The object of this research is to investigate the use of metal and dielectric nanostructures to enable, via optical scattering, improved absorption in thin-film solar cells, and to explore applications in terrestrial and aerospace photovoltaic systems. The approach is based on GaAs/InGaAs/InAs quantum well-based heterostructure solar cells, which have been postulated to offer maximum power conversion efficiencies of ~45% to over 60%. Use of metal and dielectric nanostructures for scattering and light trapping within devices will enable absorption efficiency to be improved over selected wavelength ranges in very thin device layers. The University of Texas at Austin and Boeing will collaborate in exploring both basic device physics and photovoltaic system applications.
Fundamental issues in the physics, design, and fabrication of quantum-well and related multiband solar cells will be elucidated. Understanding of the interaction between metal and dielectric nanostructures and semiconductor devices, particularly with regard to the design of nanophotonic structures and achieving control over photon propagation and absorption within thin-film semiconductors, will be advanced.
The proposed work could have a major impact on renewable power generation, particularly in concentrating photovoltaic systems, and could advance a broad range of device concepts in which semiconductor, metal, and dielectric nanostructures are integrated. Introduction of freshman electrical engineering students to photovoltaics and solar energy, senior-level design projects, and university-industrial collaboration will provide a broad range of students with exposure to state of the art energy technology and provide a path for rapid technology transfer.
This research project addressed issues in the design of solar cells and associated components that impact their effective use in nontraditional settings such as airborne systems and man-portable solar energy harvesting. Specific priorities for exploration and development were informed by interactions with engineers at Boeing, which has a particular interest in solar energy harvesting for airborne systems. A key issue in this project was the optimization of solar energy harvesting for sunlight reaching a packaged solar cell at a large angle of incidence, i.e., at a glancing angle rather than perpendicular to the surface of the solar cell. In typical applications, e.g., large-area flat solar panels, sunlight is typically incident normal to the light collection surface, or within angles of incidence around 20-30 degrees off normal incidence, and the cells and panels can be optimized for these conditions. In airborne (and also, often, man-portable) applications, much larger angles of incidence are frequently accounted, and efficient operation over all angles of incidence is imperative. This is challenging because typically, a large portion of sunlight that is incident at a large angle is reflected from the surface of either the solar cell itself, or of glass or polymer packaging material on top of the cell. In this project, we were able to design, and fabricate at low cost and over substantial areas (several square centimeters), nanostructures on the top surface of both the solar cell and typical polymer packaging materials that dramatically reduced the light reflected from both surfaces, and correspondingly increased the light absorbed by the solar cell and converted to electrical power. At large angles of incidence (around 80 degrees from perpendicular), the efficiency with which incident light was converted to electricity increased by approximately 70 percent. As a result of this work, we are implementing systems incorporating these ideas in collaboration with a small company, and have also provided samples to a government research laboratory for potential use in systems they are developing. This program also had a large impact in education and personnel development. Three graduate students were supported in part by this program, and two of them have received their Ph.D. degrees to date. Both are actively involved in research and development at companies in the United States. One is at a leading company in the high-efficiency photovoltaics industry, and the other is at a major semiconductor electronics company. The principal investigator for this project has also taken a very active role in advancing freshman-level electrical engineering education at the University of Texas at Austin, introducing new laboratory components, content relevant to renewable energy and nanotechnology, and extensive online content (as a complement to in-person interactions) into a freshman electrical engineering course that is now taken by over 400 students annually.