Computational modeling of perovskites ppt

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Computational modeling of perovskites for
photovolatic application, opportunities and
challenges
Tewodros Adaro
July 23, 2021
Computational modeling of perovskites for photovolatic
application, opportunities and challenges
Tewodros Adaro Computational modeling of perovskites for photovolatic applicatio

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Introduction
One of the crucial problems confronting our community today
is the need for eco-friendly and renewable energy sources to
overcome the increasing electricity demand concerning the
swelling population and manufacturing
One of the promising technologies is sun cellular generation,
which is considered eﬀicient for clean electricity at low cost
and minimal pollution.
Tetrahedral coordination structures (TCS), with silicon as
prototype, have been dominating solar cell research and
market, since the first semiconductor solar cell was fabricated
based on silicon absorber in the 1950s and followed by GaAs
and CdTe.
The development of low-cost, high-eﬀiciency solar cells has
become a central issue in recent years

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Introduction
Recently, with appeared the fourth generation of photovoltaic
technology, Perovskite Solar Cells (PCE) have appeared,
which exceeded expectations for Power Conversion Efficiency
(PCE) within short term.
Perovskite Solar Cells (PCEs) attracted widespread attention
from the solar cell research community due to their fantastic
improvement in device efficiency with a significant increase
from an initial value of3.8 percent in 2009, to 15 percent in
2013, up to 23.3 − 25.2percent recently (as shown in fig-1)
and decade due to high absorption coefficient, excellent
bipolar charge mobility, long carrier diffusion length, low
exciton binding energy, low trap state density, and tunable
bandgap

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Introduction
Figure: . A gradual increase in performance of P SC ( the eﬀiciencies of
PSC during last few year)

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Introduction
In contrast to TCS, in this class of perovskites, corner-sharing
octahedra are key structural characteristics (octahedral
coordination structure (OCS)), and a unique feature is its
defect tolerance.
As prototype compounds for TCS and OCS, respectively,
silicon and α-phase CH3NH3PbI3 have the highest symmetry
as three-dimensional crystals.
Alternatively, previous interest had also been directed to
low-symmetry structures, which included Zn3P2 (Eg 1.5 eV,
PCE 6.08 percent), Cu2S (Eg 1.21 eV, PCE 11percent) , and
FeS2 (Eg 0.95 eV, PCE 2.8 percent).
Inspired by the recent explosion in perovskite solar cell
research, efforts are also being made to include perovskite
derivatives in the search of improved stability, such as edge- or
face-shared A3B2X9 formula (Cs3Bi2I9 (PCE 1.0 percent),
Cs3Sb2I9 (PCE < 0.1percent)), A2BX4 compounds, and
Ruddlesden-Popper phases.

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Introduction
However, none of them has exhibited sufficiently promising
properties to survive in the race for novel solar cell materials,
and to date the search for novel stable high-efficiency solar
cell absorbers with earth-abundant and environment friendly
elements remains an active area in materials modeling
However, there exist unavoidable shortcomings in these
Pb-based high-efficiency PSCs , that is, the element lead is
toxic to the environment and organisms and difficult to
discharge from the body.
Therefore, to guarantee human’s safe and pollution free
natural environment it is necessary to develop some non- or
low-toxic metal ions to replace lead as perovskite absorbers of
PSCs.
This situation has stimulated considerable interest in research,
and many proposals have been made to provide suitable
solutions.

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Introduction
Unlike the success in TCS, chemical mutations on Octahedral
coordination structure (OCS) are not able to result in
functional semiconductors with performance comparable to
CH3NH3PbI3.
Despite extensive theoretical and experimental research,
unfortunately, both the stability and toxicity issues of those
perovskites have not yet been solved.
Nowadays, many experimental and theoretical researchers are
engaged in extending the understanding of the fundamental
physicochemical properties of MHPs, which is crucial for
increasing their stability.
Computational modeling has proven to be a valuable tool to
this endeavor, since it can provide essential insights about the
fundamental properties of materials that are diﬀicult, if not
impossible, to obtain experimentally,

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Introduction
Density functional theory (DFT) calculations are widely used
to investigate the structural and electronic properties of
materials in various fields, including photocatalysis, lithium
battery materials, dye-sensitized solar cells and so on .
In this context, computational techniques are extremely useful
in explaining the features of MHPs at the microscopic level.
There are diverse computational techniques that exhibit
advantages and limitations to investigate processes at
different sizes and time scales.
A proper choice of the most suitable technique is often
challenging because the performance of computational
methods for novel or complex materials is still unknown .
In this report, Computational modeling of perovskites for
photovolatic application, opportunities and challenges to
materials design for solar energy applications will be discuss.

