ZnSe etching of Zn-Rich Cu₂ZnSnSe₄: An oxidation route for improved solar-cell efficiency

Cu₂ZnSnSe₄ kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na₂S solution. Three different oxidizing agents were tested: H₂O ₂, KMnO₄, and K₂Cr₂O₇, combined with different concentrations of H₂SO₄. With all of these agents Se^2- from the ZnSe surface phase is selectively oxidized to Se⁰, forming an elemental Se phase, which is removed with the subsequent etching in Na₂S. Using KMnO₄ in a H ₂SO₄-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density (J_sc) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage (V_oc), shunt resistance (R_sh), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction.