Mixing Anions in Metal Chalcogenides for Effective Band Gap Engineering with Temperature: Density Functional Theory and Experimental Study

Toward the high efficiency of solar energy conversion and well-controlled functionalities of optoelectronic devices, huge amounts of effort are devoted to optimizing the band levels of semiconductors. In many cases, desired band levels and band gap values of semiconductors in tandem cell devices can be obtained via solid solution formation in candidate materials. However, most alloying-based band engineering relies on exhaustive trial-and-error experiments. As a case study of rational optimization of band levels and band gaps, we predict the equilibrium compositions and band gaps of Cu2GeS3 (CGS) and Cu2Ge(S1–xSex)3 (CGSSe) by combining computations and experiments. From our density functional theory calculations and thermodynamics modeling, we find the temperature-dependent miscibility gaps of CGSSe and the consequent change of the band gap, which enable a theoretical prediction of an optimum processing temperature for an ideal band gap. Our theoretical predictions suggest that the band gap of 1.16 eV, close to the ideal value, would be obtained at an annealing temperature of 900 K, which is in good agreement with our experimentally measured value of 1.24 eV. This study suggests a systematic way to design alloyed semiconductive materials via the rational control of the equilibrium composition with temperature.