The enhancement of CZTSSe solar cell performance through active construction of the double-layer absorber

The absorbers of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) that have undergone rapid thermal process (RTP) selenization typically exhibit a double-layer structure, with a fine crystal layer at the bottom, which degrades the performance of devices. We propose an efficient strategy for actively constructing the double-layer absorber to improve the quality of the small grains in the bottom layer. Compared to traditional methods, this method ensures uniform element distribution, particularly for Se element, and enables a more desirable Cu-poor and Znrich composition with a Zn/Sn ratio closer to the standard stoichiometric ratio, resulting in a reduction in harmful internal defects. This strategy eliminates small grains at the bottom and decreases grain boundary formation, resulting in a dense double-layer structure. The high-quality CZTSSe absorber not only reduces carrier recombination during back transmission, further reducing reverse saturation current (J0) and increasing optical current density (JL), but also enhances shunt resistance (Rsh) and decreases series resistance (RS). The estimated contribution percent for PCE improvement due to changes in JL, J0, Rsh, RS, and A are 17.20 %, 120.01 %, 1.15 %, 6.50 %, and -44.86 %, respectively. Ultimately, the highest photoelectric conversion efficiency (PCE) increased from 8.73 % to 10.49 %, and device uniformity improved considerably. This study highlights the significance of optimizing the microstructure of CZTSSe absorber layer in enhancing the performance of CZTSSe solar cells, and will provide guidance for producing high-quality CZTSSe absorber layers.