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)­Se₂ (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.