Band Gap Changes of the CdS Buffer Induced by Post-Annealing of Cu2ZnSn(S,Se)4 Solar Cells

Changes in band gap energy of the buffer layer of kesterite Cu
2
ZnSn(S,Se)
4
solar cells were analyzed in dependence of different post-annealing temperatures and times using electroreflectance (ER). In order to measure the band gap of the buffer, the ER experiments were performed in “Brewster geometry” (angle of incidence and angle of detection equal Brewster's angle) allowing to reduce line shape distortions induced by interference effects and hence allowing reliable analysis of the ER spectra. Due to post-annealing, an irreversible reduction of the buffer layer's band gap is detected that increases for longer annealing times and higher temperatures. The decrease of band gap energy is attributed to an interdiffusion of atoms between the buffer and absorber layers.