from the (h00) alkyl chain stacking planes for crystals of both P3HT and DPP blocks shows texture towards the out-of-plane direction, with the (010) pi-stacking vector in the film plane (edge-on). In addition, solution SAXS and UV-Vis spectroscopy are used to probe the size and conformation of casting solution aggregates as a function of temperature. Fibrillar aggregates of DPP blocks direct the crystallization of P3HT-b-DPP following film casting and enable the formation of wormlike domains following annealing in the P3HT melt. The understanding of casting solution and thin film structure in this materials class will lead to the formation of optimal organic electronic morphologies.

Pi-conjugated polymers have attracted considerable attention over the past decade due to the advantageous combination of their electrical, optical and film-forming properties. A major milestone for this emerging technology consists in achieving functional device fabrication by means of large-area printing.[1] As printing involves liquid solutions of functional substances from a limited variety of solvents, multi-layer devices can, in most cases, only be fabricated by alternating printing and curing steps. The latter can both be achieved by cross-linking or removal of solubilizing groups and can be initiated by chemical, thermal or photochemical stimuli or a mixture thereof.
Our work aims at the immobilization of semiconducting molecules by the external stimulus of heat. Inspiration for our approach is acknowledged to work by Fréchet[2] and Krebs[3] who introduced tertiary esters of poly(thiophen-3-carbocyclic acid-2,5-diyl)s which can be transformed into native polythiophene above 300 °C for use in organic solar cell devices. A major drawback of these materials remains the need for high processing temperatures, which delicately limit their application in functional devices. Thus, we focused on developing new thermally cleavable solubilizing groups that allow immobilization of functional polymers.
We developed a variety of new polymers and intermediates bearing thermocleavable groups employing an easily scalable synthetic route. Our contribution will discuss synthetic aspects of these new materials as well as studies related to their future application in printable organic electronics.
[1] F. C. Krebs, Solar Energy Materials & Solar Cells 2009, 93, 394.
[2] J. M. J. Fréchet et al., J. Am. Chem. Soc. 2004, 126, 9486.
[3] Bjerring, M.; Nielsen, J. S.; Nielsen, N. C.; Krebs, F. C. Macromolecules 2007, 40, 6012-6013.; S. A. Gevorgyan and F. C. Krebs, Chem. Mater. 2008, 20, 4386.

Bulk heterojunction (BHJ) type photoactive layer is widely used for the organic solar cells. The blended solution of electron donating polymers and accepting C₆₀ derivatives is used to form BHJ layer. The performance of BHJ solar cell is strongly influenced by the formation of nano scale morphology between donor and acceptor. The nano morphology formation is sensitive to various optimization conditions such as donor/acceptor blending ratio, processing additive and solvents.
In contrast, bilayer heterojunction structure prepared by the sequential deposition of electron donating and accepting layer is conceptually more straightforward since each layer can be optimized independently. Two main requirements should be fulfilled for the bilayer heterojunction organic solar cell. First, the polymer layer (bottom layer) should not be destroyed by the solvent of the PCBM layer (top layer). Second, the polymer and PCBM should form nano scale heterojunction with large surface area for the efficient exciton dissociation.
Efficient bilayer heterojunction organic solar cells are prepared by controlling the crystallinity and surface roughness of the polymer layer. The poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) is used for the bottom layer and the phenyl-C71-butyric-acid-methyl ester (PC₇₁BM) is used for the top layer. The crystallinity of PCDTBT was controlled by incorporating additives, such as 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN), to the polymer solution. Furthermore, it is found that the surface roughness of the PCDTBT film is increased after processing with additives. The enhanced crystallinity and roughness of PCDTBT bottom layer enable to construct PCDTBT/PC₇₁BM bilayer with high heterojunction interfacial area. The best device shows an open circuit voltage (V_OC) of 0.89 V, a short circuit current density (J_SC) of 11.73 mA cm^-2, and a fill factor (FF) of 0.67, corresponding to power conversion efficiency (eta;) of 7.0%.

A ring protected chromophore based small molecule was synthesized and was used for photovoltaic application. Solution processable small molecule with fullerene derivative was used as bulk heterojunction in conventional device structure. A high open-circuit voltage of 0.97 V, was observed with power conversion efficiency of 2.15 % . A 12-carbon ring introduced on the backbone of the small molecule is supposed to provide a certain order of molecular stacking in the active layer morphology. The active layer was optimized for different processing condition, resulting in smooth films with low roughness of <1 nm. Transient photo-voltage and photocurrent measurements were carried on fabricated device for determining charge carrier lifetime and charge collection time.

Abstract

Solution processed inverted polymer Bulk heterojunction (BHJ) solar cells are considered to be the most promising technology for low cost, flexible, and roll to roll processed solar cell applications. The power conversion efficiency of polymer solar cell has been reached over 9% [1]. For commercialization of such technology further improvements are required, especially in terms of power conversion efficiency. The further improvement in solar cell performances can be achieved by using additional processing such as doping of additives in the active layer [2], efficient and air stable buffer layers [3], thermal annealing [4], and interfacial engineering [5] respectively.
In this paper we investigate the simple and effective way of improving the overall performances of inverted BHJ solar cells by tuning the surface property of solution processed Zinc oxide (ZnO) layer. Thin non-conjugated polymer (NCP) poly-vinylpyrrolidone (PVP) layer is used to tune the hydrophilic surface of ZnO. The PVP is environmentally stable, non-toxic and easily soluble in water and other polar solvents due to their polar amide group and non-polar methylene groups in the backbone and in the ring. The modified ZnO surface using thin PVP layer in inverted polymer solar cells based on the blend of low band gap semiconducting polymer thieno[3, 4-b]thiophene/benzodithiophene (PTB7) and fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) photoactive layer exhibited a enhanced power conversion efficiency (PCE) as high as 7.34% under simulated AM1.5G illumination with intensity of 100 mW/cm2. The overall 10% PCE enhancement is obtained after the incorporation of thin PVP layer between ZnO and PTB7:PC70BM-based photoactive layer in inverted solar cells. Such enhanced performances are attributed to improved charge extraction and transport, suppress charge recombination at the interface and decreased contact resistance. We also found that PVP is one of best non conjugated polymer layer for interfacial engineering in organic solar cell.
We believe that such water/alcohol soluble NCP could be an excellent alternative for improving the interfacial contact area between n-type metal oxide and photoactive layer in future BHJ solar cells.
Acknowledgments
This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (MSET) (NRF-2009-0093323).
References
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2. S. J. Lou, J. M. Szarko, Tao Xu, L. Yu, T. J. Marks, L. X. Chen, J. Am. Chem. Soc. 2011, 133, 20661-20663.
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Using ultraviolet photoelectron spectroscopy (UPS), we have measured energy level offset at the planar junction interface of poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM) in ultrahigh vacuum. The planar junction (PCBM/P3HT) was formed in high vacuum by the electrospray vacuum deposition (EVD) of PCBM onto spin-coated P3HT. The EVD-prepared planar interface exhibited 0.91 eV for the offset between P3HT HOMO and PCBM LUMO which can be considered as the upper limit of Voc for the given material combination. This value is different from the one measured for P3HT:PCBM bulk heterojunction mixture.

Charge carrier mobility is still the most challenging issue that should be overcome to realize everyday-organic electronics in near future. In this communication, we show that introducing a smart side chain engineering to polymer semiconductors can facilitate intermolecular electronic communication. Two new polymers, P-29-DPPDBTE and P-29-DPPDTSE, which consist of a highly conductive diketopyrrolopyrrole backbone and a branching-position-adjusted side chain, showed unprecedented record high hole mobility of 12 cm^2/Vs. From photo-physical and structural studies, we found that moving the branching position of the side chain away from the backbone of these polymers resulted in increased intermolecular interactions with extremely short π-π stacking distances, without compromising solubility of the poly-mers. As a result, high hole mobility could be achieved even in devices fabricated using the polymers at room temperature.

Conventional Pt counter electrodes used in dye-sensitized solar cells (DSSCs) are expensive and thus there have been efforts in replacing Pt counter electrodes with other more cost-effective counter electrode materials yet still with good electrochemical activity. Conducting polymers, such as PEDOT:PSS, have been considered as one possible replacement of Pt counter electrodes in DSSCs. However, earlier attempts in replacing Pt counter electrodes with PEDOT:PSS mostly resulted in substantially degraded DSSC efficiencies, due to poorer conductivity and electrochemical activity of PEDOT:PSS. Taking advantage of the development of high-conductivity PEDOT:PSS in recent years, in this work we report the preparation of high-conductivity PEDOT:PSS (up to ~1000 S/cm) and its application as counter electrodes to achieve DSSC efficiencies ( >9%) comparable to those achieved with Pt electrodes. High conductivity of PEDOT:PSS was achieved by adding polar co-solvents into formulations of PEDOT:PSS. We investigated and optimized the influences of co-solvents on conductivity, morphologies and electrocatalytic activity of PEDOT:PSS, as well as their effects on DSSC characteristics such as short-circuit current density (Jsc), open- circuit voltage (Voc), fill factor (FF) and power conversion efficiency. As a result, the conversion efficiency of DSSCs can be significantly enhanced from 4.5% (with using the conventional PEDOT:PSS) to >9% (with using the high-conductivity PEDOT:PSS).

Organic polymer photovoltaic solar cells (OPVs) have achieved huge breakthroughs in past years. The power conversion efficiency (PCE) has been improved from 5% to 8% due to the synthesis of several novel low band gap polymers (1.4 eV-1.88 eV). The PCE of OPVs was further improved to 9.2% by introducing the novel electron transporting layer PFN. This PCE can be further improved since organic solar cells still suffered from the low charge mobility and low exciton diffusion length. High work function metal oxides are good candidates for hole transport. Meanwhile, incorporation of the transition metal oxide interlayer is one of the strategies to improve the PCE as well as the device stability.
In this work, high work function metal oxides including molybdenum oxide (MoO3) and vanadium oxide (V2O5) are used as the anode interlayer in high efficiency OPVs (such as PTB7:PC71BM system) with both normal and inverted structure. It is expected to exhibit improved device performance in particular JSC and FF, since the efficient charge extracting ability of these metal oxides. Meanwhile, we will also investigate the effect of different metal oxide preparing method on device performance, and the process compatibility will be discussed. The effect of the film morphology will be investigated by AFM and SEM.

The primary objective of this study is to synthesize novel fullerene derivatives and evaluate morphology when blended with homo polymers and block copolymers as an approach to understand the donor/acceptor behavior in blended system toward achieving optimum device performance. To this end, we synthesized novel fullerene derivative C60- fused N-(3-methoxy propyl) -2- (carboxyethyl) -5-(4-cyanophenyl) fulleropyrrolidine (NCPF) and characterized it by 1H NMR, 13C NMR, MALDI-TOFMS, FT-IR, UV-Vis and CV. The LUMO energy level of highly soluble NCPF was found to be greater than that of parent fullerene and similar to PCBM suggesting that electronic property of newly synthesized NCPF is comparable to that of PCBM. UV-Vis absorption spectra of NCPF/P3HT thin films show a red shift of nearly 10 nm for P3HT in the visible region compared to PCBM/P3HT system suggesting the improved packing of P3HT chains. GIWAXS of NCPF/P3HT thin films showed intense higher order reflections of P3HT indicating an enhancement in P3HT crystallinity upon annealing which is highly desirable for improved OPV properties. Upon annealing, the PCE of NCPF/P3HT and PCBM/P3HT was found to increase by 33% and 27% in relation to as-cast blend films. The improvement in PCE is attributed to enhanced crystallinity of P3HT in blend films. However, this blend showed sub-micron size aggregates suggesting phase separation, which could be further improved by using self assembled block copolymer as compatibilizer. The synthesized NCPF and PS-P3HT block copolymer having different block ratio will be evaluated in blended system processed via thermal and solvent vapor annealing conditions so as to achieve desired phase morphology to obtain optimum OPV performance.
Acknowledgements : U.S. Department of Energy, Division of Basic Energy Sciences under contract No. DE-FG02-10ER4779

While the field of organic electronics is ever expanding the application of these new materials into devices is held back by the scarcity of n-type molecules. Indeno[1,2-b]fluorenediones are electron accepting molecules that show promise as n-type materials for organic electronics. We are investigating the optical and electronic properties of Donor-Acceptor-Donor triads using the indeno[1,2-b]fluorenedione as the acceptor unit and a series of aryl donating groups attached at the 5 and 11 positions. These triads are synthesized via Sonogashira or Stille cross coupling in order to allow for donor groups with or without an ethynyl linker. Further work is focusing on thiophene and furan based donor groups, as well as quantifying the effect of the ethynyl linker.

Felix Penningsfeld, Jiyun Song, and Changhee Lee
Department of Electrical and Computer Engineering, Global Frontier Center for Multiscale Energy Systems, Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea
E-mail: [email protected]
The concept of ternary solar cells with an added low band gap polymer to extend spectroscopic response into the near infrared region brings the development of efficient and cost-effective solar cells closer to realization by utilizing a broad spectrum of sunlight in a single cell. We investigated the morphology and electrical characteristics of bulk heterojunction solar cells with ternary blends of PTB7 and P3HT as donors and PCBM as an acceptor. By adding small amounts of PTB7 in the P3HT:PCBM blend, we demonstrated enhanced photon absorption in the near infrared spectrum as well as 15% improvement of device efficiency. The use of 1,8-diiodooctane (DIO) as processing additive also gives better control over film morphology. We expect that the concept of two donors and one acceptor with suitable low band gap polymer offers a great opportunity for future research on maximizing light absorption.

We report the use of polymeric self-assembled monolayers (PSAMs) as hole injecting interface layer for polymer solar cells. The asymetric diblock copolymers composed of relatively long polystyrene (PS), poly(n-hexylisocyanate) (PHIC) and poly(3-hexylthiophene) (P3HT) with short sticky poly[(3-trimethoxysilyl) propyl methacrylate] (PTMSM) moieties were used for formation of PSAMs. The polymer photovoltaic cells using Poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as an active layer were fabricated on the different thickness (10, 30 and 50 nm) of molybdenum oxide (MoO3) coated with these ultra-thin PSAMs as a modified hole transporting layer. The PSAM treated surfaces were prepared from covalent bonding of trimethoxysilyl groups at their chain end with hydroxylated MoO3 layer by immersion coating method. The effects of the type of PSAMs and the thickness of MoO3 layers on the device performance were investigated. Interestingly, devices containing PSAM treated hole transporting layer showed little-to-no dependence on the thickness of the MoO3 layer. 35-60% higher efficiencies were observed for the devices with PSAMs over the control devices on top of a 50 nm thick of MoO3 layer, due to the control on the series and shunt resistance (Rs & Rsh).

