There is a worldwide effort to increase power generation through solar cells, to meet targets in reducing greenhouse gases. One requirement is for high efficiency multijunction solar cells (MJSCs) to extract power from concentrated solar power (CSP) plants, which are expected to become central to the delivery of solar power to national and super-grid systems. At present such MJSCs must combine different materials systems, and are usually limited by the requirement to lattice-match the individual cells to avoid efficiency losses due to defects. In this proposal we aim to circumvent these problems by investigating solar cells based on InxGa1-xN, which has a direct band gap of 0.7-3.4 eV, spanning most of the visible spectrum, thus promising MJSCs from a single materials system. To avoid the problems of lattice mismatch and of material quality, which limit prototype solar cells based on InxGa1-xN epilayers to low x (x<0.3), we will grow the InxGa1-xN in nanorod form, merging the nanorods using methods we have already developed to provide a solar cell template. The team assembled, which combines complementary expertise in growth and device fabrication (U. Nottingham), structural characterization (U. Bristol), nanoscale optical and electrical characterization (Arizona State U.) and solar cell design and characterization (NREL), aims to explore the properties of InxGa1-xN single junction cells over the full composition range (0

Potential Impact:
Concentrated solar power plants (CSPs, up to 1000 suns) have undergone rapid expansion in the last few years, and are expected to become central to the delivery of solar power to national and super-grid systems. A vital component for power extraction is the development of robust, high efficiency multijunction solar cells (MJSCs). The research here aims to establish InxGa1-xN as a viable material to produce efficient MJSCs by solving the problems encountered in high In, or lower bandgap, devices by using nanorod structures. Achieving MJSCs in this single materials system will lead to cost reduction. There is also greater potential for optimum design of MJSCs (with the best combination of band gaps), and for optimum design of the CSPs (as the necessary concentration factor and MJSC cost are linked). Our schedule is designed such that, at the end of the project, we should have sufficient information on growth and cell performance to attract commercial development of InxGa1-xN nanorod-based solar cells. This potentially benefits UK manufacturers of MJSCs, including e.g. Sharp, Quantasol and (potentially) IQE The research programme aims to clarify the fundamental science behind carrier migration along InxGa1-xN nanorods, and how this depends on composition, nanorod dimensions and the presence of defects. Studies by cathodoluminescence, electron holography and time-resolved photoluminescence will give information on the electronic properties, and on the role of electric fields which are lttle known for nanorod structures. This is needed firstly to understand solar cell performance. By revealing these fundamental properties, our research will also benefit the understanding of InxGa1-xN nanorods as potential sensors, and for light emitting applications such as light emitting diodes. By clarifying how defect-free growth can be maintained as the composition changes, and how defects are eliminated, the research will also benefit efforts to grow better quality continuous InxGa1-xN layers for devices, including, for example, light emitting diodes operating at green and longer wavelengths. In total there are planned to be 6 PhD students involved in this project, 4 in the UK of whom only 1 student is funded on the grant, and 2 in the US. By maximising exchange visits, and through the regular grant meetings, these students will benefit from gaining a broad range of research skills, and from active participation in an international research project. The provision of PhDs with such broad cross-disciiplinary skills is important for the future expansion of solar power.