Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 192-198). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 107-111). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 107-111). Also available on the Internet.

M.Sc. / Current solar cell research programmes in general aim to develop a high conversion efficiency photovoltaic (PV) module from high quality thin films. In this study, Cu (In,Ga)Se2 (CIGS) thin films were grown and characterized. These films were grown by selenization of Cu-In-Ga precursors. These precursors were prepared by co-sputtering In and (Cu, Ga). All the precursors were grown on Mo coated soda lime glass substrates. The selenization was conducted under different conditions in Ar/H2Se atmosphere, i.e. taking different values of flow rate of H2Se (5.00, 1.00, 0.25 mol%) in Ar, temperature (350, 450, 550 ºC) and time (10, 20, 30, 40, 50, 60 min). At each selenization condition, two samples were placed at different positions in the chamber. The structural properties of the produced films were analyzed by the techniques of X-ray Diffraction (XRD) for phases, Scanning Electron Microscopy (SEM) for morphology and Energy Dispersive Spectroscopy (EDS) for the bulk composition. The effect of temperature variation, the effect of flow rate variation and the effect of time variation were analyzed by comparing the structural properties as analyzed by the techniques mentioned. All in all this specific study delivers important information about the sensitivity of Cu(In,Ga)Se2 (CIGS) thin films to the temperature, gas flow rate and exposure time of the selenization step. / Doctor C.A. Engelbrecht Professor Vivian Alberts

M.Sc. / The efficiency of commercial polycrystalline silicon solar cells is currently 12% and 15% in the case of single crystalline cells. It is possible to lose about half of the open circuit voltage due to inferior contacts on the cell. It is thus clear that inferior contacts can seriously impede the relative low efficiency and care should be taken to make good ohmic contacts. Experiments were done to evaluate the influence of several factors on the quality and stability of the contacts. 1 C2•cm p-type polycrystalline silicon and 3 52.cm n-type single crystalline silicon were primarily used for these experiments. Results of molybdenum contacts on n-type silicon are also presented and the problems with silver epoxy contacts are discussed. It was found that aluminium contacts on p-type polycrystaline silicon improve with temperature and time, while those on single crystaline n-type degrade with temperature and time. These changes are already present at room temperature and are attributed to solid state diffusion of the aluminium into the silicon. This results in a p + layer. In the case of contacts on p-type, the behaviour is that of a Schottky diode. After the solid state diffusion, it becomes possible for the charges to quantum mechanically tunnel through the p+ layer. This results in an improvement of the contact. The contacts on n-type however, are ohmic just after evaporation. Similar to the p-material, the p+ layer causes a p+-n-junction with the depletion layer primarily in the n-type material. This causes a degradation in the contact quality. It is possible to achieve good quality contacts on polycrystaline p-type material, by annealing the contacts above 500°C for one minute. These contacts however, are non-ideal. SEM photographs show that the silicon surface is crated by pits due to solid state diffusion. It is only at these pits that conduction through the Schottkybarrier is possible. Since the area of the pits constitutes only a portion of the total area, only a portion of the surface will partake in conduction. Contact resistance is always present. For pm sized contacts on integrated circuits, the spesific resistance is of the order of 10 -6 Q.cm2. Contacts on solar cells, however, are of millimetre dimensions and the spesific resistance can be four orders of magnitude larger. The conduction through the surface can be modelled as conduction through a surface that is constituted of a mixture of minute ohmic and diode surfaces.

