Highly efficient Cu2ZnSn(S,Se)4 bifacial solar cell via a composition gradient strategy through the molecular ink

The use of transparent conducting oxide (TCO) as a substrate in Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells allows for advanced applications, such as bifacial, semi-transparent, and tandem solar cells with the capability to increase power density generation. However, the efficiency of this kind of solar cell is still below 6% based on the low-cost solution process. In this work, we develop a composition gradient strategy and demonstrate a 6.82% efficient CZTSSe solar cell on F:SnO2 (FTO) substrate under the ambient condition. The composition gradient is realized by simply depositing the precursor inks with different Zn/Sn ratios. To verify that the high performance of the solar cell is attributed to the composition gradient strategy rather than the sole change of the Zn/Sn ratio, devices based on absorbers with varied Zn/Sn ratios are fabricated. Furthermore, the structure and surface morphology of the CZTSSe films with/without composition gradients are examined. The presence of elemental gradient through the depth of the CZTSSe films before and after annealing is confirmed by secondary ion mass spectroscopy analysis. It is found that the composition gradient enhances the crystallinity of the absorber, reduces the surface roughness as well as device parasitic losses, contributing to a higher fill factor, open-circuit voltage and conversion efficiency.