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Summary As the power conversion efficiency of organic photovoltaic has been dramatically improved to over 18%, achieving long-term stability is now crucial for applications of this promising photovoltaic technology. Among the high-efficiency systems, most are using BTP-4F and its analogs as acceptors. Herein, we determine the thermal transition temperatures (Tg) of seven BTP analogs to develop a structure-Tg framework. Our results point out an unresolved molecular design conundrum on how to simultaneously achieve high performance and intrinsic stability with BTP-based acceptors. We also show that PC71BM has miscibility above the percolation threshold in PM6 and can maintain local charge percolation and improved stability in ternary devices. However, PC71BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that develop during aging still degrade performance. This points to a second thermodynamic conundrum. A compound with differential miscibility in the donor polymer can only impact percolation, and a compound with differential miscibility with the BTP only impacts diffusion.

Nonfullerene all-small molecule organic solar cells (OSCs) have attracted significant attention due to their competitive performance compared to the well-studied polymer-based OSCs. Small molecules are considered to be materials of choice for industrial scale OSC production due to their relatively high purity, reproducibility, and well-defined molecular weights. Despite the recent progress in all-small molecule solar cells, the mesoscale morphology of these devices remains elusive. A fundamental understanding of the structure–property relationships of these systems is thus critical for further device performance enhancement. Here, we report a dramatic change in device performance of a nonfullerene all-small molecule bulk heterojunction system upon changing the side chains of the electron donor small molecules. Morphology analysis via resonant soft X-ray scattering shows that the average composition variation and the size of the smaller domains of the all-small molecule blends are the most influential factors that derive the photogeneration and collection efficiency of charges in efficient all-small molecule OSCs. The observed side-chain and processing initiated morphological changes resulted in distinct device efficiencies in these systems. Despite the fact that side chains are mainly meant to facilitate material dissolution, the current study clearly shows that they can also impose a significant effect on material organization and photovoltaic properties. The findings will give a clear understanding of the role of side chains and processing in material organization and device functionality, and the knowledge gained in this study is quite crucial in designing next-generation materials and devices.

Solution-processed Cu(In,Ga)(S,Se)

High-performance low-band-gap polymer semiconductors are visibly colored, making them unsuitable for transparent and imperceptible electronics without reducing film thickness to the nanoscale range. Herein, we demonstrate polymer/insulator blends exhibiting favorable miscibility that improves the transparency and carrier transport in an organic field-effect transistor (OFET) device. The mesoscale structures leading to more efficient charge transport in ultrathin films relevant to the realization of transparent and flexible electronic applications are explored based on thermodynamic material interaction principles in conjunction with optical and morphological studies. By blending the commodity polymer polystyrene (PS) with two high-performing polymers, PDPP3T and P (NDI2OD-T2) (known as N2200), a drastic difference in morphology and fiber network are observed due to considerable differences in the degree of thermodynamic interaction between the conjugated polymers and PS. Intrinsic material interaction behavior establishes a long-range intermolecular interaction in the PDPP3T polymer fibrillar network dispersed in the majority (80%) PS matrix resulting in a ca. 3-fold increased transistor hole mobility of 1.15 cm2 V-1 s-1 (highest = 1.5 cm2 V-1 s-1) as compared to the pristine material, while PS barely affects the electron mobility in N2200. These basic findings provide important guidelines to achieve high mobility in transparent OFETs.

