Enhanced performance of Solution-processed Cu(In,Ga)(S,Se)2 solar cells via separate control of surface and bulk Cu concentrations

Abstract

Cu(In,Ga)(S,Se)2 (CIGSSe) is a promising material for next-generation photovoltaic cells due to its abundance, high absorption coefficient, and long-term stability. To enhance its commercial viability, significant efforts have been made to improve its efficiency through cost-effective fabrication methods. In particular, alcohol-based solution processing offers a more economical and environmentally friendly alternative to conventional CIGSSe manufacturing techniques. In CIGSSe solar cells, copper (Cu) concentration plays a critical role in determining both electrical and material properties. Since surface and bulk Cu concentrations influence device performance in distinct ways, their individual effects must be thoroughly investigated. In this study, we fabricate CIGSSe absorber layers via sulfoselenization of spin-coated precursor films and systematically examine the impact of bulk and surface Cu concentrations by adjusting the Cu content in the precursor layers. Our findings indicate that both Cu distributions significantly affect the chalcogen supply and elemental composition within the absorber. When bulk Cu concentration is excessive, an oversupply of sulfur (S) leads to the formation of secondary phases and an increased bandgap, thereby degrading photovoltaic performance. Conversely, insufficient bulk Cu results in the formation of an ordered vacancy compound (OVC) layer, which further deteriorates device efficiency. Additionally, the influence of a Cu-deficient (CuD) surface layer is dependent on bulk Cu concentration. In Cu-rich conditions, the CuD layer moderates S incorporation, ensuring the formation of an appropriately thick OVC layer. This layer functions as a charge carrier barrier, effectively enhancing solar cell parameters. By independently optimizing surface and bulk Cu concentrations, we achieve a notable power conversion efficiency of 12.6 %, surpassing devices where Cu distribution remains uncontrolled.