Solution-processed bandgap tunable kesterite absorbers for low-cost and eco-friendly thin film solar cells

Cu2ZnSnS4 (CZTS) is a promising kesterite semiconductor for sustainable photovoltaic applications, offering advantages such as high optical absorption, bandgap tunability, and eco-friendly, earth-abundant elements. However, while solution-processed CZTS has shown potential, key gaps remain in understanding their optoelectronic properties, particularly how sulfur (S) and selenium (Se) influence phase segregation and bandgap grading. Furthermore, the wide-bandgap CZTS suffers from a significant open-circuit voltage (VOC) deficit, limiting its efficiency. In this study, we fabricated solution-processed wide-bandgap Ag0.1(Cu0.9)2 ZnSnS4 solar cells, achieving a remarkably high VOC of 770 mV [58.5% of (VOCSQ)]. We compared the pure-sulfide wide-bandgap films, which formed a single kesterite layer, with narrow-bandgap Ag0.1(Cu0.9)2ZnSn(S,Se)4 films, which exhibited a dual-layer structure. Advanced characterization techniques, including scanning electron microscopy and scanning transmission electron microscopy, revealed Zn-rich and Sn-rich phase segregation for narrow-bandgap films, while back-side Raman spectroscopy showed depth-dependent compositional gradients. The incorporation of Se in the narrow-bandgap films led to improved carrier dynamics, reduced defect density, and enhanced device performance, with a significant increase in efficiency compared to the wide-bandgap films. These findings emphasize how S and Se tuning can modulate phase behavior, enabling the design of CZTSSe materials with tailored bandgaps and optimized optoelectronic properties for high-efficiency, environmentally sustainable solar cells.