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

This article was publised in "Solar Energy 226 (2021) 251-259". 

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 E_g. However, the evaporation of Se together with NaF results in higher E_g and open circuit voltage V_OC, 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 E_g of 2 eV are obtained.