Localized state distribution and its effect on recombination in organic solar cells

Transient photocurrent (TPC) and transient photovoltage (TPV) measurements are reported in bulk heterojunction organic solar cells. TPC is used to measure the band tail and deep trap density of states within the interface band gap and to identify the carrier species from the relative contribution of drift and diffusion. Steep exponential band tails are observed near the donor valence band edge, changing to a broader distribution of states at larger trap energies. The effect of the localized state distribution on the recombination is analyzed---particularly for geminate recombination, where it is shown that band tails can substantially enhance the probability that geminate pairs ionize to form free carriers. The TPV response is shown to have completely different characteristics from those of TPC, and the explanation is provided.