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Energy distributions ͓density-of-states ͑DOS͔͒ of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly͑3-hexylthiophene͒ ͑P3HT͒ ͓6,6͔-phenyl C 61 -butyric acid methyl ester ͑PCBM͒ solar cells. The analysis, which was performed on blends of different composition, reveals the presence of defect bands exhibiting Gaussian shape located at E Ϸ 0.38 eV above the highest occupied molecular orbital level of the P3HT. The disorder parameter , which accounts for the broadening of the Gaussian DOS, lies within the range of 49-66 meV. The total density of defects results of order 10 16 cm −3 .

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In this work we study the different electrical loss pathways occurring during the operation of bulk heterojunction solar cells by using a variety of electrical and optical characterization techniques beyond the current density-voltage curve (J-V): Impedance Spectroscopy (IS), Charge Extraction (CE) and Transient Photovoltage (TPV). Two sets of devices are analyzed: the first is based on the donor polymer P3HT, known to provide efficient cells using thick active layers (i.e. 270 nm), and the recently developed PTB7 which offers maximum efficiencies for devices with thinner layers (i.e. 100 nm). Devices fabricated with P3HT:PC 60 BM are not limited by transport of carriers and large active layer thickness may be used.Importantly, increasing the active layer thickness does not modify the contact selectivity. This is supported by analysis of the diode curve measured in the dark (similar leakage currents) and by capacitance-voltage measurements (similar fullerene content covering the cathode). Under these conditions the current density curve under illumination is mainly defined by the recombination processes taking place in the bulk of the active layer. In contrast, transport of carriers and contact selectivity are both limiting factors for the PTB7:PC 60 BM system. In this case, best efficiencies are obtained with a low active layer thickness and a high fullerene ratio. Reduced active layer thickness minimizes undesired electrical resistances related to carrier transport through the bulk of the active layer. High fullerene content enhances the amount of fullerene molecules at the cathode leading to decreased leakage currents. Then, the overall device efficiency will be a combination of the recombination kinetics in the bulk of the active layer, undesired resistance to transport of carriers and leakage current present due to low selectivity of the contact. The use of additives has also been explored which enhances charge generation and extraction. Overall, this work provides a comprehensive guide on how to interpret results obtained from some of the most widely used optoelectronic techniques employed to analyse operating devices.

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, JSC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in JSC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.

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