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What Is Moving in Hybrid Halide Perovskite Solar Cells?
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† Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
‡ Global E3 Institute and Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
*E-mail: [email protected]
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Accounts of Chemical Research

Cite this: Acc. Chem. Res. 2016, 49, 3, 528–535
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https://doi.org/10.1021/acs.accounts.5b00431
Published February 9, 2016

Copyright © 2016 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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Conspectus

Organic–inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. High-efficiency solar cells can be produced with solution-processed active layers. The materials are earth abundant, and the simple processing required suggests that high-throughput and low-cost manufacture at scale should be possible.

While these materials bear considerable similarity to traditional inorganic semiconductors, there are notable differences in their optoelectronic behavior. A key distinction of these materials is that they are physically soft, leading to considerable thermally activated motion.

In this Account, we discuss the internal motion of methylammonium lead iodide (CH3NH3PbI3) and formamidinium lead iodide ([CH(NH2)2]PbI3), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron–hole recombination rates, and current–voltage hysteresis. Multiscale simulations, drawing from electronic structure, ab initio molecular dynamic and Monte Carlo computational techniques, have been combined with neutron diffraction measurements, quasi-elastic neutron scattering, and ultrafast vibrational spectroscopy to qualify the nature and time scales of the motions. Electron and hole motion occurs on a femtosecond time scale. Molecular libration is a sub-picosecond process. Molecular rotations occur with a time constant of several picoseconds depending on the cation. Recent experimental evidence and theoretical models for simultaneous electron and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to fast-ion conducting metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action.

The temporal behavior displayed by hybrid perovskites introduces a sensitivity in materials characterization to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials modeling and device simulations. There are large differences between the average and local crystal structures, and the nature of charge transport is too complex to be described by common one-dimensional drift-diffusion models. Herein, we critically discuss the atomistic origin of the dynamic processes and the associated chemical disorder intrinsic to crystalline hybrid perovskite semiconductors.

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Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”.

1 Introduction

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Methylammonium lead iodide was first reported in the cubic perovskite crystal structure in 1978. (1) The first solar cell was published in 2009, (2) with high-efficiency (≥10%) cells since 2012. There have been many detailed discussions and reviews on the development, progress, and limitations of this technology. The focus of this Account is the dynamic behavior of hybrid organic–inorganic perovskites, which has been linked to the performance, degradation, and unusual physics of these materials and their associated devices. Our principal concern is the atomistic origin of these processes, and their description by first-principle and multiscale materials modeling. For brevity, we refer to the methylammonium compound CH3NH3PbI3 as MAPI and the formamidinium compound [CH(NH2)2]PbI3 as FAPI.

2 Molecular Motion

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Weber first reported MAPI in the cubic perovskite crystal structure (Oh symmetry), (1) which may be surprising considering the anisotropy of the molecular building block (CH3NH3+ is of C3v symmetry). However, as early as 1985, it was understood that these molecules were orientationally disordered in the crystal, giving rise to an effective higher lattice symmetry on average. (3) In 1987, Poglitsch and Weber linked this dynamic disorder to the temperature and frequency dependence of the complex permittivity. (4) Despite being crystalline solids, the dielectric response of these materials has one component that is akin to a polar liquid due to the rotational freedom of the dipolar molecules. (5) For MAPI, at 300 K, the contribution of CH3NH3+ rotations to the static dielectric response can be estimated as ∼ 9, as will be discussed below.
Early work on the characterization of the crystal structure identified three phases of MAPI: orthorhombic, tetragonal, and cubic Bravais lattices in order of increasing temperature. (6) While the position of Bragg peaks in X-ray diffraction can distinguish between the three phases, the peak intensities arising from CH3NH3+ relative to PbI3– are too weak to assign accurate molecular orientations. The recent application of high-resolution powder neutron diffraction (with a more even distribution of scattering cross-section between light and heavy atoms) has provided a quantitative description of the temperature dependent average structures (summarized in Figure 1). (7) At high temperatures, the structure of MAPI can be represented by the cubic space group Pm3̅m, whereas below ca. 327 K it is better described by the tetragonal space group I4/mcm. The cubic-tetragonal transition is second order (Ehrenfest classification) and continuous over 165–327 K. This transition is linked to the anisotropy of the molecular disorder, as well as inorganic octahedral deformation and tilting, which changes from predominately two dimensions (tetragonal) to three dimensions (cubic). The energy balance between the phases can be affected by epitaxial or uniaxial strain. At 165 K, there is a first-order transition to the orthorhombic Pnma phase, with associated discontinuities in the observable physical properties. The transition entropy suggests an ordering of the molecular species, (8) and a full powder neutron solution shows columnar antiferroelectric ordering (head-to-tail arrangement of the CH3NH3+ cations in 1D channels). (7) It is possible that other low-temperature orderings can form with different epitaxial strain, temperature quench regimes, applied electric fields, and sample processing.
For FAPI, analysis of powder neutron diffraction has assigned a room temperature space group of Pm3̅m, which is isostructural to the high-temperature MAPI structure. (9) Faulted or twinned phases at the nanoscale can appear as hexagonal to single-crystal diffraction methods, which explains the earlier assignment of a hexagonal space group. (10) A distinction of FAPI is that the corner-sharing perovskite structure (black color) is in competition with a face-sharing δ phase (yellow color); (10) this is also the case for CsSnI3. (11, 12) In contrast to MAPI, knowledge of the low temperature phase behavior of FAPI is currently lacking.
A point to consider regarding structural characterization by diffraction, which is particularly relevant to perovskite structured materials, is that standard techniques probe only the average crystal structure. The intensities of Bragg peaks are related to the average positions of single atoms within the unit cell. The relative positions of atoms (e.g., bond lengths and angles) requires higher-order correlation functions. These can be accessed through using X-ray or neutron total scattering techniques. Disordered crystalline materials display large atomic displacement parameters. This is the case for both MAPI and FAPI at room temperature. (7, 9) The material is continuously distorting through this average position as a function of time; the thermal vibrations are large relative to the interatomic spacings.

Figure 1

Figure 1. Temperature-dependent (100–352 K) powder neutron-diffraction pattern of CH3NH3PbI3. From ref 7. Adapted by permission of the RSC. The space groups of the average crystals structures are shown with a schematic of the extent of disorder in the CH3NH3+ sublattice.

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We have attempted to address the issue of molecular motion using three scales of materials modeling: (i) static lattice, (ii) molecular dynamic, and (iii) Monte Carlo approaches, which we will now address in turn.

Static Lattice

The standard approach in density functional theory calculations of solids is to first minimize all forces on the system by structural relaxation. In the absence of temperature, the size and shape of the crystallographic unit cell is relaxed with respect to the quantum mechanical forces. This should include all internal degrees of freedom. It is for this “equilibrium” athermal crystal structure that the electronic structure is calculated, and so the total energy and other properties are extracted.
As a starting point we assessed the total energy difference of different molecular orientations within a single “quasi-cubic” unit cell of MAPI. (13) We note that any electronic structure calculation on such unit cells implicitly assumes that the methylammonium molecules are arranged into an infinite ferroelectric domain; although, it should be noted that there is no long-range electric field created with the standard “tinfoil” boundary conditions. We found that the energy difference between the low index ⟨100⟩ (cube face), ⟨110⟩ (cube edge), and ⟨111⟩ (cube diagonal) orientations of the CH3NH3+ ions are similar (within 15 meV) with small barriers between them (<40 meV), but with the facial ⟨100⟩ orientation being preferred. Even from this static description, one can envisage molecular orientational disorder driven by thermal energy (kbT = 26 meV at 300 K).

Molecular Dynamics

The most simple method to incorporate temperature into first-principles simulations is through molecular dynamics (MD). The forces on the ions are calculated quantum-mechanically but the ion velocities are classical and integrated numerically with Newton’s laws of motion to positions. Finite temperature is maintained by rescaling ion motion against a thermostat. Such simulations should display the full range of anharmonic behavior, but due to finite computational resources, rare events are undersampled.
Renderings of our MD simulations (14) on supercells of MAPI and FAPI are available online. (15, 16) Without any deep analysis of the trajectories, it is clear to an observer that the molecules are rotationally mobile at room temperature, with a time scale of motion on the order of picoseconds. A more detailed statistical treatment gives time constants of 3 ps (MA) (17) and 2 ps (FA) (9) for molecular reorientations at 300 K.
There are a range of complex motions displayed, involving relative twisting of the head and tail groups, libration of a molecular in a single orientation, and the rotation of molecules between orientations. The results support and expand upon the picture obtained using static lattice techniques: a statistical analysis (14) reveals the extent of the disorder as well as the preference for facial ⟨100⟩ orientations that are also evident in neutron diffraction measurements. (7, 9)
A surprising result from the MD simulations is that the PbI6 octahedra are far from ideal, even for a cubic lattice. While the “average” structure may appear cubic the local structure is distorted significantly. Pb–I–Pb bond angles of 180° would be expected for an ideal network, while values of 165–172° are found in the relaxed cubic structure. (18) Pb(II) is a prototype “lone pair” cation with a ground-state electronic configuration of 5d106s26p0. In a centrosymmetric coordination environment, on-site sp hybridization is forbidden by group theory; however, local distortions allow for hybridization that can result in electronic stabilization. In effect, these systems display a dynamic second-order Jahn–Teller instability, (19) in addition to displacive instabilities associated with rigid titling of the octahedra. These displacive motions are particularly important as the electronic bands at the gap relevant for device operation are hybridized Pb and I atomic orbitals, and will be directly affected by such distortion. These predictions are supported by measurements of the local structure of CH3NH3SnBr3 by X-ray scattering that indicated lone pair distortions. (20)
A limitation with ab initio molecular dynamics simulations is that they are restricted to small simulation cells, which include a limited set of phonon wavevectors. A subtlety of pervoskite molecular dynamics is that the expansions should be even in the number of octahedra, to avoid falsely constraining the simulation by preventing zigzag tilts through the periodic boundaries. The development of an analytical interatomic potential model can provide access to much larger simulation cells and integration times, and hence describe temporal correlations over larger areas.

Monte Carlo

In order to simulate disordered materials on a scale relevant to photovoltaic devices, we constructed a model Hamiltonian that describes the intermolecular dipole interactions and solved for the temperature dependent equilibrium structure using on-lattice Monte Carlo (MC). (14) Since our original report, the Starrynight codes have been generalized to three-dimensional boundary conditions (17) and to include defect structures. (21) We note that the as-parametrized model considers rotating molecular dipoles, but these can be considered an effective dipole of the unit cell including local lattice distortions, which is also applicable to model fully inorganic perovskites. By universality arguments, systems described with the same symmetry and form of Hamiltonian should have identical transitions and equilibrium behavior.
The strength of the methylammonium dipole moment is comparable to the electric polarization calculated for simple models of the room temperature tetragonal phase. (22) A molecular dipole of 2.2 D ≈ 3 μC cm–2 is predicted from quantum chemistry; by calculating the torque generated around the center of mass due to an applied electric field, the magnitude of the dipole is rotation and position invariant even for a charged molecule. For formamidinium, the value is reduced to 0.2 D; however, the smaller molecular dipole can be compensated by larger structural distortions in the solid-state owing to the steric effect of the additional amine group.
The general behavior obtained from the MC simulations is easy to follow: ordering of molecular dipoles at low T minimizes the total energy of the system by maximizing the dipole–dipole interactions, while disorder at high T is driven by configurational entropy. The electrostatic potential resulting from dipole alignment suggests that electrons and holes may be segregated by this structure. This hypothesis has been subsequently confirmed by direct electronic structure calculation (23) and applied in a device model. (24)
The molecular motion discussed above has been supported by recent quasi-elastic neutron scattering (QENS) investigations. (17, 25) This technique is particularly sensitive to the motion of hydrogen atoms due to the high incoherent neutron-scattering cross-section of hydrogen nuclei. To get sufficient signal, over 10 g of material was required for these studies, relying on bulk synthesis procedures which may not be representative of the active material (thin film) in devices. Analysis in one of these studies (25) suggests that only ca. 20% of the molecules were fully rotationally active in the time scale of the measurement (200 ps), which could be explained by the presence of ordered domains.
Ultrafast pump–probe vibrational spectroscopy applied to device-relevant films offers a direct realspace measurement of methylammonium motion within MAPI: a fast libration (“wobbling in a cone”) motion at 300 fs, and a slower rotation (reoriention) with a 3 ps signature. (26) Here the decay of the polarization anisotropy within the 10 ps measurement window indicates that 80% of the molecules were rotationally active during this time, and that any ordered domains must therefore be continuously interconverting. (26) The time constant for reorientation in FAPI is calculated as 2 ps at 300 K, (9) with no direct measurements available at this time. The associated frequency range for molecular rotations of 0.3–0.5 THz overlaps with the low end of the vibrational spectrum, associated with distortions of the octahedral cage.
In order to further understand the device behavior, we need to understand the nature and motion of electrical carriers.

