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Abstract: This study focusses on the surface and bulk properties of Ti–O thin film photoanodes for water splitting to generate green hydrogen. Here, TiOx thin films were deposited by reactive RF magnetron sputtering of Ti in an Ar + O2 atmosphere. The oxygen flow rate ηO2, was varied to grow a sequence of TiO, Ti2O3 and TiO2 layers, as determined by X-ray diffraction. The spectral dependence of the optical absorption coefficient reveals a significant colour evolution, which is due to the interference of light, as well as black appearance, resulting from strong absorption within the visible range. Electrical resistivity from impedance spectroscopy increased from 5.2 × 10−2 for black TiO (ηO2 = 5%) to 9 × 104 ohm cm for transparent anatase TiO2 (ηO2 = 30%). X-ray photoelectron spectra were collected at different photon energies, 200 and 1200 eV above the O 1s and Ti 2p core levels, probing the surface and subsurface states, respectively. The depth distribution of the OH–Ti3+ defects indicated their increased surface/subsurface concentration at higher ηO2. X-ray absorption spectroscopy (XAS) showed that the crystal field splitting increased from 1.7–2.1 eV to 2.2–2.3 eV as the amount of Ti3+ states decreased from 20% to 10%. Surface photovoltage (SPV) and the photoelectrochemical performance were correlated. The anatase/rutile mixture or pure anatase TiO2 photoanodes with the highest SPV values of about 270 mV demonstrated the best combination of high negative flat band potential (−650 mV), photocurrent density (350 μA cm−2 at 0 V vs. Ag/AgCl) and a reasonable shape factor (0.75). These findings highlight the critical role of surface-sensitive characterization in optimizing TiOx photoanodes for efficient solar-driven hydrogen development.

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Abstract: This thesis explores the influence of close space sublimation (CSS) growth conditions on antimony selenide solar cells as well as the possible benefits of post-growth processing approaches including an assessment of protective layer annealing. Single and two-step growth approaches involving the use of seed layers to modify film coverage and grain structure were investigated as a way of improving the solar cell performance. Both isolated layers and complete device structures were fabricated to allow investigation of the interrelation of preferred ribbon orientation with device efficiency. It was identified that, whilst the use of a seed layer was an important step to achieve good film coverage and grain morphology, the ribbon orientation appeared to have minimal influence on performance. The developed CSS growth approaches were then expanded to produce antimony sulfoselenide films and devices for a single phase source material. It was demonstrated that the approach was feasible, allowing the formation of material with a notably higher bandgap than for the base selenide. This indicated that material did not completely degrade during sublimation with the resulting devices achieve >4% efficiency and with a notably higher open circuit voltage than selenide counterparts. There were however significant issues with the formation of large oxide phase regions within the absorber. These served to reduce the device performance with the cause being attributed to sulphur loss and reaction with oxygen, the growth ambient during deposition. Post growth annealing approaches to improve antimony selenide solar cell efficiency were systematically investigated. Air, selenium, and nitrogen environments were initially compared across a broad temperature range. The results highlighted the degree of sensitivity of the material to post growth annealing with bot air annealing and selenization causing minimal changes to film morphology but drastic performance loss. Nitrogen annealing appeared more favourable with some minor open circuit voltage increases, both again the overall trend was a decrease in cell performance. To overcome these limitations the nitrogen ambient annealing approach was expanded to a protective layer annealing approach. A series of capping layers CdS, ZnO and P2O5 were deposited on the back surface prior to the annealing process to protect the antimony selenide layers and then etched off prior to device completion. The CdS capping layer was found to protect the surface from oxidation but frustratingly still resulted in performance decreases. There was however one “outlier” device series which showed a marked improvement for all device parameters. This result was not reproducible despite many attempts but seemed to indicate the potential of the approach so other materials were investigated. ZnO was considered but it was quickly determined it was unsuited as a capping layer. P2O5 however was tested and despite the limited number of samples being able to be prepared, it was found to notably improve device performance even with short time and low temperature anneals. Secondary ion mass spectrometry analysis showed significant quantities of phosphorus had been incorporated in the film during annealing. This finding demonstrates there is high potential from the protective layer annealing approach and indicates additional work in this area could leave to improved device efficiencies.

