என் அய்யாவிற்கு சமர்ப்பணம்
தேடிச் தசாறுநிேம் தின்று பல சின்னஞ்சிறு கதேகள் தபசி மனம் வாடிே் துன்பமிக உழன்று பிறர் வாட பலசசயல்கள் சசய்து நதர கூடி கிழப்பருவ சமய்தி சகாடும் கூற்றுக் கிதரசயனப்பின் மாயும் பல தவடிக்தக மனிேதர தபாதல நான் வீழ்வேனனன்று நினனத்தாவ ா - சுப்பிரமணிய பாரதிTable of Contents
List of Abbreviations and Acronyms……….... 10
List of Symbols……….... 12
1. Introduction………
13
1.1 Semiconductors: Heart of Electronic Devices……… 13
1.2 Metal-Halide Perovskites: The Game Changer………... 15
1.2.1 Crystal Structure of Metal Halide Perovskites………... 15
1.2.2 Limitations of 3D Metal-Halide Perovskites……….... 18
1.3 Two-Dimensional Layered Metal-Halide Perovskites………….... 19
1.4 Optoelectronic Properties of 2D Metal-Halide Layered
Perovskites…….………....………
24
1.4.1 Band Structure and Optical Properties……….. 24
1.4.2 Excellent Defect Tolerance………...
28
1.4.3 Tunable Exciton Binding Energy……….. 29
1.4.4 Charge-transport Dynamics…..……….…..
30
1.4.5 Narrow and Broadband Emission………...……..
31
1.5 Functionalizing the Organic Layer in 2D Metal-Halide Layered
Perovskites……….………
33
1.6 Brief Introduction to Layered Double Perovskites…………...….
37
1.7 Outline of this Thesis…………...………...…...
40
2. Low-cost and Eco-friendly Synthesis Route for the Fabrication of
Emitting Two-Dimensional Layered Perovskite Crystals...…… 51
2.1 Simple One-pot Synthesis of 2D Layered Perovskites……… 52
2.2 Structural Characterization of the 2D Lead-Bromide Layered
Perovskite Ensembles………
56
2.3 Optical Characterization of the 2D Lead-Bromide Layered
Perovskite Ensembles…………..………..………....
58
2.4 Mechanical Exfoliation of 2D Lead-Bromide Layered Perovskite
Ensembles...
64
2.5 Conclusions………..………..…………
71
Bibliography………...………..……… 72
3. Mapping the Fundamental Vibrational Modes of 2D Layered
Perovskites
by
Polarized
Ultralow
Frequency
Raman
Spectroscopy………...………
75
3.1 Preparation of 2D Layered Perovskite Single Platelets……….…
76
3.2 Raman Spectroscopy Analysis of 2D Layered Perovskite Single
Platelet………….……….…….. 80
3.3 Orientation and Polarization Dependent Raman Spectroscopy…..
83
3.4 Conclusions……..………....…..
91
Bibliography...……….……. 92
4. Designing Metal-Organic Frameworks towards Efficient White
Light-Emitting 2D Layered Perovskites………....……..……. 94
4.1.1 One-pot Synthesis of 2D Layered Perovskites……..………... 95
4.1.2 Structural Characterization of 2D Layered Perovskites ……... 97
4.1.3 Optical Characterization of the 2D Layered Perovskites….… 99
4.1.4 Density Functional Theory Calculations……..…...………… 104
4.1.5
Surface Characterization and Power Dependent PL Analysis.. 106
4.2 Effect of the Organoammonium Chain Length in the 2D Layered
Perovskites Microcrystals………... 110
4.3 Conclusions………...……….………… 121
Bibliography………..………..……. 121
5. Orientation-related Emission Switching in Deep-blue 2D Layered
Perovskite-Polymer Free-Standing Films ………...…. 125
5.1 Synthesis and Film Fabrication………..………… 127
5.2 Structural and Optical Properties of the (PMA)
2PbBr
4-PDMS
Free-standing Films……… 129
5.3 In situ Opto-mechanical Study on the (PMA)
2PbBr
4-PDMS
Films………..………. 131
5.4 In situ Opto-mechanical Study on Mn-doped (PMA)
2PbBr
4-PDMS Films………..………. 140
5.5 Conclusions………....
143
Bibliography………. 143
Appendix I: Thermal and Optical Stability of 2D Layered
Perovskites…... 146
Appendix II: Synchrotron Single Crystal X-ray Diffraction
Refinement Data collected at 100 K……… 148
Appendix III: Opto-mechanical study on 2D Layered
Perovskites-Polymer Composites………..……….. 152
A3.1 Opto-mechanical Response of (PMA)
2PbBr
4-PCL
Composite Films……….. 153
A3.2 Opto-mechanical Response of (PMA)
2PbBr
4-PMMA
Composite Films……….. 155
Appendix IV: List of Chemicals……….. 157
Appendix V: Characterization Techniques………
158
A5.1 Structural Analysis………... 158
A5.1.1 X-Ray Diffraction………... 158
A5.1.2 Synchrotron Single Crystal Diffraction………... 158
A5.2 Morphological Characterization……….. 159
A5.2.1 Optical Microscopy……… 159
A5.2.2 Scanning Electron Microscopy………... 159
A5.2.3 Atomic Force Microscopy……….. 159
A5.3 Optical and Vibrational Characterization………. 160
A5.3.1 Absorption and Diffused Reflectance Spectroscopy... 160
A5.3.2 Emission Spectroscopy………... 160
A5.3.4 Photoluminescence Quantum Yield……… 160
A5.3.5 Time-resolved Photoluminescence………. 161
A5.3.6 Color Coordinates………... 161
A5.3.7 Raman Spectroscopy……….. 161
A5.3.8 Excitation Power Dependent Photoluminescence…... 162
A5.3.9 In situ PL Recording………... 162
A5.4 Surface Analysis……….. 163
A5.4.1 X-ray Photoelectron Spectroscopy (XPS)…………... 163
A5.5 Elemental Analysis……….. 163
A5.5.1 Micro X-ray Fluorescence Spectroscopy……… 163
A5.5.2 Energy Dispersive X-Ray Spectroscopy………. 163
A5.6 Theoretical Modeling………... 164
A5.6.1 Modeling on Raman Vibrational Modes………. 164
A5.6.2 Modeling on Broadband Emitting Crystals…………. 164
Bibliography………... 164
List of Scientific Contribution………. 166
List of Abbreviations and Acronyms
µ-XRF Micro x-ray fluorescence
ACI Alternating Cations in the Interlayer space AFM Atomic Force Microscopy
BA Butylammonium
CCD Charge-Coupled device detector CCT Correlated Color Temperature
CIE Comission Internationale de l’Eclairage
CRI Color Rendering Index CRI Color Rendering Index
DA Decylammonium
DFT Density Functional Theory DJ Dion–Jacobson
DMF Dimethylformamide DMSO Dimethyl sulfoxide DopA Dopammonium DOS Density Of States
DSSC Dye-Sensitized Solar Cells
EDS Energy-Dispersive X-ray spectroscopy EQE External Quantum Efficiency
FA Formamidinium FE Free Exciton
FWHM Full-Width-Half-Maximum
GGA Generalized Gradient Approximation JDOS Joint Density Of States
LEDs Light Emitting Diodes LHP Lead Halide Perovskites MA Methylammonium
MHLP Metal-Halide Layered Perovskites MHPs Metal-Halide Perovskites
NCs Nanocrystals
N-MDDA N-methyldodecylammonium NMF N-methylformamide
N-MODA N-methyloctadecylammonium N-MTDA N-methyltetradecylammonium OctA Octylammonium
OLED Organic Light Emitting Diodes PBE Perdew–Burke–Ernzerhof PCE Power Conversion Efficiency PCL Polycaprolactone
PEA Phenylethylammonium PL Photoluminescence
PLE Photoluminescence Excitation PLQY Photoluminescence Quantum Yield PMA Phenylmethylammonium
PMMA Poly(methyl methacrylate) PVCs Photovoltaic Cells
P-XRD Powder X-Ray Diffraction
QLED Quantum dot Light Emitting Diodes RD Rashba–Dresselhaus
RP Ruddlesden–Popper
RPLP Ruddlesden-Popper layered perovskite SEM Scanning Electron Microscopy
SOC Spin–Orbit Coupling STEs Self-Trapped Excitons
TCSPC Time-Correlated Single-Photon Counting UDA undecylammonium
UPS Ultraviolet Photoelectron Spectroscopy UV Ultra-Violet
XPS X-ray photoelectron spectroscopy XRD X-Ray Diffraction
List of Symbols
µ octahedral factor
A1, A2, A3 pre-exponential weights
Eg bandgap
Im ε imaginary part of the dielectric permittivity IPL integrated PL intensity
k power-law fitting constant
Knr non-radiative recombination rate
Knr Inc. percentage increase in Knr
Kr radiative recombination rate
Kr Inc. percentage increase in Kr
L excitation power
m type of organic molecule
n thickness of metal-halide inorganic layers n emission efficiency
t tolerance factors ε dielectric constant
λoct octahedral quadratic elongation parameter
τ1, τ2, τ3 three-exponential time decay τavg average lifetime
l0 distances between Pb and Br in the octahedra in the bulk structure
Introduction
