The development of photovoltaic (PV) technologies based on earth-abundant materials is a cornerstone for sustainable and cost-effective energy generation. Numerous solar-to-X applications, including single junction, indoor, tandem, and semi-transparent PV devices, as well as photoelectrocatalysis, require a broad spectrum of light absorption and band position. This wide variety of solar-driven niche markets necessitates the development of a PV technology with customizable band gaps for specific applications. Mature PV technologies such as crystalline Si, CdTe, and GaAs have fixed bandgap absorbers of 1.1 eV, 1.45 eV, and 1.3 eV, respectively, even though exhibit impressive power conversion efficiencies exceeding 22%. Some emerging PV absorbers like perovskite, organic, and Sb₂(S,Se)₃ also exhibit bandgap tuning properties from 1.2 eV to 1.95 eV, 1.49 eV to 2.0 eV, and 1.2 eV to 1.7 eV, respectively, enhancing the feasibility of tandem solar cell design and other PV applications. However, it is challenging for these materials groups to tune the bandgap below 1.0 eV, limiting the utilization of infrared photons. Moreover, these materials face environmental, cost, and stability concerns, limiting their potential for widespread deployment.

Kesterite semiconductors are gaining attention as they are earth-abundant, nontoxic, and have excellent stability properties and bandgap tuning ability. Recently, this technology has achieved 15% power conversion efficiency using a simple molecular ink process, demonstrating significant potential for widespread deployment. In this work, we will present the high adaptability of solution-based processes to achieve single-phase kesterite materials with high crystalline quality, resulting in devices with efficiencies beyond 10% under AM1.5G, with band gaps ranging from 0.9 to 1.7 eV. A systematic isovalent cationic (Ag, Cd, Ge) substitution for the selenide-based (Cu₂ZnSn(S,Se)₄) and sulfur-based kesterite (Cu₂ZnSnS₄) using advanced molecular ink solutions is the key strategy and will be detailed in the presentation. For the first time, we present a lower bandgap of 0.9 eV kesterite absorber with an efficiency exceeding 12%. A champion device with a 14.4% efficiency kesterite solar cell is achieved with an absorber bandgap of 1.15 eV. Furthermore, we showcase kesterite solar cells with decent efficiency under AM1.5G, with wide bandgaps up to 1.9 eV to 2.2 eV, ideal for indoor and underwater PV applications. Systematic photoluminescent, Time-Resolved Photoluminescence, phase structural, composition, and element distribution analysis, as well as optoelectronic analysis of the device, are performed in this work. This provides valuable insight into reducing recombination in the bulk of the different bandgap absorbers and at the heterojunction interface with different band alignment structures.

Furthermore, two important case studies of bandgap tuning will be presented. Firstly, the performance of the devices in indoor conditions will be measured using a tunable LED solar simulator, to systematically investigate the behavior of wide bandgap kesterite solar cells under a wide range of simulated indoor conditions (from 6000K to 2700K illumination). Over 18% efficiency kesterite solar cells under indoor conditions will be presented. Secondly, the series bandgap-tuned kesterite absorber are used as photocathode for investigating photoelectrochemical properties for water reduction.

In conclusion, this work will demonstrate that the low cost and high tolerance isovalent cationic substitution can lead to a very wide range of bandgap tunability in kesterite absorbers, with their customizable bandgaps enabling them to suit various specific application scenarios. This work provides critical insights into the widespread deployment of kesterite PV technology, offering a roadmap for future advancements.