Strain-Engineered Defect Suppression in Sn Perovskite Photovoltaics

Tin-based halide perovskite solar cells offer a compelling lead-free alternative for sustainable photovoltaics; however, their performance is intrinsically limited by the facile oxidation of Sn 2+ and the resulting formation of tin vacancies. Here, we introduce an interface-engineered thermodynamic strategy to suppress intrinsic defect formation by imposing compressive lattice strain during film growth. A deliberately mismatched self-assembled monolayer (SAM) is employed as an active mechanical template, forcing Sn perovskite films to adopt a compressed tetragonal lattice. Structural analysis reveals in-plane lattice contraction accompanied by out-of-plane expansion, while first-principles calculations show that such strain increases the formation energy of Sn vacancies, enabling thermodynamic defect suppression. As a result, the strained films exhibit strongly reduced nonradiative recombination and a doubled photoluminescence quantum yield. In inverted SAM devices, this approach delivers a substantial open-circuit voltage enhancement and a power conversion efficiency of 14.8%. This work establishes mechanical strain engineering related to SAM as a general route to regulate defect chemistry in lead-free perovskites.