November 13, 2015
Colloquium Speaker: Daniel Friedman
Dr. Daniel Friedman is the manager of NREL’s High Efficiency Crystalline Photovoltaics Group. His interests have been in the development of next-generation multijunction high-efficiency solar cells for concentrator systems. He has studied novel semiconductor materials for photovoltaics, such as GaInNAs for potential application in a 1-eV-bandgap device on conventional GaAs or Ge substrates, as well as developing concepts for junctions suitable for fabrication on Si. His current work centers on development of future generations of lattice-mismatched multijunction cell structures for ultrahigh-efficiency operation under concentration, resulting in several recent efficiency records for inverted metamorphic multijunction cells. Dr. Friedman has authored or coauthored more than 150 papers and four book chapters and been awarded two patents.
Colloquium Topic: Progress and opportunities for next-generation ultrahigh-efficiency multijunction solar cells
Multijunction solar cells based on III-V materials are the most efficient solar cells ever developed, with demonstrated efficiencies of roughly 39% for operation under standard terrestrial one-sun conditions, 46% for operation in concentrator applications, and 36% for operation in space. Furthermore, the materials and design architectures provide a remarkable degree of versatility, offering a clear path to significant further improvements in performance and cost, as well as the option of making the cells extremely lightweight and flexible. Such advances can have significant impact in the future of cutting edge national security and civil space missions, as well as the deployment of UAVs in a growing mission space. In this talk, we first review the fundamental limitations to the efficiency of conventional single-junction solar cells. We then introduce the multijunction cell concept (illustrated schematically in the figure below), which provides a way to transcend these limitations, and lay out the basic design and operating principles for multijunction cells. We discuss the materials and device physics underlying these devices, and the resulting challenges and opportunities for the most promising next-generation cell device architectures.