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Muhammad AamirPhD Chemistry

AU840146Supervisor: Dr. Muhammad SherCo-supervisor: Dr. Javeed Akhtar

Allama Iqbal Open University Islamabad, Pakistan

THE SPELL OF PEROVSKITES

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2Requisites Of Talk:1. PEROVSKITE2. APPLICATION 3. STRUCTURE OF PEROVSKITE4. DISTORTION OF STRUCTURE5. PHASE TRANSITION6. CLASSIFICATION OF PERVOSKITE7. CHEMICAL BONDING IN HYBRID PEROVSKITE8. OPTICAL PROPERTIES 9. DECOMPOSITION IN WATER 10. SYNTHESIS OF PERVOSKITE 11. UV AND PL STUDIES OF PEROVSKITE

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PEROVSKITES Perovskite is a calcium titanium oxide mineral with the chemical formula of CaTiO3.

The mineral was discovered in the Ural mountains of Russia by Gustav Rose in 1839. Named after the Russian mineralogist, L. A. Perovski (1792–1856) Class of compounds, which have the same type of crystal structure as CaTiO3, known as the perovskites.

(Perovskite Webmineral)

( Comte Lev Alexeïevich Perovski)

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Why Perovskites Are So Important: •Perovskites are the most abundant materials in the Earth’s Lower Mantle (MgSiO3). Pervoskites thought to be up to 93% in Earth’s Lower Mantle and are probably the most abundant minerals on Earth.

Scientific Reports 3, Article number: 3381

Scientific Reports 3, Article number: 3381

(Scientific Reports 3, Article number: 3381)

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1. Colossal Magnetoresistance • They have interesting magnetic properties depending on the elements they contained (LaMnO3) .( electrical resistance changes when they are put in a magnetic field – really useful for microelectronics and telecoms).Application: • Magnetic data storage • Sensors;• Hybrid cells

2. Superconductors!Applications:(a) Electric power transmission and transformers.(b) Superconducting electromagnets. (c) Superconducting digital circuits. Cryotron switches, RF and microwave filters, 3. PIEZOELECTRIC & FERROELECTRIC PEROVSKITESApplications:(a) High voltage and power sources (piezo-based ignition systems, piezoelectric transformer)(b) Sensors(c) Hybrid cells(d) Vibration dampers;(e) Frequency standards (quartz clocks, frequency multipliers);

4. they can be ionic conductors which means atoms inside the structure can move around without the whole thing falling apart this is particularly useful for energy materials such as those in batteries and fuel cells.

5. Thermopower generation; 6. Catalytic materials. Co-based perovskite material is as a replacement for Pt in

catalytic converters in diesel vehicles;7. Photovoltanics.

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PEROVSKITES

Colossal Magnetoresistance

Superconductors Photovoltanics

Catalytic materials

Thermopower Generation Ionic Conductors

Piezoelectric & Ferroelectric

WHY PEROVSKITES ARE SO IMPORTANT:

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STRUCTURE OF PEROVSKITE Typical Calcium titatnate (CaTiO3) has cubic geometry.

Pervoskites are generally represented as AMX3.

A= Cation M= Metal Cation X= Anion

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DISTORTION FROM IDEAL STRUCTURE:

Perovskites distorted from ideal structure. CaTiO3 is pseduo-cubic due to tilting of octahedral.

Tilting and rotation of MX3 in lattice produces tetragonal, orthorhombic, trigonal and monoclinic.

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MECHANISM OF DISTORTION:

Distortion variables:

Jahn Teller Effect ( distortion of MX6)

Size effect

Changing the composition of AMX3

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1. JAHN-TELLER EFFECT:

If the ground electronic configuration of a non-linear complex is orbitally degenerate, the complex will distort so as to remove the degeneracy and achieve a lower energy. This is called the Jahn-Teller Effect

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In the ideal cubic case the cell axis, a, is geometrically related to the ionic radii (rA, rm and rx)

2. SIZE EFFECTS

a = ra+rx = √2 (rm+rx) (where rA, rM and rX are the ionic radii of A, M and X, respectively) Degree of distortion is determined by Goldschmidt's tolerance factor (t)

(rA + rX)

√2(rM + rX)t =

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Ideal cubic has t = 1.00

where rA = 1.44Ao rM= 0.605Ao and rX = 1.40 Ao

It means t α rA

t-value Effect Possible Structures> 1 A-cations are too large to fit into

their interstices or small MHexagonal perovskites

~0.9- 1.0

Ideal conditions Cubic perovskites

0.71 - 0.9

A-cations are too small to fit into their interstices

•Several possible structures.Among them: orthorhombic pervoskites•rhombohedra variants

< 0.71 A-cations are of same size as M-cations ( 1/—2 = 0.71 )

•Possible close-packed structures

Low values of the tolerance factor t = 0.81

If t is larger than 1 due to a large A or a small B ion

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PHASE TRANSITION OF PEROVSKITES:

Three Factors Controla) Temperature orthorhombic to tetrahedral occurs at 160 K while cubic is stable at

330K

b) Pressure

c) Applied Field

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Classification of perovskites

1. Inorganic perovskites: All the bonded atoms in AMX3 are inorganic in nature e.g: CaTiO3, GdFeO3, BaNiO3, CaRbF3 etc

Stoichiometry Of Perovskites ( Composition Of Perovskites)

A-M-X3

1. I-V-VI3 ( KTaO3)2. II-IV-VI3 ( SrTiO3)3. III-III-VI3 ( GdFeO3) 4. I-II-VII3 (CsSnI3)

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2. ORGANIC-INORGANIC PEROVSKITES (HYBRID PEROVSKITES) A is replaced by organic cation

organic cations contain three or less C-C or C-N bonds (3D)

The hybrid halide perovskites are of type I−II−VII3 (R-NH3MX3).

