Strain engineering and epitaxial stabilization of halide perovskites

Strain engineering is a powerful tool with which to enhance semiconductor device performance 1 , 2 . Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties 3 – 5 . Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization 6 – 8 , electrostriction 9 , annealing 10 – 12 , van der Waals force 13 , thermal expansion mismatch 14 , and heat-induced substrate phase transition 15 , the controllable and device-compatible strain engineering of halide perovskites by chemical epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here we report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. We investigated strain engineering of α-formamidinium lead iodide (α-FAPbI 3 ) using both experimental techniques and theoretical calculations. By tailoring the substrate composition—and therefore its lattice parameter—a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI 3 thin film. We demonstrate that this strain effectively changes the crystal structure, reduces the bandgap and increases the hole mobility of α-FAPbI 3 . Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI 3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI 3 -based photodetector. A method of deposition of mixed-cation hybrid perovskite films as lattice-mismatched substrates for an α-FAPbI 3 film is described, giving strains of up to 2.4 per cent while also stabilizing the metastable α-FAPbI 3 phase for several hundred days.