1.1 Semiconductors: Heart of Electronic Devices
In the 20th century, the ground-breaking discoveries in the semiconducting field empowered the electronic revolution, taking technology and industry to a new era, the
Information Age. Semiconductor materials have been the backbone of modern electronic
systems. Starting from simple diodes and transistors, to satellites and Mars rovers, passing through the devices that we use every day, such as mobile phones and computers, all require semiconductors.1-2 As the name denotes, the electrical properties of semiconductors
lie between those of a conductor and an insulator. A perfect semiconductor is, indeed, an insulator, but by introducing impurities to it (doping) - in the form of chemical elements with different valences - it allows the material to conduct electricity. Depending on the dopant elements, two types of semiconductor materials can be obtained, either with excess of holes (p-type) or excess or electrons (n-type). These two materials can be brought together to form a p-n junction, which acts as the basic building block for all the semiconductor devices. The direction of the flow of electrons can be controlled by this junction within an electronic circuit. The same semiconducting structure can also be extended to another emerging branch of physics that deals with the behaviour and properties of the light, so called optoelectronics, to convert one form of energy to other: for instance, light energy will be converted into useful electrical energy in solar cells and, vice versa, electricity can be transformed into light in light emitting diodes (LEDs) and lasers. These materials are also at the base of other applications that include sensors and actuators, thermoelectric, piezoelectric, photodetectors, memory switchers, and many more.3
In the field of optoelectronics, there is a high demand for new and efficient semiconducting materials towards the commercialization of optoelectronic devices that can be produced through easy processing techniques and at low cost. Especially, the devices such as lasers, LEDs, photovoltaic cells (PVCs) and sensors have attracted the attention of the scientific community to address the current challenges on greenhouse gas emission, energy saving,
lighting and display applications.4 For example, PVCs can be the potential candidates for alternating energy sources to meet the worldwide energy demand towards the decline in fossil fuel supply. Likewise, lasers, LEDs, and sensors can find application in communication, lightings and displays, manufacturing, data storage, automotive, aerospace, metrology, medical treatments, military, and many other areas of modern science.
Semiconducting materials exhibit fascinating properties when the materials are at the atomic scale.5-6 For instance, upon decreasing their size below the exciton Bohr radius,
more discrete energy levels at the band edges are created, increasing the material’s band gap energy. This phenomenon is called the ‘quantum confinement effect’.5 Furthermore,
the electronic properties of these materials become highly dependent on their shape. One can tune the material’s properties for a specific application by just varying its nanoscale size, which has driven the scientific community towards a new class of materials called nanocrystals (NCs). These are objects with a size of less than 100 nm, defined shape, structure, and chemical composition. These objects, if properly prepared, can be dispersed and stabilized in a liquid medium, creating a colloidal solution. Colloidal NCs represent an important technological boost, since they allow the preparation of films through simple deposition techniques, such as spray coating or inkjet printing, paving the way towards the realization of efficient and flexible electronic devices, as well as unlocking new possibilities in the field of nano-biotechnology, including drug delivery, cancer treatments, and in vivo imaging.7-8 Semiconducting colloidal NCs have been extensively studied in the last few decades and great developments have been achieved.9 Up to now, a large range of sizes and shapes have been reported for different NCs, such as CdS, CdSe, PbS, ZnS, InAs, Cu2-xS, CuInS2. Furthermore, NCs with anisotropic shapes and their self-assembled
super-lattices display peculiar optical and electronic properties.10
The development of the first commercial silicon solar cell and visible-light LEDs based on gallium arsenide phosphide (GaAsP) prompted the materials research industry to evolve numerous other optoelectronic materials, including inorganic and organic semiconductors, quantum dots, polymers, and hybrid materials.11-12
1.2 Metal-Halide Perovskites: The Game Changer
Recently, the introduction of semiconducting metal-halide perovskites (MHPs) attracted the attention of the optoelectronic community, both in academia and industry, due to their easy and low-cost fabrication, along with their exceptional electronic and optical properties.13 For instance, perovskite-based solar cells have achieved a power conversion efficiency (PCE) close to their theoretical limit in less than 15 years, while it took several decades for the silicon-based solar cell technology.14 The MHPs have also found application in many other fields, such as solid-state lighting, catalysis, CO2 reduction,
piezoelectric, ferro- and flexo- electrics, memory switching, mechatronics, gas sensing, and light sensing.15-19 In the field of lightings and displays, these materials exhibit excellent optical properties that outperform the silicon-based technologies commercially available, such as organic LEDs (OLED), which suffer of lower colour purity, narrower colour gamut, and expensive processing techniques. Though quantum dot LEDs (QLEDs) can offer better colour purity, their complex processing and expensive precursors hinder their commercial applications.20 Moreover, the MHPs have been also studied for the conversion of CO2 in the form of useable fuels as a solution towards the current environmental issues.21
1.2.1 Crystal Structure of Metal Halide Perovskites
The name ‘perovskite’ originally refers to the mineral of calcium titanate (CaTiO3), named
after the Russian mineralogist Lev Alekseevich Perovski. The structure of this oxide family was first described by Victor Moritz Goldschmidt in 1926, henceforth, the name perovskite has been given as a generic name for all the compounds having the same type of crystal structure as CaTiO3.22-23
Figure 1.1. Crystal structure of cubic AMX3 metal halide perovskites. (a) Single cubic unit cell
The general chemical formula for the metal halide perovskites is derived as AMX3, where
A is a monovalent cation, M is a divalent cation (e.g. Pb2+, Cu2+, Cd2+, Mn2+, Mg2+, Ni2+, Eu2+ or Sn2+), and X is a halide (e.g. Cl-, Br- or I-). The A cation can be either organic, such as methylammonium (MA+) and formamidinium (FA+), or inorganic, such as caesium (Cs+) or rubidium (Rb+). An ideal three-dimensional (3D) lead halide perovskite presents
a crystallographic lattice formed by an ionic bonding, where the A cation is positioned at the centre of a cubic cage made by eight corner-sharing octahedra, each formed by a M2+
cation in the centre that is coordinated by six halide atoms, as depicted in Figure 1.1.
