Abstract

In recent years the power conversion efficiency of organic solar cells based on polymer:fullerene blends improved to beyond 10%. These new material compositions deserve particularly consideration in order to understand the crucial steps from photon absorption to photocurrent, to gain a deeper understanding and aid the ongoing optimisation. I will present some of our work concerning loss mechanisms in P3HT :PC60BM and PTB7 :PC70BM solar cells. For the latter, the power conversion efficiency changes dramatically from about 3% to about 7% upon adding 1,8-diiodooctane (DIO) as a co-solvent to the chloroform solution are investigated in view of the dominating loss processes. By combining transient photovoltage (TPV), voltage dependent charge extraction (CE) and time delayed collection field (TDCF) measurements, we show that in the devices processed from chlorobenzene solution severe geminate and nongeminate recombination occur, while the use of DIO facilitates charge photogeneration and strongly reduces geminate losses. From the thus measured charge carrier recombination rate we determine the nongeminate loss current of the device, allowing us to reconstruct the current/voltage characteristics across the whole operational range of the 7% PTB7 :PC71BM solar cells. In contrast, the reconstruction deviates from the measured j/V characteristics for the lower performance devices without DIO . Based on our experiments, we propose that this discrepancy is due to a combination of field dependent photogeneration and trapped charge carriers in isolated domains of pure fullerene phases. The nongeminate recombination dynamics are explained in detail, and are discussed in view of Langevin-like and trap-assisted recombination.