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  Ruiz-Perona, A.; Gurieva, G.; Sun, M.; Kodalle, T.; Sánchez, Y.; Grossberg, M.; Merino, J.M.; Schorr, S.; León, M.; Caballero, R.: Routes to develop a [S]/([S]+[Se]) gradient in wide band-gap Cu₂ZnGe (S,Se)₄ thin-film solar cells. Journal of Alloys and Compounds 868 (2021), p. 159253/1-9

10.1016/j.jallcom.2021.159253
Open Access Version

Abstract:
Wide band-gap kesterite-based solar cells are very attractive to be used for tandem devices as well as for semi-transparent photovoltaic cells. Here, Cu2ZnGe(S,Se)4 (CZGSSe) thin films have been grown by sulfur-ization of co-evaporated Cu2ZnGeSe4. The influence of a NaF precursor layer and of a Se capping film on CZGSSe absorbers and solar cells has been investigated. It has been found that the distribution of [S]/([S] +[Se]) through the CZGSSe absorber layer is strongly dependent on the Na content. Na promotes the dif-fusion of S towards the bulk of the absorber layer. Thicker NaF layers >6 nm lead to a higher S content in the bulk of the absorber layer, but to a decreased accumulation of sulphur at the surface, as detected by GIXRD, GD-OES, and Raman spectroscopy measurements. A relationship between Jsc, FF and Na-content supplied was found; higher Na content resulted in improved solar cell efficiencies. It has also been possible to modify the [S]/([S]+[Se])-gradient throughout the CZGSSe film by the absence of the Se capping layer, achieving devices with 2.7% performance and Eg = 2.0 eV. This work reveals two ways to control the [S]/([S]+[Se]) depth-profile to produce wide band gap CZGSSe absorber layers for efficient solar cells.