Lattice Anharmonicity: A Double-Edged Sword for 3D Perovskite

Jul 5, 2019 - Chen, X.; Lu, H.; Yang, Y.; Beard, M. C. Excitonic Effects in Methylammonium Lead Halide Perovskites. J. Phys. Chem. Lett. 2018, 9, 2595...
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Lattice Anharmonicity: A Double-Edged Sword for 3D Perovskite-Based Optoelectronics Kyle T. Munson, John R. Swartzfager, and John B. Asbury*

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Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Halide perovskites have attracted much attention due to their remarkable optoelectronic properties, such as long carrier lifetimes and diffusion lengths. However, current state-of-the-art perovskite optoelectronics lack longterm stability and are prone to ion or defect migration. In this Perspective, we discuss the structural properties and dynamics of the perovskite lattice that extend carrier lifetimes, but that also may facilitate ion migration. We describe changes of the vibrational properties of the anharmonic perovskite lattice in both the ground and excited electronic states and reveal the influence that these changes have on carrier recombination and ion transport under the influence of optical and electrical excitation. Finally, we suggest modifications of the phonon dynamics of the perovskite lattice that may allow these properties of the materials to be tuned to strike the right balance between carrier dynamics and ion migration.

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transport dynamics in this class of material are critical for the rational design of future perovskite optoelectronics. In this Perspective, we describe how the soft nature of the perovskite lattice that arises from its large anharmonicity underpins many of the material’s unique optoelectronic properties. We begin by highlighting recent experimental studies examining the vibrational dynamics of the halide perovskite lattice in both the ground and excited electronic states. We then describe how charge carrier recombination and ion transport mechanisms in halide perovskites are affected by structural fluctuations of the perovskite lattice, indicating that the novel electronic properties and instabilities of the material both share a common origin. Finally, we suggest opportunities to use the information gained from these studies to tune the optoelectronic properties of perovskite-based devices, offering opportunities to optimize both their performance and stability. The three-dimensional halide perovskites used to make current state-of-the-art devices share a common ABX 3 structure shown in Figure 1A. This structure consists of an A-site cation (e.g., methylammonium (MA), formamidinium (FA), or cesium (Cs)), a B-site metal dication (e.g., Pb2+ or Sn2+), and X-site halide ions (e.g., I, Br, or Cl).17 The B-site metal dication and the X-site halide ions form a corner-sharing inorganic framework (BX3−). The A-site cation is located within this framework and interacts with the BX3− lattice mainly via ion−ion, ion−dipole, and hydrogen bonding interactions (in the cases of MA or FA).18

ince the discovery of perovskite-based photovoltaics in 2009,1 halide perovskites have emerged as leading candidates for photovoltaic technologies because of their ease of fabrication and high photovoltaic performance.2 Currently, the record power conversion efficiency (PCE) of perovskite-based photovoltaics has exceeded 23%,3 approaching the PCE of single-crystalline silicon (∼27%). Halide perovskites also show significant promise in other optoelectronic applications including photodetectors4 and optically pumped lasing structures,5 pushing this class of material further into the forefront of innovative optoelectronics research. The record efficiencies of perovskite photovoltaics are mainly due to the micrometer carrier diffusion lengths (LDs) and microsecond carrier lifetimes realized in this material class.6,7 These properties enable halide perovskites to support large photogenerated carrier densities with high open-circuit voltages and photoluminescence (PL) quantum yields.2,6 Halide perovskites also exhibit other favorable optoelectronic properties such as small exciton binding energies8 and high absorption coefficients,9 making them ideal materials for a wide range of optoelectronic applications. Despite their favorable optoelectronic properties, current perovskite photovoltaic devices suffer from instabilities including current−voltage hysteresis,10 time-dependent performance evolution,11,12 and degradation.13 For example, ion (or defect) transport is widely observed in mixed halide perovskites after exposing the materials to continuous optical or electrical excitation,14−16 which limits the long-term stability of devices made from these materials. Consequently, research efforts aimed at understanding photoexcitation and carrier © XXXX American Chemical Society

