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Inverted bulk-heterojunction organic solar cells (OSCs) using solution-processed layers possess significant advantages compared to the usual noninverted devices. To investigate the full potential of this type of OSC, we have carried out some optical modeling by rigorous coupled wave analysis. The influence of the thickness of several different layers in the device has been quantified, as well as the maximum possible number of photons absorbed in the poly(3-hexyltiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer for both conventional and inverted structures. It appears that the thickness of the hole injecting layer placed in front of the metallic mirror can influence the electromagnetic field distribution in the OSC, but no additional beneficial optical spacer effect is observed. The thickness of the electron injecting layer deposited on the semitransparent electrode also has a negligible influence on the photons absorbed in the active layer for the inverted structure. [ABSTRACT FROM AUTHOR]

We have carried out detailed optical simulations of tandem solar cells based on the following organic semiconductors: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), 1-(3- methoxycarbonyl) propyl-1-phenyl[6,6] C61 (PC60BM), and 1-(3-methoxycarbonyl) propyl-1- phenyl[6,6] C71 (PC70BM). We demonstrate that out of the many possible combinations of the component materials, one specific combination emerges as the best to reduce the spectral overlap of the two bulk heterojunction blends and thereby to ensure an optimized short-circuit current density (Jsc). Furthermore, the calculations allow us to predict the maximum Jsc achievable in tandem cells based on P3HT and PCPDTBT. Finally, we show that the efficient tandem cell realized and described recently by Kim et al. [Science 317, 222 (2007)] ensures balanced absorption in the top and bottoms cells. [ABSTRACT FROM AUTHOR]

The realization of highly efficient organic solar cells requires the understanding and the optimization of the light path in the photoactive layer. We present in this article our approach to measure and model the optical properties of our bulk-heterojunction devices, and to control them in order to enhance the photovoltaic performances. We report our recent observations on the dependence of the external quantum efficiency (EQE) on the incidence angle of the light, and our results on the determination of internal quantum efficiency based on EQE measurement and optical modeling cross-checked by reflection measurements. We investigate poly(3-hexylthiophene): 1-(3-methoxy-carbonyl) propyl-1-phenyl[6,6]C61 based solar cells with two different thicknesses of the active layer (170 and 880 nm), and show that in the thin ones the absorption is enhanced for oblique incident radiation. [ABSTRACT FROM AUTHOR]

Copper sulfides and copper selenides have recently been reported as new and promising low-cost and environmentally friendly thermoelectric materials. Here, it is shown that these materials have actually been studied for more than 190 years and the absence of commercial thermoelectric modules based on them stems from some major intrinsic issues related to these chalcogenides. Further development of these semiconductors will require addressing and solving these problems before large scale utilization can be considered. [ABSTRACT FROM AUTHOR]

Abstract: The realization of highly efficient organic solar cells requires the understanding of all optical losses in the solar cell. In this paper, we demonstrate the use of a nano-replicated moth eye anti-reflection coating which functions as an effective medium at the air-substrate interface. We show that the reflection losses of the substrate are compensated, yielding an increase of the peak EQE values by 3.5%. This improvement is valid for a wide range of incidence angles and preserves the insensitivity of organic solar cells against oblique incident light. [Copyright &y& Elsevier]

Abstract: We report herein large area (>10cm2), interconnected organic solar cell modules both on glass substrates as well as on flexible ultra-high barrier foils, reaching 1.5% and 0.5% overall power conversion efficiency under AM1.5 conditions. Series connection is described, as these modules consist of up to three cells. Using our flexible barrier material, a shelf lifetime of polythiophene-based solar cells of 6000h could be realized. Furthermore, we compare the photovoltaic performance of efficient conjugated polymer:fullerene solar cell modules with established technologies. Under typical indoor-office lighting, our modules are competitive with these systems. [Copyright &y& Elsevier]

