Seminar: Dr. Andrei Sergeev, Army Research Laboratory

Wednesday, February 22, 2017
10:00 a.m.
ERF 1207, Large Conference Room
tprender@umd.edu


TITLE: Photovoltaic Conversion Beyond Shockley – Queisser Approximations

ABSTRACT: The fundamental Shockley - Queisser (S-Q) limit establishes a maximum solar conversion efficiency of 33.7% for a single p-n junction with a semiconductor bandgap of 1.34 eV.  Gallium arsenide and silicon cells have maximum theoretical efficiencies about 33% and 32% correspondingly.  However, even record values of measured efficiencies are by 5 – 6% below the theoretical limits. These losses are associated with imperfect tailoring of photons and photoelectrons in the critical region near the bandgap. Shockley and Queisser showed that high photovoltaic efficiency requires high concentration of photoelectrons near the band edge and strong emission of the bandgap photons. To keep high density of photoelectrons the emitted bandgap photons should be re-absorbed and re-emitted. While the S-Q model assumes the 100% absorption of all above bandgap photons, including absorption near the bandgap, in semiconductors the near-bandgap absorption is small. Numerous attempts to increase efficiency due to the photon trapping near the semiconductor bandgap show a very limited success.

New approach for improving efficiency of photovoltaic conversion that will be addressed in this talk is the deviations from chemical equilibrium under the solar light. The S-Q model postulates the chemical equilibrium in the whole photoelectron subsystem as well as the equilibrium between photoelectrons and emitted photons (all electrons and photons are described by the same chemical potential).  Engineering specific photoelectron distributions favorable for PV conversion is a new pathway with a potential for 10-13% efficiency improvement above the S-Q limit. The corresponding solar cell design and operating regimes will be discussed in this talk.   

BIO: Dr. Andrei Sergeev is currently a National Research Council Senior Fellow at Army Research Laboratory.  He conducted research on various topics, including hot-electron nanodetectors, single-photon counters, quantum nanocalorimeters, wide-band mixers, quantum dot and quantum well photodetectors, and nanostructured solar cells. His design of advanced optoelectronic devices is based on multiscale modeling, which includes quantum transport, hydrodynamic transport, and Monte-Carlo simulations. He has authored or co-authored more than 100 scientific papers in refereed journals and 8 book chapters (the Google Scholar h-index = 30). His current research interests include energy conversion, quantum kinetics, heat and energy transfer, and noise in nanostructures. 

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