Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu₂ZnSnS₄ solar cells

Although Cu₂ZnSnS₄ (CZTS) has attracted attention as an alternative absorber material to replace CuInGaSe₂ (CIGS) in solar cells, the current level of understanding of its characteristic loss mechanisms is not sufficient for achieving high power conversion efficiency. In this study, which aimed to minimize the characteristic losses across the devices, we examined the relations between the compositional ratio distribution in the absorber layer, subsequent defect formation, and surface electrical characteristics. A high-temperature sulfurization process was used to improve the crystallinity of the absorber layer, which increased the uniformity of the compositional ratio distribution and consequently suppressed the formation of a ZnS secondary phase on the CZTS/MoS₂ interface. Because defects and defect clusters generated in the absorber layer are shallower when the compositional ratio distribution is uniform, the electron-hole recombination loss is reduced. These characteristics were confirmed by measuring the defect energy level using admittance spectroscopy and by analyzing the surface potential and current characteristics. These measurements revealed that improving the compositional ratio distribution suppresses the formation of deep-level defects and reduces the rate of carrier recombination. In addition, improving the compositional ratio distribution substantially contributes to improving the series resistance and short circuit current density characteristics.