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 (Se 2 S) 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 Cu 2-x Se 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.