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

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 photocurrent 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 recombination using transient absorption spectroscopy and highlight that in order to achieve optimal fill factors in organic solar cells, minimizing free carrier recombination is an important consideration