Growth of Semiconductor Nanowires for Solar Cell Applications
Abstract: Nanowires have the ability to absorb light much more efficient than conventional thin film layers. This makes them candidates for the development of new types of solar cells that have higher efficiency and lower material usage than current technologies. In this thesis fabrication of nanowires with techniques suitable for large area applications are investigated. The nanowires are grown by either Metal Organic Vapor Phase Epitaxy (MOVPE) or a novel technique called Aerotaxy. When using MOVPE nanowires are nucleated on a substrate. The position of nucleation can be determined with nanoimprint lithography by which several square centimeter surfaces can be covered with nanoscale patterns. The local environment will thus be similar for all nanowires during synthesis which is a prerequisite to achieve homogenous nanowire properties. During synthesis we have developed an optical technique to monitor the growth in situ. In situ monitoring can be used to precisely control the length of different sections, increase reproducibility, and achieve a better understanding of the complex nanowire growth process. Aerotaxy grown nanowires nucleate from gold particles which are suspended in a stream of gas. This can increase the production rate substantially compared to substrate based techniques. By controlling the gold particle size and the reaction conditions in the growth chamber the nanowire properties can be tuned. By incorporating a p-n junction within each nanowire it is possible to fabricate solar cell devices. We have connected two p-n junctions in series within a single nanowire by using a tunnel junction. This type of structure can in the future be used to achieve higher efficiencies of nanowire solar cells by implementing a multi-junction structure, with several sub-cells of different absorbing material. In a radial solar cell design carriers are extracted orthogonal to the direction of incoming light. With this geometry we could compare a vertical array device with that of a single nanowire device. With the array device an efficiency of 5.3% was achieved.
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