As described by Goldschmidt24 in his work on tolerance factors (t), the stability of the final AMX3 perovskite structure can be predicted by using the chemical formula and the ionic
radii of the A, M and X ions (Eq. 1.1).
Goldsmith tolerance factor, 𝑡 = 𝑟𝐴+𝑟𝑋
√2(𝑟𝑀+𝑟𝑋) (1.1)
Only when this tolerance factor falls in between the range of 0.76 to 1.13, stable 3D perovskite structures are formed, which limits the number of possible A-cations that can be accommodated in the perovskite structure.25 Most of the divalent elements, indeed, are either too small or too large to fit into this structure. For instance, smaller A-site cations result in the formation of tilted or non-perovskite structures due to the excessive negative chemical pressure in the A-site cavity. Likewise, larger cations produce non-perovskite structures due to the excessive positive chemical pressure applied on the inorganic M−X framework that results in M-X bond lengths beyond its limit. Moreover, A-site cation with tolerance factor below the stable limit can lower the symmetry of the structure from pure cubic, which is the standard and often depicted perovskite phase, to tetragonal and orthorhombic phases, by slightly rotating the [MX6]2-octahedron. Higher tolerance factor,
instead, influences the stability of higher symmetry phases. Similarly, the octahedral factor (µ) further determines the final stability of the [MX6]2- octahedron that depends again on
the ionic radii of M and X ions (Eq. 1.2). A stable metal-halide octahedron is formed when µ falls in the range from 0.44 to 0.9.26
Octahedral Factor, µ = 𝑟𝐵
Though novel perovskites combinations can be predicted using the tolerance and octahedral factors, this model is limited only to the more electronegative and smaller anions and to spherical cations.26 Non-spherical and linear A+ cations tend to rotate in different directions within the metal-halide framework, minimizing their effective atomic radius. Regarding the types of halide perovskites, they can be classified into two sub-categories as all-inorganic and hybrid (organic-inorganic), depending on the type of A-site cation. The first all-inorganic (CsPbX3) perovskite crystal structure was reported back in 1958,27 and
this study included its ferroelectric behaviours. However, their applications in thin-film transistors and LEDs were demonstrated only in 199528 and 2001,29 respectively. Interestingly, they observed photovoltaic properties from their LEDs and anticipated to study these materials in solar cells, but the Pb toxicity and the instability of Sn-based compounds prevented further studies at that time. On the other hand, Miyasaka and co-workers, for the first time, implemented the hybrid perovskite materials (MAPbI3, MA =
CH3NH3+(methylammonium)) in solar cells as a replacement for the organic dye in
dye-sensitized solar cells (DSSC), since the perovskite materials could absorb light over a broad wavelength range and display efficient charge-transport properties.30 Initially, these systems did not attract the attention of the PVC community due to their poor stability caused by the dissolution of the perovskites in the electrolyte, and thus their low PCE of ca. 3.8%. Later, in 2012, the PVC research community turned its focus on the investigation of low-cost solution processed perovskite solar cells. This occurred after a breakthrough work reported simultaneously by two independent research groups on solid-state mesoscopic solar cells. The authors employed MAPbI3 and achieved a great improvement
in the device stability, as well as a higher PCE of 9.7% (Park’s group)31 and 10.9%
(Snaith’s group).32 Thereafter, an extensive amount of research work have been dedicated
to study the fundamental material properties and commercialization of perovskite-based optoelectronic devices. Recently, lead-halide perovskite-based solar cells achieved a PCE (25.2%), which is close to that of crystalline silicon solar cells and the state-of-the art CdInGaSe and CdTe thin-film solar cells.33-34 Benefiting from the knowledge obtained on
the materials properties of MHPs especially the lead-based systems, and solution-processed solar cells devices, the optoelectronic community expanded the application of
these materials into other fields including LEDs,35-37 lasers,38-39 photodetectors,40-41 sensors, catalysis,42 greenhouse gas reduction,43 etc. An example is the combination of high colour purity, tunable optical band gaps, high charge carrier mobility, and increased radiative recombination rates for efficient LEDs. This set of properties, along with efficient, simple and low-cost device processing, has enable the perovskite-based LEDs to reach external quantum efficiency (EQE) higher than 20% for green, red, and near-infrared devices, and about 10% for the blue-emitting devices.35, 37, 44-45
Figure 1.2. Fabrication process of different CsPbX3 perovskites NCs produced by using organic
acid and amine ligands. Nanorods crystalized through dodecylamine and acetic acid; spherical quantum dots are mediated by octylamine and hexanoic acid; for nanocubes, dodecylamine and oleic acid are employed whereas for few-unit-cell-thick nanoplatelets, octylamine, and oleic acid are used in the synthesis. Adapted and modified with permission from ref 46. Copyright © 2016
American Chemical Society.
Furthermore, like with other conventional semiconductor materials, to exploit the potential of quantum confinement effect, large surface-to-volume ratio, and anisotropic geometry of NCs in optoelectronic applications, many studies have been performed to reduce their dimensionality and produce nanoplatelets, nanosheets, nanorods, nanowires, and quantum dots (Figure 1.2).
1.2.2 Limitations of 3D Metal-Halide Perovskites
Besides exceptional optical performance of MHP-based optoelectronic devices, the operational and environmental stability of both the MHP material and the device, remains a big challenge, hindering their commercial applications.20 For example, green and red emitting perovskite-based LEDs have showed only few hundreds of working hours (to
reach 50% of their emission intensity (T50) with low luminescence of ca. 100 cd/cm2),
whereas blue LEDs last for a few minutes. Currently, only near infrared devices have shown longer operational stability time. The major factors influencing the stability of these devices regard the perovskites themselves. These factors are ion migration, crystal structure instability, interfacial instability, and thermal instability. Indeed, the MHP materials decompose into precursors, such as PbI2, I2, HI, and CH3NH2, for MAPbI3 in the presence
of water, oxygen, and light, which further aggravates at elevated temperatures.1 Moreover,
phase transformation occurs when these perovskites are exposed to different chemical environment and NCs agglomerate due to ligand desorption from their surfaces during purification, which also limits their operational stability.2 Efforts have been made to address this stability issue.47 For example, bringing the Goldsmith tolerance factor closer to 1 through A-site and B-site doping results in a more stable phase; on the other hand, polymer encapsulation and surface coating with materials offering large steric hindrance have been also used for improving the stability. Though these techniques provide stable devices, they significantly limit the device performance.
