Open circuit voltage decay transients and recombination in bulk-heterojunction solar cells

The internal loss mechanisms in polymer:fullerene bulk-heterojunction solar cells can be fruitfully studied using open circuit voltage decay (OCVD). For OCVD transients of poly (3-hexylthiophene-2, 5-diyl) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells, we observe that the open circuit voltage as a function time t changes from initially being nearly constant to being proportional to ln(t) for most part of the decay before eventually decaying to zero. We demonstrate that the transients can be fully described over eight orders of magnitude in time using a simple model of decay based on a diode coupled to a capacitor. The fitting to the analytical model solution enables true determination of the diode ideality factor and saturation leakage current. The ideality factor is observed to vary between 1.52 and 1.68 depending on excess carrier concentration and temperature. The technique is used to isolate the diode current in presence of excess carriers, and hence to independently determine the intensity dependence of the light-induced recombination current and shunt resistance.