An experimental study of graphitic carbon nitride-based materials and selected metal-based semiconductors in organic solar cells combined with a computational study of perovskite solar cells.

Details of contributions to publications that form part of and/or include research presented in this thesis (includes manuscripts in preparation, submitted, in press and published articles. The details of each author's contribution to the experimental and computational work and writing of each publication are indicated). A theoretical study of the effect of the hole and electron transport materials on the performance of a lead-free perovskite solar cell based on CH3NH3SnI3.

CONFERENCE CONTRIBUTIONS

ACKNOWLEDGEMENTS

I am sincerely grateful to my sons Kelvin Kipkirui and Emanuel Kiprotich for their love, patience during my absence, laughter and wonderful video calls. I am also grateful to other family members (cousins, aunts, in-laws and uncles), the "Braii family" in Durban and friends who have supported me along the way.

DEDICATION

LIST OF ABBREVIATIONS

51 Figure 2.13: (a) An inverted bulk heterojunction polymer solar cell consisting of ITO/ZnO/C3N4:active layer/PEDOT:PSS (or MoO3)/Ag. 161 Figure 5.14: (a) J-V dark curves with different nanoparticles and (b) the SCLC obtained from various BHJ-OSC devices fitted to Equation (3).

LIST OF TABLES

278 Table 10.1: Properties of ETL, absorber and different HTL materials………..294 Table 10.2: Optimized thicknesses for different layers of devices using WS2 as. Cadmium Sulfide Doped Carbon Nitride Graphite Nanoplates for Enhanced Active Layer Photon Collection in Organic Solar Cells.

CHAPTER ONE Introduction

Third-generation solar cells

Organic solar cells
Perovskite solar cells
Dye-sensitised solar cells
Tandem solar cells

Efficiency parameters of a photovoltaic cell
Factors affecting the performance of solar cells
Computational modelling of solar cells
Problem statement
Aim and objectives
Thesis synopsis

The PCE of the devices was significantly improved over the pristine device (without the dopants). The effect of using different ETL and HTL materials on device performance is highlighted.

Figure 1.1: A schematic diagram of a typical bilayer (a) and (b) BHJ OSC [20]

Yan, Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors. Hakala, First principles modeling of perovskite solar cells based on TiO2 and Al2O3: Stability and electronic interfacial structure. Amin, A comprehensive defect study of tungsten disulfide (WS2) as electron transport layer in perovskite solar cells by numerical simulation.

White, interfacial passivation using ultrathin polymer-fullerene films for high-efficiency perovskite solar cells with negligible hysteresis.

CHAPTER TWO

A review of the current status of graphitic carbon nitride

Abstract

Introduction

As well-established sensors, g-C3N4-based materials have advantages such as high sensitivity and analyte selectivity, large surface area, and chemical stability [27]. Graphitic carbon nitride has been synthesized by various routes, but essentially g-C3N4 has been obtained by pyrolysis of any nitrogen-rich precursor such as melamine. Unfortunately, bulk g-C3N4 has poor electronic properties, such as high recombination rate of photogenerated electrons and holes, small specific surface area, and low quantum yield [25, 32].

Exfoliation methods, such as liquid phase exfoliation [36, 37], thermal etching exfoliation [38, 39], ultrasound-assisted exfoliation [40], chemical blasting [41] and chemical (using strong acids) exfoliation [42] have been used to achieve nanostructured materials with few sheets of g-C3N4.

Structures of graphitic carbon nitride

Geometrical structure
Electronic structure

72] reported the assimilation of g-C3N4 dots into the active layer of bulk heterojunction (BHJ) polymer solar cells (PSCs), and the power conversion efficiency (PCE) of the cell was significantly improved. However, it is worth noting that the use of g-C3N4 in polymer solar cells is very narrow in the literature [72]. The lone pair of electrons from nitrogen is responsible for the formation of the lone pair valence band and thus the band structure [84].

The combination of the lone pair state and π-bonding stabilizes the lone pair position, so it is worth noting that it is the nitrogen lone pair that is central to the electronic structure of g-C3N4 [85].

