Simple Visualization of Universal Ferroelastic Domain Walls in Lead Halide Perovskites

Abstract

Domain features and domain walls in lead halide perovskites (LHPs) have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of solution‐processable semiconductors. Using nonpolarized light and simple imaging configurations, ferroelastic twin domains and their switchings through multiple consecutive phase transitions are directly visualized. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, optically anisotropic domains inside the material bulk. The findings show these domain walls serve as internal reflectors and steer energy transport inside halide perovskites optically. First‐principles calculations show universal low domain‐wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile moving observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of nonvolatile on–off switching and tunable positioning endowed by their universal ferroelasticity.

Ferroics in Hybrid Organic–Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

“…[44] (Me 3 NCH 2 Cl)MnCl 3 (mmmFm-type ferroelectric) has four crystallographically equivalent polar directions exhibiting herringbone-shaped non-180°d omain textures along both lateral and vertical directions. [45] As exemplified in Figure 2c, the topology of domain textures is associated with the ferroic order vectors and equivalent order directions, which can be stripe-type, [26,44,46] hexagonal-type, [47] herringbone-type as well as vortex-antivortex-shaped. [45,48] The ferroelastic twin domains in HOIPs are often stripe-like which involve the slight rotation of crystal lattice at the domain boundaries to minimize the strain energy (Figure 2c(i)).…”

Robust Ferroelasticity and Carrier Dynamics Across the Domain Wall in Perovskite‐Like van der Waals WO2I2

“…The POM image of WO 2 I 2 clearly shows the striped optical contrast that is less visible in non-polarized optical microscopy, where the optical contrast in the POM image originates from the birefringence phenomenon associated with the orientation of the anisotropic crystal. [40][41][42][43][44][45] The refractive index of WO 2 I 2 depends on the direction of the linear polarization of the incident light versus the principal optical axes, which determines the intensity of reflective light under linear polarized light. WO 2 I 2 flakes with the same orientation and thickness exhibit the same optical contrast in POM.…”

Strain Regulation and Photophysical Properties in Halide Perovskite

“…The volume of PbI 2 expands twofold as a result of conversion to MAPbI 3 perovskite on the ZnO surface, which brings perovskite decomposition into MAI and PbI 2 as a consequence of methylammonium cations in MAPbI 3 deprotonating on ZnO. [27] The microstrain observed in perovskite arises from lattice disorder, which can be attributed to ion radii mismatch, [28,29] local phase transition, [30,31] and phase separation. [32,33] Recently, the application of strain control to reduce unwanted energy loss in perovskite photovoltaic device absorbers has resulted in enhanced performance and stability by minimizing defects and nonradiative recombination.…”

“…The volume of PbI 2 expands twofold as a result of conversion to MAPbI 3 perovskite on the ZnO surface, which brings perovskite decomposition into MAI and PbI 2 as a consequence of methylammonium cations in MAPbI 3 deprotonating on ZnO. [ 27 ] The microstrain observed in perovskite arises from lattice disorder, which can be attributed to ion radii mismatch, [ 28,29 ] local phase transition, [ 30,31 ] and phase separation. [ 32,33 ]…”