A- or X-site mixture on mechanical properties of APbX3 perovskite single crystals

Organic-inorganic halide perovskites (OIHPs) have the typical composition of APbX(3), in which A is a cation such as methlyamine (MA) and formamidine (FA) and X is a halide anion such as Cl, Br, or I. The mixture at the A or X site in OIHPs provides greatly improved versatility in their compositions and therefore allows the enhancement of their performance in LEDs and solar cells. In real application circumstances and deformable devices, the mechanical properties of OIHPs are of great importance. In this work, the mechanical properties of three series of mixed OIHP single crystals, MAPbI(x)Br(3-x), MAPbBr(x)Cl(3-x), and FA(y)MA(1-y)PbBr(3), are studied by nanoindentation. The results are summarized according to the composition of these mixtures. With the increase in the FA content at the A site, the average Young's modulus (E) of FA(y)MA(1-y)PbBr(3) decreases greatly from 19.2 GPa to 11.5 GPa, which indicates that the influence of organic cations on the mechanical properties of OIHPs is as important as that of Pb-X bonds. The mixture at the A or X site could also increase the hardness (H) and the wear resistance (H/E ratio). The average values of hardness and wear resistance of MAPbI(0.1)Br(2.9) are almost double (0.63 GPa, 0.033) the values for undoped MAPbBr(3) (0.32 GPa, 0.017). The dynamic mechanical responses of the OIHP single crystals show reduced creep stress exponents and thus increased strain rate sensitivities in the mixture at the X site, thereby improving the ductility. The nanoindentation sites are characterized using a scanning electron microscope and slip bands are observed, suggesting the plastic deformation mechanism governed by the activation of dislocations.