Charge recombination in polymer/fullerene photovoltaic devices

A model is proposed for the mechanism of charge recombination in polymer-fullerene blend photovoltaic films. The model explains recombination kinetics determined by transient optical spectroscopy and their dependence on light intensity, temperature and fullerene concentration. We show that under solar illumination conditions, charge recombination is limited by the activation of positive polarons out of deep traps, but carrier collection competes successfully with recombination in thin films. The model is supported by comparison with data on cells and independent measurements of charge transport.