All-Layer Sputtering-Free Cu2Zn1-xCdxSnS4 Solar Cell with Efficiency Exceeding 7.5%

Cu2ZnSnS4 (CZTS) is a promising photovoltaic material drawing worldwide attention due to its remarkable photovoltaic performance and earth-abundant element constitution, making it a good candidate for cost-effective photovoltaic application. The power conversion efficiency (PCE) of CZTS solar cell has been increasing rapidly during last few years. Although the photovoltaic materials can be synthesized and fabricated with solution-processed methods, the conventional CZTS solar cells still rely heavily on vacuum-based techniques such as sputtering, evaporating or electron-beam deposition, which however, make CZTS solar cells still costly. A few attempts on sputtering-free CZTS solar cells have been reported, yet for now their PCEs are lagging behind the leading edges. Here, we demonstrated a 7.5% efficient sputtering-free Cd-alloyed CZTS solar cells by using sulfur-free precursor recipe. This recipe is proven to modify the morphology of CZTS film, leading to enhanced crystallinity and absorber/buffer junction quality. The mechanism behind the above-mentioned improvements is that applying S powder-free recipe can eliminate the problem of initial elemental distribution inhomogeneity within ball-milling processed precursor film, promoting vertically aligned grain growth to form compact and flat absorber film. Our work shows the vital importance of tuning the proportion of sulfur within solution-processed precursor ink to achieve high-performance of sputtering-free CZTS solar cells.