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

Abstract 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 Cu 2 ZnSn(S,Se) 4 (CZTSSe) films. Here, it has successfully achieved a double gradient bandgap in the CZTSSe absorber by spin‐coating the K 2 S solution during the preparation process of the precursor film. The K 2 S 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 K 2 S 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 K 2 S solution and its deposition stage, thereby avoiding the electronic potential barrier produced by an inadvertent notch position and depth. In addition, the K─Se 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.