Effect of external bias on nongeminate recombination in polythiophene/methanofullerene organic solar cells

Much recent literature suggests that nongeminate recombination is the loss mechanism that predominantly determines the bias dependence of the photocurrent in efficient organic solar cells. Here, we report a new experimental technique based on measuring the quasi-steady-state current-voltage characteristics during illumination by two pulsed lasers and observing how the current-voltage characteristics change as a function of the time delay between the two pulsed lasers. This technique unequivocally demonstrates a bias dependence of nongeminate recombination and reveals the dwell time of charge carriers in a photovoltaic device. We relate the results of our pulsed experiment to devices under solar illumination and find that the reduction of the charge carrier dwell time with increasing internal electric field explains the observed bias dependence of the device photocurrent under constant illumination and consequently affects the fill factor of high-performance organic solar cells.