Comparison of optical and electrical gap of electrodeposited CuIn(S,Se)_2 determined by spectral photo response and Ⅰ-Ⅴ-T measurements

摘要

The knowledge of the absorber band gap of heterojunction solar cells is of crucial importance, because it helps to investigate the band offsets at the buffer/absorber heterointerface. Usually this gap is deduced from optical techniques such as Spectral Photo Response (SPR). However, for inhomogeneous materials, this method of estimating the band gap is dominated by the lowest value somewhere inside the absorber layer, and SPR alone cannot give any indication about the spatial location of the region concerned with this measured value. Nevertheless, to determine the nature of band offsets at the heterointerface, we need a more accurate and local value of the gap next to the interface. In this study, dark Ⅰ-Ⅴ-T measurements were performed to characterize the gap within the CuIn(S,Se)_2 (CISS) Space Charge Region (SCR) near the CdS buffer layer. Under forward bias conditions, several recombination mechanisms occur in the CdS/CISS solar cells, each of them being characterised by a specific activation energy. One of these activation energies is related to the gap of the CISS SCR near the buffer/absorber heterointerface. We present a detailed study of the gap of electrodeposited CISS, determined from Ⅰ-Ⅴ-T measurements and its evolution with various CdS deposition conditions. The obtained gap values are found to be 200 to 300 meV higher than those derived from SPR. We explain this difference by the occurrence of graded CISS gap, with a gap value next to the heterointerface higher than the bulk gap value.