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The Intertubes lit up today with news of a new, 190% efficient solar cell that could finally send fossil fuels packing once and for all. The research is still in the proof-of-concept stage, but other solar cells that shoot past the 100% mark are already in development, so anything is possible. However, if you’re thinking this blows the Shockley-Queisser theoretical limit to bits, well, guess again.
Solar Cells Can Shoot Past 100% Efficiency, Depending On What That Means
The Shockley-Queisser limit refers to the ability of solar cells to convert sunlight to electricity. The theory emerged in the 1960s to describe the upper limit of basic silicon photovoltaic technology. The initial limit was determined to be 30%, later revised upward to 33.7%.
That leaves quite a bit of sunlight hanging around without a purpose, but more complex solar cells can easily surpass 33.7%. For example, in 2022 researchers at the Fraunhofer Institute for Solar Energy Systems in Germany achieved 47.6% for a solar cell that combines a layer of gallium indium phosphide and aluminum gallium arsenide, together with another layer of gallium indium arsenide phosphide and gallium indium arsenide.
Photovoltaic technology can also be tweaked to achieve higher efficiencies by harvesting heat as well as light, and that describes the goings-on over at the laboratory of Lehigh University physics professor Chinedu Ekuma in Pennsylvania.
Ekuma’s team developed the proof of concept for a new solar cell that captures energy from reflected light and heat, as well a direct sunlight, to get to the 190% level for external quantum efficiency, which is not the same thing as conversion efficiency.
“In traditional solar cells, the maximum EQE [external quantum efficiency] is 100%, representing the generation and collection of one electron for each photon absorbed from sunlight. However, some advanced materials and configurations developed over the past several years have demonstrated the capability of generating and collecting more than one electron from high-energy photons, representing an EQE of over 100%,” Lehigh University explained in a press statement.
What Is External Quantum Efficiency?
For an explanation of quantum efficiency, let’s turn to the online course offered by MIT Professor Tonio Buonassisi back in 2011.
“‘Quantum efficiency (QE)‘ is defined as the number of electrons out per incident photon,” Buonassisi explains.
“Note that QE is simply a census: it does not take into consideration the energy of the electron or photon. QE is generally reported as a function of wavelength. QE is a useful troubleshooting tool to identify why a device is underperforming,” Buonassisi adds.
“QE values can be quite high (between 60 and 99% for certain wavelengths), and thus can be used by devious individuals to misrepresent the conversion efficiency of their solar cell device,” he concludes.
Devious individuals! That certainly does not include the media office at Lehigh. They are very careful to explain that the new proof of concept involves quantum efficiency, not conversion efficiency. It’s right there in the headline, “New quantum material promises over 190% quantum efficiency in solar cells.”
Why Solar Cells With 190% Quantum Efficiency Matter
Turning to the trade organization Solar Manufacturing, we also learn that external quantum efficiency is not the same as internal quantum efficiency.
“There are two types of quantum efficiency: internal and external,” they explain. “The external quantum efficiency (EQE) includes the reflection losses of the solar cell. The internal quantum efficiency (IQE) is corrected for the optical losses due to reflection at the front of the solar cell.
In a conventional solar cell, the EQE can reach up to 100%, meaning that each incoming photon produces one electron. The Lehigh media team points out that surpassing the 100% limit is not uncommon in today’s world of atomic-level tailoring and two-dimensional advanced materials.
The leap to 190% is something else again. To get there, Ekum and his team took a two-dimensional compound of germanium selenide and tin sulfide and stuffed it with atoms of zerovalent copper, referring to a nanoscale form of copper commonly used in environmental remediation.
“It’s a promising candidate for the development of next-generation, high-efficient solar cells, which will play a crucial role in addressing global energy needs,” Ekuma explains. “Its rapid response and enhanced efficiency strongly indicate the potential of Cu-intercalated GeSe/SnS as a quantum material for use in advanced photovoltaic applications, offering an avenue for efficiency improvements in solar energy conversion.”
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From Promising Candidates To The Solar Cells Of The Future
As Professor Ekuma makes clear, the new quantum material is a material, not a full-fledged solar cell. That will be the next step in the R&D process. In the meantime, you can get all the details about the new material from the journal Science Advances under the title, “Chemically tuned intermediate band states in atomically thin CuxGeSe/SnS quantum material for photovoltaic applications.”
Tin sulfide is commonly used in photovoltaic applications, so it’s no surprise to see it pop up in the new research. The tin angle has also come up in relation to perovskite solar technology, a new class of photovoltaic materials based on the structure of the naturally occurring mineral perovskite.
As for Lehigh University, the school does not grab the media spotlight as often as the other top schools in the US. However, the institution is an important energy research facility and it crosses the CleanTechnica radar on a fairly regular basis.
The latest addition to the school’s research resources is a new solar thermal concentrator installed at the Mountaintop campus.
“The new equipment is the result of a collaboration involving Lehigh’s Energy Research Center (ERC; Dr. Romero is the director), the Department of Mechanical Engineering and Mechanics (MEM), and industry partner Solarflux Energy Technologies,” Lehigh explains.
Solarflux is also a familiar face on CleanTechnica, so stay tuned for more on the Mountaintop installation. As described by Lehigh, the company’s parabolic trough thermal concentrating technology is paired with a high tech solar tracking system to align with the sun for maximum efficiency throughout the day.
The school lists solar thermal energy, thermal energy storage, and thermochemical solar water splitting among the research projects already planned for the new Solarflux system, so stay tuned for more on that.
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Photo (cropped): Proof of concept for a new photovoltaic material could lead to next-generation solar cells (courtesy of US Department of Energy).
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