PROF. MWABORA JULIUS M.

Substitutional Nb donor and Cr acceptor states in Anatase and Rutile TiO2 have been studied using generalized gradient approximation (PBE-GGA) employing pseudopotentials and plane wave basis sets in bulk. The calculations reveal that, on doping the Rutile structure with Cr and Nb atoms, new states were found to occur within the band gap, principally between 8.67 eV and 10.56 eV. These states are due to Nb_4d and Cr_3d orbitals. For the anatasse structure, states due to the dopants occurred between 6.663 eV and 8.939 eV. It was also observed that during the 2% doping with Cr and Nb, there were fewer new states in the band gap compared to many new states realized during the 4% doping and this happened in both Rutile and Anatase phases of TiO2. This shows that a higher doping concentration of 4% results in more energy states and hence more carriers, thus making TiO2 a better conductor than either 2% doping or pure TiO2. This study found that doping TiO2 (Anatase and Rutile) with either Cr or Nb at 2% and 4%, resulted in the removal of the energy band gap, implying improved conductivity compared to pure TiO2 [1] which exhibits insulating properties.

Titanium dioxide (TiO2) is used as semiconductor in the dye sensitized solar cell (DSSC), amongst many other applications. Thus coupled with a suitable sensitizer such as catechol, the study of surface electronic structure of TiO2 will improve light harvesting and electron transfer processes in DSSC. The distribution of states in clean and catechol terminated four and five layer TiO2 (110) rutile surfaces were investigated. All calculations in this work were done by quantum espresso code which uses plane waves and pseudopotentials. The slabs were modelled by four and five layers with vacuum width of 20 Å. The results showed that the (110) stoichiometric TiO2 four layer surface had band gap of 2.1 eV, a value less than band gap value of 2.2 eV of similar catechol bound TiO2 surface. There was an increase in the band gap value of 0.32 eV for the catechol bound TiO2 (110) rutile five layer surface compared to that of clean stoichiometric TiO2 (110) surface. The HOMO in four and five layered TiO2 (110) surfaces was found to lie above the valence band edge. The LUMO in both surfaces was located in the conduction band, and hence the band gap of the molecule was in the range of 4.0 eV. These findings have showed that the energy level alignment of catechol coupled to TiO2 is a suitable model to study electron transfer processes that occur in dye sensitized solar cell.

This study intends to realize a novel thin film material for photovoltaic applications. TiO2 that has a large band gap of 3.2eV is sensitized to visible light via the use of dyes in the Gratzel cell. The dye monolayer when excited by light photons, electron-hole pairs are generated, electrons are injected into the conduction band of TiO2, while the holes are transported to the counter electrode by diffusion. The use of dye and wet electrolyte material has associated instability problems which threatens the suitability of this type of solar cell for commercialization purposes.

The objective of this proposed study is to come up with a semiconductor material of a smaller band-gap which can be used to fabricate a solar cell. This is to be achieved by doping the metal oxide (TiO2) with germanium utilizing the property of the semiconductor nanodot band gap variation with the size. The reduction of the band gap is expected to broaden the wavelength range of the incident light that can be absorbed by the material. This involves the use of large band gap materials (TiO2,) in the form of a thin film that acts as the matrix within which atoms of Ge are added by doping. This enables the tailoring of the band gap of the matrix semiconductor (TiO2,) to absorb incident radiation of a wide range of wavelengths. Film deposition will be done using the sputtering method. Substrate temperatures will be varied for deposition in order to vary the phase. Annealing of the deposited films will be done at different temperatures. The films will then be investigated using various techniques to establish their structural, optical, electrical and opto-electrical properties. The results of the investigation will help to optimize the material performance for fabrication of the solar cells of high efficiency and low cost.

Limited energy resources and growing pollution associated with conventional energy production have stimulated the search for cleaner, cheaper and more efficient energy technologies. Hydrogen as a fuel is seen as one of the promising energy technologies alternative to fossil fuel. Metal hydrides have been suggested as potential candidates for the bulk storage of hydrogen. In this study, ab-initio calculations of metal hydrides that are promising candidates for hydrogen storage applications, that is, magnesium hydride (MgH2) and lithium hydride (LiH) was carried out using the Quantum Espresso computer code. The calculated quantities were the equilibrium structural parameters namely, the electronic properties as well as the thermodynamic properties. The calculated lattice parameters for MgH2 were a = 4.54 Å and c = 3.019 Å. Both values of a and c are in good agreement with experimental values of a = 4.501 Å and c = 3.01 Å. The calculated lattice parameter for LiH was a = b = c = 3.93 Å. The lattice parameter of LiH shows a correlation of approximately -3.79% with the experimental value of 4.083 Å. Thermodynamic properties of LiH were investigated by performing density functional theory within the quasi harmonic approximation. The temperature dependence of the heat capacity at constant volume CV, the Helmholtz free energy ∆F, the internal energy ∆E and the entropy ∆S was obtained. The thermodynamic properties and formation enthalpies are in good agreement with the experimental data.