M.Sc. / CuInSe2 (CIS) is considered to be one of the most promising candidates for high efficiency thin film solar cells. The reaction of metallic alloys to a reactive selenium atmosphere (H2Se/Ar or elemental Se vapour) is a promising growth technique to produce CIS thin films of high crystalline quality. However, up to now, the control of the final film quality has been critically influenced by the loss of material and subsequent formation of detrimental binary phases during the high temperature selenization stages. In this study, it is shown that this phenomenon is strongly related to the selenization temperature and, in particular, the ramping procedure followed to the final selenization temperature. Metallic alloys which were selenized in H2Se/Ar at 400°C or slowly heated in 20 minutes to temperatures around 400°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to have nonuniform surface morphologies, highly defected 0.8-2 !lm sized grains and to contain Cuselenide binary phases. Energy dispersive X-ray spectroscopy (EDS) analysis confirmed the generally reported sharp increase in the Cu/In atomic ratio for these classes of samples. In contrast, rapid heating (in 2 minutes) of identical metallic alloys to temperatures above 400°C, resulted in uniform, dense films with low defect density 1 !lm sized grains void of any evidence of secondary phases. X-ray fluorescence (XRF) Kal,2 measurements of metallic alloys at different stages of selenization revealed no evidence of material losses. XRF depth profiles, however, explained this discrepancy by revealing a pronounced segregation of In towards the Mo back contact when the samples were selenized at 400°C, or slowly heated to temperatures around 400°C. This segregation was dramatically reduced in films rapidly heated and selenized at temperatures above 400°C. For the purpose of comparison, metallic alloys were also reacted to elemental Se vapour. The structural features (grain size and preferred orientation) ofthese films differed significantly from those selenized under similar conditions in H2Se/Ar. The results from this study, including photoluminescence (PL) measurements obtained from these films, were used to affect the fabrication of CIS absorbers with excellent material properties and solar cell devices with moderate conversion efficiencies.

M.Sc. / Techniques for the fabrication of polycrystalline silicon solar cells have advanced in recent years with efficiencies exceeding 17%. The major advantage of polycrystalline silicon is its low cost relative to single-crystalline silicon. The disadvantage is the significantly smaller minoritycarrier bulk diffusion length and inhomogeneous nature of the material. These two drawbacks are due to the presence of grain boundaries as well as high concentrations of dislocations and other physical and chemical defects. In this study the experimental conditions were determined to fabricate solar cells on polycrystalline silicon substrates. The controlled diffusion of phosphorous into silicon and subsequent evaluation of the doped layers (by spreading resistance profiling and chemical staining) were important aspects of this study. From these results the diffusion parameters (i.e. temperature and reaction times) could be optimized in order to improve the solar cell output parameters. Additional material improvement (increase in surface- and bulk minority carrier lifetimes) was demonstrated by the hydrogen passivation of electrically active defects in polycrystalline silicon. However. measurements on hydrogenated silicon samples also indicated that excess passivation can result in surface damage and subsequent reduction in the minority carrier lifetimes. Preliminary solar cells were fabricated on polycrystalline silicon with efficiencies ranging between 0.5 and 6% (total area = 16 cm2).

Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.

Titanium dioxide, TiO2, has potential applications as a photoelectrode for photoelectrochemical generation of hydrogen by splitting water using solar energy and as a photocatalyst for water purification. This study is part of the UNSW research program to process TiO2-based oxide semiconductors as high-performance photoelectrodes and photocatalysts. This study investigates the effect of defect disorder on semiconducting properties of polycrystalline TiO2. This study involved the processing of high-purity polycrystalline TiO2 and determination of its semiconducting properties through measurement of electrical conductivity and thermoelectric power at elevated temperatures (1073-1323K) in controlled oxygen activities [1x10-13 Pa < p(O2) < 75 kPa]. The study included two types of experiments: Determination of electrical properties under conditions of gas/solid equilibrium. The data obtained was used to derive defect disorder and related semiconducting properties Monitoring of electrical properties during equilibration. This data was used to determine the chemical diffusion coefficient. The data obtained under equilibrium conditions indicates that oxygen may be used as a dopant to impose controlled semiconducting properties. In reduced conditions TiO2 is an n-type semiconductor and under oxidizing conditions TiO2 is a p-type semiconductor. The n-type behaviour is associated with oxygen vacancies as the predominant defects and titanium interstitials as the minority defects. The p-type behaviour is closely related to titanium vacancies that are formed during prolonged oxidation. Charge transport at elevated temperature was shown to involve substantial contribution from ions. Analysis of electrical properties enabled determination of several defect-related quantities including the activation enthalpy for oxygen vacancy formation, and the activation energy of the electrical conductivity components related to electrons, holes and ions. The kinetic data obtained during gas/solid equilibration enabled determination of the chemical diffusion coefficient which exhibited a complex dependence on nonstoichiometry. In addition, prolonged oxidation showed that equilibration occurred in two kinetic regimes. One for highly mobile oxygen vacancies and titanium interstitials which quickly reached an ??operational equilibrium?? within hours and another slow kinetic regime for equilibration of titanium vacancies over many thousand hours. The determined chemical diffusion coefficient data may be used to select the processing conditions required to impose uniform concentration of defects within a TiO2.

Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.