Kesterite Cu2ZnSn(S,Se)4 is one of the most promising next‐generation thin‐film photovoltaic materials. Along with the efficiency advancement of kesterite solar cells, a cost‐effective fabrication process with low carbon footprint plays an increasingly important role considering the near‐future industrialization of CZTSSe solar cells. In this study, we proposed an environmental friendly process to recover copper and zinc from the scrap brass, which can be directly used as the precursor salts to fabricate efficient CZTSSe thin‐film solar cells. The best power conversion efficiency (PCE) of the device using the proposed eco‐friendly process is 11.39% (certified at the National PV Industry Measurement and Testing Center) for CZTSSe solar cells.

Suppressing the band tailing and nonradiative recombination caused by massive defects and defect clusters is crucial for mitigating open-circuit voltage (V oc ) deficit and improving the device performance of CZTSSe thin film solar cells. Cation substitution is one of the most commonly used strategies to address the above issues. The latest world record efficiency of 13.0% is obtained through this strategy (Ag substitution for Cu). Nevertheless, the importance of the approach to implementing metallic ion doping is easily overlooked by researchers. Here, different approaches are adopted to realize Ga doping and the differences in the efficacy and mechanism are thoroughly investigated. It is found that the secondary phase easily emerged when the physical-based method is employed, and thus challenging to regulate the doping effect. In the case of the chemical-based method, Ga doping can enlarge the depletion region widthand lower the defect activation energyand Urbach energy. Furthermore, Ga Sn defects located at grain boundaries can expand the energy band bending between GBs and grain interiors (GIs), thereby suppressing the deep defect states and nonradiative recombination. Consequently, power conversion efficiency as high as 12.12% with a V oc of 522 mV is achieved at 2% Ga doping.

Solar cells offer clean and abundant power sources for directly photo‐charging rechargeable batteries, which shows great potential for the development of integrated power supply. In order to deepen the understanding of the novel type of charging process, this research takes silicon solar cells and lithium cobalt oxide batteries as examples to compare the performance difference between photo‐charging and conventional constant current charging in detail. Surprisingly, the photo‐charging turns out to be superior in charging efficiency, polarization mitigation, and cyclic performance even with the intrinsic instability of the solar cells when compared with constant current charging mode. Finally, this research optimizes the calculation method of energy storage efficiency in the integrated power supply by calculating the actual power during the photo‐charging process by recording the voltage and current change with time. Besides, the power matching degree is quantified by the ratio of η1____ to η1, providing new ideas for the matching method of solar cells and rechargeable batteries in integrated power supplies.