Influence of Side Chains on Geminate and Bimolecular Recombination in Organic Solar Cells

The effects of the side chains on the loss processes in three polymer:PC[61]BM organic blends are studied by comparing transient absorption spectroscopy and solar cell measurements. It is shown that, after efficient photoinduced charge transfer, charge pairs are the only significant species present in the blend and that their recombination can be followed by monitoring the PIA kinetics. Charges are found to decay following two regimes. The subnanosecond decay of the charge signal, responsible for the losses of 55-75% of the charges, is well described by an Onsager-Braun geminate recombination model. The rates are found to be strongly affected by the presence and position of side chains, We attribute these changes to a better aggregation of the side-chain-less polymer that provides a better delocalization of the holes and a better mobility. At longer times (after about 10 ns) the charges undergo bimolecular recombination. By modeling the time and intensity dependence of this process, we measure the rate constant and find that it is similar for the three systems and slightly lower than the theoretical Langevin rate. By comparing these spectroscopic results with the light-intensity and bias dependence of the photocurrent, we conclude that bimolecular recombination does not play an important role under solar intensity. Geminate recombination is the main loss mechanism and explains the difference between all the devices, their strong bias dependence, and their generally low performance.