Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites

Abstract

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.

Progress of Surface Science Studies on ABX3‐Based Metal Halide Perovskite Solar Cells

“…Recently, Qi, Yan, and co‐workers revealed the exact locations of I and Cl anions in real space with atomic resolution on the surfaces of the MAPbBr 3− y I y and MAPbBr 3− z Cl z mixed halide perovskites by STM. The surface reconstruction of a pristine MAPbBr 3 surface has been previously described to show the characteristic dimmer and zig‐zag structures, with preference of the dimmer structure (Figure c) . The incorporation of I and Cl leads to an additional randomly distribution of bright and dark protrusions (Figure d,e).…”

“…Perovskites are considered to possess a “soft” (or “plastic”) crystalline structure formed by ions exhibiting rich dynamical processes such as organic cation rotations (Figure c), which leads to temporal‐dependent deformation in the inorganic lattice 29a,b,30. For instance, the deformation and dynamics of a metal‐halide lattice is associated with several local and eventually averaged global effects such as (i) dynamic direct‐to‐indirect transitions in bandgap suppressing charge recombination; (ii) spatially separated localization of valence and conduction bands; and (iii) nanoscale ferroelectric domains (due to the MA + cation dipole moment) can assist separation of electrons and holes reducing their recombination.…”

“…Various thermodynamically stable MAPbI 3 surfaces along the different crystallographic directions such as (110), (100), (101), and (001) were characterized . Corroborated by STM studies,11,26,27,38,42a–d DFT calculations revealed that the (001) MAPbI 3 surface was the most thermodynamically stable, hence it has been extensively modeled. Two different terminations for the (001) MAPbI 3 surface found to be stable at room temperature were proposed: PbI 2 and the MAI‐terminated surface ( Figure 11 a) .…”

Nanoscale properties of lead halide perovskites by scanning tunneling microscopy

“…Hsu et al studied photoinduced changes in a MAPbBr 3 single crystal by cross-sectional STM and STS. 50 Figure 3G,H show the surface structures which were obtained in the dark and under 532 nm illumination. While a 2 × 2 dimer structure is seen without illumination in the dark, the structure changes to a dominant 4 × 2 one under 532 nm illumination with a modulation along the [1 10 ] and [110] directions, respectively.…”

“…The authors could also show that this transition is reversible, which was attributed to a photodriven reordering of the MA dipole orientation, which is most likely caused by excitation of electron-hole pairs in spatially displaced orbital states. 50 Shih et al studied the spatial distribution of carriers in a MAPbI 3 crystal under 532 nm illumination. 51 The obtained dI/dV spectra in Figure 3I depicts the differences in conductance in the dark and under illumination where a change in the position of the CB onset relative to the Fermi level was seen (denoted as E D and F I G U R E 3 Influence of surface states, electric field and light-induced changes in lead halide perovskites.…”