Received: May 17, 2019 Accepted: July 5, 2019 Published: July 5, 2019 1888

DOI: 10.1021/acsenergylett.9b01073 ACS Energy Lett. 2019, 4, 1888−1897

Perspective

Cite This: ACS Energy Lett. 2019, 4, 1888−1897

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authors identified two distinct motions of the MA cations for all three samples, corresponding to wobbling-in-cone (∼0.3 ps) and rotational motion (∼3 ps). A depiction of these motions is shown in Figure 1D. We note that polarizationresolved 2DIR spectroscopy was similarly used to investigate the reorientation dynamics of FA ions in FAPbI3 thin films.30 Because of the fast rotational motion of organic cations within the perovskite lattice’s inorganic framework, the authors of these studies concluded that long-lived ferroelectric domains involving head-to-tail alignment of the organic cations are unlikely to form within the material when no charge carriers are present.31 Instead, organic cations are thought to be dynamically disordered within the inorganic framework. The observations presented in Figure 1 have led several investigators to propose that many of the optoelectronic properties of halide perovskites are due to the highly anharmonic properties of the material coupled with the dipolar motion of the organic ions, although this is still a topic of debate.19,32,33 For example, the polarizability of the BX3− framework and the orientational motion of the A-site cation have been thought to screen the Coulomb potential between bound electron−hole pairs,34 potentially reducing the binding energy of excitons to ∼10 meV. Likewise, charge carrier mobility in halide perovskites is thought to be limited by the coupling between carriers and the electric field produced from lattice fluctuations.35,36 The connection between lattice fluctuations and electrical properties has encouraged investigation of the structural dynamics of the perovskite lattice following photoexcitation. For example, sub-10 fs pump−probe experiments of MAPbX3 films were recently used to examine electron−phonon coupling.37,38 In the experiment, impulsive excitation of the bandgap using ultrashort laser pulses was used to launch coherent wavepackets among the optical phonons of the perovskite lattice at ∼100 and 300 cm−1. Figure 2 displays time-domain Raman (TDR) spectra of a CH3NH3PbBr3 film collected after such resonant (pump generates electron−hole pairs) or nonresonant (pump generates no electron−hole pairs) conditions.25 The spectra highlight the polar optical phonon modes of the PbBr3− framework that are excited by changes of the geometry of the perovskite lattice after charge generation. This observation indicated that the presence of charge carriers shifts the equilibrium positions and vibrational energies of the perovskite lattice. Similar results were obtained in the time domain using optical Kerr effect (OKE) spectroscopy, which monitored the relaxation of the Pb−X modes following optical excitation.26,39 The vibrational dynamics of the perovskite lattice following photoexcitation have also been examined using time-resolved mid-infrared (TRIR) spectroscopy.22,30,40 Figure 3A displays baseline-corrected TRIR spectra collected in the symmetric N−H bend region of MA ions in a MAPbI3 perovskite film.22 The corresponding ground-state FTIR spectrum appears in the lower panel of Figure 3A. We note that the TRIR spectrum presented in Figure 3A does not show negative features centered at the vibrational frequency of the ground-state N−H bend mode. From the excitation intensity used for the TRIR experiments presented in Figure 3A and the corresponding FTIR spectra of the film, the authors estimated that the amplitude of the ground-state bleach signal would be ∼200 nanoO.D., which was near the limit of detection of the spectrometer used in the investigation. Instead, the TRIR spectra shown in Figure 3A demonstrates that the amplitude of

Figure 1. (A) Cartoon representation of the perovskite unit cell. Adapted from ref 17. (B) Refined structure of a MAPbI3 single crystal obtained from neutron diffraction measurements showing the disordered nature of the MA cation and the large displacement (ellipsoids) of the halides. Adapted from ref 28 with permission from The Royal Society of Chemistry. (C) 2DIR transient anisotropy signals of the MA ion’s N−H bend vibrational mode in MAPbI3 thin films. Reprinted from ref 21. (D) Depiction of the wobbling-in-cone (top) and rotational (bottom) motions of the MA cation obtained from the 2DIR anisotropy measurements. Reprinted from ref 21.

