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Dynamical Phase Transitions and Cation Orientation-Dependent Photoconductivity in CH(NH2)2PbBr3 Eve M. Mozur,† Julia C. Trowbridge,† Annalise E. Maughan,† Matthew J. Gorman,† Craig M. Brown,‡,¶ Timothy R. Prisk,‡ and James R. Neilson*,† Downloaded via 109.94.174.58 on July 20, 2019 at 09:23:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States NIST Center for Neutron Research National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ¶ Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡
S Supporting Information *
ABSTRACT: The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH(NH2)2+) perovskites exhibit unusual temperature-dependent trends in photoluminescence, dielectric constant, and phase behavior that are hypothesized to relate to CH(NH2)2+ reorientations. This contribution describes five distinct, temperaturedependent phase transitions in CH(NH2)2PbBr3 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(NH2)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(NH2)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.
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gradually over a large temperature range, with little dependence on crystallographic transitions and octahedral tilt patterns.9−11 The gradual change in molecular dynamics correlates to similar trends in optoelectronic behavior.12,13 The differences between the molecular dynamics of CH3NH3+ and CH(NH2)2+, which could arise from the increased steric bulk of CH(NH2)2+, the additional amine capable of hydrogen bonding, the resonant π system, or the weaker dipolar moment and the stronger quadrupolar moment relative to CH3NH3+, are associated with differences in the overall structure and phase behavior of formamidinium perovskites. CH(NH2)2+ reorientations and the resulting local strain have been implicated in the complex phase behavior of CH(NH2)2PbI3. Depending on sample preparation or thermal history, CH(NH2)2PbI3 undergoes a reentrant phase transition, a phase transition between a perovskite cubic phase and a non-perovskite hexagonal phase, or retains the cubic phase between 400 and 8.2 K.9,14−16 While formamidinium perovskites show great promise for photovoltaic
ybrid perovskites have emerged as high performing semiconductors, with applications in photovoltaics and solid-state lighting.1,2 Several studies suggest that their transformative optoelectronic properties arise in part from the formation of giant polarons, formed by transient structural fluctuations of the coupled lead-halide octahedral framework and the organic sublattice.3−5 The topologically under-constrained nature of the corner connected octahedral framework and the stochastic, liquidlike reorientation of the organic cations methylammonium (CH3NH3+) and formamidinium (CH(NH2)2+) that occupy the cuboctahedral void render their potential energy landscape highly complex and time-dependent relative to conventional inorganic semiconductors.5−7 The interplay of the functional properties and structural dynamics motivates investigation into their relationships, such as the role that structural rigidity plays in efficient luminescence.8 Formamidinium-based perovskites exhibit complex structural and dynamic behavior when compared to the archetypal methylammonium perovskites. Unlike CH3NH3PbX3 perovskites, in which the octahedral tilt patterns couple strongly to CH3NH3+ dynamic degrees of freedom to give a unique tilting pattern,9 changes in CH(NH2)2+ molecular dynamics appear © XXXX American Chemical Society
Received: June 6, 2019 Accepted: July 9, 2019 Published: July 9, 2019 260
DOI: 10.1021/acsmaterialslett.9b00209 ACS Materials Lett. 2019, 1, 260−264
Letter
Cite This: ACS Materials Lett. 2019, 1, 260−264
Letter
ACS Materials Letters applications,17−19 the implications and driving forces of the unusual phase behavior are not well understood. Here, we demonstrate that the molecular reorientations of CH(NH2)2+ modify the steady-state light-induced charge carrier separation in CH(NH2)2PbBr3 by characterizing the temperature-dependent photoconductivity, crystallography, calorimetry, and dynamics of CH(NH2)2PbBr3. In addition to the two previously reported crystallographic phase transitions, specific heat and neutron scattering data reveal three phase transitions that are not resolved crystallographically. We demonstrate that the additional three transitions pertain to changes in the molecular reorientations of CH(NH2)2+ and also influence the photoconductivity spectrum and intensity. This work indicates that the dynamic degrees of freedom of the organic sublattice influence steadystate optoelectronic properties in hybrid perovskites and highlights the importance of studying domain structure in these complex materials. Photoconductivity data collected on a single crystal of CH(NH2)2PbBr3, presented in Figure 1, resolve several
Figure 2. (a) Excitonic peak center (left axis, gray circles), excitonic peak intensity (right axis, green squares), and interband intensity (right axis, pink triangles) of the photoconductivity data presented in Figure 1. (b) Heat capacity data for CH(NH2)2PbBr3. The crystallographically-observed phase transitions occur at 266 and 153 K. While the crystallographically resolvable transition at 266 K is not observed in the heat capacity, it is observed in differential scanning calorimetry (Figure S10). (c) Mean squared displacement (MSD) of hydrogen determined from fixed window elastic neutron scattering (collected on the instrument HFBS) of CH(NH2)2PbBr3 measured on heating (orange circles) and cooling (blue squares). Dotted gray lines indicate crystallographically resolvable phase transition temperatures and dashed gray lines indicate crystallographically unresolvable phase transition temperatures determined from heat capacity and differential scanning calorimetry data. Estimates of the uncertainties are smaller than the symbols used to represent the data in all subplots.
(Figures S3, S5, and S9), in contrast to the resolvable peak splitting and discontinuities in lattice parameters observed in CH3NH3PbBr3 and CsPbBr3.22,23 The remaining three phase transitions at T = 182, 162, and 118 K cannot be resolved in either high resolution SXRD or NPD (Figure S7). However, all five phase transitions involve changes in the dynamics of CH(NH2)2+. The mean squared displacements (MSD) of hydrogen determined from temperature-dependent fixed window neutron scattering (Figure 2c)24 show clear discontinuities at all five transitions. Quasielastic neutron scattering (QENS) spectra indicate that all five phase transitions change the relaxation times of CH(NH2 ) 2 + reorientations and extent of hydrogen motion (by jump distance or by fraction participating in reorientations), evidenced by changes in peak width in the QENS spectra and the slope of the extracted elastic incoherent structure factors (EISF) (Figures S13 and S14). Therefore, the crystallographically unresolvable phase transitions must relate to cooperative changes in the dynamics of the CH(NH2)2+ sublattice. Comparison of the MSD and photocurrent data demonstrates that cation dynamics correlate with steady-state optoelectronic properties; this relationship is most obvious around the T = 182 K phase transition. At 182 K, the CH(NH2)2+ motion changes rapidly, exhibiting an abrupt increase in slope, and the photoconductivity excitonic peak center transitions from red-shifting (>180 K) to blue-shifting (
