Surface Sulfurization of Cu(In,Ga)Se2 Solar Cells by Cosputtering In2S3 in the One-Step Sputtering Process

The one-step sputtering process by using a quaternary Cu(In,Ga)Se-2 (CIGSe) target has been demonstrated to be a simple route to form an absorber without post-selenization. To achieve a higher efficiency, sulfurization is known as a promising method to modify the front interface, which has been carried out by using H2S to obtain a Cu(In,Ga)(S,Se)(2) (CIGSSe) layer. To relieve the environmental concerns, we propose an alternative approach to perform surface sulfurization of CIGSe absorbers without H2S. We cosputter a CIGSe target with an In2S3 target at the final stage of the absorber deposition in the one-step sputtering process and demonstrate the surface sulfurization, that is, a thin CIGSSe layer is formed on the CIGSe absorber, resulting in an enlarged surface band gap and increased open-circuit voltage. With the optimized surface sulfurization, the surface Se vacancies (V-Se), a donor state commonly existing in the one-step sputtered CIGSe films, can be effectively passivated. In addition, the formed thin CIGSSe layer results in a downshift of the valence band; therefore, the recombination at the interface and in the depletion region can be significantly reduced, leading to substantial efficiency enhancement from 11.74% (without In2S3 cosputtering) to 14.10%. Our results reveal that by using cosputtering, one-step sputtering can effectively form a surface sulfurization layer on a CIGSe absorber without any post-treatment of H2Se or H2S, which can be essentially beneficial for environmental considerations.