Routes to develop a [S]/([S] plus [Se]) gradient in wide band-gap Cu2ZnGe (S,Se)4 thin-film solar cells

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 sulfurization 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 diffusion 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 J(sc), 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 E-g = 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. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

The study investigates the influence of NaF precursor layer and Se capping film on Cu2ZnGe(S,Se)4 absorbers and solar cells. It reveals the strong dependence of [S]/([S]+[Se]) distribution on Na content and the relationship between Na content and solar cell efficiencies. The absence of Se capping layer can modify the [S]/([S]+[Se])-gradient throughout the CZGSSe film to produce efficient solar cells with wider band gap.