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Bending Fracture Behavior of Flexible Organic Lead Halide Perovskite Films Depending on Composition and Morphology

Organic-inorganic halide perovskites receive great attention due to their superb optoelectronic properties such as high efficiency over 25% demonstrated on a glass-based solar cell. Since flexible optoelectronics are considered to be more beneficial to territorial and space applications than the rigid counterparts due to lightweight and bendable characteristics, flexible perovskite optoelectronics need to be developed. However, mechanical properties of perovskite films depending on compositions has not been studied. We report here on bending fracture behavior of the flexible perovskite films depending on the substitution ratio (x) in four different compositions of (FAPbI3)1-x(CsPbBr3)x, (FAPbI3)1-x(MAPbBr3)x, FA1-xCsxPbI3, and FA1-xMAxPbI3 (FA = formaminidinium). The spin-coated perovskite films on the indium-tin-oxide (ITO)-coated polyethylene naphthalate (PEN) substrates show different morphology depending on x and composition. At certain values of x, that is mixed cation and/or mixed anion, a wrinkled morphology is obtained with different amplitude and wavelength in the sinusoidal surface structure. Based on applied and critical strains, fracture energy and fracture toughness are estimated, where the mixed compositions with enhanced wrinkled morphology show an improved bending fracture behavior as compared to the pure FAPbI3 or MAPbBr3. The highest fracture energy of 186.2 J/m2 along with fracture toughness of 3.559 GPa∙(m1/2) is obtained from (FAPbI3)0.75(CsPbBr3)0.25 due to highly enhanced wrinkled morphology.