3 Electron and Hole Transport

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There has been significant debate concerning the nature of photogenerated electron–hole pairs in MAPI: free carriers or excitons? It is now clear that the small effective mass of the electron and hole combined with the large static dielectric constant result in long-time scale exciton binding energies, EB ≪ kbT at operating temperatures. (27-29) The situation can be different in lower dimensional perovskite networks where exciton binding is enhanced.
The exciton is a quasi-particle—a bound state of an electron polaron and a hole polaron. The long-term energetic stability of these particles is dependent upon the dielectric function. The most simple effective mass approximation takes the static dielectric constant (ϵ0), giving a binding energy , where m* represents the reduced carrier mass. Due to the frequency dependence of this dielectric function (i.e., the range of response times of the lattice and molecular motion), excitons may be instantaneously stable yet decay away on the time scales of the dielectric response with EB ∼ 6 meV at 300 K. (29) The differing time scales of the dielectric response lead to non-hydrogenic energy levels of the bound exciton. (28)
The calculated high-frequency (optical) dielectric constant of 5 increases to 24 when the harmonic phonon response of the lattice is included. (27) The additional molecular rotational contribution can further increase the static dielectric constant to 33 (assuming an unhindered dipolar liquid at 300 K). These values represent a bulk response and exclude any effects from space charges or conductivity.
An early dielectric study (5) shows the strong frequency dependence (and therefore response time) of the permittivity above 165 K, which can fitted with a Kirkwood–Fröhlich model for a dipolar liquid, indicating the increasing hindrance of motion (and thus slower response time, greater dielectric response) of the methylammonium ions upon cooling toward the 165 K transition. In the pristine low temperature phase, the molecules are fixed and the dielectric response is limited to the electronic and ionic components with EB ∼ 16 meV. (29) The dielectric response will have less frequency spread, and more normal excitonic behavior will be recovered, with the potential for long-lived excitons in the orthorhombic perovskite phase.
As with any semiconductor, variation in the photophysics and electrical transport properties reported in the literature can be related to sample variation (stoichiometry, surface, morphology, point and extended defect structures). Solution processed materials are particularly susceptible to such variation. Care must be taken not to draw general conclusions from a single experiment or data source on the hybrid perovskites. Some recent reports have used high-quality single crystals whose properties should be more transferable.
Hall effect measurements suggest intrinsic dark carrier mobilities of 8–66 cm2 V–1 s–1. (10, 30) This is comparable to other solution-processed semiconductors. (31) The field-effect mobility is much smaller, 10–2–10–4 cm2 V–1 s–1. (32) These figures are relatively low considering the high dispersion and well-defined bands in the electronic structure of MAPI. CdTe, which has a similar band structure to MAPI, displays an electron mobility exceeding 1000 cm2 V–1 s–1 at low temperature. High performance photovoltaics do not directly require high mobility active materials, rather a sufficient mobility to extract photogenerated charges before recombination.
Much more remarkable are the minority-carrier diffusion lengths which exceed 1 μm. (33) This value is large for a direct band gap semiconductor, particularly one which is solution processed (and therefore structurally defective). The value is comparable to high-quality vacuum processed CdTe. This long carrier diffusion length, being larger than the film thickness necessary to absorb the full solar spectrum, makes MAPI and FAPI such high-performance photovoltaic materials.
From temperature-dependent transport measurements, the dominant carrier scattering mechanism has been attributed to acoustic phonons. (34, 35) From studying the time constants of transient recombination, monoexponential (suggesting a monomolecular process) electron–hole recombination is dominant under normal (sunlight, low fluence) operating conditions, while biexponential recombination (suggesting a bimolecular process) is a better match to higher light intensities. (36) The data can be fitted with moderate bulk and interface trap densities, but more importantly they suggest the existence of an internal mechanism that suppresses direct electron–hole interactions.

Figure 2

Figure 2. Bimolecular electron–hole recombination rates in hybrid perovskites are anomalously low. Two mechanisms are illustrated: (i) Relativistic splitting of the band edge states suppresses electron–hole recombination at the valence and conduction band edges due to momentum selection rules. The band edges are separated in reciprocal space. (ii) Fluctuations in electrostatic potential caused by molecular arrangements/octahedral distortion could cause electrons and holes to separate as proposed in ref 39. The band edges are separated in real space. These are two models that could reduce recombination rates.

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There are two likely causes of the non-Langevin recombination: (i) the local structure of molecular dipoles and octahedra distortions (leading to an inhomogeneity of the electrostatic potential) separates electron and holes in real space; (ii) the relativistic splitting of the conduction band (a Rashba–Dresselhaus effect (37) driven by the internal crystal field) results in separation of electrons and holes in reciprocal space. Polarized domains have been observed from piezoforce microscopy. (38) Both effects, illustrated in Figure 2, could contribute to the long minority-carrier lifetimes and deserve more detailed attention from theory, computation, and experiment.
A critical question is the spatial extent of the electron and hole carriers in comparison to local molecular order, as this affects both mechanisms discussed above. Within the large-polaron theory of Fröhlich we previously estimated a coupling constant (α) of 1.2. (14) However, this was based on an estimate for the longitudinal optical phonon frequency of 9 THz, while phonon calculations (40) have revealed a lower lying optical mode at 2.25 THz, which increases α to 2.4. We can therefore revise our estimate of the effective electron and hole radius to ∼4 unit cells and the associated phonon-drag increase in the bare carrier effective mass to 40%.

4 Drift and Diffusion of Ions

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Ion transport in inorganic perovskites is well established, with large anion vacancy diffusion coefficients. The activation energy in CsPbCl3 was measured to be 0.29 eV (0.69 eV including the vacancy formation energy) with a diffusion coefficient of 2.66 × 10–3 cm2 s–1. (41)
In methylammonium lead iodide, we have shown that the Schottky defect formation energy (the energy required to form a stoichiometric amount of isolated charged vacancies) is low (0.14 eV per defect); in Kröger–Vink notation:(1)Even for a stoichiometric material, there is a thermodynamic driving force for the formation of a high concentration of vacant lattice sites independent of the growth conditions. (42) The enthalpic cost of defect formation (0.14 eV per defect) is offset by the gain in configurational entropy of the system to an extent beyond the dilute defect limit. Fortunately, these defects do not result in deep electronic states in the band gap, and thus Shockley–Read–Hall type nonradiative recombination is avoided.
A reservoir of available charged point defects in the lattice could support a significant ionic current, as evidenced in impedance spectroscopy of MAPI. (43) However, further to the presence of defects, the mobility of each species depends on the activation energy for solid-state diffusion (ΔHdiff), with a hopping rate given by(2)where ν represents an effective frequency of the diffusing species in the direction of the saddle point.
The transport of ions and electrons can be approximated as two separate processes owing to the difference in time constants (see Table 1), with the total current density being a sum of the ionic and electron partial current density:(3)However, the two processes are invariably coupled. Considering a material placed under an applied voltage with ion blocking electrodes, jionic = 0 under steady-state conditions. After the current is turned on ions and electrons will flow, but the ions will gradually cease to be available for transport. The initial equilibration of the ionic distribution relies on the solid-state diffusion pathways discussed above, which are activated processes that are slow and can give rise to a temporal change in the current–voltage behavior before equilibrium is reached. Electromigration of ions due to momentum transfer from the photovoltaic current in an operating solar cell can also occur, while ion leakage into the electron and hole contacts is also a possibility. Ion transport can now be considered the most likely origin of the slow component of the reported hybrid perovskite current–voltage “hysteresis” (44) and the reversible photocurrents. (45)
The measured frequency-dependent permittivity of a hybrid perovskite film is compared with the outer-yellow skin of a banana in Figure 3. Both samples show a rapidly increasing dielectric response with decreasing frequency of the applied electric field. The banana is a good ionic conductor, and an example of a lossy dielectric composite, that shows an apparent hysteresis in current–voltage measurements at low-frequencies in the regime where charging and discharging processes at the electrode interfaces are dominant. (46) The similarity with the response of CH3NH3PbI3 is notable, with the added complication of contributions from electronic carriers whose concentration are influenced by the light intensity. (47)
Planar photovoltaic devices will have a build up in electric field (space charge region) from ion accumulation apply evenly across the charge carriers. In contrast, mesoporous devices will still offer diffusive extraction pathways. Rates of ion diffusion will be strongly affected by material stoichiometry (defect concentration and self-healing ability) and material composition (diffusion along grain boundaries and interfaces). This combination of factors may have led to the large variation in reports of hysteresis time constants and the “hysteresis-free” devices. (48)
The microscopic origin of ion transport is currently controversial with recent proposals of dominant: (a) proton diffusion; (49) (b) methylammonium diffusion; (50) (c) iodide diffusion. (51, 52) While the observable response is most likely a combination of several processes, the three key factors that determine the contribution of individual ions are the concentration, activation energy, and attempt frequency (eq 2). Given the structural disorder of these materials discussed previously, the hybrid perovskites are expected to exhibit nonideal diffusion behavior due to the complex kinetics associated with a range of local coordination environments and migration pathways. Unlike electron transport with electrons and hole wave functions delocalized over many unit cells, ion transport is dependent on and more sensitive to the local structure.

Figure 3

Figure 3. Comparison of the frequency-dependent dielectric response of a banana (Adapted from ref 46 with permission by IoP, Copyright 2008) and CH3NH3PbI3 (Adapted with permission from ref 47. Copyright 2014 American Chemical Society.). Broadband spectra of (a) real and (b) imaginary dielectric permittivity and (c) conductivity of a banana skin at room temperature. The (d) real and (e) imaginary permittivity of a CH3NH3PbI3 thin film under dark and 1 sun illumination conditions. The free carrier concentration is increased with above band gap illumination.

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Concerning proton diffusion, free H+ ions are unlikely to present in abundance since methylammonium is a weak acid. The reaction CH3NH3+ ⇌ CH3NH2 + H+ has an equilibrium constant Ka of ∼4 × 10–11 at 300 K. Taking into account the density of MAPI, this would correspond to a nominal H+ concentration of 1011 cm–3, which may be affected by the preparatory conditions (e.g., humidity) and reactions with iodine but nonetheless is minor compared to the other defects present.
There should be an adequate supply of charged Pb2+, I– and CH3NH3+ vacancies with predicted concentrations of 1017–1020cm–3 within the assumption of thermal equilibrium and noninteracting defects. (42) Due to the orientational disorder of the molecular cations, long-range diffusion of CH3NH3+ should be slower with transport between successive cages being a relatively complex process, thus lowering the attempt frequency. The effective frequency should be largest for the iodide ions, as diffusion consists of short jumps over small distances, which is aided by the high polarizability and large thermal displacements for anions in a perovskite lattice. There is evidence of fast vacancy-mediated anion diffusion in inorganic halide perovskites with a low activation energy, and thus we consider this to be the dominant process in MAPI and FAPI. Further evidence is provided by the rapid anion exchange process between the chloride, bromide and iodide perovskites, which itself is an exceptional phenomenon. A cation exchange process has also recently been observed between MAPI and FAPI, (53) which is slower than anion exchange and hence consistent with anion diffusion being a faster process.

5 Conclusions

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The time scales of the various processes discussed in this Account are summarized in Table 1. There is still much work to do concerning correlated movements, including the formation of ordered molecular domains, the nature of electron and hole polarons, and the true coupling between electronic and ionic charge carriers. Our discussion has largely focused on what is moving in CH3NH3PbI3, but we have also shown that comparable behavior is apparent in [CH(NH2)2]PbI3. It is expected that other hybrid perovskites will behave similarly, but there will be differences, for example, due to different molecular size, shape, and/or polarization. In particular, the high-performance mixture of (MA)1–x(FA)xPbI3–3xBr3x could exhibit additional dynamic processes relating to the cation and anion distributions on their respective sublattices. Inorganic perovskites would be expected to exhibit many of these phenomena, with the exception of molecular rotations, and hence could provide a platform to investigation dynamic disorder with one less degree of freedom. While halide perovskites may be relatively simple to synthesize, they still pose great challenges for a rigorous fundamental understanding.
Table 1. Summary of Dynamic Processes and Estimates of Their Associated Time Constants in CH3NH3PbI3
processmicroscopic origintime scalefrequencydiffusion coefficient
lattice vibrationsvibrational entropy10 fs to 1 ps1–100 THz 
molecular librationvibrational entropy0.5 ps2 THz 
molecular rotationrotational entropy3 ps0.3 THz 
electron transportdrift and diffusion∼1 fs∼1000 THz10–6 cm2 s–1
hole transportdrift and diffusion∼1 fs∼1000 THz10–6 cm2 s–1
ion transportdrift and diffusion∼1 ps∼1 THz10–12 cm2 s–1

Author Information

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  • Corresponding Author
    • Aron Walsh - Global E3 Institute and Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea;  Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom;  Email: [email protected]
  • Author
    • Jarvist M. Frost - Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
  • Notes
    The authors declare no competing financial interest.

Biographies

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Jarvist Moore Frost is presently a Research Associate at the Department of Chemistry at the University of Bath. He was awarded his PhD in Physics at Imperial College London under the supervision of Prof. Jenny Nelson FRS on the simulation of bulk heterojunction solar cells. For research on perovskite solar cells, he received the 2014 young researcher prize from the UK SUPERSOLAR Hub. Jarvist maintains an interest in statistical physics, multiscale simulation techniques, and bicycles.

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Aron Walsh holds the Chair of Materials Theory in the Centre for Sustainable Chemical Technologies at the University of Bath, and is dual professor with the Global E3 Institute at Yonsei University. He was awarded his BA and PhD from Trinity College Dublin (Ireland), completed a postdoctoral position at the National Renewable Energy Laboratory (USA), and held a Marie Curie fellowship at University College London (UK). Aron maintains an interest in atomistic processes at the interface of solid-state chemistry and physics, and existential Japanese cinema.

Acknowledgment

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This work has benefited from collaboration at Bath, Imperial College and Kings College London, and was supported by EPSRC Grants EP/K016288/1 and EP/J017361/1, and the ERC (Grant 277757).

References

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This article references 53 other publications.

