Nanoconfined Metal Halide Perovskite Crystallization within Removable Polymer Scaffolds

Nanoconfining crystallization to access metastable polymorphs and prescribe crystal orientations typically involves filling inert nanoporous scaffolds with target compounds, resulting in isolated nanocrystals. Such crystal-scaffold composites are unsuitable for optoelectronic devices that require interconnected crystalline pathways for charge transport. Here, we reverse the order of fabricating crystal-scaffold composites by first electrospinning interconnected networks of amorphous methylammonium lead iodide (MAPbI₃) precursor nanofibers, then introducing a poly(methyl methacrylate) (PMMA) scaffold by spin coating from an antisolvent for MAPbI₃. PMMA suppresses MAPbI₃ crystal blooming from the fiber surface during thermal annealing, instead promoting the formation of densely packed polycrystalline networks of MAPbI₃ crystals at the fiber/PMMA interface. Near-IR photodetectors comprising densely packed MAPbI₃ nanocrystals grown within a PMMA scaffold in a coplanar electrode geometry exhibit photocurrents up to 60 times larger than those comprising fibers annealed without PMMA. These results indicate that MAPbI₃ crystals form a percolated network for charge carriers to flow through PMMA-confined fibers, resulting in significantly improved photodetector performance.

False-colored TEM images of electrospun MAPbI₃ fibers highlighting MAPbI₃ crystals that formed during thermal annealing in the absence and presence of a confining polymer scaffold. Large, isolated crystals bloomed from the surfaces of unconfined fibers, while confined fibers exhibited small, interconnected crystals at the fiber surface.