Hyperactive Selenium Source Yields Kesterite Solar Cells with 12.86% Efficiency

One of the main issues that limits the efficiency of kesterite solar cells is the low diffusion and chemical activity of selenium clusters. Here, this work proposes a simple and effective pre-selenization strategy using Na-2(Se2S) solution, which enables the direct introduction of hyperactive Se-2, Se-3, and Se-4 into the precursors. The results demonstrate that Se-2, Se-3, and Se-4 promote the formation of the Cu2-xSe liquid phase and enhance the diffusion of elements from different micro-regions. Consequently, the ratios of Cu/(Zn + Sn) and Sn/Zn in different micro-regions of the absorber are controlled within the optimal range, exhibiting reduced fluctuations. The controlled environment suppresses the formation of Cu-Zn and Sn-Zn defects, as well as [2Cu(Zn) + Sn-Zn] defect clusters. Finally, a power conversion efficiency (PCE) of 12.86% is achieved, which is the highest PCE for kesterite solar cells made under ambient pressure and with N,N-dimethylformamide solvent.