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AB INITIO ATOMISTIC INSIGHTS INTO
LEAD-FREE PEROVSKITES FOR
PHOTOVOLTAICS AND
OPTOELECTRONICS
Md Roknuzzaman
B.Sc. (Hons.) in Physics, M.Sc. in Solid State Physics
Submitted in fulfilment of the requirements for the degree of
Doctor of Philosophy
School of Chemistry and Physics
Science and Engineering Faculty
Queensland University of Technology
2020
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics i
Keywords
Perovskites, Inorganic Perovskites, Organic Perovskites, Organic-Inorganic
Perovskites, Hybrid Perovskites, Cs-based Perovskites, MA-based Perovskites, FA-
based Perovskites, Double Perovskites, Hybrid Double Perovskites, Lead-free
Perovskites, Hybrid Semiconductors, Organic Semiconductors, Structural Properties,
Electronic Properties, Optical Properties, Transport Properties, Elastic Properties,
Mechanical Properties, Optoelectronics.
ii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Abstract
Methylammonium lead iodide (CH3NH3PbI3) and some other hybrid perovskites
have drawn significant attention to the science community because of their high power
conversion efficiency in solar cells. In addition, this group of semiconductors has the
potential to be used in a wide range of optoelectronic devices like light-emitting
diodes, lasers, field-effect transistors, photodetectors, photoluminescent,
electroluminescent devices as well as light-emitting electrochemical cells.
Commercialization of perovskite materials may revolutionize the global energy sector
as these materials are abundant in nature and inexpensive, as a result it would be
cheaper and more efficient than silicon-based technology. However, the insufficient
long-term stability and toxicity of lead (Pb) are two major barriers for Pb-based hybrid
perovskites to be adopted in large-scale industrial applications. Therefore, it is utmost
important to find non-toxic Pb-free stable perovskites for the further development of
perovskites based optoelectronic technology. A detailed atomistic insight of the
fundamental properties of perovskite materials can help to understand the basic
characteristics of the materials and it can guide research to find non-toxic stable
materials for photovoltaics and optoelectronics.
This thesis presents a first-principles Density Functional Theory (DFT)
investigations of the structural, electronic, optical and mechanical properties of
caesium (Cs) based inorganic perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) as
well as methylammonium (MA) and formamidinium (FA) based organic-inorganic
hybrid perovskites MABX3 (MA = CH3NH3, B = Pb, Sn, Ge; X = I, Br, Cl) and FABX3
[FA = CH(NH2)2; B = Pb, Sn, Ge; X = I, Br, Cl]. The results suggest that the considered
perovskites are semiconductors with direct energy band gap and are mechanically
stable. Also, the calculated high absorption coefficient, low reflectivity and high
optical conductivity suggest that the considered Pb-free materials have potential to be
used in solar cells and other optoelectronic energy devices. The optical properties of
the considered inorganic perovskites indicate that germanium (Ge) would be a better
replacement of Pb. Indeed, Ge containing compounds have higher optical absorption
and optical conductivity than that of Pb containing compounds. A complementary
analysis of the electronic, optical and mechanical properties of the Cs-based
compounds reveals that CsGeI3 is the best Pb-free inorganic metal halide
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics iii
semiconductor for the solar cell application while the solid solution CsGe(I0.7Br0.3)3 is
found to be mechanically more ductile than CsGeI3. However, in case of organic-
inorganic hybrid perovskites, tin (Sn) containing compounds show better
optoelectronic properties compared to the Ge-containing counterparts. More
specifically, MA and FA based Sn containing perovskites such as MASnI3 and FASnI3
have superior properties compared to other Pb-free options as the materials have
excellent electronic, optical and mechanical properties. MASnI3 is found to be one of
the best Pb-free materials considering its promising optoelectronic properties as well
as the unique mechanical property of MASnI3 makes this compound flexible and easy
to be fabricated into thin films. In case of FA based perovskites, FASnI3 would be the
preferred Pb-free material for photovoltaic application because of its low carrier
effective mass and high absorption coefficient along with good material ductility.
Furthermore, the optoelectronic properties of a new group of compounds called
organic-inorganic hybrid double perovskites, ABiCuX6 [A = Cs2, (MA)2, (FA)2,
CsMA, CsFA, MAFA; X = I, Br, Cl] have been investigated using the same
methodology to predict their suitability in photovoltaic and optoelectronic
applications. The considered hybrid double perovskites are found as semiconductors
with a tunable band gap characteristics that are suitable for devices like light emitting
diodes. Moreover, the high dielectric constant, high absorption, high optical
conductivity and low reflectivity suggest that the materials have the potential to be
used in a wide range of optoelectronic applications including solar cells. Furthermore,
the organic-inorganic hybrid double perovskite (FA)2BiCuI6 has been predicted as the
best candidate in photovoltaic and optoelectronic applications as this material has
superior optical and electronic properties.
The findings of this study can help the understanding of structure-property
relationships in perovskite materials. Therefore, it is expected that these results will
benefit the development of Pb-free non-toxic sustainable optoelectronic devices.
iv Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
List of Publications
The published journal and conference papers during the PhD candidature are
listed below.
Journal Papers:
1. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Aijun Du,
Tuquabo Tesfamichael, Towards lead-free perovskite photovoltaics and
optoelectronics by ab-initio simulations, Scientific Reports, 7 (2017) 14025.
IF: 4.122, SJR: 1.41 [Q1]. (Included in the thesis)
2. Md Roknuzzaman, Kostya (Ken) Ostrikov, Kimal Chandula
Wasalathilake, Cheng Yan, Hongxia Wang, Tuquabo Tesfamichael, Insight
into lead-free organic-inorganic hybrid perovskites for photovoltaics and
optoelectronics: A first-principles study, Organic Electronics, 59 (2018) 99-
106. IF: 3.495, SJR: 0.94 [Q1]. (Included in the thesis)
3. Md Roknuzzaman, Jose A. Alarco, Hongxia Wang, Aijun Du, Tuquabo
Tesfamichael, Kostya (Ken) Ostrikov, Ab initio atomistic insights into lead-
free formamidinium based hybrid perovskites for photovoltaics and
optoelectronics, Computational Materials Science, 169 (2019) 109118 . IF:
2.292, SJR: 0.81 [Q1]. (Included in the thesis)
4. Md Roknuzzaman, Chunmei Zhang, Kostya (Ken) Ostrikov, Aijun Du,
Hongxia Wang, Lianzhou Wang, Tuquabo Tesfamichael, Electronic and
optical properties of lead-free hybrid double perovskites for photovoltaic
and optoelectronic applications, Scientific Reports, 9 (2019) 718. IF: 4.122,
SJR: 1.41 [Q1]. (Included in the thesis)
5. M. Roknuzzaman, M.A. Hadi, M.A. Ali, M.M. Hossain, N. Jahan, M.M.
Uddin, J.A. Alarco, K. Ostrikov, First hafnium-based MAX phase in the 312
family, Hf3AlC2: A first principles study, Journal of Alloys and Compounds,
727 (2017) 616-626. IF: 3.779, SJR: 1.07 [Q1].