In order to realize enhanced spectral coverage in organic photovoltaic devices, the authors have stacked a zinc phthalocyanine:C60 based cell on the top of a poly-3-hexyl-thiophene:[6,6]-phenyl C61-butyric acid methyl ester layer using a 1 nm thick Au intermediate recombination layer. Such tandem devices comprising active materials with complementary absorption spectra exhibit a short circuit current (Isc) of 4.8 mA cm-2, an open circuit voltage (Voc) of 1020 mV, and a fill factor of 0.45. Measurements of the photocurrent versus wavelength of the incident light show that photons are converted into charge carriers from 400 to more than 800 nm. Further optimization of the respective layer thicknesses may lead to high efficiency devices. [ABSTRACT FROM AUTHOR]

The converse electromechanical response in an organic Schottky contact follows a power law dependence S∝Vα of the mechanical strain S versus the applied voltage V, with an exponent α=1.5, in between linear piezoelectricity and quadratic electrostriction. The experimental result is discussed within the frame of a model, where the Coulomb attraction between charged impurities present in the depletion zone of the Schottky contact and the charges accumulated in the metal at the interface with the semiconductor is considered. Electromechanical responses of such devices appear to be of fundamental interest for the investigation of nonuniform electric field distributions and for potential practical applications as electromechanical transducers. [ABSTRACT FROM AUTHOR]

Organic polymeric semiconductor diodes based on poly(paraphenylene vinylene) exhibit electromechanical strain under reverse and forward bias operation. Under reverse bias, the strain in the organic diode is created by Maxwell forces (“electrostrictive” actuation). Under forward bias, the large electrical current results in Joule heating and thus in a thermally induced electromechanical strain. These electromechanical effects might be used for transducer applications of organic electronic materials. [ABSTRACT FROM AUTHOR]

The motivations of the present work are to investigate the exfoliation of montmorillonite within a linear low-density polyethylene matrix and to control its orientation during the cast extrusion process. The first part is focused on the exfoliation of the montmorillonite through the melt extrusion process. The accuracy and relevance of each method used to determine the exfoliation state of montmorillonite have been examined, thanks to X-ray diffraction, transmission electronic microscopy, and rheology. All these methods have presented limitations, but the combination of all leads to a better estimation of the exfoliation state. Finally, the orientation of the montmorillonite is quantified systematically by X-ray texture analysis and correlated with process parameters to discern which one is affecting their orientation. The results have demonstrated an enhancement of the "in-plane" orientation of the montmorillonite with the exfoliation, especially at high concentration and when combined with cast extrusion. Finally, in the multi-nano-layer polymer film configuration, the reduction of the individual layers 29 nm thickness leads to some orientation improvements. However, these improvements are almost at the same level as the concentration effect in a monolayer system. This work gives an overview of all the parameters needed to achieve a significant organo-modified montmorillonite "in-plane" orientation. [ABSTRACT FROM AUTHOR]

Organic solar cells have great potential, but an unseemly race to report record efficiencies without proper care is damaging the field. [Copyright &y& Elsevier]

Intragrain charge carrier mobilities measured by time-resolved terahertz spectroscopy in state of the art Cu2ZnSn(S,Se)4 kesterite thin films are found to increase from 32 to 140 cm²V-1 s-1 with increasing Se content. The mobilities are limited by carrier localization on the nanometer-scale, which takes place within the first 2 ps after carrier excitation. The localization strength obtained from the Drude-Smith model is found to be independent of the excited photocarrier density. This is in accordance with bandgap fluctuations as a cause of the localized transport. Charge carrier localization is a general issue in the probed kesterite thin films, which were deposited by coevaporation, colloidal inks, and sputtering followed by annealing with varying Se/S contents and yield 4.9%-10.0% efficiency in the completed device. [ABSTRACT FROM AUTHOR]

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500 μW K-2m-1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K-2m-1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl3. The obtained results are plotted on a semilogarithmic log (r) versus S graph to demonstrate that a very strong linear trend that limits the power factor around 500 μW K-2m-1 exists. Further improvement of Bi2S3 as thermoelectric material will require finding new doping modes that will break through the observed trend. The results of stability tests demonstrate that properties of optimally doped Bi2S3 are stable. [ABSTRACT FROM AUTHOR]

