Growth Mechanism and Properties of Nanostructure Cu₂ZnSnSe₄ Thin Films and Solar Cells

The quality of the absorber layer is considerably important in thin-film solar cells. The precursor stacking with uniform distribution of metal elements is usually considered to be critical while preparing high-quality kesterite-structured thin-film solar cells. However, the Mo/Zn/Cu/Sn/Cu stacking order may be more reasonable after considering the growth mechanism of films. Herein, the growth mechanism and thus the solar cells prepared with these two stacking sequences Mo/Zn/Cu/Sn/Cu and Mo/Sn/Cu/Zn/Sn/Cu are studied in-depth. The cross-sectional images reveal that the Cu 2 ZnSnSe 4 (CZTSe) film prepared with the precursor stacking order of Mo/Zn/Cu/Sn/Cu shows large grains without nanoscale small grains at the bottom of the film, which is attributed to a top-to-bottom growth mechanism. However, the CZTSe film prepared with the precursor stacking order Mo/Sn/Cu/Zn/Sn/Cu has a bilayer structure. The crystallization of CZTSe at the top of the film is good, but its crystallization at the bottom of the film is poor because the SnSe 2 liquid phase that assists the growth of CZTSe grains is absent in the bottom layer. The difference in nanoscale morphology and the growth mechanism of CZTSe films is mainly due to the difference in the locations of Zn layers in the precursor films. Therefore, the best CZTSe solar cells have been fabricated with the selenized Mo/Zn/Cu/Sn/Cu films and exhibit an efficiency of 10.28%. Further development of a high-quality kesterite-structured thin film is expected based on the growth mechanism studied herein.