6. M.T. Nasir, M.A. Hadi, M.A. Rayhan, M.A. Ali, M.M. Hossain, M.
Roknuzzaman, S.H. Naqib, A.K.M.A. Islam, M.M. Uddin, K. Ostrikov,
First-Principles Study of Superconducting ScRhP and ScIrP Pnictides,
Physica Status Solidi B, 254 (2017) 1700336. IF: 1.454, SJR: 0.52 [Q2].
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics v
7. M.A. Hadi, M. Roknuzzaman, A. Chroneos, S.H. Naqib, A.K.M.A. Islam,
R.V. Vovk, K. Ostrikov, Elastic and thermodynamic properties of new
(Zr3−xTix)AlC2 MAX-phase solid solutions, Computational Materials
Science, 137 (2017) 318-326. IF: 2.292, SJR: 0.81 [Q1].
8. Kimal Chandula Wasalathilake, Md Roknuzzaman, Kostya (Ken)
Ostrikov, Godwin A. Ayoko, Cheng Yan, Interaction between
functionalized graphene and sulfur compounds in a lithium-sulfur battery: a
density functional theory investigation, RSC Advances, 8 (2018) 2271. IF:
3.049, SJR: 0.81 [Q1].
9. M.A. Ali, M. Anwar Hossain, M.A. Rayhan, M.M. Hossain, M.M. Uddin,
M. Roknuzzaman, K. Ostrikov, A.K.M.A. Islam, S.H. Naqib, First-
principles study of elastic, electronic, optical and thermoelectric properties
of newly synthesized K2Cu2GeS4 chalcogenide, Journal of Alloys and
Compounds, 781 (2019) 37-46. IF: 3.779, SJR: 1.07 [Q1].
10. Mehri Ghasemi , Miaoqiang Lyu, Md Roknuzzaman, Jung-Ho Yun,
Mengmeng Hao, Dongxu He, Yang Bai, Peng Chen, Paul V. Bernhardt,
Kostya (Ken) Ostrikov, Lianzhou Wang, Phenethylammonium bismuth
halides for low-toxic, stable and solution-processable optoelectronics
beyond lead halide perovskites, Journal of Materials Chemistry A, 7 (2019)
20733. IF: 10.733, SJR: 3.37 [Q1].
Conference Presentations:
1. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Tuquabo
Tesfamichael, Towards lead-free inorganic and hybrid perovskites for
photovoltaics and optoelectronics, Oral Presentation, 47th Chemeca
Conference, 30 Sep to 3 Oct 2018, Queenstown, New Zealand.
2. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Tuquabo
Tesfamichael, Insight into lead-free hybrid double perovskites for
photovoltaics and optoelectronics, Oral Presentation, 48th Chemeca
Conference, 29 Sep to 2 Oct 2019, Sydney, Australia.
vi Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Table of Contents
Keywords .................................................................................................................................. i
Abstract .................................................................................................................................... ii
List of Publications .................................................................................................................. iv
Table of Contents .................................................................................................................... vi
List of Figures ......................................................................................................................... ix
List of Tables .......................................................................................................................... xv
List of Abbreviations ............................................................................................................ xvii
Statement of Original Authorship ......................................................................................... xix
Acknowledgements ................................................................................................................ xx
Chapter 1: Introduction ...................................................................................... 1
1.1 Background .................................................................................................................... 1
1.2 Research Problem .......................................................................................................... 3
1.3 Aims and Objectives ...................................................................................................... 3
1.4 Thesis Outline ................................................................................................................ 6
1.5 References ...................................................................................................................... 7
Chapter 2: Literature Review ........................................................................... 15
2.1 Concept of Perovskite Materials .................................................................................. 15 2.1.1 Perovskite and Perovskite Structure .................................................................. 15 2.1.2 Crystal Structure of Perovskite Materials .......................................................... 15 2.1.3 Classification of Perovskite Materials ............................................................... 17
2.2 Physical Properties and Critical Parameters ................................................................ 18
2.3 Potential Applications of Perovskites .......................................................................... 18
2.4 Double Perovskites ...................................................................................................... 21 2.4.1 Crystal Structure of Double Perovskites ............................................................ 21 2.4.2 Properties and Potential applications of Double Perovskites ............................. 22
2.5 References .................................................................................................................... 23
Chapter 3: Insight into Cs based Inorganic Perovskites CsBX3 (B = Pb, Sn,
Ge; X = I, Br, Cl) ...................................................................................................... 29
3.1 Statement of Contribution ............................................................................................ 30
3.2 Abstract ........................................................................................................................ 32
3.3 Introduction .................................................................................................................. 32
3.4 Results and Discussion ................................................................................................. 34 3.4.1 Structural Properties .......................................................................................... 34 3.4.2 Electronic Properties .......................................................................................... 35 3.4.3 Optical Properties .............................................................................................. 37 3.4.4 Mechanical Properties ........................................................................................ 39
3.5 Lead Free Perovskites .................................................................................................. 41
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics vii
3.6 Conclusion ....................................................................................................................43
3.7 Computational Methods................................................................................................43
3.8 References ....................................................................................................................44
3.9 Supplementary Information ..........................................................................................47
Chapter 4: Insight into MA based Hybrid Perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl) ...................................................................................................... 63
4.1 Statement of Contribution .............................................................................................64
4.2 Abstract .........................................................................................................................66 4.2.1 Graphical Abstract ..............................................................................................66
4.3 Introduction ..................................................................................................................67
4.4 Computational Methods................................................................................................68
4.5 Results and Discussion .................................................................................................69 4.5.1 Structural Properties ...........................................................................................69 4.5.2 Electronic and Optical Properties .......................................................................71 4.5.3 Mechanical Properties ........................................................................................76
4.6 Conclusions ..................................................................................................................79
4.7 Acknowledgements.......................................................................................................80
4.8 References ....................................................................................................................80
4.9 Supplementary Information ..........................................................................................85
Chapter 5: Insight into FA based Hybrid Perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl) ...................................................................................................... 91
5.1 Statement of Contribution .............................................................................................92
