PhD Defense: "Design, Fabrication and Characterization of III-N Based Solar Cells"

Carl J. Neufeld

September 23rd (Friday), 11:00am
Elings Hall 1605

Solar energy represents a vast potential clean, cheap energy. Harnessing this vast source of energy in an economical way such that it can compete with conventional fossil fuels is not a trivial matter. To date, triple junction solar cells based on GaP/InGaAs/Ge or InGaP/GaAs/InGaAs technologies are the most efficient in the world with conversion efficiencies of over 40% at high concentration. To increase the conversion efficiency over 50% it will be necessary to move to multijunction solar cells with increased number of junctions and to find materials with larger band gap energies than is attainable in the III-P or III-As material systems. The III-N material system has several characteristics which give it key advantages over the existing materials currently used for high efficiency solar cells. The most important of which is an extremely wide range of band gap energies available, <0.7 eV for InN to 6.2 eV for AlN, which spans the entire solar spectrum. This wide range offers excellent opportunities to integrate III-N solar cells with existing multijunction devices to closely match the solar cell absorption to the solar spectrum and increase the efficiency. The III-N materials are also direct band gap for the entire range of In and Al compositions and thus has exceptionally high absorption coefficients on the order of 1x105 cm-1 for GaN which is an order of magnitude greater than for GaAs and several orders of magnitude higher than for an indirect band gap material such as silicon. While III-Ns are appealing, there are challenges for device design which are unique to this material system, namely: polarization charges at hetero-interface, high defect density, and significant lattice mismatch. While these properties offer unique challenges, they can be overcome by careful device design and material optimization. This dissertation covers our work on the design, fabrication and characterization of III-N based solar cells. The unique design challenges are outlined and our progress on improving InGaN device performance is covered. Topics include: design and characterization of both p-i-n, and multi quantum well (MQW) devices, exploration of the effects of polarization-induced electric fields on carrier collection, and the thermal performance of InGaN based solar cells. We demonstrate p-i-n devices with fill factors of 75-80 %, VOC of greater than 2 V, and peak external quantum efficiency of over 80%. MQW solar cells are reported with EQE response beyond 500 nm and VOC of 2 V. Positive thermal power coefficients are also reported for both p-i-n and MQW solar cells. These results highlight the potential for future III-N based solar cell progress. Hosted by: Professor Umesh K. Mishra

Hosted by: Professor Umesh K. Mishra