3. Electronic Theses and Dissertations (ETDs) - All submissions
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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.Item Structure and optical properties of natural low dimensional, semiconducting, organic inorganic hybrids(2013-02-06) Black, Robert ShewanThe aim of investigating the 2D PbX6 inorganic organic hybrids was to study octahedral distortions, short interlayer spacing’s, and the effect of functionalized aliphatic’s terminal halogen on idealizing or destabilizing the octahedral arrangements and their effect on the band gap of the single layer 2D hybrid systems. It was found that the PbX6 metal centred distortions do display some impact on the band gap, the greater the distortion experienced in the Ieq-Pb-Ieq cis bond angles, the wider the band gap, as we suspect a decrease in I 5p antibonding character which lowers the top of the valence band. The terminal halogen interaction specifically in (BrC2)PbI4, clearly displayed some Br 4p/s character at the bottom of the conduction band, which may further explain the reduction of the band gap of this compound. This in conjunction with the shorter interlayer spacing serve to stabilize more idealized bridging angles, as seen in both the lead iodide and bromide analogues. In the short interlayer spacing compounds large idealizations of the Pb-X-Pb bridging angles are observed however display a large metal centred octahedral distortions in order accommodate the spatial occupation of the lone pair on lead. It was generally observed that the lead bromide hybrids appear to have a greater sensitivity to exciton lattice interactions, which give rise to red shifted emissions and absorptions with decreasing temperature. Structurally this behaviour is counterintuitive; because the structures increase in inorganic distortions with decreasing temperature and therefore a blue shift in the exciton absorption is expected. It should be noted that compounds displaying this phenomenon most, (C4, C6, C7)PbBr4 do display a large amount of structural disorder in their lower temperature phases. In the 1D systems investigated further structure to property correlations were made. Optically it was found that unlike the corner-shared perovskite type 1D wires of [NH2C(I)=NH2]3PbI5 and [CH3SC(=NH2)NH2]3PbI5 the first exciton absorption of the octahedral face sharing wires of (A)PbI3 appear to be largely insensitive to the inorganic structural distortions experienced as a result of the low temperature phase transitions. In one instance however a low temperature phase transition did result in a polaron emission which was directly related to a discontinuity in the inorganic wires. More generally experimental links between the STE luminescence emissions and the inter-wire spacing, organic dielectric constant, and the density of the crystal, were shown to influence the STE lattice interactions to a greater degree. This effect is increased through a decrease in crystal density and organic dielectric constant, with an associated increase in the inter-wire spacing. Therefore as the exciton lattice interactions increase, a red shift in the STE emissions is observed. In another series of systems strong 1- and interactions were present in particularly two 1D charge transfer compounds. It was noted that the inorganic wires promote interactions between the organic templates as has also been established in literature. Structurally it was also observed that the CT transitions of these compounds begin to largely coincide with the STE emission arising from the inorganic wire. Even though the CT compounds structurally have strong interactions the current experiments do not ascertain to what degree this interaction assists in electron transport. It was also established that as intermolecular interactions are absent in previously published MV and Et compounds with the dominant CT interaction was the I…N interaction which functions over a large range (4.9A). This long distance is substantiated from the strong covalent character of the I…N interaction observed in IR experiments completed on (MV)Pb2I6. It was also observed in our compounds that the position of the LUMO of the organic cation relative to the valence band of the inorganic wires appears to be largely dependent on the N…I distance and largely independent of the electron accepting templates HOMO-LUMO gap. The increased wire thickness observed in these compounds does appear to display a pronounced effect on the PL emissions as seen in three chain wide wires produced. The thicker chains begin to allow higher energy emission’s to occur i.e. the desired first exciton emission begins to become favored due to the relaxing of the wires spatial confinement on the electron-hole’s orbit. Further investigations are needed into even thicker chain wires, in order to ascertain the ideal size of the wire to obtain the desired high energy first exciton emission. To date the wire thickness that does give rise to the first exciton emission appears to still need at least six coordinated PbI6 octahedral units.