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- and bromide-rich regions with different bandgap energy (E_g). 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 E_g, which is of crucial importance for tandem applications where such PVKs are likely to be used.<br/><br/>Recently, research has focused on the use of lower-dimensional PVKs, such as RPPs, to mitigate halide segregation. In RPPs, bulky organic spacers are used to separate inorganic layers of PbI₂in quantum-well structures that are usually described by an <i>n</i>-value, which is the number of octahedral sheets between the spacers. In our work, we first investigated halide segregation in pure 2D perovskite PEA₂Pb(I_0.67Br_0.33)₄ (<i>n</i> = 1, 33% Br content). We studied its photostability by continuously measuring photoluminescence (PL) spectra under 3-sun illumination for 10⁴ s and found that the pure 2D system is relatively resilient to halide segregation. The PL emission peak, in fact, displayed only a marginal redshift from 2.46 to 2.41 eV, which indicates that only a small amount of lower-E_g, I-rich perovskite phases formed over time. To gain more insights on the photostability of RPPs with <i>n </i>= 1, we 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 (2.13 eV) compared to 3D MAPbI₃(1.87 eV) and that their diffusion barrier is more than 2x higher. Interestingly, DFT calculations also showed that both Br ions and vacancies are preferentially located at the equatorial position of the lead halide octahedra, while I ions are mostly at the axial position.<br/><br/>Once we clarified the higher photostability of pure 2D PVKs, we investigated RPPs with higher <i>n</i>-values, as they are more relevant for device integration due to their broad absorption profile and higher stability and efficiency. However, when forming an RPP with <i>n </i>&gt; 1 (quasi-2D) it is common to form a multidimensional system where multiple phases with different <i>n</i>-values co-exist. Furthermore, these PVK phases form a vertical 2D-3D gradient along the thickness of the film, which we finely tuned by means of solvent engineering.<br/><br/>Firstly, we fabricated a strongly graded 2D-3D film, with <i>n </i>= 1 and 2 phases coexisting with 3D ones, to study halide segregation of multiple phases at the same time. For these films, we measured PL under constant illumination and observed that both RPPs with <i>n </i>= 2 and (quasi-)3D perovskite phases undergo halide segregation. By analyzing their recovery, however, we found that while 3D perovskites shift back to the original state in the dark, <i>n</i> = 2 RPPs do not. This is because recovery in the dark is mostly dictated by an entropic contribution, which is hindered by the high activation energy of ion migration in lower-dimensional systems. To confirm this, we studied the recovery of <i>n </i>= 2 phases in the dark at higher temperature and were successful in inducing a return to the original state. From these findings, it can be concluded that the energetic barrier for halide redistribution is sufficiently high in segregated <i>n </i>= 2 phases and requires the application of additional energy to induce recovery in the dark. Finally, again by solvent engineering, we produced films with a less distinct 2D-3D gradient, producing PVKs that undergo negligible changes under illumination, showing that RPPs are a valuable means to prevent halide segregation.