Titanium dioxide has been intensively studied as a wide band gap transition metal oxide due to its n-type semi-conducting property which makes it to have many applications in industry. Some of the observed conductivity arises from its intrinsic point defects. The structural properties and electronic band structures of TiO2 (rutile and anatase) phases, have been investigated using ab-initio methods. The structural properties were obtained using generalized gradient approximation (GGA) employing pseudopotentials and plane wave basis sets. For the two phases of TiO2, the calculated equilibrium lattice constants, bulk moduli and bond lengths were found to be in good agreement with other recent theoretical calculations and also with experimental data. After introduction of various defects to the perfect super cell, the Ti-O bond lengths were altered greatly. The apical bond lengths changed from a constant 1.959 Å to a range of values (1.718 - 1.861) Å, and the equatorial bond lengths changed from a constant 2.006 Å to a range of values (2.072 - 2.231 ) Å for rutile TiO2. The apical bond lengths changed from a constant 1.956 Å to a range of values (1.782 - 1.830) Å, and the equatorial bond lengths changed from a constant 2.050 Å to a range of values (2.112 - 2.214) Å, for anatase TiO2. Also altered were Ti-O-Ti angles, from the two constants (99.93, 131.04)° to a range of values (88.86 - 95.69 and 132.01 - 143.49)° for rutile TiO2. For anatase TiO2, Ti- O-Ti angles changed from the two constants (103.81, 152.39)° to a range of values (93.59 - 149.91 and 156.74 - 176.05)°. Electronic properties were investigated too. Perfect rutile and anatase super cells gave band gaps of 2.24 eV and 2.44 eV, respectively, underground-state conditions. Valence bandwidths (VB) and conduction bandwidths (CB) were also obtained for both phases. VB of 5.6 eV and CB of 1.654 eV were observed for rutile TiO2, while VB of 4.76 eV and CB of 2.35 eV were observed for anatase TiO2; all in good agreement with experimental values. This study also investigated the defect formation enthalpies of Frenkel and Schottky defects in both rutile and anatase phases of TiO2. This study also considered point defect stability in rutile and anatase phases of TiO2. The formation energies for oxygen and titanium atoms, defects were found to be in agreement with the experimental values, especially the case of rutile oxygen atom vacancy. Both Frenkel and Schottky defects were found to induce new energy states in titanium dioxide. Normally band gaps are reduced in defective TiO2 crystals, and in this study, reduced energy band gaps were reported for all the defective super cells. In rutile, the metal oxide gaps were found to almost vanish due to the presence of oxygen atom vacancy, oxygen atom Frenkel and titanium Frenkel defects. These gave direct energy band gaps: 0.35 eV, 0.207 eV and 0.327 eV, respectively. Defects in anatase phase showed a similar trend, with the least energy band gap being reported for the case titanium interstitial (0.041 eV, which is indirect). With such infinitesimal gaps, these otherwise insulating oxides can with ease become conducting metal oxides, by either increasing the temperatures or pressure since these calculations were done at 0 K and 0 pressure. It can thus be said that intrinsic point defects in titanium dioxide do contribute to the improvement of the electrical conductivity of this oxide.

Solar cell has the potential of being the main drive to economic prosperity as it is one of the most promising sources of cheap, environmentally friendly and renewable energy. Crystalline silicon based technology currently dominate the solar energy market. However, it is generally expensive and the cell efficiency has reached 24.7% hence approaching theoretical expected maximum of 30%. In order to reduce cost of production, focus is shifting towards thin film based I-III-VI family chalcopyrite compounds where cheaper CuInxGa1-xSe2 absorber semiconductor is reported to have attained the highest efficiency of 20.3 %. This study intends to fabricate and characterize a compound of copper, aluminum, boron and selenium (CuAlxB1-x Se2 ) thin film. The compound is based on I-III-IV family of chalcopyrite which has generated a lot of interest as an absorber material for solar cells due to their high absorption coefficients. The research procedure will involve deposition of CuAlxB1-xSe2 thin film by DC and RF magnetron sputtering of CuAlB alloy and selenium targets respectively. The deposition is done using Edwards Auto 360 RF and DC magnetron vacuum system. Characterization of the resulting thin film based on structural and optoelectronic properties is done using X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Scanning Electron microscopy (SEM), UV-Visible-IR Spectrometer, and the Hall Effect. The outcome of this research will provide fundamental practical science and engineering knowledge base on structural and optoelectronic properties of CuAlxB1-x Se2 compound as solar absorber material among other optoelectronic applications. In general the study will contribute towards achieving greater efficiency in production of “green” energy.

Light soaking characterization on complete SnO2:F/TiO2/ln(OH)xSy/PEDOT:PSS/Au, Pb(OH)xS)pEDOT:PSS/Au, eta solar cell structure
as well as on devices which do not include one or both TiO2 and/or PEDOT:PSS layers has been conducted. Additionally,
studies of SnO2:F/In(OH)xSy/PEDOT:PSS/Au solar cell have been performed. The power conversion
efficiency and the short circuit current density have been found to increase with light soaking duration by a factor of
about 1.6 - 2.7 and 2.1 - 3, respectively. The increase in these two parameters has been attributed to the filling up of trap
states and/or charge-discharge of deep levels found in In(OH)xSy. These effects take place at almost fill factor and open
circuit voltage being unaffected by the light soaking effects.