Investigating the Formation Dynamics of Mixed 2D/3D Perovskite Films via Multimodal In Situ Analysis

<br/>Within the current landscape of solar energy materials, hybrid metal halide perovskite thin films have rapidly emerged as an extremely interesting photovoltaic technology due to their continually improving power conversion efficiencies and ease of fabrication [1]. However, instability under conditions of thermal stress or high humidity still remains a significant obstacle to the production of optimized, high-quality hybrid perovskite solar cell devices [2]. The deposition of bulky, organic cations onto the 3D perovskite layer during or after thin film synthesis generating 2D perovskites has recently emerged as a potential solution for this lack of stability [3]. As the method of incorporating these bulky molecules into the 3D hybrid perovskite film has yet to be optimized [4], and considering the known impact that varying properties of these bulky cations and their counterions have on the formation of the 2D layer [3], it is thus of significant interest to gain mechanistic insights into the formation dynamics and the influence of the concentration of the bulky molecules used on these mixed 2D/3D perovskite thin films.<br/>In this study, in-situ transmission, photoluminescence (PL), and grazing incidence wide angle X-ray scattering (GIWAXS) measurements were employed during spin-coating and annealing of the perovskite films to investigate the evolution of the optical and structural properties.The deposition of the 2D material was combined with the antisolvent (AS) drop in lieu of the typical two-step process in which the 3D perovskite layer is completed prior to deposition of the 2D material to reduce both the processing time and the complexity of the deposition process. By varying the concentration of phenethylammonium chloride (PEACl) being deposited during the AS-step we find that the 2D-treated samples exhibit slower and smoother formation dynamics across all growth stages with nucleation and growth rates inversely proportional to the PEACl concentration. For concentrations higher than 1.5 mg/ml, we see a slight shift in the diffraction peaks of the precursor-solvent phase towards larger q-values, indicating that the intermediate solvate-phase has smaller lattice parameters than in the reference case. In the PEACl-treated samples, the PbI2-related diffraction peak temporarily disappears upon attainment of the maximum annealing temperature, and diffraction peaks corresponding to the n=1 and n=2 phases of the layered 2D-structures emerge. The duration of interruption of the PbI₂ signal is longer with increasing PEACl concentration, which we propose derives from consumption of excess PbI2 via reaction with PEACl during formation of the n=1 2D-layer. In addition, the GIWAXS images acquired for the 2D-treated samples indicate strongly pronounced crystal orientation, demonstrating that the simplified combined antisolvent and PEACl drop promotes the formation of 2D-structures. Our results demonstrate the utility of in-situ techniques for obtaining a detailed mechanistic understanding of the 2D layer formation and its subsequent interaction with the 3D triple cation perovskite.<br/>References:<br/>[1] T. Wu et al., Nano-Micro Lett. 152(13), 2021.<br/>[2] J. A. Christians et al., J. Am. Chem. Soc. 137(4), 2015.<br/>[3] T. Kodalle et al., Adv. Energy Mater. 2201490, 2022.<br/>[4] Q. Jiang et al., Nat. Photonics 460(13), 2019.