Attaining 15.1% Efficiency in Cu2ZnSnS4 Solar Cells Under Indoor Conditions Through Sodium and Lithium Codoping

The rising demand for sustainable low-power devices has driven interest in indoor photovoltaic (IPV) technologies for Internet of Things (IoT) applications. Composed of earth-abundant and non-toxic elements, Kesterite-based Cu₂ZnSnS₄ (CZTS) solar cells are highly attractive for IPV. This study systematically investigates the effects of sodium (Na), lithium (Li), and Na–Li co-doping on solution-processed CZTS devices. A comprehensive analysis reveals that Na-doping substantially improves crystallinity and grain morphology, significantly boosting efficiency, whereas Li alone has minimal impact. Notably, Na–Li co-doping achieves a 10.1% efficiency under AM 1.5G illumination, outperforming both the reference and singly doped devices. The co-doping synergy arises from Na-induced grain growth and Li-induced defect passivation and carrier concentration regulation. These devices exhibit high adaptability under 20 different indoor lighting conditions representative of real-world environments, achieving up to 15.1% power conversion efficiency under 3000 K illumination at 2.93 mW cm⁻²;—the highest reported indoor efficiency for CZTS cells. Their stable open-circuit voltage, high fill factor, and consistent efficiency across various color temperatures and intensities underline their suitability for IPV applications. Future work should focus on improving bandgap alignment with indoor light spectra to further enhance the efficiency of this eco-friendly technology for IoT energy solutions.