64 research outputs found
Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited
The application of ferroelectric materials (i.e. solids that exhibit
spontaneous electric polarisation) in solar cells has a long and controversial
history. This includes the first observations of the anomalous photovoltaic
effect (APE) and the bulk photovoltaic effect (BPE). The recent successful
application of inorganic and hybrid perovskite structured materials (e.g.
BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasises that polar semiconductors
can be used in conventional photovoltaic architectures. We review developments
in this field, with a particular emphasis on the materials known to display the
APE/BPE (e.g. ZnS, CdTe, SbSI), and the theoretical explanation. Critical
analysis is complemented with first-principles calculation of the underlying
electronic structure. In addition to discussing the implications of a
ferroelectric absorber layer, and the solid state theory of polarisation (Berry
phase analysis), design principles and opportunities for high-efficiency
ferroelectric photovoltaics are presented
Band alignment of the hybrid halide perovskites CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3
Valence band energies of three organic–inorganic perovskites are compared from electronic structure theory.</p
Descriptors for Electron and Hole Charge Carriers in Metal Oxides
Metal oxides can act as insulators, semiconductors, or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Fröhlich electron–phonon coupling. Numerical analysis allows us to assign p- and n-type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behavior. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides
Phonon anharmonicity, lifetimes, and thermal transport in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> from many-body perturbation theory
Lattice vibrations in CHNHPbI are strongly interacting, with
double well instabilities present at the Brillouin zone boundary. Analysis
within a first-principles lattice dynamics framework reveals anharmonic
potentials with short phonon quasi-particle lifetimes and mean-free paths. The
phonon behaviour is distinct from the inorganic semiconductors GaAs and CdTe
where three-phonon interaction strengths are three orders of magnitude smaller.
The implications for the applications of hybrid halide perovskites arising from
thermal conductivity, band-gap deformation, and charge-carrier scattering
through electron-phonon coupling, are presented
Perspective: Theory and simulation of hybrid halide perovskites
Organic-inorganic halide perovskites present a number of challenges for first-principles atomistic materials modeling. Such “plastic crystals” feature dynamic processes across multiple length and time scales. These include the following: (i) transport of slow ions and fast electrons; (ii) highly anharmonic lattice dynamics with short phonon lifetimes; (iii) local symmetry breaking of the average crystallographic space group; (iv) strong relativistic (spin-orbit coupling) effects on the electronic band structure; and (v) thermodynamic metastability and rapid chemical breakdown. These issues, which affect the operation of solar cells, are outlined in this perspective. We also discuss general guidelines for performing quantitative and predictive simulations of these materials, which are relevant to metal-organic frameworks and other hybrid semiconducting, dielectric and ferroelectric compounds
Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
Lead halide perovskites such as methylammonium lead triiodide (MAPI) have
outstanding optical and electronic properties for photovoltaic applications,
yet a full understanding of how this solution processable material works so
well is currently missing. Previous research has revealed that MAPI possesses
multiple forms of static disorder regardless of preparation method, which is
surprising in light of its excellent performance. Using high energy resolution
inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and
find direct evidence for another form of disorder in single crystals: large
amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra
that persist to room temperature and above, left over from structural phase
transitions that take place tens to hundreds of degrees below. Phonon
calculations show that the orientations of the methylammonium couple strongly
and cooperatively to these modes. The result is a non-centrosymmetric,
instantaneous local structure, which we observe in atomic pair distribution
function (PDF) measurements. This local symmetry breaking is unobservable by
Bragg diffraction, but can explain key material properties such as the
structural phase sequence, ultra low thermal transport, and large minority
charge carrier lifetimes despite moderate carrier mobility.Comment: 30 pages, 11 figure
Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural
behaviour, with successive transitions between orthorhombic, tetragonal and
cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles
lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The
equilibrium structures compare well to solutions of temperature-dependent
powder neutron diffraction. By following the normal modes we calculate infrared
and Raman intensities of the vibrations, and compare them to the measurement of
a single crystal where the Raman laser is controlled to avoid degradation of
the sample. Despite a clear separation in energy between low frequency modes
associated with the inorganic PbI3 network and high-frequency modes of the
organic CH3NH3+ cation, significant coupling between them is found, which
emphasises the interplay between molecular 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 temperature.Comment: 9 pages, 4 figure
Multi-Pulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites
The behavior of hot carriers in metal-halide perovskites (MHPs) present a
valuable foundation for understanding the details of carrier-phonon coupling in
the materials as well as the prospective development of highly efficient hot
carrier and carrier multiplication solar cells. Whilst the carrier population
dynamics during cooling have been intensely studied, the evolution of the hot
carrier properties, namely the hot carrier mobility, remain largely unexplored.
To address this, we introduce a novel ultrafast visible pump - infrared push -
terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivity
dynamics of cooling carriers in methylammonium lead iodide. We find a decrease
in mobility upon optically depositing energy into the carriers, which is
typical of band-transport. Surprisingly, the conductivity recovery dynamics are
incommensurate with the intraband relaxation measured by an analogous
experiment with an infrared probe (PPP- IR), and exhibit a negligible
dependence on the density of hot carriers. These results and the kinetic
modelling reveal the importance of highly-localized lattice heating on the
mobility of the hot electronic states. This collective polaron-lattice
phenomenon may contribute to the unusual photophysics observed in MHPs and
should be accounted for in devices that utilize hot carriers.Comment: 28 pages, 4 figures, 77 reference
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