Wide band gap Cu2ZnGe(S,Se)(4) thin films and solar cells: Influence of Na content and incorporation method

The influence of the Na content and incorporation procedure into Cu2ZnGe(S,Se)4 (CZGSSe) thin films and solar cells is investigated. The effects of the presence/absence of Se during the deposition of a 15 nm thick NaF layer before and/or after the Cu2ZnGeSe4 (CZGSe) co-evaporation, are compared. Both the Na content, and Na-supply method significantly influence the incorporation of S into the CZGSe lattice and its distribution in the absorber. A [S]/([S] + [Se])-gradient throughout the CZGSSe layers is observed for all the samples and correlated with effects induced by the Na incorporation procedure. For instance, the evaporation of Se together with NaF leads to an increased S concentration in the surface-region of the CZGSSe layer and a modified surface morphology. CZGSSe-based solar cells with band gap energies of about 2 eV are obtained, regardless of the NaF addition method used, while the absence of the NaF layer reduces the S incorporation and the Eg. However, the evaporation of Se together with NaF results in higher Eg and open circuit voltage VOC, probably related to a higher S accumulation near the surface, demonstrating the importance of the [S]/([S] + [Se]) distribution. CZGSSe-based photovoltaic devices with efficiency of 3.2 % and Eg of 2 eV are obtained.