Influence of exponential tail states on photovoltaic parameters and recombination of bulk heterojunction organic solar cells: an optoelectronic simulation

Abstract

Recombination is an important loss mechanism in organic solar cells. Here, both free charge and trapped charge carriers are taken into account in order to calculate the recombination rate. Trapped charge carriers are approximated by an exponential tail between the conduction band and valence band, and recombination is calculated as the sum of bimolecular recombination and trap-assisted recombination, the latter being described by recombination via exponential tail states. Then, an optoelectronic model including the effect of tail states is employed by finite element method to simulate a common structure of P3HT:PCBM organic solar cell. We investigate the influence of characteristic temperature on device performance, charge carrier densities and recombination rate. Simulation results reveal that the dominant recombination process could be the Langevin recombination or the recombination via exponential tail, depending on the characteristic temperature and applied voltage. Calculated open circuit voltage versus generation rate demonstrates a good agreement between our modeling prediction for the dominant recombination and experimental reports.