Polymer solar cells (PSC) have attracted a lot of interest due many advantages including low production cost, light weight, good processability and flexibility. Several low bandgap polymers have been designed and used in PSC to harvest more sunlight. Poly(3-hexylthiophene)s (P3HT)s are presently among the most promising candidates for use in PSCs as an electron donor material due to their high charge carrier mobility, strong absorption in the visible and near-infrared region.
The binary blends of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are extensively employed as an active layer in PSC because of high power conversion efficiency. However, the photovoltaic performance of the bulk heterojunction (BHJ) solar cells of P3HT: PCBM is still limited as compared with those obtained from inorganic solar cells due to the insufficient charge carrier mobility and the narrow absorption range of donor polymers, as well as the relatively short exciton diffusion length of only a few nanometers. They also show limited device stability because oxygen can diffuse into the active layer leading to its degradation. In order to overcome these limitations and improve the photovoltaic performance, the electron-transporting interfacial layers of metal oxides, e.g. TiO2 and CuO have been introduced between the active layers and the electrodes. These metal oxides harvest electrons from many of the electron acceptor components of PSCs and block holes and limit surface recombination at the contact. It is also reported that the interfacial interlayer of these metal oxides located between the active layer and the electrode acts as optical spacer to shift the maximum of light intensity which increase the exciton generation rate and improve the performance of PSCs.
In search of new materials suitable for the electron-transporting interlayers in PSCs, the surfaces of TiO2 nanoparticle (TiO2NP) are modified and introduced between the active layers and the electrodes in the present study. The surface-modified TiO2 nanoparticle (mTiO2NP) interlayer improves the organic-metal interface in the BHJ solar cells of P3HT:PCBM without the need for surfactants, and their film-to-air surfaces becomes smooth without any pinholes or other gaps. It plays a crucial role of an electron-harvesting layer of the BHJ solar cells of P3HT:PCBM and blocks holes the organic-metal interface. In addition, mTiO2NPs also play an additional role of diffractive layer to provide an efficient way of achieving light trapping in PSCs. The deposition of mTiO2NPs an interlayer is relatively simple and convenient, and can be processable through spin-coating.

Most conjugated polymers have been synthesized via various cross-coupling methods such as Suzuki, Kumada, Stille and Yamamoto reactions. However, they suffer from a number of disadvantages such as low catalytic activity, high reaction temperature and prolonged reaction time that limit their extensive usage in many industrial applications. Direct heteroarylation polymerization has been recently proposed to offer a facile route to synthesize aromatic polymers in relatively high yields with simple purification of the final polymer product.
In this study, we employed direct heteroarylation polymerization to synthesize a novel low bandgap polymer that was used as p-type material for the preparation of the active layer of polymer photovoltaic cells. To reduce the bandgap of poly(3-alkylthiophene)s, the electron donor-acceptor (D-A) alternating repeat unit which consisted of electron-withdrawing cyano- thiophene and electron-donating 3-alkylthiophene were designed and polymerized via direct heteroarylation polymerization. Cyanothiophene as a commoner of the polymer backbone enhanced the intermolecular interaction between neighboring chains and improved the structural perfection of the crystal structure on the substrate.
The bulk heterojunction (BHJ) solar cell devices of the prepared polymer with PCBM were fabricated on ITO-coated glass substrates with a sheet resistance of 20Omega;/sq. ITO-coated glass substrates were first cleaned by ultrasonic agitation in D. I. water, acetone and isopropanol alcohol sequentially and were dried in a vacuum oven overnight. After ITO underwent a UV-ozone treatment for 15 min, the 1:1(v/v) mixture of poly(3,4-ethylene dioxythiophene)(PEDOT):sulfonated polystyrene (PSS) was spun onto the ITO substrates at 2500 rpm for 45 sec and were then annealed at 130oC for 10 min. The thickness of the PEDOT:PSS layer was approximately 40 nm. The active layers was spun in a N2 filled glove box from the solution of P3HT:PCBM (weight ratio of 1:0.7) in chlorobenzene with the overall concentration of 40 mg/ml. After mixing for 2 days, the solution was spun at 1300 rpm for 30 sec on the PEDOT:PSS layer, after which the samples were annealed at 150 oC for 10 min. Finally Al with an average thickness of ~120 nm was deposited under 1.0X10-6 Torr by thermal evaporation through a shadow mask. The mean film thickness was measured from the scratched AFM image in tapping mode. For each film, at least three of these measurements were performed and averaged.

Countries around the globe are pursuing electricity generated from renewable resources in order to satisfy growing energy demand and alleviate global warming. The cost of solar power generation has decreased dramatically over the last few years and is now a viable energy source. However for most applications, consumption profile does not match the availability of sunlight. This brings up the necessity for energy storage, and integration with photovltaics. We hereby present an integrated solar cell, supercapacitor and LED in a single substrate. Traditionally, solar cells are connected to external storage devices through metal wires. However, the long distance decreases storage efficiency because of the higher connection resistance and results in a higher bill of materials and increased size and weight of the whole power system.
We combine multiple organic solar cells (energy generation), a nano-carbon supercapacitor (energy storage) and an organic LED (energy sink) into a single intergrated pack. Because these devices are solution processable, it is possible to make the power pack in large areas using roll-to-roll processes. Herein, we use P3HT:PCBM hybrid system as the solar cell materials(efficiency is about 3%) and carbon nanotube or graphene as capacitor materials (capacitance is about 4 mF) to manufacture the integrated power pack. Furthermore, we also used parallel (current is about 2mA) and series pattern (open circuit voltage is over 4 volt) for solar cells to turn on the optoelectronic devices such as light emitting diode.

In normal structure of polymer solar cells, active layer is sandwiched between a transparent anode and a metal cathode. Generally, for efficient hole collecting to transparent anode, poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as an hole transport layer (HTLs) due to its high work function and high conductivity. However, several groups have tried to replace PEDOT:PSS because of several problems such as high acidity and hygroscopic properties, leading to poor long-term stability of devices. Therefore, as an alternative HTLs for the replacement of PEDOT:PSS, interests in p-type like metal oxides such as vanadium oxide (V2O5), and molybdenum oxide (MoO3) has been increased. In this study, we investigate the effect of V2O5 HTLs fabricated by simple brush painting, which is very compatible with roll-to-roll process on a various substrates. Fabrication process was optimized by controlling several conditions such as precursor concentrations and temperatures of substrate during brush painting. The device with brush painted V2O5 at 50 °C showed higher power conversion efficiency of 3.8 % than that of conventional PEDOT:PSS contained PSC having 3.5 %. Optical, electrical, and structural properties of brush painted V2O5 were characterized and their effects on the performance and stability of PSC will be discussed.

Organic and nanocrystalline solar cells comprising novel materials require a considerable amount of effort in optimization for various parameters. Current experimental methods, in which several devices must be fabricated in serial to test the effect of a single variable potentially introduces variability, which paints an incomplete picture of parameters that determine the photovoltaic properties of devices. Such conventional methods also consume copious amounts of precious, non-commercial π-conjugated polymers and other materials. Due to the resource intensiveness of these methods, only a small number of variables for a new system of materials can be optimized. This paper describes the use of gradients in parameters to measure spatially resolved photovoltaic and charge-transport properties through films of organic semiconductors. Gradients in one or more parameters—e.g., film thickness and temperature of the post-processed thermal annealing—can be used as a universal optimization process for any solution-processed organic solar cell system. Our results demonstrate the main strengths of the technique: the efficient use of precious materials and the experimenter&’s time and the generation of a single substrate comprising many devices that share the same processing history.

Graphene, a two-dimensional carbon atom sheet with a honeycomb lattice, has shown outstanding physical, chemical, and optoelectrical properties. Specially due to the high conductivity and excellent transmittance, the chemical vapor deposition (CVD) based graphene films have been received significant attention as a promising transparent electrode for a indium tin oxide (ITO)_alternative. However, for real commercialization of graphene films and for the use as a key material in semiconductor based technologies, control of optoelectronic properties of graphene films via a controllable graphene structural modification should be studied.
Herein, we introduce an electron-beam irradiation on CVD-based graphene films to modify the graphene structure and thus to study changes in optoelectrical properties of graphene. With various irradiation conditions such as dose intensity, time, and ambient, the changes in conductivity, work-function, transmittance, chemical composition, and microstructure of graphene films have been systemically investigated using a four-probe measurement, kelvin probe, UV-vis absorption measurement, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). In addition, we have also studied their effects as transparent electrodes and interfacial materials on cell-performances in organic solar cells, respectively. In conclusion, we mention the possibility that CVD-based graphene is possible to be a ITO alternate electrode in organic solar cells.

To enhance efficiency through prevention of recombination of carriers and leakage current, electron-blocking, hole-transporting layers (HTLs) are commonly used to modify the transparent conducting oxide (TCO) anode in organic solar cells (OSCs). The most commonly used HTL in OSCs is PEDOT:PSS, which is typically spin coated between the transparent anode and active layer. PEDOT:PSS provides good coverage over the indium-tin oxide (ITO) surface and reduces the surface roughness while simultaneously acting to align the electronic energy levels of ITO and the active layer for efficient hole collection. Several weakness have also been reported concerning the electrical inhomogeneity of electrical properties and microstructure in the PEDOT:PSS thin film as well as poor electron blocking properties. Finally, PEDOT:PSS is strongly acidic and it has been shown that slow etching of the underlying ITO electrode promotes diffusion of indium atoms into the active layer, thereby degrading device performance. Herein we synthesized novel GO and water-soluble conjugated polymer composite and developed electrical conductivity of this novel composite through radiation technology. Electrical conductivity was improved by radiation. To investigate mechanism of enhanced electrical conductivity of, GO-water-soluble polymer composite was measured UV-Vis absorption and Raman spectroscopy. As a result, new absorption peak appeared from 300 nm to 380 nm. This new peak indicated possibility to make a longer conjugation depending on the radiation energy. In Raman spectroscopy, D-band at 1350 cm-1 gradually reduced by radiation due to the fact of enhanced conjugation between GO and water-soluble polymer. GO-water-soluble polymer composite was used as a hole-transporting layer in organic solar cells (OSCs). The performance of OSCs with GO-water-soluble composite significantly enhanced. Specially, efficiency of device with composite used as a hole-transporting layer was significantly enhanced

Fiber-shaped organic solar cells (OSCs) have received a great attention due to their excellent potentials such as low-cost, solution-based fabrication process, light weight, flexibility, and multifunctional fabrics. The structure of fiber-shaped OSCs can be separated into two types. One type is a single fiber structure, which consists of primary electrode/hole transport layer/active layer/electron transport layer/metal electrode. Another type is a twisting fiber structure, which is composed of primary electrode and secondary electrode. The twisting fiber systems have shown the highest efficiency (3.87%), and fiber-shaped OSCs are generally fabricated on metal fiber. However, in order to achieve more focus on device efficiency, primary or secondary electrodes should be further studied, especially in terms of conductivity, resistance, roughness, solution ability process. Moreover, fiber-shaped OSCs based on non-metal primary electrodes was less studied compared to metal primary electrode.
In this work, we have fabricated fiber-shaped OSCs by using metal and non-metal primary fiber electrodes. In addition, the surface of the metal and non-metal primary fiber electrodes was further smoothened with an additional layer, due to reduced shorting and improved layer contact. We also examined the effects of morphology and electrical properties of fiber electrodes on device performance in fiber-shaped OSCs. The other detailed information such as tensile strength and bending flexibility have been also studied and will be also fully discussed.

We report novel plasmonic polymer solar cells by embedding a nanobump assembly (NBA) constructed with molybdenum oxide (MoO3) layer covering Ag nanoparticles (NPs) under the active layer. The Ag NPs were deposited on an indium tin oxide anode by an aerosol process at atmospheric pressure and covered by thermally evaporated MoO3 layer, resulting in the Ag NBA structure. The active layer, spin-coated from the PCDTBT:PC70BM blend solution, has an undulated structure conformably covering the Ag NBA structure. The combination of undulated active layer and Ag NBA structure leads to 18% improvement in the power conversion efficiency. Compared to devices with Ag NPs incorporated in the flat poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS), the NBA-structured plasmonic solar cells show better performance due to strong forward scattering, higher ratio of scattering cross-section to absorption ratio, and longer exciton lifetime originating from the NPs isolated by MoO3. Thus, the NBA plasmonic structure provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells.

Photovoltaic technology is recognized as one of the best methods to prevent the exhaustion of fossil fuels and deal with the problem of global climate change. In particular, organic photovoltaic (OPV) has recently attracted much attention because of various advantages such as light-weight, flexibility, transparency, and low cost by roll-to-roll process. Toward its practical uses, our effort focus mainly on the development of highly performances OPV by means of a tandem device in which a properly designed materials with different absorption bands are stacked in a tandem configuration providing a broader absorption of the solar radiation spectrum compared to the single layer based devices.
In this respect, the development of novel D-A copolymers by a synthetic manipulation of polymer energy levels is fundamental to enhance both light absorption and voltage generation in a bulk device. Herein we present our recent progress in the development of highly efficient organic solar cells by the use of two different band gap polymers. Recently we have synthesized a narrow bang gap polymer which absorbs light up to 900 nm by the use of alternating donor-acceptor (D-A) π-conjugated structure. So we newly synthesized a wide band gap (WBG) polymer (lambda;th= 750 nm) which has low lying HOMO energy level (-5.6 eV) leading to high open circuit voltage (1.06 V) in the corresponding fabricated single cell. The representative polymers were applied to tandem organic solar cell giving a power conversion efficiency of 10% with high Voc (1.76 V). Optimization of the device structure is currently under investigation and it will be presented as well.
This research was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through its program of "High Performance PV Generation Systems for the Future".

Polymer solar cells (PSC) have attracted attention as a renewable energy source due to their low cost, ease of manufacture, and compatibility with flexible substrates. To improve efficiency of PSC, donor polymers which have adequate energy level related to acceptor materials should be developed. Recently, donor polymers which were adopted fluorinated unit show enhanced performanceof PSC, especially VOC, because the electron withdrawing property of F atom lowers the energy level of polymer without optical band gap changing. Based on this motivation, we synthesized donor polymers containing fluorinated benzothiadiazole unit to control the energy level of polymers. Thermal properties of donor polymers were measured by thermal gravimetric analysis and differential scanning calorimeter. To estimate energy level of polymers electrochemical property of polymers was measured by cyclic voltammetry. UV-vis absorption spectrum of polymers was measured to confirm the absorption range of polymers. PSCs were fabricated using the polymer as an electron donor material and [6,6]-phenyl-C61 butylic acid methyl ester (PC61BM) and [6,6]-phenyl-C71 butylic acid methyl ester (PC71BM) as an electron acceptor material.

