Band alignment at Sb_2S_3/Cu(In,Ga)Se_2 heterojunctions and electronic characteristics of solar cell devices based on them

摘要

Band offsets at Sb_2S_3/Cu(In,Ga)Se_2 heterojunctions have been studied by x-ray and ultraviolet photoemission spectroscopy. The valence and conduction band offset have been estimated to -(0.6 ± 0.3) eV and (0.2 ± 0.3) eV, respectively. This result suggests Sb_2S_3 as a potential buffer layer material for chalcopyrite based solar cells. However, Cu(In,Ga)Se_2/Sb_2S_3/ZnO solar cells have been investigated. While the open circuit voltage ranged up to ～0.4-0.5 V, the short circuit current was limited to ～1.8-4.9 mA/cm~2. A photocurrent of about 30 mA/cm~2 was found for negative bias. On the basis of bias dependent quantum efficiency measurements and calculations, limiting mechanisms are discussed.