Session Y20: Hybrid Perovskites -- Lattice Vibrations

The Impact of Lattice and Charge Fluctuations on Carrier Dynamics in Halide Perovskites

While halide perovskites (HaPs) have shown great technological promise for use as materials in energy devices, several of their unique optoelectronic properties remain incompletely understood. We develop a microscopic theory that is aimed at describing pertinent physical effects related to the carrier transport and optical properties of HaPs. It captures the nuclear displacements to all orders and goes beyond assuming linear electron-phonon coupling in calculating the dynamics of carriers and band gap characteristics. We combine the theory with first-principles calculations and apply it to the paradigm HaP compound MAPbI₃. Our results explain intriguing experimental findings related to the charge-carrier mobility and optical properties, including their temperature dependencies. The findings of our work demonstrate that orbital overlap fluctuations in the lead-halide structure play a leading role in determining the optoelectronic features of HaPs.   

Effects of Electron-Phonon Coupling on Electronic Properties of Methylammonium Lead Iodide Perovskites

Temperature can have a dramatic effect on the solar efficiency of methylammonium lead iodide (CH₃NH₃PbI₃) absorbers due to changes in the electronic structure of the system even within the range of stability of a single phase. Herein using first principles density functional theory, we investigate the electron band structure of the tetragonal and orthorhombic phases of CH₃NH₃PbI₃ as a function of temperature due to electron-phonon coupling and thermal expansion. Our results show that the band gap increases with temperature for the two CH₃NH₃PbI₃ phases in excellent agreement with experimental results. We also found that temperature have a significant effect on the effective masses and Rashba coupling especially in the tetragonal phase. At room temperature, electron-phonon coupling is found to enhance the effective mass by a factor of two, and to diminish the Rashba coupling by the same factor compared to T=0 K values.   

Breakdown of the static picture of defect energetics in halide perovskites: the case of the Br vacancy in CsPbBr3

We consider the Br vacancy in CsPbBr₃ as a prototype for the impact of lattice dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the V_Br level is found to be dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at their atomic sites. The unusually strong coupling of lattice dynamics and defect energetics bears important implications for both experimental and theoretical analysis of defect characteristics in HaPs and may hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaics.   

Lattice dynamical properties of halide-perovskites from infra-red spectroscopy and first-principles calculations

Halide perovskites (HaPs) are intriguing optoelectronic materials. It is of particular interest to understand how the outstanding optoelectronic properties of HaPs are related to their vibrational properties, especially in light of the possibility that phonons might be the dominant source of scattering for charge carriers at room temperature. Studying the lattice dynamics of the all-inorganic CsPbBr₃ as well as the hybrid MAPbBr₃, we here present our findings on how the A-site cation as well as its potential orientation effects influence the vibrational properties of HaPs. To this end, we show results from first-principles calculations, based on density functional theory, and compare these with experimental data obtained from far infra-red spectroscopy. Finally, we present results related to the LO phonons that are not infra-red active, but inherently contained in our calculations and experimentally accessible through reflectance measurements.   

Vibrational Dynamics of Hybrid Organic-Inorganic Perovskites

Vibrational modes, which are connected to the electronic properties of hybrid organic-inorganic perovskites (HOIPs) through the structural transition, electron-phonon coupling, and hot-phonon bottleneck effect, is an important topic for perovskite solar cells. This talk focuses on the phonon modes of two perovskite materials, MAPbI₃ (MA = CH₃NH₃) and FAPbI₃ (FA = HC(NH₂)₂). We have performed inelastic neutron scattering, Raman scattering, and density-functional theory (DFT) calculations on both systems. Our analysis well reproduces the low-temperature inelastic neutron scattering spectra for MAPbI₃ and FAPbI₃, and illuminates the connection between the vibrational dynamics and the organic molecules.   

Rotational Dynamics of Organic Cations in the [CH3(CH2)7NH3]2PbI4 Perovskite

The biggest challenge of the 3D high-efficiency perovskite solar cells, such as CH₃NH₃PbI₃ and CH(NH₂)₂PbI₃, is their device instability. 2D perovskite compounds such as butylammonium methylammonium lead iodide perovskite, [CH₃(CH₂)₃NH₃]₂(CH₃NH₃)_n-1Pb_nI_3n+1, present a solution to this problem. Here we present our recent quasi-elastic neutron scattering on a powder sample of the 2D 1-layer system ([CH₃(CH₂)₇NH₃]₂PbI₄) as a function of temperature. Upon cooling, this system undergoes an orthorhombic phase(Acam) to another orthorhombic phase(Pbca) and further to a monoclinic phase(P2₁/a). Our group theory and density functional theory analysis show that the rotational dynamics is dominated by C₃ rotation of NH₃ and CH₃ groups. The relaxation rate of the C₃ modes decreases with decreasing temperature through the orthorhombic-to-orthorhombic transition. On the other hand, in the low-temperature monoclinic phase, the C₃ rotation becomes very slow to detect. The relevance of this finding to electronic properties will also be discussed.   

