Microstructural and chemical studies of interfaces between Cu(In,Ga)Se2 and In2S3 layers

Microstructural and chemical properties of the interfaces between Cu(In,Ga)Se2 (CIGS) and In2S3 layers in dependence on the In2S3 deposition temperature and Na concentration were investigated. The In2S3 layers were deposited by atomic layer deposition on CIGS layers at substrate temperatures ranging from 140°C to 240°C. Interfaces were investigated by means of scanning electron microscopy, bright-field and high-resolution transmission electron microscopy, electron diffraction, and energy-dispersive x-ray spectrometry. An orientation relationship between CIGS {112) and In2S3 {103) planes was found for the sample deposited at 210°C, whereas no orientation relationship was detected for the 240°C sample. Cu diffusion from CIGS into In2S3 was detected, as well as Cu depletion and In enrichment on the CIGS side of the interface. All three effects are enhanced with increasing deposition temperature. These results indicate the formation of a buried junction in the CIGS layer. In addition, a Na-free solar cell was investigated. The results show that In2S3 grain sizes are smaller than in solar cells containing Na. Also, enhanced Cu and Ga diffusion from the CIGS absorber into the In2S3 buffer as well as enhanced Cu depletion and In enrichment on the CIGS side of the interface were detected. This may indicate that both Na and Cu occupy vacancies and In sites in the In2S3 tetragonal spinel structure.