Local Polar Fluctuations in Lead Halide Perovskite Crystals

Yaffe, Omer; Guo, Yinsheng; Tan, Liang Z.; Egger, David; Hull, Trevor D.; Stoumpos, Constantinos C.; Zheng, Fan; Heinz, Tony F.; Kronik, Leeor; Kanatzidis, Mercouri G.; Owen, Jonathan S.; Rappe, Andrew M.; Pimenta, M. A.; Brus, L. E.

doi:10.1103/physrevlett.118.136001

Cited by 555 publications

(777 citation statements)

References 57 publications

(71 reference statements)

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“…The energy locations and the temperature dependence of the Raman data presented are generally in agreement with previous studies 32,33,[43][44][45][46] . At low energies below ∼ 50 meV, Raman and neutron spectroscopy are in agreement with both showing a significant temporal broadening of the excitations upon heating from the orthorhombic phase, indicative of faster fluctuations with shortened lifetimes.…”

Section: Discussionsupporting

confidence: 91%

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

et al. 2017

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Lead-halide organic-inorganic perovskites consist of an inorganic host framework with an organic molecule occupying the interstitial space. The structure and dynamics of these materials have been heavily studied recently due to interest in their exceptional photovoltaic properties. We combine inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron scattering to study the temperature dependent dynamics of the molecular cation in CH3NH3PbBr3. By applying high resolution quasielastic neutron scattering, we confirm the [CH3NH3]+ ions are static in the low temperature orthorhombic phase yet become dynamic above 150 K where a series of structural transitions occur. This molecular melting is accompanied by a temporal broadening in the intra-molecular modes probed through high energy inelastic spectroscopy. Simultaneous Raman measurements, a strictly |Q|=0 probe, are suggestive that this broadening is due to local variations in the crystal field environment around the hydrogen atoms. These results confirm the strong role of hydrogen bonding and also a coupling between molecular and framework dynamics.

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Section: Discussionsupporting

confidence: 91%

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

et al. 2017

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“…The different increases in lifetime in the two systems are due to the different geometries of the linear CH 3 2 + molecules to change their orientations in response to a presence of photoexcited charge carriers to maximize the screening effect due to the dipole and quadrupole moments, yielding the observed more enhanced increase in the charge carrier lifetime than in CH 3 NH 3 + . We note that there have been recent papers that suggest the role of screening by the inorganic atoms (33)(34)(35) or acousticoptical phonon up-conversion involving both organic and inorganic atoms (36) in increasing the carrier lifetime in metal halide perovskites. These mechanisms may contribute to the long carrier lifetime.…”

Section: Discussionmentioning

confidence: 89%

Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites

Chen

Foley

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

161

179

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Long carrier lifetime is what makes hybrid organic-inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.organic-inorganic hybrid perovskite | carrier lifetime | photoluminescence | polaron T he record efficiency of hybrid organic-inorganic perovskite (HOIP)-based solar cells has reached above 22% (1-4), which is comparable to that of silicon solar cells. The most dominant contribution to the high photovoltaic performance of HOIPs comes from their long carrier lifetimes (≥ 1 μs), which translates to large carrier diffusion lengths despite their modest charge mobilities (5). Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of ferroelectric domains (6-9), Rashba effect (10-12), photon recycling (13), and large polarons (14-16). When the HOIPs are replaced with all inorganic perovskites in the solar cell architecture, the device can still function as a solar cell. This indicates that the photons excite electrons and holes out of the inorganic metal halide atoms, which is consistent with the density functional theory (DFT) calculations that the corner interstitial cations, whether organic or inorganic, do not directly contribute to the band-edge states (17). However, the efficiency of the purely inorganic perovskites is currently at ∼11% (18-20), which is far below 22% of HOIP-based solar cells. This suggests that the presence of organic cation may be the key for achieving high solar cell efficiency. It is, however, yet to be understood how the organic cations enhance the efficiency.Among the aforementioned microscopic mechanisms, three are based on the role of organic cations. First, in the ferroelectric domain theory, nanoscale ferroelectric domains are formed due to alignment of organic cations (6-9). Such domains can spatially separate the photoexcited electron and holes and thereby reduce their recombination. Second, in the Rashba effect theory (10-12), the spin and orbit degrees of freedom of the inorganic atoms are coupled with the electric field generated by the organic cations. This results in the electronic band splitting for different spins and leads to an effectively in...

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“…22,[48][49][50]53,[56][57][58]83 We hope that new results such as these may inform a key point of contention: Are the molecular cations simply larger monocations than any (non-radioactive)…”

Section: Resultsmentioning

confidence: 99%

“…50 These hypotheses are somewhat misaligned with the growing body of evidence suggesting an important role for local distortions of the inorganic sublattice. 22,[51][52][53][54][55][56][57][58][59] Indeed, there are now numerous indications that the appealing optoelectronic properties of the hybrid perovskites are qualitatively matched by their all-inorganic analogues. 54,[60][61][62][63][64] In order then to delineate the properties and functionality of the molecular cations from those of the inorganic framework, one must first complete the description of the structure and dynamics of each.…”

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confidence: 99%

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

et al. 2017

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The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI 3 ) and formamidinium lead iodide (FAPbI 3 ), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Here, we establish the dynamics of the molecular cations in FAPbI 3 between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons of the reported temperature dependence of the dynamics of MAPbI 3 are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes.For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule-cage interaction in FAPbI 3 produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom-atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.2

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Local Polar Fluctuations in Lead Halide Perovskite Crystals

Cited by 555 publications

References 57 publications

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

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