Effect of Na2S soaking on the properties of Cu2ZnSn(S,Se)4 films and solar cells.

• Na 2 S soaking process was proposed for non-vacuum prepared Cu 2 ZnSn(S,Se) 4 films. • Fine-grain layer can be eliminated by soaking precursor film in Na 2 S solution. • Short-circuit current of the solar cell was improved by the Na 2 S soaking. • 11.5% efficient Cu 2 ZnSn(S,Se) 4 solar cell was prepared. In this paper, we propose a Na 2 S soaking process (used as post-treatment process for the precursor films) for the non-vacuum prepared Cu 2 ZnSn(S,Se) 4 (CZTSSe) films. The CZTSSe film prepared without this process show triple-layer structure in which fine-grain layer and obvious layer-boundaries exist. By soaking the precursor film in Na 2 S aqueous solution with suitable concentration, the fine-grain layer in the triple-layer structure is substituted by a small-grain layer and the layer-boundaries become continuous. We find the quantum efficiencies of the solar cells in long wavelength region are improved by the Na 2 S soaking. Therefore, the short-circuit currents of the solar cells are significantly increased. By using the Na 2 S soaking process, CZTSSe solar cells with the best efficiency of 11.5% have been prepared. [ABSTRACT FROM AUTHOR]