Modulation Effect of the Zn/Sn Ratio on Performance of Chloride-Fabricated CZTSSe Cells for Boosting 10%-Beyond Efficiencies

The cation ratio in Cu-poor and Zn-rich CZTSSe absorbers is key to decide the photovoltaic performance of CZTSSe solar cells, while its toleration and the influence mechanism is still not clear. In this work, for single-layer-structured CZTSSe absorbers spin-coated from metal chloride precursors with constant Cu and Zn values, the Zn/Sn ratio is regulated in the range of 0.8∼1.6. It is found that the Zn/Sn ratio has great influence on the morphology and electrical properties of the CZTSSe absorbers and thus the performance parameters of the CZTSSe cells. Comparatively, the results show that a 1.2 Zn/Sn ratio can boost the photovoltaic efficiency to 10.65% with a relatively high open-circuit voltage (VOC) of 481 mV and a reduced VOC-deficit of 585 mV. Detailed studies indicate that the 1.2 Zn/Sn ratio could favor the large-grain growth, increase the carrier concentration, suppress the carrier recombination, and reduce the band-tail effects and Urbach energy, thus leading to the highest efficiency with superior photovoltaic performance. The optimization of the Zn/Sn ratio in chloride precursor-coated CZTSSe absorbers would be beneficial for in-depth understanding of the effect mechanism of the element ratio on device performance.