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.
| Identifer | oai:union.ndltd.org:ADTP/188055 |
| Date | January 2005 |
| Creators | Jiang, Chu-Wei, School of Photovoltaic Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Photovoltaic Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Chu-Wei Jiang, http://unsworks.unsw.edu.au/copyright |
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