Voltage data acquired after probe signal transmitted through the organic film and reflected off the film surface as a function of 0.36 mW millimeter wave signal frequency in the range 110–160 GHz. Five different organic photovoltaic (OPV) materials and one 95:5 blend produced at 2 spin rates are used. These materials are a) fluorinated 2-alkyl-benzol[d] [1–3]triazole (FTAZ), a high hole-mobility polymer used for transistors and photovoltaics, b) diketopyrrolopyrrole (DPP3T), an acceptor polymer used in field-effect transistors (FET), c) Y5(PffBT4T-2OD) film that possesses remarkable temperature controllable morphology, d) a neat conjugated polymer P3HT, Poly(3-(hexylthiophene-2,5diyl) film that is used in optoelectronic devices and as a conductive binder for Li-ion batteries, e) phenyl-C61-butyric acid methyl ester (PCBM) films and its soluble derivatives used as n-type organic semiconductors, and f) excitonic photovoltaic material 95%:5% donor-acceptor blend P3HT:PCBM produced by 2 different spin rates. Measurement of direct-current (dc) transmitted and reflected power (RF voltage signal) are measured using a newly developed continuous wave (CW) D-waveguide band probe (110–160 GHz) apparatus named time-resolved millimeter wave conductivity (TR-mmWC) [1]. Transmission and first surface reflection voltages are captured by a zero-bias Schottky barrier diode (ZBD) and converted to relevant dc voltages. Original voltage signal datasets attached with this can be utilized for photovoltaic, dielectric property estimation, and other semiconductor physics applications. A manually collected dataset of transmission and reflection coefficient at incident probe power level ∼0.9 mW for 95:5 P3HT:PCBM films produced at 2 different spin rates, and one separately only for the neat P3HT film are also presented here in tabular form.

The performance of polymer–fullerene bulk heterojunction (BHJ) solar cells is highly affected by the morphology of the blend film. Though the structure of the BHJ morphology is well-understood, the relationship between relevant material interactions and BHJ morphological evolutions is poorly understood and seldom explored. In this report, we discuss the impact of polymer molecular weight (MW) on thermodynamic and kinetic phenomena which have a drastic influence on the nanoscale BHJ morphology. The blend film comprises a highly aggregating low bandgap diketopyrrolopyrrole-based polymer PDPP3T and the fullerene electron acceptor molecule PC71BM, cast from a 1,2-dichlorobenzene (DCB) solution with and without the additive 1,8-diiodooctane (DIO). The pair-wise interactions among the components were evaluated by Flory–Huggins interaction parameters (χ). The BHJ blend of PDPP3T and PC71BM exhibited liquid–liquid (L–L) phase separation as a result of strong polymer–fullerene interactions (large χPDPP3T–fullerene) in DCB solution. In contrast, addition of 3% DIO into the DCB solution is found to stimulate polymer aggregation which gives rise to liquid–solid (L–S) phase separation. Large χPDPP3T–fullerene and χPDPP3T–solvent were observed as a result of increasing polymer MW, and these changes promote strong L–L phase separation and polymer aggregation in the blend solution, respectively. The latter interactions have led to low molecular mobility, with an end result of reduced crystallinity and smaller domain size of the BHJ films. The observed dramatic MW-dependent morphological changes were also manifested in solar cell outputs as well as charge carrier dynamics calculated by transient absorption measurements.

Charge transport is a central issue in all types of organic electronic devices. In organic films, charge transport is crucially limited by film microstructure and the nature of the substrate/organic interface interactions. In this report, we discuss the influence of active layer thickness on space-charge limited hole transport in pristine polymer and polymer/fullerene bulk heterojunction thin films (∼15-300 nm) in a diode structure. According to the results, the out-of-plane hole mobility in pristine polymers is sensitive to the degree of polymer chain aggregation. Blending the polymers with a fullerene molecule does not change the trend of hole mobility if the polymer tends to make an amorphous structure. However, employing an aggregating polymer in a bulk heterojunction blend gives rise to a marked difference in charge carrier transport behavior compared to the pristine polymer and this difference is sensitive to active layer thickness. In aggregating polymer films, the thickness-dependent interchain interaction was found to have direct impact on hole mobility. The thickness-dependent mobility trend was found to correspond well with the trend of fill factors of corresponding bulk heterojunction solar cells. This investigation has a vital implication for material design and the development of efficient organic electronic devices, including solar cells and light-emitting diodes.

Solution-processed Cu(In,Ga)(S,Se)2 (CIGS) has a great potential for the production of large-area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum-processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron-hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution-processed CIGS solar cells.

