Investigating the origin of the N1 signature in admittance spectroscopy of Cu(In,Ga)(S,Se)[formula omitted] thin-film solar cells by a variation of individual cell layers

Abstract

Electronic defect levels in semiconductor devices are often investigated by thermal admittance spectroscopy, a method in which the device capacity is measured as a function of frequency and temperature. When applied to Cu(In,Ga)(S,Se)2-based thin film solar cells, these commonly show a characteristic signature which has an activation energy of roughly 0.1 eV and has been termed the “N1” signature. However, even though this feature has been observed for at least two decades, the origin of the N1 signature is still under quite some debate in the community. Explanations include a defect level in the absorber bulk or near the heterointerface, a transport barrier at the back contact or at the buffer/i-layer interface, as well as mobility freeze-out. In order to contribute to an answer on this issue, in this study we fabricated several Cu(In,Ga)(S,Se)2 devices in which the individual layers of the cell stack were systematically varied, so that between two variations only one layer was different. This approach allowed us to test different hypotheses on the origin of the N1 signature by measuring admittance spectroscopy on the variations and checking whether a change in a specific layer led to a change in the N1 signature. Additionally, we investigated the role of the N1 signature in non-radiative recombination losses. The results suggest that the N1 signature is caused by a transport barrier in the buffer/absorber region in the investigated devices.