Off-stoichiometry in I-III-VI₂ chalcopyrite absorbers: a comparative analysis of structures and stabilities

Chalcopyrite Cu(In,Ga)Se₂ (CIGSe) solar absorbers are renowned for delivering high solar power conversion efficiency despite containing high concentration of lattice defects amounting to copper deficiencies of several atomic percent. The unique ability to incorporate this deficiency without triggering decomposition (i.e. "tolerance to off-stoichiometry") is viewed by many as the key feature of CIGSe. In principle, this property could benefit any solar absorber, but remarkably little attention has been paid to it so far. In this study, we assess the tolerance to off-stoichiometry of thin-film photovoltaic materials by carrying out ab initio analysis of group-I-poor ordered defect compounds (ODCs) in the extended family of I-III-VI systems (where I = Cu, Ag, III = Al, Ga, In, and VI = S, Se, Te). We analyze convex hulls and structural evolution with respect to group-I content, link them with experimental phase diagrams, and determine two empirical principles for the future identification of solar energy materials with high tolerance to off-stoichiometry. Practical implications for the deposition of I-III-VI absorbers are also discussed in light of our computational results and recent experimental findings.