Solution processable conjugated organic materials have gained tremendous interest motivated by their potential of low cost, lightweight and especially easy manufacturing of large-area and flexible electronics. Toxic halogen-containing solvents have been widely used in the processing of organic electronics, particularly organic photovoltaics (OPVs). To transition this technology to more commercially attractive manufacturing approaches, removing these halogenated solvents remains one of the key challenges. Our morphological (hard/soft X-ray scattering) and calorimetric characterizations reveal that using o-methylanisole, a certified food additive, as processing solvent can achieve similar crystalline properties and domain spacing/purity with that achieved by widely used binary halogenated solvents (chlorobenzene and 1,8-diiodooctane), thus yielding comparable photovoltaic performance in spin-casted films. To move a step forward, we further present the potential of o-methylanisole as processing solvent in th...

Corrugated structures are integral to many types of photoelectronic devices, used essentially for the manipulation of optical energy inputs. Here, we have investigated the gains and losses incurred by this microscale geometrical change. We have employed nanostructured electrode gratings of 600 nm pitch in PbS colloidal quantum dot (PbS-CQD) solar cells and investigated their effect on photovoltaic properties. Solar cells employing grating structure achieved a 32% and 20% increase in short-circuit current density (Jsc) and power conversion efficiency, respectively, compared to nonstructured reference cells. The observed photocurrent increase of the structured devices mainly stems from the enhancement of photon absorption due to the trapping of optical energy by the grating structures. This optical absorption enhancement was particularly high in the near-infrared portion of the sun spectrum where PbS-based solar cells commonly present poor absorption. We have interestingly observed that the open-circuit voltage of all the devices increase with the increase in the absorbed photon energy (at a fixed light intensity), indicating a significant shift in Fermi energy level due to localization of low photon energy generated carriers in the tail of the density of states. We elucidate the role of the grating structure on charge dynamics and discuss the feasibility of these structures for construction of cheap and efficient photovoltaic devices.

This work was supported by the Office of Naval Research Grant N00014-17-1-2242, National Science Foundation Award CBET-1639429, and NextGen Nano.

The role of optical charge generation and nongeminate recombination on the photocurrent of upright and inverted colloidal-PbS quantum-dot solar cells is investigated. With a controlled active layer thickness, upright (PbS/fullerene) devices are found to present overall better photovoltaic performance relative to inverted devices, notwithstanding the better NIR photoconversion efficiency in the latter. Through detailed analysis and numerical optoelectronic simulations, we show that beyond incidental differences, these two device architectures have fundamentally dissimilar properties that stem from their particular optical generation characteristics and the nature of the recombination processes at play, with the inverted devices affected only by trap-assisted losses and the upright ones suffering from enhanced bimolecular recombination. This study unveils the role of device geometry and inherent material properties on the carrier generation and collection efficiency of the light-generated photocurrent in co...

Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption.

In this report, a high-performance all-polymer organic photodetector that is sensitive to linearly polarized light throughout the visible spectrum is demonstrated. The active layer is a bulk heterojunction composed of an electron donor polymer PBnDT-FTAZ and acceptor polymer P(NDI2OD-T2) that have complementary spectral absorption resulting in efficient detection from 350 to 800 nm. The blend film exhibits good ductility with the ability to accommodate large strains of over 60% without fracture. This allows the film to undergo large uniaxial strain resulting in in-plane alignment of both polymers making the film optically anisotropic and intrinsically polarization sensitive. The films are characterized by UV–vis spectroscopy and grazing incidence wide-angle X-ray scattering showing that both polymers have similar in-plane backbone alignment and maintain packing order after being strained. The films are integrated into devices and characterized under linear polarized light. The strain-oriented detectors have maximum photocurrent anisotropies of 1.4 under transverse polarized light while maintaining peak responsivities of 0.21 A W −1 and a 3 dB cutoff frequency of ≈1 kHz. The demonstrated performance is comparable to the current state of the art all-polymer photodetectors with the added capability of polarization sensitivity enabling new application opportunities.

