Suppressing the Formation of High n-Phase and 3D Perovskites in the Fabrication of Ruddlesden–Popper Perovskite Thin Films by Bulky Organic Cation Engineering

Two-dimensional perovskites (2D PVKs) were first studied because of their dielectric and magnetic properties for dielectric and transistor applications in the electronic field. As the optical properties were elucidated with the change of n in function of organic and metal components, their fundamental features such as exciton binding energy, quantum and dielectric confinement, and higher thermal and moisture resistance allowed them to position as one of the most promising materials for a wide and diverse range of applications such as in solar cells. However, the preparation of quasi-2D PVK thin films containing single-phase materials corresponding just to the desired n value (n = the number of neighboring inorganic layers in a quasi-2D structure) instead of the wide mixture of different n and 3D PVK phases, usually obtained, is a very challenging topic to overcome. Until now, a few methods to reach that goal have been proposed; however, they imply the use of additional reagents or fabrication steps, increasing the number of precursor materials and the complexity of the process. Here, we report the fabrication of quasi-2D PVK thin films with n = 2, where the formation of undesired higher n-phases, including 3D PVKs, was effectively reduced without any extra reagent/additive by only making use of the particular features of the molecules in each of the evaluated formulation. Powder X-ray diffraction, absorption, and photoluminescence spectroscopy, at room temperature (RT) and low temperature (LT), reveal the fabrication of thin films with reduced phase polydispersity and the absence of the 3D PVK phase. Dynamic light scattering results show that the use of bulky cations possessing robust intermolecular bonding interactions in the precursor solution promotes the formation of colloidal nanoparticles by diminishing the number of particles out of the colloidal size, thus resulting in the desired higher concentration of low n-phases as compared to high-n and 3D phases in PVK thin films. A correlation between dynamic light scattering and 1H Nuclear Magnetic Resonance studies at RT and high temperature (HT) shows that the stronger the intermolecular interactions between bulky cations and the metal halide (in the precursor solution), the better the control on n-phase polydispersity. Interestingly, despite the potential steric hindrance owing to the large substituents on the main organic chain of the ammonium cation, the feasibility to suppress the formation of large n-phase and 3D PVKs during the fabrication of quasi-2D PVK thin films using a cation with stronger intermolecular interactions without additives or extra fabrication steps will promote an accessible progress of the technology based on this kind of materials.