5.2 Abstract .........................................................................................................................94 5.2.1 Graphical Abstract ..............................................................................................94
5.3 Introduction ..................................................................................................................95
5.4 Computational Methods................................................................................................97
5.5 Results and Discussion .................................................................................................98 5.5.1 Structural Properties ...........................................................................................98 5.5.2 Electronic and Optical Properties .......................................................................99 5.5.3 Elastic Properties ..............................................................................................105
5.6 Conclusion ..................................................................................................................107
5.7 Acknowledgements.....................................................................................................108
5.8 References ..................................................................................................................108
5.9 Supplementary Information ........................................................................................113
Chapter 6: Insight into Double Perovskites ABiCuX6 [A = Cs2, (MA)2, (FA)2,
CsMA, CsFA, MAFA]............................................................................................ 121
6.1 Statement of Contribution ...........................................................................................122
6.2 Abstract .......................................................................................................................124
6.3 Introduction ................................................................................................................124
6.4 Results and Discussion ...............................................................................................126 6.4.1 Structural Properties .........................................................................................126
viii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
6.4.2 Electronic Properties ........................................................................................ 127 6.4.3 Optical Properties ............................................................................................ 131
6.5 Conclusion ................................................................................................................. 133
6.6 Computational Methods ............................................................................................. 133
6.7 Acknowledgements .................................................................................................... 134
6.8 References .................................................................................................................. 134
6.9 Supplementary Information ....................................................................................... 138
Chapter 7: Conclusions.................................................................................... 149
7.1 Conclusions ................................................................................................................ 149
7.2 Limitations and Future Recommendations ................................................................ 151
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics ix
List of Figures
Figure 1.1. Periodic table showing the possible replacements of Pb (red colour)
and I (yellow colour) in inorganic CsPbI3 and hybrid MAPbI3 and
FAPbI3 perovskites. ................................................................................. 4
Figure 1.2. Periodic table showing the possible trivalent (blue boxes) and
monovalent (red box) elements to form double perovskite structure
because of the replacement of Pb from perovskite by complex
substitution. ............................................................................................. 5
Figure 2.1. Unit cell of cubic ABX3 perovskite structure. ......................................... 15
Figure 2.2. The unit cell of CH3NH3PbI3 material [30]. ............................................ 16
Figure 2.3. Structure of a perovskite with a chemical formula ABX3. The red
spheres are X atoms, the blue spheres are B-atoms and the green
spheres are the A-atoms [4]. .................................................................. 16
Figure 2.4. Octahedron in perovskite crystal structure [7]......................................... 17
Figure 2.5. Variation of efficiency over time in different solar cell technology
[19]. ....................................................................................................... 18
Figure 2.6. The change in Power Conversion Efficiency of perovskite solar cells
compared to other types of photovoltaics over recent years. (Data
collected from “www.ossila.com”) [1].................................................. 19
Figure 2.7. Potential optoelectronic properties and applications of typical
perovskite ABX3 (acronyms: QD: quantum dots, NW: nanowire,
NR: nanorod, NC: nanocrystal, MC: millimeter-scale crystal, NP:
nanoparticle, PL: photoluminescence, EL: electroluminescence,
LEDs: light-emitting diodes, FET: field-effect transistor and LECs:
light-emitting electrochemical cells) [25]. ............................................ 20
Figure 2.8. Polyhedral model of the conventional unit cell of a double
perovskites [43]. .................................................................................... 22
Figure 3.1. Unit cells of the considered cubic metal halide perovskites CsBX3 (B
= Sn, Ge; X = I, Br, Cl) as compared with Pb-based compounds
CsPbX3 (X = I, Br, Cl). Replacement of halogen atoms is showing
from left to right while the replacement of Pb is showing from top
to bottom. The molecular models are optimized by DFT
calculations. ........................................................................................... 34
Figure 3.2. Variation of lattice parameter due to the replacement of atoms by
similar atoms. The lattice parameter is seen to change periodically
depending upon the size of atoms in the unit cell. ................................ 35
Figure 3.3. Variation of the electronic band gap due to the replacement of atoms
by similar atoms. ................................................................................... 36
Figure 3.4. Calculated optical absorption of perovskites CsBX3 (B = Pb, Sn, Ge;
X = I, Br, Cl) as a function of incident photon energies. ...................... 38
x Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Figure 3.5. Calculated optical conductivity of perovskites CsBX3 (B = Pb, Sn,
Ge; X = I, Br, Cl) as a function of incident photon energies. ................ 38
Figure 3.6. Variation of the Pugh’s ratio of the perovskite with different
composition. The pink dashed line separates the ductile materials
from brittle. ............................................................................................ 41
Figure 3.7. Variation of Pugh’s ratio for different combination of I and Br in
CsGe(I1-xBrx)3. CsGe(I0.7Br0.3)3, CsGe(I0.1Br0.9)3 and CsGeBr3 are
ductile while the others are brittle and the maximum ductility is
found in CsGe(I0.7Br0.3)3. ....................................................................... 42
Figure S3.1. Variation of the unit cell volume due to the replacement of atoms
by similar atoms. ................................................................................... 48
Figure S3.2. Variation of optical band gap due to the replacement of atoms by
similar atoms. The data of optical band gap are taken from
references [24], [32], [22] and [16]. ...................................................... 50
Figure S3.3. Electronic band structure of perovskites CsBX3 (B = Pb, Sn, Ge; X
= I, Br, Cl). ............................................................................................ 51
Figure S3.4. Total and partial densities of states of perovskites CsBX3 (B = Pb,
Sn, Ge; X = I, Br, Cl). The Fermi level EF is set at 0 eV. ..................... 52
Figure S3.5. Calculated real part of dielectric function of perovskites CsBX3 (B
= Pb, Sn, Ge; X = I, Br, Cl) as a function of incident photon energy.
............................................................................................................... 53
Figure S3.6. Calculated imaginary part of dielectric function of perovskites
CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) as a function of incident
photon energy. ....................................................................................... 53
Figure S3.7. Calculated reflectivity of perovskites CsBX3 (B = Pb, Sn, Ge; X =
I, Br, Cl) as a function of incident photon energy. ................................ 54
Figure S3.8. Calculated refractive index of perovskites CsBX3 (B = Pb, Sn, Ge;
X = I, Br, Cl) as a function of incident photon energy. ......................... 54
Figure S3.9. Calculated extinction coefficient of perovskites CsBX3 (B = Pb, Sn,
Ge; X = I, Br, Cl) as a function of incident photon energy. .................. 55
Figure S3.10. Periodic change of optical properties due to replacement of
halogen atoms. (a) Calculated optical absorption of perovskites
CsGeX3 (X = I, Br, Cl) as a function of incident photon energies.