We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells. The increase in EQE is not fully explained by the difference in the optical transparency of electrodes, interference effects due to an optical spacer effect of the metal-oxide electrode buffer layers, or variation in charge generation profile. We propose that a large additional splitting of excited states at the ZnO-Al/polymer interface leads to the considerably large photocurrent yield in inverted cells. Our finding provides new insights into the benefits of n-type metal-oxide interlayers in bulk heterojunction solar cells, namely the splitting of excited states and conduction of free electrons simultaneously.

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases – a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) bilayer film – the evolution of the P3HT–PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.

Solid-state Dye-Sensitized Solar Cells (ss-DSSCs) are promising candidates for future low cost photovoltaic energy generation and are based on polymer/metal oxide donor/acceptor heterojunctions. However, a crucial drawback of hybrid solar cells is the use of environmental unfriendly solvents, such as toluene, chloroform, chlorobenzene, etc. in the phase of preparation. In this work towards eco-friendly processing, we use water as a solvent in the preparation of the photo-active layer for hybrid solar cells. We demonstrate eco-friendly hybrid polymer/titania solar cells consisting of water soluble polythiophene as light-absorber, donor and Hole Transporting Layer (HTL), above a TiO 2 layer that acts as an acceptor and electron conductor. The water soluble conjugated polymer materials are studied in terms of their opto-electrical and morphological properties, leading to a better understanding of the resulting photovoltaic performance. An alternative new processing method in device preparation is introduced; yielding prototype solar cells with an efficiency of 0.7%. This promising solar cell device performance can be considered as a proof-of-principle for future eco-friendly solar cells.

Efficient polymer solar cells typically contain the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which promotes dissociation of excited states and enhances charge transpo ...

The morphological, bipolar charge-carrier transport, and photovoltaic characteristics of poly(3-alkylthiophene) (P3AT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side-chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3-butylthiophene) (P3BT, m = 4), poly(3-pentylthiophene) (P3PT, m = 5), and poly(3-hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo-current yield (exceeding 10 mA cm−2) irrespective of side-chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole-only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field-effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.

Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]-phenyl C61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground-state charge-transfer complexes in the studied polymer–fullerene blends. The effective band gap (Eg) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open-circuit voltage (Voc) scales linearly with Eg. The redshift of the CT band upon thermal annealing of regioregular poly(3-hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of high-temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of Eg, proportional with the observed changes in Voc for different PPV:PCBM ratios. As Eg corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.

In high performance polymer: fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to 'High T-g PPV':PCBM based solar cells, where the 'High Tg PPV' is characterized by its high glass transition temperature (138 degrees Q. In situ monitoring of the photocurrent-voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms. (c) 2008 Elsevier B.V. All rights reserved.

A New Donor-Acceptor-Donor Polyfluorence Copolymer with Balanced Electron and Hole Mobility

A prototype solar cell with a transparent polymer cathode and an ITO/poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) anode was fabricated. As an active layer, a thin film of a bulk heterojunction of polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) and an electron acceptor mol. [6,6]-phenyl-C61-butyric acid Me ester (PCBM) (1:4 by wt.) was sandwiched between the 2 transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymd. PEDOT (VPP PEDOT) with a cond. 102-103 S/cm. The cathode is supported on an elastomeric substrate and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells. [on SciFinder (R)]

We investigate charge formation in bulk-heterojunction solar cells based on conjugated polymers in the form of alternating polyfluorene copolymers and the methanofullerene PCBM. Using transient absorption spectroscopy we show that optimal charge formation is obtained with 20-50 wt% PCBM. This is in contrast to the maximum short circuit current density obtained at similar to 80 wt% PCBM as determined by steady state current density-voltage characterization. Hence, we show explicitly that the solar cell performance of these interpenetrating polymer networks containing PCBM is limited by charge transport rather than by formation of charges. (c) 2006 Elsevier B.V. All rights reserved.

Charge transport in a near infrared absorbing polyfluorene copolymer (APFO-Green1) and its blends with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and 3′-(3,5-bis-trifluorom ...