Lead halide perovskites are polaronic materials

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. Here we explain the essential physics in this class of materials based their dielectric functions or dynamic symmetry breaking on microscopic level. We show that the dielectric function of a hybrid organic-inorganic lead halide perovskite (LHP) possesses combined characteristics of a polar liquid and a ferroelectric material. The latter response in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential and introduce potential barriers to charge carrier scattering. The ferroelectric large polaron may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites, as well as providing a design principle for high performance semiconductors from nano, molecular, and hybrid materials. [Science, 2016, 353, 1409; Science Adv. 2017, 3, e1701217; Nature Mat. 2018, 17, 379].   

Large Polaron Formation and its Effect on Electron Transport in Hybrid Perovskite

Many experiments have indicated that large polaron may be formed in hybrid perovskite, and its existence is proposed to screen the carrier-carrier and carrier-defect scattering. However, detailed theoretical study of the large polaron and its effect on carrier transport at the atomic level is still lacking. In this work, using CH₃NH₃PbI₃ as an example, we implement tight-binding model fitted from the density-functional theory to describe the electron large polaron ground state and to understand the large polaron formation and transport at its strong-coupling limit. We find that the formation energy of the large polaron is around -12 meV for the case without dynamic disorder, and -55 meV by including dynamic disorder. By performing the explicit time-dependent wavefunction evolution, together with the rotations of CH₃NH₃⁺ and vibrations of PbI₃^- sublattice, we studied the diffusion constant and mobility of the large polaron state. On one hand, the vibration of the sublattice provides additional driving force for carrier mobility, on the other hand, the large polaron polarization further localizes the electron, reducing its mobility.   

Impact of the exchange-correlation functional on the analysis of static and dynamical structural properties of MAPbI3

Hybrid organic-inorganic halide perovskites (HaPs) have shown many properties that are advantageous for efficient renewable energy applications. Several of the open questions for these systems can be addressed by first-principles calculations applying density functional theory (DFT). However, in order to achieve reliable insight from such calculations, it is important to understand which of the various microscopic effects in the different DFT-related approximations play an important role in the calculations. Here, we present an investigation on the importance of various theoretical aspects in the DFT calculations of the structural properties for MAPbI₃. We focus on a comparison of calculations varying the DFT functional, the account of van-der-Waals interactions as well as the inclusion of spin-orbit coupling. Our computed unit-cell volumes and bulk moduli are compared to results of experiments, addressing the relative importance of these effects on calculations of structural properties of MAPbI₃. Finally, we also present results on the impact of temperature-induced structural fluctuations on calculating the structural properties of MAPbI₃.   

Magneto-Raman Spectroscopy on multiferroic metal-organic framework [(CH3)2NH2]Co(HCOO)3

Metal-organic frameworks (MOFs) are a class of nanoporous compounds where organic groups are used in combination with transition metal ions to obtain multifunctional materials. The family of MOFs perovskites ABX₃ comprised of methylammonum (A= (CH₃)₂NH₂) and metal (B=Co, Cu, Fe, Mn, Ni) cations with a formate (X=HCOO₃) anion are studied because of their multiferroic properties [1]. Therefore, several efforts have been made to understand the nature and strength of exchange interactions in these materials including magnetization at high magnetic fields up to 60 T and infrared spectroscopy at magnetic fields up to 35 T [2,3]. Concurrently, we performed magneto-Raman spectroscopy on [(CH₃)₂NH₂]Co(HCOO)₃ at magnetic fields up to 31T to probe the magneto-elastic coupling, the results of our investigations and its implications will be discussed.
  

Acoustic phonon instabilities and central peaks at the Brillouin zone boundary in the organic-inorganic perovskite CH3NH3PbCl3

Lead halide hybrid perovskites consist of an inorganic framework hosting a molecular cation located in the interstitial space. These compounds have been extensively studied as they have been identified as promising materials for photovoltaic applications. In particular, the interaction between the molecular cation and the inorganic framework has been suggested to influence the electronic properties. CH₃NH₃PbCl₃ undergoes several structural transitions associated with both distortion of the octahedra and orientational ordering of the CH₃NH₃ cation. We have measured the low-frequency lattice dynamics using neutron spectroscopy. We report an anomaly in the acoustic phonon linewidth towards the high-symmetry point Q_X= (2, 1/2, 0) when approaching the transitions, and a hardening of the entire phonon branch with decreasing temperature. Measurements at the Brillouin zone edges Q_X= (2, 1/2, 0), Q_M= (3/2, 1/2, 0) and Q_R= (3/2, 3/2, 5/2) show central peaks appearing at the lower temperature transition. We discuss the presence of both central peaks and acoustic phonon instabilities as evidence of a strong coupling between the inorganic framework and the molecular cation.   

Diffuse Neutron and X-ray Scattering from Cesium Lead Bromide

Cesium lead bromide has attracted recent attention, along with other lead halide perovskites, due to its optoelectronic properties. It has been proposed that the long carrier life supporting these properties is due to local polar fluctuations. To investigate the short-range lattice distortions that such fluctuations would imply, diffuse neutron and x-ray scattering experiments were performed on single crystals of CsPbBr₃ across range of temperatures spanning two structural phase transitions. Significant diffuse scattering indicating short-range displacement correlations was found in all three structural phases. In particular, diffuse neutron scattering from CsPbBr₃ shows distinct elastic and inelastic features, distinguishing between static and dynamic forms of local displacement correlations.