Abstract
Applying mechanical strain and strain engineering to halide perovskites has endowed them with intriguing properties. However, an in-depth understanding of mechanical strain, including residual strain in halide perovskites, remains incomplete, coupled with the critical challenge of decoupling strain effects from other interferences. Here, we examine the relaxation of residual tensile strain in three-dimensional (3D) halide perovskites through 2D/3D perovskite heterojunction formation. The 2D perovskite induces structural fragmentation in 3D perovskites, facilitating plastic relaxation of tensile strain. By isolating extrinsic crystalline phase interference and exciton-related optical disturbances, we observe that 3D perovskites retain high crystallinity only with moderate tensile strain relaxation. This moderate relaxation enhances optoelectronic properties in 3D perovskites, including broadened band-to-band absorption and prolonged charge carrier lifetime, markedly contributing to an increase in the maximum attainable power conversion efficiency in photovoltaic devices. Our findings outline conditions for strain relaxation that optimize optoelectronic properties, advancing strain engineering in halide perovskites.