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    Two organolead halide perovskite nanocrystals, MeNH3PbBr3 and MeNH3PbI3, efficiently sensitize TiO2 for visible-light conversion in photoelectrochem. cells. When self-assembled on mesoporous TiO2 films, the nanocryst. perovskites exhibit strong band-gap absorptions as semiconductors. The MeNH3PbI3-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The MeNH3PbBr3-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.
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    Wasylishen, R.; Knop, O.; Macdonald, J. Cation rotation in methylammonium lead halides Solid State Commun. 1985, 56, 581– 582 DOI: 10.1016/0038-1098(85)90959-7
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    3
    Cation rotation in methylammonium lead halides
    Wasylishen, R. E.; Knop, Osvald; Macdonald, J. B.
    Solid State Communications (1985), 56 (7), 581-2CODEN: SSCOA4; ISSN:0038-1098.
    2H and 14N NMR spectra of the simple perovskites MeNH3PbX3 (X = Cl, Br, I) reveal the existence of several phases. In the high-temp. phase I the long spin-lattice relaxation times T1 of both nuclei and the absence of quadrupole splitting indicate extremely rapid overall reorientation of the C-N axis of the cation in a potential of cubic symmetry. In phase II of the bromide and iodide, both T1 and the small quadrupole splitting show unusual variation with temp. In the lowest-temp. phase, rotations of the C-N axis are restricted.
  4. 4
    Poglitsch, A.; Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates observed by millimeter-wave spectroscopy J. Chem. Phys. 1987, 87, 6373 DOI: 10.1063/1.453467
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    4
    Dynamic disorder in methylammonium trihaloplumbates(II) observed by millimeter-wave spectroscopy
    Poglitsch, A.; Weber, D.
    Journal of Chemical Physics (1987), 87 (11), 6373-8CODEN: JCPSA6; ISSN:0021-9606.
    The temp.-dependent structure of cryst. methylammonium trihaloplumbates(II) CH3NH3+PbX3- (X = Cl, Br, I) as detd. by x-ray diffraction, was compared with measurements of the temp.-dependent complex permittivity at frequencies of 50-150 GHz. The dielec. measurements reveal a ps relaxation process which corresponds to a dynamic disorder of the methylammonium group in the high-temp. phases of the trihaloplumbates.
  5. 5
    Onoda-Yamamuro, N.; Matsuo, T.; Suga, H. Dielectric study of CH3NH3PbX3(X = Cl,Br,I) J. Phys. Chem. Solids 1992, 53, 935– 939 DOI: 10.1016/0022-3697(92)90121-S
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    5
    Dielectric study of CH3NH3PbX3 (X = Cl, Br, I)
    Onoda-Yamamuro, Noriko; Matsuo, Takasuke; Suga, Hiroshi
    Journal of Physics and Chemistry of Solids (1992), 53 (7), 935-9CODEN: JPCSAW; ISSN:0022-3697.
    Complex dielec. permittivities of CH3NH3PbX3 (X = Cl, Br, I) were measured at frequencies between 20 Hz and 1 MHz and at 20-300 K (15-350 K for the iodide). Discontinuities or a sharp bend of the real part of the dielec. permittivity occurred at the phase transitions, except at the tetragonal (I4/mcm)-cubic phase transition where the permittivity showed no apparent change. The dielec. behaviors in the cubic and tetragonal (I4/mcm) phases are described well by a modified Kirkwood-Froehlich equation. Dielec. dispersions were found in the orthorhombic phase of CH3NH3PbBr3 and CH3NH3PbI3 at 30-120 K.
  6. 6
    Onoda-Yamamuro, N.; Yamamuro, O.; Matsuo, T.; Suga, H. p-T phase relations of CH3NH3PbX3(X = Cl,Br,I) crystals J. Phys. Chem. Solids 1992, 53, 277– 281 DOI: 10.1016/0022-3697(92)90056-J
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    6
    The pressure-temperature phase relations of methylammonium haloplumbate CH3NH3PbX3 (X = chlorine, bromine, iodine) crystals
    Onoda-Yamamuro, Noriko; Yamamuro, Osamu; Matsuo, Takasuke; Suga, Hiroshi
    Journal of Physics and Chemistry of Solids (1992), 53 (2), 277-81CODEN: JPCSAW; ISSN:0022-3697.
    Pressure-temp. phase relations of CH3NH3PbX3 (X = Cl, Br, I) crystals were studied by using a high pressure DTA app. in the range between 0.1 Pa and 200 MPa. A triple point was found in each compd. below 100 MPa. By pressurization, the low pressure phase disappeared at the triple point in the chloride and bromide while a new high-pressure phase appeared in the iodide. The pressures and temps. of the triple points are 75.1 MPa, 175.7 K for CH3NH3PbCl3, 43.2 MPa, 152.9 K for CH3NH3PbBr3, and 84.8 MPa, 176.2 K for CH3NH3PbI3. All the boundaries between the cubic and tetragonal phases are upward convex and that of the iodide has a max. at about 120 MPa. Other phase boundaries are essentially straight lines in the measured pressure and temp. ranges. By the use of the Clausius-Clapeyron equation, the transition vols. were calcd. from the slopes of the phase boundaries and the transition entropies obtained in a previously published calorimetric expt. (authors, 1990).
  7. 7
    Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K Chem. Commun. 2015, 51, 4180– 4183 DOI: 10.1039/C4CC09944C
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    7
    Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K
    Weller, Mark T.; Weber, Oliver J.; Henry, Paul F.; Di Pumpo, Antonietta M.; Hansen, Thomas C.
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (20), 4180-4183CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3]- framework and migrates towards the cavity center with increasing temp. Crystallog. data and at. coordinates are given.
  8. 8
    Onoda-Yamamuro, N.; Matsuo, T.; Suga, H. Calorimetric and IR spectroscopic studies of phase transitions in methylammonium trihalogenoplumbates J. Phys. Chem. Solids 1990, 51, 1383– 1395 DOI: 10.1016/0022-3697(90)90021-7
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    8
    Calorimetric and IR spectroscopic studies of phase transitions in methylammonium trihaloplumbates(II)
    Onoda-Yamamuro, Noriko; Matsuo, Takasuke; Suga, Hiroshi
    Journal of Physics and Chemistry of Solids (1990), 51 (12), 1383-95CODEN: JPCSAW; ISSN:0022-3697.
    Heat capacities of CH3NH3PbX3(X = Cl, Br, I) were measured at 13-300 K (365 K for the I). Two anomalies were found in the Cl and the I, and 3 in the Br. All the phase transitions were of the 1st order, although the highest temp. transitions in the Br and the I were close to 2nd order. Their temps. and entropies are given.
  9. 9
    Weller, M. T.; Weber, O. J.; Frost, J. M.; Walsh, A. Cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298K J. Phys. Chem. Lett. 2015, 6, 3209– 3212 DOI: 10.1021/acs.jpclett.5b01432
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    9
    Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K
    Weller, Mark T.; Weber, Oliver J.; Frost, Jarvist M.; Walsh, Aron
    Journal of Physical Chemistry Letters (2015), 6 (16), 3209-3212CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The structure of black formamidinium lead halide, α-[HC(NH2)2]PbI3, at 298 K was refined from high resoln. neutron powder diffraction data and found to adopt a cubic perovskite unit cell, a 6.3620(8) Å. The trigonal planar [HC(NH2)2]+ cations lie in the central mirror plane of the unit cell with the formamidinium cations disordered over 12 possible sites arranged so that the C-H bond is directed into a cube face, whereas the -NH2 groups H bond (NH···I = 2.75-3.00 Å) with the iodide atoms of the [PbI3]- framework. High at. displacement parameters for the formamidinium cation are consistent with rapid mol. rotations at room temp. as evidenced in ab initio mol. dynamic simulations. Crystallog. data and at. coordinates are given.
  10. 10
    Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: Phase transitions, high mobilities, and near-infrared photoluminescent properties Inorg. Chem. 2013, 52, 9019– 9038 DOI: 10.1021/ic401215x
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    10
    Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties
    Stoumpos, Constantinos C.; Malliakas, Christos D.; Kanatzidis, Mercouri G.
    Inorganic Chemistry (2013), 52 (15), 9019-9038CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A broad org.-inorg. series of hybrid metal iodide perovskites AMI3, where A is the methylammonium (MeNH3+) or formamidinium (HC(NH2)2+) cation and M is Sn (1 and 2) or Pb (3 and 4) are reported. The compds. were prepd. through a variety of synthetic approaches, and the nature of the resulting materials is discussed in terms of their thermal stability and optical and electronic properties. The chem. and phys. properties of these materials strongly depend on the prepn. method. Single crystal x-ray diffraction anal. of 1-4 classifies the compds. in the perovskite structural family. Structural phase transitions were obsd. and studied by temp.-dependent single crystal x-ray diffraction in the 100-400 K range. The charge transport properties of the materials are discussed in conjunction with diffuse reflectance studies in the mid-IR region that display characteristic absorption features. Temp.-dependent studies show a strong dependence of the resistivity as a function of the crystal structure. Optical absorption measurements indicate that 1-4 behave as direct-gap semiconductors with energy band gaps distributed at 1.25-1.75 eV. The compds. exhibit an intense near-IR luminescence (PL) emission in the 700-1000 nm range (1.1-1.7 eV) at room temp. Solid solns. between the Sn and Pb compds. are readily accessible throughout the compn. range. The optical properties such as energy band gap, emission intensity, and wavelength can be readily controlled for the isostructural series of solid solns. MeNH3Sn1-xPbxI3 (5). The charge transport type in these materials was characterized by Seebeck coeff. and Hall-effect measurements. The compds. behave as p- or n-type semiconductors depending on the prepn. method. The samples with the lowest carrier concn. are prepd. from soln. and are n-type; p-type samples can be obtained through solid state reactions exposed in air in a controllable manner. In the case of Sn compds., there is a facile tendency toward oxidn. which causes the materials to be doped with Sn4+ and thus behave as p-type semiconductors displaying metal-like cond. The compds. appear to possess very high estd. electron and hole mobilities that exceed 2000 cm2/(V s) and 300 cm2/(V s), resp., as shown in the case of MeNH3SnI3 (1). The authors also compare the properties of the title hybrid materials with those of the all-inorg. CsSnI3 and CsPbI3 prepd. using identical synthetic methods.
  11. 11
    Chung, I.; Song, J. H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnI3: Semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material J. Am. Chem. Soc. 2012, 134, 8579– 8587 DOI: 10.1021/ja301539s
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    da Silva, E. L.; Skelton, J. M.; Parker, S. C.; Walsh, A. Phase stability and transformations in the halide perovskite CsSnI3 Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 144107 DOI: 10.1103/PhysRevB.91.144107
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    12
    Phase stability and transformations in the halide perovskite CsSnI3
    da Silva, E. Lora; Skelton, Jonathan M.; Parker, Stephen C.; Walsh, Aron
    Physical Review B: Condensed Matter and Materials Physics (2015), 91 (14), 144107/1-144107/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We employ the quasiharmonic approxn. to study the temp.-dependent lattice dynamics of the four different phases of cesium tin iodide (CsSnI3). Within this framework, we obtain the temp. dependence of a no. of structural properties, including the cell vol., bulk modulus, and Gr.ovrddot.uneisen parameter. The Gibbs free energy of each phase is compared against the temp.-dependent Helmholtz energy obtained from the equil. structure within the harmonic approxn. We find that the black tetragonal perovskite phase is not dynamically stable up to at least 500 K, with the phonon dispersion displaying neg. optic modes, which pass through all of the high-symmetry wave vectors in the Brillouin zone. The main contributions to the neg. modes are found to be motions of the Cs atom inside the perovskite cage. The black cubic perovskite structure shows a zone-boundary instability, indicated by soft modes at the special q points M and R. These modes are present in calcns. at the equil. (0 K) lattice const., while at finite temp. addnl. neg. modes develop at the zone center, indicating a ferroelec. instability. The yellow crystal, composed of one-dimensional (SnI6)n double chains, has the same heat of formation as the orthorhombic perovskite phase at 0 K, but becomes less energetically favorable at higher temps., due to its higher free energy.
  13. 13
    Brivio, F.; Walker, A. B.; Walsh, A. Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles APL Mater. 2013, 1, 042111 DOI: 10.1063/1.4824147
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    13
    Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles
    Brivio, Federico; Walker, Alison B.; Walsh, Aron
    APL Materials (2013), 1 (4), 042111/1-042111/5CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    The performance of perovskite solar cells recently exceeded 15% solar-to-electricity conversion efficiency for small-area devices. The fundamental properties of the active absorber layers, hybrid org.-inorg. perovskites formed from mixing metal and org. halides [e.g., (NH4)PbI3 and (CH3NH3)PbI3], are largely unknown. The materials are semiconductors with direct band gaps at the boundary of the first Brillouin zone. The calcd. dielec. consts. and band gaps show an orientation dependence, with a low barrier for rotation of the org. cations. Due to the elec. dipole of the methylammonium cation, a photoferroic effect may be accessible, which could enhance carrier collection. (c) 2013 American Institute of Physics.
  14. 14
    Frost, J. M.; Butler, K. T.; Walsh, A. Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells APL Mater. 2014, 2, 081506 DOI: 10.1063/1.4890246
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    14
    Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells
    Frost, Jarvist M.; Butler, Keith T.; Walsh, Aron
    APL Materials (2014), 2 (8), 081506/1-081506/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    We report a model describing the mol. orientation disorder in CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic structure calcns., with the nature of the motions informed by ab initio mol. dynamics. We investigate the temp. and static elec. field dependence of the equil. ferroelec. (mol.) domain structure and resulting polarisability. A rich domain structure of twinned mol. dipoles is obsd., strongly varying as a function of temp. and applied elec. field. We propose that the internal elec. fields assocd. with microscopic polarisation domains contribute to hysteretic anomalies in the current-voltage response of hybrid org.-inorg. perovskite solar cells due to variations in electron-hole recombination in the bulk. (c) 2014 American Institute of Physics.
  15. 15
    https://www.youtube.com/watch?v=PPwSIYLnONY (accessed 25.12.15).
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    https://www.youtube.com/watch?v=jwEgBq9BIkk (accessed 25.12.15).
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    Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kochelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O'Regan, B. C.; Nelson, J.; Cabral, J. T.; Barnes, P. R. F. Dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells Nat. Commun. 2015, 6, 7124 DOI: 10.1038/ncomms8124
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    17
    The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells
    Leguy Aurelien M A; McMahon Andrew P; Nelson Jenny; Barnes Piers R F; Frost Jarvist Moore; Walsh Aron; Sakai Victoria Garcia; Kochelmann W; Law ChunHung; Li Xiaoe; O'Regan Brian C; Foglia Fabrizia; Cabral Joao T
    Nature communications (2015), 6 (), 7124 ISSN:.
    Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
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    https://github.com/WMD-group/hybrid-perovskites (accessed 25.12.15).
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    Walsh, A.; Payne, D. J.; Egdell, R. G.; Watson, G. W. Stereochemistry of post-transition metal oxides: revision of the classical lone-pair model Chem. Soc. Rev. 2011, 40, 4455– 4463 DOI: 10.1039/c1cs15098g
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    Worhatch, R. J.; Kim, H. J.; Swainson, I. P.; Yonkeu, A. L.; Billinge, S. J. L. Study of local structure in selected cubic organic-inorganic perovskites Chem. Mater. 2008, 20, 1272– 1277 DOI: 10.1021/cm702668d
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    20
    Study of Local Structure in Selected Organic-Inorganic Perovskites in the Pm‾3m Phase
    Worhatch, Richard J.; Kim, HyunJeong; Swainson, Ian P.; Yonkeu, Andre L.; Billinge, Simon J. L.
