Charge Generation and Recombination in Organic Solar Cells
Organic solar cells promise cheap, flexible and lightweight modules for the conversion of solar energy to electricity. Power conversion efficiencies >11% can now be achieved. To further increase this performance, it is essential to gain a deeper understanding of the photophysical processes occurring in organic photovoltaic devices. In particular, it must be clarified how charges are generated, which involves light absorption, the splitting of the exciton at a donor–acceptor junction and separation of the ensuing electron–hole pair to charges that can be transported to the electrodes and extracted as a photocurrent. At the same time, geminate and non-geminate recombination losses must be characterised because these compete with the generation of usable charges and reduce the efficiency of the solar cells. All these processes have been thoroughly studied in recent years, but many controversies and open questions remain. In this chapter, we review the latest insights and emerging pictures concerning charge generation and recombination in organic solar cells, with a focus on blends of conjugated polymer electron donors with fullerene electron acceptors. The role of delocalisation, hot states and the structure and phase morphology of solid state thin films are at the centre of our discussion.