Distinguishing limits on the fill factor in organic solar cells processed from different solvents: Charge recombination kinetics vs. charge extraction

We use transient photovoltage (TPV) and transient photocurrent (TPC) techniques to understand charge recombination kinetics in a highly efficient organic solar cells (OSCs) system based on a blend of benzodithiophene terthiophene rhodanine (BTR) and [6,6]-phenyl C-71-butyric acid methylester (PC71BM), processing different solvents. The OSCs fabricated from mixed solvents show the highest PCE of 10.2%, mainly due to an improved FF of 75.4% as compared to the devices from pure organic solvent, while J(SC) and V-OC remain unchanged. Despite of difference in FF, non-geminate recombination is found to be the dominant photocurrent loss mechanism in all of the devices, regardless of processing solvents. Introducing a figure of merit, which is ratio between charge extraction rate and recombination rate by using the recombination kinetics data, we are able to correlate the competition between them to the fill factor of the devices.