    Chemistry of Materials (2008), 20 (4), 1272-1277CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The local structures of the inorg. component of selected org.-inorg. perovskites (OIPs) were studied by analyzing the x-ray pair distribution function. Whereas the long-range structure of each perovskite is the untilted Pm3‾m phase, all the OIPs showed significant internal distortion of the octahedra. Varying the halide has a significant impact on the lattice const. There is evidence of local lone-pair distortions for certain compns. The most complex case of disorder appears to be that of CH3NH3SnBr3.
  21. 21
    Grancini, G.; Srimath Kandada, A. R.; Frost, J. M.; Barker, A. J.; De Bastiani, M.; Gandini, M.; Marras, S.; Lanzani, G.; Walsh, A.; Petrozza, A. Role of microstructure in the electronhole interaction of hybrid lead halide perovskites Nat. Photonics 2015, 9, 695– 702 DOI: 10.1038/nphoton.2015.151
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    21
    Role of microstructure in the electron-hole interaction of hybrid lead halide perovskites
    Grancini, Giulia; Srimath Kandada, Ajay Ram; Frost, Jarvist M.; Barker, Alex J.; De Bastiani, Michele; Gandini, Marina; Marras, Sergio; Lanzani, Guglielmo; Walsh, Aron; Petrozza, Annamaria
    Nature Photonics (2015), 9 (10), 695-701CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Org.-inorg. metal halide perovskites have demonstrated high power conversion efficiencies in solar cells and promising performance in a wide range of optoelectronic devices. The existence and stability of bound electron-hole pairs in these materials and their role in the operation of devices with different architectures remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modeling as a function of the degree of polycrystallinity and temp., that the electron-hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycryst. disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same compn. demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphol. strongly influence perovskite behavior, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.
  22. 22
    Stroppa, A.; Quarti, C.; De Angelis, F.; Picozzi, S. Ferroelectric polarization of CH3NH3PbI3: A detailed study based on DFT and symmetry mode analysis J. Phys. Chem. Lett. 2015, 6, 2223– 2231 DOI: 10.1021/acs.jpclett.5b00542
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    22
    Ferroelectric Polarization of CH3NH3PbI3: A Detailed Study Based on Density Functional Theory and Symmetry Mode Analysis
    Stroppa, Alessandro; Quarti, Claudio; De Angelis, Filippo; Picozzi, Silvia
    Journal of Physical Chemistry Letters (2015), 6 (12), 2223-2231CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Ferroelectricity in halide perovskites currently represents a crucial issue, as it may have an important role for the enhancement of solar cells efficiency. Simulations of ferroelec. properties based on d. functional theory are conceptually more demanding compared with conventional inorg. ferroelecs. due to the presence of both org. and inorg. components in the same compd. Here the authors present a detailed study focused on the prototypical CH3NH3PbI3 perovskite. By using d. functional theory combined with symmetry mode anal., the authors disentangle the contributions of the methylammonium cations and the role of the inorg. framework, therefore suggesting possible routes to enhance the polarization in this compd. The authors' est. of the polarization for the tetragonal phase at low temp. is ∼4.42 μC/cm2, which is substantially lower than that of traditional perovskite oxides.
  23. 23
    Ma, J.; Wang, L.-W. Nanoscale charge localization induced by random orientations of organic molecules in hybrid perovskite CH3NH3PbI3 Nano Lett. 2015, 15, 248– 253 DOI: 10.1021/nl503494y
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    23
    Nanoscale Charge Localization Induced by Random Orientations of Organic Molecules in Hybrid Perovskite CH3NH3PbI3
    Ma, Jie; Wang, Lin-Wang
    Nano Letters (2015), 15 (1), 248-253CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues of the hybrid perovskites were not fully understood. Exptl., it is known that in CH3NH3PbI3 the org. mols. CH3NH3 are randomly oriented at the room temp., but the impact of the random mol. orientation was not studied. Because of the dipole moment of the org. mol., the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. Using linear scaling ab initio methods, the charge densities of the conduction band min. and the valence band max. are localized in nanoscales due to the potential fluctuations. The charge localization causes electron-hole sepn. and reduces carrier recombination rates, which may contribute to the long carrier lifetime obsd. in expts.
  24. 24
    Sherkar, T.; Koster, J. A. Can ferroelectric polarization explain the high-performance of hybrid-halide perovskite solar cells? Phys. Chem. Chem. Phys. 2016, 18, 331– 338 DOI: 10.1039/C5CP07117H
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    Chen, T.; Foley, B. J.; Ipek, B.; Tyagi, M.; Copley, J. R. D.; Brown, C. M.; Choi, J. J.; Lee, S.-H. Rotational dynamics of organic cations in CH3NH3PbI3 perovskite Phys. Chem. Chem. Phys. 2015, 17, 31278– 31286 DOI: 10.1039/C5CP05348J
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    Bakulin, A. A.; Selig, O.; Bakker, H. J.; Rezus, Y. L. A.; Müller, C.; Glaser, T.; Lovrincic, R.; Sun, Z.; Chen, Z.; Walsh, A.; Frost, J. M.; Jansen, T. L. C. Real-time observation of cation reorientation in methylammonium lead iodide perovskites J. Phys. Chem. Lett. 2015, 6, 3663– 3669 DOI: 10.1021/acs.jpclett.5b01555
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    26
    Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites
    Bakulin, Artem A.; Selig, Oleg; Bakker, Huib J.; Rezus, Yves L. A.; Mueller, Christian; Glaser, Tobias; Lovrincic, Robert; Sun, Zhenhua; Chen, Zhuoying; Walsh, Aron; Frost, Jarvist M.; Jansen, Thomas L. C.
    Journal of Physical Chemistry Letters (2015), 6 (18), 3663-3669CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The introduction of a mobile and polarized org. moiety as a cation in 3D Pb-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the org. cation and the mol. mechanism behind its motion remain unclear, with different exptl. and computational approaches providing very different qual. and quant. description of the mol. dynamics. Ultrafast 2D vibrational spectroscopy of methylammonium (MA) Pb iodide was used to directly resolve the rotation of the org. cations within the MAPbI3 lattice. The results reveal 2 characteristic time consts. of motion. Using ab initio mol. dynamics simulations, the authors identify these as a fast (∼300 fs) wobbling-in-a-cone motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the mol. dipole with respect to the iodide lattice. The obsd. dynamics are essential for understanding the electronic properties of perovskite materials.
  27. 27
    Brivio, F.; Butler, K. T.; Walsh, A.; van Schilfgaarde, M. Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 155204 DOI: 10.1103/PhysRevB.89.155204
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    27
    Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers
    Brivio, Federico; Butler, Keith T.; Walsh, Aron; van Schilfgaarde, Mark
    Physical Review B: Condensed Matter and Materials Physics (2014), 89 (15), 155204/1-155204/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Solar cells based on a light absorbing layer of the organometal halide perovskite CH3NH3PbI3 have recently surpassed 15% conversion efficiency, though how these materials work remains largely unknown. We analyze the electronic structure and optical properties within the quasiparticle self-consistent GW approxn. While this compd. bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: Band gaps are much larger than what is predicted by the local-d. approxn. (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addn., spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The av. hole mass is small, which partially accounts for the long diffusion lengths obsd. The surface ionization potential (work function) is calcd. to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO2 and Au elec. contacts.
  28. 28
    Menéndez-Proupin, E.; Beltrán Ríos, C. L.; Wahnón, P. Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3 Phys. Status Solidi RRL 2015, 9, 559– 563 DOI: 10.1002/pssr.201510265
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    28
    Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3
    Menendez-Proupin, E.; Rios, Carlos L. Beltran; Wahnon, P.
    Physica Status Solidi RRL: Rapid Research Letters (2015), 9 (10), 559-563CODEN: PSSRCS; ISSN:1862-6254. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The excitons in the orthorhombic phase of the perovskite CH3NH3PbI3 are studied using the effective mass approxn. The electron-hole interaction is screened by a distance-dependent dielec. function, as described by the Haken potential or the Pollmann-Buettner potential. The energy spectrum and the eigenfunctions are calcd. for both cases. The results show that the Pollmann-Buettner model, using the corresponding parameters obtained from ab initio calcns., provides better agreement with the exptl. results.
  29. 29
    Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.-W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites Nat. Phys. 2015, 11, 582– 587 DOI: 10.1038/nphys3357
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    29
    Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites
    Miyata, Atsuhiko; Mitioglu, Anatolie; Plochocka, Paulina; Portugall, Oliver; Wang, Jacob Tse-Wei; Stranks, Samuel D.; Snaith, Henry J.; Nicholas, Robin J.
    Nature Physics (2015), 11 (7), 582-587CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
    Solar cells based on the org.-inorg. tri-halide perovskite family of materials have shown significant progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temps., over three times smaller than has been previously assumed. In the room-temp. phase we show that the binding energy falls to even smaller values of only a few millielectron volts, which explains their excellent device performance as being due to spontaneous free-carrier generation following light absorption. Addnl., we det. the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estd. exptl. but in good agreement with recent calcns. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.
  30. 30
    Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites Adv. Mater. 2014, 26, 1584– 1589 DOI: 10.1002/adma.201305172
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    30
    High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites
    Wehrenfennig, Christian; Eperon, Giles E.; Johnston, Michael B.; Snaith, Henry J.; Herz, Laura M.
    Advanced Materials (Weinheim, Germany) (2014), 26 (10), 1584-1589CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors find that methylammonium lead trihalide perovskites are particularly well-suited as light absorbers and charge transporters in photovoltaic cells because they allow for an unexpected combination of both low charge recombination rates and high charge-carrier mobilities. The authors establish lower bounds of-10 cm2 V-1 s'1 for the high-frequency charge mobility, which is remarkably high for a soln.-processed material. The planar heterojunction photovoltaic cells may only be achieved because the ratio of bimol. charge recombination rate to charge mobility is over four orders of magnitude lower than that predicted from Langevin theory. Such effects are likely to arise from spatial sepn. of opposite charge carriers within the metal-halide structure or across a cryst. domain. Modeling and tuning recombination channels, e.g. through halide and metal substitutions, or crystallite size, will hold the clue to raising material performance.
  31. 31
    Mitzi, D. B.; Kosbar, L. L.; Murray, C. E.; Copel, M.; Afzali, A. High-mobility ultrathin semiconducting films prepared by spin coating Nature 2004, 428, 299– 303 DOI: 10.1038/nature02389
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    31
    High-mobility ultrathin semiconducting films prepared by spin coating
    Mitzi, David B.; Kosbar, Laura L.; Murray, Conal E.; Copel, Matthew; Afzali, Ali
    Nature (London, United Kingdom) (2004), 428 (6980), 299-303CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient soln.-based deposition is also an important research direction, with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resoln. displays and electronic paper) and lower-cost device fabrication. Here, the authors demonstrate a technique for spin coating ultrathin (∼50 Å), cryst., and continuous metal chalcogenide films, based on the low-temp. decompn. of highly sol. hydrazinium precursors. The authors fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS2-xSex films, which exhibit n-type transport, large current densities (>105 A cm-2), and mobilities greater than 10 cm2V-1s-1 - an order of magnitude higher than previously reported values for spin-coated semiconductors. He spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelecs. and memory devices).
  32. 32
    Labram, J. G.; Fabini, D. H.; Perry, E. E.; Lehner, A. J.; Wang, H.; Glaudell, A. M.; Wu, G.; Evans, H.; Buck, D.; Cotta, R.; Echegoyen, L.; Wudl, F.; Seshadri, R.; Chabinyc, M. L. Temperature-dependent polarization in field-effect transport and photovoltaic measurements of methylammonium lead iodide J. Phys. Chem. Lett. 2015, 6, 3565– 3571 DOI: 10.1021/acs.jpclett.5b01669
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    32
    Temperature-Dependent Polarization in Field-Effect Transport and Photovoltaic Measurements of Methylammonium Lead Iodide
    Labram, John G.; Fabini, Douglas H.; Perry, Erin E.; Lehner, Anna J.; Wang, Hengbin; Glaudell, Anne M.; Wu, Guang; Evans, Hayden; Buck, David; Cotta, Robert; Echegoyen, Luis; Wudl, Fred; Seshadri, Ram; Chabinyc, Michael L.
    Journal of Physical Chemistry Letters (2015), 6 (18), 3565-3571CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid org.-inorg. perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI3) as the active material and conducted a series of temp.-dependent measurements. Field-effect transistor, capacitor, and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temp.-dependent polarization susceptibility in (MA)PbI3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI3 photovoltaic cells is obsd. to reduce drastically with decreasing temp., suggesting that such polarization effects could be a prerequisite for high-performance device operation.
  33. 33
    Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber Science 2013, 342, 341– 344 DOI: 10.1126/science.1243982
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    33
    Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber
    Stranks, Samuel D.; Eperon, Giles E.; Grancini, Giulia; Menelaou, Christopher; Alcocer, Marcelo J. P.; Leijtens, Tomas; Herz, Laura M.; Petrozza, Annamaria; Snaith, Henry J.
    Science (Washington, DC, United States) (2013), 342 (6156), 341-344CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Org.-inorg. perovskites showed promise as high-performance absorbers in solar cells, 1st as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, the authors report transient absorption and photoluminescence-quenching measurements to det. the electron-hole diffusion lengths, diffusion consts., and lifetimes in mixed halide (MeNH3PbI3-xClx) and triiodide (MeNH3PbI3) perovskite absorbers. The diffusion lengths are >1 μm in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. But the triiodide absorber has electron-hole diffusion lengths of about 100 nm. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a crit. parameter to optimize for future perovskite absorber development.
  34. 34
    Karakus, M.; Jensen, S. A.; D’Angelo, F.; Turchinovich, D.; Bonn, M.; Canovas, E. Phonon-electron-scattering limits free charge mobility in methylammonium lead iodide perovskites J. Phys. Chem. Lett. 2015, 6, 4991– 4996 DOI: 10.1021/acs.jpclett.5b02485
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    34
    Phonon-Electron Scattering Limits Free Charge Mobility in Methylammonium Lead Iodide Perovskites
    Karakus, Melike; Jensen, Soeren A.; D'Angelo, Francesco; Turchinovich, Dmitry; Bonn, Mischa; Canovas, Enrique
    Journal of Physical Chemistry Letters (2015), 6 (24), 4991-4996CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The nature of the photocond. in soln.-processed films of methylammonium lead iodide perovskite was studied by detg. the variation of the photoconductive response with temp. Ultrabroadband terahertz (THz) photocond. spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temp., where free charge carrier motion was characterized by an av. scattering time. The scattering time detd. from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm2 V-1 s-1) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm2 V-1 s-1). For the tetragonal phase (temp. range 150 < T < 300 K) the scattering time shows a approx. T-3/2 dependence, approaching the theor. limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.
