Crystallization of Low-Dimensional Lead-Free (R/S-MBA)₂CuBr₄ Chiral Perovskite Thin Films

Low-dimensional halide perovskites with chiral organic spacer cations possess unique properties that are appealing for next-generation optoelectronic and spintronic applications. However, a complete understanding and effective control over the thin-film crystallization kinetics of these materials have not yet been established. Here, we unravel a plausible diffusion-controlled crystallization mechanism with spontaneous nucleation for lead-free (R/S-MBA)₂CuBr₄ thin films by estimating the Avrami exponent, n ≈ 0.45, from time-dependent UV-vis absorption spectra. Additionally, by exploring the influence of processing conditions on the phase purity and morphology of the (R/S-MBA)₂CuBr₄ thin films, we demonstrate the formation of an oriented 2D crust at the air-ink interface, followed by continuous crystallization toward the substrate. Furthermore, the influence of trapped residual solvent and the effect of atmospheric conditions on the air-ink interface were explored to understand the manifestation of a 1D impurity phase within the 2D matrix. We report controlled solvent removal using a vacuum process for suppressing the occurrence of the 1D phase in the bulk of the films triggered by the presence of residual solvent molecules. Our findings elucidate the importance of film processing conditions for controlling phase purity in chiral 2D (R/S-MBA)₂CuBr₄ thin films. The insights presented in this work will advance fabrication methods for achieving chiral low-dimensional perovskite-based chiroptical and spintronic devices.