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Abstract: This thesis addresses challenges in the development of lithium-sulfur (Li-S) batteries using metallic MoS2 nanosheets as functional host materials. While our prior research established lithiated MoS2 (LixMoS2) as an effective sulfur host for Li-S batteries, two major barriers remain to practical implementation of LixMoS2: incomplete understanding its stability and its scalable synthesis. Therefore, the initial focus of this thesis was to comprehensively investigate the thermal and environmental stability of LixMoS2. This part of the thesis revealed that lithiation dramatically enhances the thermal stability of the meta-stable metallic 1T phase, preserving it up to 250 °C in dry air—well above the conventional phase transformation temperature of ~100 °C. We establish that moisture is the primary degradation factor, enabling conventional fabrication methods in dry-room conditions rather than requiring costly fully inert environments. Second part of the thesis involves microwave-assisted chemical exfoliation (MWCE), a novel synthesis approach that dramatically reduces reaction time from 48-72 h to just 30 s while achieving near-complete conversion to the desirable 1T phase. By incorporating carbon-based microwave susceptors and replacing conventional conduction heating with microwave heating, effective reaction temperature increases from 66 °C to 144 °C, significantly accelerating phase transformation kinetics. The enhanced understanding of stability mechanisms and improved synthesis methods enable high-performance Li-S batteries. We employ melt-diffusion method to incorporate sulfur into the 1T MoS2 host structure, creating composite cathodes that maintain excellent electrochemical performance even under commercially relevant conditions. The scalability of MWCE further enables the fabrication of Ampere-hour pouch cells that deliver impressive initial capacities of 1245 mAh g⁻¹ with stable cycling under lean electrolyte conditions.

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Abstract: Performance losses in positive–intrinsic–negative architecture perovskite solar cells are dominated by nonradiative recombination at the perovskite/organic electron transport layer interface, which is particularly problematic for wider bandgap perovskites. Large endeavours have been dedicated to the replacement of fullerenes, which are the most commonly used class of electron transport layers, with limited success thus far. In this work, we demonstrate blending the fullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and indene-C60 bis-adduct (ICBA) as a thin interlayer between 1.77 eV bandgap perovskite and an evaporated C60 layer. By tuning the fullerene blend to a trace 2% by mass of PCBM in ICBA, we remarkably form an interlayer which features improved energetic alignment with the perovskite and the PCBM[thin space (1/6-em)]:[thin space (1/6-em)]ICBA fullerene mixture, together with a stronger molecular ordering and an order of magnitude higher electron mobility than either neat PCBM or ICBA. Additional molecular surface passivation approaches are found to be beneficial in conjunction with this approach, resulting in devices with 19.5% steady state efficiency, a fill factor of 0.85 and an open-circuit voltage of 1.33 V, which is within 10% of the radiative limit of the latter two device parameters for this bandgap. This work highlights the complex nonlinear energetic behaviour with fullerene mixing, and how control of the energetics and crystallinity of these materials is crucial in overcoming the detrimental recombination losses that have historically limited perovskite solar cells.

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Abstract: Thermoelectric materials, enabling direct waste-heat to electricity conversion, need to be highly electrically conducting while simultaneously thermally insulating. This is fundamentally challenging since electrical and thermal conduction usually change in tandem. In quasi-two-dimensional conjugated coordination polymer films we discover an advantageous thermoelectric transport regime, in which charge transport is defect-tolerant but heat propagation is defect-sensitive; it imparts the ideal mix of antithetical properties—temperature-activated, exceptionally low lattice thermal conductivities of 0.2 W m−1 K−1 below Kittel’s limit originating from small-amplitude, quasi-harmonic lattice dynamics with disorder-limited lifetimes and vibrational scattering length on the order of interatomic spacing, and high electrical conductivities up to 2000 S cm−1 with metallic temperature dependence, notably in poorly crystalline structures with paracrystallinity >10%. These materials offer attractive properties, such as ease of processing and defect tolerance, for applications, that require fast charge, but slow heat transport.