Controllable Double Gradient Bandgap Strategy Enables High Efficiency Solution-Processed Kesterite Solar Cells

The double gradient bandgap absorber has the potential to enhance carrier collection, improve light collection efficiency, and make the performance of solar cells more competitive. However, achieving the double gradient bandgap structure is challenging due to the comparable diffusion rates of cations during high-temperature selenization in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) films. Here, it has successfully achieved a double gradient bandgap in the CZTSSe absorber by spin-coating the K2S solution during the preparation process of the precursor film. The K2S insertion serves as an additional S source for the absorber, and the high-affinity energy of K-Se causes the position of the spin-coated K2S solution locally Se-rich and S-poor. More importantly, the position of the bandgap minimum (notch) and the depth of the notch can be controlled by varying the concentration of K2S solution and its deposition stage, thereby avoiding the electronic potential barrier produced by an inadvertent notch position and depth. In addition, the KSe liquid phase expedites the selenization process to the elimination of the fine grain layer. The champion CZTSSe device achieved an efficiency of 13.70%, indicating the potential of double gradient bandgap engineering for the future development of high-efficiency kesterite solar cells. The controllable preparation of double gradient bandgap structure is realize by spin-coating K2S solution during the preparation of precursor films to obtain high-efficiency CZTSSe devices with high crystalline quality and low defect density.image