Study of Changes of Electronic and Structural Nature of CBD-CDS/CIGS Interface with Ga Concentration

Band alignments and microscopic structures at interfaces between CdS buffer layer grown by chemical bath deposition and Cu(In_1-x Ga_x)Se₂ (CIGS) absorbing one by three stage co-evaporation have been studied as a function of Ga substitution ratio x by means of photoemission, inverse photoemission spectroscopy, Kelvin probe force microscopy and in-situ X-ray photoemission spectroscopy (XPS). For the interfaces over the In-rich CIGS, conduction band offset (CBO) was positive, where conduction band minimum of CdS was higher than that of CIGS. The CIGS region adjacent to the interface of these specimens had band gap energy much wider than that of corresponding bulk. In in-situ XPS measurement, as-grown surfaces of the vacuum-transported In-rich CIGS showed a wide band gap feature with a surface composition around Cu:(ln+Ga):Se = 1:3.5:5.5~1:2.5:3.5, which indicates that the CIGS region adjacent to the interface should be modified in the final stage of the growth of CIGS. An increase of x resulted in a decrease of CBO. On the specimen with x = 0.40~0.45, an almost flat conduction band alignment was realized. This decreasing tendency of CBO continued even in the Ga rich region. Further rise of x resulted in an inversion of sign of CBO; for the specimens with x = 0.60 and 0.75, CBO were about -0.2 and -0.3, respectively. Around a center region of the interface of the Ga rich samples, a decrease of Fermi level, which corresponded to a rise of conduction band minimum, was locally observed together with a local rise of oxygen impurities in this region. This phenomenon also enhanced a rise of CBM of CIGS beyond that of CdS. The observed changes of the band alignments are consistent with Ga substitution ratio dependence of performances of the cells fabricated over the studied interfaces