Robust Heterostructures in Two‐Dimensional Perovskites by Threshold‐Dominating Anion Exchange

Abstract

Heterostructures play an irreplaceable role in high‐performance optoelectronic devices. However, the preparation of robust perovskite heterostructures is challenging due to spontaneous interdiffusion of halogen anions. Herein, a vapor‐phase anion exchange method universally suitable for the preparation of robust 2D Ruddlesden–Popper perovskite (RPP) heterostructures is developed. A variety of heterostructures are fabricated based on exfoliated RPP microplates (MPs). Depending on the specific organic cations, the heterostructures can be either sharp and uniform, or broad and gradient, suggesting a new anion diffusion behavior different from that in 3D perovskites. Further experimental studies reveal that the lateral transport of anions follows a threshold‐dominating mechanism, while the vertical transport can be partially or completely suppressed by organic cations. Subsequently, quantitative investigation of anion diffusion in 2D perovskites is conducted. The lateral diffusion coefficient of halogen anions is calculated to be 6 to 7 orders of magnitude larger than the vertical coefficient, consistent with the observed highly anisotropic anion diffusion. In addition, it is shown that the anion exchange threshold can also enhance the thermodynamic stability of the heterostructures at elevated temperature. These results provide a general method to fabricate robust lateral RPP heterostructures, and offer important insights into anion behavior in low‐dimensional perovskites.

Ramifications of Ion Migration in 2D Lead Halide Perovskites

“…The bulky 2T spacer is more efficient in preventing halide ion diffusion in these vertical heterostructures. A similar spacer cation dependence of anion exchange has also been shown in X 2 PbBr 4 /​X 2 PbI 4 heterostructures, with PEA showing a slower diffusion compared to BA …”

“…A similar spacer cation dependence of anion exchange has also been shown in X 2 PbBr 4 /X 2 PbI 4 heterostructures, with PEA showing a slower diffusion compared to BA. 56 Our group compared anion exchange between physically paired 2D perovskite films with bromide and iodide. 48 We found that when a n = 10 bromide perovskite and a n = 10 iodide perovskite with PEA spacers were annealed together (facing each other), halide ion exchange occurred, giving a mixed halide perovskite as shown in Figure 4A.…”

“…However, the direct synthesis of such heterostructures with well-defined interfaces becomes challenging owing to facile ion migration. Vapor-phase anion exchange is an effective method to fabricate perovskite heterostructures with sharp interfaces. − In a recent work, 2D iodide perovskite microplates were subjected to anion exchange by exposing them to HBr vapor to form heterostructures with n = 1 and n = 2 layers . However, for n > 3, the heterostructures formed were more diffuse and did not have sharp interfaces, as the halide migration activation energy significantly dropped with increasing the layer number ( n ).…”

Field-Induced Transport Anisotropy in Single-Crystalline All-Inorganic Lead-Halide Perovskite Nanowires

“…Single-crystalline perovskite nanostructures offer an intrinsic platform for the investigation of ionic movement on a microscopic scale. Research has shown that the reduction of dimensionality may cause significant difference in ionic behavior in one-dimensional (1D) nanowires (NWs) and 2D nanoplates. , In addition, combined with scanning probe measurements, the coupled ionic and optoelectronic transport process can be spatially and temporally resolved, , so that the microscale ionic transport mechanism can be revealed.…”

“…Research has shown that the reduction of dimensionality may cause significant difference in ionic behavior in one-dimensional (1D) nanowires (NWs) and 2D nanoplates. 32,33 In addition, combined with scanning probe measurements, the coupled ionic and optoelectronic transport process can be spatially and temporally resolved, 34,35 so that the microscale ionic transport mechanism can be revealed. In this work, we investigated the ion-drift-induced giant switchable photovoltaic effect in all-inorganic perovskite NW devices.…”