  35. 35
    Zhu, X.-Y.; Podzorov, V. Charge carriers in hybrid-organicinorganic lead-halide perovskites might be protected as large polarons J. Phys. Chem. Lett. 2015, 6, 4758– 4761 DOI: 10.1021/acs.jpclett.5b02462
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    Charge Carriers in Hybrid Organic-Inorganic Lead Halide Perovskites Might Be Protected as Large Polarons
    Zhu, X.-Y.; Podzorov, V.
    Journal of Physical Chemistry Letters (2015), 6 (23), 4758-4761CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Large polaron that provides the protection of charge carrier is explained in terms of carrier diffusion length,electron hole recombination rates, charge carrier and Hall mobility and low carrier scattering rate.
  36. 36
    Stranks, S. D.; Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices Nat. Nanotechnol. 2015, 10, 391– 402 DOI: 10.1038/nnano.2015.90
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    Metal-halide perovskites for photovoltaic and light-emitting devices
    Stranks, Samuel D.; Snaith, Henry J.
    Nature Nanotechnology (2015), 10 (5), 391-402CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    A review. Metal-halide perovskites are cryst. materials originally developed out of scientific curiosity. Unexpectedly, solar cells incorporating these perovskites are rapidly emerging as serious contenders to rival the leading photovoltaic technologies. Power conversion efficiencies have jumped from 3% to over 20% in just four years of academic research. Here, we review the rapid progress in perovskite solar cells, as well as their promising use in light-emitting devices. In particular, we describe the broad tunability and fabrication methods of these materials, the current understanding of the operation of state-of-the-art solar cells and we highlight the properties that have delivered light-emitting diodes and lasers. We discuss key thermal and operational stability challenges facing perovskites, and give an outlook of future research avenues that might bring perovskite technol. to commercialization.
  37. 37
    Kepenekian, M.; Robles, R.; Katan, C.; Sapori, D.; Pedesseau, L.; Even, J. Rashba and Dresselhaus effects in organic-inorganic perovskites: from basics to devices ACS Nano 2015, 9, 11557– 11567 DOI: 10.1021/acsnano.5b04409
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    Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of ferroelectric domains in solution-processed CH3NH3PbI3 thin films J. Phys. Chem. Lett. 2014, 5, 3335– 3339 DOI: 10.1021/jz501697b
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    Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films
    Kutes, Yasemin; Ye, Linghan; Zhou, Yuanyuan; Pang, Shuping; Huey, Bryan D.; Padture, Nitin P.
    Journal of Physical Chemistry Letters (2014), 5 (19), 3335-3339CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelec., almost nothing is known about their ferroelec. properties exptl. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelec. domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new soln.-processing method. The size of the ferroelec. domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelec. domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelec. behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.
  39. 39
    Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells Nano Lett. 2014, 14, 2587– 2590
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    Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 144308 DOI: 10.1103/PhysRevB.92.144308
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    40
    Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide
    Brivio, Federico; Frost, Jarvist M.; Skelton, Jonathan M.; Jackson, Adam J.; Weber, Oliver J.; Weller, Mark T.; Goni, Alejandro R.; Leguy, Aurelien M. A.; Barnes, Piers R. F.; Walsh, Aron
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (14), 144308/1-144308/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equil. structures compare well to solns. of temp.-dependent powder neutron diffraction. By following the normal modes, we calc. IR and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degrdn. of the sample. Despite a clear sepn. in energy between low-frequency modes assocd. with the inorg. (PbI3-)n network and high-frequency modes of the org. CH3NH3+ cation, significant coupling between them is found, which emphasizes the interplay between mol. orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temp.
  41. 41
    Mizusaki, J.; Arai, K.; Fueki, K. Ionic conduction of the perovskite-type halides Solid State Ionics 1983, 11, 203– 211 DOI: 10.1016/0167-2738(83)90025-5
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    Ionic conduction of the perovskite-type halides
    Mizusaki, Junichiro; Arai, Kimiyasu; Fueki, Kazuo
    Solid State Ionics (1983), 11 (3), 203-11CODEN: SSIOD3; ISSN:0167-2738.
    The ionic conduction in the perovskite-type halides CsPbCl3, CsPbBr3, and KMnCl3 was studied. Measurements were made of a.c. cond. from 150° to the m.p., and of ionic transport no. by using the Tubandt, emf., and ion-blocking methods. The effects of impurity doping on the ionic cond. of CsPbCl3 were also studied by using samples of compn. CsPb0.99M0.01Cl2.99 (M = Li, Na, K, Ag). These materials are halide-ion conductors. The ionic conductivities of CsPbCl3 and CsPbBr3 are close to those of the well known halide-ion conductors, PbCl2 and PbBr2. The ionic transport nos. are >0.9 for CsPbCl3 and CsPbBr3, and ∼0.99 for KMnCl3. The conduction is caused by the migration of halide-ion vacancies VX (X = Cl, Br). The activation energies for the migration of VX are 0.29 eV for CsPbCl3, 0.25 eV for CsPbBr3, and 0.39 eV for KMnCl3. The vacancy diffusion coeffs. of these materials are very large. However, the impurity doping does not increase the ionic cond. markedly because of small dopant soly.
  42. 42
    Walsh, A.; Scanlon, D. O.; Chen, S.; Gong, X. G.; Wei, S.-H. Self-regulation mechanism for charged point defects in hybrid halide perovskites Angew. Chem., Int. Ed. 2015, 54, 1791– 1794 DOI: 10.1002/anie.201409740
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    Self-regulation mechanism for charged point defects in hybrid halide perovskites
    Walsh, Aron; Scanlon, David O.; Chen, Shiyou; Gong, X. G.; Wei, Su-Huai
    Angewandte Chemie, International Edition (2015), 54 (6), 1791-1794CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concns. despite being processed at low-temps. from soln. We demonstrate, through quantum mech. calcns., that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concn. of charge carriers through ionic compensation of charged point defects. The equil. charged vacancy concn. is predicted to exceed 0.4 % at room temp. This behavior, which goes against established defect conventions for inorg. semiconductors, has implications for photovoltaic performance.
  43. 43
    Yang, T.-Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. Significance of ion conduction in a organic-inorganic lead-iodide-based perovskite photosensitizer Angew. Chem., Int. Ed. 2015, 54, 7905– 7910 DOI: 10.1002/anie.201500014
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    Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T. W.; Wojciechowski, K.; Zhang, W. Anomalous hysteresis in perovskite solar cells J. Phys. Chem. Lett. 2014, 5, 1511– 1515 DOI: 10.1021/jz500113x
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    Anomalous Hysteresis in Perovskite Solar Cells
    Snaith, Henry J.; Abate, Antonio; Ball, James M.; Eperon, Giles E.; Leijtens, Tomas; Noel, Nakita K.; Stranks, Samuel D.; Wang, Jacob Tse-Wei; Wojciechowski, Konrad; Zhang, Wei
    Journal of Physical Chemistry Letters (2014), 5 (9), 1511-1515CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current-voltage curves. The authors identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. The authors also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. The authors hypothesize 3 possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.
  45. 45
    Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices Nat. Mater. 2015, 14, 193– 198 DOI: 10.1038/nmat4150
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    Giant switchable photovoltaic effect in organometal trihalide perovskite devices
    Xiao, Zhengguo; Yuan, Yongbo; Shao, Yuchuan; Wang, Qi; Dong, Qingfeng; Bi, Cheng; Sharma, Pankaj; Gruverman, Alexei; Huang, Jinsong
    Nature Materials (2015), 14 (2), 193-198CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, soln.-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small elec. field of <1 V μm-1. The switchable photocurrent, generally obsd. in devices based on ferroelec. materials, reached 20.1 mA cm-2 under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelec. photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and elec.-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in elec. and optically readable memristors and circuits.
  46. 46
    Loidl, A.; Krohns, S.; Hemberger, J.; Lunkenheimer, P. Bananas go paraelectric J. Phys.: Condens. Matter 2008, 20, 191001 DOI: 10.1088/0953-8984/20/19/191001
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    Bananas go paraelectric
    Loidl, A.; Krohns, S.; Hemberger, J.; Lunkenheimer, P.
    Journal of Physics: Condensed Matter (2008), 20 (19), 191001/1-191001/3CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
    Using a banana as an example, we demonstrate how the ferroelec.-like hysteresis loops measured in inhomogeneous, conducting materials can easily be identified as non-intrinsic. With simple expts., the response of a banana to elec. fields is revealed as characteristic for an inhomogeneous paraelec. ion conductor. Not even abs. beginners in dielecs. should identify this biol. matter as ferroelec.
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    Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Photoinduced giant dielectric constant in lead halide perovskite solar cells J. Phys. Chem. Lett. 2014, 5, 2390– 2394 DOI: 10.1021/jz5011169
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    Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells
    Juarez-Perez, Emilio J.; Sanchez, Rafael S.; Badia, Laura; Garcia-Belmonte, Germa; Kang, Yong Soo; Mora-Sero, Ivan; Bisquert, Juan
    Journal of Physical Chemistry Letters (2014), 5 (13), 2390-2394CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Org.-inorg. lead trihalide perovskites have emerged as an outstanding photovoltaic material that demonstrated a high 17.9% conversion efficiency of sunlight to electricity in a short time. We have found a giant dielec. const. (GDC) phenomenon in these materials consisting on a low frequency dielec. const. in the dark of the order of ε0 = 1000. We also found an unprecedented behavior in which ε0 further increases under illumination or by charge injection at applied bias. We observe that ε0 increases nearly linearly with the illumination intensity up to an addnl. factor 1000 under 1 sun. Measurement of a variety of samples of different morphologies, compns., and different types of contacts shows that the GDC is an intrinsic property of MAPbX3 (MA = CH3NH3+). We hypothesize that the large dielec. response is induced by structural fluctuations. Photoinduced carriers modify the local unit cell equil. and change the polarizability, assisted by the freedom of rotation of MA. The study opens a way for the understanding of a key aspect of the photovoltaic operation of high efficiency perovskite solar cells.
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    Bryant, D.; Wheeler, S.; O’Regan, B. C.; Watson, T.; Barnes, P. R.; Worsley, D. A.; Durrant, J. Observable hysteresis at low temperature in hysteresis free lead halide perovskite solar cells J. Phys. Chem. Lett. 2015, 6, 3190– 3194 DOI: 10.1021/acs.jpclett.5b01381
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    Observable Hysteresis at Low Temperature in "Hysteresis Free" Organic-Inorganic Lead Halide Perovskite Solar Cells
    Bryant, Daniel; Wheeler, Scot; O'Regan, Brian C.; Watson, Trystan; Barnes, Piers R. F.; Worsley, Dave; Durrant, James
    Journal of Physical Chemistry Letters (2015), 6 (16), 3190-3194CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    In this paper the JV hysteresis behavior is addressed of planar org.-inorg. lead halide perovskite solar cells fabricated using PC60BM as the cathode. At room temp., these devices exhibit apparently hysteresis free JV scans. It is obsd. that cooling to 175 K results in the appearance of substantial JV hysteresis. Employing chronoamperometric measurements, it is demonstrated that the half-time for the relaxation process underlying this hysteresis slows from 0.6 s at 298 K to 15.5 s at 175 K, yielding an activation energy of 0.12 eV. It is further demonstrated that by cooling a cell to 77 K while held under pos. bias, it is possible to "freeze" the cell into the most favorable condition for efficient photovoltaic performance. The changes to device architecture that appear to remove room temp. JV hysteresis may not remove the underlying process(es), but rather shift them to time scales not readily observable in typical room temp. JV scans.
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    Egger, D. A.; Kronik, L.; Rappe, A. M. Theory of hydrogen migration in organic-inorganic halide perovskites Angew. Chem., Int. Ed. 2015, 54, 12437– 12441 DOI: 10.1002/anie.201502544
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    Theory of Hydrogen Migration in Organic-Inorganic Halide Perovskites
    Egger, David A.; Kronik, Leeor; Rappe, Andrew M.
    Angewandte Chemie, International Edition (2015), 54 (42), 12437-12441CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Solar cells based on org.-inorg. halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current-voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calcns. based on d. functional theory, we study the properties of an important defect in hybrid perovskites-interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin-Corbett mechanism. Our anal. highlights the structural flexibility of org.-inorg. perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells.
  50. 50
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    Solar cells based on org.-inorg. halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behavior such as current-voltage hysteresis and a low-frequency giant dielec. response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chem. origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extd. from the current-voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and exptl. study suggest that hybrid halide perovskites are mixed ionic-electronic conductors, a finding that has major implications for solar cell device architectures.
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    Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calcns. The calcd. activation energies of the anionic and cationic vacancy migrations clearly show that I- anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that obsd. in ion-conducting materials. More interestingly, typical MA+ cations and larger FA+ cations both have rather low barriers as well, indicating that the cation mols. can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by expts. With the dil. diffusion theory, we discuss that smaller vacancy concns. (higher crystallinity) and replacement of MA+ with larger cation mols. will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.
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    Cation exchange for thin film lead iodide perovskite interconversion
    Eperon, Giles E.; Beck, Clara E.; Snaith, Henry J.
    Materials Horizons (2016), 3 (1), 63-71CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
    We report a new technique for tuning the bandgap of hybrid org.-inorg. halide perovskite materials. By dipping films of methylammonium or formamidinium lead triiodide (MAPbI3 or FAPbI3) in solns. of formamidinium or methylammonium iodide (FAI or MAI) at room temp., we are able to inter-convert through cation exchange between perovskite materials, allowing us to carefully tune the bandgap between 1.57 and 1.48 eV. We observe uniform conversion through the entirety of the bulk film, with no evidence for a "bi-layered" or graded structure. By applying this technique to solar cell devices, we are able to enhance the performance of the single cation devices. Furthermore, we demonstrate that this technique allows us to form pure black phase FAPbI3 infiltrated into mesoporous scaffolds; this is normally infeasible since the pores confine the FAPbI3 in a yellow non-perovskite phase with a much wider bandgap, which is not of practical use in solar cells or other optoelectronic devices. Addnl., this work provides evidence for mol. cation mobility in the halide perovskites, indicating that the cations play a role in ionic conduction as well as the mobile anions.

