CuInSe2 and related I–III–VI2 chalcopyrite compounds for photovoltaic application

CuInSe2 and its related I–III–VI2 chalcopyrite compounds have been studied for about 50 years for application to absorbers in polycrystalline thin-film solar cells. Several research groups developed Cu(Ga,In)Se2, Cu(Ga,In)(S,Se)2, and (Cu,Ag)(Ga,In)Se2 solar cells with conversion efficiencies higher than 20%. The bandgap energies of these absorber materials in high-efficiency solar cells are less than 1.2 eV. Currently, wide bandgap chalcopyrite compounds, such as Ga-rich Cu(Ga,In)Se2, Cu(Ga,In)S2, and Ga-rich (Cu,Ag)(Ga,In)Se2, have gained attention as absorber materials for the top cells in tandem structure solar cells. However, the conversion efficiency of these wide bandgap solar cells cannot reach 20%. I–III–VI2 chalcopyrite compounds are composed of three or more elements and their crystal and electronic structures are more complicated than III–V or II–VI compound semiconductors. In particular, phase diagrams containing the I–III–VI2 chalcopyrite compounds are complex and vary greatly depending on the material systems. This article provides an overview of the current state of our materials science understanding of I–III–VI2 chalcopyrite compounds. In particular, we discuss the differences between CuInSe2 and the other chalcopyrite compounds based on their phase diagrams of I2VI–III2VI3 pseudobinary systems such as Cu2Se–In2Se3.