Discriminating between surface and bulk recombination in organic solar cells by studying the thickness dependence of the open-circuit voltage

Abstract

In case of thin-film solar cells, it is often rather difficult to determine what the dominant recombination mechanism is. In particular, it is difficult to distinguish recombination at the interface between the absorber layer and the electrodes (typically called surface recombination) from recombination in the bulk of the absorber—or in case of organic solar cells at the internal donor-acceptor interfaces. Here, we suggest a method to distinguish surface and bulk recombination in thin-film solar cells based on the thickness dependence of the saturation current density, which we derive from the open-circuit voltage and the photocurrent at short circuit or reverse bias. By means of numerical simulations, we show that surface and bulk recombination currents scale differently with thickness assuming the material properties to be unchanged. We test our predictions on a range of organic solar cell data from our laboratory and from literature and show that in the field of organic photovoltaics the whole range of cases, from mostly surface limited to purely bulk limited, is observed.