Controlling Solvate Intermediate Growth for Phase-Pure\nOrganic Lead Iodide Ruddlesden–Popper (C₄H₉NH₃)₂(CH₃NH₃)_<i>n</i>−1Pb<i>_n</i>I_3<i>n</i>+1 Perovskite Thin Films

Abstract

The growth of Ruddlesden–Popper perovskite thin films of\norganic lead halides is complicated by the existence of multiple crystallization pathways available\nto precursors in solution. During thin-film growth processes, such\nas spin-coating or blade-coating, solvents can evaporate too quickly\nto clearly resolve different reaction intermediates and products that\nform during crystallization. Here, we resolve multiple reaction products\nand intermediates that form during growth of (C₄H₉NH₃)₂(CH₃NH₃)_<i>n</i>−1Pb<i>_n</i>I_3<i>n</i>+1 Ruddlesden–Popper compounds by studying\ndrop-cast precursor solutions through the evolution of X-ray diffraction,\nphotoluminescence, and optical micrographs in situ over long timescales\nin a thin-film geometry. We found that methylammonium-rich solvate\nintermediates play a crucial role in directing the bulk optical properties\nof the films and form simultaneously with smaller regions of Ruddlesden–Popper\nphases during growth. The microstructure and optical properties of\nthese sub-phases were characterized during growth and after annealing,\nrevealing that discrepancies between thin-film and single-crystal\noptical properties originate from solvate intermediates. These lower-band-gap\nminority phases dominate the optical emission spectrum by means of\nrapid energy migration and contribute to sub-band-gap electronic states\nin photovoltaic devices. Processing routes to yield thin films with\noptical properties similar to single crystals of Ruddlesden–Popper\nphases were developed by tuning the precursor stoichiometry and deposition\nkinetics.