(b) Calculated optical conductivity of perovskites CsGeX3 (X = I,
Br, Cl) as a function of incident photon energies. ................................. 55
Figure S3.11. Change in Poisson ratio due to the replacement of atoms by similar
atoms...................................................................................................... 56
Figure S3.12. Change in Poisson ratio for the solid solutions CsGe(I1-xBrx)3,
where x = 0-1 at the increment of 0.1. ................................................... 57
Figure 4.1. Crystal structure of MASnI3 as an example of the crystal structure of
organic-inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br,
Cl). (a) Schematic view of the unit cell of MASnI3. (b) The two
dimensional view of the primitive cell of MASnI3. .............................. 70
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xi
Figure 4.2. The uniform variation of the structural properties of the considered
organic-inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br,
Cl). (a) Variation of the lattice parameter because of the
replacement of atoms by similar atoms. (b) Periodic reduction of
the unit cell volume because of the replacement of atoms by similar
atoms. This change of unit cell volume can affect other properties
of these materials. .................................................................................. 70
Figure 4.3. (a) Calculated electronic band gap of the considered organic-
inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). The
electronic band gap is seen to increase due to the replacement of I
by Br and Cl. (b) Calculated dielectric constant of the considered
organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). The
maximum dielectric constant is observed for MASnI3 among the
considered nine perovskites. The dielectric constant is seen to
decrease due to the replacement of I by Br and Cl. ............................... 73
Figure 4.4. The calculated (a) real part of the dielectric function and (b)
imaginary part of the dielectric function of perovskites MABX3 (B
= Pb, Sn, Ge; X = I, Br, Cl) as a function of incident energies up to
30 eV. .................................................................................................... 74
Figure 4.5. Calculated optical properties of the considered organic-inorganic
perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) The
calculated optical absorption as a function of incident photon
energies. (b) The calculated optical conductivity as a function of
incident photon energies. ....................................................................... 75
Figure 4.6. Graphical representation to separate ductile materials from brittle for
the considered organic-inorganic perovskites MABX3 (B = Pb, Sn,
Ge; X = I, Br, Cl). The red dashed line separates the ductile
materials from brittle. (a) The calculated Pugh’s ratio for the
considered compounds. (b) The calculated Poisson ratio for the
considered perovskites. ......................................................................... 78
Figure S4.1. Calculated electronic band structure along the highly symmetric
directions of the Brillouin zone of perovskites MABX3 (B = Pb, Sn,
Ge; X = I, Br, Cl). Electronic states in the valence band are
indicated by blue colour while the electronic states of the
conduction bands are indicated by green colour. The red dashed line
along the energy of 0 eV indicates the Fermi level, EF. The
considered compounds are direct band gap semiconductors where
the lowest band gap is observed at Z point. ........................................... 86
Figure S4.2. Total and partial densities of states of perovskites MABX3 (B = Pb,
Sn, Ge; X = I, Br, Cl). The black dashed line along the energy of 0
eV indicates the Fermi level, EF. The contribution to the total DOS
of C-2s and N-2s near the Fermi level is not significance, therefore
it is not shown in the figure. The total DOS at the upper part of the
valence band mainly comes from the p orbital of halogen atoms (I-
5p, Br-4p, Cl-3p) indicated by blue colour. Also, the total DOS at
the lower part of the conduction band mainly comes from the p
orbital of B atoms (Pb-6p, Sn-5p, Ge-4p) indicated by green colour.
............................................................................................................... 87
xii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Figure S4.3. The calculated (a) reflectivity (b) refractive index and (c) extinction
coefficient of perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl)
as a function of incident photon energies up to 30 eV. ......................... 88
Figure 5.1. Structural properties for the investigated FA-based hybrid
perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) A model
crystal structure of the unit cell of FA-based hybrid perovskites. (b)
Periodic change in the calculated lattice parameter for the
investigated compounds. ....................................................................... 98
Figure 5.2. Variation of the studied electronic parameters for the selected FA-
based perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a)
Calculated band gap. (b) Calculated dielectric constant. ...................... 99
Figure 5.3. Comparison of the calculated carrier effective masses for the studied
FA-based hybrid perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br,
Cl). ....................................................................................................... 101
Figure 5.4. Calculated dielectric function of the studied FA-based hybrid halide
perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) Calculated
real part of the dielectric function. (b) Calculated imaginary part of
the dielectric function. ......................................................................... 102
Figure 5.5. Variation of the calculated optical properties for the studied FA-
based hybrid perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl).
(a) Calculated absorption spectra. (b) Calculated optical
conductivity. ........................................................................................ 103
Figure 5.6. Identification of ductile and brittle materials for the considered FA-
based hybrid halide perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br,
Cl). (a) Calculated Pugh’s ratio. (b) Calculated Poison ratio. ............. 107
Figure S5.1. Calculated electronic band structure for FA-based Pb-free non-toxic
hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well
as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl) along high
symmetry directions of the Brillouin zone. The considered path in
reciprocal space is X(0.5,0,0)-R(0.5,0.5,0.5)-M(0.5,0.5,0)-(0,0,0)-R(0.5,0.5,0.5). Electronic states corresponding to the valence band
maximum (VBM) are indicated by the green colour line, while the
electronic states in corresponding to the conduction band minimum
(CBM) are indicated by the pink colour line. The red dashed line
along the energy of 0 eV indicates the Fermi level. Both the VBM
and CBM are seen for the same k-vector (at the R point of the
Brillouin zone) indicating that the considered compounds are direct
band gap semiconductors. ................................................................... 114
Figure S5.2. Calculated total and partial densities of states for FA-based Pb-free
non-toxic hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br,
Cl) as well as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl).
The partial DOSs from H-1s, C-2s and N-2s are not contributed
significantly to the total DOS near the Fermi level, therefore it is
not shown in the figure. The total DOS at the valence band
maximum (VBM) is primarily contributed by the p states of X
atoms (5p states of I, 4p states of Br, 3p states of Cl) and the
corresponding curves are indicated by green colour. Also, the total
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xiii
DOS at the conduction band minimum (CBM) is primarily
contributed by the p states of B atoms (6p states of Pb, 5p states of
Sn, 4p states of Ge) and the corresponding curves are indicated by
the pink colour. .................................................................................... 115
Figure S5.3. Calculated (a) reflectivity (b) refractive index and (c) extinction
coefficient for FA-based Pb-free non-toxic hybrid halide
perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well as for its Pb-
based counterparts FAPbX3 (X = I, Br, Cl) with respect to the
energy of incident photons with energies ranged from 0 to 15 eV. .... 116
Figure 6.1. Unit cell of double perovskites Cs2BiCuI6 as an example of the
crystal structure of the considered double perovskites ABiCuX6 [A
= Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl]. .............. 127
Figure 6.2. Comparison of the electronic band gap and dielectric constant of the
selected group of double perovskites ABiCuX6 [A = Cs2, (MA)2,
(FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl]. (a) Electronic band
gap of the materials determined by using GGA-PBE approach. (b)
Calculated dielectric constant or static dielectric function for the
considered double perovskites. ........................................................... 128
Figure 6.3. Calculated electronic properties of the considered double perovskite
(FA)2BiCuI6. (a) Electronic band structure along the high symmetry
direction of the Brillouin zone having path (0,0,0)-F(0,0.5,0)-
Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The bands calculated by GGA-PBE are indicated by blue color whereas the bands calculated by
HSE06 are indicated by pink color. The valence band maximum
(VBM) is seen at F point whereas the conduction band minimum
(CBM) is observed at Z point of the Brillouin zone indicating that
it is an indirect band gap semiconductor. (b) Calculated total and
