This article aims to review the recent progress and future aspects of organic solar cells (OSCs). Different device designs proposed for efficient light harvesting and their advancements have been discussed. We have discussed various parameters limiting their efficiency and their possible solutions. Special attention has been paid to understand the device physics and its fundamental facets. Open circuit voltage (Voc) is an important parameter in the determination of their efficiency, but the picture behind the real origin of Voc is not very much clear; therefore, this issue has been discussed in detail. Short lifetime is another important issue; therefore, recent progress in stability and related issues has also been discussed. The progress in roll to roll (R2R) processing and demonstration for large‐area OSCs has been paid particular attention. The areas, where further advances will help in the commercial realization of this technology, have also been discussed. Copyright © 2011 John Wiley & Sons, Ltd.

There is no signature of the trap filled limit voltage (VTFL) in the J-V characteristics of a sample containing exponentially distributed traps. We show that VTFL and the voltage at which V2 dependence sets in (VMott) can be determined accurately. These voltages are independent of the energy distributions of the traps and depend strongly on the trap density Hb. Contrary to the literature results, it turns out that VTFL is significantly smaller than the VMott. In a specific case with Hb=1.6×1018cm−3 and for a 5% accuracy in the current the value of VMott is about 400V whereas VTFL is 13.5V. Universal J-V curves in reduced units are derived and plotted. The reduced value of VTFL is 0.5. These curves are valid for all organics and inorganic semiconductors and for all energy distributions of traps. It is shown mathematically that all J-V curves approached Mott’s V2 law asymptotically as V increases to infinity. To validate the theory, the experimental J-V curves in polycrystalline undoped and Al doped ZnO thin films are made. The experimental results show good agreement with the theory. In the undoped ZnO films the traps are exponentially distributed and the trap concentration is calculated to be 1.7×1017cm−3. The trap distribution in Al doped ZnO films is found to be discrete at a single level with trap concentration of 8×1016cm−3.

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Current-voltage ͑J-V͒ characteristics of an organic bulk heterojunction solar cell have been modeled and compared with the measured characteristics of solar cell based on the blend of poly͑3-hexylethiophene͒ ͑P3HT͒ and phenyl ͓6,6͔ C 61 butyric acid methyl ester ͑PCBM͒. In an undoped organic double Schottky junction diode, for V Ͻ V bi the electric field remains constant and is given by ͑V bi − V͒ / d, where V bi is the built-in voltage, V is the applied voltage, and d is sample thickness. We considered the effect of this constant electric field on the charge carrier transport and solved the drift and diffusion equations to model the J-V characteristics. For V Ͻ V bi the current is found to be dominated by diffusion. A comparison of the theoretical results with the experimental data measured in dark and under different illumination intensities shows good agreement.

The effect of illumination intensity and temperature on open circuit voltage ͑V oc ͒ in organic photovoltaic devices has been investigated. V oc is observed to saturate at high illumination intensities. The illuminated J-V characteristics at different intensities intersect the dark characteristic at a single point. This intersection point is shown to be equal to the built-in voltage ͑V bi ͒ in the sample. A reduction in temperature shows increment in saturated V oc . This increment in saturated V oc is attributed to the variation of V bi with temperature. A model has been presented that explains the observed behavior of V bi at different temperatures. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3129194͔Organic photovoltaic ͑OPV͒ devices can play an important role in generating long-term clean and cheap energy. Power conversion efficiencies and lifetime of these devices are not yet high enough for their commercial viability. Efforts are being made all over the world to understand the physics behind the operation and to improve the performance of these devices. The interpenetrating bulk-heterojunction tandem devices have shown a power conversion efficiency ͑͒ of ϳ6.5%.2 In order to optimize the power conversion efficiency and their reliability in various operating conditions, the complete understanding of device physics is of prime importance. There are various fundamental aspects regarding the physics of OPV devices that require attention. For example, contrary to the Si photovoltaic devices the forward bias current in OPV devices under illumination intersects the dark current and becomes more than the dark current. And another important issue is the origin of V oc . For the design of future solar cells, it is important to understand whether the V oc is a bulk property or an electrode property or a combination of both. It has been suggested that V oc in OPV devices depends either on the difference of energies of highest occupied molecular orbital of the donor and lowest occupied molecular orbital of the acceptor 3,4 or on the difference of the work functions of anode and cathode ͑⌬W͒ used. 5The variation of V oc with temperature 6 and illumination intensity 7,8 cannot be explained by either of these models. Various models have been presented to explain the experimental observations in OPV devices 5,7 yet the physics behind the operation is not well understood and requires more work to be done. Schilinsky et al. 7 extended the standard Si one diode model and a good agreement with the experimental data at different illumination intensities was observed. By self-consistent calculations, Schilinsky et al. 7 showed the existence of a constant electric field in the OPV devices. This finding is an important contribution. The correction due to this electric field was applied to the observed short circuit current ͑J sc ͒. However, the effects of the electric field on the dark currents were not investigated. We have recently presented a model considering the effect of constant electric field on both the dark an...

We demonstrate organic photovoltaic devices incorporating two donors, namely, copper phthalocyanine (CuPc) and boron sub-phthalocyanine chloride (Sub-Pc) in association with single acceptor fullerene (C60) with sensitivity extending across the visible solar spectrum. It has been found that the absorption in different spectral regions in CuPc and Sub-Pc results in efficient harvesting of incident light photons which leads to enhanced power conversion efficiency (η). An enhancement in η from 0.64%, in the device architecture indium-tin-oxide (ITO)/CuPc(20 nm)/C60(40 nm)/bathophenanthroline (BPhen) (8 nm)/Al(150 nm), to ∼1.3% in the optimized device having a 2 nm layer of Sub-Pc in the geometry ITO/CuPc(18 nm)/Sub-Pc(2 nm)/C60 (40 nm)/BPhen (8 nm)/Al(150 nm) has been observed. This enhancement in η is dominantly attributed to the increment in short circuit current density (Jsc) due to efficient photon harvesting by incorporation of dual donors.

Natural melanin with many interesting properties has potential applications in cosmetics, drug delivery, semiconductors, etc. However, conventional production methods are not efficient, resulting in its high cost (350−650 USD g −1 ), which has been a bottleneck for its efficient commercial utilization. To explore a faster extraction method with a higher yield, a melanin-producing endophytic bacterium was isolated from the halophyte Salicornia brachiata and further identified as Bacillus subtilis 4NP-BL by phylogenetic analysis of 16S rRNA gene sequences. The maximum melanin yield of up to 1.5 g dry wt L −1 of production media was obtained through central composite design (CCD). The isolated melanin belonged to the eumelanin class with an irregular structure on the basis of elemental analysis, UV−vis, Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and NMR studies. Furthermore, purified melanin displayed antioxidant activity and antimicrobial activity against pathogens Xanthomonas campestris and Alteromonas macleodii. Thus, this study further suggests a probable role of endophytes that produce melanin in aiding host plant protection from environmental stress and other pathogens.

Authenticity of conventional circuit model, to interpret the characteristics of polymer solar cells (PSCs) is examined. Conventional circuit model is found to be quite limited, and various assumptions used there are not valid for PSCs. By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different environmental conditions. Investigations are carried out on the solar cells, made of the blend of regioregular poly(3‐hexylethiophene) (P3HT) and phenyl [6,6] C61 butyric acid methyl ester (PCBM). The model is developed by treating both the dark and illuminated characteristics separately, even the characteristics were dealt with separately in reverse and forward biases. The formulated equivalent circuit model helps us in explaining many other important features, observed in the characteristics of PSCs. Copyright © 2013 John Wiley & Sons, Ltd.