110 research outputs found
What is moving in hybrid halide perovskite solar cells?
Organic-inorganic semiconductors, which adopt the perovskite crystal
structure, have perturbed the landscape of contemporary photovoltaics research.
In this Account, we discuss the internal motion of methylammonium lead iodide
(CHNHPbI) and formamidinium lead iodide ([CH(NH)]PbI),
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. Multi-scale simulations - drawing from electronic structure, ab
initio molecular dynamic and Monte Carlo techniques - have been combined with
neutron scattering and ultra-fast vibrational spectroscopy to qualify the
nature and timescales of the motions. Recent experimental evidence and
theoretical models for simultaneous electron transport and ion transport in
these materials has been presented, suggesting they are mixed-mode conductors
with similarities to metal oxide perovskites developed for battery and fuel
cell applications. We expound on the implications of these effects for the
photovoltaic action. The temporal behaviour found in hybrid perovskites
introduces a sensitivity in materials characterisation 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 and device simulations. Herein,
we critically discuss the atomistic origin of the dynamic processes.Comment: 29 pages, 3 figure
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
Predicting polaron mobility in organic semiconductors with the Feynman variational approach
We extend the Feynman variational approach to the polaron problem
\cite{Feynman1955} to the Holstein (lattice) polaron. This new theory shows a
discrete transition to small-polarons is observed in the Holstein model.
The method can directly used in the FHIP \cite{Feynman1962} mobility theory
to calculate dc mobility and complex impedance. We show that we can take matrix
elements from electronic structure calculations on real materials, by modelling
charge-carrier mobility in crystalline rubrene. Good agreement is found to
measurement, in particular the continuous thermal transition in mobility from
band-like to thermally-activated, with a minimum in mobility predicted at 140
K.Comment: 8 pages, 6 figure
Spontaneous Octahedral Tilting in the Cubic Inorganic Caesium Halide Perovskites CsSnX and CsPbX (X = F, Cl, Br, I)
The local crystal structures of many perovskite-structured materials deviate
from the average space group symmetry. We demonstrate, from lattice-dynamics
calculations based on quantum chemical force constants, that all the
caesium-lead and caesium-tin halide perovskites exhibit vibrational
instabilities associated with octahedral titling in their high-temperature
cubic phase. Anharmonic double-well potentials are found for zone-boundary
phonon modes in all compounds with barriers ranging from 108 to 512 meV. The
well depth is correlated with the tolerance factor and the chemistry of the
composition, but is not proportional to the imaginary harmonic phonon
frequency. We provide quantitative insights into the thermodynamic driving
forces and distinguish between dynamic and static disorder based on the
potential-energy landscape. A positive band gap deformation (spectral
blueshift) accompanies the structural distortion, with implications for
understanding the performance of these materials in applications areas
including solar cells and light-emitting diodes
Impact of non-parabolic electronic band structure on the optical and transport properties of photovoltaic materials
The effective mass approximation (EMA) models the response to an external perturbation of an electron in a periodic potential as the response of a free electron with a renormalized mass. For semiconductors used in photovoltaic devices, the EMA allows calculation of important material properties from first-principles calculations, including optical properties (e.g., exciton binding energies), defect properties (e.g., donor and acceptor levels), and transport properties (e.g., polaron radii and carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band nonparabolicity on four common thin-film photovoltaic materials - GaAs, CdTe, Cu2ZnSnS4 and CH3NH3PbI3 - at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasilinear dispersion to quantify the extent of nonparabolicity and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite CH3NH3PbI3 as a model system to assess the impact of nonparabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of 1020cm-3 the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport properties of other semiconductors with significant band nonparabolicity
The cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K
The structure of black formamidinium lead halide, -[HC(NH2)2]PbI3, at 298 K has been refined from high resolution 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 while the -NH2 groups hydrogen bond (NH…I = 2.75 – 3.00Å) with the iodide atoms of the [PbI3]− framework. High atomic displacement parameters for the formamidinium cation are consistent with rapid molecular rotations at room temperature as evidenced in ab initio molecular dynamic simulations
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