Strain engineering in metal halide perovskite materials and devices: Influence on stability and optoelectronic properties

Metal halide perovskites (MHPs) have been extensively studied for their promising applications in solar cells and other devices due to their extraordinary optoelectronic properties, low cost, and easy fabrication by versatile processes. Different from bulk crystals grown from solutions, polycrystalline perovskite films deposited on substrates generally are strained due to multiple mechanisms, which significantly impact their optoelectronic properties, defect physics, and photostability. The fabrication and operation of perovskite solar panels inevitably introduce strains in perovskite. Strain has been broadly applied to stabilize the photoactive phase of several perovskite compositions that would otherwise show a thermodynamically stable photoinactive phase at room temperature. There is increasing research on strain engineering of MHPs to enhance device performance. However, a systematic review and understanding of strain engineering in MHP is still lacking. Herein, an overview of strain engineering on MHP materials and solar cells is provided. In this review, we start with a general review on strain in semiconductors, including the characteristics of strain, characterization techniques, and the effects of strain on the lattice structure, electronic, and optical properties of semiconductors. We then summarize progress in understanding the generation of strain categorized by local and global strains and their impacts on the multi-faceted properties of MHPs, including phase stability, photostability, and other optoelectronic properties. Both positive and negative impacts have been observed on these properties. Strain engineering has shown to be promising in making much more efficient and stable perovskite solar cells.