Synergistic engineering of back interface and bulk defects via Mo:Na layer incorporation for efficient directly sputtered Cu(In,Ga)Se2 solar cells

Abstract

Cu(In,Ga)Se₂ (CIGS) thin-film solar cells fabricated via direct sputtering of a single quaternary CIGS target face challenges in back-contact engineering, particularly MoSe₂ formation under Se-deficient conditions, which distributes high-density detrimental bulk defects. Herein, interface band alignment and defect control for a high-quality quaternary-sputtered CIGS solar cell is reported by introducing a Mo:Na layer on the back electrode. By depositing a Mo:Na layer prior to Mo sputtering, we achieve controlled Na incorporation, suppressing deep defects while facilitating MoSe₂ crystallization. XPS and UPS analyses reveal that Na-induced surface Cu depletion enhances the preferential (112) oriented grain growth, and the formed MoSe₂ produces a favorable energy band structure at the Mo/CIGS interface. In addition, Na segregation on the absorber surface promotes lateral grain growth, facilitating the growth of the uniform CdS thin films. Combining the results of experiment and SCAPS simulation, the insertion of the Mo:Na layer simultaneously engineers back interface and bulk absorber, the power conversion efficiency of CIGS solar cells increases from 10.17 % to 13.38 %. This work emphasizes the effect of Mo:Na layer in tailoring grain growth, defect level and band arrangement, which paves a convenient and efficient way to realize high performance sputtered solar cells and photovoltaic devices.