3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Iinvestigation of ZnO, and AZnO and rare earth doped ZnO thin films for spectral conversion and application to solar cells(2018) Otieno, Francis OtienoRecently Zinc oxide has drawn a resurgent attention in semiconductor industry due to its interesting properties with diverse application potential. These properties include high exciton binding energy, high resistance against radiation, high breakdown voltage, insensitivity to visible light, and easy wet chemical etching. The high quantum efficiency for emission by ZnO has seen it being considered a strong candidate for solid-state white lighting applications as well as transparent conductor electrode in solar cells. In order to realize efficient utilization of the multi-functional properties of ZnO for electronic and opto-electric applications, ZnO is usually doped with different elements. Such doping is aimed at enhancing and controlling its electrical, optical and multi-functional properties. Typical dopants widely used are trivalent atoms categorized as group III in the periodic table (Al, In, Ga) through substitution of cations. The as-grown ZnO thin film is usually n-type semiconductor with structural, electrical and optical properties that can be varied depending on the growth conditions as well as post deposition treatment such as thermal annealing. The use of RF sputtering for ZnO deposition has been explored in this work through varying deposition time, RF power and the partial pressure of oxygen. The films were then subjected to ex-situ thermal annealing in Argon filled furnace leading to a significant increase in grain size. Rare earth (RE) doping of materials has been widely investigated owing to the prominent and desirable optical and magnetic properties. Typically trivalent rare earths elements such as Sm+, Tb3+ and Eu3+ are investigated in this research project. ZnO doped with RE has exhibited electroluminescence, thus highlighting its potential for photovoltaic applications as a bi-functional layer. A doped ZnO layer is thus simultaneously utilized as transparent conducting electrode and as a spectral conversion layer. The RE doped luminescent materials provide an opportunity to effectively use the high energy and sub-band gap energy photons from the solar spectrum that would have otherwise been lost in direct band gap absorbers. In solar cells, they have been applied with an intention to reduce the fundamental thermalization losses arising as a result of the intrinsic properties of the semiconductor material namely: (a) sub-bandgap photon loss (b) thermalization of charge carriers resulting from absorption of high energy photons. From the X-ray diffraction (XRD) patterns both pristine and doped ZnO thin films showed growth along the c-axis of the wurtzite structure. The peaks were found to match the reflection planes of (100), (002) and (102) with all the diffraction peaks being well indexed to the wurtzite structure of ZnO of the space group P63mc, which is consistent with the standard values reported in JCPDS, card no. 03-0888. The structural properties of the material were investigated using a -scanning electron microscope (SEM) and Atomic force microscopy (AFM) where the particle size, roughness, skewness and kurtosis were found to change with growth condition and annealing temperature. Most importantly, the results indicated that the photoluminescence (PL) properties reflect the quality of the pristine and doped ZnO. The films were then used in the fabrication of the solar cells as a bi-functional layer and thus as a proof of concept of good transparent conducting oxides (TCOs) and for spectral conversion. RBS measurements indicated the depth profile distribution of Zn, O and various rare earths which showed homogeneity in depth distribution without any external impurity.Item Enhancement of photo-conversion efficiency of organic solar cells by plasmon resonance effect(2016) Otieno, Francis OtienoOrganic Photovoltaic (OPVs) is a promising alternative technology to provide clean and inexhaustible energy due to their excellent optoelectronic properties of the active polymer blends. The organic polymers have low weight, tunable electrical and optical properties besides being relatively insensitive to film imperfections which in the long run enable low-cost high-throughput roll-to-roll processing. However, their photo-conversion efficiency (PCE) and instability to air remains their greatest drawback as these preclude their commercialization. Indeed the highest power-conversion efficiency reported in literature is between 10-12 % compared to their inorganic counterparts (40 %). Therefore there is great need for improvement to make them competitive with grid parity. In this thesis, the major factors limiting the efficiency of organic solar cells such as light absorption, exciton diffusion and dissociation as well as charge collection are investigated and discussed. Despite the high thickness dependent absorption coefficients (>105 cm-1) within the visible spectrum the materials exhibit short exciton diffusion lengths (10-20 nm) which limit the optimal active layer thickness to a few nanometers. Improving optical absorption within this thickness forms the basis of this project. We report the use of surface Plasmons synthesized by both thermal evaporation and Radio Frequency (RF) magnetron sputtering system to tune and enhance optical absorption and scattering using the surface Plasmon resonance effect. The NPs were annealed at various temperatures and for different times to reconstruct and modify their shapes, sizes as well as the inter-particle distance (coverage). Stability is of paramount importance in organic semiconductor devices. Serious degradation in air constrains their applications potential. The study further investigates the mechanisms that determine the stability of organic photovoltaic devices. Our results depict the degradation mechanisms and their circumvention through the use of high mobility pentacene to improve stability.