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The 2016 Paint Branch Distinguished Lecture



A New Type of Heat Engine, Using LEDs as Refrigerators


Eli Yablonovitch

Electrical  Engineering and Computer Sciences Dept., University of California, Berkeley

Tuesday, October 25, 2016

Light snacks & refreshments will be served at 3:30PM

The lecture will begin at 4PM

1412 John. S. Toll Physics Building, University of Maryland



Very efficient light emitting diodes (LEDs) actually become colder as they operate because the emitted light carries away entropy.  This surprising effect requires superb LED efficiency, which is achieved by using 2D photonic crystal structures to extract the luminescence.  2D photonic crystals have likewise been employed in photovoltaic cells to trap incident light.  We now know that the photovoltaic cell and the LED are reciprocals of one another. The maxim that “a great solar cell has to be a great LED” has led to solar cells with record efficiency.

What if the electrical output of a photovoltaic cell drives an LED, and the LED light in turn drives the photovoltaic cell?  You might fear that it would become a perpetual motion machine. Instead it becomes a heat engine in which a small amount electricity can efficiently provide refrigeration, or conversely a small temperature difference can generate electricity. Such an electro-luminescent heat engine, in which photons are the working fluid, can be more efficient than thermo-electrics, in which electrons are the working fluid.



Eli Yablonovitch is Director of the NSF Center for Energy Efficient Electronics Science (E3S), a multi-University Center headquartered at Berkeley.  Yablonovitch introduced the idea that strained semiconductor lasers could have superior performance due to reduced valence band (hole) effective mass. With almost every human interaction with the internet, optical telecommunication occurs by strained semiconductor lasers.  In his photovoltaic research, Yablonovitch introduced the 4n2 (“Yablonovitch Limit”) light-trapping factor that is in worldwide use, for almost all commercial solar panels.  Based on his mantra that “a great solar cell also needs to be a great LED”, his startup company Alta Devices Inc. has, since 2011, held the world record for solar cell efficiency, now 28.8% at 1 sun.  He is regarded as a Father of the Photonic BandGap concept, and he coined the term "Photonic Crystal".  The geometrical structure of the first experimentally realized Photonic bandgap, is sometimes called “Yablonovite”.  His startup company Ethertronics Inc., has shipped over one billion cellphone antennas. He has been elected to the NAE, the NAS, and as Foreign Member, UK Royal Society.  Among his honors is the Buckley Prize of the American Physical Society, and the Isaac Newton Medal of the UK Institute of Physics.