partial densities of states. The Cu-3d states (green color curve) and
the I-5p (blue color curve) states are seen as the main contributors
towards VBM whereas the Bi-6p (pink color curve) and I-5p (blue
color curve) states are mostly contributed towards CBM. .................. 130
Figure 6.4. Comparison of the optical properties of double perovskites ABiCuX6
[A = Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl] along
the incident electromagnetic radiation of energy from 0 to 5eV. (a)
Calculated dielectric function (real part). (b) Calculated absorption
coefficient. (c) Calculated optical conductivity. ................................. 132
Figure S6.1. Crystal structure of organic-inorganic double perovskites. The
structure of inorganic double perovskite Cs2BiCuI6 was initially
drawn. Then the Cs atoms were replaced by MA
(methylammonium) or FA (formamidinium) to get the structure of
required organic-inorganic double perovskites. (a) Three and two
dimensional views of the unit cell of inorganic double perovskite
Cs2BiCuI6. (b) Three and two dimensional views of the primitive
cell of inorganic double perovskite Cs2BiCuI6. (c) Molecular
structure of CH3NH3 (or, MA) and CH(NH2)2 (or, FA). The carbon-
nitrogen double bond is also shown in the figure. ............................... 139
xiv Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Figure S6.2. Calculated electronic band structure of the considered double
perovskites ABiCuI6 [A = Cs2, (MA)2, (FA)2; X = I, Br, Cl] along
the high symmetry direction of the Brillouin zone having path
(0,0,0)-F(0,0.5,0)-Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The green band indicates the highest energy valence band in which the
valence band maximum (VBM) is observed. On the other hand, the
pink band indicates the lowest energy conduction band in which the
conduction band minimum (CBM) is observed. ................................. 141
Figure S6.3. Calculated electronic band structure of the considered double
perovskites ABiCuI6 [A = CsMA, CsFA, MAFA; X = I, Br, Cl]
along the high symmetry direction of the Brillouin zone having path
(0,0,0)-F(0,0.5,0)-Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The green band indicates the highest energy valence band in which the
valence band maximum (VBM) is observed. On the other hand, the
pink band indicates the lowest energy conduction band in which the
conduction band minimum (CBM) is observed. ................................. 142
Figure S6.4. Calculated total and partial densities of states of the considered
double perovskites ABiCuI6 (A = Cs2, (MA)2, (FA)2; X = I, Br, Cl).
The Cu-3d states (blue colour curve) and the p states of halogen (I-
5p, Br-4p, Cl-3p) are seen to contribute towards valence band
maximum (VCM). The p states of halogen is also contributed to
conduction band minimum (CBM). However, the maximum
contribution towards the CBM is mainly come from Bi-6p states
(pink colour curve). ............................................................................. 143
Figure S6.5. Calculated total and partial densities of states of the considered
double perovskites ABiCuI6 (A = CsMA, CsFA, MAFA; X = I, Br,
Cl). The Cu-3d states (blue colour curve) and the p states of halogen
(I-5p, Br-4p, Cl-3p) are seen to contribute towards valence band
maximum (VCM). The p states of halogen are also contributed to
conduction band minimum (CBM). However, the maximum
contribution towards the CBM is mainly come from Bi-6p states
(pink colour curve). ............................................................................. 144
Figure S6.6. Calculated (a) Imaginary part of dielectric function and (b)
Reflectivity of the considered double perovskites ABiCuI6 (A = A
= Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl). The
variation of the imaginary part of the dielectric function is quite
similar for the considered compounds except an intense peak of
(FA)2BiCuI6 and (FA)2BiCuI6 at the energy 3.75 eV and 4.3 eV,
respectively. The reflectivity of the considered compounds is seen
to vary between 12 to 30 % implies that the materials have low
reflectivity for incoming solar radiation. ............................................. 145
Figure S6.7. Calculated (a) Refractive Index and (b) Extinction Coefficient of
the considered double perovskites ABiCuI6 (A = Cs2, (MA)2, (FA)2,
CsMA, CsFA, MAFA; X = I, Br, Cl). The variation of the refractive
index is quite similar for the considered compounds. However, an
intense peak is observed in the extinction coefficient spectrum of
(FA)2BiCuI6 double perovskite, this suggests the potential of the
material to be used in solar cell and other optoelectronic devices. ..... 146
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xv
List of Tables
Table 3.1. The calculated and the available theoretical and experimental values
of elastic constants Cij (GPa), bulk modulus B (GPa), shear modulus
G (GPa), Young’s modulus Y (GPa), Pugh’s ratio B/G and Poisson
ratio 𝜐 of cubic perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .... 40
Table 3.2. Summary of the key properties to predict lead free perovskites. .............. 41
Table S3.1. The calculated and the available theoretical and experimental values
of lattice parameter a (in Å) and the calculated unit cell volume V (in Å3) of perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .............. 48
Table S3.2. The calculated electronic band gap using GGA-PBE along with
available theoretical results and experimental optical bandgap for
perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) in eV. .................... 49
Table S3.3. The calculated elastic constants Cij (in GPa), bulk modulus B (in
GPa), shear modulus G (in GPa), Young’s modulus Y (in GPa),
Pugh’s ratio B/G and Poisson ratio 𝜐 of perovskites solid solutions CsGe(I1-xBrx)3. ....................................................................................... 56
Table 4.1. The calculated elastic constants Cij (GPa), bulk modulus B (GPa),
shear modulus G (GPa), Young’s modulus Y (GPa), Pugh’s ratio
B/G and Poisson ratio 𝜐 of hybrid perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .................................................................................. 77
Table S4.1. The calculated and the available theoretical and experimental values
of lattice parameter a (in Å) and the calculated unit cell volume V (in Å3) of organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). ......................................................................................................... 85
Table S4.2. The calculated electronic band gap, Eg in eV and dielectric constants,
1(0) of organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). ......................................................................................................... 85
Table 5.1. The calculated independent elastic constants C11, C12 and C44 in GPa,
bulk modulus B in GPa, shear modulus G in GPa, Young’s modulus
Y in GPa, Pugh’s ratio B/G and Poisson ratio 𝜐 for the considered FA-based hybrid halide perovskites FABX3 (B = Pb, Sn, Ge; X = I,
Br, Cl). ................................................................................................. 106
Table S5.1. Calculated values of lattice parameter a in Å and unit cell volume V in Å3, along with available theoretical and experimental results and enthalpy of formation H in KJ/mol for FA-based Pb-free non-toxic
hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well
as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl). ................... 113
Table S5.2. Electronic band gap Eg in eV, calculated by GGA-PBE and HSE06
approach as well as the available experimental results, dielectric
constants 1(0), carrier effective mass for electrons 𝑚𝑒 ∗ and holes 𝑚ℎ ∗ in electron mass units for FA-based Pb-free non-toxic hybrid
xvi Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well as for
its Pb-based counterparts FAPbX3 (X = I, Br, Cl). ............................. 113
Table S6.1. The calculated enthalpy of formation, H (KJ/mol), electronic band
gap, Eg (eV) for GGA-PBE and hybrid HSE06 potential, and
dielectric constants, 1(0) of inorganic-organic double perovskites
ABiCuX6: [A = Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I,
Br, Cl]. ................................................................................................. 140
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xvii
List of Abbreviations
Ab initio From the beginning (Latin Term “ab initio”)
AO Atomic Orbital
BF Bloch Function
BFGS Broyden–Fletcher–Goldfarb–Shanno
CASTEP Cambridge Serial Total Energy Package
CO Crystalline Orbital
DFT Density Functional Theory
DOS Density of States
EHTB Extended Huckel Tight-Binding
EL Electroluminescence
ETB Empirical Tight Binding
FET Field Effect Transistor
GGA Generalized Gradient Approximation
GTF Gaussian Type Function
HF Hartree-Fock
HSE Heyd-Scuseria-Ernzerhof
HTM Hole Transporting Material
IR Infra-Red
LDA Local Density Approximation
LEC Light Emitting Electro-Chemical Cell
LED Light Emitting Diode
MC Millimeter-Scale
NC Nanocrystal
NP Nano Particle
NR Nanorod
NW Nanowire
PAW Projector Augmented Wave
PBE Perdew-Burke-Ernzerhof
PCE Power Conversion Efficiency
PL Photoluminescence
xviii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
PP-PW Pseudo-Potential Plane Wave
QSGW Quasiparticle Self-consistent GW
UV Ultra Violet
VASP Vienna Ab initio Simulation Package
VCSEL Vertical-Cavity Surface-Emitting laser
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xix
Statement of Original Authorship
The work contained in this thesis has not been previously submitted to meet
requirements for an award at this or any other higher education institution. To the best
of my knowledge and belief, the thesis contains no material previously published or
written by another person except where due reference is made.
