Metastable interphase induced pre-strain compensation enables efficient and stable perovskite solar cells

High-efficiency metal halide perovskite solar cells (PSCs) include the rigid substrates with low thermal-expansion coefficients (TECs), resulting in a significant TEC mismatch with the perovskites with high TECs at the buried interface. This mismatch leads to the thermally induced residual tensile strain in the perovskite films after annealing during film fabrication, which facilitates the ion migration and the defect formation, thereby compromising the performance and stability of PSCs. In this study, we present a pre-strain compensation strategy by introducing an in-situ generated metastable Pb(CH3NH2)2Cl2 (PMC) phase at the buried substrate/perovskite interface, which will transform to PbCl2 upon the annealing of formamidinium lead iodide (FAPbI3)-based perovskite films. This phase transformation provides a source of compressive stress for the perovskite films to counteract the adverse residual tensile strain during cooling from annealing. This strategy is demonstrated to be able to effectively reduce the defect formation and the non-radiative recombination rate in the perovskite films, while enhance the charge-carrier mobility, lower the exciton binding energy, and weaken the electron-phonon coupling interactions. As a result, the corresponding modified n-i-p PSCs achieve a champion efficiency of 25.83% (certified at 25.36%) and exhibit improved stability.