Analysis of the Heavy Alkali Element Postdeposition Treatment: Which Factors Determine the Electronic Structure and Transport Properties of the Heterojunction in CIGS Thin Film Solar Cells

The remarkable efficiency of CIGS solar cells has been continuously broken in recent years due to the postdeposition treatment of heavy alkali elements (Alkali-PDT). At present, a large number of experiments have been carried out to study the influence of heavy Alkali-PDT in the electronic structure of the absorber surface. However, there are still some controversies. This paper takes CsF-PDT as an example to study the change in the electrical structure of the absorber surface. It can be found that the surface valence band of the absorber moves downward after CsF-PDT, which is consistent with the literature reports. However, for the position of the conduction band on the absorber surface after CsF-PDT, there are two situations. If the absorber surface has low Ga content (GGI < 0.5), a small amount of CsInSe2 can cause the conduction band edge of the absorber surface to exceed that of the No-PDT absorber surface after CsF-PDT. This may induce a carrier transportation barrier that deteriorates the device’s performance. However, for the absorber surface with high Ga content (GGI ≥ 0.5), even lots of CsInSe2 do not easily induce the conduction band edge of the absorber surface to move upward, and the transportation barrier is unlikely to form. Meanwhile, for the absorber with a high surface Ga content, the shrinking of the valence band on the absorber surface after CsF-PDT can play a more important role in reducing the interfacial recombination. Therefore, the heavy Alkali-PDT should work better for the devices with a high Ga content on the absorber surface.