Colloquium: Eli Yablonovich

    Thursday, September 13, 2012 - 3:30pm - 5:00pm
    Meinel 307

    Eli Yablonovich, professor of electrical engineering and computer sciences at the University of California, Berkeley, and director of the National Science Foundation Center for Energy Efficient Electronics Science, will present "The Optoelectronic Physics That Just Broke the Efficiency Record in Solar Cells."


    High-efficiency solar cells are more about optics than about electronics. Counter-intuitively, efficient external fluorescence is a necessity for approaching the ultimate limits. Now new efficiency records are being broken; Alta Devices has reached 28.8 percent. A great solar cell also needs to be a great light emitting diode.

    We are finally approaching the Shockley-Queisser limit for single junction solar cell, approximately 33.5 percent efficiency under the standard solar spectrum. Previously, the record had been stuck at 25.1 percent, during 1990-2007. There an 8 percent discrepancy between the theoretical limit of 33.5 percent versus the previously achieved efficiency. It is usual to blame material quality. But in the case of GaAs double heterostructures, the material is almost ideal with an internal fluorescence yield of greater than 99 percent. This deepened the puzzle as to why the full theoretical SQ efficiency had not been achieved.

    The single-crystal thin film technology that is breaking these records is created by epitaxial liftoff and can be produced at cost well below the other less efficient thin film solar technologies. The path is now open to a 30 percent-efficient photovoltaic technology, which can be produced at low cost.

    Speaker Bio(s): 

    Eli Yablonovitch is the director of the National Science Foundation Center for Energy Efficient Electronics Science, a multi-university center based at the University of California, Berkeley.

    He received his Ph.D. degree in applied physics from Harvard University in 1972. In his photovoltaic research, Yablonovitch introduced the 4n2 light-trapping factor that is in worldwide use for almost all commercial solar panels. This factor increased the theoretical limits and practical efficiency of solar cells. The 4n2 figure is based on statistical mechanics, and is sometimes called the “Yablonovitch Limit.”

    He introduced the idea that strained semiconductor lasers could have superior performance due to reduced valence band (hole) effective mass. Today, almost all semiconductor lasers use this concept, including for optical telecommunications, in most mouse-clicks, for DVD players and in the ubiquitous red laser pointers. Yablonovitch is regarded as one of the fathers of the photonic bandgap concept, and coined the term “photonic crystal.” The geometrical structure of the first experimentally realized photonic bandgap is sometimes called “Yablonovite.”