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.

An important milestone in the development of photovoltaic thin-film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin-film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n-ZnO/n-CdS/p-CuInSe2 deposited on a soda-lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large-grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

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