Bulk heterojunction solar cells have attracted considerable attention over the last years due to their potential as a low cost photovoltaic technology. The control over the morphology of the active layer in fullerene based solar cells (bulk heterojunctions) is critical in order to improve and control the efficiency of the optoelectronic devices. While maintaining the interface of the donor and acceptor subphases as large as possible it has to be guaranteed that continuous networks of donors and acceptors are formed. These are required for the transport of electrons and holes to the electrodes. So far all attempts to control this mode of specific phase separation failed. We propose an approach where we want to force the packing of fullerenes within the percolation networks by covalent preclustering.
It is to be expected that within these clusters and between neighboring clusters (especially in the solid state) favorable π-π-stacking interactions between fullerenes, enabling facile transport of electrons, will take place.
We report the synthesis of some new fullerene clusters, where different approaches were pursued. In general these clusters exist of a core, a linker and one or more fullerenes. We were successful synthesizing a cluster with three fullerene monoadducts linked to a benzene core both with a C6-alkyl chain and a polyethylene glycol chain. Furthermore it was possible to synthesize a hexaphenylbenzene derivative with solubility generating groups, which was planarized afterwards to give the corresponding hexabenzocoronene compound. Afterwards we functionalized the hexabenzocoronene derivative to get the appendant fullerene molecule (X-ray structure available). Actually we work with first success on molecules consisting of a hexabenzocoronene core and three fullerene monoadducts. In addition the work on clusters with a naphtalene core is currently in progress. So we synthesized another novel fullerene cluster, in which we have a naphthalene unit aligned with four malonate linkers, where in each case two of them form a bisadduct with the fullerene (π-π-stacking suggested). First devices for photovoltaic measurements were already built.

P-type π-conjugated polymers for organic photovoltaic (OPV) application are synthesized by coupling electron accepting monomers such as isoindigo, thienoisoindigo, and diketopyrrolopyrrole with thiophene-based donor units. To improve polymer crystallinity, we select alkyl-substituted thiophene-based donor units due to their known potentials for inducing order, leading to controlled phase-separation in the polymer:PCBM blend, better ordered morphology, and higher mobility. We reveal the degree of crystallinity of these polymers using x-ray diffraction (XRD), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). This family of polymers exhibits favorable properties for OPV application: 1) broad absorption spectra extending into the near infrared region (400 to 1,200 nm); 2) high extinction coefficients (32,200 to 63,400 M-1cm-1); and 3) well-aligned energy levels for exciton dissociation at the donor-acceptor interface as determined by differential pulse voltammetry (DPV). We first use spin-coating to optimize polymer thin-film deposition. We then utilize scalable processing methods such as blade coating and slot-die coating. These roll-to-roll compatible deposition methods play an important role in advancing OPV development. The thin-films deposited by the different methods are subjected to morphological and transport characteristic studies to unveil the effect of processing methods on film quality and OPV performance.

Organic solar cells (OSCs) are of great commercial interest due to their light weight, robustness, and ease of manufacturing. However, the low efficiency and the poor stability of the OSC devices hinder their application. To overcome the instability issue associated with the normal structured OSC, one feasible approach is to construct an inverted structure device, where indium tin oxide (ITO) serves as the cathode and a high work function metal as the anode. In this work, an inverted structure organic solar cell of ITO/fullerene (C60) (50nm)/boron subphthalocyanine chloride (SubPc) (15nm)/MoO3 (30nm)/Ag was prepared and characterized without any further encapsulation and it gave a power conversion efficiency (PCE) of merely 0.9%. To improve the performance, a bathocuproine (BCP) buffer layer was inserted between ITO and C60 and it enhanced the PCE up to 1.5% and the external quantum efficiency (EQE) exceeded from 20% to 30%. A further improvement of PCE of 1.74% and EQE of 37% were witnessed by the application of Ag doped BCP (1:1 in molar ratio) buffer layer instead of pure BCP. The improvement of the device performance is due to the increase of the open circuit voltage and fill factor. The results revealed that the poor performance of the device without buffer layer mainly due to the enhanced recombination at the interface of ITO/C60. And the insertion of the BCP layer suppressed the recombination path while the Ag doped BCP layer further improved the solar cells performance not only by inhibiting the recombination path but also by optimizing the conductivity and facilitating the transport of the electrons. This work identifies an effectiveness of buffer layer in inverted structure organic solar cells which increased the open circuit voltage and fill factor leading to higher power conversion efficiency and has significant implications in improving the performance of organic solar cells.

During the past two decades, the performance of solution processed bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) has been remarkably improved due to the development of highly efficient photoactive materials. Most of BHJ type OPVs were fabricated using polymeric donor materials, since their good film property after coating. However, the difficulty in synthetic reproducibility and purification of polymeric donors were main drawback to overcome. In this study, we designed and synthesized D-A-D-A-D (donor-acceptor-donor-acceptor-donor) type small molecular donor materials containing alternatively linked benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Also, we studied on the effect of substituent at both terminal position of these small molecular donors with variation of alkyl chain length and electron donating power. Finally, 4.29% of power conversion efficiency can be obtained using a mixture of synthesized small molecular donor and PCBM as an active layer with a Voc of 0.84 V, a Jsc of 9.72 mA/cm2, and a fill factor of 0.52. We will show how one can decide what to adjust to get the ideal morphology and thereby obtain high energy conversion efficiency in organic photovoltaic cells. Also, the performance of inkjet and slot-die printed OPVs using a mixture of titled donor and PCBM as an active layer will be discussed.

Research into the field of small molecular organic photovoltaics has experienced steady growth over the past decade. This growth can be attributed to the wide range of potential donor/acceptor materials available, the ease with which the materials can be tuned, the potential for low cost manufacturing, and the possibility for high power conversion efficiencies. Metal phthalocyanines (Pc) have long been the archetype donor material for small molecular organic photovoltaics due to their high absorption coefficients, high thermal and electrochemical stability, and high hole mobility. Unfortunately with most Pc materials device performance is limited by short exciton diffusion lengths and poor energy level alignment with fullerene acceptors. In this presentation we demonstrate the simultaneous enhancement in fill factor, open circuit voltage, and short circuit current in in planer heterojunction devices of metal phthalocyanines and C60 through the introduction of organic anode interfacial layers. By using these organic templating layers, we observed enhancement in exciton diffusion efficiency through exciton blocking at the anode interface, by choosing materials with appropriate energy levels, as well as improving the crystallinity and preferentially orienting the molecules of the donor material. Furthermore, by utilizing such a templating layer of Rubrene in a SubPc/C60 heterojunction we achieved a VOC above 1.1V while maintaining a fill factor of 51%, yielding a power conversion efficiency over 3.5%.

[6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) are the most commonly used fullerene derivatives in bulk heterojunction (BHJ) organic solar cells. Almost all high efficiency polymer solar cells are fabricated with PC70BM due to its better absorption. However, researchers have recently reported that BHJ solar cells with some polymer donors show an increase in power conversion efficiency (PCE) when blended with PC60BM instead of PC70BM. However, the reason is not yet well understood.
Herein, we investigated the solar cell performance of an NT-based polymer with two common fullerene derivatives, PC60BM and PC70BM. Solar cells based on the latter show lower PCEand short circuit current (Jsc) than PC60BM-based solar cells, but show higher performance when 1,8-diiodooctane (DIO) is used as an additive. Grazing incidence wide angle X-ray scattering (GIWAXS) results indicate the degree of NT polymer order is interrupted by PC70BM in the blend film and reappears in blend films cast from solution containing the DIO additive. Transient photocurrent measurements showed that processing with DIO leads to decreased recombination rates in both cells, allowing for more efficient charge extraction. It also significantly increased carrier generation in NT:PC70BM film, consistent with the restored order. The results indicate that fullerene derivatives and processing conditions play important roles in determining the solar cell performance.

Organic solar cells (OSCs) fabricated by versatile printing methods such as the doctor blade, inkjet, and roll-to-roll methods are inexpensive, lightweight, and highly solution-processable. Over the last few years, considerable effort has been focused on improving the OSC performance, with the aim of achieving a power conversion efficiency (PCE) of 10%. The following strategies have been adopted for this purpose: 1) development of photoactive materials such as pi-conjugated semiconducting polymers and fullerenes, 2) use of functional layers for buffering, charge transport, optical spacing, etc., and 3) tuning the morphology of the photoactive film by post-annealing, solvent drying, or by using processing additives. Particularly considerable research has been recently focused on developing efficient small-molecule organic semiconductors to improve the performance of solution-processed small-molecule OSCs (SMOSCs), with the near-term goal of achieving a PCE comparable to that above 8% in polymer solar cells (PSCs). We have also developed various molecular structures for efficient small-molecule organic semiconductors and reported their unique photovoltaic characteristics in solution-processed SMOSC. Herein, we wish to introduce the research results to figure out the structure-performance relationship for rational molecular design of organic semiconductors in solution-processed SMOSC.

In the field of solar cells, photovoltaic conversion is an attractive process to supply green and sustainable energy in a more and more critical environmental context. The use of nanostructured materials in the form of nanowires is very promising. By using a radial junction surrounding nanowires according to core shell heterostructures, light absorption (related to nanowire length) can be separated from charge carrier collection (depending on nanowire diameter). Nanostructured materials can therefore induce light trapping in nanowire ensemble, hence improving absorption with respect to conventional planar layers. Furthermore, charge carrier collection can be more efficient owing to the much smaller collection distance and to the better crystalline quality of nanowires presenting a lower density of defects. In the present work we designed and studied a Photovoltaic and thermoelectric system based on ZnO and CdTe nanowires surrounded by an absorbing organic self assembled semiconductor known as CBI materials that has been developed in order to efficiently convert UV-visible and IR energy into electricity.
The hot anode of n-type ZnO nanowires was fabricated using a thermal process on pre-seeded layer and results to be crystalline with a transmittance up to 95 % and a bandgap of ~ 3.32 eV. The optoelectronic properties of the nanowires from experimental studies were related to the aspect ratios and the crystal surface defects. Conductivity measurements reveal diode-like behavior for the ZnO nanowires.
The visible-UV light-active organic layer was deposited between the anode and cathode at room temperature using a layer-by-layer deposition onto ITO and SiO2 substrates and ZnO and Bi2Te3 nanowires from aqueous solution. The organic layer is composed of oppositely charged porphyrin metal (Zn(II) and Sn(IV)(OHacirc;euro;‘)2) derivatives that are separately water soluble, but when combined form a virtually insoluble solid. The electron donor/acceptor properties (energy levels, band gaps) of the solid can be controlled by the choice of metals and the nature of the peripheral substituent groups of the porphyrin ring.
The highly thermoelectric structure, which acts as a cold cathode, is composed of p-type Bi2Te3 nanowires with a thermoelectric efficiency (ZT) between ~0.7 to 1, values that are twice that expected for bulk Bi2Te3.
The organic material presents a better layering onto hydrophilic surfaces. The role of water molecules and the surface defect was studied using high resolution TEM and absorbance spectroscopy.
Properties of the integrated materials is presented in terms of photo- and thermo-generated current and advantages of the low cost fabrication process is discussed.

We have investigated the influence of metal-oxide interlayers on the degradation dynamics of polymer-based organic solar cells. Whereas for high work function metals in general the intrinsic device layer stack integrity is a sensitive issue, for low work function metals extrinsic stability is controlling device lifetime. We present via specifically designed degradation experiments in combination with standard characterizations and imaging analysis improved understanding for the underlying degradation mechanisms acting on the electrodes of OPV devices and how these may be modified via metal-oxide interlayers.

Herein we present a methodology for improving the power conversion efficiency of organic solar cells made by photoactive layers of poly(3-hexylthiophene) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) of non-optimized microstructure. In our study we achieve a 47% improvement in the power conversion efficiency (PCE) of the device by utilizing a thiophene-based quinoid (QBT) moiety as the third component in the P3HT:PCBM:QBT photoactive layers. Based on a set of independent characterization experiments we elucidate the origin of the PCE improvement by addressing the QBT composition dependent photophysical, electrical, thermal, structural and morphology-related properties of the triple-bulk heterojunction of P3HT:PCBM:QBT. Unipolar P3HT:PCBM:QBT devices are electrically characterized for revealing the impact of QBT addition on the charge transport properties of the P3HT:PCBM:QBT blend films. The hole mobility values correlate well with the crystallinity of the P3HT matrix as evidenced by the X-ray diffraction patterns of P3HT:PCBM:QBT layers with increased QBT content. The positive effect of adding QBT is not observed in the device performance and hole mobility when a P3HT matrix with lower molecular weight is used [1]. Our results suggest that QBT serves as nucleation agent that modifies the non-optimized P3HT microstructure. In addition, the relatively small optical gap of QBT facilitates a resonant energy transfer step from the photoexcited PCBM to the QBT followed by a charge transfer process between QBT and the P3HT matrix. Our findings offer general guidelines for the design of next generation functional additives to be used in organic photovoltaics [2].
[1] Zen, A., et al. Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene). Macromolecules (2006)39(6): 2162-2171.
[2] Z.Kan,et al., Triple bulk heterojunctions as means for recovering the microstructure of photoactive layers in organic solar cell devices, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.08.007i

We used Stille coupling polymerization to synthesize a series of new two-dimensional conjugated D-π-A polymers—PBDTTTBO, PBDTTTBS, PBDTTSBO, PBDTSTBO, PBDTTSBS, PBDTSTBS, PBDTSSBO and PBDTSSBS that incorporated selenium atoms located in π-bridge, side chain or main chain of the polymers that consist of BDT (benzo[1,2-b:4,5-b]dithiophene) as donor units and BO (alkoxy-modified 2,1,3-benzooxadiazole) as the acceptor units. The presence of selenium atom altered the solubility, conformations and electronic properties of the synthesized conjugated polymers, allowing fine-tuning of their photovoltaic properties when blended with fullerenes. Density functional theory calculations revealed that the presence of the selenium atom in these π-bridge linkers, side chain groups and main chain units affected the torsion angles between the side chain groups and the conjugated main chains and the dihedral angles between the π-bridge linkers and the conjugated main chains. These polymers displayed excellent thermal stability and broad spectral absorptions. We found that the active layer&’s morphology varied substantially with the polymers that have different selenium atoms location when the films were examined with transmission electron microscopy. As a result, these bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided tunable power conversion efficiency. In particular, the photovoltaic device incorporating the PBDTSTBO/PC71BM (w:w, 1:2) blend system as an additive exhibited good performance, under AM 1.5 G irradiation (100 mW cm-2), with a value of open-circuit voltages of 0.86 V, a short-circuit current density of 10.6 mA cm-2, a fill factor of 0.68, and a promising power conversion efficiency of 6.2%.