We report a comparative study of plastic photodiodes using four different copolymers of fluorene, with a variation of alkyl side chain length and chemical structure. Photodiode materials are formed by blending the polymers with a fullerene derivative and spincoating the blend solution. A photovoltage of 1 V is obtained in devices, where the anode is a doped polymer and the cathode is LiF/Al. Monochromatic quantum efficiencies are better than 40% over most of the absorption range, and under solar light AM 1.5 simulation, we reach energy efficiencies beyond 2%. The high fill factors obtained in some of the devices indicate that these are of interest for more elaborate optimisation. Reasons for the benign electrical transport are discussed.

A Schottky contact is made from a single layer polymer poly [3-(4-octylphenyl)-2,2 0 -bithiophene] (PTOPT) in its neutral state and a low work function metal (Al). The electrical and optical properties have been investigated by means of I–V measurements in the dark and under illumination. Various parameters such as the reverse saturation current density, barrier height, and diode quality factor were determined from the I–V curves in the dark of Al/PTOPT/ITO sandwich structure using thermionic emission theory. Spectral response of the device was measured at various wavelengths giving a peak at 500 nm. The IPCE% was obtained for illumination through both the Al and ITO sides. By illuminating the diode with a monochromatic light of wavelength 500 nm, the open-circuit voltage, short-circuit current density, power conversion efficiency, and fill-factor (FF) were obtained. The dependence of photocurrent on light intensity was also recorded and analyzed. # 2002 Elsevier Science B.V. All rights reserved.

Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.

We investigated the electrical properties of solution processed Al-doped ZnO (AZO) nanoparticles, stabilized by mixing with a TiOx complex. Thin solid films cast from the solution of AZO-TiOx (AZOTi) (Ti/Zn ∼0.4 in the bulk and ∼0.8 on its surface) is processable in inert environment, without a need for either ambient air exposure for hydrolysis or high temperature thermal annealing commonly applied to buffer layers of most metal-oxides. It was found that the electronic structure of AZOTi matches the electronic structure of several electron acceptor and donor materials used in organic electronic devices, such as solar cells. Inverted solar cells employing a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester, cast on an indium-tin-oxide/AZOTi electrode, and capped with a tungsten oxide/aluminum back electrode, give rise to a nearly 70% fill factor and an optimized open-circuit voltage as a result of efficient hole blocking behavior of AZOTi. The resulting electron collecting/blocking capability of this material solves crucial interfacial recombination issues commonly observed at the organic/metal-oxide interface in most inverted organic bulk heterojunction solar cells.

A scalable procedure for nano-patterning the bulk heterojunction layer in organic photovoltaic (OPV) devices is reported. Nano-patterning is shown to increase light absorption in poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT : PCBM) devices (ITO\WO3\P3HT : PCBM\Ca\Al). Nano-patterning also modifies electric fields in OPV devices, thus affecting charge harvesting. Nano-patterned OPV devices with a power conversion efficiency of 4% are presented. Comparable efficiencies are also obtained by optimization of thicknesses in a flat-layer device. Trade-offs between absorption enhancement and charge harvesting deterioration induced by nano-patterning are discussed as well as possible optimization strategies.

The study of the influence of different anode buffer layers on the performance of solar cells based on blends of polyfluorene copolymers, acting as electron donor, and [6,6]-phenyl-C61-butyric acid ...

We demonstrated a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. The interface modification with N719 changes the charge injection levels as indicated by ultraviolet photoemission spectroscopy. The efficiency of inverted solar cells comprising a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C 61-butyric acid methyl ester has increased from ∼2.80% to 3.80% upon employing the dye modification of the electrode interface.

International audience; The influence of fullerene side chain functionalization on both the morphology and electro-optical properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[2-methoxy-5- (3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) as donor and fullerene molecules with different side chains as the acceptor. The varying side chain of the fullerenes was found to induce morphological changes as confirmed by different analytical techniques such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Nuclear Magnetic Resonance (NMR). The fullerene with the shorter side chain, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), forms crystalline nanophases in the blend, while this is not the case for the alternative diphenylmethanofullerene acceptor, [6,6]-1,1-bis(4, 4′-dodecyloxyphenyl)methanofullerene (DPM-12). The introduction of NMR allows us to estimate the fraction of crystalline fullerene. The morphological changes have a profound effect on the characteristics of charge transfer states (CT) formed at the polymer:fullerene interfaces. Crystallization of fullerene molecules shifts the energy of the CT state. This shift in energy is directly manifested in the open-circuit voltage of solar cells based on the fullerene acceptors under investigation.

