Effects of d0 and s2 cations on optical properties of silicate glasses

There is a growing requirement worldwide for low-cost, reliable, and green electricity. From 2000 to 2015, the total installed capacity worldwide of solar photovoltaics (PV) increased from 4 GW to 227 GW, and is worth more than £75 billion annually. Solar photovoltaics are available in a multitude of technologies such as various morphologies of silicon, perovskite, organics, and other semiconducting technologies. However, a common issue regardless of technology is a spectral mismatch, where the incident solar irradiance does not equally match the range in which the semiconductor efficiently absorbs photons, and a second issue is degradation of polymeric components from UV photons. Ultimately critical failure of a solar module can occur due to the degradation of polymeric glues in the module, which allows for the ingress of water which rapidly leads to failure. Even before the critical failure, transmission of light is reduced due to the polymeric components becoming discoloured under UV irradiation to a yellow and brown colour due to the formation of organic radicals and short chain alkenes. Glass front sheets are used for the transmission of light to the active semiconducting material in a PV module whilst providing environmental, chemical and physical protection. Spectral mismatch and polymeric damage can be ameliorated through absorption of ultraviolet (UV) photons in the glass layer of the PV module. Incorporation of particular cations, in specific oxidation states into a glass matrix can afford strong UV absorption, and no visible or infrared (IR) absorption allowing for the protection of the polymeric species within the module with no reduction in transmission of lower-energy photons required by the PV cell for conversion to electric current. Furthermore, broadband visible emission can be induced from the absorption of UV photons with careful selection and preparation of the cations in the glass matrix, which allows a better spectral matching 8 from the incident (and re-emitted) radiation and the absorption profile of the semiconductor. This thesis describes the effects of certain cations with d0, d10 and s2 electron configurations in silicate glasses. Investigations into the optical, structural and chemical properties of doping silicate (soda lime silica and borosilicate) glasses with cations of titanium, zirconium, hafnium, niobium, tantalum, molybdenum, and tungsten (d0), zinc (d10), bismuth, lead and tin (s2) ions. Shifts in the absorbance profiles of doped and base glasses were measured by UV Visible IR absorption spectroscopy. These measurements were conducted in conjunction with UV Visible IR fluorescence emission and excitation spectroscopy measurements, by which the oxidation state(s) and fluorescence profiles of the dopants can be elucidated. X-Ray diffraction (XRD) was undertaken to confirm the amorphous nature of the materials prepared. Raman spectroscopy was used to investigate the structure of the glasses and to determine whether changes occurred upon addition of the dopants studied. Electron paramagnetic resonance spectroscopy (EPR) and X-Ray Absorption Near Edge Structure (XANES) measurements were performed to elucidate the oxidation state/s of the dopants within the glasses. X-Ray fluorescence (XRF) spectroscopy was carried out to measure the proportions of oxides within the glasses and confirm that the melt-quench regime did not result in excessive volatilisation of materials or other contamination. Differential scanning calorimetry (DSC) was used to determine the glass transition temperature (Tg) of the prepared glasses

Optical and structural properties of d0 ion-doped silicate glasses for photovoltaic applications

Optical and structural properties of float-type soda lime silicate (SLS) glasses doped with 0.2 mol % TiO2, ZrO2, HfO2, Nb2O5, Ta2O5, MoO3 or WO3 have been studied. Under UV excitation all d0 doped glasses exhibit broadband visible emission centred between 19,000 cm-1 and 25,000 cm-1 (400nm – 525nm) due to a transition from the 2p orbital of O2- to the metal d0 orbital. Dopant additions lead to shifts in the UV absorption edge to lower energies, with doped glasses having an absorption edge 2,000 cm-1 (~20nm), and in the case of MoO3, 4,000 cm-1 (~40nm), lower than the corresponding undoped glass. Combined UV-Vis absorption and X-band EPR spectroscopy analyses confirm that dopant cations occur in the studied glasses in the expected oxidation states of Ti4+, Zr4+, Hf4+, Nb5+, Ta5+, Mo6+ and W6+, although very low levels of Mo5+ are also observed, as demonstrated by the EPR resonance at g=1.92 (3.7T). The incorporation of the studied dopants into SLS glasses may find applications as cover glasses in photovoltaic (PV) applications, providing UV protection of polymers and solar cell materials in PV units whilst enhancing solar cell efficiency through downconversion / fluorescence of absorbed UV photons with re-emission as visible photons, available for absorption and conversion by the solar cell material

Non-isothermal crystallization kinetics and stability of leucite and kalsilite from K2O-Al2O3-SiO2 glasses

The crystallization mechanisms and elemental stability of leucite and kalsilite formed from K2O-Al2O3-SiO2 glasses were investigated by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Raman spectroscopy and differential scanning calorimetry (DSC). Glass samples with compositions along the leucite-kalsilite tie-line were produced by melt processing; and were then heat treated at 850ºC, 950ºC and 1250ºC for times ranging from 5 minutes to 1000 hours. Kalsilite is an unstable phase that behaves as an intermediate precursor to leucite. Crystalline materials in which kalsilite is the major phase lose potassium upon prolonged heat treatment (1000 hours at 1250ºC), in contrast to those with leucite, in which little or no compositional alteration is detected. The formation of leucite from stoichiometric kalsilite is accompanied by the formation of potassium doped alumina. The activation energies for leucite and kalsilite crystallization, determined via application of the Kissinger equation to thermal analysis data, were 579 kJ/mol and 548 kJ/mol respectively. Finally, production of pure leucite can be achieved with more favourable crystallization kinetics when starting with off-stoichiometric compositions