Inner Strain Regulation in Perovskite Single Crystals through Fine-Tuned Halide Composition

Halide-composition engineering has been recognized as an effective means to stabilize formamidine (FA)-based perovskite (FAPbI3) in thin-film devices. However, the influence of a halide mixture on the lattice strain, trap density, carrier lifetime, and phase stability remains unclear. Herein, FAPbBrxI3–x single crystals (SCs) are grown as a model to study how halide composition would affect the inner strain and the optoelectronic properties. It is found that by gradually increasing the Br/I ratio from 0:1 to 1:0, the inner strain can be regulated. X-ray diffraction results show excellent phase stability of FAPbBrxI3–x SCs (stable in an ambient atmosphere over 1.5 years when x ≥ 0.5) because of the released expansion stress in the cubic lattice. The relevant photodetector presents good performance with an on–off ratio exceeding 103, a dark current as low as 10–10 A, a linear dynamic range (LDR) of 111 dB, detectivity of 4 × 1011 Jones, and long-term photostability. The present work paves a path toward the design of high-quality devices based on composition-optimized FAPbBrxI3–x SCs with predictable optoelectronic properties.