04 January 2012
Researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have demonstrated the first solar cell that produces a photocurrent with an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) greater than 100%.
NREL’s news release pins down a process called Multiple Exciton Generation (MEG)—or Carrier Multiplication (CM)—as the key to achieving a peak quantum efficiency of 114%, where a single absorbed photon bearing at least twice the bandgap energy can produce two or more electron-holes.
Could this research breakthrough mark a timely leap toward third-generation solar cells? “Presently, it’s only a potential game-changer,” says Arthur J. Nozik, Senior Research Fellow at NREL and one of the co-authors of a paper on the research work, “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” published in the December 16 issue of Science Magazine.
Nevertheless, the scientist with a Ph.D. in Physical Chemistry from Yale University makes a critical point regarding semiconductor quantum dots (QDs): “The carrier multiplication effect occurs within the solar spectrum as compared to conventional bulk PV materials, where it occurs in the UV region outside the solar spectrum and, hence, is useless for solar applications.” Furthermore, quantum confinement in such incredibly tiny crystals of semiconductors allows the MEG process to be two times more efficient than in bulk semiconductors. By confining charge carriers within their nanoscale volume of 1-20 nanometers, quantum dots can harvest excess energy that would otherwise be lost in heat.
Scientists across the world have achieved positive MEG results in different semiconductor quantum dots, but never before in the external photocurrent of a solar cell. The difference here is that “Nearly all previous MEG measurements were made using time-resolved transient spectroscopic measurements to determine the effect of electron-hole population on light,” Nozik says, bringing into consideration that this indirect measurement is subject to variation, depending on data analysis and research techniques. External photocurrent quantum yield measurements in an operating QD solar cell, on the other hand, are “a direct counting of the collected electrons/incident photon and require no analysis; just precise measurement of the photocurrent generated per unit area and incident photon fluence (photons/cm2/sec) and wavelength,” Nozik elaborates on his team’s new approach.
Now that the MEG effect has been established and confirmed, what’s next? “MEG and QD solar cells have not yet been optimized to produce a power conversion efficiency [watts out/solar power in] greater than present day PV cells,” Nozik says. His “ultimate goal” is to obtain 200% internal quantum yield (electrons generated in the QDs/photons absorbed in the QD region of the cell) while collecting photons with twice the bandgap values in the external circuit at a yield of around 100%, meaning without losses due to electron-hole recombination during carrier transport. If successful, the scientist predicts a 35-100% increase in power conversion efficiency (total power generated/input power) over present day PV cells, depending on the semiconductor material used. What’s more, quantum dot solar cells can be manufactured inexpensively. And high efficiency along with low cost per unit area is indeed what defines a third-generation solar cell.
Written by Sandra Henderson, Contributing Editor, Solar Novus Today