Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Abstract

The electron–phonon (e–ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e–ph interaction in LHPs. Herein, we advance a fundamental understanding of the e–ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure–property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)­PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)­PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e–ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e–ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites

“…On the other side, the increase in optical bandgap from MA + to ATA + arises mainly from the Sn–I bond length elongation due to the oversized A-cation, which also weakens the spin–orbit coupling and decreases the band dispersion, and therefore increases the optical bandgap . Similar optical bandgap bowing has also been observed in both 3D and 2D Pb-based perovskites. , …”

“…Lead halide perovskites (LHPs) are an emerging class of semiconductors that exhibit a set of properties including soft and dynamic lattices, defect tolerances, spontaneous lattice polarizations, nonlinear optical responses, and spin–orbit coupling, which have enabled breakthroughs in various applications such as photovoltaics, optoelectronics, ferroelectrics, and spintronics. − Central to these successes is the fundamental solid-state chemistry study on how structural chemistry dictates the specific physical properties of LHPs. A particularly important structure–property relationship discovered in recent studies is related to the stereochemical expression of the 6s 2 lone pair on the Pb 2+ cation, ,− which causes dynamic or static distortion of the local octahedral geometry, as the resulting lone pair occupies one of the coordination sites. The dynamic lone-pair expression is likely responsible for the strong lattice anharmonicities and local polar fluctuations in LHPs, ,− whereas the static manifestation may give rise to inversion symmetry breaking and noncentrosymmetric structuresa prerequisite to properties such as ferroelectricity, even-order nonlinear optical properties, and Rashba–Dresselhaus effect. ,,− …”

“…A particularly important structure–property relationship discovered in recent studies is related to the stereochemical expression of the 6s 2 lone pair on the Pb 2+ cation, ,− which causes dynamic or static distortion of the local octahedral geometry, as the resulting lone pair occupies one of the coordination sites. The dynamic lone-pair expression is likely responsible for the strong lattice anharmonicities and local polar fluctuations in LHPs, ,− whereas the static manifestation may give rise to inversion symmetry breaking and noncentrosymmetric structuresa prerequisite to properties such as ferroelectricity, even-order nonlinear optical properties, and Rashba–Dresselhaus effect. ,,− …”

Mixology of MA1-xEAxPbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics and Photoluminescence

“…Further studies involving more precise diffraction methods are necessary to probe this phase in more detail. As recently discussed by Huang et al for the related MA 0.13 EA 0.87 PbBr 3 perovskite, 44 the disappearance of the low-temperature phase transition may be also related to the tolerance factor approaching unity upon mixing, which prevents large octahedral tilts associated with the orthorhombic P nma phase. Upon further increase of x, we observed phase separation indicating that the EA solubility limit in MAPbI 3 is about 40%.…”

“…The hybrid compounds containing EA cations are gaining significant attention for their improved stability and photovoltaic performance. [24][25][26][27][28][29][42][43][44] Pure EAPbI 3 crystallizes into the orthorhombic (P nma) symmetry [45][46][47] with the anionic component having a 1D architecture composed of [PbI 3 ]n columns which are built of face-sharing octahedra, however no information is available on the structural phase transitions of this compound. The band gap of this compound is 2.2 eV making it suitable for photovoltaic applications.…”