Signature:
Date: _________________________
QUT Verified Signature
xx Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics
Acknowledgements
I would like to express my sincere and deepest gratitude to my principal
supervisor Prof. Kostya (Ken) Ostrikov for giving me the opportunity to conduct this
research as well as his continuous support, excellent motivation and constructive
guideline throughout my PhD. He always provided quick feedback and it helped me a
lot to go smoothly in this research journey to reach my goal.
I would like to convey my gratitude to my associate supervisors Prof. Hongxia
Wang and Dr. Tuquabo Tesfamichael for their support and feedback throughout my
PhD candidature. The critical and constructive feedback from Dr. Tuquabo is
appreciated and it helped me a lot for the improvement of my papers.
Furthermore, I would like to thank my collaborators Prof. Aijun Du, Prof. Jose
Alarco, Prof. Cheng Yan, Dr. Kimal Chandula Wasalathilake and Ms. Chunmei Zhang
for their support and feedback to my papers. Special thanks to Prof. Jose Alarco for
providing me the access of Computational Chemistry Lab and access to Materials
Studio simulations software. Also, I wish to express my gratitude to my collaborator
Prof. Lianzhou Wang from University of Queensland for his comments and feedback
on our paper.
I would also like to acknowledge the Queensland University of Technology
(QUT), Science and Engineering faculty (SEF), School of Chemistry, Physics and
Mechanical Engineering (CPME) for their facility and financial support through
QUTPRA scholarship and HDR Tuition Fee sponsorship. The High Performance
Computing Facility at QUT is sincerely acknowledged. In addition, I would like to
acknowledge the Nanoscience Discipline at QUT for their additional financial support
to attend in conferences.
I wish to express my thanks to my colleagues at QUT, Dr. Mohammad Saidul
Islam, Dr. Fawad Ali, Mr. Jickson Joseph and Dr. Karthika Prasad for their helpful
discussion and suggestion. Special thanks to my colleague, Dr. Kimal for his
cooperation, help and support at QUT. Also, the partial proofreading of my thesis by
Dr. Kimal is greatly appreciated. I also acknowledge my friends, colleagues at QUT
who gave me support and encouragement throughout my PhD candidature.
Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xxi
I wish to extend my sincere thanks to my family, friends, colleagues and well-
wishers in Bangladesh and in Brisbane for their inspiration. Special thanks to my
parents for their endless support and prayers, not only in this PhD journey, but also in
my whole life. I also wish to express my thanks to my younger brother for his love and
support.
Last but not least, I wish to express my immense love and gratitude to my wife
for her unconditional support, love and care. I wish to express my heartfelt love and
blessing to my son, his love, thinking and activities always give me pleasure and
refresh my mind and it gave be mental support throughout this PhD journey.
Chapter 1: Introduction 1
Chapter 1: Introduction
1.1 BACKGROUND
Inorganic and organic-inorganic hybrid halide perovskites having the general
formula ABX3 (A = inorganic or organic cation, B = metal cation, and X = halogen
anion) have great potential for commercial applications due to natural abundance and
low cost. [1]. Furthermore, the materials can easily be converted into thin films as well
as some other suitable crystalline forms for device applications such as nanocrystals,
nanorods, nanowires and nanoparticles [1-5]. Therefore, it is expected that the
technology based on perovskites would be more cheaper and efficient than silicon-
based technology [6]. On the other hand, halide perovskites are an emerging class of
materials because of their extraordinary physical properties such as direct band gap
with tunable bandgap characteristics, high absorption coefficient with broad
absorption spectrum, small electron and hole effective masses, high charge carrier
mobility with long charge diffusion lengths, low non-radiative recombination rates,
high photoconductivity, and dominant point defects [2-7]. Because of these
exceptional optoelectronic properties, this group of semiconductors has the potential
to be used in a wide range of device applications including solar cells, light-emitting
diodes (LEDs), lasers, catalysts, x-ray detectors, field-effect transistors (FET),
photodetectors, photoluminescence (PL), electroluminescence (EL), light-emitting
electrochemical cells (LECs) and solar-to-fuel conversion devices [2-17].
Perovskites have been well known for many years [18, 19], but they have
recently become more popular after the discovery of perovskites solar cells by
Miyasaka et al. in 2009 [20]. Following the discovery, the organic-inorganic hybrid
methylammonium lead trihalides CH3NH3PbX3 (X = Cl, Br, I) have been subjected to
a huge number of studies to demonstrate their potential in photovoltaic application
[21-34]. The perovskite solar cell, also known as the third generation solar cell, has
the potential to bring about a major change in the solar cell technology. Recently, the
halide hybrid perovskites have drawn significant attention from the science community
because of their fastest growing power conversion efficiency (PCE) in solar cells, as
the PCE of the perovskite solar cells has jumped from 3.8 % [20] in 2009 to 23.7%
[35] in 2018. The highest efficiency reported for perovskite solar cells so far have been
2 Chapter 1: Introduction
found mainly with methylammonium lead (Pb) halide materials [32-39]. Efficient
photovoltaic cells using inorganic–organic lead halide perovskite materials appear
particularly promising for next generation solar cell devices due to their high power
conversion efficiency and low production cost [28].