Figure 2.1: (a) s-Triazine and (b) tri-s-triazine-based structures of g-C 3 N 4 [76]

Different morphologies of graphitic carbon nitride

Bulk material
Nanosheets

Exfoliation of bulk material into nanosheets

Nanotubes
Nanowires
Quantum dots

Various methods, such as chemical exfoliation (using strong acids), thermal exfoliation, liquid ultrasonic peeling, and chemical blasting (using blowing agents) have been used to "peel" bulk g-C3N4. Challagulla and co-workers [105] used both thermal and chemical etching of bulk g-C3N4 to form nanosheets for the photocatalytic reduction of nitrobenzene. Initially, bulk g-C3N4 was prepared by thermal decomposition of melamine and the resulting material was exfoliated with deionized water and sonication.

Bulk g-C3N4 was prepared by thermal etching of melamine, after which it was ultrasonically exfoliated for 10 h.

Figure 2.3: A representation of the chemical exfoliation of bulk g-C 3 N 4 in which (a) bulk g-C 3 N 4 is intercalated with sulfuric acid and (b) an exfoliated g-C 3 N 4 nanosheet is formed [102]

Modifications to improve efficiency

Doping
Constructing heterojunctions
Coupling with carbonaceous materials
Introduction of defects

The 3D architecture offers more sites for photoreactions and better light absorption. 142] prepared a porous carbon/carbon nitride composite with improved photocatalytic degradation of methylene blue (MB) due to the synergistic effects of both carbon and carbon nitride, such as better light absorption efficiency, improved adsorption, and improved charge separation. The heat treatment induced many defects in the engineered electronic structure and thus increased the light absorption efficiency of the material.

The synergistic effects of both modifications give better charge separation, light absorption (red shift) and high surface areas, which are essential in photo-driven reactions.

Figure 2.8: A graphical representation of flower-like nanostructured phosphorus doped g-C 3 N 4 used in the photolysis of water to yield hydrogen [138]

Applications

Photocatalysis

Magnetic photocatalysts

Sensors
Energy storage
Photovoltaic cells

151] synthesized a C-doped ZrO2/g-C3N4/Ni2P (C-ZrO2/g-C3N4/Ni2P) composite based on the UiO-66-NH2 material for hydrogen evolution in water splitting. The composite was rapid and selective in the detection of glutathione in living cells and aqueous solutions. 163] used graphitic carbon nitride quantum dots (g-CNQDs) directly to detect hydroquinone in the environment (water and air systems).

Although this is the first report of the use of graphitic carbon nitride quantum dots in polymer cells, it shows that g-C3N4 has great potential for incorporation into more polymer cells.

Figure 2.10: Effects of altering bulk g-C 3 N 4 into nanosheets on the photocatalytic destruction of tetracycline where DCN is bulk g-C 3 N 4 , NDCN-4N is N-doped bulk g-C 3 N 4 , DCN-S is g-C 3 N 4 nanosheets and NDCN-4S is N-

Computational studies

Theoretical estimation of HOMO-LUMO band gap of carbon nitride oligomers and polymers

The calculated and experimental (UV-Vis and XPS) band gap of triazine-based g-C3N4 was between 1.6-2.0 eV. 171] theoretically calculated the band gap energy of the most stable g-C3N4 phase; the calculated band gap was 2.87 eV. For comparison purposes, the HOMO-LUMO band gap should be estimated experimentally and theoretically.

The LUMO is calculated by adding the HOMO values to the band gap energy obtained from UV-Vis absorption spectroscopy [201].

Figure 2.14: Theoretical decomposition process of NO with g-C 3 N 4 as a catalyst [179]

Summary and outlook

Experimentally, HOMO-LUMO apertures can be measured by both cyclic voltammetry (CV) [200] and photoelectron spectroscopy in air (PES). PES measurement can be done on solid thin films, determining the work function, and this corresponds to the HOMO. Similarly, CV measurements can be performed to determine the HOMO energy and the values obtained are added to the band gap values obtained by UV-VIS absorption spectroscopy to also determine the LUMO energies [202].

