Routes to develop a [S]/([S]+[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.