Solar cells based on CCSVT-grown CuGaSe 2—absorber and device properties

Abstract

For solar cells based on CuGaSe 2 (CGSe) polycrystalline thin films, a novel efficient chemical close-spaced vapor transport (CCSVT) technique is used to deposit the CGSe absorber. Clean and Mo-coated soda lime glass (SLG) substrates are used for the CGSe deposition. The CGSe thickness ranges from 1.6 to 1.9 μm and the corresponding [Ga]/[Cu] ratio of the thin films is adjusted within the range of 0.9–1.3. The high bulk homogeneity of the as-grown CGSe films is shown. Transmittance and reflectance measurements were performed to monitor the changes in the CGSe band gap as a function of composition. The optical spectra reveal a shift of the absorption edge towards longer wavelengths with increasing Ga content, as well as a broadening of the distinct structure corresponding to three band-to-band transitions characteristic for CGSe. These results are used for a discussion of the behaviour of the ZnO/CdS/CuGaSe 2 solar cell photovoltaic parameters as well of the current transport. The transport mechanism is analysed on an our best solar cell device with an active area efficiency of 8.7%. For the first time, a thermally activated Shockley–Read–Hall recombination mechanism is observed for the CGSe-based solar cells in a large temperature region.