Effect of transparent polymer encapsulation overlayers on bending fracture behavior of flexible organic lead halide perovskite thin films

Although halide perovskites have significant potential for flexible devices, the bending mechanical behavior of halide thin films has not been investigated in detail. Herein, we report the quantitative bending fracture behavior of flexible methylammonium lead triiodide (MAPbI₃) thin films encapsulated with an overlayer of either polystyrene (PS) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The significantly retarded cracking behavior of the halide films demonstrated that adjusting the polymer overlayers with different thicknesses resulted in substantial mechanical enhancements. For example, the minimum strain for crack initiation was ~1.96% for a highly concentrated PS overlayer with a thickness of ~50 nm, which corresponds to an increase of ~40% relative to the non-encapsulated sample. In addition to quantifying the fracture behavior with fracture energy and film strength, the chemical effects of encapsulation were also investigated in terms of the phase dissolution of the perovskite, which was correlated with changes in the Hall electrical properties.