Morphology and Photoluminescence of CH₃NH₃PbI₃ Deposits on Nonplanar, Strongly Curved Substrates

Organic–inorganic metal halide perovskites have recently attracted increasing attention as highly efficient light harvesting materials for photovoltaic applications. The solution processability of these materials is one of their major advantages on the route toward fabrication of low-cost solar cells and optoelectronic devices. However, the precise control of crystallization and morphology of organometallic perovskites deposited from solutions, considered crucial for enhancing the final photovoltaic performance, still remains challenging. In this context, here, we report on growing microcrystalline deposits of methylammonium lead triiodide perovskite, CH₃NH₃PbI₃ (MAPbI₃), by one-step solution casting on cylinder-shaped quartz substrates (rods) having diameters in the range of 80 to 1800 μm. We show that the substrate curvature has a strong influence on morphology of the obtained polycrystalline deposits of MAPbI₃. Specifically, a marked size reduction of MAPbI₃ microcrystallites concomitant with an increased crystal packing density was observed with increasing the substrate curvatures. In contrast, although the crystallite width and length markedly decreased for substrates with higher curvatures, the photoluminescence (PL) spectral peak positions did not significantly evolve for MAPbI₃ deposits on substrates with different diameters. The crystallite size reduction and a denser coverage of microcrystalline MAPbI₃ deposits on cylinder-shaped substrates with higher curvatures were attributed to two major contributions, both related to the annealing step of the MAPbI₃ deposits. In particular, the diameter-dependent variability of the heat capacities and the substrate curvature-enhanced solvent evaporation rate seemed to contribute the most to the crystallization process and the resulting morphology changes of MAPbI₃ deposits on cylinder-shaped quartz substrates with various diameters. The longitudinal geometry of cylinder-shaped substrates provided also a facile solution for checking the PL response of the deposits of MAPbI₃ exposed to the flow of various gaseous media, such as oxygen (O₂), nitrogen (N₂), and argon (Ar). Specifically, under excitation with λ_exc = 546 nm, the rapid and pronounced decreases and increases of PL signals were observed under intermittent subsequent exposures to O₂ and N₂, respectively. Overall, the approach reported herein inspires novel, cylinder-shaped geometries of MAPbI₃ deposits, which can find applications in low-cost photo-optical devices, including gas sensors