On the other hand, the possibility of inorganic perovskites in device applications
cannot be ignored as it has the similar properties like organic perovskites and the
inorganic perovskites are expected to be more stable than organic one. However, it is
true that inorganic perovskites show a less PCE than its organic counterparts [40].
Research on inorganic metal halide perovskites also going on and a lot of works have
been done in recent years [8-11, 40-47] to reveal its properties and potential to be used
in a wide range of optoelectronic devices beyond solar cells.
In general, a good light absorbing material is also a good emitter of light. So, the
science community is searching other optoelectronic applications of organometallic
perovskite materials and they have already found the potentiality of these materials in
some other applications [48-51]. Perovskite materials may be good candidates for use
in light-emitting diodes (LEDs) because of their high photoluminescence quantum
efficiencies [27]. Moreover, it is possible to get colourful light emitting diodes as the
band gap of the perovskite materials can be tuned to cover almost the entire visible
spectrum by changing the composition of the content of halogens [38]. Also, the mixed
halide perovskite materials show surprisingly clean semiconducting behaviour and can
be used as optically pumped vertical-cavity surface-emitting lasers (VCSELs) [50].
Furthermore, a new class of materials known as double perovskites having
general formula A2BʹBʹʹX6 (A is a relatively large cation, Bʹ and Bʹʹ are either trivalent
or monovalent cations, and X is either oxygen or halogen) have similar
crystallographic structure like halide perovskites [52]. Therefore, it is expected that
double perovskites also possess similar optoelectronic properties like halide perovskite
and have the potential to be used in a wide range of optoelectronic applications
including solar cells. In addition, halide double perovskites have become more popular
in the community of photovoltaic research because of its potential to overcome the
instability and toxicity issues associated with Pb-based hybrid perovskites [52]. As a
result, several works have been reported on double perovskites [53-61], however the
findings are still far from expectations [62, 63].
https://en.wikipedia.org/wiki/Light-emitting_diodehttps://en.wikipedia.org/wiki/Quantum_efficiencyhttps://en.wikipedia.org/wiki/Quantum_efficiency
Chapter 1: Introduction 3
1.2 RESEARCH PROBLEM
Although a substantial amount of research has been done within relatively short
time, the halide perovskites are under development as some difficulties are still
remaining. For the commercial applications of perovskite materials, low cost, high
efficiency and especially long term stability is needed [64]. The studied literature
suggests that halide hybrid perovskite MAPbI3 and other Pb-containing hybrid
perovskites have been studied more compared to others [21-37] as the materials show
high power conversion efficiencies (PCEs). However, the problem with the materials
is that they do not show sufficient stability [65, 66] as well as they contains lead (Pb)
which is toxic [67] and a potential threat to the environment [68]. Also, it has been
recently reported that self-degradation of iodine may limit the lifetime of iodine-
containing perovskites [69]. Therefore, it is essential to obtain Pb-free perovskites,
which could be used for energy applications. Furthermore, Pb-containing organic-
inorganic perovskites MAPbX3 (X = I, Br, Cl) have a low relative dielectric constant.
This is a significant limitation for solar cell application because the low dielectric
constant increases the charge carrier recombination rate and affects the overall
performance of solar cells [70].
Therefore, the materials still face a huge challenge in large scale
commercialization because of the structural instability against moisture/air and
temperature as well as the toxicity of lead (Pb) [64, 71]. It is of utmost importance to
find stable and nontoxic perovskites for the further development of perovskites solar
cells. The insatiability issue of the halide perovskites can be reduced by using carbon
encapsulation, multi-cation substitution as well as incorporation of hydrophobic
moieties [72]. However, the only way to address the toxicity of perovskite materials is
the substitution of Pb by non-toxic elements [71].
1.3 AIMS AND OBJECTIVES
The main aim of the present study is to develop Pb-free non-toxic perovskites
for photovoltaic and optoelectronic applications. To achieve this goal it is required to
replace Pb from perovskites by other suitable elements. There are two possible routes
of substitution of Pb by non-toxic elements such as simple substitution (or, direct
substitution) and complex substitution.
4 Chapter 1: Introduction
Route 1 (simple substitution or direct substitution): Pb is a group IVA
element in the periodic table. Therefore, the easiest way is to use other group
IVA elements like tin (Sn) and germanium (Ge) for the replacement of Pb.
Substitution of Pb by Sn and Ge can be done for both inorganic and hybrid
perovskites. Pb and its possible replacements are highlighted by red colour
in the periodic table as shown in Figure 1.1. In addition, the halogen atom
can be changed by other suitable halogens for the tuning of the properties of
perovskites. The potential possible replacement of iodine (I) are bromine
(Br) and chlorine (Cl) and are highlighted by yellow colour in Figure 1.1.
Also, the considered A site occupants inorganic caesium (Cs) and Organic
methylammonium (MA) and formamidinium (FA) are indicated by green
colour in Figure 1.1.
Figure 1.1. Periodic table showing the possible replacements of Pb (red colour) and I
(yellow colour) in inorganic CsPbI3 and hybrid MAPbI3 and FAPbI3
perovskites.
Route 2 (complex substitution): A complex substitution of Pb in perovskites
can be addressed by a combination of a trivalent and a monovalent cations
to form a new structure known as double perovskites which can be
represented by the general formula, A2BʹBʹʹX6, where A is a relatively large
cation (typically Cs+), Bʹ and Bʹʹ are either trivalent or monovalent cations,
Chapter 1: Introduction 5
and X is either oxygen or halogen. For example, the replacement of Pb from
perovskite CsPbI3 by a combination of a trivalent element Bi (bismuth) and
a monovalent element Cu (copper) form a new structure Cs2BiCuI6 and it is
a double perovskite. The possible trivalent elements can be scandium (Sc),
yttrium (Y), antimony (Sb) and bismuth (Bi), the elements are indicated by
blue colour boxes in the periodic table as shown in Figure 1.2. Also, the
possible monovalent elements can be copper (Cu), silver (Ag) and gold (Au)
and are indicated by red colour box in Figure 1.2.
Figure 1.2. Periodic table showing the possible trivalent (blue boxes) and monovalent
(red box) elements to form double perovskite structure because of the
replacement of Pb from perovskite by complex substitution.
To achieve the goal of this study, the specific objectives can be summarised as below:
1. Investigations of the structural, electronic, optical and mechanical properties
of potential inorganic perovskites such as CsBX3 (B = Pb, Sn, Ge; X = I, Br,
Cl) to find best possible Pb-free inorganic perovskites for photovoltaics and
optoelectronics.
2. Studying the structural, electronic, optical and mechanical properties of MA
and FA based organic-inorganic hybrid perovskites ABX3 (A = MA, FA; B
= Pb, Sn, Ge; X = I, Br, Cl) to obtain effective alternatives of Pb in
photovoltaic and optoelectronic applications.
6 Chapter 1: Introduction
3. Calculations of the essential optoelectronic properties of a range of potential
Pb-free double perovskites to predict their suitability in solar cells and other
optoelectronic devices.