Connections between the crystal structure and electronic properties of halide perovskites have been extensively explored.19,20 Unlike other inorganic semiconductors such as Si and GaAs that have stiff covalent networks, the halide perovskite lattice is mechanically soft and dynamically disordered.21,22 In part, this mechanical softness arises from the low energy of the B−X phonon modes that are occupied by many vibrational quanta at room temperature.23 Similarly, the vibrational modes of the A-site cation also couple to the motion of the inorganic BX3− framework.24 Experimental evidence for the anharmonic nature of the perovskite lattice has been obtained using a variety of methods.25−27 For example, Raman spectroscopy and molecular dynamics simulations have been used to investigate the polar modes of the BX3− framework that undergo largeamplitude displacements at moderate temperatures.23,24 Neutron diffraction measurements have been used to investigate the disordered nature of the MAPbX3 lattice.28 For example, Figure 1B shows the refined structure of a MAPbI3 perovskite obtained from such neutron diffraction measurements. The structure highlights the disordered nature of the MA cations that appear as a “sphere” in the center of the BX3− framework. Similarly, the atomic displacements of the iodine atoms that occur at room temperature arise from the anharmonic nature of the perovskite lattice and are represented as ellipsoids in the figure. Polarization-resolved 2DIR spectroscopy has been used to examine the reorientation dynamics of MA cations within the BX3− framework by monitoring the dynamics of the molecules’ N−H bend mode in the ground electronic state.21,29 Figure 1C shows the transient anisotropy signal of the MA cation N−H bend mode for a series of MAPbI3 thin films fabricated using different deposition methods. From these measurements, the 1889

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Figure 2. Time-domain Raman (TDR) spectra of a MAPbBr3 perovskite film obtained after resonant (Epump = 2.46 eV) or nonresonant (Epump= 1.86 eV) excitation. The 2D maps were obtained by taking the Fourier transform of a transient absorption signal’s oscillating component. The vibrational spectra in the center of the maps were obtained by averaging the TDR maps over different pump wavelengths. The spectra highlight the differences between the ground-state (red) and excited-state (green) vibrational dynamics of the PbBr3− polar optical modes. Reprinted from ref 25.

Figure 3. (A) (left) Chemical structure of the MA cation. (right) Time-resolved infrared (TRIR) spectra of a MAPbI3 film collected in the film’s N−H bend region following 532 nm excitation. The lower panel shows the corresponding ground-state FTIR spectra. The black dashed line highlights the center frequency of the ground-state N−H bend mode. Reprinted with permission from ref 22. (B) (left) Chemical structure of the FA cation. (right) TRIR spectra collected in the CN stretch region of the FA cation obtained at several time delays following 760 nm excitation. The dashed lines represent the center frequency of the ground- and excited-state vibrational modes, respectively. Reprinted from ref 30.

the excited-state N−H bend vibration is significantly larger (∼30 microO.D.) than the corresponding bleach signal. The larger oscillator strength of the excited-state N−H bend vibration arises from coupling of the vibrational and electronic coordinates of the material, similar to infrared activated vibrational modes that have been reported in conjugated polymers.41 Because MA cations interact with the inorganic framework through local hydrogen bond and ion−dipole interactions, which are highly distance-dependent, the N−H bend provides a sensitive probe of the structural fluctuations of the perovskite lattice following photoexcitation. In particular, the broader line width of the symmetric N−H bend in the excited state indicates that the vibrational dynamics of the excited electronic state are markedly different from that of the ground state. The broader line width reveals faster fluctuations of the hydrogen bond interactions following photoexcitation, indicating that the perovskite lattice undergoes larger-amplitude fluctuations in the presence of charge carriers. Additionally, the blue shift of the N−H bend mode in the excited state arises from both the weakening of hydrogen bonding interactions of the MA ions with iodide ions in the BX3− framework combined with the effects of the vibrational Stark effect. TRIR spectra collected in the CN stretch region of a FAPbI3 thin film also displayed similar vibrational dynamics, suggesting that both MA and FA ions respond in a similar manner to the presence of charge carriers (Figure 3B).30 The observations of distinct lattice fluctuations in the electronic excited states suggest that structural fluctuations of the lattice are strongly coupled to the electronic states of halide perovskites. For example, distortions of the halide perovskite

