One of the technologies with the greatest scope for reducing greenhouse gas emissions is photovoltaics. The most important step in promoting photovoltaics is to bring down the cost of solar electricity to a level lower than electricity from fossil fuels. This can be achieved by simultaneously increasing the device efficiencies and reducing the volume of semiconductor material used for absorbing the solar radiation. III-V semiconductor nanowires maximise absorption of sunlight per unit volume of material. In fact, modelling suggests that III-V semiconductor nanowire solar cells require only a small fraction of semiconductor material used in conventional solar cells to achieve similar or higher efficiencies.
Achieving high efficiencies in solar cells also requires efficient separation of photogenerated electron-hole pairs to prevent recombination and generate current in an external circuit. The most widely used approach for electron-hole separation is to create electric fields at a junction formed between p- and n-type materials, in what is known as a p-n junction, to force electrons and holes into different regions of the device (Figure). Fabricating good quality p-n junctions at the nanoscale is very challenging. Moreover, eliminating the need for p-n junctions will reduce the number of process optimization steps. Thus, photovoltaic device structures that do not rely on p-n junctions for separation of photogenerated electrons and holes, are a promising alternative to fulfil the potential of the nanowire array device geometry.
This project will develop alternative approaches to separate photogenerated electron-hole pairs to exploit the full potential of nanowire solar cells. This involves the search for materials that have a type-II bandgap alignment with III-V semiconductors (Figure). For heterostructures with type-II bandgap alignment the minimum in conduction band and the maximum in valence band occur in different materials. Hence electrons and holes diffuse to different sides of the heterostructure and are spatially separated. Such structures are ideally suited for photovoltaic applications.
This project involves aspects of materials development, nano-scale fabrication and device characterisation and will result in products that will have a direct impact on the solar cell commercial market. The materials development aspect involves identifying correct material combinations that will maximise electron-hole separation; nano-scale fabrication is required to fabricate these materials in the correct geometry/configuration and the device characterisation steps are necessary to evaluate the performance of the final device. Students may choose to work on any aspect of this project that interests them.