Surface morphology control of In2S3 buffer layer by Sn incorporation and its application to cadmium-free Cu(In,Ga)Se2 thin-film solar cells

Sn atoms were incorporated into In2S3 thin-film to modify the morphology of the In2S3 buffer layer for Cd-free Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The incorporation of Sn atoms into the In2S3 films was carried out by adding a SnCl2-dissolved solution to an InCl3-and CH3CSNH2-dissolved solution during chemical bath deposition process. With Sn incorporation into In2S3 film, the surface morphology of In2S3 film was significantly changed showing more uniform and smooth film. Raman spectroscopic analysis shows that the backbone of In2S3 lattice disappear completely with 10% Sn incorporation in the film. In addition to effect of Sn content on the surface morphology of In2S3 film, we found that the morphology of Sn-incorporated In2S3 film strongly depends on the pH value in the reaction solution. At the pH value of 2.53, the film became very dense without pores and secondary phases. The film was completely covered the surface of CIGS substrate. Photoluminescence analysis indicates that the shallow acceptor levels at the CIGS surface might be passivated by Sn doping during chemical bath deposition process. As a result, the efficiency of CIGS solar cell with 4.9%-Sn doped In2S3 buffer was improved from 9.98 to 12.7%.