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

Microstructural and chemical properties of the interfaces between $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) and ${\mathrm{In}}_2{\mathrm{S}}_3$ layers in dependence on the ${\mathrm{In}}_2{\mathrm{S}}_3$ deposition temperature and $\mathrm{Na}$ concentration were investigated. The ${\mathrm{In}}_2{\mathrm{S}}_3$ layers were deposited by atomic layer deposition on CIGS layers at substrate temperatures ranging from $140°\mathrm{C}$ to $240°\mathrm{C}$ . 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 ${\mathrm{In}}_2{\mathrm{S}}_3$ {103) planes was found for the sample deposited at $210°\mathrm{C}$ , whereas no orientation relationship was detected for the $240°\mathrm{C}$ sample. $\mathrm{Cu}$ diffusion from CIGS into ${\mathrm{In}}_2{\mathrm{S}}_3$ was detected, as well as $\mathrm{Cu}$ depletion and $\mathrm{In}$ 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 $\mathrm{Na}$ -free solar cell was investigated. The results show that ${\mathrm{In}}_2{\mathrm{S}}_3$ grain sizes are smaller than in solar cells containing $\mathrm{Na}$ . Also, enhanced $\mathrm{Cu}$ and $\mathrm{Ga}$ diffusion from the CIGS absorber into the ${\mathrm{In}}_2{\mathrm{S}}_3$ buffer as well as enhanced $\mathrm{Cu}$ depletion and $\mathrm{In}$ enrichment on the CIGS side of the interface were detected. This may indicate that both $\mathrm{Na}$ and $\mathrm{Cu}$ occupy vacancies and $\mathrm{In}$ sites in the ${\mathrm{In}}_2{\mathrm{S}}_3$ tetragonal spinel structure.