Bandgap-Tunable Kesterite Solar Cells via Isovalent Cation Substitution—High-Efficiency Solutions for Diverse Photovoltaic Applications

The development of photovoltaic (PV) technologies based on earth-abundant and non-toxic materials is essential for sustainable and cost-effective energy generation. Kesterite semiconductors, recognized for their excellent stability and tunable bandgaps, have emerged as promising candidates for a broad spectrum of solar-driven applications—including single-junction, indoor, tandem, and semi-transparent PV devices, as well as photoelectrocatalysis.
In this work, we demonstrate the high adaptability of solution-based processes to synthesize single-phase kesterite materials with high crystalline quality and customizable bandgaps from 0.9 to 2.2 eV. By employing systematic isovalent cationic substitutions (Ag, Cd, Ge) in selenide-based (Cu2ZnSn(S,Se)4) and sulfur-based (Cu2ZnSnS4) kesterites using advanced molecular ink solutions, we achieve devices with efficiencies exceeding 10% under AM1.5G illumination across a wide range of bandgaps. Notably, we present—for the first time—a kesterite absorber with a low bandgap of 0.9 eV achieving an efficiency exceeding 12%, and a champion device with 14.1% efficiency at a bandgap of 1.15 eV. Additionally, we showcase kesterite solar cells suitable for indoor and underwater PV applications, featuring wide bandgaps of 1.7 eV to 2.2 eV. Comprehensive analyses—including photoluminescence, time-resolved photoluminescence, phase structural characterization, composition and elemental distribution studies, and optoelectronic device analysis—provide valuable insights into reducing recombination in the bulk material and at heterojunction interfaces with different band alignment structures.
Additionally, We present 3 other key case studies demonstrating the practical applications of bandgap tuning: 1) Indoor PV Performance: Utilizing a spectrum-tunable LED solar simulator, we systematically investigate the behavior of wide-bandgap kesterite solar cells under 20 different indoor lighting conditions with color temperatures ranging from 6000 K to 2700 K. Finally, kesterite solar cells achieved a 15.1% efficiency under indoor illumination conditions. 2) Tandem Solar Cells: Through optical modeling and experimental validation, we study the efficiency of perovskite/kesterite tandem solar cells combining narrower bandgap kesterite (1.1 eV) with wider bandgap perovskite (1.7 eV). We achieve beyond 23% power conversion efficiency (PCE) in a 4T tandem solar cell configuration. 3)Photoelectrochemical Applications: Bandgap-tuned kesterite absorbers are employed as photocathodes to explore their photoelectrochemical properties for water reduction.
In conclusion, our work demonstrates that low-cost and high-tolerance isovalent cationic substitution enables a wide range of bandgap tunability in kesterite absorbers. This bandgap customization allows kesterite semiconductors to be tailored for various specific applications, offering critical insights into the widespread deployment of kesterite PV technology and providing a roadmap for future advancements.