Alternatively, incorporating large hydrophobic cations such as organoammonium molecules, which prevent the water molecules to enter into the structure, have been demonstrated as a potential strategy for improving the environmental stability of perovskite-based optoelectronic devices. Moreover, functionalizing these materials with organic cations that possess diversified physical and chemical properties could unlock many new fascinating optoelectronic properties.48
1.3 Two-Dimensional Layered Metal-Halide Perovskites
When large cations, which cannot be accommodated within the metal-halide framework, are employed in the perovskite structure, the [MX6]2- octahedra cannot be connected in all
three directions, creating lower dimensional metal-halide structures, such as layered 2D, 1D, 0D derivatives.49-51 In the last few years, extensive research has been carried out, especially on 2D layered structures, as these have a relaxed Goldsmith tolerance factor and, hence, there is an ample choice of A-site cation that can be used, opening up many paths for designing efficient and stable optoelectronic devices.
In general, the corner sharing AMX3 geometry cannot be maintained by [MX6]2- octahedral
in MHPs when A-site cation becomes larger than the volume of the cubic cage made by the metal-halide framework. This initiates the formation of 2D metal-halide layered perovskites (MHLPs), where 2D layers of corner-sharing [MX4]2- octahedra are separated
by a layer of large A-site cation, as shown in Figure 1.3. Typically, long aliphatic or aromatic organoammonium molecules, which can pack efficiently, are incorporated in the 2D MHLPs. These crystals are the thinner case (n =1) of the layered perovskites family with a general chemical formula of LmAn-1MnX3n+1 (n = 1, 2, 3, 4, ... and m = 1 or 2) where
L represents a large (aliphatic or aromatic) organic cation, usually derived from the
ammonium family, which coordinates well with the halide atoms primarily through hydrogen bonding (however, the halide vacancies can affect the coordination level, thus promoting more distorted structure)52-53 and separates the layers formed by the smaller monovalent cation (A) and [MX4]2- inorganic cages. Whereas ‘m’ represents the type of
organic molecule, such as monocations (m = 1) or dications (m = 2) and ‘n’ refers to the thickness of metal-halide inorganic layers that can be tuned by adjusting the L and A compositions: the resulting structure is usually addressed as quasi-2D MHLPs (Figure 1.3).
Figure 1.3. Two-dimensional metal halide layered perovskites with different ‘n’ values in a general
chemical formula L2An-1MnX3n+1.
When ‘n’ approaches infinity, the structure becomes 3D MHPs. The cumbersome L organic molecules in MHLPs act as a barrier, preventing water molecules in the atmosphere to enter into the perovskite structure. This explains the excellent stability of the 2D MHLPs under ambient conditions, as compared to their 3D counterparts. It is worth mentioning
that, though these layered structures no longer display the typical AMX3 perovskite
structure, the similarities on the properties and applications of these materials allow to call them with the name perovskites.
In addition, different dimensionalities, such as zero- (0D) and one- (1D) crystallographic structures of organic-inorganic metal halides can be obtained by properly selecting the organic molecules and metal-halides.51 For instance, metal-halide octahedra connected in one direction, either in corner-, face-, or edge-sharing configuration, and passivated by organic cations form 1D perovskite crystals. This connecting method and the selected organic cation determine the final chemical formula of the 1D crystals. Whereas, if a single layer or a cluster of metal-halide octahedra layers that are completely passivated by organic cations, are arranged in a periodical lattice, it is called as 0D crystals. The general chemical formula for the 0D crystals is A4MX6. All these low-dimensional hybrid organic-inorganic
metal-halides are structurally different from the conventional 2D nanoplatelets/nanosheets, 1D nanorods/nanowires and 0D quantum dots/nanocrystals that are derived morphologically from the 3D AMX3 structure. The present thesis focuses on 2D layered
perovskite materials; thus, the details on other low-dimensional materials such as 1D and 0D perovskites can be found elsewhere.49-50, 54-55
Synthesis and general studies on 2D layered hybrid halide perovskites were reported nearly a century before the 3D hybrids, prior to the introduction of X-Rays. This made difficult to investigate their atomic arrangements and to assign a definite birth period for this class of materials.56 Initially, most of the studies were carried out on the layered transition-metal hybrid perovskites, which were based on Cu and Cr due to their dielectric and ferromagnetic properties at low temperatures.57-59 Other divalent transition metals, such as
Mn, Fe, Cd, and Pd were also employed to fabricate layered halide perovskites.60-61 It is worth noting that, in 1962, red photoluminescence was observed from the (CH3NH3)2MnCl4 and (C2H5NH3)2MnCl4 compounds.61 Mitzi and co-workers laid a great
foundation for the exploration of different functional 2D MHLPs semiconducting materials by providing thorough insight on their crystallographic structure. First reports on layered perovskites based on group 14 hybrids, such as Pb-I, Sn-I and Ge-I, can be found back in early 1990s where the crystal structure and optical properties of these compounds were
studied.62-66 Strong room-temperature excitonic recombination luminescence was observed from the Pb-I based perovskites by Ishihara and colleagues.63 Optical investigation carried out by Kitazawa et al.,67-68 on the first 2D Pb-based mixed-halide layered perovskites demonstrated the wide tunability of their optoelectronic properties by properly employing different ratio of halides in their synthesis. Furthermore, more recent studies on the effect of different organic cations on the structure and optical properties of MHLPs showed the opportunity to design new optoelectronic device with desired properties by employing various functional organic molecules.69-74 In addition, the electrical conductivity of the 2D
MHLP materials was improved by changing the number of inorganic layers (i.e., n > 1).28,
65
The crystallographic structure of the 2D MHLPs can be expanded further depending on the orientation of the inorganic slab, which is strongly related to lattice distortions that can be induced by the organic cations. These materials can show a (100)-oriented structure or a (110) crystal orientation (so called corrugated structures) or a (111)-oriented structure, that can be conceptually realized by chemically cutting the 3D perovskites lattice in the corresponding crystallographic planes (Figure 1.4).
Figure 1.4. Derivation of (100)-, (110)- and (111)-oriented 2D metal halide layered perovskites
realized by chemically cutting the corresponding crystallographic planes of 3D perovskites lattice. Adapted and modifiedwith permission from ref 75. Copyright © 2018 American Chemical Society.
The most commonly studied 2D MHLP is the (100)-oriented structure whereas there are only few examples of the (110)- and (111)-oriented ones. Mitzi and co-workers28 reported the first corrugated (110)-oriented tin-based perovskites using iodoformamidinium cation. On the other hand, only group 15 M3+ ions (e.g. Bi, Sb, As) are able to form (111)-oriented crystals with application as solar light absorbers due to their relatively small effective charge carrier masses and p-type-like character.76-79 Besides, the relative size of the cations also dictates the connectivity of the octahedra in the final structure. Different connectivity modes of metal-halide octahedra such as corner-sharing80, edge-sharing81, and face-sharing82 were
observed on these 2D layered perovskite materials.
Figure 1.5. Different phases of 2D layered metal halide layered perovskites formed by employing
different organic cations. The formation of Ruddlesden–Popper phase is strongly related to the presence of monovalent cations with a single anchor head group (i.e., NH3) whereas the Dion–
Jacobson phase will be formed through divalent organic cations. ACI phase requires two different cations of different size.
The 2D MHLPs can be further classified into two major types depending on the organic molecule used in their synthesis: Ruddlesden–Popper (RP) phases (Figure 1.5) in which the two adjacent inorganic layers are separated by a bilayer of monovalent cations (such as alkylammonium ones). In this case, the inorganic layers are shifted each other by half unit cell due to relatively weak van der Waals interactions between the organic molecules and the need to accommodate the organic bi-layer. Dion–Jacobson (DJ) phases (Figure 1.5), meanwhile, are formed by a monolayer made of divalent organic ions, in general diammoniums including 3-(aminomethyl)piperidinium and 4-(aminomethyl) piperidinium, that links via hydrogen or ionic bonding two adjacent inorganic layers.