The mono positively charged ion is used so that it fit in the void and give 3D structure If the size of A cation is too large then low dimensional geometry (2D, 1D) is obtained that give layered structure (R-NH3)2MX4 or (NH3-R-NH3)MX4 .

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STABILITY OF PEROVSKITES Madelung electrostatic potential is used to explain the stability of ionic and heteropolar crystals.

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INORGANIC PEROVSKITES HYBRID PEROVSKITES

Spherical Symmetric A site ( inorganic) is replaced by reduced symmetry (organic ) .

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CHEMICAL BONDING IN PEROVSKITES:

The chemical bonding in hybrid perovskites with AMX3 stoichiometry can be separated into three distinct components.

It should be noted that these materials are organic−inorganic but not organometallic as there is no direct bond between a metal and carbon atom

Pb+  : [Xe] 4f14 5d10 6s2 6po 6do

I-   : [Kr] 4d10 5s2 5p6

R-NH3+ : [He] 2s2 2p6 }SP3d2 hybridization and

form a octahedron

There is hybridization between the filled Pb 6s band with I 5p that result in antibonding states at the top of the valence band.

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The CH3NH3+ molecules in neighboring cages are 6 Å apart.∼

The dominant bonding between the molecule (A site) and framework is electrostatic in nature. CH3NH3+ is a positively charged ion inside a negatively charged cage, so there is a strong electrostatic potential

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• Electrostatic contribution to the chemical bonding between the molecular dipole and the PbI6 octahedra is the charge−dipole interaction, which is dependent on the dipole orientation.

• Polarizability of the I−ions an induced dipole interaction is expected (the so-called Debye force). Due to these interactions, a correlation is expected between molecular orientation and octahedral deformation.

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The strong polarization of the lattice has two potential advantages for photovoltaic operation: (i) enhanced charge separation and improved carrier lifetimes; (ii) open circuit voltages above the band gap of the material

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OPTICAL PROPERTIES Energy difference between valence band maxima and conduction band minima is called band gap. All inorganic perovskites that generally have a large indirect energy band gap of around 3 eV . Three-dimensional (3D) organic–inorganic halide perovskites exhibit strong absorbance in the visible wavelength range with a direct energy band gap of around 1.5 eV .

 if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon is called Band Gap Direct

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BAND GAP TUNING Band gap tuning is required to extend the absorption longer wavelength without sacrificing the absorption coefficient. changing of any of the A, M and X in AMX3 changes the band gap.

A-site Substitution Smaller molecular cations (A) lower band gap. The replacement of CH3NH3

+ by NH4+ in the pervoskite lattice

reduces the band gap by 0.3 eV. The replacement of CH3NH3

+ by NH4+ in the pervoskite lattice

reduces the band gap by 0.3 eV. HPbI3 has a band gap of less than 0.3 eV.

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An A-site ion too small for the BX3 framework results in an instability of the octahedral networks with respect to tilting, which can change the electronic properties The choice of A-site ions that are too large for the BX3 framework can result in layered pervoskite structures

B-SITE SUBSTITUTION: Substitution on the B site can be used to directly alter the conduction band.

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X-Site Substitution: the anion (X site) dictates the valence band energy.The observed band gap changes upon halide substitution are influenced by the electronic states of the anion; i.e., from Cl to Br to I the valence band composition changes from 3p to 4p to 5p with a monotonic decrease in electron binding energy

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DECOMPOSITION OF PEROVSKITES: Hybrid perovskites reacting with Lewis bases with the most notable being irreversible degradation in the presence of H2O and temporary bleaching in the presence of ammonia.

Hybrid perovskites incorporating aprotic organic ions (such as tetramethylammonium, (CH3)4N+). Such a material would not be capable of the reaction mechanism as above and so may be more chemically stable.

Single water molecule is sufficient to degrade the Material.

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HYBRID PERVOSKITES SOLAR CELLS

organic−inorganic lead halide perovskite has yielded photovoltaic efficiencies of 20.1% (methylammonium lead iodide (CH3NH3PbI3) a sharp band gap close to the ideal, high absorbance, low exciton binding energy with the excited state composed primarily of free carriers, near perfect (CH3NH3PbBr3) the Br perovskite yielding lower efficiency.(2.3 eV band gap) CH3NH3PbCl3, 3.11 eV Mixed halide precursor solutions with varying Br content (CH3NH3Pb(I1−xBrx )3with 0 <x< 1).The mixed halide system allows a band gap tuning from 786 nm (1.58 eV, forx= 0) to 544 nm (2.28 eV, forx= 1)

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7.37% for CH3NH3Sn0.25Pb0.75I3 10.1% was obtained with CH3NH3SnaPb1−aI3−xClx in a planar device, with Cl addition credited for improved film coverage, effective exciton dissociation and charge transport.

Pervoskites Efficiency

CH3NH3PbI3 20.1 %

CH3NH3PbBr3 10.45%

CH3NH3Sn0.25Pb0.75I3 7.37%

CH3NH3SnaPb1−aI3−xClx 10.1%

CH3NH3SnI3 6.4%

CsGeX3 3.2%

(CH3NH3)2CuCl0.5Br3.5 0.02%

HC(NH2)2PbI3 16.01%

(CH3NH3)0.6(HC-(NH2)2)0.4PbI3 14.9%

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(Jung, et al; 2015)

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(Jung, et al; 2015)

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(Jung, et al; 2015)

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