The open-circuit voltage (V-oc) of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer (CT) states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to V-oc is deduced and shown to be valid for photovoltaic devices comprising three commonly used conjugated polymers blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). V-oc is proportional to the energy of the CT states E-CT. The energetic loss q Delta V between E-CT and qV(oc) vanishes when approaching 0 K. It depends linearly on T and logarithmically on illumination intensity. Furthermore q Delta V can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss q Delta V of similar to 0.6 eV at room temperature and under solar illumination conditions, of which similar to 0.25 eV is due to radiative recombination via the CT state and similar to 0.35 eV is due to nonradiative recombination. Original Publication:Koen Vandewal, Kristofer Tvingstedt, Abay Gadisa, Olle Inganäs and Jean V Manca, Relating the open-circuit voltage to interface molecular properties of donor:acceptor bulk heterojunction solar cells, 2010, PHYSICAL REVIEW B, (81), 12, 125204.http://dx.doi.org/10.1103/PhysRevB.81.125204Copyright: American Physical Societyhttp://www.aps.org/

Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from 12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to 25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers, a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T, P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved to be a potentially competitive material compared to P3HT.

New low band gap alternating polyfluorene copolymers were synthesized for use in plastic solar cells and their optical, electrochem., and photovoltaic characteristics were detd. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The HOMO-LUMO values were estd. from electrochem. studies. By varying the donor and acceptor strength and position of the solubilizing substituents, similar HOMO values were obtained. These values were also found to correlate well with the open circuit voltage (VOC) values detd. from photovoltaic data of the polymers blended with the acceptor PCBM. Despite similar HOMO values, the absorption spectra of the polymers differ significantly. This prompted the prepn. of photovoltaic devices consisting of blends of two polymers with complementary absorptions in combination with PCBM to harvest more photons in the polymer solar cells. [on SciFinder (R)]

The authors report a simple method for the artificial fabrication of the complex three-dimensional (3D) ultrastructures of butterfly wing scales. This method uses chemical vapor deposition, UV lithography, and chemical etching to create the ultrastructures over a large area surpassing previously used focused ion beam techniques that are limited to microscopic areas. Furthermore, this method shows flexibility to modify nanostructure types and can precisely control shapes and dimensions and periodicity. Fabricated 3D ultrastructures are also replicated using a nanoimprint method into soft polymer materials. Reflectivity measurements and simulations of the master and polymer replicas show selective UV reflection consistent with the length scales used in such butterfly-like nanostructures.

We review nano-structures used as light trapping schemes in organic photovoltaic (OPV) cells. Light management via nano-structures enables one to exploit sub-diffraction limit properties, thereby giving access to enhanced light absorption in OPV cells. Light trapping schemes have been demonstrated in the form of surface plasmons, anti-reflection coatings, and photonic crystal patterns. Versatile methods are now available to facilitate fabrication of such nano-structures on sufficiently large areas for OPV applications.

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼0.1 and ∼0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT=ECT0−0.2f eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc=ECT/q−0.6 V.

The existence of a ground-state charge-transfer (CT) complex in a conjugated polymer:metal oxide nanoparticle bulk heterojunction photovoltaic cell is demonstrated by Fourier-transform photocurrent spectroscopy (FTPS). The CT complex between poly(3-hexylthiophene) (P3HT) and titanium dioxide (TiO2) is characterized by a weak additional photocurrent band (onset 1eV) in the FTPS spectra, situated below the conjugated polymer bandgap of 2eV. The presence of CT interaction between P3HT and TiO2 in relation to frontier orbital alignment is discussed, as well as the contribution of a sub-bandgap interfacial CT state to the electron transfer process in P3HT:TiO2 solar cells.