Solid-solution-induced lattice hardening reinforces perovskite stiffness for stable solar cells

The operational instability of perovskite photovoltaics is mainly attributed to the soft, ionic nature of its lattice. However, the quantification and intrinsic tuning strategy of perovskite lattice stiffness, and its correlation with device performance and stability have not been well explored. Here, we propose a solid-solution-induced lattice hardening strategy to enhance the stiffness of perovskite through partial substitution of lead ions with lanthanum ions. The reinforcement mechanism stems from the beneficial lattice contraction and bond-strengthening induced by the lanthanum dopant with smaller ionic radius and stronger chemical interaction, which collectively yield a denser and more rigid framework. Consequently, the strategy obviously enhances the perovskite stiffness with Young's modulus increasing from 27.96 to 35.71 GPa. The enhanced lattice stiffness reduces the thermal expansion coefficient, suppresses defect formation and ion migration, thereby mitigating non-radiative recombination and structural degradation under external stress. By employing 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl) borate as a stable p-dopant for the hole transport layer, the resulting device achieves a champion power conversion efficiency of 26.54% and a certified steady-state efficiency of 26.03%. The optimized devices retain >95% and >90% of their initial efficiencies after 1000 h of continuous 1-sun illumination at 65°C and under damp-heat conditions, respectively.