Coordination-regulated defect suppression enables stable wide-bandgap perovskites for efficient perovskite/CIGS tandem solar cells

Wide-bandgap (WBG) perovskites are essential for high-efficiency tandem photovoltaics, yet their deployment is hindered by intrinsic phase instability driven by defect-assisted halide migration. Here, we report a coordination-regulated strategy that introduces bis(2-pyridylmethyl) sulfide (2PyS) to reduce undercoordinated Pb²⁺ defects and associated halide vacancies, thereby limiting ion migration and mitigating photoinduced halide segregation. In situ photoluminescence measurements reveal that 2PyS modulates crystallization kinetics and improves structural stability. As a result, the perovskite films exhibit enhanced phase stability under combined illumination and thermal stress. The optimized WBG perovskite solar cells achieve a power conversion efficiency of 22.21% with an open-circuit voltage of 1.20 V, and retain over 91% of their initial efficiency after 2000 h of continuous operation. Furthermore, a four-terminal perovskite/CIGS tandem solar cell delivers an overall efficiency of 29.71%. This work highlights the critical role of coordination chemistry in controlling defect formation and phase stability, offering an effective route toward stable and high-efficiency WBG perovskites for tandem photovoltaic applications.