NREL solar cell produces external quantum efficiency above 100%

NREL researchers devised a solar cell that produces a photocurrent with an external quantum efficiency >100% when photoexcited with photons from the high energy region of the solar spectrum.

December 22, 2011 — US Department of Energy (DOE) National Renewable Energy Laboratory (NREL) researchers devised a solar cell that produces a photocurrent with an external quantum efficiency >100% when photoexcited with photons from the high energy region of the solar spectrum.

The external quantum efficiency for photocurrent is the number of electrons flowing per second in the external circuit of a solar cell divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell. NREL reports that this is the first solar cell to exhibit external photocurrent quantum efficiencies above 100% at any solar wavelength.

The external quantum efficiency reached a peak value of 114%. NREL researchers used multiple exciton generation (MEG) or carrier multiplication (CM), where a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon. They created a layered solar cell with antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode. Quantum dots (QD) confine charge carriers and therefore harvest excess energy instead of allowing it to be wasted as heat.

NREL scientist Arthur J. Nozik first predicted in a 2001 publication that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. In a 2006 publication, NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35% relative to today?s conventional solar cells.

The fabrication of quantum dot solar cells could be performed on inexpensive, high-throughput roll-to-roll manufacturing equipment and processes.

Since MEG was demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE?s Los Alamos National Laboratory, researchers have confirmed MEG in many different semiconductor quantum dots based on ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions. NREL’s research shows MEG manifested as an external photocurrent quantum yield greater than 100 percent measured in operating quantum dot solar cells at low light intensity.

These cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5% with simulated sunlight. While these solar cells are un-optimized and thus exhibit relatively low power conversion efficiency (which is a product of the photocurrent and photovoltage), the demonstration of MEG in the photocurrent of a solar cell opens new and unexplored approaches to improve solar cell efficiencies.

Another important aspect of the new results is that they agree with the previous time-resolved spectroscopic measurements of MEG and hence validate these earlier MEG results. Excellent agreement follows when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell. In this case, the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incident photons are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

Results are published in the Dec. 16 issue of Science Magazine, “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand and Matthew C. Beard.

The research was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences. Semonin and Nozik are also affiliated with the University of Colorado at Boulder.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by the Alliance for Sustainable Energy, LLC. Visit NREL online at www.nrel.gov.

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