Title: Chemical Strain Engineering of MAPbI3 Perovskite Films

Abstract: This study introduces a new chemical method for controlling the strain in methylammonium lead iodide (MAPbI3) perovskite crystals by varying the ratio of Pb(Ac)2 and PbCl2 in the precursor solution. To observe the effect on crystal strain, a combination of piezoresponse force microscopy (PFM) and X-ray diffraction (XRD) is used. The PFM images show an increase in the average size of ferroelastic twin domains upon increasing the PbCl2 content, indicating an increase in crystal strain. The XRD spectra support this observation with strong crystal twinning features that appear in the spectra. This behaviour is caused by a strain gradient during the crystallization due to different evaporation rates of methylammonium acetate and methylammonium chloride as revealed by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and grazing incidince x-ray diffraction (GIXRD) measurements. Additional time-resolved photoluminescence (TRPL) show an increased carrier lifetime in the MAPbI3 films prepared with higher PbCl2 content, suggesting a decreased trap density in films with larger twin domain structures. The results demonstrate the potential of chemical strain engineering as an easy method for controlling strain-related effects in lead halide perovskites.