Strain-Induced Intrinsic Constraint Boosts Slow-Thermalization and Fast-Transfer of Carriers in FAPbI Quantum Dot Solar Cells.

Formamidinium lead iodide quantum dots (FAPbI QDs) are extensively utilized in photovoltaic applications due to their superior optoelectronic characteristics. Nonetheless, the weak ionic bonds within their soft lattice structure lead to structural deformation, which causes a disordered charge distribution of FAPbI QDs. Stress engineering not only can mitigate the inherent soft lattice by reinforcing ion bonds but also can promote electron localization, thus enhancing charge carrier transfer. This work introduces a strain-induced intrinsic constraint (SIC) strategy that employs steric bulk modulation of nitrogen-rich ligands to induce anisotropic surface strain (ɛ = 0.53-0.78) in FAPbI QDs. By systematically designing nitrogen-coordinating ligands, guanidinium acetate (GA-acid) is demonstrated to facilitate controlled anisotropic lattice strain by filling A-site vacancies while simultaneously establishing a self-reinforcing stress, which effectively strengthens the antibonding interaction of Pb-O/I and reduces Pb-Pb orbital overlap, resulting in "slow-thermalization and fast-transfer" synergy for enhanced charge transfer. The PQDSCs engineered using the SIC approach achieve a photoelectric conversion efficiency of 17.11% and a highest short-circuit current density of 20.96 mA·cm. It is anticipated that stress-induced modulation of nanocrystals offers a critical insight for advancing the photovoltaic performance of perovskite solar cells.