While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...

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A photopolymerized nematic liquid‐crystal gel, which is visible‐light absorbing and electron‐donating, is used to template a nanoporous surface. This forms a distributed interface to an overlying electron acceptor in the device. An electron‐blocking layer lies between the gel and bottom electrode (see figure). An open‐circuit voltage of 1.1 V and a power conversion efficiency of 0.6% are obtained for a monochromatic irradiance of 45 mW cm–2.

Three solution processable n-type semiconducting perylene bisimides (PBI) with an unsymmetrical substitution pattern are evaluated in terms of their charge transport properties, morphology, and crystal structure. The nature of the substituents is varied from hydrophobic alkyl chains to hydrophilic oligoethyleneglycol (OEG) chains to control intermolecular interactions and to tune self-assembly properties of the compounds. A correlation of structure and morphology with charge transport properties is attempted. Bulk X-ray diffraction (XRD) data are indicative for a lamello-columnar packing motif in the case of PBI 1 and PBI 3 and a columnar hexagonal packing for PBI 2. Further, OEG chains induce liquid crystalline phases while the alkyl substituted compound is crystalline. In the amorphous state after film formation all three materials have a low electron mobility in the range of 10–5 cm2 V–1 s–1. After annealing in the ordered state the mobility of the liquid crystalline compounds increases by 2 orders of magnitude up to 7 × 10–3 cm2 V–1 s–1, while the mobility of the crystalline material decreases by a factor of 4.

COMMUNICATION 1091 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.dePhotovoltaic cells [ 1 ] based on organic and polymer materials (OPVs) are celebrated as being a possible solution to the energy needs of the future, [ 1b ] and, with record effi ciencies having breached the 10% milestone accompanied by emerging involvement from the materials industry, expectations are rapidly approaching reality. [ 1c ] However, in order to move OPVs beyond the individual laboratory and into a generally applied setting, scientists need to limit the gap between carefully prepared hero devices and the large-scale manufacture of thousands of devices. We propose that this is done by urgently attending to the challenges of scalability and reproducibility. In this communication, we demonstrate an approach using round-robin testing as a method to validate effi ciency measurements of OPVs based on semitransparent electrodes on fl exible substrates, with and without indium tin oxide (ITO). ITO-free substrates were rollto-roll coated under ambient conditions and were truly scalable. Our results demonstrate inherent uncertainties in the device-effi ciency data, with variations in the carefully measured effi ciency data for the same device between highly qualifi ed laboratories as high as 25%, depending on the substrate and its active area.Thus the concrete needs of society impose a broadening of the scientifi c perspective, but also a requirement for the validation and verifi cation of reports. The view should thus be that it is no longer enough to report very high effi ciency unless several independent laboratories report it. In a few instances, laboratories have obtained certifi ed effi ciency data for record devices, but, in practical terms, this effi cient solution represents a bottleneck, as very few laboratories have the capacity to offer certifi cation, and it would not be possible to certify all efficiency reports with the currently available laboratories. Instead, concerns of the validity of effi ciency data and reproducibility should be tackled by employing interlaboratory studies and round robins, in an effort to gain a consensus and gradually approach standardization. [2][3][4] Regarding the important issue of scalability, the average effi ciency of many devices prepared by large-scale methods is unlikely to rival current record effi ciencies. It has been argued that it is no longer enough to present high effi ciencies, focusing on the decimals of the reported numbers, without seriously addressing which processes and which materials can reasonably be included when fabricating polymer solar cells on what will eventually be a GW p per day scale. To do this, considerations of both the economic and environmental impact should be made. [ 1b ] In this regard, a systematic approach to such considerations as, for example, offered by life-cycle assessments is very important at this point. [ 1 , 5-7 ] Such studies reveal the favorite transparent electrode material, ITO, as being the most-critical bottleneck for state-...