Carrier Transport and Recombination in Organic Solar Cells

This talk will describe experiments and the analysis of electronic conduction in organic solar cells, with the aim of a better understanding of how they operate so that we can find ways to improve the solar cell efficiency.

An understanding of the physics of organic bulk heterojunction solar cells requires knowledge of the electronic transport and recombination mechanisms. We show that time-of-flight (TOF) transient photoconductivity can be used to obtain a wide range of information about the opto-electronic processes in the cells, including carrier mobility, recombination lifetimes and recombination mechanisms. Measurements of carrier mobility complicated because the solar cells are optically thin. The usual TOF model has to be adapted to carriers created throughout the material with both electrons and holes contributing to the transport. We show that the two carriers can be distinguished and their mobility measured except when the response is limited by the RC time constant of the measurement.  Dispersive mobility is observed and we discuss the relation of the mobility to the material disorder.  When the internal voltage is small, recombination dominates and the transient photoconductivity provides a measure of the recombination time. The transient photoconductivity measurements are able to rule out geminate recombination as a significant mechanism and together with dc photoconductivity indicate that recombination through interface states is the dominant process in the P3HT/PCBM and PCDTBT/PCBM cells measured.