Structural fluctuations of the lattice are strongly coupled to the electronic states of halide perovskites. lattice around a charge carrier create a polarization cloud that spans several unit cells.42 This polarization cloud, also called a polaron, has a delocalization length at room temperature of approximately 9 nm.22,40 Therefore, polarons in halide perovskites are called “large” polarons in the literature.26,35,43 Figure 4A shows a cartoon that illustrates the nuclear distortions of the perovskite lattice that form around selftrapped electrons and holes (polarons) in the perovskite lattice. For clarity of presentation, the illustration depicts polarons that are far more localized than those actually present in halide perovskites even at room temperature.22,40 Several experimental and computational reports have investigated polaron formation mechanisms in halide perovskites.44,45 For example, ultrafast spectroscopy collected in the THz region has been used extensively to investigate electron−phonon coupling in halide perovskites because it is capable of probing correlations between low-frequency vibrational modes and intraband charge carrier motion.46 This capability has been demonstrated in work by Bonn and co-workers47 and Stagira and co-workers,48 who used THz spectroscopy to examine charge carrier coupling to the low-frequency bending modes of 1890

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mechanisms in halide perovskites are still debated in the literature. The formation of large polarons in halide perovskites has also been confirmed by directly probing the spectroscopic signatures of large polarons in the mid-infrared (mid-IR) region using TRIR spectroscopy.22 Figure 4C displays TRIR spectra of a MAPbI3 film measured at several time delays between 30 ns and 1 μs following 532 nm excitation. The spectra show two distinct types of spectroscopic features. The first is a broad electronic transition spanning the entire mid-IR region that arises from the photoionization of charge carriers from large polaronic states to unbound continuum (band) states. Figure 4D displays a state diagram that depicts the fundamental processes leading to the broad electronic transitions, which we will refer to as large polaron absorption spectra. Because the absorption maximum of the large polaron absorption spectra is predicted to occur at 3 times the polaron binding energy,51 the sharp onset of the large polaron absorption at ∼0.15 eV (1200 cm−1) indicates that large polarons in MAPbI3 have self-trapping energies of ∼0.05 eV, or about 2 times the thermal energy at room temperature. The second spectroscopic signature observed in the TRIR spectra corresponds to narrow vibrational features of the MA cation. The gray shaded boxes in Figure 4C highlight the vibrational frequencies of the N−H bend and N−H stretch modes of the MA cation. These vibrational features arise from perturbations of the N−H bend and N−H stretch modes following photoexcitation, as was previously discussed in the context of Figure 3. The ability to directly probe the spectroscopic signatures of large polarons and the vibrational dynamics of MA ions opens the opportunity to examine the interplay between anharmonic fluctuations of the perovskite lattice and charge carrier recombination. The top panel of Figure 5A depicts groundstate FTIR absorption spectra of a MAPbI3 film measured at different temperatures in the region of the N−H bend vibrational mode.40 The data reveal that the line width of the N−H bend mode changed little within the 190−310 K temperature range, indicating that the changes in occupation of phonon modes between 190 and 310 K had little effect on the MA cation in the ground electronic state. Conversely, the bottom panel of Figure 5A displays baseline-corrected TRIR spectra collected in the N−H bend region of the MA ion measured at 310 and 190 K following optical excitation of the MAPbI3 film at 532 nm.22 As previously discussed, the line width of the N−H bend mode increases in the excited state because the perovskite lattice undergoes larger-amplitude or higher-frequency fluctuations following photoexcitation. Unlike the ground state, the data in Figure 5A reveal that the line width of the N−H bend of the electronic excited state of MAPbI3 varies significantly with temperature. By fitting the transient vibrational features with Lorentzian functions, the line width of the N−H bend mode of the excited electronic state was found to decrease from 41 to 27 cm−1 as the temperature decreased from 310 to 190 K. The decreased line width of the transient vibrational spectra indicated that the N−H bend mode underwent slower vibrational dephasing at lower temperatures in the excited electronic state of the perovskite film. The temperature dependence of the N−H bend mode indicated that the perovskite’s PbI3− framework underwent significantly smaller amplitude fluctuations at lower temperature. These smaller-amplitude fluctuations caused smaller variations of the hydrogen bonding interactions of

Figure 4. (A) Depiction of polaron formation in halide perovskites. Reprinted with permission from ref 22. (B) THz-TDS measurements of a MAPbI3 perovskite film. For resonant excitation at the material’s band-edge (800 nm, red line), the photoconductivity rises faster compared to above bandgap excitation (400 nm, blue line) due to the occurrence of both carrier cooling and polaron formation in the latter. Reprinted with permission from ref 50. (C) TRIR spectra of a MAPbI3 film collected from 30 to 1000 ns following pulsed excitation at 532 nm. The spectra correspond to the broad electronic absorption of large polarons formed within the perovskite film. The gray shaded boxes highlight the vibrational features of the film’s MA cations. Reprinted with permission from ref 22. (D) Schematic diagram depicting the optical transitions observed in the TRIR spectra. Reprinted with permission from ref 22.

