Understanding Structural Differences in Completely Dry-Processed MAPbI₃ Thin Films by Detailed Analyses of Temperature-Dependent Optical Spectroscopy

In recent years, halide perovskites have attracted much attention due to remarkable increase in associated solar cell efficiencies, which were achieved by processing high quality thin films. Here, even small differences in film morphology seem to be relevant for optimized optoelectronic functionality and stability. However, a detailed understanding of how small morphological differences in perovskite films affect related structural and optoelectronic properties has not yet been fully developed.<br/>Here, we investigate the influence of small morphology differences induced in MAPbI₃ thin films by (hot-)pressing, characterized by small increase in crystallographic orientation and grain size, on the structural properties phase transition, defects and energetic disorder. To this end, we perform detailed analyses of temperature-dependent absorption and PL measurements between 300 K and 5 K. We use MAPbI₃ thin films prepared by a completely dry processing approach, comprising film formation based on mechanochemically synthesized MAPbI₃ powder via powder aerosol deposition method [1,2], followed by (hot-)pressing. The latter has emerged as an attractive method for manufacturing and post-treatment of perovskite films [3,4]. Detailed temperature-dependent absorption and PL analyses reveal lower strain, decreased defect density and less energetic disorder in films with higher out-of-plane orientation and grain size. To resolve even small differences in static energetic disorder, suitable, powerful analysis methods of the Urbach energy are presented.<br/>Our results demonstrate the importance of considering even small morphology differences in halide perovskite films regarding related structural and optoelectronic film properties. The gained knowledge will facilitate identifying structural differences in halide perovskites to further optimize their functionality and stability.<br/><br/>[1] Leupold, N. et al. High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices. <i>ACS Appl. Mater.</i> 2019, 11 (33), 30259-30268.<br/>[2] Panzer, F. et al. Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process—Uncoupling Material Synthesis and Layer Formation. <i>Materials</i> 2016, 9 (4), 277.<br/>[3] Witt, C. et al. Impact of Pressure and Temperature on the Compaction Dynamics and Layer Properties of Powder-Pressed Methylammonium Lead Halide Thick Films. <i>ACS Appl. Electron. Mater.</i> 2020, 2 (8), 2619-2628.<br/>[4] Pourdavoud, N. et al. Room-Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films. <i>Adv. Mater.</i> 2019, 31, 1903717.