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Register a new userFirst principles study of ferroelastic twins in halide perovskites
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Added by
Andrew Warwick BSc MSc
Publication date
2019
fields
Physics
and research's language is
English
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We present an ab initio simulation of $90^{circ}$ ferroelastic twins that were recently observed in methyl ammonium lead iodide. There are two inequivalent types of $90^{circ}$ walls that we calculate to act as either electron or hole sinks which suggests a possible route to enhancing charge carrier separation in photovoltaic devices. Despite separating non-polar domains, we show these walls to have a substantial in-plane polarisation of $sim 6 phantom{|} mu text{C}phantom{|}text{cm}^{-2}$, due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect and create efficient pathways for photocurrents within the wall.
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Solar cells based on hybrid perovskites have shown high efficiency while possessing simple processing methods. To gain a fundamental understanding of their properties on an atomic level, we investigate single crystals of CH3NH3PbI3 with a narrow transition (~5 K) near 327 K. Temperature dependent structural measurements reveal a persistent tetragonal structure with smooth changes in the atomic displacement parameters (ADPs) on crossing T*. We show that the ADPs for I ions yield extended flat regions in the potential wells consistent with the measured large thermal expansion parameter. Molecular dynamics simulations reveal that this material exhibits significant high asymmetries in the Pb-I pair distribution functions. We also show that the intrinsically enhanced freedom of motion of the iodine atoms enables large deformations. This flexibility (softness) of the atomic structure results in highly localized atomic relaxation about defects and hence accounts for both the high carrier mobility as well as the structural instability.
Lead halide perovskite semiconductors are soft, polar, materials. The strong driving force for polaron formation (the dielectric electron-phonon coupling) is balanced by the light band effective-masses, leading to a strongly-interacting large-polaron. A first-principles prediction of mobility would help understand the fundamental mobility limits. Theories of mobility need to consider the polaron (rather than free-carrier) state due to the strong interactions. In this material we expect that at room temperature polar-optical phonon mode scattering will dominate, and so limit mobility. We calculate the temperature-dependent polaron mobility of hybrid halide perovskites by variationally solving the Feynman polaron model with the finite-temperature free-energies of =Osaka. This model considers a simplified effective-mass band-structure interacting with a continuum dielectric of characteristic response frequency. We parametrise the model fully from electronic-structure calculations. In methylammonium lead iodide at 300 K we predict electron and hole mobilities of 133 and 94 cm^2/V/s respectively. These are in acceptable agreement with single-crystal measurements, suggesting that the intrinsic limit of the polaron charge carrier state has been reached. Repercussions for hot-electron photo-excited states are discussed. As well as mobility, the model also exposes the dynamic structure of the polaron. This can be used to interpret impedance measurements of the charge-carrier state. We provide the phonon-drag mass-renormalisation, and scattering time constants. These could be used as parameters for larger-scale device models and band-structure dependent mobility simulations.
Hybrid halide perovskite semiconductors exhibit complex, dynamical disorder while also harboring properties ideal for optoelectronic applications that include photovoltaics. However, these materials are structurally and compositionally distinct from traditional compound semiconductors composed of tetrahedrally-coordinated elements with an average valence electron count of silicon. As discussed here, the additional dynamic degrees of freedom of hybrid halide perovskites underlie many of their potentially transformative physical properties. Neutron scattering and spectroscopy studies of the atomic dynamics of these materials have yielded significant insights to the functional properties. Specifically, inelastic neutron scattering has been used to elucidate the phonon band structure, and quasi-elastic neutron scattering (QENS) has revealed the nature of the uncorrelated dynamics pertaining to molecular reorientations. Understanding the dynamics of these complex semiconductors has elucidated the temperature-dependent phase stability and origins of the defect-tolerant electronic transport from the highly polarizable dielectric response. Furthermore, the dynamic degrees of freedom of the hybrid perovskites provides additional opportunities for application engineering and innovation.
Complex quantum coupling phenomena of halide perovskites are examined through ab-initio calculations and exact diagonalization of model Hamiltonians to formulate a set of fundamental guiding rules to engineer the bandgap through strain. The bandgap tuning in halides is crucial for photovoltaic applications and for establishing non-trivial electronic states. Using CsSnI$_3$ as the prototype material, we show that in the cubic phase, the bandgap reduces irrespective of the nature of strain. However, for the tetragonal phase, it reduces with tensile strain and increases with compressive strain, while the reverse is the case for the orthorhombic phase. The reduction can give rise to negative bandgap in the cubic and tetragonal phases leading to normal to topological insulator phase transition. Also, these halides tend to form a stability plateau in a space spanned by strain and octahedral rotation. In this plateau, with negligible cost to the total energy, the bandgap can be varied in a range of 1eV. Furthermore, we present a descriptor model for the perovskite to simulate their bandgap with strain and rotation. Analysis of band topology through model Hamiltonians led to the conceptualization of topological influencers that provide a quantitative measure of the contribution of each chemical bonding towards establishing a normal or topological insulator phase. On the technical aspect, we show that a four orbital based basis set (Sn-${s,p}$ for CsSnI$_3$) is sufficient to construct the model Hamiltonian which can explain the electronic structure of each polymorph of halide perovskites.
For the intrinsic carriers of MAPbBr$_{3}$, the temperature $T$ dependent mobility $mu(T)$ of behaves like $mupropto T^{-1/2}$ in piezoelectric tetragonal phase, $mupropto T^{-1.4}$ in non-piezoelectric cubic phase. But for the photo-generated carriers in other halide perovskites ABX$_{3}$, $mupropto T^{-3/2}$ behavior is typical. Due to the strong interaction of carrier with longitudinal optical phonon, in ABX$_{3}$ the carriers mainly exist as optical polarons. The softness of ABX$_{3}$ renders it without inversion center in tetragonal phase, which allows piezoelectric effect at low carrier concentration. The variations of $mu(T)$ behavior results from (1) the wave vector dependence of the piezoelectric interaction of polarons with acoustic phonons is different from that of ordinary polaron-acoustic phonon interaction; (2) the residual interaction of polaron with 2 longitudinal optical phonons can be ignored at low temperature, but is important at higher temperature; and (3) the concentration of intrinsic carriers is determined by temperature, while the concentration of photo-generated carriers is determined by the incident flux of photons.
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