Direct Spectroscopic Observation of the Hole Polaron in Lead Halide Perovskites

Liu, Cunming; Tsai, Hsinhan; Nie, Wanyi; Gosztola, David J.; Zhang, Xiaoyi

doi:10.1021/acs.jpclett.0c01708

Cited by 20 publications

(21 citation statements)

References 58 publications

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“…[ 109 ] Alternative spectroscopic approaches have included optical Kerr effect spectroscopy and 2D spectroscopy, [ 84,110,111 ] both of which are also able to provide sub‐picosecond time resolution, as well as slower techniques such as X‐ray transient absorption, which can give insights into structural distortions in a material. [ 112–114 ] We would caution that, although these techniques can provide valuable information regarding ultrafast processes in metal‐halide semiconductors, in general there can be a variety of contributing signals at early times and great care needs to be applied when interpreting specific transient experimental signals.…”

Section: Experimental Approaches For Observing Polaronic Effectsmentioning

confidence: 99%

“…Recent work has highlighted the importance of structural distortions in inducing polaron formation across MHPs, [ 25,114,138 ] and a crucial question with regards nanocrystals is whether such structural distortions, and thus polaron formation, are facilitated or inhibited by nanoscale structures, as is the case for quasi‐0D metal halides (discussed below). Given that temperature‐dependent PL studies have found LO‐phonon energies between ℏ ω LO = 14–33 meV for (FA/Cs)PbBr 3 nanocrystals, [ 139–141 ] similar to the value of 15 meV obtained for bulk FAPbBr 3 by Wright et al., [ 79 ] as well as a similar LO‐phonon couplings (45 meV in nanocrystal FAPbBr 3 , [ 140 ] as opposed to 60 meV in bulk [ 79 ] ), it is not fully clear how much fundamental change is caused by the formation of nanocrystals.…”

Section: Polarons In Metal‐halide Semiconductorsmentioning

confidence: 99%

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Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

Buizza

Herz

2021

Advanced Materials

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Metal‐halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light‐emitting diodes, and detectors. In this review the role of charge–lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden–Popper layered perovskites, nanocrystals, vacancy‐ordered, and other novel structures, is summarized. The formation of Fröhlich‐type “large” polarons in archetypal bulk metal‐halide ABX3 perovskites and its dependence on A‐cation, B‐metal, and X‐halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal‐halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge‐carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.

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Section: Experimental Approaches For Observing Polaronic Effectsmentioning

confidence: 99%

Section: Polarons In Metal‐halide Semiconductorsmentioning

confidence: 99%

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

Buizza

Herz

2021

Advanced Materials

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“…30,31 Although the polaron hypothesis was frequently invoked to rationalize experimental observations in both organic and inorganic perovskites, the quantification of the associated local structural rearrangement is still missing. In hybrid organic− inorganic lead halide perovskites, local distortions around the Pb 32 and Br 33 sites were separately reported in time-resolved Xray absorption spectroscopy (TR-XAS) studies and ascribed to polaron formation, but an unambiguous identification of the associated structural fingerprint was not provided. Ultrafast electron diffraction on a MAPbI 3 thin film showed evidence of local rotational disorder of the PbI 6 octahedra arising from optical excitation, as a consequence of hot carrier−phonon coupling.…”

Section: ■ Introductionmentioning

confidence: 99%

Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals

et al. 2021

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The development of next-generation perovskitebased optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr 3 perovskites that the Pb−Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron−phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of timeresolved and temperature-dependent studies at Br K and Pb L 3 X-ray absorption edges with refined ab initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperaturedependent XAS results rules out thermal effects as the primary source of distortion of the Pb−Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr 3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

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“…The one mechanism up to now neglected in our analysis is carrier‐lattice interaction, which can induce the formation of large charged polarons, as well documented in literature. [ 11–20 ] Dressing charged carriers by lattice deformations brings two main consequences: the first one is that polarons are energetically stabilized with respect to free electrons and holes, meaning that they sit at lower energy and therefore effectively reduce the exciton binding energy; the second one is the heavier mass of polarons than free carriers, which increases the density of states making them entropically favored with respect to excitons. Both effects reduce the probability of exciton formation, as also inferred from the analytical expression of the modified Saha constant

n_{eq} = {(\frac{k_{normalB} T {\overset{m}{true}}_{normalp}}{2 π ℏ^{2}})}^{\frac{3}{2}} e^{- \frac{E_{normalb} - (E_{p +} + E_{p -})}{k_{normalB} T}}

, in which the exciton binding energy E b is reduced by the stabilization energy for the two polarons ( E p + + E p − ), and

{\overset{m}{true}}_{eh}

is replaced by the much larger mass

{\overset{m}{true}}_{normalp} = \frac{m_{p +} m_{p -}}{m_{normale} + m_{normalh}},

where m p+, p− and m e,h are the polaron and free carrier masses, respectively (see Experimental Section for derivation).…”

Section: Discussionmentioning

confidence: 99%

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Simbula¹,

Pau²,

Wang³

et al. 2021

Advanced Optical Materials

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spectrum featuring a marked excitonic resonance, the majority optical excitation in prototypical HP materials for photovoltaics are not bound excitons, but unbound charge carriers. Therefore, electrons and holes excited by solar light can be directed to the electrodes at a negligible energy cost, without the need to split tightly bound excitons as in organics. Taking advantage of the flexibility of the materials class, layered 2D HPs are obtained by inserting bulky organic cations into the formulation, leading to materials inherently more stable than their 3D counterparts against degradation. [8][9][10] However, in 2D HPs the exciton binding energy can be as large as 400 meV, [8] so that it is commonly assumed that their excited states are mostly excitons.A second peculiar characteristic of the excited states in perovskites is the formation of large polarons, that is, charge carriers coupled to lattice deformations and delocalized over many crystal lattice sites. [11][12][13][14][15][16][17][18][19][20] Unlike small polarons in organics, localized in a single molecule, large polarons are compatible with band transport, but are also able to screen the excited states from scattering with defects and reduce non-radiative recombination through trap states, resulting in large mobilities and long lifetimes. Large polarons are also believed to reduce scattering with phonons and have been proposed as an explanation for hot carriers persisting for several nanoseconds at temperatures significantly higher than the lattice one. [12,16,17,[21][22][23][24][25][26] Large polarons may therefore be the enabling microscopic mechanism for efficient solar cells, including innovative architectures that exploit photoconversion with hot carriers. [27,28] Theoretical estimations forecast that the energy associated with polaron formation is comparable with the binding energy gained by forming an exciton, maybe even larger in some materials. [12,14,24,[29][30][31][32] When do polarons form and whether excitons or polarons are the lowest-energy optical excitations is still an open question. The issue is particularly relevant for layered 2D HPs, where polaronic effects have been demonstrated, although it is not clear if small or large polarons are formed. [33][34][35][36][37] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons.

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Direct Spectroscopic Observation of the Hole Polaron in Lead Halide Perovskites

Cited by 20 publications

References 58 publications

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

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