This project is directed toward the synthesis, processing, and characterization of GaAs/GaInAs/GaInNAs quantum-well and quantum-dot p-i-n heterostructure materials for application in high-efficiency photovoltaic devices. Fundamental issues in optimization of epitaxial heterojunction interface quality and controlled formation of self-assembled quantum dot structures by molecular-beam epitaxy are explored in the context of photovoltaic devices but with broad-ranging material and device implications. In addition, the basic physics and performance potential of quantum-well/dot and related photovoltaic device concepts is examined and employed as a driver for exploration of specific materials issues. Integration of metal and dielectric nanostructures with semiconductor heterostructures, which is expected to enable key advances in engineering of photon propagation behavior in semiconductor heterostructures, is also investigated. Ultimately, it is anticipated that the advances in materials physics enabled by the project will contribute substantially to the development of highly efficient quantum-well and quantum-dot based solar cells, which have been predicted theoretically to enable power conversion efficiencies well in excess of those attainable using more conventional solar cell technologies. This work entails an international collaborative effort between researchers at the University of California, San Diego (UCSD) and at the University of Karlsruhe in Germany. Researchers at Karlsruhe, led by Dr. Daniel Schaadt, focus on epitaxial material growth and optimization, while UCSD researchers focus on materials processing, characterization, and eventual incorporation into photovoltaic device structures.
This Materials World Network project enables collaboration between researchers in the Unites States and Germany that is expected to provide invaluable experience for graduate student and postdoctoral researchers in different cultural and scientific environments, and to help seed future international collaborations and connections between rapidly growing research and commercial activities in solar energy and solid-state nanostructures generally. Germany is a particularly appropriate partner in this regard, in light of its strong commitment to and activity in solar energy technologies. The technological and economic impact of high-efficiency solar energy conversion that could be enabled by the materials advances emerging from this project is potentially dramatic. In addition, the project makes use of facilities and expertise associated with major research centers at each institution ? the California Institute for Telecommunications and Information Technology (CalIT2) at UCSD and the DFG-Center for Functional Nanostructures (CFN) at Karlsruhe. UCSD researchers visiting Karlsruhe benefit from the availability of extensive facilities for materials synthesis and characterization, and from exposure to the broad range of research at CFN on nanostructured materials for electronics, photonics, and biology. Researchers from Karlsruhe visiting UCSD gain exposure to the broad, multidisciplinary research environment that engages issues in basic materials, devices, and systems for telecommunications and information technology, and will benefit from the availability of state-of-the-art experimental materials processing and nanofabrication facilities at CalIT2.
This project involved research on the development and advancement of approaches for realizing high-efficiency solar cells that provide high power conversion efficiency under a broad range of illumination conditions – specifically, in the presence of variations in the spectral distribution of sunlight that occur as a function of time of day, time of year, geographic location, and general atmospheric conditions. Current high-efficiency solar cell technologies, particularly multijunction tandem solar cells, can suffer from severe spectral sensitivity in concentrating photovoltaic applications leading to reduced efficiency and increased cost of solar-generated electricity. Thus, the concepts explored in this program are expected to have a substantial impact on the performance of concentrating photovoltaic systems that are under development for utility-scale harnessing of solar power, by enabling high power conversion efficiency to be realized over a greater fraction of operating time than possible with current high-efficiency solar cell technologies. A key element of the research program was a close collaboration with a research group at the Clausthal University of Technology in Clausthal, Germany. This collaboration, in which semiconductor crystal growth capability at Clausthal was combined with materials characterization, processing, and computational modeling capability at the University of Texas at Austin, led to substantial new advances in high efficiency solar cell technology. In particular, improved approaches were developed and demonstrated for controlling the path of light within a solar cell to dramatically increase the probability that the light would be absorbed and produce electrical current and power. These advances are expected to enable, in future work, realization of new types of solar cells that provide very high efficiency under the broad range of conditions required by terrestrial solar power harvesting applications such as concentrating photovoltaic systems, thereby leading to broad technological and economic impact via corresponding reductions in the cost of utility-scale solar power. This program also had a large impact in education and personnel development. Five graduate students were supported in part by this program, and three of these students have received their Ph.D. degrees to date. All three are actively involved in the solar energy industry in the United States. One is the lead solar cell designer for the company that currently holds the world record for solar cell efficiency, and played a major role in the successful demonstration of this record-holding technology. 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 most recently extensive online content (as a complement to in-person interactions) into a freshman electrical engineering course taken by over 350 students annually.