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REVIEW LITERATURE
Perovskites
Perovskites were characterized to have an ABX3 structure
where X can be halogen or oxygen and A and B are 12 and 6
coordinated with X. An empirical value for the stability and
distortion of the perovskite structure is given by the ratio of
the (A-X) distance with respect to the (B-X) distance, called
the tolerance factor
t =
RA + Rx
√
2(RB + Rx)
(1)
The cubic phase is stabilized when t=1 and the structure is
distorted when t deviates from 1.
The band gap energy of CH3NH3PbI3 and CH3NH3PbBr3 was
measured using ultraviolet photoelectron spectroscopy (UPS)
and UV-vis spectroscopies to be about 1.5 and 2.25eV,
respectively.

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Perovskites
While the energetics of TiO2/perovskite/spiro-MeOTAD
suggests a good matching for charge transfer to happen (like
for liquid DSSCs), there are pieces of evidence showing that
perovskites on their own act as electron and hole transporters.
The former was proved by utilizing Al2O3 instead of TiO2
which lead to devices that performed with a significant
efficiency of 10 percent, and the latter was proved when
devices with pure perovskite and no HTM were made, yielding
a considerable efficiency of 5.5percent.
Later, Seok’s group improved the efficiency up to 12percent
using the wider band gap CH3NH3PbBr3, which was expected
to perform less good considering its lower photocurrent. The
reason was due to the high photovoltage that compensated
the lack in photocurrent[7].

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Perovskites
Metal halide perovskites (MHPs) are novel semiconductors
that have gained great scientific attention in the recent years
due to their excellent optoelectronic properties, which make
them suitable for applications such as perovskite solar cells
(PSCs) and light emitting diodes.
MHPs have the chemical formula ABX3, where A is a
monovalent organic or inorganic cation, B is a metal divalent
cation and X are halide anions.
Combining these compounds results in a semiconductor that
exhibits suitable band gaps, high light absorption
performance, low exciton binding energies, long carrier
diffusion lengths, and high charge carrier mobility.
In addition, MHPs exhibit a competitive fabrication cost
together with a simple route to synthesize. Despite all these
desirable properties, instability issues critically hamper their
industrial application[5].

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Perovskites
The last ten years have yielded an explosion of interest in the
broad family of materials based on metal halide perovskite
frameworks.
Interest in these systems (most notably, those based on
germanium, tin, and lead) derives from a confluence of
attractive semiconductor traits, including:
1 electronic structures that provide direct tunable bandgaps,
strong light absorption, relatively small and balanced electron/
hole effective masses, and defect resistance (i.e., dominant
defects that do not cause substantial nonradiative
recombination);
2 unprecedented flexibility to independently and synergistically
tune structural, optical, and electronic properties using both
organic and inorganic components of hybrid members of this
family, pointing to outstanding promise for “organic−inorganic
electronics” application; and
3 readily accessible synthesis of high-quality crystals and films,
enabling facile structure−property correlation and rapid device
prototyping/optimization

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Perovskites
Recent breakthroughs have been made through doping the
electron transporter TiO2-EL layer which yielded 19.3 percent
efficiency. Seok’s group took another approach and mixed
CH3NH3PbI3 with formamidinium organic cation perovskites
FAPbI3 to obtain an efficiency of 20.1percent , the highest
efficiency to date.
FAPbI3 has carrier diffusion lengths that are longer than those
of CH3NH3PbI3 and in addition it has a shallow band gap,
which makes it a better light harvester.
The only unsolved issue with this compound is its instability,
since the perovskite phase of FAPbI3 is unstable at room
temperature.
With the fast pace in perovskite DSSC, future improvements
to overcome this problem do not seem far to reach

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Perovskites
Figure: Structures of different perovskites. (a )Simple perovskite
cubic crystal structure and (b )Double perovskite crystal structure.

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Perovskites
PSCs have a perovskite photosensitive film that bounded
between two electrodes. A surface buffer layer is used the
active and electrode layers to make easy charge processing.
The Electron Transport Materials (ET M ) and Hole Transport
Materials (HT M ) are the two types of interface layers.
In fundamental, one of the electrodes should be a transparent
conductive oxide, such as anitride containing indium in fig
2(a), to show the two PSCs’ device structure.

