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Organic-inorganic lead halide perovskites have gained a lot of attention over the past six years as a desirable class of materials for various devices which has been shown to possess desirable properties such as long diffusion lengths, large carrier mobility, low trap states. At present, some important topics in this research have not been adequately addressed, like interface energetics, hot carrier dynamics and problems with UV degradation. This thesis is dedicated to address these issues. Firstly, the interface energetics of FAPbI3-based perovskites with organic charge extraction layers showed apparent upward band bending, but its origin is unexpectedly due to presence of δ-FAPbI3. This finding implicates solar cell operation as it causes inefficient extraction of photoexcited electrons to the PCBM. In the second part of the work, a successful homemade TR-2PPE system was set up, and preliminary studies on CsPbBr3 single crystals found that hot carriers generated through carrier-carrier scattering ∼1 eV above the conduction band edge possess long lifetimes ∼3 ns under resonant excitation due to strong band-filling and long lifetimes of band edge carriers in single crystals which hinder hot carrier cooling. This observation in CsPbBr3 may point towards using perovskite single crystals to generate and extract hot carriers in solar cells. The last part of this work addresses the issue of UV instability of perovskites in MAPbI3 through the addition of Hindered Amine Light Stabilisers (HALS) to suppress photodegradation by UV light through photo-oxidation and free radical formation of perovskite. A systematic study was performed by doping MAPbI3 through antisolvent treatment with four representative HALS compounds with varying functional groups and molecular sizes to study their effects on film formation, morphology and resulting photostability enhancement under UV light. Surprisingly, doping with different HALS compounds provides varying photostability enhancements and may even result in more photodegradation in some cases due to the influence of functional group and size on surface morphology, crystallinity and surface coverage. Among the HALS compounds, the polymeric one gave the best photostability enhancement with reduction of photodegradation by 59.2%. This method would not only be potentially useful for solar cell applications, but also in optical spectroscopy that uses UV light like TR-2PPE. Doctor of Philosophy

Heralded as the next miracle material, graphene oxide (GO) has the potential to be used in a variety of next generation devices. However, there are still gaps in the fundamental knowledge on this material and even now, a canonical model of GO has not been developed. The two aims of this thesis are to support the development of the canonical model by studying the O 1s1/2 spectra of GO and to develop the use of reduced GO for electronic device applications. Here, the identities of peaks in the O 1s1/2 spectrum were identified by chemical and mathematical analysis and the fabrication processes were made environmentally friendly and industrially scalable by the use of ascorbic acid for low temperature reduction of GO and ultrasonic spray coating for large area deposition respectively in transparent conducting electrodes (TCE). Further to this, the conductive character of reduced GO was also utilized in a bilayer device design with phthalocyanines for a NO2 sensing application. The binding energy of O 1s1/2 electrons in the carbonyl, carboxyl, hydroxyl and epoxy functional groups were found to be 530.9, 532.3, 533.1 & 534.4 eV respectively. With this information, metastability in GO was understood to result in the preferential formation of the carboxyl functional group, which cause vacancies in the graphene flake and are difficult to remove, during the thermal reduction of GO. This was estimated to occur between the temperatures of 543 & 561 K and it is recommended that thermal reduction methods keep below this temperature range. A TCE with a figure of merit of 189.9 was fabricated when GO reduced by ascorbic acid was used in conjunction with silver nanowires (AgNW) in a bilayer TCE. The reduced GO over layer was observed to retard AgNW degradation caused by capillary instability and this was achieved by protecting the surface of the nanowire from oxygen. This reduced surface diffusion of the silver atoms and extended the lifetime of the nanowires. Finally, a new bilayer gas sensing device utilizing phthalocyanines on reduced GO was described and tested. The bilayer design improved device currents, with no loss in normalized sensitivity and a theoretical model that was developed to describe it. In summary, a deficiency in the literature of GO was identified & solved and this finding will be a useful tool in the development of a canonical model for GO. The identification of the O 1s1/2 peaks of the different functional groups will also aid in the characterization of reduced GO with low C/O ratios, typically found in highly reduced GO. The main challenges preventing the widespread use of GO in electronic devices were identified to be device design and fabrication techniques. GO is a versatile material platform which can be used in a variety of applications, but a deep understanding of both GO properties and the properties of complementary materials is required so that the right device design can be employed to harness the desirable characteristics of these materials. Future work will focus on the development of the canonical model, consider methods of increasing the FOM of TCEs fabricated using reduced GO, confirm the stability of the bilayer device and expand the use of GO into other devices. DOCTOR OF PHILOSOPHY (SPMS)