Copper indium gallium diselenide (CIGS) solar cells are the most efficient thin film photovoltaic devices. In this work, we investigate CIGS/organic hybrid solar cells comprising semi-transparent metal top electrodes and wide-band gap organic semiconductors as buffer layer. Depositing the organic semiconductors from solution, we fabricate Cd-free CIGS solar cells exhibiting about the same efficiency as their fully inorganic counterparts comprising CdS, and significantly higher open-circuit voltages as compared to buffer-free devices. Although the organic molecules do not cover the CIGS surface homogeneously, their use enables prolonged charge carrier lifetimes according to impedance spectroscopy measurements.
Combining wide and narrow band gap absorbers in tandem solar cells is a promising approach to improve the energy conversion of sunlight. We therefore present hybrid tandem devices comprising monolithically connected copper indium gallium diselenide (CIGS) bottom cells and polymer top cells. The thin polymer:fullerene bulk heterojunction absorber layers were transferred onto the rough CIGS surface by a soft-contact lamination technique. Sputtered or solution deposited top-cathodes complete the tandem devices with enhanced open circuit voltages.
Finally, we present copper indium gallium diselenide (CIGS) solar cells incorporating solution-deposited transparent top cathodes. The utilized highly conductive polymer blend and the silver nanowire mesh exhibit excellent transparency in the near-UV region and form pinhole-free, homogeneous layers.
[Appl. Phys. Lett., 103 (2013) 143904; Appl. Phys. Lett. 102 (2013) 063304; Org. Electron. 14 (2013) 273-277]

Bulk heteojunction organic solar cell has attracted worldwide attention due to its great potential as a green, flexible and low-cost renewable energy source. P3HT-PCBM blend is the most used binary system for organic bulk heterojunction solar cells. Recent works that investigated vertical phase separation of P3HT/PCBM blend have revealed that unfavorable vertical distribution of donor-acceptor components exists after thermal annealing, in which electron-acceptor component PCBM dominates near both anode and cathode. In contrast, the ideal vertical phase separation requires donor-rich phase near the anode and acceptor-rich phase near the cathode for efficient electron and hole transport and collection. In this work, computer simulation using SETFOS was conducted to further verify that vertical configuration is one of the key factors that influences the performance of OPV. The J-V curves from simulation match well with experimental ones and almost 1% efficiency enhancement was observed from blend film with addition of PS-b-P3HT block co-polymer for optimal vertical composition profile. The comparison of absorbency indicates that the addition of small amount of block copolymer does not affect light absorption of blend film. The band diagram clearly illustrates the aggregation of PCBM near ITO substrate will block hole transport, leading to a poor hole mobility in the original structure. With ideal composition profile in the vertical direction, i.e. P3HT rich phase near the ITO anode and PCBM rich phase near cathode, hole mobility increases, resulting in significant improvement of power-conversion efficiency.
Key words: P3HT-PCBM, efficiency, vertical composition profile, hole transport

In this study, we demonstrate the introduction of imprinted periodic nanostructures can effectively improve electrical property of inverted polymer solar cells (PSCs). The introduction of nano-imprinted periodic nanostructures into the bulk heterojunction (BHJ) film composed of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) improved the internal quantum efficiency (IQE) and fill factor (FF). Formation of periodic nanostructure induced favorable vertical component distribution and more finely intermixed PTB7 and PC₇₁BM, enhancing the IQE of the inverted PSC significantly. Consequentially, the power conversion efficiency of PSCs with the periodic nanostructure was enhanced by approximately 12 % due to increased Jsc and FF compared to the flat PSC, suggesting the incorporation of periodic nanostructures on active layers of PSCs can be effective to control the nanomorphology of BHJ film.

It has been noted that an interfacial layer of organic solar cells (OSCs) in cascade structures could increase the charge transfer (CT) state energy, helping dissociate the bound CT state, minimizing exciton recombination at the donor-acceptor interface, and hence increasing internal quantum efficiency (IQE).[1], [2] We report enhanced IQE and absorption of parallel-like bulk heterojunction (BHJ) and trilayer OSCs in broad spectral regions by inserting multi-functional layers (MFL).[3] The inserted MFL has an energy level between main donor and acceptor levels, assisting exciton dissociation of the CT state. Furthermore, two donors having complementary absorption spectra yield higher and broader absorption efficiency. Experimentally, when a ClAlPc:C60 layer was inserted as an MFL on top of CuPc:C60, the short circuit current (Jsc) was improved from 10.35 mA/cm2 to 11.62 mA/cm2, leading to an increase of the PCE from 2.34 % to 2.71 %. In addition, when a ClAlPc layer was inserted as an MFL between CuPc and C60 in a trilayer OSC, the Jsc was enhanced from 5.04 mA/cm2 to 6.08 mA/cm2, resulting in an increase of the PCE from 1.33 % to 1.69 %.
[1] T. D. Heidel, D. Hochbaum, J. M. Sussman, V. Singh, M. E. Bahlke, I. Hiromi, J. Lee, and M. A. Baldo, J. Appl. Phys. 109 (10), 104502 (2011).
[2] C. Deibel, T. Strobel, and V. Dyakonov, Adv. Mater. 22 (37), 4097 (2010).
[3] J. W. Seo, S. H. Lee, J. Y. Lee, Appl. Phys. Lett. 103 (12), 123301 (2013).

The bulk heterojunction (BHJ) solar cells have been a very attractive research topic because of their advantages like light-weight, flexibility, low-cost, and simple fabrication for large processing area as renewable energy resources. However in the fullerene and polymer blend, due to comparatively low charge carrier mobility and short exciton diffusion length, the solar energy conversion efficiency of the devices is limited. Recently, the use of surface plasmon resonance (SPR) effects to scatter light has attracted much attention as a means for increasing the photocurrents of OPVs. We report improved performances of organic solar cell obtained by focusing on the effect of various kinds of metal oxide nanoparticles in the buffer layers. PEDOT:PSS (Poly(3,4 ethylenedioxy-thiophene) poly(styrenesulfonate)) used as a buffer layer was mixed with MoO3 (molybdenum oxide), Fe2O3 (iron oxide), or WO3 (tungsten oxide) nanoparticles (NPs). We studied the effects of the concentrations of metal oxide NPs (100 nm) from 0.1 to 2wt% in the PEDOT: PSS buffer layers on the solar cell performance. Increasing concentrations of all NPs showed good light absorption ability, but comparatively MoO3 NPs showed better properties. The three different oxide nanoparticles have different energy band. For MoO3 NPs, the lowest unoccupied molecular orbital (LUMO) level is 2.3 eV and highest occupied molecular orbital (HOMO) level is 5.3 eV. For Fe2O3 and WO3, the LUMO levels are 4.78 eV and 5.24 eV whereas the HOMO levels are 6.98 eV and 7.94 eV, respectively. MoO3 acts as an electron-blocking layer between the active layer and silver electrode. Due to the reduction of recombination, the short circuit current density increased. However, Fe2O3 and WO3 may prevent the transport of the holes. A stoppage in holes migration reduced the current density directing to a lower PCE. We also validated the role of metal oxide NPs in improving the electrical properties by analyzing the external quantum efficiency (EQE), charge collection probability and reduction in the recombination loss.

A simple, environmentally friendly approach for the synthesis of gold nanoparticle-decorated graphene oxides (AuNP-GOs) is reported herein. When these nanocomposites were employed as anode buffer layers in organic photovoltaic devices, surface plasmonic effects induced by the Au NP-GOs improved their efficiencies.

Since Grätzel and his colleagues reported the dye-sensitized solar cell (DSSC), it had attracted much attention due to its various merits, such as the relatively high efficiencies, simple device structures, facile fabrication, potential low cost, and variety and flexibility in applications. A typical DSSC consists of a transparent conductive substrate, a porous thin-film photoelectrode composed of TiO₂ nanoparticles, dye, electrolyte, and a platinum counter electrode. Each part of the cell has crucial effects on overall cell efficiency. The counter electrode, as one important component in DSSCs, is usually composed of a platinum conductive catalytic layer. The role of the conductive catalytic layer as the counter electrode of the DSSC is to catalyze the reduction of the I₃^- ions in the electrolyte. The requirements for the counter electrode in a DSSC are thus low charge-transfer resistance and high exchange current densities for reduction of the oxidized species, and good chemical/electrochemical stability in the electrolyte systems used in DSSCs.
Novel carbon materials such as graphene nanosheets have the advantages of high specific surface area, excellent conductivity, thermal stability, corrosiveness resistance, good catalytic activity, and low cost. Thus, graphene nanosheets are expected to be ideal materials as the counter electrodes for dye-sensitized solar cells and have the potential to replace high price platinum materials. In this paper, we investigated the fabrication methods and material characteristics of graphene nanosheets. Graphene nanosheets were prepared on substrates as counter electrodes for DSSCs by using spin coating method. Counter electrodes having graphene-platinum nanocomposites were characterized for their physical and electrochemical properties and were subjected to device studies to establish the correlation between graphene-platinum nanocomposites and DSSC characteristics. The photocurrent-voltage (I-V) characteristics of the DSSCs were measured under illumination of the simulated AM 1.5G solar simulator. The short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF) of the DSSC based on conventional platinum counter electrode are 14.93 mA/cm², 0.70 V and 0.69, respectively, yielding an overall conversion efficiency of 7.21%. Under the same conditions, the DSSC based on graphene nanosheets-platinum counter electrodes show enhanced Jsc and conversion efficiency of (16.68 mA/cm², 8.06%). Power conversion efficiency of the DSSC was enhanced by ~12% with using the graphene nanosheets-platinum counter-electrodes.

Low band gap semiconducting polymers are found to be interesting in applications such as organic electronics and photovoltaics due to its tunable optical properties and high processability in common organic solvents. Herein, a sereis of π-conjugated low bandgap copolymers based on 4,4&’-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moiety as donor and a few acceptors were successfully prepared by palladium-catalysed Sonogashira polymerization. All polymers were characterized using 1H NMR spectroscopy, Gel Permeation Chromatography (GPC), UV-visible absorption spectroscopy and fluorescent spectrometer. It was found that all polymers possess high average molecular weights (Mn) ranging from 16 000 to 89 000 g mol-1 and good solubility in common organic solvents. The polymers absorb light in deep-red region with maximum absorptions in the range between 630 nm to 760 nm in solution and exhibit significant red-shifts (up to 70 nm) in solid-state thin films. Polymers showed a narrow optical bandgaps of 1.35 eV, which is significantly lower than the corresponding BODIPY monomer and homopolymers. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of polymers were estimated from cyclic voltammetry (CV) data. The LUMO energy levels of BODIPY-based alternating copolymers were found to be independent of acceptor choices, which suggest that the major factor on tuning LUMO energy levels of the polymers could be the nature of BODIPY core.
Acknowledgement: CPS thanks the National University of Singapore for a research scholarship and SV acknowledges the Department of Chemistry and National University of Singapore for funding and technical support.

Since charge separation in organic photovoltaics takes place between n- and p-type semiconductors, their interface should be maximized within the active layer, while charge percolation pathways to the electrodes should be ensured. In this regard, “bulk heterojunction” materials prepared by mixing n- and p-type semiconductors contain a metastable interpenetrating network of both components, with a morphology that is difficult to control. Thermodynamically stable “ordered heterojunctions” with direct charge percolation pathways may be obtained from covalently linked n- and p-type semiconductors, provided a sufficiently high internal order is achieved. To this end, we prepared a perylene-quaterthiophene-perylene triad substituted with poly(isobutene) segments that induce order at the nanoscale via microphase segregation. The relation between molecular packing, phase morphology and charge separation under illumination was investigated by steady state and transient absorption spectroscopy, atomic force and transmission electron microscopy, and grazing incidence X-ray diffraction. The material showed lamellar structures in thin films, in which the π-conjugated cores give rise to spectroscopic aggregates. Photoluminescence was quenched due charge separation within the triad. In solution, these charges quickly recombined, but were shown to have longer lifetimes in films, which is beneficial for charge collection in a photovoltaics device.

Dialysis in water is commonly used for purification of conjugated polyelectrolytes to remove low molecular fractions or inorganic salts. We recently synthesized an anionic, narrow band gap conjugated polyelectrolyte, poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b&’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBTSO3K) via Suzuki polymerization, and it was found to be doped after dialysis. The doping level can be further tuned via additions of acids or bases. Formation of radical cations (polarons) is supported by studies of UV-Vis-NIR absorptions, electron paramagnetic resonance spectroscopies, and electrical conductivity measurements. The proposed doping mechanism involves double protonations of the polymer backbone, followed by electron transfer from a neutral chain, to generate radical cations in both polymers, which are stabilized by the pendant sulfonate anions by Coulombic stabilization. That PCPDTBTSO3K is doped under acidic conditions that leave its cationic counterpart (PCPDTBT-Pyr+BIm4-) unperturbed highlight the importance of the pendant group charges. Effects of the specific structural unit, cyclopentadithiophene (CPDT), on this facial doping process are studied. We found that other polymers with CPDT units, which provide low oxidation potentials, can also be doped during dialysis. More importantly, this doping process provides a pH-neutral conductive polymer with comparable electrical conductivity as the widely used PEDOT:PSS. Although PEDOT:PSS is commonly used in organic electronic devices, its strong acidity is known to be harmful for metal contacts. The successful applications of PCPDTBTSO3K as hole-transporting layers replacing PEDOT:PSS in organic solar cells will also be discussed. The devices perform comparable or better than those with PEDOT:PSS.

We demonstrate the high efficiency inverted organic solar cells of Glass/ITO/LZO/P3HT:ICBA/PEDOT:PSS+WOx/Al. The devices with post annealing exhibit improved photovoltaic performance with high Voc of 0.86V, high Jsc of 9.37mA/cm2 and high FF of 68.8% along with 5.54% PCE, which was achieved after 150C annealing. It is noted that the fabricated cell exhibited Voc of 0.81V, Jsc of 8.68 mA/cm2, FF of 64.0% and PCE of 4.47%. The dark current density is also decreased by one order of magnitude by post annealing.

We utilized an innovative single-cantilever beam testing technique to investigate the role of the molecular weight (MW) of the photoactive polymer poly(3-hexylthiophene) (P3HT) on the temperature dependent decohesion kinetics within organic photovoltaics (OPVs) containing a bulk heterojunction (BHJ) layer. Decohesion kinetic behavior is an important indicator of thermomechanical stability and reliability for OPVs and both MW and temperature have proven extremely important to device performance. The MW is directly coupled to charge carrier field effect mobilities and external operating temperatures affect photoactive polymer arrangement within the BHJ layer. Surface analysis reveals that even under inert environmental conditions, at room temperature with mechanical loads well below the threshold for critical failure, mechanical decohesion through the BHJ layer readily occurs. By increasing the MW of P3HT, the BHJ layer gains a greater resistance to mechanical decohesion due to an increase in plasticity, but it still proves to be the weakest layer within the device stack. It was also determined that the decohesion kinetics were sensitive to ambient temperature, with the general trend of increased decohesion rate with increasing temperature. However, when the device temperature was increased above the glass transition temperature (~41oC) of the BHJ layer, decohesion towards the BHJ layer adjacent interfaces was more favorable. Our study aims to understand and assess the fundamental parameters that affect the long-term mechanical reliability of OPVs which will be pertinent to their implementation and commercialization.

An organic solar cell based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) doped poly[N-9‘‘-hepta-decanyl-2,7-carbazole-alt-5,5-(4‘,7‘-di-2-thienyl-2‘,1‘,3‘-benzothiadiazole) (PCDTBT) was fabricated. By incroprating a thin layer TFSA of 10 nm instead of poly(3,4-ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS), We found the hole mobility (mu;h) of PCDTBT increased significantly two orders from 1.15×10E-5 cm2/Vs to 1.23×10E-2 cm2/Vs and results in a largely enhanced device performance from 4.70% of the control sample to 5.98%. Our findings suggest that TFSA could functions not only as a doping molecular but also may acting as an potential anode interfacial layer to replace PEDOT:PSS.

