Effect of Nongeminate Recombination on Fill Factor in Polythiophene/Methanofullerene Organic Solar Cells

A key factor in solar cell efficiency is the dependence of the photo-current on applied-bias. With respect to organic solar cells it is often suggested that this factor is governed by the field dependence of charge-transfer state separation. Here, we demonstrate that this is not the case in benchmark polythiophene/methanofullerene solar cells. By examining the temperature and light intensity dependence of the current-voltage characteristics, we determine that (1) the majority of free charge generation is not dependent on the field or temperature and (2) the competition between extraction and recombination of free charges principally determines the dependence of photocurrent on bias. These results are confirmed by direct observation of the temperature dependence of charge separation and recombinations using transient absorption spectroscopy and highlight that in order to achieve optimal fill factors in organic solar cells minimizing free carrier recomibation is an important consideration.