1.4 THESIS OUTLINE
This thesis represents a first-principles density functional theory (DFT)
investigations of the structural, electronic, optical, transport and mechanical properties
of different perovskite materials to find Pb-free materials for photovoltaic and
optoelectronic applications. Herein, the essential optoelectronic properties of
inorganic perovskites (Chapter 3), organic-inorganic hybrid perovskites (Chapter 4
and Chapter 5) and inorganic and hybrid double perovskites (Chapter 6) have been
investigated.
Chapter 1 introduces this research including the background of the research
topic, current problems and challenges as well as the aims and objectives of
this study.
Chapters 2 presents a literature review explaining the background and
current advancement in this field of research. The literature review starts
with the structure, classifications and properties of perovskite materials.
Following this, it presents a wide range of possible potential applications of
perovskites beyond solar cells. Furthermore, the advancement in the
discovery of Pb-free suitable perovskites for photovoltaics and
optoelectronics have been discussed here. In addition, the chapter examines
the up to date literature on the properties and potential applications of a new
group of materials known as double perovskites.
Chapter 3 reports the structural, electronic, optical and mechanical
properties of caesium (Cs) based inorganic perovskites, CsBX3 (B = Pb, Sn,
Ge; X = I, Br, Cl). This section is focused on the investigations of the
fundamental properties of perovskite materials to understand the
characteristics of the materials at atomic level. It also reports the possible
effective alternative of Pb in inorganic perovskites for photovoltaic and
optoelectronic applications.
Chapter 4 presents the studied structural, electronic, optical and mechanical
properties of methylammonium (MA) based organic-inorganic hybrid
Chapter 1: Introduction 7
perovskites, MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). This section is focused
on finding of the effective Pb-free alternatives of the most commonly
studied potential perovskites MAPbI3 for solar cell application. It also
describes other possible optoelectronic applications of MA based Pb-free
perovskites.
Chapter 5 demonstrates the structural, electronic, optical, transport and
mechanical properties of formamidinium (FA) based hybrid perovskites,
FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). This chapter explores possible
alternatives of Pb-based perovskites for photovoltaic and optoelectronic
applications. This section also describes transport properties like electron
and hole effective masses of the considered FA based hybrid perovskites.
Chapter 6 presents the optoelectronic properties of a new group of 18 non-
toxic lead-free organic-inorganic halide double perovskites, ABiCuX6 [A =
Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl] to predict their
suitability in photovoltaic and optoelectronic applications. This section also
describes the possible solution of finding Pb-free non-toxic materials for
photovoltaic and optoelectronic applications. The chapter also represents a
new approach based on the investigations of the optoelectronic properties
for a group of new hypothetical compounds to find suitable materials for
specific applications. Particularly, the incorporation of organic MA or FA
into Cs based inorganic double perovskites is described here and this
approach can provide opportunities to tune and enhance the photovoltaic
and optoelectronic properties of the materials.
Finally, chapter 7 summarizes the major findings of the present
investigations. The contribution towards achieving Pb-free non-toxic
perovskites for photovoltaics and optoelectronics are highlighted.
Considering the findings of the study, the important recommendations for
the further advancement of the field are presented.
1.5 REFERENCES
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(2015) 917-917.
8 Chapter 1: Introduction
[2] J. Chen, S. Zhou, S. Jin, H. Li, T. Zhai, Crystal organometal halide perovskites
with promising optoelectronic applications, Journal of Materials Chemistry C, 4
(2016) 11-27.
[3] Y. Yang, J.J.N.N. You, Make perovskite solar cells stable, Nature, 544 (2017) 155-
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[4] M.V. Kovalenko, L. Protesescu, M.I. Bodnarchuk, Properties and potential
optoelectronic applications of lead halide perovskite nanocrystals, Science, 358
(2017) 745-750.
[5] W.-J. Yin, T. Shi, Y. Yan, Unique Properties of Halide Perovskites as Possible
Origins of the Superior Solar Cell Performance, Advanced Materials, 26 (2014)
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[6] W. Zhang, G.E. Eperon, H.J.J.N.E. Snaith, Metal halide perovskites for energy
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Chapter 2: Literature Review 15
Chapter 2: Literature Review
2.1 CONCEPT OF PEROVSKITE MATERIALS
2.1.1 Perovskite and Perovskite Structure
A perovskite is a mineral that was first found in the Ural Mountains of Russia
by Gustav Rose in 1839 and is named after Russian mineralogist von Perovski who
was the founder of the Russian Geographical Society [1, 2]. On the other hand, a
perovskite structure or a perovskite material is a compound that has the same structure
like the perovskite mineral. The first discovered perovskite mineral is calcium titanium
oxide (CaTiO3) [2, 3]. However, a perovskite material is a material that has the same
crystallographic structure as perovskite mineral and having the form ABX3, where 'A'
and 'B' are two cations of very different sizes, and X is an anion that bonds to both [4].
2.1.2 Crystal Structure of Perovskite Materials
The inorganic and organic halide perovskites have been found in different phases
depending on the temperature [5]. However, the phases are equivalent to each other
according to the orientation of atoms in unit cell. At high temperature, these materials
have a cubic perovskite ABX3 structure with space group 𝑝𝑚3̅𝑚 having space group
number 221 [6]. The unit cell contains one formula unit, where the “A” atoms occupy
the corner positions (0, 0, 0) of the cube, “B” atoms occupy the body centred (½, ½,
½) positions and the “X” atoms occupy the face centred (½, ½, 0) positions as shown
in Figure 2.1.
Figure 2.1. Unit cell of cubic ABX3 perovskite structure.
https://en.wikipedia.org/wiki/Calcium_titanium_oxidehttps://en.wikipedia.org/wiki/Calcium_titanium_oxidehttps://en.wikipedia.org/wiki/Cationhttps://en.wikipedia.org/wiki/Anion
16 Chapter 2: Literature Review
Figure 2.2. The unit cell of CH3NH3PbI3 material [30].
The unit cell of CH3NH3PbI3 is shown in Figure 2.2 as an example of organic
perovskite materials. On the other hand, the ideal cubic-symmetry structure has the
“B” cation in 6-fold coordination, surrounded by an octahedron of anions and the “A”
cation in 12-fold cuboctahedral coordination as shown in Figure 2.3 [4].
Figure 2.3. Structure of a perovskite with a chemical formula ABX3. The red spheres
are X atoms, the blue spheres are B-atoms and the green spheres are the
A-atoms [4].
https://en.wikipedia.org/wiki/Cationhttps://en.wikipedia.org/wiki/Octahedronhttps://en.wikipedia.org/wiki/Anionhttps://en.wikipedia.org/wiki/Cuboctahedron
Chapter 2: Literature Review 17
Figure 2.4. Octahedron in perovskite crystal structure [7].
The slight buckling and distortion can produce several lower-symmetry distorted
versions, in which the coordination numbers of A cations, B cations or both are
reduced because the requirements of relat