Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2

A new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented. In situ photoemission measurements of CuInSe2 clearly show the existence of an In-rich n-type surface layer on samples relevant for solar-cell devices. Furthermore, this layer has been identified as an ordered vacancy compound (OVC) with a band gap of about 1.3 eV. The previous model of the CuInSe2/CdS solar cell with a p-n heterojunction between p-type CuInSe2 and n-type CdS is replaced by the model of a chalcopyrite/defect chalcopyrite heterojunction between p-type bulk CuInSe2 and the In-rich n-type OVC. The existence of this junction was proven directly by evaporating an ohmic metal contact onto the surface n-type layer and measuring the spectral quantum efficiency and electron-beam-induced current of this device. The band offsets of CuInSe2-based devices have been determined.