Phase-Pure Hybrid Layered Lead Iodide Perovskite Films Based on a Two-Step Melt-Processing Approach

Layered lead halide perovskites have recently been heavily investigated due to their versatile structures, tunable electronic properties, and better stability compared with 3D perovskites and have also been effectively incorporated into photovoltaic and light-emitting devices. They are often prepared into thin film form by solution methods and typically contain a mixture of phases with different inorganic layer thicknesses (denoted by n). In addition, melt-processing has recently been introduced as an option for film deposition of n = 1 lead iodide-based perovskites. Here, we study the thermal properties of higher n (n > 1) layered perovskites in the family (beta-Me-PEA)(2)MA(n-1)Pb(n)I(3n+1), with n = 1, 2, and 3 and where beta-Me-PEA = beta-methylphenethylammonium and MA = methylammonium, and reveal that they do not melt congruently. However, they can still be melt-processed in air by using a two-step process that includes a lower temperature post-annealing step after the initial brief melting step. While typically higher n films contain a mixture of the different n phases, the resulting two-step melt-processed films are highly crystalline and phase pure. Optical and electrical properties of these films were further characterized by time-resolved photoluminescence and dark/illuminated transport measurements, showing the same order of magnitude single-exciton recombination rates compared to previous single crystal results and >2 orders of magnitude higher conductivity compared to conventional spin-coated films. These results offer new pathways to study the layered perovskites and to integrate them into electronic and optoelectronic devices.