We present a method to directly study temperature induced structural and morphological changes in the active layer of organic solar cells by capacitance determination. At sufficiently high frequencies the change in device capacitance with temperature is dominated by the expansion of the organic layer. Variations in the expansion coefficient or the permittivity are visible in the capacitive response. We have studied thin film devices based on poly(3- hexyl)thiophene (P3HT) as well as organic solar cells made from blends of P3HT with a fullerene derivative. Temperature induced transitions are correlated to structural changes in the polymer (e.g., glass transition) as well as morphological changes of the blend; the morphology transition driven by phase separation sets in around 120 °C. The advantage of our capacitance method is substantiated as we have demonstrated that confinement due to the substrate and the top electrode alters the effect of temperature on thin films as compared to the bulk material. [ABSTRACT FROM AUTHOR]

Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(Sx,Se1-x)4 compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In1-y,Gay)Se2 and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open-circuit voltage ( Voc) never better than 60% of the Voc max, which is expected from the Shockley-Queisser radiative limit (S-Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the Voc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain-to-grain and other non-uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the Voc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S-Q limit. The implications and future ways for improvement are discussed. [ABSTRACT FROM AUTHOR]

Thin film solar cells with Al/ITO/ZnO/CdS/CZTSSe/Mo-glass structure are fabricated employing a fast and low-cost preparation procedure using an aqueous ink deposited by nonpyrolytic spray, followed by high temperature crystallization and selenization steps. Capacitance-voltage measurements on previously reported devices with >8% efficiency under 1 sun irradiation show a charge carrier density of the order of 1017 cm−3. Moreover, admittance spectroscopy indicates the presence of mid-bandgap defects that are tentatively attributed to a Sn deficit in the film. In order to reduce the number of these deep defects within the active layer of our solar cells, the Sn content is tuned in the precursor ink. Their morphology, elemental composition, crystal phases, capacitance-voltage profiling, admittance, photoluminescence, and photovoltaic performances are characterized. The results indicate that tuning the Sn content offers a strong leverage upon some key properties of the active layer, in particular the grain size, and the charge carrier and defect density. By employing this leverage to optimize the performance of our CZTSSe layers, the cell performances are increased to 10.0% without antireflection coating (ARC) and to 10.8% (on 0.25 cm2) with an ARC. [ABSTRACT FROM AUTHOR]

Abstract: Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than ∼100nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices. [Copyright &y& Elsevier]

Abstract: We have studied organic bulk heterojunction photovoltaic devices based on a bridged-bithiophene donor–acceptor type low-band gap polymer blended with PCBM and bis-PCBM. The impact of the molecular arrangement is discussed in terms of the correlation between the solar-cell performance and the degree of crystallization. Differential scanning calorimetry (DSC) and grazing-incidence X-ray diffraction (GIXRD) prove that films with bis-PCBM typically result in more amorphous blends than comparable films with PCBM. Electron tomography (ET) is used to visualize the three dimensional morphology of photoactive layers, confirming the presence of nanofibers, formed in different scales through the thickness in the blended films with mono and bis-fullerenes. [Copyright &y& Elsevier]

Abstract: Here we report on organic photovoltaic''s (OPV) suitable for low light applications. In this paper, we illustrate the impact of R s and R p for indoor and outdoor applications. In addition, we propose a simple physics approach to predict the behavior of organic solar cells under various illumination intensities through electrical modeling. The combination of simulation and modeling allows to define a set of design rules for OPVs under low light illumination. The performance of various organic solar cells under low light intensity is compared with our predictions and excellent correlation is found. OPV shows high performance under low light conditions. [Copyright &y& Elsevier]

The characteristics of a power producing flexible wire based on organic photovoltaics (OPV) and the processes by which they are produced are described in this paper. A set of materials and coating formulations used on the electrode wires are very similar to those used in the development of two dimensional photovoltaic cells and modules. The active layer of the primary electrode wire comprises the bulk heterojunction-forming P3HT/PCBM (1:1 weight ratio) that has been extensively studied in planar cells. A second wire, which is wrapped around the coated, primary electrode wire, serves as the counter electrode. Ray tracing analysis indicates that light incident on the wires is focused by the cladding onto to the active layer, coated, primary electrode wire even when it is completely shadowed by the counter electrode. Furthermore, when the counter electrode is in a position that partially shadows the primary wire, a significant percentage of the light is reflected by the counter electrode onto the primary electrode. Many hundreds of feet of OPV wire have been produced continuously for experimental purposes, and the process is capable of producing any length of PV wire desired. Efficiency values of a 200 foot spool of PV wire ranges from 2.79% to 3.27%. [ABSTRACT FROM AUTHOR]