Another possible structure can be generated when smaller A-site cations are allocated inside the octahedral cage and also in between the layers made by larger cations, combining both RP and DJ phase’s structure characteristics. This is called as alternating cations in the interlayer space (ACI) phase (Figure 1.5). So far, the ACI structure is reported only for the guanidinium cations.
In principle, the distance between inorganic layers increases according to the length of the organic molecules incorporated. Thus, larger interlayer distance can be achieved in the RP phase due to the bilayer nature of the organic spacer cations. The factors influencing the choice of suitable organic cations for the 2D MHLPs are net positive charge at the anchoring site and degree of substitution, hydrogen-bonding and space-filing ability, and, eventually, stereochemical configuration. For instance, primary ammonium groups form more stable structures as compared to secondary, tertiary or quaternary ammoniums.83 As evident, there is a vast variety of organic molecules that can fulfil these requirements, and thus this can be seen as key drivers for the design of materials with unique properties.
1.4 Optoelectronic Properties of 2D Metal-Halide Layered Perovskites
1.4.1 Band Structure and Optical Properties
Apart from the structural properties, investigation on optical and electrical properties from both material and device perspective provides a complete insight on designing efficient and advanced optoelectronic devices. The energy band structure, impurity levels, excitons, localized defects, and lattice vibrations are the material factors that determines the optical properties of semiconducting materials. Typically, the group 14 p-block metals based 2D MHLPs, are direct bandgap semiconductors where metal and halides are participating in the formation of valence and conduction band (Figure 1.6 (a-b)).56, 84
In contrast to the conventional semiconductor materials such as Si, GaAs, CdTe, etc., where the bandgap is formed between bonding and antibonding orbitals, interestingly, both the valence and conduction band of MHLPs are formed by antibonding interactions through strong hybridization of M ns-states and halide outer p-orbitals in the case of upper valence band (i.e. 5p for I, 4p for Br, and 3p for Cl), and of M np-orbitals and the same halide p-orbitals in the case of conduction band.
Figure 1.6. (a) The molecular and atomic orbital contributions of BA2PbBr4 (BA =
butylammonium) determined at the Heyd–Scuseria–Ernzerhof hybrid functional and spin-orbit coupling level of theory. (b) Calculated band structure of BA2PbBr4 at the generalized gradient
approximation (GGA)/Perdew–Burke–Ernzerhof (PBE) level with and without account of SOC effects. (c) Illustration of natural quantum well formed in the 2D layered perovskites due to the quantum and dielectric confinement exerted by the organic cations. (d) The DOS of BA2PbI4 and
MAPbI3 near the band edge determined by ultraviolet photoelectron spectroscopy (UPS) He-I. The
arrows indicate the valence band edge determined through density functional theory (DFT) calculations. Reproduced with permission from ref 84. Copyright © 2018 WILEY. (e) Absorbance
and (f) emission profiles acquired from 2D metal halide layered perovskites with different ‘n’ values. Reproduced with permission from ref 85. Copyright © 2015 American Chemical Society.
The large organic molecule will contribute to the density of states (DOS) only if there is a charge transfer between the organic and inorganic layers due to the overlap between the energy levels or if the energy between valence band maximum and conduction band minimum falls below the energy gap of the [MX4]2- inorganic unit (Figure 1.6 (a)).70, 84
Similar to the 3D perovskites, the A-site cations play a vital role in both the internal and external [MX4]2- octahedra distortions, as well as in confining the charge carriers within a
two-dimensional range, that is within the inorganic layer. Moreover, the organic layers also provide the dielectric confinement on the excited charge carriers, modulating the material exciton binding energy (Figure 1.6 (c-d)).
The energy bandgap of 2D MHLPs is higher than that of their 3D counterparts due to the quantum-confinement of the charge carriers within the nanometric-thick inorganic layers (ca. 0.6 nm). The bandgap can be tuned, down to its 3D counterparts energy value, by increasing the number ‘n’ of octahedra layers that are sandwiched between organic spacers, from 1 to infinity. For instance, the (BA)2MAn-1PbnI3n+1 (BA = butylammonium, MA =
methylammonium) perovskite exhibits 2.24 eV of bandgap when n = 1 and it decreases to 1.99 eV, 1.85 eV and 1.60 eV for n = 2, 3 and 4, respectively, and finally converges to 1.52 eV when n reaches infinity (Figure 1.6 (e, f)).85 Furthermore, the optical properties of the 2D MHLPs can be varied by employing different B-site cations, such as Sn2+, which show strongly red-shifted bandgap due to its higher electronegativity.86
Further, the Mn2+-based system also shows red-shifted emission centred in the red region due to the d-d transition of Mn energy states (Figure 1.7).87 Compared to other metal-based perovskites, the lead-based system exhibits exceptional optical performance due to its unique atomic configuration, such as the Pb 6s lone pair, inactive Pb 6p, and strong spin– orbit coupling (SOC).88
The transition matrix elements and the joint DOS (JDOS) are the two main factors that determine the optical absorption of a semiconductor material.88 For example, the indirect
bandgap nature of the silicon (first-generation absorber used in PVCs) results in low optical absorption due to the forbidden transitions between band edges. Whereas, the direct bandgap materials, such as GaAs (second-generation absorber) and 2D layered halide perovskites, which allow transitions between band edges, show stronger optical absorption.
As compared to the GaAs, one order of magnitude of difference have been demonstrated from 2D layered halide perovskites due to the less dispersive nature of Pb 6p bands in the conduction band minimum level and much higher JDOS.88-89 Hence, the high optical absorption coefficient of MHLP provides possibility to achieve higher PCEs than the other second-generation absorbers including GaAs, CuInSe2 (CIS), and Cu2ZnSnSe4 (CZTS).89
Figure 1.7. (a) Crystal structure of Mn-doped 2D layered perovskite. (b) Emission spectra acquired
from pristine (blue) and Mn-doped (orange) butylammonium-based 2D layered perovskites. Reproduced with permission from ref 87. Copyright © 2017 American Chemical Society. (c)
Schematic illustration of energy transfer from the pristine perovskites to Mn2+-doped system.
Reproduced with permission from ref 56. Copyright © 2019 American Chemical Society.
Besides the high optical absorption coefficients, the presence of organic bilayers provide structural and moisture stability to the inorganic layers, which is critical for device performance and lifetime.90-91 Superior optoelectronic properties of the 2D MHLPs, such as quantum and dielectric confinement, fast radiative recombination rates, long carrier diffusion lengths and narrow- and broadband emission along with low cost processing directly, reflected on the excellent device performances.48, 74, 92-108
1.4.2 Excellent Defect Tolerance
One of the fascinating features of MHLPs, in particular of the lead based system, is their ability to retain their electronic structure even with a large concentration of defects.13 Intrinsic point defects, such as halide and A-site vacancies, interstitials and Pb–halide anti-site occupations, or higher-dimensional defects, such as dislocations and grain boundaries, are responsible for the formation of non-radiative trap states within the crystal. It is desirable to have shallow-level defect states that generate free carriers, rather than deep-levels that create pathways to non-radiative recombination, limiting significantly optoelectronic device performances.
