A research team at the National Renewable Energy Laboratory (NREL) report in Science the first photovoltaic solar cell that produces a photocurrent that has an external quantum efficiency greater than 100% when photoexcited with photons from the high energy region of the solar spectrum.
For comparison external quantum efficiency for photocurrent is usually expressed as a percentage where the number of electrons flowing per second in the external circuit of a solar cell is divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell. So far no other solar cells show external photocurrent quantum efficiencies above 100% at any wavelength from the solar spectrum.
The NREL team has reached an external quantum efficiency peak value of 114%. At these efficiencies both solar electricity and solar fuels may be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.
The paper at Science Magazine entitled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard.
The mechanism for producing quantum efficiency above 100% with solar photons is based on a process called Multiple Exciton Generation (MEG). In a MEG a single absorbed photon of appropriately high energy can produce more than one electron-hole pair in the solar cell.
The idea that would be so came from NREL scientist Arthur J. Nozik who first predicted in a 2001 publication that a MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors.
On the technical side quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range from 1-20 nm. One nm equals one-billionth of a meter. At these dimensions semiconductors exhibit dramatic activity due to the quantum physics.
Most importantly quantum physics applied to the nano semiconductors forms correlated electron-hole pairs (called excitons) at room temperature, which enhances coupling of electronic particles (electrons and positive holes) through Coulombic forces. As the quantum dots are reduced in size a rapidly increasing bandgap occurs.
The physics set up the quantum dots to confine the charges and harvest excess energy.
Quantum dots are still semiconductor crystals with tiny volumes. When an electrical charge is confined in such a space the energy stays electric instead of becoming heat. That’s how the high efficiency starts. Spilling out more than one electron per photon strike pushes the result even higher.
The NREL team achieved the 114% external quantum efficiency with a layered cell consisting of 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. Other than the gold, the raw materials are low cost. It is looking like fabrication of quantum dot solar cells may apply to inexpensive, high-throughput roll-to-roll manufacturing.
At this stage one has to ask is plus 100% even believable. MEG was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory. By 2006 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 percent compared to the cells of the day.
Meanwhile many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dot designs. But nearly all those experiments used ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions. No power out was measured.
The new NREL team result is a MEG built with an external photocurrent quantum yield greater than 100 percent. The reporting on the study points out the cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight.
Still these new solar cells are un-optimized and thus exhibit relatively low power conversion efficiency, which is a product of the photocurrent and photovoltage.
This is still stage one. The MEG demonstration has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.
As well as being a milestone the new results confirm the previous time-resolved spectroscopic measurements of MEG and hence validate those earlier MEG results. The confirmation improves when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell.
In actual absorption 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 incoming 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.
That brings us to the questions. If every incoming photon was to drive an exciton pair the potential efficiency would seem to be 200%, leaving a lot on the cell, so to speak. Then the matter of a photon driving even more than just a pair comes to mind. Follow this with the reality, the NREL team has made the measurement from lab built first successful experimental materials leaving a full range of innovation to come – where those levels could get to isn’t offered, yet.
Lastly, the baseline and materials are geared towards the high-energy end of the solar radiation arriving at the surface. What can be done to improve the range of the useful spectrum is another intriguing question.
The best perspective is today’s solar cell at 10 to 20% efficiency and marketed with some success now could have a 5 fold increase in the future if the science milestone of this week can scale to commercial production. And the prospects can only get better with more research and engineering.