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Published February 9, 2016

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  • Abstract

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    Figure 1

    Figure 1. Temperature-dependent (100–352 K) powder neutron-diffraction pattern of CH3NH3PbI3. From ref 7. Adapted by permission of the RSC. The space groups of the average crystals structures are shown with a schematic of the extent of disorder in the CH3NH3+ sublattice.

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    Figure 2

    Figure 2. Bimolecular electron–hole recombination rates in hybrid perovskites are anomalously low. Two mechanisms are illustrated: (i) Relativistic splitting of the band edge states suppresses electron–hole recombination at the valence and conduction band edges due to momentum selection rules. The band edges are separated in reciprocal space. (ii) Fluctuations in electrostatic potential caused by molecular arrangements/octahedral distortion could cause electrons and holes to separate as proposed in ref 39. The band edges are separated in real space. These are two models that could reduce recombination rates.

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    Figure 3

    Figure 3. Comparison of the frequency-dependent dielectric response of a banana (Adapted from ref 46 with permission by IoP, Copyright 2008) and CH3NH3PbI3 (Adapted with permission from ref 47. Copyright 2014 American Chemical Society.). Broadband spectra of (a) real and (b) imaginary dielectric permittivity and (c) conductivity of a banana skin at room temperature. The (d) real and (e) imaginary permittivity of a CH3NH3PbI3 thin film under dark and 1 sun illumination conditions. The free carrier concentration is increased with above band gap illumination.