the perovskite lattice’s PbX3− framework. Similarly, work by Cooke et al. recently used multi-THz spectroscopy to investigate the coupling of charges to longitudinal optical phonons in MAPbI3 single crystals.46 Terahertz time-domain spectroscopy (THz-TDS) has also been used to investigate charge carrier cooling and polaron formation in perovskite films with different A-site cations.49 In their report, the authors used a sequential model to describe the rise of the THz-TDS signal, which accounted for both carrier cooling and polaron formation. Figure 4B shows THzTDS kinetics obtained for a MAPbI3 film following photoexcitation at 800 (red line) and 400 nm (blue line). For resonant excitation above the band-edge of the material (400 nm), the THz-TDS signal rose faster in comparison to excitation at the band-edge (800 nm), and this was interpreted as arising from the effects of both carrier cooling and polaron formation. From the fits to the data using the sequential model, the authors determined that polaron formation occurs within ∼400 fs of photoexcitation irrespective of the A-site cation used to fabricate the perovskite film (e.g., MA, FA, or Cs). Conversely, carrier cooling rates were found to depend on the film’s A-site cation, consistent with recent results from Bakulin et al., who used ultrafast intraband spectroscopy to investigate carrier cooling in lead halide perovskites.50 These results suggest that while carrier cooling rates may depend on the A-site cation50 polaron formation does not.49 Instead, distortions of the inorganic perovskite lattice may dominate polaron formation in halide perovskites. However, we note that the roles of the Asite cation in polaron formation and carrier cooling 1891

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Figure 5. Comparison of (A) FTIR spectra (top) and TRIR spectra (bottom) of a MAPbI3 film measured at several temperatures between 190 and 310 K. The spectra highlight the N−H bend region of the film. TRIR spectra were obtained 30 ns after optical excitation at 532 nm. The smooth curves overlaid onto the TRIR data represent the Lorentzian functions used to quantify the line width of the vibrational features. Reproduced from refs 22 and 40. (B) Polaron absorption decay kinetics measured at 190 and 300 K plotted in an αb/ΔA vs time representation. The dashed black lines represent fits to the decays obtained from eq 1. (C) Experimentally determined bimolecular recombination coefficients obtained from the fits of the polaron absorption decays compared to the vibrational dephasing times of the N−H bend vibrations measured at different temperatures. (D) Depiction of the influence that fluctuations of the perovskite lattice have on charge recombination at elevated temperatures. Fluctuations of the perovskite lattice cause charge carriers to become self-trapped in large polaronic states that reduce electron−hole wave function overlap, slowing down charge recombination. Reproduced form ref 40.

MA ions with the PbI3− framework, leading to slower dephasing of the N−H bend mode at lower temperatures in the presence of photogenerated charge carriers. To better understand how fluctuations of the MAPbI3 perovskite lattice affect charge carrier recombination, the temperature dependence of the dephasing dynamics of the MA ion’s N−H bend was compared to charge recombination kinetics obtained from the large polaron absorption signal. Figure 5B displays polaron absorption decay kinetics measured at 190 and 300 K following excitation at 532 nm of a MAPbI3 film.40 The kinetics are plotted in an αb/ΔA versus time representation, where α is the polaron absorption’s molar extinction coefficient at the probe frequency and b is the film thickness. In this representation, bimolecular decay processes such as polaron−polaron recombination exhibit a linear dependence on time (black dashed lines in Figure 5C). This linear dependence of αb/ΔA versus time is described by eq 1 αb αb = + k bit ΔA ΔA 0

