Photovoltage Tomography in Polycrystalline Solar Cells

Abstract

To date, the performance of all polycrystalline photovoltaics is limited by their open-circuit voltage (Voc), an indicator of charge carrier recombination within the semiconductor layer. Thus, the successful implementation of high-efficiency and low-cost solar cells requires the control and suppression of nonradiative recombination centers within the material. Here, we spectrally and spatially resolve the photovoltage of polycrystalline thin-film Cu(In,Ga)Se2 (CIGS) solar cells. Micro-Raman and energy-dispersive X-ray spectroscopy maps obtained on the same grains showed that the chemical composition of the CIGS layer is very uniform. Surprisingly, we observed concurrent spatial variations in the photovoltage generated across the device, strongly indicating that structural properties are likely responsible for the nonuniform mesoscale behavior reported here. We build a tomography of the photovoltage response at 1 sun global illumination, mimicking the operation conditions of solar cells. Furthermore, we spatially resolve the voltage within the CIGS grains, where we found variations >20%. Our functional characterization could be realized to identify where nonradiative recombination preferentially takes place, enabling the implementation of nonuniform materials for future devices with higher Voc.