Finally, the authors believe that more research should be conducted on g-C3N4 to expand its potential applications.

Acknowledgements

Yang, Constructing simultaneous K-doping and exfoliation into graphitic carbon nitride (g-C3N4) for enhanced photocatalytic hydrogen production. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Zhai, Characteristic defects in graphitic carbon nitride for highly extended visible light photocatalytic hydrogen.

Yan, Soft-templating synthesis of mesoporous graphitic carbon nitride with enhanced photocatalytic evolution of H2 under visible light.

CHAPTER THREE

A theoretical study of the optoelectronic properties of heptazine-based graphitic carbon nitride

Introduction

For this reason, heptazine-based g-C3N4 has been extensively studied both theoretically and experimentally by many researchers [12, 13]. From the computational point of view, heptazine-based g-C3N4 has a hexagonal crystal structure with a lattice parameter “a” = 7.13 Å, which is close to an in-plane distance of repeating units of 6.788 Å as deduced from experiments [19]. Although heptazine-based g-C3N4 has been extensively studied both experimentally and computationally, most of its optical and electronic properties have not been fully elucidated, thus hindering its application in optoelectronics and nanotechnology.

In this study, the electronic and optical properties of heptazine-based g-C3N4 have been computationally investigated with the Wien2K computational platform.

Computational details

The Monkhorst-pack k-points mesh with distributions of reciprocal lattice vectors in the irreducible part of the Brillouin zone was used [31]. Ideally, a denser k-dot mesh is crucial because it reduces parasitic effects when calculating various properties of the material [31-33]. The optical properties of the heptazine system were derived using a complex dielectric function ε(ω) = ε1(ω) + ε2(ω), where ε1(ω) is the real part and ε2(ω) is the imaginary part.

The real part, ε1(ω), as determined by the Kramers-Kronig transformation, connects the two parts of the dielectric constant [29, 34].

Results and discussion

Structural optimisation and geometrical properties
Optical properties

The real and the imaginary dielectric function
The optical absorption

It can be observed that the band gap obtained from GGA-PBE was small and related to the band structure of the material (see Fig. 3.3a). In general, the two approaches (GGA-PBE and mBJ) showed similar results for both real and imaginary parts of the dielectric constant in both the ⊥ (xx direction) and // (zz direction) directions of the incident photons. The letters xx and zz stand for the ⊥ and // directions of the incident electromagnetic radiation, respectively.

The anisotropic behavior of the material is more pronounced when the mBJ approximation is used than the GGA-PBE approximation.

Figure 3.2: Geometrical structures of optimised bulk heptazine-based g-C 3 N 4 supercells:

Conclusions

Pietrowski, Structure Elucidation of Graphitic Carbon Nitride Nanomaterials via X-ray Photoelectron Spectroscopy and X-ray Powder Diffraction Techniques. Ghambarian, Tuning the Conductivity of Charged Triazine Quantum Dots and Graphitic Carbon Heptazine Nitride (g-C3N4 ) via nonmetal doping (B, O, S, P): DFT calcns. Hu, Corrugation matters: Structure models of single-layer heptazine-based carbon nitride from first-principles studies.

Umezawa, Crystal Structure Determination of Graphitic Carbon Nitride: Ab Initio Evolutionary Search and Experimental Validation.

CHAPTER FOUR

A comparative study between thermal etching and liquid exfoliation of bulk graphitic carbon nitride to nanosheets for solar harvesting

Introduction

To improve the performance of bulk g-C3N4, various modifications such as doping [27, 28] and exfoliation [29] on nanosheets have been performed. The exfoliated nanosheets have been shown to have better performance in a range of applications, such as the photocatalytic degradation of pollutants [30], hydrogen production [25, 31] and biosensing [32]. Currently, many attempts have been made to exfoliate bulk g-C3N4 into nanosheets with extensive light absorption properties [34].

Here, we have compared two methods of bulk g-C3N4 exfoliation; namely, thermal etching and liquid exfoliation.