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  • References

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      Stoumpos, Constantinos C.; Malliakas, Christos D.; Kanatzidis, Mercouri G.
      Inorganic Chemistry (2013), 52 (15), 9019-9038CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A broad org.-inorg. series of hybrid metal iodide perovskites AMI3, where A is the methylammonium (MeNH3+) or formamidinium (HC(NH2)2+) cation and M is Sn (1 and 2) or Pb (3 and 4) are reported. The compds. were prepd. through a variety of synthetic approaches, and the nature of the resulting materials is discussed in terms of their thermal stability and optical and electronic properties. The chem. and phys. properties of these materials strongly depend on the prepn. method. Single crystal x-ray diffraction anal. of 1-4 classifies the compds. in the perovskite structural family. Structural phase transitions were obsd. and studied by temp.-dependent single crystal x-ray diffraction in the 100-400 K range. The charge transport properties of the materials are discussed in conjunction with diffuse reflectance studies in the mid-IR region that display characteristic absorption features. Temp.-dependent studies show a strong dependence of the resistivity as a function of the crystal structure. Optical absorption measurements indicate that 1-4 behave as direct-gap semiconductors with energy band gaps distributed at 1.25-1.75 eV. The compds. exhibit an intense near-IR luminescence (PL) emission in the 700-1000 nm range (1.1-1.7 eV) at room temp. Solid solns. between the Sn and Pb compds. are readily accessible throughout the compn. range. The optical properties such as energy band gap, emission intensity, and wavelength can be readily controlled for the isostructural series of solid solns. MeNH3Sn1-xPbxI3 (5). The charge transport type in these materials was characterized by Seebeck coeff. and Hall-effect measurements. The compds. behave as p- or n-type semiconductors depending on the prepn. method. The samples with the lowest carrier concn. are prepd. from soln. and are n-type; p-type samples can be obtained through solid state reactions exposed in air in a controllable manner. In the case of Sn compds., there is a facile tendency toward oxidn. which causes the materials to be doped with Sn4+ and thus behave as p-type semiconductors displaying metal-like cond. The compds. appear to possess very high estd. electron and hole mobilities that exceed 2000 cm2/(V s) and 300 cm2/(V s), resp., as shown in the case of MeNH3SnI3 (1). The authors also compare the properties of the title hybrid materials with those of the all-inorg. CsSnI3 and CsPbI3 prepd. using identical synthetic methods.
    11. 11
      Chung, I.; Song, J. H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnI3: Semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material J. Am. Chem. Soc. 2012, 134, 8579– 8587 DOI: 10.1021/ja301539s
      There is no corresponding record for this reference.
    12. 12
      da Silva, E. L.; Skelton, J. M.; Parker, S. C.; Walsh, A. Phase stability and transformations in the halide perovskite CsSnI3 Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 144107 DOI: 10.1103/PhysRevB.91.144107
      12
      Phase stability and transformations in the halide perovskite CsSnI3
      da Silva, E. Lora; Skelton, Jonathan M.; Parker, Stephen C.; Walsh, Aron
      Physical Review B: Condensed Matter and Materials Physics (2015), 91 (14), 144107/1-144107/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We employ the quasiharmonic approxn. to study the temp.-dependent lattice dynamics of the four different phases of cesium tin iodide (CsSnI3). Within this framework, we obtain the temp. dependence of a no. of structural properties, including the cell vol., bulk modulus, and Gr.ovrddot.uneisen parameter. The Gibbs free energy of each phase is compared against the temp.-dependent Helmholtz energy obtained from the equil. structure within the harmonic approxn. We find that the black tetragonal perovskite phase is not dynamically stable up to at least 500 K, with the phonon dispersion displaying neg. optic modes, which pass through all of the high-symmetry wave vectors in the Brillouin zone. The main contributions to the neg. modes are found to be motions of the Cs atom inside the perovskite cage. The black cubic perovskite structure shows a zone-boundary instability, indicated by soft modes at the special q points M and R. These modes are present in calcns. at the equil. (0 K) lattice const., while at finite temp. addnl. neg. modes develop at the zone center, indicating a ferroelec. instability. The yellow crystal, composed of one-dimensional (SnI6)n double chains, has the same heat of formation as the orthorhombic perovskite phase at 0 K, but becomes less energetically favorable at higher temps., due to its higher free energy.
    13. 13
      Brivio, F.; Walker, A. B.; Walsh, A. Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles APL Mater. 2013, 1, 042111 DOI: 10.1063/1.4824147
      13
      Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles
      Brivio, Federico; Walker, Alison B.; Walsh, Aron
      APL Materials (2013), 1 (4), 042111/1-042111/5CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      The performance of perovskite solar cells recently exceeded 15% solar-to-electricity conversion efficiency for small-area devices. The fundamental properties of the active absorber layers, hybrid org.-inorg. perovskites formed from mixing metal and org. halides [e.g., (NH4)PbI3 and (CH3NH3)PbI3], are largely unknown. The materials are semiconductors with direct band gaps at the boundary of the first Brillouin zone. The calcd. dielec. consts. and band gaps show an orientation dependence, with a low barrier for rotation of the org. cations. Due to the elec. dipole of the methylammonium cation, a photoferroic effect may be accessible, which could enhance carrier collection. (c) 2013 American Institute of Physics.
    14. 14
      Frost, J. M.; Butler, K. T.; Walsh, A. Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells APL Mater. 2014, 2, 081506 DOI: 10.1063/1.4890246
      14
      Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells
      Frost, Jarvist M.; Butler, Keith T.; Walsh, Aron
      APL Materials (2014), 2 (8), 081506/1-081506/10CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      We report a model describing the mol. orientation disorder in CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic structure calcns., with the nature of the motions informed by ab initio mol. dynamics. We investigate the temp. and static elec. field dependence of the equil. ferroelec. (mol.) domain structure and resulting polarisability. A rich domain structure of twinned mol. dipoles is obsd., strongly varying as a function of temp. and applied elec. field. We propose that the internal elec. fields assocd. with microscopic polarisation domains contribute to hysteretic anomalies in the current-voltage response of hybrid org.-inorg. perovskite solar cells due to variations in electron-hole recombination in the bulk. (c) 2014 American Institute of Physics.
    15. 15
      https://www.youtube.com/watch?v=PPwSIYLnONY (accessed 25.12.15).
      There is no corresponding record for this reference.
    16. 16
      https://www.youtube.com/watch?v=jwEgBq9BIkk (accessed 25.12.15).
      There is no corresponding record for this reference.
    17. 17
      Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kochelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O'Regan, B. C.; Nelson, J.; Cabral, J. T.; Barnes, P. R. F. Dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells Nat. Commun. 2015, 6, 7124 DOI: 10.1038/ncomms8124
      17
      The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells
      Leguy Aurelien M A; McMahon Andrew P; Nelson Jenny; Barnes Piers R F; Frost Jarvist Moore; Walsh Aron; Sakai Victoria Garcia; Kochelmann W; Law ChunHung; Li Xiaoe; O'Regan Brian C; Foglia Fabrizia; Cabral Joao T
      Nature communications (2015), 6 (), 7124 ISSN:.
      Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
    18. 18
      https://github.com/WMD-group/hybrid-perovskites (accessed 25.12.15).
      There is no corresponding record for this reference.
    19. 19
      Walsh, A.; Payne, D. J.; Egdell, R. G.; Watson, G. W. Stereochemistry of post-transition metal oxides: revision of the classical lone-pair model Chem. Soc. Rev. 2011, 40, 4455– 4463 DOI: 10.1039/c1cs15098g
      There is no corresponding record for this reference.
    20. 20
      Worhatch, R. J.; Kim, H. J.; Swainson, I. P.; Yonkeu, A. L.; Billinge, S. J. L. Study of local structure in selected cubic organic-inorganic perovskites Chem. Mater. 2008, 20, 1272– 1277 DOI: 10.1021/cm702668d
      20
      Study of Local Structure in Selected Organic-Inorganic Perovskites in the Pm‾3m Phase
      Worhatch, Richard J.; Kim, HyunJeong; Swainson, Ian P.; Yonkeu, Andre L.; Billinge, Simon J. L.
      Chemistry of Materials (2008), 20 (4), 1272-1277CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The local structures of the inorg. component of selected org.-inorg. perovskites (OIPs) were studied by analyzing the x-ray pair distribution function. Whereas the long-range structure of each perovskite is the untilted Pm3‾m phase, all the OIPs showed significant internal distortion of the octahedra. Varying the halide has a significant impact on the lattice const. There is evidence of local lone-pair distortions for certain compns. The most complex case of disorder appears to be that of CH3NH3SnBr3.
    21. 21
      Grancini, G.; Srimath Kandada, A. R.; Frost, J. M.; Barker, A. J.; De Bastiani, M.; Gandini, M.; Marras, S.; Lanzani, G.; Walsh, A.; Petrozza, A. Role of microstructure in the electronhole interaction of hybrid lead halide perovskites Nat. Photonics 2015, 9, 695– 702 DOI: 10.1038/nphoton.2015.151
      21
      Role of microstructure in the electron-hole interaction of hybrid lead halide perovskites
      Grancini, Giulia; Srimath Kandada, Ajay Ram; Frost, Jarvist M.; Barker, Alex J.; De Bastiani, Michele; Gandini, Marina; Marras, Sergio; Lanzani, Guglielmo; Walsh, Aron; Petrozza, Annamaria
      Nature Photonics (2015), 9 (10), 695-701CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Org.-inorg. metal halide perovskites have demonstrated high power conversion efficiencies in solar cells and promising performance in a wide range of optoelectronic devices. The existence and stability of bound electron-hole pairs in these materials and their role in the operation of devices with different architectures remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modeling as a function of the degree of polycrystallinity and temp., that the electron-hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycryst. disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same compn. demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphol. strongly influence perovskite behavior, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.
    22. 22
      Stroppa, A.; Quarti, C.; De Angelis, F.; Picozzi, S. Ferroelectric polarization of CH3NH3PbI3: A detailed study based on DFT and symmetry mode analysis J. Phys. Chem. Lett. 2015, 6, 2223– 2231 DOI: 10.1021/acs.jpclett.5b00542
      22
      Ferroelectric Polarization of CH3NH3PbI3: A Detailed Study Based on Density Functional Theory and Symmetry Mode Analysis
      Stroppa, Alessandro; Quarti, Claudio; De Angelis, Filippo; Picozzi, Silvia
      Journal of Physical Chemistry Letters (2015), 6 (12), 2223-2231CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Ferroelectricity in halide perovskites currently represents a crucial issue, as it may have an important role for the enhancement of solar cells efficiency. Simulations of ferroelec. properties based on d. functional theory are conceptually more demanding compared with conventional inorg. ferroelecs. due to the presence of both org. and inorg. components in the same compd. Here the authors present a detailed study focused on the prototypical CH3NH3PbI3 perovskite. By using d. functional theory combined with symmetry mode anal., the authors disentangle the contributions of the methylammonium cations and the role of the inorg. framework, therefore suggesting possible routes to enhance the polarization in this compd. The authors' est. of the polarization for the tetragonal phase at low temp. is ∼4.42 μC/cm2, which is substantially lower than that of traditional perovskite oxides.
    23. 23
      Ma, J.; Wang, L.-W. Nanoscale charge localization induced by random orientations of organic molecules in hybrid perovskite CH3NH3PbI3 Nano Lett. 2015, 15, 248– 253 DOI: 10.1021/nl503494y
      23
      Nanoscale Charge Localization Induced by Random Orientations of Organic Molecules in Hybrid Perovskite CH3NH3PbI3
      Ma, Jie; Wang, Lin-Wang
      Nano Letters (2015), 15 (1), 248-253CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues of the hybrid perovskites were not fully understood. Exptl., it is known that in CH3NH3PbI3 the org. mols. CH3NH3 are randomly oriented at the room temp., but the impact of the random mol. orientation was not studied. Because of the dipole moment of the org. mol., the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. Using linear scaling ab initio methods, the charge densities of the conduction band min. and the valence band max. are localized in nanoscales due to the potential fluctuations. The charge localization causes electron-hole sepn. and reduces carrier recombination rates, which may contribute to the long carrier lifetime obsd. in expts.
    24. 24
      Sherkar, T.; Koster, J. A. Can ferroelectric polarization explain the high-performance of hybrid-halide perovskite solar cells? Phys. Chem. Chem. Phys. 2016, 18, 331– 338 DOI: 10.1039/C5CP07117H
      There is no corresponding record for this reference.
    25. 25
      Chen, T.; Foley, B. J.; Ipek, B.; Tyagi, M.; Copley, J. R. D.; Brown, C. M.; Choi, J. J.; Lee, S.-H. Rotational dynamics of organic cations in CH3NH3PbI3 perovskite Phys. Chem. Chem. Phys. 2015, 17, 31278– 31286 DOI: 10.1039/C5CP05348J
      There is no corresponding record for this reference.
    26. 26
      Bakulin, A. A.; Selig, O.; Bakker, H. J.; Rezus, Y. L. A.; Müller, C.; Glaser, T.; Lovrincic, R.; Sun, Z.; Chen, Z.; Walsh, A.; Frost, J. M.; Jansen, T. L. C. Real-time observation of cation reorientation in methylammonium lead iodide perovskites J. Phys. Chem. Lett. 2015, 6, 3663– 3669 DOI: 10.1021/acs.jpclett.5b01555
      26
      Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites
      Bakulin, Artem A.; Selig, Oleg; Bakker, Huib J.; Rezus, Yves L. A.; Mueller, Christian; Glaser, Tobias; Lovrincic, Robert; Sun, Zhenhua; Chen, Zhuoying; Walsh, Aron; Frost, Jarvist M.; Jansen, Thomas L. C.
      Journal of Physical Chemistry Letters (2015), 6 (18), 3663-3669CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The introduction of a mobile and polarized org. moiety as a cation in 3D Pb-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the org. cation and the mol. mechanism behind its motion remain unclear, with different exptl. and computational approaches providing very different qual. and quant. description of the mol. dynamics. Ultrafast 2D vibrational spectroscopy of methylammonium (MA) Pb iodide was used to directly resolve the rotation of the org. cations within the MAPbI3 lattice. The results reveal 2 characteristic time consts. of motion. Using ab initio mol. dynamics simulations, the authors identify these as a fast (∼300 fs) wobbling-in-a-cone motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the mol. dipole with respect to the iodide lattice. The obsd. dynamics are essential for understanding the electronic properties of perovskite materials.
    27. 27
      Brivio, F.; Butler, K. T.; Walsh, A.; van Schilfgaarde, M. Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 155204 DOI: 10.1103/PhysRevB.89.155204
      27
      Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers
      Brivio, Federico; Butler, Keith T.; Walsh, Aron; van Schilfgaarde, Mark
      Physical Review B: Condensed Matter and Materials Physics (2014), 89 (15), 155204/1-155204/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Solar cells based on a light absorbing layer of the organometal halide perovskite CH3NH3PbI3 have recently surpassed 15% conversion efficiency, though how these materials work remains largely unknown. We analyze the electronic structure and optical properties within the quasiparticle self-consistent GW approxn. While this compd. bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: Band gaps are much larger than what is predicted by the local-d. approxn. (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addn., spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The av. hole mass is small, which partially accounts for the long diffusion lengths obsd. The surface ionization potential (work function) is calcd. to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO2 and Au elec. contacts.
    28. 28
      Menéndez-Proupin, E.; Beltrán Ríos, C. L.; Wahnón, P. Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3 Phys. Status Solidi RRL 2015, 9, 559– 563 DOI: 10.1002/pssr.201510265
      28
      Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3
      Menendez-Proupin, E.; Rios, Carlos L. Beltran; Wahnon, P.
      Physica Status Solidi RRL: Rapid Research Letters (2015), 9 (10), 559-563CODEN: PSSRCS; ISSN:1862-6254. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The excitons in the orthorhombic phase of the perovskite CH3NH3PbI3 are studied using the effective mass approxn. The electron-hole interaction is screened by a distance-dependent dielec. function, as described by the Haken potential or the Pollmann-Buettner potential. The energy spectrum and the eigenfunctions are calcd. for both cases. The results show that the Pollmann-Buettner model, using the corresponding parameters obtained from ab initio calcns., provides better agreement with the exptl. results.
    29. 29
      Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.-W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites Nat. Phys. 2015, 11, 582– 587 DOI: 10.1038/nphys3357
      29
      Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites
      Miyata, Atsuhiko; Mitioglu, Anatolie; Plochocka, Paulina; Portugall, Oliver; Wang, Jacob Tse-Wei; Stranks, Samuel D.; Snaith, Henry J.; Nicholas, Robin J.
      Nature Physics (2015), 11 (7), 582-587CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
      Solar cells based on the org.-inorg. tri-halide perovskite family of materials have shown significant progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temps., over three times smaller than has been previously assumed. In the room-temp. phase we show that the binding energy falls to even smaller values of only a few millielectron volts, which explains their excellent device performance as being due to spontaneous free-carrier generation following light absorption. Addnl., we det. the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estd. exptl. but in good agreement with recent calcns. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.
    30. 30
      Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites Adv. Mater. 2014, 26, 1584– 1589 DOI: 10.1002/adma.201305172
      30
      High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites
      Wehrenfennig, Christian; Eperon, Giles E.; Johnston, Michael B.; Snaith, Henry J.; Herz, Laura M.
      Advanced Materials (Weinheim, Germany) (2014), 26 (10), 1584-1589CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors find that methylammonium lead trihalide perovskites are particularly well-suited as light absorbers and charge transporters in photovoltaic cells because they allow for an unexpected combination of both low charge recombination rates and high charge-carrier mobilities. The authors establish lower bounds of-10 cm2 V-1 s'1 for the high-frequency charge mobility, which is remarkably high for a soln.-processed material. The planar heterojunction photovoltaic cells may only be achieved because the ratio of bimol. charge recombination rate to charge mobility is over four orders of magnitude lower than that predicted from Langevin theory. Such effects are likely to arise from spatial sepn. of opposite charge carriers within the metal-halide structure or across a cryst. domain. Modeling and tuning recombination channels, e.g. through halide and metal substitutions, or crystallite size, will hold the clue to raising material performance.
    31. 31
      Mitzi, D. B.; Kosbar, L. L.; Murray, C. E.; Copel, M.; Afzali, A. High-mobility ultrathin semiconducting films prepared by spin coating Nature 2004, 428, 299– 303 DOI: 10.1038/nature02389
      31
      High-mobility ultrathin semiconducting films prepared by spin coating
      Mitzi, David B.; Kosbar, Laura L.; Murray, Conal E.; Copel, Matthew; Afzali, Ali
      Nature (London, United Kingdom) (2004), 428 (6980), 299-303CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient soln.-based deposition is also an important research direction, with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resoln. displays and electronic paper) and lower-cost device fabrication. Here, the authors demonstrate a technique for spin coating ultrathin (∼50 Å), cryst., and continuous metal chalcogenide films, based on the low-temp. decompn. of highly sol. hydrazinium precursors. The authors fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS2-xSex films, which exhibit n-type transport, large current densities (>105 A cm-2), and mobilities greater than 10 cm2V-1s-1 - an order of magnitude higher than previously reported values for spin-coated semiconductors. He spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelecs. and memory devices).