temperature. Figure 5C shows the temperature-dependent variation of the bimolecular recombination coefficients.40 The data reveal that the recombination coefficients decrease with increasing temperature, leading to longer charge recombination lifetimes at 300 K.52,53 Self-trapping of charge carriers in large polarons leads to the formation of energetic barriers to charge recombination, which inhibits electron and hole recombination because their polarization clouds screen the Coulomb potentials between the carriers.42 The screening effects that cause long carrier lifetimes in halide perovskites may also protect charge carriers from becoming trapped at ionized defects, making them defect-tolerant despite being processed from solution.20 It is instructive to overlay in Figure 5C the temperature dependence of the recombination coefficients of the MAPbI3 film with the vibrational dephasing times of the N−H bend mode of MA cations measured at the same temperatures in the presence of charge carriers. The similarity of the temperature dependence of these metrics reveals that the structural fluctuations of the perovskite lattice that cause the N−H bend mode to undergo faster dephasing dynamics are also correlated with the formation of large polarons, which slows charge carrier recombination by an order of magnitude. It was noted that the temperature dependence of the bimolecular recombination coefficients could not be accounted for solely by the temperature-dependent variation in charge carrier mobility observed in previous Hall effect and THz measurements.36,54 Instead, the authors determined that fluctuations of the soft perovskite lattice inhibit delocalization of charge

(1)

where ΔA is the absorption signal of the large polaron at time t following excitation, ΔA0 is the initial change in absorption signal of the large polaron at time t = 0, and kbi is the bimolecular recombination coefficient, which depends on the diffusivity of the polarons and their capture cross section. From the slope of the polaron absorption decays plotted in this representation, the authors obtained the bimolecular recombination coefficients of a MAPbI3 film as a function of 1892

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vacancies lies between 0.3 and 0.6 eV (∼28−58 kJ/mol) in halide perovskites.14,60 Figure 6A shows the calculated

carriers, causing them to self-trap into large polarons. This reduces their recombination cross sections by introducing energetic barriers to charge recombination (Figure 5D). These energetic barriers arise because of the repulsion of the polaron’s polarization clouds, as was described by Emin.43

Fluctuations of the soft perovskite lattice inhibit delocalization of charge carriers, causing them to self-trap into large polarons. This reduces their recombination cross sections by introducing energetic barriers to charge recombination. We recognize that the presence of Rashba band splitting, by which the perovskite band-edge could become slightly indirect, has been suggested as a plausible explanation for the long carrier lifetimes observed in halide perovskites.54−56 In this view, spin−orbit coupling caused by the presence of heavy elements and dynamic inversion symmetry breaking within the perovskite lattice could produce two spin-split bands. As a result, fast radiative recombination from the indirect band-edge state produced from the Rashba effect could become momentum-forbidden,54 although such splitting would not necessarily affect the rate of nonradiative recombination. Some investigators have suggested that second-order charge recombination in halide perovskites is thermally activated on the basis of temperature-dependent microwave conductivity and PL measurements54,56 and interpreted the results as being in support of the Rashba effect in halide perovskites. However, other experimental reports have observed the opposite dependence of the bimolecular recombination constant with temperature,40,53,57,58 including the temperature-dependent bimolecular recombination constants shown in Figure 5C. Moreover, second harmonic generation rotational anisotropy experiments of a MAPbI3 single crystal have shown no sign of symmetry breaking in the material.59 Finally, recent PL measurements have suggested that the perovskite bandgap is direct.57,58 Therefore, future work aimed at identifying the presence (or absence) of Rashba-like effects in halide perovskites is warranted.

Figure 6. (A) Calculated activation energy for iodide migration at iodide vacancies in MAPbI3. Reprinted form ref 61. (B) PL spectra of a MAPb(I:Br)3 perovskite nanoplatelet after continuous wave illumination. The spectra highlight the emergence of emission at 750 nm following continuous wave light soaking. Reprinted from ref 64.

activation energy of iodine vacancies obtained by Haruyama and co-workers.61 Moreover, this activation energy is likely much lower in the presence of charge carriers on the basis of the increased amplitude of the lattice fluctuations that cause faster vibrational dephasing in the excited state (Figure 5). Ion (or defect) migration has also been observed experimentally in a variety of halide perovskites when the material is electrically polled or photoexcited.16 For example, light-induced halide segregation is commonly observed in mixed halide perovskites (e.g., MAPb(I:Br)3).15,62 Halide segregation in these systems, usually after several minutes of exposure to continuous illumination, causes the emission of the mixed halide material to red shift to ∼750 nm (1.68 eV), near where MAPbI3 emits regardless of the bandgap of the original perovskite film.63 Figure 6B shows PL spectra of a mixed halide perovskite film collected before and after light soaking with continuous illumination.64 The changes observed in the PL spectra after light soaking result from the formation of iodiderich domains leading to the longer-wavelength emission. Interestingly, this process is reversible. That is, the material’s bandgap recovers after it is left in the dark for a few hours at room temperature. While numerous studies have investigated ion migration in halide perovskites, a complete understanding of the mechanism of this process is still lacking.16 Recently, work by Ginsberg and co-workers used nanoscale imaging and multiscale modeling to suggest that ion migration in mixed halide perovskites is caused by charge carrier localization into large polaronic states.65 Figure 7 shows coarse-grained dynamics simulations obtained for a MAPb(I:Br)3 film following the addition of multiple

