Synergistic A-Site Codoping Enables Lattice Stabilization and Strain Balance in Wide-Bandgap Perovskites

Wide-bandgap (WBG, ∼1.75–1.80 eV) perovskites are essential for tandem solar cells, but suffer from rapid crystallization, defects, and instability. Single-cation strategies face intrinsic trade-offs: methylammonium (MA) accelerates lattice formation but volatilizes under thermal stress, whereas guanidinium (GA) strengthens lattice interactions yet introduces tensile strain. Here, we uncover a synergistic mechanism through MA–GA A-site codoping, which balances lattice strain, reinforces ion–lattice interactions, and optimizes crystallization kinetics. Comprehensive experiments confirm codoping suppressed lattice distortions, reduced nonradiative recombination, and markedly enhanced thermal and operational stability. Consequently, 1.77 eV perovskite solar cells achieve an average PCE increase from 18.4% to 21.2%, with a champion device reaching 21.5% and retaining >95% of initial performance after 1400 h of continuous operation. Integration into tandem cells with 1.25 eV Sn–Pb subcells yields a PCE of 28.5%, demonstrating that antagonistic cation pairing can cooperatively stabilize the lattice, offering a general route to resolve the efficiency–stability trade-off in perovskite photovoltaics.