Figure 1.8. (a) Formation of typical point defects in the 2D lead halide layered framework. (b)
Illustration of electronic band structure of defects generated within the band gap for conventional GaAs and CdSe semiconductors and within the conduction and valence band for lead halide semiconductors. The nature of forming bandgap between bonding (σ) and antibonding (σ*) orbitals in the conventional semiconductors, the defect or dangling bond states are formed within the bandgap due to the emergence weak bonding or non-bonding states. Whereas, in the case of 2D lead halide layered perovskites, the bandgap is created between two antibonding orbitals that promotes mostly the shallow defect states with in the valence or conduction band. Adapted and modified with permission from ref 13. Copyright © 2018 Springer Nature.
Intriguingly, the lead-based MHLPs show excellent defect tolerance nature by producing mostly shallow-level trap states, as well as forming large polarons that prevent the photo-generated charge carriers to recombine through deep-level defect states (Figure 1.8).13 The studies on all the possible intrinsic point defects reveal that the dominant ones in MHLPs are shallow defects since the deep transition level defects (originates from, for example, surface halide vacancies) have high formation energies.53, 109 Moreover, depending on the
chemical potential, the conductivity of MHLPs can be changed from p-type to n-type. Combined properties of high crystal symmetry, strong antibonding coupling between Pb lone-pair s and I p orbitals, large atomic size and ionic nature are attributed to the unique defect tolerance nature of 2D layered perovskites.88, 109
1.4.3 Tunable Exciton Binding Energy
In contrast to 3D perovskites, which show smaller exciton binging energy, 2D MHLPs show higher exciton binging energy due to the dielectric mismatch and quantum confinement effect of charge carriers (Figure 1.9).
Figure 1.9. (a) The quantum and dielectric confinement provided by the organic layers on 2D
layered perovskite crystals that (b) increases the binding energy of the system as compared the 3D system. Reproduced with permission from ref 110. Copyright © 2016 American Chemical Society. (c) Illustration of tuning the binding energy of the 2D layered perovskites by increasing the ‘n’
values. Reproduced with permission from ref 111. Copyright © 2018 Springer Nature.
Organic molecules with a polarizable group reduce the dielectric mismatch between the insulating (organic) and conducting (inorganic) layers. For example, exciton binding energy of 320 meV is reduced to 220 meV when the dielectric constant (ε) of organic molecule change from 2.44 (decylamine) to 3.32 (phenethylamine).74 Moreover, when
organic molecules with high dielectric constants, such as ethanolamine (ε = 37), are employed in the fabrication of 2D layered perovskites, they reduce the Coulomb attraction between electrons and holes, leading to exciton binding energies 20 times smaller than the conventional 2D layered perovskites.112 This proves that the exciton binding energy of 2D system can be tuned by the choice of organic molecules with different dielectric constants. The exciton binding energy can also be further decreased by changing the halide from bromide to iodide.113 Alternatively, increasing the number of inorganic layers reduces both
the dielectric and quantum confinement that modulates the bandgap and exciton binding energies of 2D MHLPs (Figure 1.9 (c)). For example, the exciton binding energy of 220 meV is reduced to 170 meV when n is increased from 1 to 2 in the (PEA)2(MA)n−1PbnI3n+1
(PEA = phenethylammonium, MA = methylammonium) family.
1.4.4 Charge-transport dynamics
The organic molecules present relatively high resistivity and low charge carriers mobility as compared to the inorganic network, effectively confining the charge carriers and resulting in anisotropic conductivity and carrier mobility in 2D MHLPs.114-115 The charge transport between the adjacent inorganic layers are promoted by the tunnelling process through organic layers. Thus, better distribution of the quantum well within the layers provided by varying the thickness of inorganic layer (n > 1) can modulate the charge-transfer behaviour of 2D perovskite materials.116 For example, creating thicker inorganic layers (i.e., n > 1) improves the plane to plane (i.e., out-of-plane direction) charge transport that combined with the in-plane transport properties, it provides devices that are more stable and show better performance. Alternatively, different strategies to grow the 2D layered perovskite crystals perpendicular to the substrate, such as hot casting, solvent engineering and small and large cation engineering, can be used to achieve better charge transport properties.116 For example, Tsai et al., observed that the hot casting method
promotes the growth of (BA)2(MA)3Pb4I13 (n = 4) films with octahedra layers vertical to
the substrate and this alignment showed 10% of PCE, along with excellent stability in 2D MHLPs based solar cells.117 Adding ammonium chloride and ammonium thiocyanate in
the system, Fu et al., produced highly crystalline and vertically oriented (PEA)2(MA)4Pb5I16 (n = 5) films with higher PCE of 14.1%.118
1.4.5 Narrow and Broadband Emission
The 2D MHLPs, especially the lead-based systems, display light emission ranging from extremely narrow101 to extremely broadband119 at room temperature. Their emission full-width-half-maximum (FWHM) can reach up to 1 eV depending on the composition of the structure (Figure 1.10).
Their emission originate from the inorganic layers due to the tightly bound excitons that exhibits intense, sharp and minimally Stokes shifted free-excitonic emission characteristics. Upon cooling to cryogenic temperatures, the emission of these materials becomes sharper and stronger. Moreover, structural distortions, such as octahedral tilting and bond length variations, generated by the organic molecules have a direct effect on the band structure of the resulting materials.56 Mitzi and co-workers observed an increment in the bandgap from Sn-X based 2D MHLPs when significant octahedral tilting was present in the structure. This is related to the energetically stabilization of the valence band maximum because of the decrement in the antibonding Sn 5s and X np orbitals overlap. In addition, the distortions also produce a low structural symmetry that destabilizes the nonbonding Sn 5p states at the conduction band minimum due to the introduction of antibonding character by mixing with X orbitals. Similarly, small increment in the bandgap is reported when producing longer in-plane Sn−X bond lengths.69 Furthermore, an increment in optical efficiency is observed from highly distorted 2D MHLP structures, which exhibit fast radiative recombination rate by reducing the exciton Bohr radius because of higher exciton effective mass.120 The emission of 2D MHLPs is also affected by the excited-state coupling to the phonon vibrations of organic molecules that modulates the radiative recombination pathways.121-122 At room temperature, asymmetrical low-energy
tail is observed in the emission spectrum from the (CnH2n+1NH3)2PbX4 family, which splits
into different emission components at low temperatures and that are ascribed to phononic contributions.122
More recent studies also show how devices can be fabricated from white light emitting 2D MHLP. This emission is strongly associated to the structural deformation of the inorganic layer that are generated by the organic cations in the system. This was first reported by Karunadasa and co-workers119, 123 in 2014. And it is attributed to elastic distortions in the lattice that are
caused by inter-octahedral distortions, as depicted in Figure 1.10 (e), together with strong electron-phonon interactions. The later creates transient lattice defects that trap the strongly bound excitons (exciton self-trapping).
Figure 1.10. (a) 2D layered perovskite crystals oriented in (100) structure exhibits blue emission
and (b) their corresponding crystal structure. (c) More distorted (110) structure exhibits white-light emission and (d) the corresponding crystal structure. Reproduced with permission from refs 119, 124.
Copyright © 2014-2016 American Chemical Society. (e) Schematic illustration of in-plane and out-of-plane distortions generated in the inorganic framework upon excitation. Reproduced with permission from ref 125. Copyright © 2017 Royal Society of Chemistry.