The anharmonicity that leads to charge screening and that slows charge carrier recombination can also cause the B−X bonds in the material to be relatively weak, which can facilitate ion migration. The ability of the perovskite lattice to screen charge carriers and slow charge recombination has no doubt figured prominently in the remarkable properties of halide perovskites including their high photovoltaic PCEs. However, the anharmonicity that leads to charge screening and that slows charge carrier recombination can also cause the B−X bonds in the material to be relatively weak, which can facilitate ion migration. For example, theoretical calculations based on density functional theory have been used to estimate that the activation energy of ion transport through iodine interstitials or 1893

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migration) to primarily large polaron states at around 300 K (faster ion migration).22 This correlation confirms the hypothesis that the anharmonicity of the halide perovskite lattice that gives rise to polaron formation and the associated long carrier lifetimes and LDs also underpins their problems with ion migration. Fortunately, because large polaron formation depends on electron−phonon coupling, modifying electron−phonon interactions within mixed halide perovskites may allow investigators to mitigate the effects of ion migration while still retaining the remarkable electronic properties of the material, such as long charge carrier lifetimes. For example, recent theoretical work by Eames predicted that mixed A-site cations exhibit reduced lattice fluctuations in halide perovskites.67 These results were later observed experimentally by Gallop et al., who used 2DIR anisotropy to show that organic cation reorientation dynamics are slower in mixed A-site cation perovskite films compared to their pure A-site cation analogues.29 Such reduced lattice fluctuations may decrease electron−phonon coupling within the material, reducing ion migration. Consistent with this hypothesis, mixed A-site cation perovskites (e.g., FA0.8Cs0.2Pb(I:Br)3) exhibit enhanced photostability.68 Additionally, investigators have shown that iodide-rich domains form preferentially in halide perovskites with MA A-site cations but not in mixed systems with both MA and Cs cations in the perovskite crystals. These observations suggest that electron− phonon interactions in MA perovskites are stronger than those in in MA/Cs films, which facilitates more ion migration in the former.64,68 Taken together, these results suggest that tuning the electron−phonon coupling and anharmonicity of halide perovskites may provide an effective way to mitigate ion migration while retaining the favorable charge carrier transport properties that result from the soft perovskite lattice.

Figure 7. (top) Simulation of iodide cluster formation in a 100 nm region of a MAPb(I:Br)3 thin film. Iodide regions are shown in yellow, while bromide regions are shown in blue; ref 59. (bottom) Molecular dynamics simulation of a polaron in a MAPbI3 lattice. Polarons in mixed halide perovskites are thought to be stabilized in iodide-rich domains. This stabilization strains the lattice and causes iodide-rich domains to grow until they are the same size as the polaron’s polarization cloud, ∼8−9 nm. Reprinted from ref 65.

polarons into the perovskite lattice. From these simulations and complementary nanoscale imaging, the authors concluded that polarons are stabilized in iodide-rich clusters of the material (yellow regions in Figure 7). The stabilization of polarons in the iodide-rich regions would strain the crystalline lattice and cause the iodide-rich domains to grow until they are the same size as the polarization cloud of the large polaron, ∼8−10 nm (Figure 7). Moreover, the authors reasoned that continuous wave excitation may result in a steady-state population of polarons that induces continuous macroscopic iodide domain formation and migration. Further evidence for ion migration in mixed halide perovskites arising from the formation of large polarons was obtained from nanoscale imaging by the same authors.65 By cycling the exposure of the perovskite films, the authors found that stable iodide-rich regions did not occur in the same location after each subsequent exposure. That is, iodide-rich domains appeared randomly throughout the film, implying that the formation of these domains in MAPb(I:Br)3 may not be predetermined by static defects within the film. The link between ion migration and polarons paired with observations of the temperature dependence of large polaron formation22 suggests that the kinetics of ion migration should be strongly temperature-dependent. Recent work by Hoke et al. reported the temperature dependence of the rates of ion migration in mixed halide hybrid perovskites under steadystate illumination.66 The investigators reported a 100-fold increase of the rate of ion migration over the same temperature range at which charge carriers transitioned from primarily band-edge-free carrier states at around 200 K (slower ion

