Halide Segregation in Ruddlesden-Popper Perovskites

Ruddlesden-Popper perovskites (RPPs) have shown promise as stable alternative to conventional 3D perovskites (PVKs), which are well-known to suffer from instability introduced, among others, by light exposure. Mixed-halide PVKs, in fact, display a phenomenon known as halide segregation, where illumination is followed by ion migration and formation of iodide- (I-) and bromide-rich (Br-) regions with different bandgap energy (Eg). Halide segregation is a common problem for wide-bandgap PVKs with high content of bromide ions as the formation of I-rich regions effectively lowers Eg, which is of crucial importance for tandem applications where such PVKs are likely to be used. In our work, we first investigated halide segregation in a pure 2D perovskite (33% Br content). We studied its photostability by continuously measuring photoluminescence (PL) spectra under 3-sun illumination over time and found that the pure 2D system is relatively resilient to halide segregation. We additionally calculated via DFT simulations the formation energy of I- and Br- vacancies, which have been previously linked to halide segregation, and their corresponding diffusion barrier. Here, we found that 2D perovskites possess higher formation energy of vacancies compared to 3D MAPbI3 and that their diffusion barrier is more than 2x higher. Furthermore, we investigated halide segregation in quasi-2D perovskites consisting of a 2D-3D gradient along the thickness of the film. This allowed us to investigate halide de-mixing in multiple RPP phases and in 3D perovskites, at the same time. We found that by increasing the dimensionality of the perovskite, hence going from 2D to 3D, the tendency to suffer from halide segregation increases. Nevertheless, interesting differences in recovery behaviour for multiple RPP phases have been identified and explained by both experimental and theoretical means