Ammonia-induced surface microstructure reconstruction on ACIGS thin film at room temperature

The fabrication of Cu(In,Ga)Se2 (CIGS) solar cells has attracted wide attentions due to its low production cost, high absorption coefficient, and potentially for large-area roll-to-roll process. With the silver doped into CIGS to form (Ag,Cu)(In,Ga)Se2 (ACIGS), the film quality can be enhanced obviously. However, when the ACIGS film is exposed to air, the sample surface can be oxidized easily and it facilitates the formation of the oxides and hydroxides, especially in the summer, which can deteriorate device performance of solar cells obviously. In this contribution, through ammonia solution, we etch the sample surface and enhance the quality of the heterojunction interface. This treatment not only reduces the oxides and hydroxides on ACIGS surface significantly, but also contributes to the optimization of elements distribution. Furthermore, it is demonstrated that ammonia-treated process can effectively increase the valence band position along with the etching rate of Ga/In–Se > Ag/Cu–Se. The conversion efficiency of the optimum solar cells has a relative increase of about 7.2%. However, high concentration of ammonia solution will etch a lot of Ga–Se and In–Se phases, resulting in many suspension bonds and Ag/Cu–Se phases remaining on the film surface and deteriorating the device performance of solar cells. This work focuses on revealing the etching mechanism of ammonia solution on the ACIGS film surface.