The self-trapped excitons (STEs) are generated only upon excitation, which creates dynamic lattice deformation. They disappear once the structure is relaxed back to the ground state. Structures with such property can be seen as single sources of white-light emission with desired color chromatic coordinates and excellent color rendering index (CRI).126 As stated above, the most distorted structure is the (110)-oriented 2D layered perovskite that exhibits mostly a broad band emission.28, 81, 123, 127 Some of the (100)-oriented structures, such as (CyBMA)PbBr4
(CyBMA = 1,3‐Cyclohexanebis(methylamine))128, (2meptH
2)PbBr4 (2mept =
2-methyl-1,5-diaminopentane)129, (C
7H18N2)PbBr4130, (CH3OC6H4CH2NH3)2PbBr4131 and
(C6H11NH3)2PbBr4132 show also a broad emission profile. Likewise, due to the strong
confinement of excited charge carriers, optically efficient white-light emission is also observed in some of the metal halides formed by chains of facet or corner-sharing octahedra (1D) and hybrid organic−inorganic metal halides consisting of isolated metal halide octahedral (0D).50, 133-136 Besides, the self-tapping of excitons, which is generally accepted as the mechanism
behind the broadband emission in 2D MHLPs, emission broadening can also be possible by material defects and dopants that operate with different photo-physical phenomenon.53, 87
1.5 Functionalizing the Organic Layer in 2D Metal-Halide Layered
Perovskites
Since 2D MHLPs merge the advantages of both organic and inorganic materials, they are an excellent platform for tuning their properties by simply incorporating different combinations of organic molecules and inorganic moieties in the synthesis.137-138 As described in previous sections, the overall properties governed by the inorganic conducting layer in 2D MHLPs can be modulated by controlling the degree of interaction among them through simple functional organic molecules with different dielectric constant and chain length. Moreover, the valence band maximum and conduction band minimum of these simple organic molecules are mostly above the corresponding ones of the inorganic network. This confines the excited charge carriers within the inorganic layer and thus forming natural multiple-quantum-well electronic structures where the organic molecules serve as the dielectric barrier and inorganic layers act as quantum wells. Intriguingly, the quantum well and barriers energy levels can be varied by employing more complex, conjugated organic cations with smaller energy gap that inverts the well and barrier layers. Likewise, type II band alignment where the electrons are confined in
one layer and the holes are in a different layer can be achieved in 2D MHLPs by simply using organic molecules with specific band alignment that could potentially offset the organic and inorganic layer band energy (Figure 1.11).29, 139 For instance, no emission from the inorganic network is observed from (BTm)2PbCl4 crystals; only the BTm cation emission is observed
(Figure 1.11 (b)-(iv)). Whereas, the (2T)2PbI4 crystals show green emission due to the
confinement of excited charge carriers within the inorganic layer (Figure 1.11 (b)-(i)). Interestingly, the (4Tm)2PbI4 and (4TCNm)2PbI4 crystals have no emission due to the
efficient charge separation at the interface (Figure 1.11 (b)-(ii) and (iii)). The corresponding band alignments of these 2D MHLPs are given in Figure 1.11 (c).
Figure 1.11. Tuning the energy level alignment of organic and inorganic layers in 2D layered perovskites through proper selection of organic molecules. (a) Architecture of Organic
molecules employed in the system for the modulation of band alignment. (b) Images of 2D layered perovskite crystals under ultra-violet exposure (i) (2T)2PbI4, (2T =
2-([2,2'-bithiophen]-5-yl)ethan-1-aminium iodide)(ii) (4Tm)2PbI4 (4Tm =
2-(3''',4'-dimethyl-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)ethan-1-aminium iodide) (iii) (4TCNm)2PbI4 (4TCNm =
2-(3'',4''-dicyano-3''',4'-dimethyl-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)ethan-1-aminium iodide) (iv) (BTm)2PbI4 (BTm =
2-(4'- methyl-5'-(7-(3-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)-[2,2'-bithiophen]-5-yl)ethan-1-aminium iodide). Inset shows the monolayer thick crystals (c) The relative energy level alignment of crystals produced with different organic cations. The dark red and dark blue lines indicate the energy levels of organic and inorganic layers with in the 2D layered perovskite structures (i) (2T)2PbI4 (ii) (4Tm)2PbI4 (iii) (4TCNm)2PbI4 (iv) (BTm)2PbI4. Reproduced with permission from
Other photo-physical properties, such as fluorescence efficiency, can be further modulated in 2D MHLPs by controlling the electronic interaction between the chromophores through directing the orientation of the oligomer- or dye-containing organic cation.69, 141 (100)-oriented oligothiophene-based Pb-I 2D layered perovskite displays rotational stacking disorder in the inorganic network when 5,5′-diylbis(amino-ethyl)-[2,2′-bithiophene] (AE2T) is used as the spacer cation due to the flexibility of its anchoring group, facilitating the orientation of the organic layer in different directions. This rotational stacking disorder strongly influences the organic and inorganic energy orbitals and thus it leads to variation in the total energy and energy alignment of the system.141 Moreover, single crystalline oligomer layer can also be formed between the inorganic layers, providing higher charge carrier mobility.142-143
Figure 1.12. (a) Illustration of two different MBA (methylbenzylamine) chiral molecules (top) and
their corresponding 2D layered perovskite crystals structures (bottom). Reproduced with permission from ref 144. Copyright © 2020 Springer Nature. (b) Device architecture of circularly
polarized photodetectors showing the light with left-handed (LCP) and right-handed (RCP) circular polarization. Reproduced with permission from ref 145. Copyright © 2019 Springer Nature. (c) The
photoresponsivity of (S-MBA)- and (R-MBA)-based 2D layered perovskites. (R-MBA)-based crystals show smaller photoresponsivity and photoconductor gains for LCP than the RCP while the (S-MBA)-based crystals demonstrated opposite behaviour. Reproduced with permission from ref
The 2D layered perovskite structures can be also a potential material for spintronic, chiro-optoelectronic, and ferroelectric applications due to their flexible crystal structure that can accommodate chiral organic molecules, which impart their chiral properties to the inorganic network.144, 146-148 The chiral molecules induce the asymmetry in the structure that then promotes the non-centrosymmetry in the system through the asymmetric chemical bonding or spatial interactions between the layers. The induced symmetry breaking and strong spin-orbital coupling lead to Rashba–Dresselhaus (RD) spin-splitting, as well as other exciting properties, such as circular dichroism, optical rotation, second-harmonic generation, topological quantum properties, piezo-, pyro- and ferroelectricity. These properties can be applied to produce for example circularly polarized LEDs, photodetectors, and 3D displays. They can be also used in other fields, such as bio-responsive imaging, quantum communication, spintronic, and memory devices.116, 144, 149
In the early 2000s, Billing and coworkers150-151 demonstrated the chirality in lower dimensional metal halide perovskites for the first time. However the chiro-optical properties were studied only in 2017 by Moon group,152 where they observed oppositely-signed circular dichroism on perovskite crystals donned with S- and R-configurations of chiral organic spacers (Figure 1.12).
