Two-Step Growth of Oriented Quasi-2D Perovskite Nanocrystal Arrays with Composition Gradients

In this work, we demonstrate the oriented growth of quasi-2D metal halide perovskite nanowire and nanoplate arrays with continuous corner-sharing metal halide octahedra sheets normal to the substrate surface, the optimal crystallographic orientation for device stacks. Oriented perovskite growth is achieved through two-step deposition in which PbI2-infiltrated TiO2 nanoporous scaffolds are immersed in co-solutions of formamidinium iodide (FAI) and n-butylammonium iodide (BAI) in isopropyl alcohol to form metal halide perovskites. At 50 mol % FAI or higher relative to BAI, only 3D FAPbI3 (n = ∞) nanowires form at room temperature. BAI does not incorporate into the crystal lattice at these concentrations but stabilizes semiconducting α-FAPbI3 from converting to insulating δ-FAPbI3, the thermodynamically stable phase at room temperature. Immersion in solutions with 33 mol % FAI or lower results in quasi-2D (BA)2(FA)1Pb2I7 (n = 2) or 2D (BA)2PbI4 (n = 1) nanoplate growth. In situ photoluminescence spectra collected during crystal growth revealed the presence of intermediate n > 2 phases before conversion to the thermodynamically favored phase at some compositions. Vertical gradients in n number were also identified by monitoring photoluminescence from the tops and bottoms of films, with α-FAPbI3 and low n phases preferentially forming at the film surfaces and buried interfaces, respectively. Proof-of-concept conductivity mapping via atomic force microscopy demonstrates that these compositionally-graded nanocrystalline arrays are photoconductive, with highest conductivity normal to the substrate.