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Solution processing is mandatory to continue the development of large scale and low cost organic solar cells. Tungsten oxide (WO3) has shown good performance when used as a hole transporting layer in organic solar cells. Here this oxide has been deposited by a sol–gel technique on top of the active layer in an inverted structure device. This layer has been characterised by XPS and UPS to understand the effect of the stoichiometry and of the energy levels on the device performance. In the optimised structure, an average power conversion efficiency (PCE) of 3.5% has been achieved, which is comparable to devices with vacuum deposited transition metal oxide using poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) as an active layer. Moreover in our study no annealing is needed after the deposition of the sol–gel processed oxide, which is really promising for the development of industrial roll-to-roll production on flexible substrates.

International audience; Polymeric photovoltaic (PV) solar cells have been fabricated using six solvents: chloroform (CHCl3), toluene (T), chlorobenzene (CB), orthodichlorobenzene (ODCB), 1,2,3,4-tetrahydronaphthalene (THN) and 1,2,4-trichlorobenzene (TCB). The active layers were composed of poly(3-hexyl)thiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Special care has been taken to keep all experimental parameters constant (thickness of the active layers, donor/acceptor weight ratio, area of active surface and electrodes) in order to avoid artefacts and truly study the effect of solvents. Studies using atomic force microscopy (AFM) and optical absorption (UV-vis) showed the relationship between the photovoltaic performance and the evaporation rate of solvents. The use of solvents with high boiling point results in a higher degree of organization in the structure of P3HT. A direct comparison with devices processed with thermal treatment has also been performed. As often reported thermal annealing increases photo-conversion efficiency of devices created from common solvents, due to better separation of phase between the two materials of the blend. In the case of solvents with high boiling point such as THN and TCB, neither phase separation nor modification of P3HT crystallization induced by thermal annealing has been observed. However thermal treatment appears to enhance performance, ensuing the evaporation of remaining solvent in the active layers. An overview of the effect of solvent on the electrical properties of films containing pure P3HT and P3HT:PCBM blend reported in the literature has been completed for the discussion.

In this article, we have studied the temperature and illumination dependence of open-circuit voltage ( V OC ) in polymer–fullerene based solar cells. It has been observed that V OC at higher illumination intensities gets converged at 0 K which gives information about maximum achievable V OC in a particular donor–acceptor blend. Besides this, recombination processes have been studied by transient open-circuit voltage decay (TOCVD) and the transition between recombination regimes has been observed for the first time. At low V OC carrier lifetime exhibits a constant value around 500 μs, which is interpreted in terms of a monomolecular recombination regime. At higher V OC carrier lifetime decreases as derived from a bimolecular relaxation law. The method allows estimating the recombination coefficient, which results in 2×10 −13 cm 3 s −1 . The results have been explained by considering Gaussian density-of-states (DOS) for highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO).

The fabrication and the optimization of photovoltaic solar cells based on poly(3,3-didodecylquaterthiophene) (12-PQT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are reported. On the one hand, the effect of the blend composition shows the necessity to increase the amount of fullerene derivative compared to conventional poly(3-hexylthiophene)-based systems. On the second hand, thermal annealing of devices is optimized and discussed. Overall, in this work, the highest power conversion efficiencies are shown in the range of 0.3–0.4% which represents a lower value than that reported with poly(3-hexylthiophene). Results are discussed in terms of charge carrier mobility and phase segregation in this bicontinuous donor-acceptor network.