Variations in crystalline orientation and film thickness change the circular dichroism signal. The degree of flexibility in the 2D layered perovskites provides another interesting possibility to functionalize the organic layer through intercalating new functional molecules that bring unique electronics properties to the system, such as ionic conductivity, photocatalytic activities, photoinduced charge separation, photochromism, and non-linear optical properties (Figure 1.13). Mitzi and coworkers reported the first intercalated 2D MHLPs, which was stabilized by fluoroaryl-aryl interaction within the organic layer.153 Karunadasa’s group showed that the I
2
intercalation within the organic layer can reduce the dielectric confinement of excited charge carriers significantly in 2D Pb-I layered perovskites due to the consequence of intercalation that results in more polarizable organic layers than the inorganic one.154 The successful
incorporation of polydiacetylenes inside the 2D layered perovskites structure through thermal treatment was achieved by Ortiz-Cervantes et al., and the doping of these crystals with oxygen or iodine can reduce the bandgap from 3.0 to 1.4 eV and dramatically increase the conductivity
of the resulting crystals.155 Furthermore, 2D MHLPs can accommodate two different organic cations in a single structure, which provide more functional diversity and flexibility.156-157
Figure 1.13. Structure tunability of 2D metal halide layered perovskites demonstrated for a
diversified application in the field of optoelectronics. Reproduced with permission from ref 116.
Copyright © 2020 Royal Society of Chemistry.
Apart from all the above discussed possibilities manifested by the choice of the organic cation, these layers can be further modulated to facilitate device processing. For instance, melt-processing technique is considered as a simple technique compared to vapor-phase or solution-based one to produce high quality films in flexible electronics. The melting point of most of the semiconducting hybrid systems is close to or higher than their decomposition temperature, which hinders the use of these materials in melt-processing. Intriguingly, by using different organic cations in the synthesis of 2D MHLPs, low-melting temperature can be achieved, facilitating the fabrication of field effect transistors through this technique and by using different substrates.158-160
1.6 Brief Introduction to Layered Double Perovskites
The conventional AIBIIX3 perovskites structures reported so far have been limited due to the
restriction on the availability of BII elements with such an oxidation state and capability of stabilizing the 3D perovskite structures. The beauty of the perovskites is that, the oxidation limitation can be overruled by substituting BII metal with a combination of BI and BIII metals
that stabilizes the perovskite structure in the so called double perovskites. These structures have a general chemical formula of A2BIBIIIX6. This further widens the possibility to produce
perovskite materials with unique physical and optical properties. To date, different metal-halide double perovskites have been demonstrated as potential candidates for photovoltaics and other optoelectronic applications.161-162
Figure 1.14. Crystal structure of (a) (BA)4AgBiBr8 (n = 1) and (b) (BA)2CsAgBiBr7 layered double
perovskites (top) and their corresponding band structure (bottom). Structure with n = 2 exhibits indirect band transitions whereas, reducing the dimensionality to n = 1 demonstrated direct band transitions. The inset in the top images show the coordination of Ag sphere and the bond distances in angstroms. In the inset of (b), t and b denotes the terminal and bridging bromide. The Bi, Ag, Cs, Br, N, and C atoms are represented by Orange, white, turquoise, brown, blue, and grey spheres. H atoms are not shown for clarity. (c) Crystal structure of 3D layered double perovskites. (d) Absorption spectra collected from (BA)4AgBiBr8 (1-Bi) and (BA)2CsAgBiBr7 (2-Bi) crystals. The
inset shows their corresponding digital images taken under normal light. Reproduced with permission from ref 163. Copyright © 2018 American Chemical Society.
As in the conventional perovskites, dimensional reduction is also possible in the double perovskites by introducing into the system large organic molecules. The 2D version of double perovskites is referred as layered double perovskites. All the above discussed functionalities of 2D MHLPs, such as (100)-, (110)- and (111)- oriented structures, RP and DJ phases, and
tailoring the electronic properties through different halogens, metal doping, and organic layer functionalization, are also applicable to the layered double perovskites.
Though their synthesis was attempted in the early 1900s, using mixed-valent AuI and AuIII cations164, only in 2003 Laura et al, successfully studied the structural properties of [NH3(CH2)8NH3]2[(AuII2)(AuIIII4)(I3)2] and [NH3(CH2)7NH3]2[(AuII2)(AuIIII4)(I3)2]
compounds.165 Layered perovskites based on CuI−SbIII mixed-valence cations exhibit a direct bandgap and conductivity with excellent stability under light, moisture, and temperature.77
The change in the bandgap of layered double perovskite from direct to indirect upon increasing the thickness of inorganic layer (i.e., from n = 1 to n = 2) is observed from (BA)4AgBiBr8 and
(BA)2CsAgBiBr7 structures (Figure 1.14).163 In 2017 seven new layered halide double
perovskites were introduced by using propylamine, butylamine, 1,4-diaminobutane, and octylamine.166 Mitzi and coworkers167 reported AgBi-I based 2D double perovskites for the first time using oligothiophene cation as spacer and they observed flatter, borderline type-I/II quantum well aligned electronic bands provided by the close proximity of organic molecular orbitals to the inorganic band edges. These materials show weaker and broader optical transition due to the transition between equatorial I-p states in the maximum of valence band and hybridized equatorial p orbitals of I and Bi in the minimum of conduction band. Similarly, iodide based 2D double perovskites (AgBi-I and CuBi-I) show their potential for tandem solar cells.168-169
The abundant chemical tunability along with outstanding optoelectronic properties of the 2D layered perovskite materials, as well as the potential prospect on understanding the structure-property relationships at the molecular level, provide enormous opportunity for the scientific community on designing new and efficient 2D metal-halide layered perovskites for a specific optoelectronic application. These materials still require attention on understanding their fundamental electronic properties and on controlling their synthesis parameters to produce high quality materials. Moreover, the synergy between the organic and inorganic compounds in these systems further opens up the possibility to unlock novel optoelectronic properties by simply integrating many other available functional organic molecules into these structures.
1.7 Outline of this Thesis
This thesis is dedicated to the synthesis of 2D lead-bromide Ruddlesden–Popper layered perovskite materials through a simple synthesis technique and by using different organic molecules. It targets a full investigation of the photo-physical properties of the as-synthesized materials. Moreover, it presents detailed studies on the effect of structural rigidity and electron-phonon interaction provided by organic molecules on the emission efficiencies of 2D layered perovskites, and their emission tunability by using organic molecules with different architectures. In a more technological view, this thesis summarizes the work performed on the integration of 2D layered perovskites in polymer films and their emission enhancement by mechanical stress.
Chapter 2: Simple fabrication of two-dimensional layered perovskite crystals will
describe the facile procedure for producing the 2D layered perovskites based on two different families of organic cations by using an environmentally friendly solvent. This chapter will also present the results on their photo-physical properties and the strategy for improving their optical efficiency through mechanical exfoliation.
Chapter 3: Directionality of vibrational modes in layered perovskite flakes will
present the anisotropic nature of the 2D layered perovskite materials on fundamental phonon modes, revealed through low-temperature polarized Raman measurements. It will also discuss how to control these vibrational modes through properly selecting the organic moiety, thus the optoelectronic properties of such 2D layered materials.
Chapter 4: Tunable chromaticity through engineering the architecture of organic cations will propose the strategy for designing the 2D Ruddlesden–Popper layered
perovskites with specific optoelectronic properties for the yet emerging field of solid-state lighting from single layer materials.
Chapter 5: Mechanical switching of orientation-related photoluminescence will
discuss the in situ fabrication of 2D layered perovskites inside the polymer matrix and present the results obtained on the emission switching due to the different orientation of flakes upon applying mechanical stress in the system.
