Myth behind Metastable and Stable n-Hexylammonium Bromide-Based Low-Dimensional Perovskites

Abstract

In perovskite solar cells, passivating the surface or interface that contains a high concentration of defects, specifically deep-level defects, is one of the most important topics to substantially enhance the power conversion efficiency and stability of the devices. Long-chain alkylammonium bromides have been widely and commonly adapted for passivation treatment. However, the mechanism behind is still not well explored as the formation route and the exact structure of these alkylammonium bromide-based lowdimensional perovskites are unclear. Herein, we investigate the physical and chemical properties of an n-hexylammonium bromide (HABr)-based lowdimensional perovskite including both thin films and single crystals. First of all, the HA 2 PbBr 4 perovskite film and aged single crystal demonstrate different X-ray diffraction patterns from those of the fresh as-prepared single crystal. We found that the fresh HA 2 PbBr 4 single crystal exhibits a metastable phase as its structure changes with aging due to the relaxation of crystal lattice strains, whereas the HA 2 PbBr 4 perovskite film is pretty stable as the aged single crystal. Upon reacting with FAPbI 3 , HABr can be intercalated into the FAPbI 3 lattice to form a mixed-cation perovskite of HAFAPbI 3 Br, which is in a dynamic equilibrium of decomposition and formation. In contrast, the reaction of HABr with excess PbI 2 forms a stable HA 2 PbI 2 Br 2 perovskite. Based on such findings, we rationally develop a HA 2 PbI 2 Br 2 -passivated FACs-based perovskite by reacting HABr with excess PbI 2 , the photovoltaics based on which are more stable and efficient than those passivated by the HAFAPbI 3 Br perovskite. Our discovery paves way for a more in-depth study of bromide-containing lowdimensional perovskites and their optoelectronic applications.

Confinement of MACl guest in 2D ZIF-8 triggers interface and bulk passivation for efficient and UV-stable perovskite solar cells

“…It is well known that the photovoltage of a solar cell is directly related to its internal luminescence ability. 58 Here, we qualitatively demonstrate the reduction in trap density by recording the difference in luminescence performances of the two devices using a photophysical characterization method. 59 As shown in Fig.…”

Recent progress of two-dimensional Ruddlesden–Popper perovskites in solar cells

“…53 Subsequently, more and more studies have been reported on the long-chain organic spacer cations represented by BA ( e.g. n -hexylammonium (HA), 54 ethylammonium (EA), 55 propylammonium (PA), 56 octylammonium iodide (OA), 57 which greatly promoted the development of 2D PR PSCs.…”

Bilayer 2D-3D Perovskite Heterostructures for Efficient and Stable Solar Cells

“…Compared to their pure iodide counterparts, Br/Cl-incorporated 2D perovskites are thermodynamically more stable due to a lower formation energy from the relaxed lattice strain . For instance, the post-treatment of HABr and OABr gave rise to mixed-halide quasi-2D perovskite capping layers of HA 2 FAPb 2 Br 2 I 5 ( n = 2) and OA 2 FAPb 2 Br 2 I 5 ( n = 2), respectively. , Notably, the exposure of the 3D perovskite layer to the corrosive IPA with a high solubility of FAI tends to create a PbI 2 -rich surface that facilitates the formation of 2D perovskite of HA 2 PbI 2 Br 2 ( n = 1) by the chemical reaction between PbI 2 and HABr at the surface, while the use of nonreactive CF results in quasi-2D perovskite of HA 2 FA n ‑1 Pb n I 3 n ‑1 Br 2 ( n ≥ 2) upon the gradual intercalation of HABr into the crystal lattices of FAPbI 3 . , Moreover, oxidative Cl 2 generated in the fresh CF solvent after light soaking could offer a viable Cl source for the successful preparation of Cl-doped bilayer 2D-3D PHSs, where the identified Cl 2 /I- redox reaction triggers the Ostwald ripening for the secondary growth of 3D perovskite grains (∼5 μm) . The easy Cl 2 permeation enables effective overall defect passivation within the 3D perovskite layers as well as the buried interface, thus leading to a high PCE of 24.21% in the Cl-incorporated target devices.…”