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Perovskites
The structure of the device is widely used by depositing metal
(top) electrodes of aluminium, silver or gold.
There are two essential device architectures are used to
prepare PSCs, mesoporous and planar PSC compositions.
Therefore, the PSC charge transport channel is often
discussed based on the kind of the device structure
The perovskite layers have a mesoporous structure and are
formed by a layer of porous semiconductor metal oxide such
as titanium oxide TiO2 , which forms an interlocking network
between the two-phase interfaces.

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Perovskites
Therefore, light-induced electrons can be transported to the
cathode through the TiO2 channel, and the pores are
transported to the anode via the perovskite channel Fig. 1(b).
At the planar structure, the interface hole layer and electron
transport material are used to produce the cell.
Excitons generated in the perovskite layer drifted into the
electrode by an established electric potential or an externally
imposed electric field.

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Perovskites
Figure: Architectures of perovskite solar cells [16, 17](a) the device
structure of the top type PSC(16) (b) the energy band in the planer PSC
to show separation and collection of charge photo generating carries

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Computational Modelling of Perovskite Solar Cells
Computational Modelling of Perovskite Solar
CellsDespite the rapid progress made in the last few years in
terms of the conversion eﬀiciency, the understanding of the
fundamental electronic properties of AMX3 perovskites is
rather limited.This is especially true for realistic structures in
working devices, which often are compounds with mixtures of
more than one organic cation, metal cation or halide anion
with sophisticated surfaces and interfaces.The challenges to
fundamental understanding of the electronic structure of
AMX3 perovskites stem from their rich chemical and physical
properties and the interplay of these properties.In this context,
a first-principles computational approach capable of reliably
calculating the materials properties (electronic and
thermodynamic) is essential.

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While standard DFT provides reliable structures and stabilities
of perovskites, it severely underestimates the band gaps.
The relativistic GW approximation has been demonstrated to
be able to capture their electronic structure accurately but at
an extremely high computational cost.
Several research groups have successfully applied the GW
method to predicting electronic structures of AMX3
perovskites. Even et al. have identified the importance of a
giant spin-orbit coupling (SOC) effect (about 1.0 eV) in the
lead iodide perovskites, acting mainly on the conduction band
(mainly consisting of Pb states).
Taking SOC into account in a relativistic GW approach, Brivio
et al. predicted an unconventional band dispersion relation
and a Dresselhaus splitting at the band edges in pseudo-cubic
CH3NH3PbI3, which indicates a direct-indirect band gap
character.

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This theoretical prediction was recently confirmed by two
groups of researchers independently, revealing the mechanism
behind the slow charge recombination in these materials.
Umari et al. studied the substitution of Pb by Sn and found
the SOC effect on Sn perovskites to be much smaller (about
0.4 eV).
The same group of researchers also looked at the substitution
of halide ions in CH3NH3PbX3 and nicely reproduced
experimental findings in optical behavior, such as an increase
of the band gap when moving from I to Cl.
A few other theoretical studies at GW level include the
investigation of polar phonons, crystal structure effects and
phase transitions, exciton binding energies, and band gap
trends in AMX3.
The above mentioned GW studies have been very important
in understanding the chemistry and physics of these materials
and providing materials design inspirations.

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However, the GW calculations of realistic structures of metal
halide perovskites (figure-4) remain challenging because of
1 spin-orbit coupling effects
2 the complex structures and phase transitions in these materials
3 the necessary compromise between the size of the system
studied and the extremely high computational cost.
This leads to the quest for an accurate and cost effective
theoretical framework for electronic structure calculations of
realistic AMX3 structures and possibly the A/M/X mixed
compounds.

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Here, we apply a recently developed approximate quasiparticle
method, namely the DFT-1/2 method, which allows us to
accurately model the band gaps of AMX3 perovskites with
minimal computational cost.
The DFT-1/2 calculated electronic properties of AMX3
perovskites are compared with those calculated with the GW
method and with experimental data.
Trends in the interplay of geometrical properties and
electronic structure properties are analyzed systematically.
The results indicate that the DFT-1/2 method yields band
gaps with a GW precision, but with a computational cost
similar to standard DFT, opening the way to the study of
sophisticated structures and new materials design

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Figure: Top (a,b, and c) and side (e,f, and g) views of various
structures of metal halide perovskites, taking CH3NH3PbI3 as an
example (visualization by VESTA). From left to right: cubic,
tetragonal and orthorhombic structures. The unit cell used in the
DFT calculations are indicated by black lines

