Uncovering details behind the formation mechanisms of Cu2ZnGeSe4 photovoltaic absorbers

Among the thin film chalcogenide photovoltaic community there is an increasing interest in the study of cationic and anionic substitution in the different absorber materials, including CdTe, chalcopyrites - Cu(In,Ga)(S,Se)(2) and kesterites - Cu2ZnSn(S,Se)(4). In the last case, cationic substitution has been revealed as a key factor to solve or palliate to some extent part of the fundamental problems of the kesterite technology. Among the different possibilities, the partial or total substitution of Sn by Ge is one of the most promising options, with proved excellent results from very small up to almost complete replacement. In view of the relevance of Ge in kesterite, this work presents the complete analysis of the reaction formation of the Cu2ZnGeSe4 (CZGeSe) compound using a sequential process based on the sputtering of elemental stacked layers followed by reactive annealing under Se atmosphere, by implementing a break-off experiment. An unusual solid-liquid-vapor extended growth mechanism is observed, thanks to the previous formation of a eutectic GeSe9 liquid phase that melts at temperatures above 212 degrees C. Driven by this liquid phase, it is demonstrated that CZGeSe formation mechanisms follow a strict sequence, starting from more simple molecules (binary compounds), then evolving to the ternary one, and finally to the quaternary alloy which is formed through the reaction of Cu2GeSe3 and ZnSe solid phases. The relevance of the study is supported by the solar cells prepared with these absorbers, demonstrating conversion efficiency at the level of the best reports in the literature. Finally, possible strategies to manage this singular formation pathway are discussed.