Thin film solar cells (TFSC) such as Cu(In,Ga)Se2 (CIGS) and CdTe technology have been successfully commercialized with cost-per-watt on par with the conventional Si photovoltaics. Additional benefits of TFSC include shorter energy payback time, better light absorbance, and high flexibility (if deposited on flexible substrate) for a variety of applications. Among the industrialized TFSC, CIGS is less toxic and possesses the highest power conversion efficiency (PCE), but scarcity of indium and gallium could limit its growth potential. Therefore, research on earth-abundant and non-toxic Cu2ZnSn(S,Se)4 (CZTSSe) solar cell has generated tremendous interest in the photovoltaic community. However, large open-circuit voltage deficit (VOC,def) in CZTSSe solar cell remains unresolved even after a decade of research and development. Although open-circuit voltage (VOC) can be enhanced by increasing its bandgap (Eg) through the tuning of S/Se ratio, VOC,def (Eg/q - VOC) generally becomes more severe accompanied by significant loss in short-circuit current density (JSC) and fill factor (FF). These losses limit the power conversion efficiency (PCE) of CZTSSe solar cell. Therefore, in-depth investigations on the fundamental limitations are required to tackle the VOC,def issue. Other issues such as the JSC and FF losses, specifically in the high bandgap CZTSSe solar cells, need to be investigated as well to further improve the device performance. This thesis focuses on investigating the fundamental limitations of CZTSSe solar cells, in particular the large VOC,def, by electrical, optical and physical characterizations. Full bandgap range CZTSSe solar cells from 1.0 to 1.5 eV were studied in this study to provide a comprehensive understanding on the limitations of the CZTSSe technologies. The loss factors were investigated from two perspectives: (i) the intrinsic electrical and defect properties of the CZTSSe absorber layer, and (ii) the extrinsic loss factors arising from the interfacial secondary phases and interfacial resistances such as high shunt conductance (which lowers VOC) and high series resistance (which lowers JSC). The intrinsic limitations were investigated by self-developed electrical AC Hall measurement system and optical photoluminescence (PL) technique. From the AC Hall measurement, it was found that CZTSSe thin films (regardless of their bandgaps) typically have low carrier mobility of less than 1 cm2/V.s. This pinpoints one major factor to account for the high VOC deficit. In addition, PL studies indicate that CZTSSe material has a high defect density in the order of 1020 cm-3, which results in a deep fluctuating potential (γ) at the conduction and valence band given its low dielectric constant. The upper bound of γ for CZTSSe ranges from 115 meV (Se-rich CZTSSe) to 144 meV (S-rich CZTSSe). The fluctuating conduction and valence band (2γ) of 230 meV and 288 meV partly accounts for the VOC,def of 588 meV and 933 meV in Se-rich and S-rich CZTSSe, respectively. Adding a general thermodynamic loss of ~350 meV to 2γ, the VOC,def in Se-rich CZTSSe can be easily explained, but the VOC,def in S-rich CZTSSe remains largely unaccountable. The deep fluctuating potential will essentially reduce the energy of the photo-generated carriers and results in much lower PL photon energy compared to its bandgap. This is consistent with the low carrier mobility which partly account for the large VOC deficit. Temperature-dependent PL further suggests that non-radiative recombination centers could be the dominant defects in CZTSSe absorbers, and the defect levels are much deeper compared with the benchmark CIGS absorber. From the Arrhenius relation, the activation energy (Ea) which includes the effect of defect energy level and non-radiative recombination rate can be extracted. The Ea for CIGSSe, Se-rich CZTSSe and S-rich CZTSSe were found to be 20 meV, 48 meV and 67 meV, respectively. The particularly high Ea in S-rich CZTSSe serves as an additional factor to explain the largely unaccountable VOC,def (from the sum of thermodynamic loss and 2γ mentioned above). These undesired defect properties will exacerbate carrier recombination which already suffers from low carrier mobility. This will essentially lower the VOC of the CZTSSe solar cells. Extrinsic loss factors were investigated by electrical