Attaining 15.1% Efficiency in CZTS Solar Cells Under Indoor Conditions Through Sodium and Lithium Co-Doping

As the demand for sustainable energy sources grows, the field of indoor photovoltaics (IPV) has emerged as a promising avenue to power indoor applications, such as IoT devices and sensors, by harnessing ambient light. Indoor PV utilizes low-intensity, artificial light sources, including LED, halogen and fluorescent lighting, which are common in residential and commercial settings. Kesterite-based Cu2ZnSnS4 (CZTS) solar cells, known for their earth-abundant, low-cost, and non-toxic constituents, have demonstrated impressive efficiency gains under outdoor conditions, now surpassing 15% under AM 1.5G illumination. However, their potential for indoor applications remains largely untapped. This study investigates the effect of alkali co-doping with sodium (Na) and lithium (Li) on the performance of CZTS solar cells under both outdoor and indoor lighting conditions. We explore how Na-Li co-doping enhances crystalline and optoelectronic properties of the absorber, thereby improving device efficiency in different illumination environments. Key findings and technical details of this investigation will be presented at the conference, highlighting the advancement of CZTS technology for indoor applications.
Our comprehensive analysis reveals that Na doping significantly enhances the crystallinity and grain morphology of CZTS films, leading to improved efficiency, while Li doping alone has minimal impact. Notably, Na–Li co-doping further improves device performance, achieving a power conversion efficiency (PCE) of 10.1% under AM 1.5G illumination—surpassing both the reference and singly doped devices. The enhanced performance is attributed to the synergistic effects of Na and Li: Na promotes grain growth and reduces recombination at grain boundaries, while Li increases carrier concentration and passivates defects and grain boundaries in the absorber. When tested under 20 different indoor lighting conditions representative of real-world environments, the co-doped devices demonstrate exceptional adaptability and efficiency. They achieve a PCE of up to 15.1% under 3000 K illumination at an intensity of 2.93 mW cm-2—the highest reported value for CZTS solar cells under indoor conditions. These devices maintain high fill factors and stable open-circuit voltages across a range of color temperatures and light intensities, underscoring their suitability for IPV applications.
This study also examines the practical applicability of CZTS solar cells for powering IoT devices with different power demands. Simulations show that, under typical indoor illumination, Na-Li co-doped CZTS devices can reliably power low-consumption IoT devices (Philips Hue Motion Sensor, Honeywell HIH6130 temperature and humidity sensor) by generating sufficient power density. These findings, along with detailed performance analyses and structural characterizations, will be presented at the conference to emphasize the versatility and potential of kesterite-based solar cells in emerging indoor PV applications. The advancements presented here suggest a promising role for CZTS technology in the sustainable power landscape for indoor environments, particularly for autonomous indoor IoT devices.