Effect of Cu-In-Ga Target Composition on Hybrid-Sputtered Cu(In,Ga)Se₂ Solar Cells

High-efficiency Cu(In,Ga)Se ₂ (CIGSe) thin-film solar cells are typically fabricated by a multistage coevaporation process. The Cu-rich composition achieved during the second stage is known to favor the grain growth of the polycrystalline CIGSe film and has a beneficial effect on the solar cell performance. In this article, we analyze the effect of copper stoichiometry on the grain size of sputtered CIGSe absorbers and the efficiency of respective solar cells. CIGSe absorber layers were deposited on Mo-coated soda-lime glass substrates by pulsed hybrid reactive magnetron sputtering from Cu-poor and Cu-rich Cu-In-Ga targets and simultaneous Se evaporation. CIGSe films sputtered with the Cu-rich target show larger grain size and lead to better solar cell performance. Furthermore, we also introduce a two-stage process, which further increases the solar cell performance, consisting of an initial CIGSe layer deposited from the Cu-rich target followed by an indium-selenide postdeposition step. This two-stage process effectively eliminates the unwanted Cu _{2- x} Se impurity phase and renders the otherwise required and toxic KCN etching unnecessary.