The use of additives to improve the polymer/fullerene active layer morphology for enhancing the performances of bulk heterojunction photovoltaic devices has been extensively adopted in the recent years. The additive usually has a higher boiling point and is a better solvent for fullerene but poorer solvent for polymer, as compared to the processing solvent. The presence of a small amount of additives can affect the active layer morphology greatly for bulk heterojunction solar cells both from the kinetic and from the molecular origin during the processing and therefore influence the devices&’ performances. In this study, we used in-situ synchrotron grazing incidence small- and wide-angle X-ray scattering to monitor and to elucidate the crystallinity of the polymer poly-{bi(dodecyl)thiophene-thieno[3,4-]pyrrole-4,6-dione} (PBTTPD) and the aggregation of fullerene PC71BM cluster, respectively, during the solvent evaporation process in the presence an additive, 1,6-diiodohexane (DIH), with different concentrations. We also characterize the final morphology of the active layers that were processed at different additive concentrations with transmission electron microscopy. We found that the additive can reduce the aggregation of PC71BM cluster during the evaporating process and results in PBTTPD with higher degree of crystallinity substantially due to the slower evaporation rate of additive with an optimum concentration around 1 wt% .

It is common knowledge that the polymer conformation and its phase separation with fullerene derivatives are delicate issues crucially impacting on the photovoltaic parameters of polymer based solar cells. Whereas strongly intermixed polymer:fullerene phases provide large interfacial area and consequently a high quantum efficiency of exciton dissociation, pristine and primarily ordered polymer and fullerene domains support efficient charge transport and percolation. To study the aggregation and phase separation behaviour in polymer solar cells we investigated counterbalancing influences of polymer solution concentration and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) blending ratio on the basis of a semi-crystalline anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) copolymer statistically bearing either branched 2-ethylhexyloxy or linear octyloxy side-chains (AnE-PVstat). The polymer aggregation varied with both, the solution and the PCBM concentrations, yielding a specific maximum within the parameter range studied. We explicitly demonstrate the counterbalancing effect on charge generation and transport for increasing polymer aggregation, as predicted in theoretical considerations. Furthermore the influence of polymer aggregation on fundamental optoelectronic properties is discussed, providing detailed understanding of resulting photovoltaic parameters.

Conjugated polymers have been extensively investigated over the last few decades due to their potential applications in organic electronics such as organic photovoltaic cells (OPVs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). During the synthesis of the conjugated polymers, transition metal based materials are often used as catalysts. Residual catalysts and other organic or inorganic impurities present in the conductive polymers may act as charge traps or photoquenchers affecting strongly their intrinsic properties consequently reducing the performance of the optoelectronic devices.
In this work, we aim to investigate methods to remove metallic impurities and compare the device performances in OPVs. Here, as a preliminary investigation, high molecular weight P3HT and PCPDTBT polymers were synthesized by the Grignard metathesis method (i.e. using a Ni-based catalysis) and Suzuki cross-coupling (i.e. using a Pd-based catalysis) polymerizations processes respectively [1,2]. The polymers were purified by different purification methods: precipitation, complexation Soxhlet extraction respectively [3,4]. After each purification process, a fraction was analyzed to determine its composition in impurities by various techniques such as size exclusion chromatography (SEC), proton nuclear magnetic resonance spectroscopy (1H-NMR), Inductively coupled plasma mass spectrometry (ICP-MS), Rutherford Backscattering Spectroscopy (RBS) and Particle Induced X-ray Emission (PIXE). In addition, the effect of impurities on electro-optical properties of the studied semi-conducting polymers was analysed via the fabrication of organic bulk heterojunction solar cells (OSCs).
[1] Iovu,M.C. et al. Polymer 2005, 46, 8582-8586.
[2] Brabec et al., Adv. Mater. 2006, 18, 2884-2889.
[3] Xiao, S. S., Qiu, C., Qiu, C. X. , United States Patent Application Publication, US20040254336A1.
[4] McCullough, R. D., Iovu,M.C., WO 2008/063731 A2.

Here we investigate the lifetime and degradation mechanisms of Organic Photovoltaic (OPV) devices, based on solution processed MoOx hole transport layers (HTLs) deposited in air, using a number of techniques. The effects on device lifetime of thermally annealing the solution processed MoOx are studied, and are compared to other devices that utilise thermally evaporated MoOx or PEDOT:PSS HTLs.
Introduction
MoOx has been studied extensively as a HTL in OPVs due to its lower lying valence band (in the region of -5.30 to -6.86 eV [1]), which makes it more compatible with low energy gap polymers. MoOx and other transition metal oxides used as hole, or electron, transport layers are often deposited via thermal evaporation. Unfortunately, for low cost and large scale production of OPVs, thermal evaporation steps need to be avoided and, therefore, alternative methods of deposition need to be found.
Solution processing of metal oxide films presents a promising alternative, as this process offers the potential for low temperature, low cost manufacturing. Several groups have reported some success with solution processed MoOx HTLs [2-4], though many require high temperature annealing to form the film.
The method used herein uses a facile synthetic process based on work by S. Murase [4]. A precursor of Ammonium Molybdate Tetrahydrate was dissolved in DI water and Acetonitrile - thermal decomposition of which forms water, gaseous ammonia and MoOx in solution. Thin layers (~5nm) of MoOx were produced by spin coating in air, and were left unannealed or were thermally annealed in air. OPV devices based on an PCDTBT/PC70BM active layer were fabricated using these MoOx HTLs, and their efficiency, lifetime and degradation mechanisms were measured by a combination of device characterisation and impedance spectroscopy.
Results
Initial results show that OPV devices made with a solution processed MoOx HTL and a PCDTBT/PC70BM active layer can achieve efficiencies of up to 5.5%. This is comparable to devices with a thermally evaporated MoOx or PEDOT:PSS HTL.
Preliminary lifetime data shows that devices incorporating solution processed MoOx have shorter lifetimes than devices with thermally evaporated MoOx, with devices with thermally annealed solution processed MoOx having improved lifetimes over those with unannealed solution processed MoOx. PEDOT:PSS gives poorer device lifetimes than thermally evaporated MoOx but is comparable to solution processed MoOx.
Impedance spectroscopy has also been used to explore the mechanisms that result in device degradation and poor device lifetimes.
[1] I. Irfan, A. James Turinske, Z. Bao, and Y. Gao, Appl. Phys. Lett. 101, 093305 (2012).
[2] L. Chen, P. Wang, F. Li, S. Yu, and Y. Chen, Sol. Energy Mater. Sol. Cells 102, 66 (2012).
[3] C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B.P. Rand, ACS Appl. Mater. Interfaces 3, 3244 (2011).
[4] S. Murase and Y. Yang, Adv. Mater. 24, 2459 (2012).

The development of new organic semiconducting materials for organic photovoltaic applications has been the focus of considerable research in the past several years. One of the most highly investigated class of materials are phthalocyanines because of their low energy gaps, high extinction coefficients, and high hole mobilities. However, the poor solubility of phthalocyanines has limited device fabrication options to thermal evaporation techniques, and hence, they are incompatible with the potentially cheap solution processing techniques and the state-of-the-art solution processed acceptor materials. One recently explored class of materials, azatetrabenzoporphyrins, are isoelectonic isomers of phthalocyanine (i.e. 6-aza-13,20,27-triphenyltetrabenzoporphyrin, 6,13-D\diaza-20,27-diphenyltetrabenzoporphyrin, 20-diaza-13,27-diphenyltetrabenzoporphyrin, 6,13,27-triaza-20-phenyltetrabenzoporphyrin), in which some the nitrogen atoms at 6, 13, 20,27position in the phthalocyanine ring are replaced by carbon atoms. Like phthalocyanines, these materials have broad and strong absorption and high hole mobility, but also have the possibility of modifying the structure to improve the solubility in some common solvents, making them promising candidate as solution-processable donor materials for organic photovoltaic applications. However, previous reports have shown that achieving an appropriate synthetic yield of metal azatetrabenzoporphyrins for use in organic photovoltaic devices remains challenging. In this presentation, we introduce a new method of synthesizing azatetrabenzoporphyrins with various functional groups in high yield as well as the development of azatetrabenzoporphyrins based oligomers and polymers. The oligomers and polymers exhibit strong and broad absorption in the visible region and are compatible with solution processing techniques with common acceptor materials such as PCBM and its analogs.

Two novel polymeric semiconductor materials based on dithieno[3,2-b:2&’,3&’-d]phosphole, PDDTP-BDTTR and PDDTP-BDTSeR, were designed and synthesized for use in polymer solar cells. PDDTP-BDTTR contained thiophene substituted benzo[1,2-b:4,5-bprime;]dithiophene (BDT) units, and PDDTP-BDTSeR contained selenophene substituted BDT units. The photophysical and electrochemical measurement demonstrated that these two conjugated polymers had small optical band gaps (<1.6 eV), broad absorption, in the wavelength range of 300minus;800 nm, and low highest occupied molecular orbital (HOMO) energy levels (minus;5.60 eV). The new polymer semiconductors exhibited very promising optoelectronic performance. The field-effect mobilities of the holes varied from 6 × 10minus;4 cm2 Vminus;1 sminus;1 in PDDTP-BDTTR to 3 × 10minus;3 cm2 Vminus;1 sminus;1 in PDDTP-BDTSeR. Under optimized conditions, the dithiophenephosphole-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 4.04-5.06%. Among these copolymers studied here, PDDTP-BDTTR showed the best photovoltaic performance, with an open-circuit voltage (Voc) of 0.84 V, a short-circuit current density (Jsc) of 12.75 mA cmminus;2, a fill factor (FF) of 0.55, and a power-conversion efficiency of 5.06%, which is the champion efficiency in dithiophenephospole containing conjugated polymers.

Surface acoustic wave (SAW) atomizer is known as a promising deposition technique due to its simple and low cost experiment setup. In addition, govern by its high operation frequency (< 5MHz), SAW atomizer could possibly generate up to sub-microns size of uniform aerosol. These merits are therefore suitable for the low cost fabrication of uniform thin film. Here, we show the possibility of depositing Poly(3,4-Ethylenedioxythiophene)-Poly(Styrenesulfonate) (PEDOT-PSS) particles using standing wave surface acoustic wave and electrostatic deposition for transparent conductive film application through some parametrical studies (i.e. varied applied voltage, electrode distance, and particle concentration). Furthermore, the characterization of SAW atomizer performances (stability and throughput) are also provided. Ultimately, in this study we suggest the optimum condition for the deposition process.

Current efforts on a-Si based thin-film solar cells are directed towards the elimination of undesired scattering or reflection behaviors by maximizing the amount of light absorbed inside the absorbance layer. Light-trapping techniques utilized to this end have their own distinctive set of useful optical features. However, many such techniques are difficult to utilize in practical applications, and while light-trapping device schemes based plasmonics, surface texturing and photonic crystals effects have been documented, such designs are obtained by expensive fabrication techniques with low speed and unsuitable for mass production. Therefore, the main challenge associated with light-trapping apparatuses is not the design these structures, but their production in scales and costs demanded by the practical applications of solar cells.
In this study, low-refractive index all-polymer nanostructures are used in order to accumulate solar radiation. One dimensional nanostructures, and especially core-shell nanowires, are easily scalable up to macroscopic sizes, display a natural lattice arrangement mediated by the shell region, and are capable of broad angle perception, which makes them particularly advantageous for the efficient capture of solar radiation. The synthesis of core-shell nanowires are achieved by recently developed home-built top-down approach called “Iterative size reduction” (ISR). ISR is capable of producing large-area, indefinitely long, uniform, flexible, self-aligned core-shell nanostructures. Fabricated all-polymer core-shell nanowires are engineered to feature two complementary enhancement techniques in the same structure, such that absorption enhancement is caused by both non-resonant form of scattering and the textured surface effect. An overall enhancement of ~14% can be achieved in amorphous silicon solar cells. This value reaches around 20% when crystalline silicon is used as the absorbent material. Local enhancements are observed to exceed 300% in both cases. As a result, this technique facilitate the straightforward and cost-effective fabrication of high-throughput all-polymer nanowires as a novel light trapping platform.
[1] M. Yaman, M. Bayindir, et al., Nature Materials 10, 494 (2011).

In this paper, a novel low band gap benzodifuron and diketopyrrolopyrrole based donor-acceptor (D-A) conjugated polymer (BDF-DPP) is designed and synthesised. Optical, electrochemical, thermal stability of the polymer in addition to the device performances for polymer solar cells were investigated. Higher thermal stability was observed with the onset of decomposition temperature (Td) at ~300 °C. Broad range absorption of 300 - 900nm was observed for the polymer in film state. Polymer exhibited HOMO energy level (-5.1 eV) and with a band gap ~1.3 eV. The presence of conjugated alkylthienyl chains resulted in higher planarity, evidently improved solubility of the polymer and leading to finer phase separation morphology as evidenced by atomic force microscopy and transmission electron microscopy. Photovoltaic performance of the polymer is investigated by fabricating bulk-heterojunction polymer solar cell (PSC) device. Further, in order to increase the efficiency and light harvesting in single bulk-heterojunction solar cells, ternary organic solar cell devices were evaluated. The photovoltaic properties of the ternary organic solar cells are evaluated with a configuration (P3HT:BDF-DPP:PCBM) with various weight ratios. Preliminary results indicate a significant increase in the ternary device performance.

Solution-processed organic semiconductors to replace amorphous silicon as the active layer in field-effect transistors (FETs) have attracted great attention due to their potential applications in flexible and low-cost E-paper, smart card, radio frequency identification (RFID), and displays. Although polymer-based OFETs are easily processed using solution-based methods, carrier mobilities in polymer devices are limited due to poor polymer packing and defects on a macroscopic scale resulting from weak intermolecular van der Waals forces. The limited carrier mobilities in thin films composed of polymeric semiconductors can be improved in several ways. A central feature of high-performance OFETs composed of polymeric semiconductors is the presence of strong intermolecular interactions among adjacent donor-acceptor copolymer.
We reported the synthesis of novel polymeric semiconductors P1 for the fabrication of soluble polymer transistor, these polymers with high yield were synthesized by Stille cross coupling reactions. The polymers were soluble in chloroform, chlorobenzene, o-dichlorobenzene. The physical properties of polymers were also characterized by UV-visible, cyclic-voltametry (CV), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and X-Ray diffraction (XRD) studies. The filed-effect mobility of OFET devices based on P1 by sping-casting was found to be 1.0 ~ 1.25 cm2/Vs.

Bandgap engineering at the interface between inorganic electrodes and organic semiconductors is crucial in organic optoelectonic devices. To date, various interfacial dipole layers such as self-assembled dipole monolayer (SADM), ionic liquid molecules (ILMs), conjugated polyelectrolyte (CPE), alcohol/water-soluble conjugated polymer, and polar solvent have been exploited to reduce contact barrier between inorganic electrodes and organic semiconductors and promote enhanced carrier transport and extraction. Nevertheless, the current state-of-art OLEDs still reveal considerable room for performance improvement. Here, we demonstrate highly efficient inverted polymer solar cells with power conversion efficiencies of 8.69% by simple surface treatment using polar solvent including ethanol amine. The interfacial dipolar polarization, which comes from interactions between ZnO surface and ethanol amine end groups such as amine group and hydroxyl group to make bonds at mono layer, reduces the contact barrier after surface treatment of ethanol amine on ZnO-R layer. Various measurements such as ultraviolet photoelectron spectroscopy (UPS), scanning Kelvin probe microscopy (SKPM), charge carrier mobility obtained by Mott-Gurney space-charge-limited current (SCLC) equation, electrical impedance measurements, etc support these high efficiencies in PSCs. This method provides a promising method, and opens new possibilities for not only the organic photovoltaics but also other organic optoelectronic devices such as organic light-emitting diodes (OLEDs), organic thin film transistors, and organic laser diodes.

Thin films to used as optical sensors were prepared by using the layer-by-layer (LbL) self-assembly deposition of cationic poly(diallydimethylammonium chloride) (PDADMAC) and metallic nanoparticles modified with water soluble polyaniline. The metallic nanoparticles were prepared by chemical reduction of silver salt with sodium borohydride in the presence of various concentration of water-soluble polyaniline used as capping agent. Three types of copolymer with different ratios of styrene sulfonate to maleic acid were used in the well know interfacial polymerization of aniline to produce three kinds of water soluble PANI. The optical and electrical properties of polyaniline solution, the silver nanoparticles and the thin films assembly were measured by ultra violet-visible spectroscopy (UV-Vis). The electrical properties of the nanoparticles composite films were evaluated by 4-point probe setup while the thickness of films was measured by atomic force microscopy (AFM). The results showed that part of sulfonate and carboxylic group could provide water-soluble polyaniline and metallic nanoparticles, respectively and be used in the stabilization of the silver nanoparticles. The films optical properties and conductivity were sensitive to pH change when exposed to volatile amine compound. These films could be used as sensors for food quality.

Triplet exciton harvesting is a key issue for high efficiency organic light emitting diodes (OLEDs). Here, we demonstrate a novel pathway for efficient OLEDs by applying the general concept of thermally activated delayed fluorescence (TADF) to a host or guest with a small energy gap between singlet and triplet excited states. With TADF guest, we demonstrate a wide variety of high performance OLEDs. Also, through carrier recombination at a host molecule and formation of singlet and triplet excitons, we demonstrate a new mechanism. The triplet excitons are up-converted to the singlet state and singlet excitons in the host layer are then transferred to a fluorescent guest molecule via a Förster process, which then results in radiative decay from a singlet state of the guest molecule. Based on this concept, we demonstrate highly efficient fluorescence-based OLEDs. Based on these device architectures, we demonstrate high efficiency RGB and W emissions.

Functional materials research is one of the top priority strategic areas of development in science and technology in the 21st century. Organic and metal-organic molecules can serve as versatile building blocks for molecular-based functional materials; they can be rationally engineered and prepared, and their physical, mechanical and electronic properties are tunable with a proper understanding of structure-property relationships. Development of molecular-based materials is deemed to provide impetus and offers enormous potential for materials science research and development. In this presentation, various design and synthetic strategies together with the successful isolation of new classes of molecular materials will be described. A number of these compounds have been structurally characterized and shown to display rich chromophoric and luminescence behaviour. The luminescence properties have been studied and their excited state origins elucidated. Correlations of the luminescence behaviour with the electronic and structural effects of the compounds have also been made. Different approaches and assembly motifs have been employed to tune the electronic absorption and emission characteristics of these materials based on an understanding of their spectroscopic origins. Subtle changes in the microenvironment have been found to lead to drastic changes in both the electronic absorption and emission properties of some of these materials. Through rational design and various strategies, these molecular materials may find potential applications and functions as efficient triplet emitters in chemosensing as well as in small-molecule and solution-processable organic optoelectronics.

Organic light emitting diodes employing phosphorescent metal complexes have been widely studied over the past decade due to their ability to harvest 100% of triplet and singlet electrogenerated excitons and high external quantum efficiencies have been achieved for emission across the much of the visible range. However, achieving high power efficiencies and long device operational stability for deep blue emission remains a challenge. Such emission typically necessitates a high triplet energy which requires very large bandgap host and charge confining materials, significantly increasing the device turn on voltages and potentially reducing device operational stabilities. Recently, the development of organic emitters exhibiting thermally activated delayed fluorescence (TADF) process has demonstrated the potential to harvest both triplet and singlet excitons due to thermally activated triplet to singlet conversion. Nevertheless, the triplet to singlet conversion process is necessarily energetically unfavorable and the triplet excitons may decay non-radiatively due to the absence of a rapid phosphorescent emission process. Thus, in order to achieve efficient emission the deep blue region, the energy difference between the singlet and triplet energies must be small and the impact of using delayed fluorescent emitters to achieve deep blue emission with lower triplet energy is diminished. In this presentation we demonstrate the development and characterization of a series of Pd metal complexes capable of simultaneously exhibiting both phosphorescent emission from the triplet state and metal assisted delayed fluorescence (MADF) emission from the higher energy singlet state. Unlike heavy metal free emitters exhibiting TADF, a larger difference between singlet and triplet energies is permitted for emitters exhibiting MADF since any triplet excitons not converted the singlet state can emit through an efficient phosphorescence process, thus preserving the high efficiencies. These complexes exhibit high photoluminescent emission quantum yields and can be easily tuned to emit across the visible spectrum. Devices employing these complexes demonstrated very high external quantum efficiencies of 22% Peak EQE for the green emitting PdN3N and 26.5% Peak EQE for the blue emitting PdN1N. Furthermore, due to the lower energy of the triplet emission, these blue and yellow-green emitters are compatible with known stable devices structures and long device operational lifetimes were achieved.

Deep-blue fluorescent compounds are particularly important in organic light-emitting devices (OLEDs). We synthesized an donor-accepotor (DA) type blue-emitting compound, 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)benzene (BD3). For comparison, a non DA type compound, 1,4-bis(10-phenylanthracene-9-yl)benzene (BD1) and a weak DA type compound, 1-(10-phenylanthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)-benzene (BD2), were also synthesized. The twisted conformations of the two anthracene units in the compounds, confirmed by single crystal x-ray analysis, effectively prevent the π-conjugation, and the compound showed deep-blue photoluminescence (PL) with a high PL quantum efficiency, almost independent on the solvent polarity, resulted from the absence of intramolecular charge transfer state. The DA type molecule BD3 in a non-doped device exhibited a maximum external quantum efficiency (EQE) of 4.2% with a slight roll-off, indicating good charge balance due to the DA type molecular design. In the doped device with CBP host, the BD3 exhibited higher EQE than 10% with Commission International de L'Eclairge (CIE) coordinates of (0.15, 0.06) and a narrow full-width at half-maximum of 45 nm, which is close to the CIE of the High Definition Television standard blue.

Nanostructured electronic devices have received much attention due to enhanced or novel properties that can be induced from nanostructures employed in the electronic devices. Particularly, organic materials based devices can accommodate nanostructures with a relatively easier way than inorganic devices that require defect free high crystallinity. In many studies employing nanostructures such as nanoparticles, nanorods and nanostructured surface, liquid phase methods have been widely utilized, however, liquid based methods face problems such as possible contamination and exact positioning of nanostructures. Although gas phase evaporation methods can be used to generate nanoparticles and position at the certain location in organic electronic devices, thermal evaporation is known to damage organic materials that can deteriorate device performance. In this talk, aerosol methods are introduced for applications in the fabrication of optoelectronic devices such as organic light emitting devices and solar cells. Aerosol methods can not only effectively incorporate nanoparticles at the exact locations in organic materials without damage, but enable three dimensional nanostructuring on electrodes of the optoelectronic devices in a well ordered fashion. As examples, we present organic light emitting devices and plasmonic solar cells employing nanostructures such as nanoparticles, nanobumps, and three dimensional nanostructures that are generated, positioned and structured by aerosol engineering methods.

We herein propose an efficient, cost-effective method for producing a conducting thin film that combines efficient light extraction with enhanced stability in organic light emitting diodes (OLEDs). Continuous poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) film (PEDOT:PSS) with corrugated surface nanostructure was fabricated using an assembly of cross-linked polystyrene nanospheres (PS NSs) as template and was applied as a hole injection layer (HIL) in OLEDs. And, the 65-nm size PS NSs produce a random pattern of corrugated surface structures, which is ideal for enhancing light extraction without any spectral distortion and angle-dependence. For example, power and luminous efficiencies of the Ir(ppy)3-based solution-processable OLEDs with PS NS-PEDOT:PSS HIL were enhanced by factors of 2.0 and 1.4 respectively, compared to the control device. In addition, the improved device shows Lambertian emission pattern. It is also demonstrated that the PS NS-PEDOT:PSS HIL is highly effective in improving the ambient stability of solution-processable OLEDs. The continuous PEDOT:PSS domain in the HIL can be formed by utilizing significantly reduced amount of PEDOT:PSS (only 25 % compared to prisitine PEDOT:PSS film), which alleviates degradation induced by the acidic and hygroscopic nature of PEDOT:PSS. Stability enhancement by the factor of up to four is thus demonstrated in the proposed devices over the control device.

Due to the refractive index mismatching between layers in organic light emitting diodes (OLEDs), the external quantum efficiency (EQE) is typically about 20-25%. In order to overcome this limitation, high-refractive-index substrates were used to remove the waveguide mode. However, the efficiency is still limited by of the loss of surface plasmon (SP) mode at the interface between the organic and metal layers. While it is relatively easy to extract the substrate modes, the grand challenge in OLEDs is how to extract the surface plasma modes. In this work, we demonstrate that a Bragg diffraction grating structure with a broad distribution of periodicity can be exploited to extract the SP mode, resulting in an EQE exceeding 60%.
Phosphorescent tris(2-phenylpyridine) iridium [Ir(ppy)3]-based OLEDs were fabricated on corrugated sapphire substrates with periodicities between 0.3 and 0.5 um. Using a macrolens to extract the substrate mode, an EQE up to 60.8% was achieved. Because of the “defects” present in the gratings, the resulting electroluminescence is unchanged with an emission profile very close to a Lambertian emitter. From the emission characteristics of the OLEDs fabricated on these corrugated structures, we conclude that the use of corrugated structures on high-refractive-index substrates is very effective to extract the SP mode.

White organic light emitting diodes (WOLED) have received increasing attention over the past decade as a potentially cost effective, efficient, and high color quality replacement for incandescent or fluorescent light sources. A wide range of approaches to achieve white light from various emissive materials have been developed. In particular, multiple emissive layer structures with red, green and blue emissive dopants have drawn the most attention due to the high efficiency and high color quality of light possible with three separately tunable layers. However, the efficiency, color quality, and stability of these devices are still far from commercial application. Therefore more improvements must be made in both materials design and device architectures, especially through the development of new emissive materials. To date, most research into emissive materials has focused on Iridium based complexes. Platinum complexes, on the other hand, have received significantly less attention for WOLED applications, despite demonstrated performance of select complexes that is similar or superior to their iridium counterparts. In this presentation the development and device fabrication of red, green, and blue tetradentate Pt complexes will be shown. Devices of each individual emitter were fabricated to yield peak external quantum efficiencies (EQE) of 18.0%, 23.0%, and 21.2% for PtN3N-ptb (red), PtOO8 (green), and PtON1 (blue) respectively. These particular emitters were selected for use in WOLED devices since theoretical combinations of these emitters allow for a CRI as high as 82. WOLED samples were prepared in a device structure of three separate emissive layers within the same host material. Various thicknesses, dopant concentrations, and layer orders were studied to precisely control the charge balance and energy processes occurring within the device. Ultimately an optimal structure of a thick blue doped emissive layer with thin red and green layers on opposing sides yielded the optimal color and efficiency. This optimized structure resulted in a device with peak external quantum efficiency of 21.7%, color rendering index (CRI) of 81, and CIE coordinates (0.34, 0.37). Furthermore, another green emitter was developed which may yield a CRI as high as 85.

In this work, we found a novel high triplet energy exciplex between di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) and 5',5''''-sulfonyl-di-1,1':3',1''-terphenyl (BTPS)1) for a blue phosphorescent OLED. Importantly, the exciplex has enough high triplet energy of 2.8 eV to be used in blue phosphorescent device. The OLEDs with a structure of [ITO (130 nm) / TAPC (30 - x nm) / 4,4,4-tris(N-carbazolyl)triphenylamine (TCTA) (x nm) with x = 0, 5 nm / 11wt% iridium(III) bis[(4,6-difluorophenyl)-pyridinate-N,C2']picolinate (FIrpic) doped BTPS (10 nm) / 3,3'',5,5''-tetra(3-pyridyl)-1,1';3',1''-terphenyl (B3PyPB) (50 nm) / 8-quinolinolato lithium (Liq) (1 nm) / Al (100 nm)] were fabricated. The OLED with an exciplex showed much lower driving voltages despite the large hole injection barrier of 1.1 eV between TAPC and BTPS. Owing to a barrier free hole-electron recombination mechanism, the extremely low driving voltage of 2.9 V at 100 cd m-2 was achieved, thereby leading to a high eta;p,100 of 50 lm W-1(eta;c,100 = 46.1 cd A-1, eta;ext = 21.7%)2). While, the OLED with TCTA (5 nm) gave an power efficiency (eta;p,100) of 45.5 lm W-1 (eta;c,100 = 47.9 cd A-1, eta;ext,100 = 22.1%) with an driving voltage of 3.32 V at 100 cd m-2.
Reference:
(1) H. Sasabe, Y. Seino, M. Kimura, J. Kido, Chem. Mater., 2012, 24, 1404.
(2) Y. Seino, H. Sasabe, Y. J. Pu, J. Kido, Adv. Mater. accepted.

