Control of point defects in the Cu(In,Ga)Se₂ film synthesized at low temperature from a Cu/In₂Se₃ stacked precursor

Abstract

Low-temperature fabrication of Cu(In,Ga)Se₂ (CIGS) film is essential for flexible CIGS solar cells. A large-grained CIGS film was synthesized with a Se-deficient Cu/In,Ga)₂Se₃ stacked precursor by reacting at 500 °C in a vacuum and was then subsequently annealing in Se environment. The CIGS solar cell with the as-prepared CIGS film had a poor cell performance due to a very low Ga composition at the CIGS surface. The surface Ga composition was controlled to 0.2 by supplying In, Ga, and Se in a temperature range of 350 to 500 °C. From an analysis of the photoluminescence spectra, we found that the point defects, Se vacancy and In-in-Cu antisite, in the CIGS film were greatly reduced by annealing below 450 °C. The short-circuit current was pronouncedly increased in the CIGS cells. The open-circuit voltage depended on both the Ga composition and Cu composition at the CIGS surface. In particular, a low Cu composition at the CIGS surface was essential for the higher performance solar cells. Our results indicated that CIGSs film synthesized at high temperature must be annealed at 350 °C or below to reduce undesirable point defects.