    32. 32
      Labram, J. G.; Fabini, D. H.; Perry, E. E.; Lehner, A. J.; Wang, H.; Glaudell, A. M.; Wu, G.; Evans, H.; Buck, D.; Cotta, R.; Echegoyen, L.; Wudl, F.; Seshadri, R.; Chabinyc, M. L. Temperature-dependent polarization in field-effect transport and photovoltaic measurements of methylammonium lead iodide J. Phys. Chem. Lett. 2015, 6, 3565– 3571 DOI: 10.1021/acs.jpclett.5b01669
      32
      Temperature-Dependent Polarization in Field-Effect Transport and Photovoltaic Measurements of Methylammonium Lead Iodide
      Labram, John G.; Fabini, Douglas H.; Perry, Erin E.; Lehner, Anna J.; Wang, Hengbin; Glaudell, Anne M.; Wu, Guang; Evans, Hayden; Buck, David; Cotta, Robert; Echegoyen, Luis; Wudl, Fred; Seshadri, Ram; Chabinyc, Michael L.
      Journal of Physical Chemistry Letters (2015), 6 (18), 3565-3571CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid org.-inorg. perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI3) as the active material and conducted a series of temp.-dependent measurements. Field-effect transistor, capacitor, and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temp.-dependent polarization susceptibility in (MA)PbI3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI3 photovoltaic cells is obsd. to reduce drastically with decreasing temp., suggesting that such polarization effects could be a prerequisite for high-performance device operation.
    33. 33
      Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber Science 2013, 342, 341– 344 DOI: 10.1126/science.1243982
      33
      Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber
      Stranks, Samuel D.; Eperon, Giles E.; Grancini, Giulia; Menelaou, Christopher; Alcocer, Marcelo J. P.; Leijtens, Tomas; Herz, Laura M.; Petrozza, Annamaria; Snaith, Henry J.
      Science (Washington, DC, United States) (2013), 342 (6156), 341-344CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Org.-inorg. perovskites showed promise as high-performance absorbers in solar cells, 1st as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, the authors report transient absorption and photoluminescence-quenching measurements to det. the electron-hole diffusion lengths, diffusion consts., and lifetimes in mixed halide (MeNH3PbI3-xClx) and triiodide (MeNH3PbI3) perovskite absorbers. The diffusion lengths are >1 μm in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. But the triiodide absorber has electron-hole diffusion lengths of about 100 nm. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a crit. parameter to optimize for future perovskite absorber development.
    34. 34
      Karakus, M.; Jensen, S. A.; D’Angelo, F.; Turchinovich, D.; Bonn, M.; Canovas, E. Phonon-electron-scattering limits free charge mobility in methylammonium lead iodide perovskites J. Phys. Chem. Lett. 2015, 6, 4991– 4996 DOI: 10.1021/acs.jpclett.5b02485
      34
      Phonon-Electron Scattering Limits Free Charge Mobility in Methylammonium Lead Iodide Perovskites
      Karakus, Melike; Jensen, Soeren A.; D'Angelo, Francesco; Turchinovich, Dmitry; Bonn, Mischa; Canovas, Enrique
      Journal of Physical Chemistry Letters (2015), 6 (24), 4991-4996CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The nature of the photocond. in soln.-processed films of methylammonium lead iodide perovskite was studied by detg. the variation of the photoconductive response with temp. Ultrabroadband terahertz (THz) photocond. spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temp., where free charge carrier motion was characterized by an av. scattering time. The scattering time detd. from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm2 V-1 s-1) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm2 V-1 s-1). For the tetragonal phase (temp. range 150 < T < 300 K) the scattering time shows a approx. T-3/2 dependence, approaching the theor. limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.
    35. 35
      Zhu, X.-Y.; Podzorov, V. Charge carriers in hybrid-organicinorganic lead-halide perovskites might be protected as large polarons J. Phys. Chem. Lett. 2015, 6, 4758– 4761 DOI: 10.1021/acs.jpclett.5b02462
      35
      Charge Carriers in Hybrid Organic-Inorganic Lead Halide Perovskites Might Be Protected as Large Polarons
      Zhu, X.-Y.; Podzorov, V.
      Journal of Physical Chemistry Letters (2015), 6 (23), 4758-4761CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Large polaron that provides the protection of charge carrier is explained in terms of carrier diffusion length,electron hole recombination rates, charge carrier and Hall mobility and low carrier scattering rate.
    36. 36
      Stranks, S. D.; Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices Nat. Nanotechnol. 2015, 10, 391– 402 DOI: 10.1038/nnano.2015.90
      36
      Metal-halide perovskites for photovoltaic and light-emitting devices
      Stranks, Samuel D.; Snaith, Henry J.
      Nature Nanotechnology (2015), 10 (5), 391-402CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      A review. Metal-halide perovskites are cryst. materials originally developed out of scientific curiosity. Unexpectedly, solar cells incorporating these perovskites are rapidly emerging as serious contenders to rival the leading photovoltaic technologies. Power conversion efficiencies have jumped from 3% to over 20% in just four years of academic research. Here, we review the rapid progress in perovskite solar cells, as well as their promising use in light-emitting devices. In particular, we describe the broad tunability and fabrication methods of these materials, the current understanding of the operation of state-of-the-art solar cells and we highlight the properties that have delivered light-emitting diodes and lasers. We discuss key thermal and operational stability challenges facing perovskites, and give an outlook of future research avenues that might bring perovskite technol. to commercialization.
    37. 37
      Kepenekian, M.; Robles, R.; Katan, C.; Sapori, D.; Pedesseau, L.; Even, J. Rashba and Dresselhaus effects in organic-inorganic perovskites: from basics to devices ACS Nano 2015, 9, 11557– 11567 DOI: 10.1021/acsnano.5b04409
      There is no corresponding record for this reference.
    38. 38
      Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of ferroelectric domains in solution-processed CH3NH3PbI3 thin films J. Phys. Chem. Lett. 2014, 5, 3335– 3339 DOI: 10.1021/jz501697b
      38
      Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films
      Kutes, Yasemin; Ye, Linghan; Zhou, Yuanyuan; Pang, Shuping; Huey, Bryan D.; Padture, Nitin P.
      Journal of Physical Chemistry Letters (2014), 5 (19), 3335-3339CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelec., almost nothing is known about their ferroelec. properties exptl. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelec. domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new soln.-processing method. The size of the ferroelec. domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelec. domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelec. behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.
    39. 39
      Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells Nano Lett. 2014, 14, 2587– 2590
      There is no corresponding record for this reference.
    40. 40
      Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 144308 DOI: 10.1103/PhysRevB.92.144308
      40
      Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide
      Brivio, Federico; Frost, Jarvist M.; Skelton, Jonathan M.; Jackson, Adam J.; Weber, Oliver J.; Weller, Mark T.; Goni, Alejandro R.; Leguy, Aurelien M. A.; Barnes, Piers R. F.; Walsh, Aron
      Physical Review B: Condensed Matter and Materials Physics (2015), 92 (14), 144308/1-144308/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equil. structures compare well to solns. of temp.-dependent powder neutron diffraction. By following the normal modes, we calc. IR and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degrdn. of the sample. Despite a clear sepn. in energy between low-frequency modes assocd. with the inorg. (PbI3-)n network and high-frequency modes of the org. CH3NH3+ cation, significant coupling between them is found, which emphasizes the interplay between mol. orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temp.
    41. 41
      Mizusaki, J.; Arai, K.; Fueki, K. Ionic conduction of the perovskite-type halides Solid State Ionics 1983, 11, 203– 211 DOI: 10.1016/0167-2738(83)90025-5
      41
      Ionic conduction of the perovskite-type halides
      Mizusaki, Junichiro; Arai, Kimiyasu; Fueki, Kazuo
      Solid State Ionics (1983), 11 (3), 203-11CODEN: SSIOD3; ISSN:0167-2738.
      The ionic conduction in the perovskite-type halides CsPbCl3, CsPbBr3, and KMnCl3 was studied. Measurements were made of a.c. cond. from 150° to the m.p., and of ionic transport no. by using the Tubandt, emf., and ion-blocking methods. The effects of impurity doping on the ionic cond. of CsPbCl3 were also studied by using samples of compn. CsPb0.99M0.01Cl2.99 (M = Li, Na, K, Ag). These materials are halide-ion conductors. The ionic conductivities of CsPbCl3 and CsPbBr3 are close to those of the well known halide-ion conductors, PbCl2 and PbBr2. The ionic transport nos. are >0.9 for CsPbCl3 and CsPbBr3, and ∼0.99 for KMnCl3. The conduction is caused by the migration of halide-ion vacancies VX (X = Cl, Br). The activation energies for the migration of VX are 0.29 eV for CsPbCl3, 0.25 eV for CsPbBr3, and 0.39 eV for KMnCl3. The vacancy diffusion coeffs. of these materials are very large. However, the impurity doping does not increase the ionic cond. markedly because of small dopant soly.
    42. 42
      Walsh, A.; Scanlon, D. O.; Chen, S.; Gong, X. G.; Wei, S.-H. Self-regulation mechanism for charged point defects in hybrid halide perovskites Angew. Chem., Int. Ed. 2015, 54, 1791– 1794 DOI: 10.1002/anie.201409740
      42
      Self-regulation mechanism for charged point defects in hybrid halide perovskites
      Walsh, Aron; Scanlon, David O.; Chen, Shiyou; Gong, X. G.; Wei, Su-Huai
      Angewandte Chemie, International Edition (2015), 54 (6), 1791-1794CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concns. despite being processed at low-temps. from soln. We demonstrate, through quantum mech. calcns., that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concn. of charge carriers through ionic compensation of charged point defects. The equil. charged vacancy concn. is predicted to exceed 0.4 % at room temp. This behavior, which goes against established defect conventions for inorg. semiconductors, has implications for photovoltaic performance.
    43. 43
      Yang, T.-Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. Significance of ion conduction in a organic-inorganic lead-iodide-based perovskite photosensitizer Angew. Chem., Int. Ed. 2015, 54, 7905– 7910 DOI: 10.1002/anie.201500014
      There is no corresponding record for this reference.
    44. 44
      Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T. W.; Wojciechowski, K.; Zhang, W. Anomalous hysteresis in perovskite solar cells J. Phys. Chem. Lett. 2014, 5, 1511– 1515 DOI: 10.1021/jz500113x
      44
      Anomalous Hysteresis in Perovskite Solar Cells
      Snaith, Henry J.; Abate, Antonio; Ball, James M.; Eperon, Giles E.; Leijtens, Tomas; Noel, Nakita K.; Stranks, Samuel D.; Wang, Jacob Tse-Wei; Wojciechowski, Konrad; Zhang, Wei
      Journal of Physical Chemistry Letters (2014), 5 (9), 1511-1515CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current-voltage curves. The authors identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. The authors also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. The authors hypothesize 3 possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.
    45. 45
      Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices Nat. Mater. 2015, 14, 193– 198 DOI: 10.1038/nmat4150
      45
      Giant switchable photovoltaic effect in organometal trihalide perovskite devices
      Xiao, Zhengguo; Yuan, Yongbo; Shao, Yuchuan; Wang, Qi; Dong, Qingfeng; Bi, Cheng; Sharma, Pankaj; Gruverman, Alexei; Huang, Jinsong
      Nature Materials (2015), 14 (2), 193-198CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, soln.-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small elec. field of <1 V μm-1. The switchable photocurrent, generally obsd. in devices based on ferroelec. materials, reached 20.1 mA cm-2 under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelec. photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and elec.-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in elec. and optically readable memristors and circuits.
    46. 46
      Loidl, A.; Krohns, S.; Hemberger, J.; Lunkenheimer, P. Bananas go paraelectric J. Phys.: Condens. Matter 2008, 20, 191001 DOI: 10.1088/0953-8984/20/19/191001
      46
      Bananas go paraelectric
      Loidl, A.; Krohns, S.; Hemberger, J.; Lunkenheimer, P.
      Journal of Physics: Condensed Matter (2008), 20 (19), 191001/1-191001/3CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      Using a banana as an example, we demonstrate how the ferroelec.-like hysteresis loops measured in inhomogeneous, conducting materials can easily be identified as non-intrinsic. With simple expts., the response of a banana to elec. fields is revealed as characteristic for an inhomogeneous paraelec. ion conductor. Not even abs. beginners in dielecs. should identify this biol. matter as ferroelec.
    47. 47
      Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Photoinduced giant dielectric constant in lead halide perovskite solar cells J. Phys. Chem. Lett. 2014, 5, 2390– 2394 DOI: 10.1021/jz5011169
      47
      Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells
      Juarez-Perez, Emilio J.; Sanchez, Rafael S.; Badia, Laura; Garcia-Belmonte, Germa; Kang, Yong Soo; Mora-Sero, Ivan; Bisquert, Juan
      Journal of Physical Chemistry Letters (2014), 5 (13), 2390-2394CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Org.-inorg. lead trihalide perovskites have emerged as an outstanding photovoltaic material that demonstrated a high 17.9% conversion efficiency of sunlight to electricity in a short time. We have found a giant dielec. const. (GDC) phenomenon in these materials consisting on a low frequency dielec. const. in the dark of the order of ε0 = 1000. We also found an unprecedented behavior in which ε0 further increases under illumination or by charge injection at applied bias. We observe that ε0 increases nearly linearly with the illumination intensity up to an addnl. factor 1000 under 1 sun. Measurement of a variety of samples of different morphologies, compns., and different types of contacts shows that the GDC is an intrinsic property of MAPbX3 (MA = CH3NH3+). We hypothesize that the large dielec. response is induced by structural fluctuations. Photoinduced carriers modify the local unit cell equil. and change the polarizability, assisted by the freedom of rotation of MA. The study opens a way for the understanding of a key aspect of the photovoltaic operation of high efficiency perovskite solar cells.
    48. 48
      Bryant, D.; Wheeler, S.; O’Regan, B. C.; Watson, T.; Barnes, P. R.; Worsley, D. A.; Durrant, J. Observable hysteresis at low temperature in hysteresis free lead halide perovskite solar cells J. Phys. Chem. Lett. 2015, 6, 3190– 3194 DOI: 10.1021/acs.jpclett.5b01381
      48
      Observable Hysteresis at Low Temperature in "Hysteresis Free" Organic-Inorganic Lead Halide Perovskite Solar Cells
      Bryant, Daniel; Wheeler, Scot; O'Regan, Brian C.; Watson, Trystan; Barnes, Piers R. F.; Worsley, Dave; Durrant, James
      Journal of Physical Chemistry Letters (2015), 6 (16), 3190-3194CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      In this paper the JV hysteresis behavior is addressed of planar org.-inorg. lead halide perovskite solar cells fabricated using PC60BM as the cathode. At room temp., these devices exhibit apparently hysteresis free JV scans. It is obsd. that cooling to 175 K results in the appearance of substantial JV hysteresis. Employing chronoamperometric measurements, it is demonstrated that the half-time for the relaxation process underlying this hysteresis slows from 0.6 s at 298 K to 15.5 s at 175 K, yielding an activation energy of 0.12 eV. It is further demonstrated that by cooling a cell to 77 K while held under pos. bias, it is possible to "freeze" the cell into the most favorable condition for efficient photovoltaic performance. The changes to device architecture that appear to remove room temp. JV hysteresis may not remove the underlying process(es), but rather shift them to time scales not readily observable in typical room temp. JV scans.
    49. 49
      Egger, D. A.; Kronik, L.; Rappe, A. M. Theory of hydrogen migration in organic-inorganic halide perovskites Angew. Chem., Int. Ed. 2015, 54, 12437– 12441 DOI: 10.1002/anie.201502544
      49
      Theory of Hydrogen Migration in Organic-Inorganic Halide Perovskites
      Egger, David A.; Kronik, Leeor; Rappe, Andrew M.
      Angewandte Chemie, International Edition (2015), 54 (42), 12437-12441CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Solar cells based on org.-inorg. halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current-voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calcns. based on d. functional theory, we study the properties of an important defect in hybrid perovskites-interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin-Corbett mechanism. Our anal. highlights the structural flexibility of org.-inorg. perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells.
    50. 50
      Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defects migration in methylammonium lead iodide Energy Environ. Sci. 2015, 8, 2118– 2127 DOI: 10.1039/C5EE01265A
      There is no corresponding record for this reference.
    51. 51
      Eames, C.; Frost, J. M.; Barnes, P. R. F.; O'Regan, B. C.; Walsh, A.; Islam, M. S. Ionic transport in hybrid lead iodide perovskite solar cells Nat. Commun. 2015, 6, 7497 DOI: 10.1038/ncomms8497
      51
      Ionic transport in hybrid lead iodide perovskite solar cells
      Eames, Christopher; Frost, Jarvist M.; Barnes, Piers R. F.; O'Regan, Brian C.; Walsh, Aron; Islam, M. Saiful
      Nature Communications (2015), 6 (), 7497CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Solar cells based on org.-inorg. halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behavior such as current-voltage hysteresis and a low-frequency giant dielec. response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chem. origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extd. from the current-voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and exptl. study suggest that hybrid halide perovskites are mixed ionic-electronic conductors, a finding that has major implications for solar cell device architectures.
    52. 52
      Haruyama, J.; Sodeyama, K.; Han, L.; Tateyama, Y. First-principles study of ion diffusion in perovskite solar cell sensitizers J. Am. Chem. Soc. 2015, 137, 10048– 10051 DOI: 10.1021/jacs.5b03615
      52
      First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers
      Haruyama, Jun; Sodeyama, Keitaro; Han, Liyuan; Tateyama, Yoshitaka
      Journal of the American Chemical Society (2015), 137 (32), 10048-10051CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calcns. The calcd. activation energies of the anionic and cationic vacancy migrations clearly show that I- anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that obsd. in ion-conducting materials. More interestingly, typical MA+ cations and larger FA+ cations both have rather low barriers as well, indicating that the cation mols. can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by expts. With the dil. diffusion theory, we discuss that smaller vacancy concns. (higher crystallinity) and replacement of MA+ with larger cation mols. will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.
    53. 53
      Eperon, G. E.; Beck, C. E.; Snaith, H. Cation exchange for thin film lead iodide perovskite interconversion Mater. Horiz. 2016, 3, 63– 71 DOI: 10.1039/C5MH00170F
      53
      Cation exchange for thin film lead iodide perovskite interconversion
      Eperon, Giles E.; Beck, Clara E.; Snaith, Henry J.
      Materials Horizons (2016), 3 (1), 63-71CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
      We report a new technique for tuning the bandgap of hybrid org.-inorg. halide perovskite materials. By dipping films of methylammonium or formamidinium lead triiodide (MAPbI3 or FAPbI3) in solns. of formamidinium or methylammonium iodide (FAI or MAI) at room temp., we are able to inter-convert through cation exchange between perovskite materials, allowing us to carefully tune the bandgap between 1.57 and 1.48 eV. We observe uniform conversion through the entirety of the bulk film, with no evidence for a "bi-layered" or graded structure. By applying this technique to solar cell devices, we are able to enhance the performance of the single cation devices. Furthermore, we demonstrate that this technique allows us to form pure black phase FAPbI3 infiltrated into mesoporous scaffolds; this is normally infeasible since the pores confine the FAPbI3 in a yellow non-perovskite phase with a much wider bandgap, which is not of practical use in solar cells or other optoelectronic devices. Addnl., this work provides evidence for mol. cation mobility in the halide perovskites, indicating that the cations play a role in ionic conduction as well as the mobile anions.