Organic solar cells formed from blends of semi-conducting polymers and substituted fullerenes are gaining considerable interest for low cost solar energy conversion. The function of these solar cells is based on a photo-induced charge separation reaction between the polymer electron donor and fullerene electron acceptor. As such, elucidating the processes of charge generation, recombination and transport in blend films is of great importance in order to improve the efficiencies of the resultant devices. Transient absorption spectroscopy (TAS) has proven useful in this regard and can monitor the formation and decay dynamics of the charged species present in films. In this work, TAS has been applied to P3HT and polythiophenes with one or more selenium heteroatoms, in blends with C60, C70 and C84 derivatives. The results show that localised (‘trap’) polaron states strongly influence the recombination dynamics.

Thermal annealing significantly improves P3HT:PCBM photovoltaic device efficiency, primarily through a large increase in the short circuit current (JSC). Despite the correlations observed between the enhanced JSC and the effects of annealing (such as increases in the charge carrier mobility), no consensus has been reached regarding the origin of the improved JSC. The effect of annealing is investigated using TAS on P3HT:PCBM films. It has been found that annealing produces a significant increase in the charge generation yield: this is the probable cause of the increased JSC rather than an increase in mobility.