Dynamical Phase Transitions and Cation Orientation-Dependent Photoconductivity in CH(NH₂)₂PbBr₃

Abstract

The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH(NH 2 ) 2 + ) perovskites exhibit unusual temperature-dependent trends in photoluminescence, dielectric constant, and phase behavior that are hypothesized to relate to CH(NH 2 ) 2 + reorientations. This contribution describes five distinct, temperaturedependent phase transitions in CH(NH 2 ) 2 PbBr 3 that produce changes in the steady-state photocurrent. Three of these phase transitions do not resolve crystallographically and relate to the orientation and dynamics of CH(NH 2 ) 2 + . These crystallographically unresolvable phase transitions resemble ferroelastic transitions with the formation of nanoscale domains, which we hypothesize are mediated by the strain exerted from geometric frustration of CH(NH 2 ) 2 + quadrupoles. This work demonstrates the importance of cation orientation and dynamics, domain behavior, and their interdependence in the steady-state optoelectronic properties of hybrid perovskites.

Phase Diagram and Cation Dynamics of Mixed MA1-xFAxPbBr3 Hybrid Perovskites

“…In addition, such a distribution of energy barriers may be related to the recently proposed quadrupolar geometric frustration of the FA cations. 42 As discussed in Ref. 30, mixing reliefs this frustration causing disappearance of the isostructural transitions, which might be connected to the absence of these low-temperature dipolar relaxations in the mixed compounds.…”

“…The dielectric response of the non-mixed FAPbBr 3 perovskite exhibits four additional anomalies (Figure 3g). Three of them can be related to the isostructural transitions (Figure 2), 30,42 while one observed at about 140 K is only present in the ultrasonic data (Figure S4). The overall temperature dependence of ε * is in a good agreement with the previous dielectric studies of the same compound.…”

“…This indicates that either the symmetry change is suppressed at the maximal mixing level or the changes are too subtle to be detected with our XRD setup. We used our experimental data together with the information provided in the literature [35][36][37]41,42 to construct the temperature-composition phase diagram of MA 1-x FA x PbBr 3 (see Figure 2). The MA-rich region of the phase diagram is in a qualitative agreement with the phase diagrams reported for the related MA 1-x FA x PbI 3 perovskite system.…”

“…[39][40][41] Three additional isostructural transitions were observed at 182, 162 and 118 K that involve changes of the FA cation dynamics and are not resolved crystallographically. 30,42 A substantial degree of disorder of FA cations was found even at 10 K temperature. 41 Our results show that mixing of these compounds significantly suppresses the structural phase transitions resulting in stabilization of the cubic phase.…”

Dynamics & Spectroscopy with Neutrons—Recent Developments & Emerging Opportunities

“…A substantial part of our current understanding of the structural properties of these materials stems from Neutron Diffraction (ND) [ 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 ]. Most importantly, the use of neutron Laue diffractometers, operating both at pulsed (e.g., SXD at ISIS, UK [ 95 ]; TOPAZ at SNS [ 96 ], US; SENJU at J-PARC, JP [ 97 ]) and continuous neutron sources (e.g., KOALA at ANSTO, AU [ 98 ]; FALCON at BER-2, DE [ 99 ]—currently transferred to the spallation source SINQ, CH) provide the means to precisely explore OHP single-crystal samples from a crystallographic perspective.…”