Characterization of electronic structure of oxysulfide buffers and band alignment at buffer/absorber interfaces in Cu(In,Ga)Se2-based solar cells

The electronic structure of Zn- and In-based oxysulfide buffers formed by chemical bath deposition (CBD) in Cu(In,Ga)Se-2 (CIGS)-based solar cells, and band alignments at the interface between them and CIGS have been investigated by photoemission and inverse photoemission spectroscopy. It has been revealed that the control of anions including sulfur-oxygen mixing and OH-group concentration are dominant factors to optimize the band alignment. The partial substitution of oxygen for sulfur in both systems modifies the conduction band alignment toward flat connection. Sulfur-rich Zn(S,O,OH) and In(S, O, OH) buffers result in an adequate conduction band offset of -0.2 to +0.4 eV. For the Zn(S, O, OH) buffer, the suppression of OH-group concentration is essential to avoid the formation of a barrier against electron transport. For the In(S,O,OH)/CIGS interface, lowering of the valence band maximum of the CIGS surface and enhanced sulfur diffusion simultaneously took place. These favorable modifications are consistent with the remarkable rise of conversion efficiency in the cells using the Zn- and In-oxysulfide buffers. (C) 2014 The Japan Society of Applied Physics