Organic photovoltiacs in a flexible wire format has potential advantages that are described in this paper. A wire format requires long-distance transport of current that can be achieved only with conventional metals, thus eliminating the use of transparent oxide semiconductors. A phase-separated, photovoltaic layer, comprising a conducting polymer and a fullerene derivative, is coated onto a thin metal wire. A second wire, coated with a silver film, serving as the counter electrode, is wrapped around the first wire. Both wires are encased in a transparent polymer cladding. Incident light is focused by the cladding onto to the photovoltaic layer even when it is completely shadowed by the counter electrode. Efficiency values of the wire range from 2.79% to 3.27%. [ABSTRACT FROM AUTHOR]

Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) gradually reduces the polymer-solvent interaction, the attraction forces between polymer chains become more dominant, and the polymer chains adopt a tight and contracted conformation with more interchain interactions, resulting in a progressive aggregation in both solutions and films. This was used to fine tune the morphology of PCDTBT/PC71BM based solar cells, leading to improved domain structure and hole mobility in the active layer, and significantly improved photovoltaic performance. The power conversion efficiency increased from 6.0% to 7.1% on devices with an active area of 1.0 cm2. [ABSTRACT FROM AUTHOR]

We fabricated flexible transparent conducting electrodes by printing films of single-walled carbon nanotube (SWNT) networks on plastic and have demonstrated their use as transparent electrodes for efficient, flexible polymer-fullerene bulk-heterojunction solar cells. The printing method produces relatively smooth, homogeneous films with a transmittance of 85% at 550 nm and a sheet resistance (Rs) of 200 Ω/□. Cells were fabricated on the SWNT/plastic anodes identically to a process optimized for ITO/glass. Efficiencies, 2.5% (AM1.5G), are close to ITO/glass and are affected primarily by Rs. Bending test comparisons with ITO/plastic show the SWNT/plastic electrodes to be far more flexible. [ABSTRACT FROM AUTHOR]

In order to reduce the thermal conductivity of CoSbS, a newly developed thermoelectric semiconductor, we have aimed at intentionally induce atomic disorder in its structure. This endeavor was guided by Density Functional Theory(DFT) calculations which indicated that substituting sulfur with selenium might be easily achievable experimentally because of the low formation energy of this point defect. Thereby, CoSbS1−xSex compounds having 0 ≤ x ≤ 1 have been synthesized by solid state reaction. Besides the expected semiconducting paracostibite phase, we have observed the appearance of a semimetallic costibite phase, never reported experimentally before. This cross-fertilized theoretical and experimental approach allowed us to reduce by 50% the thermal conductivity of paracostibite and therefore reach a maximum zT of 0.62 at 730 K. This makes this entirely new CoSbS1−xSex alloy very attractive for further optimizations and potential usage in thermoelectric applications. [ABSTRACT FROM AUTHOR]

In article number 1501404 Gerardo Larramona and co‐workers present the fabrication of thin film solar cells with a fast and low‐cost procedure using an aqueous Cu‐Zn‐Sn‐S colloidal ink deposited by nonpyrolitic spray, followed by annealing/selenization. By fine tuning the stoichiometry of the CZTSSe layer spray, an efficiency of 10.8% under simulated AM1.5G is reached. The upper part of the cover represents a cross section of the CZTSSe photoactive layer. The lower part is a top view of the complete devices. [ABSTRACT FROM AUTHOR]

Single crystals of SnS (I), SnS2 (II), and Sn2S3 (III) are grown by chemical vapor transport using stoichiometric amounts of Sn and S and I2 as transport agent (evacuated silica ampule, 950→850 °C for (I), 850→600 °C for (II), and 650→500 °C for (III)). [ABSTRACT FROM AUTHOR]