On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells

Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor differentially indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage.