Randy J. Ellingson - Research Associate

phone: 303-384-6464
email: Randy.Ellingson@nrel.gov
At NREL since 1994

Among the essential processes occurring in any solar energy conversion system are light absorption, charge carrier relaxation, charge separation, charge transfer, and charge or energy transport. Randy Ellingson's research applies ultrafast  laser spectroscopy (time resolution of ~50 femtoseconds) to study these optical and electronic processes in systems consisting of molecules and semiconducting nanocrystals (quantum dots, or QDs). By measuring the ultrafast dynamics of charge carrier relaxation, charge transfer, energy transfer, and coherent lattice vibrations, one can gain remarkable insight into the nature of the inefficiencies inherent in most processes by which sunlight is converted to fuels and electricity. One of our main goals is to understand and overcome the mechanisms of energy loss, which results in a substantial fraction of absorbed sunlight being converted to heat, an undesirable byproduct of all solar cells.

Normally in a semiconductor material, absorption of a photon with energy greater than the lowest absorption energy is lost to heat by creating vibrational energy (as phonons, which make up thermal energy, or heat). Dr. Ellingson's recent research (with Matt Beard and Justin Johnson in the group of Arthur Nozik) has focused on the characterization of a unique and important charge carrier multiplication process observed in certain molecules and semiconductor QD materials. These processes are termed “singlet fission” when occurring in molecules and “multiple exciton generation” (MEG) when occurring in QDs. In QDs of PbSe and PbS, MEG results in as many as three excitons per absorbed photon at high photon energies. The result represents an important proof of the possibility of bypassing the normal phonon-based relaxation process and presents the possibility that we can generate current at significantly higher levels, which would lead to a substantial enhancement in solar energy conversion efficiency.

Dr. Ellingson has also worked recently on characterization of ultrafast processes occurring in single-walled carbon nanotubes (SWNTs) following photoexcitation. SWNTs, because of their extremely high length-to-diameter ratio, have the ability to act as either metals or semiconductors. Their wide variation in optical absorption and emission energy may serve well in solar energy conversion schemes as absorbers of light, acceptors of charge, and/or tranporters of electrical current. Through approximately the end of November 2005, Dr. Ellingson is on temporary assignment at DOE in Germantown, MD, as an assistant to the Division of Materials Sciences and Engineering in the Office of Science, Office of Basic Energy Science.

Education

Selected Publications

1. Beard, M. C., and Ellingson, R. J., “Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion,” Laser & Photon. Rev. 2, 377-399 (2008).
2. Luther, J. M., Law, M., Beard, M. C., Song, Q., Reese, M. O., Ellingson, R. J., and Nozik, A. J., “Schottky Solar Cells Based on Colloidal Nanocrystal Films,” Nano Lett. 8, 3488-3492 (2008).
3. Ellingson, R. J., “Solar cells: Slicing and dicing photons,” Nature Photon. 2, 72-73 (2008).
4. Beard, M. C., Knutsen, K. P., Yu, P., Luther, J. M., Sing, Q., Metzger, W. K., Ellingson, R. J., Nozik, A. J., “Multiple exciton generation in colloidal silicon nanocrystals,” Nano Lett. 7, 2506-2512 (2007).
5. Jones, M., Metzger, W. K., McDonald, T. J., Engtrakul, C., Ellingson, R. J., Rumbles, G., and Heben, M. J., “Extrinsic and intrinsic effects on the excited-state kinetics of single-walled carbon nanotubes,” Nano Lett. 7, 300-306 (2007).
6. Ai, X., Beard, M. C., Knutsen, K. P., Shaheen, S. E., Rumbles, G., and Ellingson, R. J., “Photoinduced charge carrier generation in a poly(3-hexylthiophene) and methanofullerene bulk heterojunction investigated by time-resolved terahertz spectroscopy,” J. Phys. Chem. B 110, 25462 (2006).
7. Ellingson, R. J., and Heben, M. J., “Tribute to Arthur J. Nozik,” J. Phys. Chem. B 110, 25125 (2006).
8. McDonald, T. J., Jones, M., Engtrakul, C., Ellingson, R. J., Rumbles, G., and Heben, M. J., “Near-infrared Fourier transform photoluminescence spectrometer with tunable excitation for the study of single-walled carbon nanotubes,” Rev. Sci. Instr. 77, 053104 (2006).

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