  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.