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Computational Modelling of Perovskite Solar Cells Using
DFT
DFTA detailed knowledge of the electrical, mechanical and
optical properties as well as chemical composition of different
materials constituting various isolated layers of solar cells is
essential for the design of novel high eﬀiciency solar
cells.Density Functional Theory (DFT) has become highly
popular in this regard owing to the existence of
well-established numerical codes capable of describing the
ground state properties of various materials.Though the Local
Density Approximation (LDA), based on uniform electron
density distribution and the Generalized Gradient
Approximation (GGA) based on non-uniform electron density
distribution, are two well established methods for estimating
the ground state electronic properties of materials, they
cannot provide accurate results for excited state DFT
calculations.

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DFT
One major drawback of the LDA is due to its self interaction
error which results in a wrong asymptotic behavior (i.e. the
spurious interaction of an electron with itself thus it yields
wrong band gaps and wrong dissociation limits for molecules.
On the other hand, Hybrid methods tries to incorporate
features from first principle methods such as Hartree Fock
methods, with some improved DFT mathematics and
computational codes.
When dealing with computational chemistry applications,
B3LYP hybrid method seems to be more reliable than other
hybrid methods of DFT calculation, even though most of the
hybrid methods depend on the materials of interest.
Another DFT calculation technique using the hybrid
functional HSE06 is good for calculating the band offsets,
bandgaps etc., but it was found to have certain disadvantages
especially when we deal with direct-indirect crossover as in the
case of the alloy, GaAsP.

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DFT
GW approach based on Many Body Perturbation Theory
(MBPT) can produce reliable results especially when
analyzing the electronic and optical properties of materials
and can be used self consistently also.
When interfaces are present, a Super Lattice Approximation is
found to be the most reliable approach as it can provide an
estimate of band line up.
Time Dependent DFT calculations can be used to calculate
the frequency dependent molecular response properties of
materials like charge carrier transport and charge diffusion.
DFT can also shed light into the concepts of electronegativity,
hardness, and chemical reactivity index of materials which are
essential for analyzing the stability of various perovskite
materials.

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DFT
In addition, the dielectric properties of materials play an
important role in determining the photon absorption and can
be calculated using Density Functional Perturbation theory.
For analyzing the optical properties, an analysis of bonding
and related properties is required and the multipole moments
of the compounds can be predicted with DFT.
It is also possible to calculate the complex second-order
optical susceptibility dispersion for the principal tensor
components and their intra-and inter-band contributions.
The theoretical circular dichroism spectra of compounds can
be calculated using Time Dependent Density Functional
Theory.

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DFT
A DFTP method combined with finite difference techniques
and symmetry analysis can provide third order components
related to physical properties such as nonlinear electrical
susceptibilities, nonlinear elasticity or photo elastic and electro
strictive effects.
The exciton binding energies and related optoelectronic
properties like the ionization potentials, electron aﬀinities and
fundamental gaps can be calculated using DFT with different
density functionals.
It is evident that a DFT study using Global Hybrid
Functionals (GH) and Periodic Boundary Conditions (PBCs)
can provide a very precise description of the electronic and
structural properties of semiconductors, as a component of
Dye Sensitized Solar Cells(DSSC) assembly.

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DFT
However, for the estimation of band gap, a well-developed
RSH functional perform better than a GH functional, but for
the description of excited state, HSE is not suitable since an
exact description of the excited state of both dye and
semiconductor is needed for the excited state calculations

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Computational model Opportunities challenges

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Conclusions
Conclusions
In summary, we have reviewed a computational modeling of
proviskates for solar cell application. opportunities and
challenges of implementing the density functional theory
(DFT) and time-dependent density-functional theory
(TDDFT) using different program package to study the
structural, electronic and optical properties of the perovskite
have been discussed in comparison with the experimental
results which helps to identify which computational modeling
is more preferable for which types of provoskites solar cell.

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Conclusions
It is evident that a DFT study using Global Hybrid
Functionals (GH) and Periodic Boundary Conditions (PBCs)
can provide a very precise description of the electronic and
structural properties of semiconductors, as a component of
Dye Sensitized Solar Cells(DSSC) assembly.
However, for the estimation of band gap, a well-developed
RSH functional perform better than a GH functional, but for
the description of excited state, HSE is not suitable since an
exact description of the excited state of both dye and
semiconductor is needed for the excited state calculations.

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Thank You
Thank You

Computational modeling of perovskites ppt

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