Organic semiconductors such as small molecules and conjugated polymers have been demonstrated as the active material in light-emitting diodes, transistors and photovoltaic cells. While innovation in new organic semiconductor materials and solution-processing techniques continues to improve the performance of organic devices, further research is required to gain crucial insights into the fundamental relationships between structure and optoelectronic properties and inform future design strategies. To achieve this goal, two main challenges must be overcome: (1) establishing an accurate and robust control over the microstructure of organic thin films and (2) developing techniques to characterise the thin-film properties at a sufficiently high resolution. In this talk, I will discuss the key advances on our understanding in these two main challenging areas. First, I will introduce a simple solution-deposition technique to print organic semiconductors in thin films with controlled structure and morphology. Second, I will demonstrate Raman spectroscopy as one of the most valuable structural probes for organic semiconductor thin films, addressing the application of non-resonant, resonant and polarized Raman spectroscopy to the characterisation of reaction, composition, crystallinity and orientation of molecules [1-6]. Based on these studies, I will discuss the impact of organic semiconductor microstructures on optoelectronic and charge transport properties.
References
[1] James et al., ACS Nano (2013) http://dx.doi.org/10.1021/nn403073d
[2] Wood et al., J. Chem. Phys. (2013) DOI: 10.1063/1.4816706
[3] Tsoi et al., ACS Nano, 6(11), (2012), 9646-9656.
[4] Tsoi et al., J. Am. Chem. Soc., 133, (2011), 9834-9843
[5] James et al., ACS Nano 5 (12), (2011), 9824-9835
[6] Kim et al., J. Am. Chem. Soc., 130, (2008), 13120-13131

Polymer bulk heterojunction (BHJ) solar cells have attracted significant attention in industry and academia because of their potential for achieving large-area, light-weight, and flexible photovoltaic devices through cost-effective solution deposition techniques. These devices consist of a blend of an absorbing polymer and an electron accepting fullerene, the molecular packing and phase segregation of which heavily influence power conversion efficiency by effecting important processes such as exciton splitting, charge transport, and recombination. Understanding and utilization of molecular interactions to predicatively control the morphology across multiple length scales, ranging from the nano to the mesoscale, is critical for the future design of devices with increased efficiencies. At present there are a number of strategies used to control this complicated morphology including the use of solvent additives, mixed solvent systems, side chain tuning and molecular weight control. However, at present there is little understanding as to which strategies are most effective for a given donor:acceptor pair.
We have used solution phase small angle x-ray scattering (SAXS) in order to investigate the self-assembly behavior of several different donor systems. We examine how the use of solvent additives, mixed solvent systems, and molecular weight tuning can induce ordering of the donor moiety within the casting solution. Using transmission SAXS on the as-prepared active layers of these BHJ systems, we observe that conditions which induce aggregation and ordering of the polymer chains within the casting solution, promote an ideal degree of phase segregation in the active layer of the device, which leads to enhanced power conversion efficiency (PCE). However, in cases in which no ordering of the donor polymer is observed in the solution phase, over phase segregated active layers are observed with corresponding low PCEs. This leads us to conclude that aggregation and ordering of the polymer chains in the casting solution is directly related to the formation of an ideal three dimensional interpenetrating network, with phase segregated domains on the order of the exciton diffusion length.

Traditionally, fullerene derivatives are the most widely used components as electron-acceptors in bulk-heterojunction (BHJ) organic solar cells. However, limitations such as high cost of synthesis, long wavelength requirement for optimal light harnessing, restrict the chemical and energetic tunability and impose serious challenges for organic solar cells to reach wide applicability as high performance, cost-effective photon harvesting devices. In this work, we demonstrate record high power conversion efficiencies over 4% for all-polymer organic solar cells utilizing an n-type polymer as electron acceptor. Although still lower than fullerene based devices, these all-polymer solar cells provides intriguing opportunities for morphology control both through material selection and control of thin film processing conditions. By using a transmission electron microscope (TEM) tomography technique, the 3D morphology of the all-polymer solar cell is investigated for the first time. This study provides full visual guidance and establishes the foundation for theoretical simulation of the charge transport and bimolecular recombination mechanisms. Here, we demonstrate a sequence of morphology evolution through controlling the polymer-polymer miscibility and the kinetics of spinodal decomposition, thus pointing out viable strategies for further performance enhancement for all-polymer solar cells.

Organic photovoltaic (OPV) cells are a promising renewable energy technology due to their flexibility and low cost. Bulk heterojunctions (BHJ) are an important class of materials for OPV that consist of a complex mixture of an electron donating polymer and an electron accepting small molecule. The performance of these blends, however, is highly dependent on the film morphology. These materials are also subject to environmental degradation. We employ a set of novel techniques using lateral devices to study degradation within BHJ materials and the effect on key charge transport parameters such as the carrier mobilities, carrier concentration, and the recombination coefficient. Unlike traditional vertical structures that have been used to study these materials, lateral BHJ devices allow us to spatially probe the channel directly and measure electrical properties along the direction of charge carrier transport. Previous experimental studies utilizing in situ potentiometry have shown that lateral BHJ devices exhibit space charge limited transport over a range of several microns under appropriate biasing conditions. Two distinct space charge regions (SCRs) develop within the device channel, separated by a central zone where recombination is dominant and which does not contribute photocurrent. The carrier mobility ratio can be measured from the voltage across these SCRs, and the conductivity within the central recombination zone is used to determine carrier concentrations and the recombination coefficient. We combine simple two-terminal photocurrent and 4-point probe potentiometric measurements from lateral BHJ devices with different channel lengths to calculate all relevant photovoltaic parameters on a short time scale. We can use this method to monitor the degradation of P3HT:PCBM films under different environmental conditions and show the resulting change in important photovoltaic parameters. A variety of P3HT:PCBM solutions are tested to show the effect of solvent choice and fabrication methodology on the degradation rate. The key advantage of this method is that we can control the extent of trapping by bleeding in exact amounts of chemical vapors (including water, ethanol, ketones, etc.) which are known to produce charge trapping in organic semiconductors. Upon removal of these vapors, we can determine if the traps introduced are temporary or permanent. We can also quantitatively study the effects of trapping on charge transport and recombination. We present the first study on the temperature dependence of lateral mobilities in organic BHJ materials.

New n-type fullerene derivatives with designed structures were synthesized by Diels-Alder reactions, enabling interactions between fullerene and the addend moieties. The LUMO energy levels of the adducts are significantly higher than that of C₆₀ by 140-300 meV, depending on the type and the number of the addends. The raised LUMO levels lead to high open-circuit voltages (V_OC) in bulk heterojunction (BHJ) polymer solar cells (0.60-0.75 V). The short-circuit current densities (J_SC) were improved by chemical modification of the Diels-Alder products, which allowed the materials to have both significant V_OC and J_SC. One of the Diels-Alder products, when poly(3-hexylthiophene) (P3HT) was used as the donor counterpart, exhibited ~15% enhancement of power conversion efficiency (PCE) compared to conventional PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester):P3HT solar cells. A suite of optical and electronic techniques was used to investigate the origin of their high performance in solar cells.

The performance of organic solar cells consisting of a donor/acceptor bulk heterojunction has rapidly improved over the past few years.1. Major efforts have been focused on developing a variety of donor materials to gain access to different regions of the solar spectrum as well as to improve carrier transport properties.2 On the other hand, the most utilized acceptors are still restricted to the fullerene family, which includes PC61BM, PC71BM and ICBA.2b, 3 The high cost of fullerenes hinders the potential commercialization of organic solar cells at industrial scale. In comparison to the existing high performing donor materials, the molecule pool of acceptors is still extremely limited, thus inhibiting the further development of organic solar cell in fundamental research and practical application. To that end, the design of new classes of cost-competitive acceptor materials is crucial.
However, despite intensive research focused on creating an efficient all polymeric solar cells for the past decade, only three different recipes with PCE only reaching 2% have been reported.4 In recent months, there have been further improvements to the PCE which were up to 3.6%.5 The fundamental understanding of all-polymer solar cells is still in its inceptive stage regarding both the materials chemistry and structure physics. Thus, rational design rules must be utilized to enable fundamental materials understanding of the all polymer solar cells.
We report high performance all-polymer solar cells employing polymeric donors and acceptor. Phase separation of the two polymers were fine-tuned by changing backbone of polymeric donors. In fact, we observed a direct correlation between the short circuit current (JSC) and the phase separation size. Additionally, with side chain engineering of the polymeric donor, the phase separation size of the all polymer blends was further reduced. We obtained a record PCE of 4.2%(avarage from 20 devices), with an average JSC of 8.4 mA cm-2. The highest PCE shoot to 4.4%, with JSC as high as 8.9 mA cm-2, and VOC of 1.05 V. It is the highest performance ever published for an all-polymer solar cell.
1. Li, G.; Zhu, R.; Yang, Y., Nat. Photon. 2012, 6 , 153-161.
2. (a) Nelson, J., Mater. Today 2011, 14 , 462-470; (b) Lin, Y.; Li, Y.; Zhan, X., Chem. Soc. Rev. 2012, 41, 4245-4272; (c) Chen, J.; Cao, Y., Acc. Chem. Res. 2009, 42, 1709-1718.
3. Sonar, P.; Fong Lim, J. P.; Chan, K. L., Energy Environ. Sci. 2011, 4, 1558.
4. Facchetti, A., Mater. Today 2013, 16 , 123-132.
5. Zhou, E.; Cong, J.; Hashimoto, K.; Tajima, K., Advanced Materials 2013, ASAP

This presentation describes the synthesis of a series of poly(isoindigo-dithiophene) based conjugated polymers with varying amounts of low molecular weight polystyrene (PS) side-chains via random copolymerization. The PS side-chain-decorated copolymers also demonstrated better thin film processability, without affecting the electronic and optical properties when the molar content of the PS-containing repeating units were le;10%. Bulk heterojunction solar cell devices fabricated with these PS-containing copolymers demonstrated significantly improved performances [maximum power conversion efficiencies (PCE) >7% and open circuit voltages (VOC) ge;0.95 V. The synthesis, processing and device performances of PS-containing copolymers represent a new approach in molecular engineering to achieve a balance between the optical/electronic properties and solubility/processability of reproducible polymeric systems.

Bulk heterojunction (BHJ) solar cells are becoming one of the most promising low-cost solutions to our need for renewable energy sources.[1] Low band-gap conjugated polymers are a key component in BHJ solar cells. In this presentation, we report on the synthesis and photovoltaic applications of a series of conjugated polymers based on ladder-type acenes with the inclusion of fluorene, carbazole or thiophene units.[2-5] The band gaps and carrier transporting properties of these conjugated polymers can be easily tuned by introducing different heteroatoms (sulfur, nitrogen) in the ladder-type fused-ring system. These polymers were used as donor materials in both conventional and inverted BHJ solar cells. Solar cells based on the ladder-type acene containing polymers exhibit open circuit voltages (Voc) as high as 1.07 V, and power conversion efficiencies as large as 7.3%. At the same time, we also report on the interface engineering approach to improve the stability of organic solar cells. By using different semiconducting metal oxides as the cathode interfacial layer, inverted solar cells with improved performance and stability were achieved.[6-7] The best PCE of the inverted devices can maintain exceeding 90% of its initial efficiency over 180 days (Voc=1.05 V), demonstrating a long-term stability which is much better than that of the conventional devices.
References
[1] Z. Yin, Q.D. Zheng, Adv. Energy Mater., 2012, 2, 179.
[2] Q.D. Zheng, B.J. Jung, J. Sun, H.E. Katz, J. Am. Chem. Soc., 2010, 132, 5394.
[3] D. Cai, S-C. Chen, Q. Zhang, Q. D. Zheng, Z. Yin, Y. Sheng, D. Zhu, C. Tang, C-Z. Lu, J. Mater. Chem., 2012, 22, 16032.
[4] Y. Ma, Q.D. Zheng, Z. Yin, D. Cai, S-C. Chen, C. Tang, Macromolecules, 2013, 46, 4813.
[5] Q.D. Zheng, S-C. Chen, B. Zhang, L. Wang, C. Tang, H.E. Katz, Org. Lett., 2011, 13, 324.
[6] J. Huang, Z. Yin, Q.D. Zheng, Energy Environ. Sci., 2011, 4, 3861.
[7] Z. Yin, Q.D. Zheng, S-C. Chen, D. Cai, ACS Appl. Mater. Interfaces, 2013, 5, 9015.

A novel n-type material, decacyclene triimide (DTI), is utilized in bulk heterojunction organic solar cells as the first non-fullerene acceptor that works efficiently with both P3HT and well-known low band gap polymers. Modeled after the well-studied naphthalene and perylene diimides, DTI exhibits a deep LUMO level that maintains the thermodynamic driving force for electron transfer from a variety of donor polymers. The promising device performance is in part ascribed to the strong absorption of DTI in the visible region, which contributes substantially to the external quantum efficiencies of the polymer:DTI devices. Furthermore, time-dependent density functional theory calculations reveal the presence of low-lying excited states in the DTI anion, and similar low-lying excited states have been postulated to aid charge transfer to the most widely used acceptor, PCBM [1]. The power conversion efficiencies of the devices are as high as 1.5% and can likely be improved with additional refinement of the processing conditions.
[1] Liu, T.; Troisi, A. Adv. Mater. 2013, 25, 1038.

Semiconducting polymers based on the extended π-conjugated fused ladder unit Indacenodithiophene (IDT) have been successfully employed in organic field effect transistors (OFET). The polymer system indacenodithiophene-co-benzothiadiazole (IDT-BT) unusually exhibits a highly face-on orientation and high mobilities of greater than 3 cm2/Vs and threshold voltages of less than 10 V.1,2 It has been suggested that the planarity of the IDT unit enables a rigid co-planar backbone, increasing conjugation, and allowing facile transport along the chain; whilst the strong dipole of the BT moiety allows for close π-π stacking for interchain charge transport.3 One can envisage changing either the π-stacking or the co-planarity would alter device performances. The substitution of carbon and hydrogen for heteroatoms (e.g. nitrogen and fluorine) offers the possibility of changing torsion angles between the donor and acceptor units along the backbone. Furthermore, the π-stacking and dipole moments will be altered by the heteroatom incorporation.
Herein we present the synthesis of several novel IDT-BT based donor-acceptor polymers that feature heteroatom substitution of carbon and hydrogen. The discussion will be focused on the influence of the different heteroatoms on the microstructure and optoelectronic performances.

The long-term stabilities of polymer solar cells (PSCs) are key requirements for commercialization, but progress has been limited in this area. Most PSCs are not stable as subsequent exposure to heat which drives further macrophase separation at the micrometer scale, and environmental stress that causes the mechanical failure at the weak interfaces of the PSCs. Here, we present various molecular designs of electroactive polymers to improve the thermal stability of PSCs. For example, we developed a series of well-defined poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers, in which the P2VP chains had different molecular weights. P3HT-g-P2VP polymers can be used as efficient compatibilizers in the active layer of PSCs and can modify the sharp interface between polymer donors and fullerenes, resulting in a dramatic enhancement of thermal stabilities and the mechanical properties of PSCs. In the second part of talk, a template approach is presented for the fabrication of semiconducting polymer blend thin films that combine the properties of electrical conductivity and enhanced stability. Nanometer-sized monodisperse polystyrene nanospheres (PS NSs) were designed as an opal template for the formation of three-dimensionally continuous PEDOT:PSS films. The resultant films were successfully applied as an anode buffer layer to produce highly-efficient PSCs with improved ambient stability. Furthermore, the PS NSs can be used as interfacial modifiers and binders in the PEDOT:PSS film, which resulted in the enhancement of mechanical durability of the PSCs.

The effect of the ratio of a small molecule donor (X2) to PC61BM acceptor on the photovoltaic performance of thin film bulk heterojunction (BHJ) solar cells was systematically investigated. Usually, the D:A (donor/acceptor) blend ratio must be optimized within narrow limits to obtain the optimal performance and presumably the best morphology. For the X2:PC61BM, its best power conversion efficiency (PCE) is approximately 6.6% at 6:4 blend ratio, however it is insensitive to the blend ratio over the range from 7:3 to 4:6. The morphology of the blend films was therefore examined using the complementary techniques of energy filtered transmission electron microscopy (EF-TEM), resonant soft X-ray scattering (RSoXS) and grazing incidence wide angle X-ray scattering (GIWAXS) to gain insight into the arrangement and order of the individual components within the thin film. It is concluded that a strongly absorbing bicontinuous network of donor and acceptor can be maintained, which provides a reasonable explanation for the constant device performance. In addition, the morphological behavior of the blends keeps constant from 80% donor to 30%. It is proposed that this is due to its liquid crystal property which suppresses the coarsening of the each domain and crystallites inside.

