Nanosecond response of organic solar cells and photodiodes: Role of trap states

The nanosecond photoresponse of organic solar cells and photodiodes based on a conjugated polymer [poly(3-hexylthiophene-2,5-diyl) (P3HT)] blended with a fullerene derivative [[6,6]-phenyl C(61)-butyric acid methyl ester (PCBM)] is found to exhibit a tail in the decay characteristics which is proportional to t(-alpha). Existing numerical drift-diffusion simulations, not including the influence of trap states in the organic materials, fail to describe the observed long tail of the current density decay up to the microsecond timescale. We have extended a numerical drift-diffusion model to account for dispersive transport phenomena. In addition to a Gaussian density of the transport states, the distribution includes an exponential tail of states acting as trap sites for the generated charge carriers. The observed decay of the photoresponse following a power law is excellently reproduced within a multiple-trapping approach taking into account nine trap states approximating the exponential tail. The mobility of carriers in the transport states is found to be three times higher than the average effective mobility.