21.15%‐Efficiency and Stable γ ‐CsPbI 3 Perovskite Solar Cells Enabled by an Acyloin Ligand

Cesium lead triiodide (CsPbI 3 ) is a promising light‐absorbing material for constructing perovskite solar cells (PSCs) owing to its favorable bandgap and thermal tolerance. However, the high density of defects in the CsPbI 3 film not only act as recombination centers, but also facilitate ion migration, leading to lower PCE and inferior stability compared with the state‐of‐the‐art organic–inorganic hybrid PSC counterpart. Theoretical analyses suggest that the effective suppression of defects in CsPbI 3 film is helpful for improving the device performance. Herein, the stable and efficient γ ‐CsPbI 3 PSCs are demonstrated by developing an acyloin ligand (1,2‐di(thiophen‐2‐yl)ethane‐1,2‐dione (DED)) as a phase stabilizer and defect passivator. The experiment and calculation results confirm that carbonyl and thienyl in DED can synergistically interact with CsPbI 3 by forming a chelate to effectively passivate Pb‐related defects and further suppress ion migration. Consequently, DED‐treated CsPbI 3 PSCs yield a champion PCE of 21.15%, which is one of the highest PCE among all the reported CsPbI 3 PSCs to date. In addition, the unencapsulated DED‐CsPbI 3 PSC can retain 94.9% of itsinitial PCE when stored under ambient conditions for 1000 h and 92.8% of its initial PCE under constant illumination for 250 h.