A numerical model of a graded band gap CdS xTer 1−x solar cell

Abstract

Photogeneration is analysed in a graded band gap thin film solar cell which has a polycrystalline structure and where the band gap is varied in the direction of film thickness. The modelled device consists of an n-type graded band gap surface layer followed by a homojunction with a p-type semiconductor of uniform band gap. A simple numerical model suitable for use with computer-aided design is derived and used to determine the distribution of photogenerated carriers with the device. The model is extended to obtain numerical expressions for current photogenerated by air mass zero illumination in the proposed solar cell. The values of the semiconductor parameters used in the model correspond to a device fabricated from alloys of CdS and CdTe. The model is then used to optimize the design of a proposed n-CdS xTe 1−x−p-CdS 0.5Te 0.5 thin film solar cell and to compare devices of both graded and uniform band gaps.