Because large polaron formation depends on electron−phonon coupling, modifying electron−phonon interactions within mixed halide perovskites may allow investigators to mitigate the effects of ion migration while still retaining the remarkable electronic properties of the material, such as long charge carrier lifetimes. In summary, halide perovskites have seen renewed interest because of their combination of long charge carrier lifetimes, long LDs, and efficient PL properties, leading to high-efficiency perovskite-based photovoltaics and light-emitting devices. This renewed interest has led investigators to examine how this class of material’s structure gives rise to this unique combination of properties. These efforts have also sought to address device instabilities that arise from ion or defect migration. In this Perspective, we focus on the mechanisms by which the soft, anharmonic nature of the perovskite lattice impacts both their carrier transport and lifetime properties as well as their ion mobility in an effort to outline future directions aimed at utilizing the best aspects of the optoelectronic properties of the materials. Polaron formation and ion migration mechanisms in halide perovskites are thought to arise from strong electron−phonon 1894

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interactions within the material. However, our understanding of electron−phonon coupling in halide perovskites is still incomplete. For example, there is an intense debate in the literature regarding the role of the A-site cation in modulating electron−phonon interactions. While several studies have suggested that the nature of the A-site cation (e.g., Cs vs MA) does not affect polaron formation or recombination mechanisms, ion migration is less pronounced in mixed A-site cation films. Likewise, CsPbX3 and MAPbX3 show different cooling rates, suggesting that the choice of A-site cation affects the phonon dynamics that influence carrier cooling. Future work that utilizes time-resolved vibrational spectroscopy to investigate the structural dynamics of the perovskite lattice following photoexcitation may allow investigators to develop design rules for tailoring the structural dynamics that underpin the carrier and ion transport properties of the material. For instance, ion migration mechanisms in halide perovskites may be further characterized by investigating the energetic distribution and dynamics of large polarons using TRIR spectroscopy in conjunction with computational efforts. Future work that investigates the spectroscopic signatures of large polarons under different applied stimuli (e.g., electrical bias or illumination) could provide investigators with a spectroscopic tool capable of unraveling ion migration mechanisms under working device conditions. Finally, structural fluctuations of the perovskite lattice are also thought to affect the optoelectronic properties of 2D Ruddlesden−Popper perovskites.69 For example, the whitelight emission observed in this class of material is thought to arise from self-trapped excitons formed by distortions of the lattice.70 Only a handful of studies have investigated vibrational dynamics in 2D perovskites.27 Therefore, future time-resolved vibrational spectroscopy experiments may also help to guide ongoing efforts to understand and control the properties of halide perovskites.

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John B Asbury is a professor of chemistry at the Pennsylvania State University and cofounder of Magnitude Instruments. His research program focuses on development of ultrafast spectroscopy and microscopy techniques to understand how the electronic properties of materials evolve from their molecular and crystalline structure.

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ACKNOWLEDGMENTS The authors are grateful for support of the work on halide perovskites from the U.S. National Science Foundation under Grant Number CHE-1464735. K.T.M. is grateful for support from the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1255832. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

John B. Asbury: 0000-0002-3641-7276 Notes

The authors declare the following competing financial interest(s): J.B.A. owns equity in Magnitude Instruments, which has an interest in this project. His ownership in this company has been reviewed by the Pennsylvania State Universitys Individual Conflict of Interest Committee and is currently being managed by the University. Biographies Kyle T. Munson is an NSF Graduate Research Fellow and doctoral candidate in the Department of Chemistry at the Pennsylvania State University working with Professor John Asbury. His research interests focus on understanding the influence that molecular structure has on the electronic properties of halide perovskites using time-resolved infrared spectroscopy. John R. Swartzfager is pursuing his Ph.D. in chemistry at the Pennsylvania State University while working in the lab of Professor John Asbury. His research interest lies in understanding the charge carrier dynamics of solution-processed optoelectronic materials including lead halide perovskites and organic electronics using ultrafast infrared spectroscopy. 1895

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