Bulk heterojunction organic photovoltaics (BHJ OPVs) have been the subject of much academic and industrial interest because of their potential to serve as a flexible, low cost, scalable source of electrical power. These devices consist of a nanoscale phase separated, bicontinuous network of electron donating and electron accepting molecules. This nanoscale morphology is necessitated by the operating principles of a BHJ OPV device. Photoexcitation of either donor or acceptor produces a bound electron-hole pair known as an exciton. Excitons can be split into free charge carriers at the type II heterojunction formed between donor and acceptor provided the exciton can diffuse to this interface before decaying to the ground state. Once the exciton has split into a hole and an electron, these free charge carriers traverse through the donor and acceptor phase, respectively, and are collected at the electrodes. The BHJ morphology must therefore consist of domains small enough to allow for efficient exciton harvesting and charge generation, but large enough to form percolating donor and acceptor networks for the efficient extraction of free charge carriers. Accordingly, BHJ performance usually correlates with the presence of donor and acceptor domain sizes that do not significantly exceed the exciton diffusion length, approximately 10 nm.
In this work, the driving forces and processes associated with the development of phase separation upon thermal annealing are investigated in solution-processed small molecule BHJ OPVs utilizing a diketopyrrolopyrrole-based donor molecule and a fullerene acceptor. In-situ thermal annealing grazing incidence x-ray scattering is used to monitor the development of thin film crystallization and phase separation and reveals that blend phase separation strongly correlates with the nucleation of donor crystallites. Additionally, in higher performing devices, donor crystallization and the development of blend phase separation lead to dramatic increases in blend electron mobility and solar cell figures of merit. These results indicate that in the system under investigation, donor crystallization is the driving force for blend phase separation. It is hypothesized that donor crystallization from an as-cast homogenous donor:acceptor blend simultaneously produces donor-rich domains, consisting largely of donor crystallites, and acceptor-rich domains, consisting of previously mixed regions of the film that have been enriched with acceptor during donor crystallization.
This work therefore emphasizes the control of donor crystallization in solution-processed small molecule BHJ OPVs as an important strategy for the engineering of the nanoscale phase separated, bicontinuous morphology necessary for the fabrication of efficient BHJ OPV devices. Accordingly we further investigate the crystallization kinetics of the donor molecule in order to determine how donor crystallization proceeds as a function of both temperature and blend ratio.

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Here, we demonstrate that excellent hole injection/extraction properties can be achieved for organic photovoltaic devices and light emitting diodes via exposure to polar non-solvent methanol. Hole extraction rates were improved two-fold in ITO/PEDOT:PSS/PBDTTT-CT:PC71BM/Al organic photovoltaic devices, leading to a high power conversion efficiency of 7.4%. Similar enhancements were observed in PV devices using bare ITO as anodes. Hole injection efficiency and luminance intensity were also increased by 3 orders of magnitude in ITO/PEDOT:PSS/F8BT/Al polymer light emitting diodes, via methanol treatment. We show, using photoelectron spectroscopy and electroabsorption spectroscopy, that the methanol treatment provides an in-situ method of increasing the work function of polar anode surfaces. In many cases, this is sufficient to cause hole doping of the organic semiconductor via charge transfer from the higher work function anode. Where the HOMO level in the organic semiconductor lies deeper, increased work function in PEDOT:PSS reduces the hole injection barrier and enables near ohmic injection of holes from the modified contact. Spectroscopic evidence from ATR-FTIR indicates that the increase in work function originates from the removal of polar hydroxyl groups from the substrate interface. These findings open new routes to device interfacial optimization using facile solvent processing techniques, and pave the way towards low cost manufacturing of efficient organic optoelectronic devices.

A typical organic solar cell is characterized by a very thin absorbing active layer consisting of donor and acceptor, often not thicker than 100 nm. However, in the overwhelming majority of investigated systems fullerene-based acceptors are used that contribute only weakly to photon-harvesting - in combination with the low thickness only a limited fraction of incident photons can be absorbed. Therefore, increasing the absorption of the photoactive layer by using an absorbing acceptor is a rational strategy that could give rise to an enhanced photon harvesting and hence an improvement of the photocurrent. Among the various classes used as alternative acceptors are perylene diimides (PDI). They show high molar extinction coefficients, good photo-stability and offer versatile synthetic variations.
An encouraging example for a solar cells making use of PDI is the recently published p DTS(FBTTh2)2:PDI system that shows efficiencies as high as 3% for optimized devices. However, the efficiency depends strongly on the use of a solvent additive, namely diiodooctane (DIO), that pushes the efficiency by a factor of 30.
In this work the influence of DIO on the overall performance is investigated by sub-picosecond to millisecond pump-probe transient absorption spectroscopy (TA) in the visible as well as the near-infrared region as well as and time-resolved photoluminescence (TRPL) spectroscopy. The former tracks the dynamics of non-radiative species whereas the latter enables the observation of emissive decay channels.
Striking differences in the processes of charge generation as well as charge recombination are observed that allow offering convincing conclusions on the positive effect of DIO. Sharper peaks point towards a higher degree of crystallinity if DIO is used and processes get more intensity dependent, owing to a more efficient generation of free charges rather than bound charge carriers. Additionally selective excitation PDI allows for further conclusions. A delayed depopulation of p-DTS(FBTTh2)2 can be attributed to diffusion limitation which does not occur in samples without DIO, suggesting smaller domains when DIO is used which, in turn, facilitates geminate recombination. This trend is also continued in charge recombination where fitting of the data reveals a respectable fraction of free charges in the optimized sample, whereas the non-optimized blend suffers from a high share of bound charges. The results of this work contribute to a better understanding which fundamental processes are connected to the improvement of solar cell devices.

Photovoltaic tandem technology has the potential to boost the power conversion efficiency (PCE) of organic solar cells approaching or even exceeding 15 % according to several independently reported predictions. The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. The intermediate layer (IML) consisting of a hole and an electron transporting layer is considered to lie at the heart of organic tandem structure. To push organic tandem solar cells towards large-scale production and commercial applications, development of reliable and low temperature processed IMLs is required.
In this contribution, we demonstrate various fully solution-processed IMLs for efficient organic tandem solar cells with either a regular or an inverted architecture [1-3]. Owing to their excellent functionality and reliability, this kind of IMLs represents a convenient way for facilitating fabrication of organic tandem solar cells comprising of double, triple or quadruple junctions. Moreover, the tandem solar cells are fabricated by using doctor-blading deposition method at fairly low temperatures (<80 °C) in air, which are fully compatible with roll-to-roll mass production. The tandem solar cells based on glass and flexible substrates achieve a PCE of up to 8 % and 6 %, respectively.
Optical and electrical simulations are performed to analyze the performance of organic tandem solar cells. The outcome of simulations is in great agreement with experimental values, indicating again the excellent functionality of the low-temperature solution-processed IMLs. Based on optical simulations, we investigate the potential of commercially available donor materials for organic tandem solar cells. A maximum PCE of 10 % is achievable for tandem solar cells based on already commercially available donors in combination with PCBM. In addition, by incorporating experimentally feasible values of FF=75 % and constant EQE=80 % into efficiency prediction, a maximum PCE of 21 % is theoretically achievable for organic tandem solar cells.
[1] Li, N. et al. Adv. Energy Mater. doi: 10.1002/aenm.201300372 (2013).
[2] Li, N. et al. Energy Environ. Sci. doi: 10.1039/C3EE42307G (2013).
[3] Li, N. et al. submitted (2013).

High resolution ultrafast laser patterning in combination with the formulation and reliability study of inks is crucial for developing stable and efficient large area slot die coated devices for organic photovoltaics. So far we demonstrated to be able to achieve high performance (PCE > 3%) large area (3500 mm2) slot die coated modules based on P3HT:PCBM absorber and transparent large area (3500 mm2) modules with transparency above 70%, opening the way to achieve high performance tandem devices and modules by successfully combining laser structuring and formulations aspects for the printed active layer.
All this is enabled by being able to precisely control the film formation by tuning the solvent composition of the active layer solution. In this presentation we focus on investigating fundamental aspects of formulating semiconductor inks suitable for the large-scale production of high-efficient single junction and tandem devices and modules. These inks have to fulfill a dual purpose: on the one hand, film formation and relatively fast drying at temperatures < 100 C, and on the other hand, forming the right microstructure of polymer and acceptor in the active layer. To fulfill the need of high geometrical fill factors (GFF) and small dead area we will present our successful approach to ultrafast laser scribing: sub 10 micron resolution and high processing speeds (up to 4 m/s) lead to GFF higher than 90% on large area modules, confirming this method as the most promising way to achieve high throughput and roll to roll compatible organic photovoltaics production.

All of the current efficient organic photovoltaic devices have a bulk heterojunction (BHJ) structure. Regular BHJ films, formed from randomly mixed donors and acceptors in solution, inevitably have many breaks and dead ends, and have uniform distribution of donors and acceptors along the vertical direction which does not match the gradient electron and hole current across the films. The nonideal morphologies inevitably cause the charge recombination both in the BHJ films and at the metal/organic interface, in forms of geminate recombination, or bimolecular recombination. Compositionally graded BHJ films, donor enriched at the anode and acceptor enriched at the cathode side, were proposed to facilitate the charge extraction and to reduce charge recombination in several highly efficient OPVs. However, the graded BHJs with preferred composition profiles were only observed in a few high efficiency material systems which require special substrate surfaces and suitable surface energy differences between donors and acceptors for its formation. Here we report a universal method to form the compositionally graded BHJ using a simple solvent-fluxing treatment of the wet BHJ films, in which the fluxing solvent brings the additives, as well as fullerene-derivatives, toward the film surface. The graded BHJ significantly reduces bimolecular charge recombination. The fluxing process also resulted in more uniform nanodomain distribution in the plane of the films and higher charge carrier mobilities. An efficiency enhancement of 15~50%, with highest efficiency of 8.6% for poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-bprime;]dithiophene-2,6-diyl]-alt-[2-(2prime;-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T):C71-butyric acid methyl ester (PC70BM) system, was achieved for all of the solution-processed BHJ systems studied compared to those devices with regular BHJ films.

The transformative potential of bulk heterojunction (BHJ) organic photovoltaics (OPVs) as inexpensive power sources rests, in part, on the ability to rapidly fabricate devices onto various flexible, stretchable, or opaque substrates without vacuum steps or ITO-based electrodes. This work examines utility of self-assembled monolayers (SAMs) of aminoalkylthiols (NH₂(CH₂)_nSH) having various alkyl chain lengths (n = 3, 6, 11, 16) bonded to Ag as part of an ITO-free, vacuum-free “flipped” device architecture where Ag acts as the cathode and a transparent conducting polymer blend serves as the anode. This SAM facilitates wetting of Ag by poly(3-hexylthiophene-2,5-diyl):phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) solutions, enhances the concentration of PCBM at the Ag/BHJ interface, and increases the work function of Ag to better match the lowest unoccupied molecular orbital of PCBM. Moreover, the use of long-chain aminoalkylthiols (n ge; 11) improved the average power conversion efficiencies by as much as an order of magnitude relative to bare Ag and smaller aminoalkylthiols (n < 11) likely due to formation of a uniform blocking layer that inhibits carrier recombination at the Ag/BHJ interface. The optimization of aminoalkylthiol functionalization and subsequent development of ink solutions led to the fabrication of entirely inkjet printed OPVs deposited on flexible polymer films. These results have direct application to the development of low-cost, printed, flexible, stretchable organic electronics and can facilitate the transition of high-efficiency laboratory-scale OPVs to high-throughput industrial processing.

In this work we present an ultrasonic spray-coating technique which is capable of being scaled up for large area device fabrication and roll-to-roll processing. This technique was used in ambient conditions to deposit a photoactive blend consisting of carbazole and benzothiadiazole based low energy gap copolymers and the fullerene derivative [6,6]-Phenyl-C71-butyric acid methyl ester (PC₇₀BM). Using a range of spray speeds, casting solvents and substrate temperatures we have been able to control film thickness and drying kinetics. Film characterization using atomic force microscopy and optical interferometry shows that spray-cast films have a comparable surface to spin-cast films.
By employing a novel solvent cleaning technique, the frequency of defects in spray-coated polymer blend films was reduced. A high-resolution light beam induced current (LBIC) technique has been used to quantify the effects of these defects on current production in organic solar cells. A power conversion efficiency of 5.7% was obtained for solar cells with PCDTBT:PC₇₀BM films cast from the cleaned solvent.
We have explored the scale-up of photovoltaic devices using spray-coating, and have conducted film depositions and device studies on a range of different substrate sizes and architectures. These include a 36 x 36 array of 6.5mm² pixels, which we compare with a single pixel of 900mm². We use such techniques to comment on the yield and efficiency of device scale-up.

Solution coating of organic semiconductors provides great potential to fabricate low-cost, large-area and flexible organic electronics. Morphology and structure control are some of the most important aspects to achieve high performance devices for a given material. For example, Diao et al. recently reported that shear-coating can enhance molecular packing of the small molecular TIPS-pentacene and consequently improves hole mobility in the organic filed effect transistor application.[1] In organic photovoltaics, molecular orientation at the donor/acceptor interface is considered to be an important factor that impacts the device performance[2] in addition to control of morphology. Local molecular orientation relative to the D/A interface of a bulk heterojunction (BHJ) device can be revealed and characterized by scattering anisotropy observed with polarized soft x-ray scattering (P-SoXS).[3] Manipulating molecular orientation at and relative to the donor/acceptor interface is a novel concept for BHJ devices. We demonstrate here for the first time that shearing methods can be used to manipulate molecular orientational correlations in polymer:fullerene devices. The shearing also modifies the domain size and average composition variations, and elongates the domains. Although correlations to devices performance still need to be established, our work indicates that manipulation of molecular orientational ordering relative to polymer:fullerene interface can be enhanced by the shear-coating methods.
[1] Y. Diao, B. C. Tee, G. Giri, J. Xu, D. H. Kim, H. A. Becerril, R. M. Stoltenberg, T. H. Lee, G. Xue, S. C. B. Mannsfeld, Z. Bao, Nat. Mater. 2013, 12, 665-671.
[2] B. P. Rand, D. Cheyns, K. Vasseur, N. C. Giebink, S. Mothy, Y. Yi, V. Coropceanu, D. Beljonne, J. Cornil, J.-L. Brédas, J. Genoe, Adv. Funct. Mater. 2012, 22, 2987-2995.
[3] B. A. Collins, J. E. Cochran, H. Yan, E. Gann, C. Hub, R. Fink, C. Wang, T. Schuettfort, C. R. McNeill, M. L. Chabinyc, H. Ade, Nat. Mater. 2012, 11, 536-543.