PhD Defense: "Design of Integrated III-Nitride/Non-III-Nitride Multijunction Photovoltaic Devices"

Nikholas Toledo

April 12th (Thursday), 12:00pm
Elings Hall, Room 1601

The integration of III-nitride and non-III-nitride materials for tandem solar cell applications can improve the efficiency of the photovoltaic device due to the added power contributed by the III-nitride (III-N) top cell to that of high-efficiency multi-junction non-III-nitride (non-III-N) solar cells if the device components are properly designed and optimized. The tandem solar cell is comprised of a III-N top cell wafer bonded to a non-III-N series-constrained multi-junction subcell. The III-N top cell is electrically isolated but optically coupled to the underlying multi-junction subcell. As the absorption edge wavelength of the III-N top cell increases, the performance of the multijunction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5G, AM1.5D and AM0), the technology to grow InGaN cells with absorption edge wavelength < 520 nm is found to be sufficient for III-N top cell applications. The device requirements for the InGaN top cell applications in terms of minimum required external quantum efficiency, maximum tolerable voltage offset and maximum tolerable degradation in fill factor and power conversion efficiency are presented. The minimum tolerable area ratio between the III-N and non-III-N subcells in a 3- or 4- terminal device is also determined. Furthermore, the effects of surface/interface reflections are also presented. Finally, the process for integrating a III-N and a non-III-N cell as well as preliminary results of wafer-bonded InGaN/GaAs tandem solar cells are presented.

Hosted by: Professor Umesh Mishra