Synthesis of bulk graphitic carbon nitride
Liquid exfoliation of B-g-C 3 N 4
Thermal exfoliation of B-g-C 3 N 4
Characterisation of the catalysts
Electrochemical measurements
Photocatalytic activity experiment

To our knowledge, this is the first time that these two exfoliation techniques have been compared in detail regarding their characteristics and the performance of the resulting materials. The contents of the box were heated, at a rate of 5 °C min-1, from room temperature to 550 °C and held at that temperature for 4 h. The thermal stability of the samples was determined with a TA Instruments Q series thermal analyzer (Q600) in air.

The system was purged for five minutes with nitrogen gas to remove dissolved oxygen in the solution that would otherwise interfere with the redox activity of the working electrode during measurements.

Results and discussion

Morphology and dimensions
Structural analysis
Surface area and porosity
Chemical structure
Chemical composition
Electrochemical properties
Optical properties and band structure
Band potentials and rate of recombination of photocharges
Photocatalytic activity
Kinetics and mechanism of photo-degradation of Rhodamine B

The crystal structures of the prepared samples were investigated by powder XRD, and the diffractograms are presented in Figure 4.4a. To further confirm the structures of the prepared samples, HRTEM analysis was performed, and the results are shown in Figure 4.4b. In order to investigate the rate of recombination of photogenerated charges, ie. efficiency of charge separation of the prepared photocatalysts, we measured the photoluminescence (PL) spectra (Figure 4.11b).

As depicted, for the first 60 minutes, adsorption of RhB on the surface of the catalyst occurred in the dark, and CNNS-TE and CNNS-LE photocatalysts showed higher adsorption than the bulk B-g-C3N4.

Figure 4.1: SEM images of (a) B-g-C 3 N 4 , (b) CNNS-TE nanosheets and (c) CNNS-LE nanosheets

Conclusions

Acknowledgement

Zhao, Porous defect modified graphite carbon nitride via a facile one-step approach with significantly enhanced photocatalytic hydrogen evolution under visible light irradiation. Parida, Facile synthesis of exfoliated graphite carbon nitride for the photocatalytic degradation of ciprofloxacin under solar irradiation. Sun, ultrathin graphitic carbon nitride nanosheet: A highly efficient fluorosensor for rapid and ultrasensitive detection of Cu2+.

Xi, Preparation of 2D graphitic carbon nitride nanosheets by green exfoliation approach and enhanced photocatalytic performance.

CHAPTER FIVE

Cadmium sulfide-doped graphitic carbon nitride nanosheets for improved photon harvesting of the active layer in organic solar cells

Introduction

The introduction of a third component into a donor-acceptor active layer has become an important strategy for enhancing the performance of OSCs [ 16 , 17 ]. 23] introduced graphitic carbon nitride quantum dots (g-C3N4 QDs) into the active layer of OSCs with a P3HT:PC71BM mixture. Devices with g-C3N4 QDs in the active layer as dopant performed better than undoped devices.

For comparison, a flawless device was also created with only P3HT:PC61BM blend in the active layer.

Experimental

Materials and chemicals
Synthesis of bulk graphitic carbon nitride
Synthesis of g-C 3 N 4 nanosheets
Synthesis of cadmium sulfide nanoparticles
Synthesis of CdS/CNNS-TE heterostructure
Characterisation of the photoactive materials
Fabrication of organic solar cells and their characterisation
Material characterisation

Surface morphology and microstructure

The SEM image of CNNS-TE shows a porous structure with disordered plate-like structures (Figure 5.2a). From Figure 5.2b, 5% CdS/CNNS-TE showed coarse particles with sizes ranging between 40 and 50 nm on porous plates. Meanwhile, a 5% CdS/CNNS-TE sample from Figure 5.3b showed agglomerated particles (5-10 nm) randomly anchored on CNNS-TE sheets.

To further investigate the microstructure of prepared samples, the interlayer distances, i.e. The d-spacings, examined from high-resolution transmission electron microscopy (HRTEM), and the images are presented in Figure 5.3 (d) and (f) and Figure S5.1 (c) and (d).