Polymer interface-driven perovskite crystallization in electrospun fibers

Polymer incorporation into metal halide perovskite films is an effective strategy to control perovskite nucleation and crystallization, passivate defects and suppress humidity-induced degradation. In this work, we use enthalpic interactions between insulating polymers and formamidinium lead iodide (FAPbI 3 ) to control FAPbI 3 crystal size, quality and location. Specifically, we electrospin interconnected networks of FAPbI 3 precursor/poly(vinylpyrrolidone) (PVP) composite nanofibers, then encapsulate the fibers in a poly(methyl methacrylate) (PMMA) or poly(phenylene oxide) (PPO) scaffold prior to thermal annealing to induce α-FAPbI 3 crystallization. Polar PMMA nucleated dense, polycrystalline FAPbI 3 crystals at the PMMA/fiber interface. Non-polar PPO scaffolds, on the other hand, suppressed FAPbI 3 nucleation at the interface, suggesting enthalpically unfavorable perovskite-polymer interactions. Transmission electron microscopy revealed that PPO drives FAPbI 3 crystallization within fiber interiors to form FAPbI 3 core/PVP shell structures. PPO-confined FAPbI 3 fibers exhibited long excited carrier lifetimes of ~1200 ns, surpassing those of bulk single crystals, but poor charge transfer to a hole transport layer due to the presence of an insulating PVP shell. Unconfined and PMMA-confined FAPbI 3 fibers, in contrast, showed rapid, defect-induced photoluminescence decay and fast charge transfer, the latter of which contributed to improved photodetector responsivity compared to PPO-confined fiber photodetectors. Polymer incorporation into these perovskite films improved their environmental stability, with α-FAPbI 3 persisting for more than seven weeks of storage in air. Collectively, these results demonstrate the use of perovskite-polymer interactions to dictate perovskite crystallization outcomes, including size, location and defect density.