High-Efficiency Solution-Processed CuInS2 Thin-Film Solar Cells with FF Exceeding 70% via Potassium Doping

Wide-bandgap chalcopyrite copper indium sulfide CuInS2 (CIS) thin-film solar cells have significant potential as top cells in tandem configuration and for indoor photovoltaic devices. Among various fabrication routes, solution-processing is particularly attractive due to its low cost, simplicity, and adaptability to large areas and flexible substrates. However, the efficiency of CIS solar cells is still lower than that achieved by vacuum-based methods. A major factor limiting the performance of CIS solar cells is the poor crystallinity of the absorber, which leads to a high density of grain boundaries and defects, facilitating nonradiative recombination. In this work, we report a highly efficient CIS solar cell by in situ potassium (K) doping via easily adding KCl into the precursor solution. The introduction of K significantly promotes grain growth and suppresses interface defects, leading to remarkable improvements in device performance. At an optimal K doping concentration (K/Cu = 7%), the champion CIS thin-film solar cell achieves an efficiency of 11.58%, enhanced by 60% compared to nondoped devices. In addition, a fill factor (FF) of 70.10% has been achieved, the highest FF reported for solution-processed CIS thin-film solar cells. Our results demonstrate a facile and efficient K doping strategy to realize high-performance wide-bandgap CIS thin-film solar cells.