Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells

The composition of Cu(In,Ga)Se₂ (CIGS) films employed in CIGS solar cells is Cu deficient. There can be point defects, including Cu vacancies, Se vacancies, and metal anti-site defects. The surface composition and defects are not well controlled right after CIGS film fabrication with a three-stage co-evaporation process. This fabrication technique can result in a large variation in cell efficiency. In order to control the CIGS film in a reproducible way, we annealed the CIGS film in air, S, or Se. With this annealing procedure, the Cu content of the CIGS surface was significantly reduced and Ga content was strongly increased. An intrinsic CIGS layer with a lower valence-band maximum and a wider ban gap was formed at the surface. By annealing the CIGS film, the open-circuit voltage and fill factor were significantly improved, which indicates that the surface intrinsic layer acts as a hole-blocking layer so that the surface recombination rate is suppressed. In addition to CIGS film annealing, with subsequent annealing of the completed devices using rapid thermal annealing, the efficiency and reproducibility of CIGS solar cells were markedly improved.