Optical Detection of SnSe₂Formation on CZTSSe Thin-Film Solar Cells

Cu2ZnSn(S1-xSex)4 (CZTSSe) is a promising candidate for the absorber layer of low-cost thin-film solar cells, thanks to the advantage of using earth-abundant, non-toxic elements. However, since the stable phase region of CZTSSe is very narrow, secondary phases are easily formed during the thin-film deposition or the post-deposition treatments, and some of the secondary phases are detrimental to the solar conversion efficiency. In this work, we investigated the influence of the SnSe2 secondary phase to the performance of a solar cell using laser-beam-induced-current (LBIC) measurements and resonance Raman spectroscopy. We found that the SnSe2 secondary phase has a critical impact on the characteristics of the solar cell even if the amount of the secondary phase is so little that it can be detected only with a resonant excitation source. We established that the points with the SnSe2 secondary phase Raman signal had a lower photocurrent. From macro-scale resonance Raman measurements, we show that the existence of the SnSe2 secondary phase directly correlated with the lower efficiency of a cell. Therefore, we conclude that controlling the formation of the SnSe2 secondary phase is a crucial factor to obtain CZTSSe solar cells with high efficiencies.