Investigating the Formation Dynamics and Stability of the 2D/3D Interface in Perovskite Thin Films

The quest for novel sustainable energy production and storage technologies is crucial for addressing the challenges associated with energy demand and the depletion of fossil fuels. In this regard, perovskite solar cells have garnered significant attention due to their high efficiency achieved within a few years, along with relatively low production costs. However, their longevity remains inferior to the conventional silicon technology due to instability under ambient conditions. One approach to address this stability issue is fabricating structured 2D/3D perovskite films; while low-dimensional 2D perovskites exhibit higher environmental stability, 3D perovskites contribute to enhanced efficiency. Despite the progress in the field of 2D/3D structures, there remains a lack of detailed understanding of the 2D/3D perovskite interface and its behavior under environmental stressors [1-3]. In light of this, we employed advanced in situ characterization techniques to monitor and evaluate the formation dynamics and stability of 2D/3D perovskites under stress conditions. Two 2D-forming cations – phenylethylammonium iodide (PEAI) and oleylammonium iodide (OLAI) – were investigated, and 2D layers were grown on CsFAMAPbI3 films. In-situ photoluminescence (PL) measurements revealed the formation of n=1 and n=2 structures for both materials. Notably, OLAI tended to convert n=1 to n=2, whereas PEAI preferentially formed n=1 after annealing. Confocal imaging indicated that the exposure of films to 485 nm light induced the degradation of 2D layers in the case of PEAI, whereas the conversion of n=1 to n=2 was observed for OLAI. In-situ X-ray diffraction (XRD) was conducted to monitor potential structural changes in the 2D/3D films under thermal stress of 85-celsius degrees for 18 hours; results suggested the amorphization of the 2D structure when using PEAI, whereas OLAI displayed improved crystallinity. Scanning electron microscopy (SEM) images of films before and after thermal stress indicated that the 2D layers mitigate the degradation of the 3D perovskite. However, OLAI formed a compact layer, which undergoes morphological changes to nanoplates covering the entire film surface after thermal stress, while PEAI showed several voids before and after the annealing. Cathodoluminescence mapping, both top-view and cross-sectional, demonstrated that 2D structures preferentially grow at grain boundaries. In-situ liquid cell transmission electron microscopy (TEM) was employed to investigate the growth of OLAI. The results revealed that after OLAI injection, 2D growth initially occurred at PbI2 on the surface of perovskite films, followed by growth at grain boundaries and facets, and eventually, growth induced by beam damage. These findings enhance our understanding of the selective nature of 2D structure formation and the superior stability of OLAI compared to PEAI.<br/><br/>[1] Sutanto, A.A., et al., <i>In Situ Analysis Reveals the Role of 2D Perovskite in Preventing Thermal-Induced Degradation in 2D/3D Perovskite Interfaces. </i>Nano Letters, 2020. <b>20</b>(5): p. 3992-3998.<br/><br/>[2] T. Kodalle, R. F. Moral, L. Scalon, R. Szostak, M. Abdelsamie, P. E. Marchezi, A. F. Nogueira, C. M. Sutter-Fella, Revealing the Transient Formation Dynamics and Optoelectronic Properties of 2D Ruddlesden-Popper Phases on 3D Perovskites. <i>Adv. Energy Mater.</i> 2023, 13, 2201490.<br/><br/>[3] Randi Azmi <i>et al.</i> Damp heat–stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions.<i>Science 2022. </i><b>376</b>,73-77