With the revision of the bandgap of InN down to about ~0.65eV, the III-Nitride material system has been touted as a promising photovoltaic material system. There is clearly potential for new and promising photovoltaic devices. However, several key challenges exist to reach this goal. Issues with mismatched tandem cells creating minority carrier recombination enhancing defects, sufficient p-type doping to demonstrate tunnel junctions, dealing with limited minority carrier diffusion lengths, and many other issues will be addressed.
It will be shown that some of these issues are best addressed by alternative substrate options such as germanium and substantially novel tandem interconnect technologies. Growth issues involving the use of Ge substrates will be identified and data for InN on Ge will be detailed.
Finally, the structural and optical behavior of InN on sapphire and Ge for various buffer topologies and oxygen content will be described. It will be shown that there can be, depending on buffer configurations used, significant evolution of the InN film defect structure with increasing thickness. This behavior is much more pronounced than in other III-Nitride materials and unfortunately is counter to the properties needed for good tandem solar cell performance.

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1. Growth Methodologies for Overcoming the Perceived Limitations of Phase Separation and p-type Doping in InGaN