colour I-V (current-voltage sweep)¬, Suns-VOC and various physical characterizations including Glancing-Incidence X-Ray Diffraction (GIXRD), Field-Emission Scanning Electron Microscopy (FE-SEM), Raman mapping, X-ray Photoelectron Spectroscopy (XPS), and XPS depth profile. As shunt conductance was severe in high bandgap S-rich CZTSSe solar cell, it further lowers the VOC by external device leakage path. Physical characterizations suggested the formation of shunting secondary phases at the CZTSSe/CdS interface which provides such leakage path. Besides the VOC deficit issue, JSC loss resulted from the high series resistance (RS) was also observed in the high bandgap CZTSSe solar cell. Suns-Voc measurement indicates the formation of non-ohmic back contact, which could be due to Schottky contact formation arising from the low carrier density in high bandgap CZTSSe (as probed by AC Hall measurement), and hole blocking secondary phases forming at the CZTSSe/Mo interface which inhibits the carrier injection into Mo electrode. The latter factor was investigated extensively by various physical characterizations, and significant presence of high bandgap secondary phases was indeed found at the back contact interface. High bandgap CZTSSe, which are vulnerable to the formation of secondary phases, could suffer more in VOC deficit, JSC loss, and FF loss. With better understanding on the intrinsic limiting factor of VOC deficit, defect engineering was attempted in order to improve the crystal quality of CZTSSe. It was demonstrated that the defect properties could be improved, specifically by reducing and passivating the non-radiative defects through extrinsic Sb doping. With the reduction in total defect density by approximately half, the depth of the potential fluctuation was reduced from ~115 meV to ~86 meV. The reduction of γ corresponded well with the increase of VOC. Other approaches such as partial or full elemental substitution could also be pursued to engineer the defect properties, as elaborated in Chapter 7. Specifically, the high concentration of the deep CuZn antisite should be inhibited and shallower defects such as Cu-vacancy (VCu) should be promoted for improved device performance. Defect engineering should lift the kesterite solar cell research into a new phase which could potentially overcome the VOC deficit bottleneck. DOCTOR OF PHILOSOPHY (SPMS)

It is very critical in the subject of surface science to discriminate the initial- and final-state effects in photoemission of supported (or isolated) metal clusters. In this thesis, Auger parameter analysis is applied to separate the contributions of the initial- and final-state effect to the binding energy shift observed. The comparison made on TiO2(001) and TiO2(110) surfaces demonstrates the importance of substrate contributions to the initial-state effect. The further analysis helps us to assign initial-state effect to a combination of eigenvalue shift in surface core-level shift and charge transfer between the Ni clusters and the TiO2 substrates. The cluster size effect and interfacial charge transfer have a collective impact on the electronic structure of Ni clusters on the TiO2 surfaces. The variation of the triplet atomic states in Ni satellite features can be used to trace the electron transfer mechanism when the dimension of Ni nano-clusters is modified because the Ni 3d electron population varies with scaling the dimension size. The correlation enhancement of electrons in Ni nano-clusters due to the confinement in reduced dimension has been observed. Both the size and shape of nano-clusters show strong influence on the Ni 2p satellite structures. Apart from the 6-eV satellite, a 3-eV satellite structure emerges in Ni 2p3/2 photoemission spectra for small clusters due to the existence of 3E and 3T triplet states. However, the 3-eV satellite becomes weakened in larger clusters due to the increased electron population in 3d level, which is achieved through the electron transfer from the free-electron-like 4sp states to the unhybridized pure 3d spin down states. DOCTOR OF PHILOSOPHY (SPMS)

Doping ZnS nanocrystals with Mn2+ ions allows it to emit in the orange region and opens up its applications in electroluminescent displays, spintronics and biomedical labeling. One of the most interesting aspects of ZnS:Mn nanoparticles is the elucidation of factors that influence the shortening of their luminescence lifetime, which is important for their applications as phosphor and sensor materials. While several researchers have investigated this, it is not well understood. The motivation for this research, is to perform a systematic study in order to gain a fundamental understanding of the factors influence the luminescence of ZnS and ZnS:Mn nanocrystals. The successful doping of Mn was confirmed by various spectroscopic characterization techniques. Theoretical calculation for particle sizes was suitable with HR-TEM images and PLE spectra, which showed the effect of quantum confinement in synthesized nanocrystals. Master of Science