No description available.

No description available.

Photovoltaics is currently the fastest growing energy source in the world. Increasing the conversion efficiency towards the thermodynamic limits is the trend in research development. ???Third generation??? photovoltaics involves the investigation of ideas that may achieve this goal. Among the third generation concepts, the tandem cell structure has experimentally proven to have conversion efficiencies higher than a standard p-n junction solar cell. The alternative hot carrier solar cell design is one of the most elegant approaches. Energy selective contacts are crucial elements for the operation of hot carrier solar cells. Besides the carrier cooling problem within the absorber, carrier extraction has to be done through a narrow range of energy to minimise the interaction between the hot carriers in the absorber and the cooler carriers in the contacts. Resonant tunnelling through localised states, such as associated with atomic defects or with quantum dots in a dielectric matrix, may provide the required energy selectivity. A new model in studying the properties of resonant tunnelling through defects in an insulator is proposed and investigated. The resulting calculations are simple and useful in obtaining physical insight into the underlying tunneling processes. It is found that defects having a normal distribution along the tunnelling direction do not reduce the transmission coefficient dramatically, which increases the engineering prospects for fabrication. Silicon quantum dots embedded in an oxide provide the required deep energy confinement for room temperature resonant tunnelling operation. A single layer of silicon quantum dots in the centre of an oxide matrix are prepared by RF magnetron sputtering. The method has the advantage of controlling the dot size and the dot spatial position along the tunnelling direction. The presence of these crystalline silicon dots in the oxide is confirmed by high resolution transmission electron microscopy (HRTEM). A negative-differential resistance characteristic has been measured at room temperature on such structures fabricated on an N-type degenerated silicon wafer, a feature that can be explained by the desired resonant tunnelling process. A silicon quantum dot superlattice can be made by stacking multiple layers of silicon quantum dots. A model is proposed for calculating the band structure of such a silicon quantum dot superlattice, with the anisotropic silicon effective mass being taken into account. It suggests a high density of silicon quantum dots in a carbide matrix may provide the bandgap and required mobility for the top cell in the stacks for the recently proposed all-silicon tandem solar cell. The resonant tunnelling modeling and silicon quantum dot experiments developed have demonstrated new results relevant to energy selective contacts for hot carrier solar cells. Building on this work, the modeling study on silicon quantum dots may provide the theoretical basis for bandgap engineering of all-silicon tandem cells.

The wide spread adoption of solar photovoltaic cells is impeded by a number of factors, the primary one of which is the cost. The technology behind the most used cells today is based on bulk single crystalline silicon wafers. These wafers subsequently undergo numerous processesto produce a finished module capableo f delivering usable direct current electricity. Even with all these processes, the single biggest contributor to production costs is the starting wafer - estimated to account for some 50% of manufacturing costs. Removing these costs by replacing the wafer is the leading topic in solar cell research today. Glass is the most convenient starting point for replacing silicon wafers - it is benign, both from an environmental and manufacturing viewpoint, and is considerably less expensive than silicon wafers for a given quantity. As an amorphous material, glass is well suited to acting as a substrate for amorphous silicon layers used in low cost cells. Amorphous silicon cells suffer from stability issues and can degrade in performance substantially over the operational lifetime of the solar cell. To overcomethese problems the amorphous silicon can be replaced with crystalline silicon material. Generally, the deposition of suitable crystalline material occurs at a temperature in excess of the softening point of glass. So however useful glass is as a substrate it is incompatible with simple, low temperature formation of crystalline silicon using most techniques. There are two outstanding issues relating to the manufacture of thin film silicon solar cells that have been researched for this thesis. One is the deposition of silicon layers at a growth rate high enough to allow for a reasonable throughput of material. The second is the production of material suited to the task i.e. structurally and electrically. In this thesis the direct deposition of high quality polycrystalline silicon( near-single orientation with suitable electrical characteristics) using electron cyclotron resonance plasma enhanced chemical vapour deposition(E CR PECVD) onto glass is demonstrated. A new visualisation of the magnetic field used in E R PECVD has given an insight into the optimisation of the deposition process using this technique. By adjusting the magnetic field appropriately, an increase in growth rate for deposition of polycrystalline silicon of 2- 25 times that reported in the literature was found. In addition to the characterisation of the deposited material, the process parameters have been fully investigated by analysing the process plasma characteristics using a Langmuir probe. An amorphous incubation layer 1 micron thick is seen when the polycrystalline material is deposited directly on glass, however this layer can be substantially reduced by depositing on a thin layer of silicon (on the glass) which has been crystallised by excimer laser irradiation. This indicatesa possible direction in combining these two approaches in future manufacturing processes for the growth of low-temperature polycrystalline silicon layers on glass to form photovoltaic devices.

No description available.

No description available.

Photovoltaics is currently the fastest growing energy source in the world. Increasing the conversion efficiency towards the thermodynamic limits is the trend in research development. ???Third generation??? photovoltaics involves the investigation of ideas that may achieve this goal. Among the third generation concepts, the tandem cell structure has experimentally proven to have conversion efficiencies higher than a standard p-n junction solar cell. The alternative hot carrier solar cell design is one of the most elegant approaches. Energy selective contacts are crucial elements for the operation of hot carrier solar cells. Besides the carrier cooling problem within the absorber, carrier extraction has to be done through a narrow range of energy to minimise the interaction between the hot carriers in the absorber and the cooler carriers in the contacts. Resonant tunnelling through localised states, such as associated with atomic defects or with quantum dots in a dielectric matrix, may provide the required energy selectivity. A new model in studying the properties of resonant tunnelling through defects in an insulator is proposed and investigated. The resulting calculations are simple and useful in obtaining physical insight into the underlying tunneling processes. It is found that defects having a normal distribution along the tunnelling direction do not reduce the transmission coefficient dramatically, which increases the engineering prospects for fabrication. Silicon quantum dots embedded in an oxide provide the required deep energy confinement for room temperature resonant tunnelling operation. A single layer of silicon quantum dots in the centre of an oxide matrix are prepared by RF magnetron sputtering. The method has the advantage of controlling the dot size and the dot spatial position along the tunnelling direction. The presence of these crystalline silicon dots in the oxide is confirmed by high resolution transmission electron microscopy (HRTEM). A negative-differential resistance characteristic has been measured at room temperature on such structures fabricated on an N-type degenerated silicon wafer, a feature that can be explained by the desired resonant tunnelling process. A silicon quantum dot superlattice can be made by stacking multiple layers of silicon quantum dots. A model is proposed for calculating the band structure of such a silicon quantum dot superlattice, with the anisotropic silicon effective mass being taken into account. It suggests a high density of silicon quantum dots in a carbide matrix may provide the bandgap and required mobility for the top cell in the stacks for the recently proposed all-silicon tandem solar cell. The resonant tunnelling modeling and silicon quantum dot experiments developed have demonstrated new results relevant to energy selective contacts for hot carrier solar cells. Building on this work, the modeling study on silicon quantum dots may provide the theoretical basis for bandgap engineering of all-silicon tandem cells.

Thesis (M. Des. (Hons.))--College of Fine Arts, University of New South Wales, 2001. / Also available online.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

Thesis (M.S. in Electrical Engineering and M.S. in Computer Science)--Naval Postgraduate School, March 2002. / Thesis advisor(s): Sherif Michael, Bret Michael, Todd Weatherford. Includes bibliographical references (p. 167-169). Also available online.