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The Intertubes have been abuzz today with news of a new, 190% efficient solar cell that could finally send fossil fuels into disposable packaging. The research is still in the proof-of-concept stage, but other solar cells that pass the 100% mark are already in development, so anything is possible. However, if you think this beats the Shockley-Queisser theoretical limit to bits, guess again.
Solar Cells Can Shoot Past 100% Efficiency Depending on What They Mean
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The Shockley-Queisser limit refers to the ability of solar cells to convert sunlight into electricity. The theory emerged in the 1960s to describe the upper limit of mainstream silicon photovoltaic technology. The initial threshold was set at 30%, later revised to 33.7%.
This leaves quite a bit of sunlight useless, but more sophisticated solar cells can easily exceed 33.7%. For example, in 2022, researchers at the Fraunhofer Institute for Solar Energy Systems in Germany achieved 47.6% for a solar cell combining one layer of gallium indium phosphide and aluminum gallium arsenide, another layer of gallium indium arsenide phosphide and gallium indium.
Photovoltaic technology can be modified to achieve greater efficiency by harvesting both heat and light, and this is what is happening in the lab of Chinedu Ekuman, a physics professor at Lehigh University in Pennsylvania.
The Ekuma team has developed a proof of concept for a new solar cell that captures energy from reflected light and heat as well as direct sunlight to reach 190% for external quantum efficiency, which is not the same as conversion efficiency.
“Maximum EQE in conventional solar cells [external quantum efficiency] 100% means the generation and collection of one electron for every photon absorbed from sunlight. However, some advanced materials and configurations developed over the past few years have demonstrated the ability to generate and collect multiple electrons from high-energy photons, representing EQEs of over 100%,” Lehigh University said in a press release.
What is External Quantum Efficiency?
For an explanation of quantum efficiency, let’s turn to an online course offered in 2011 by MIT professor Tonio Buonassisi.
“‘Quantum efficiency (QE)’ is defined as the number of electrons per photon,” Buonassisi explains.
“Note that QE is just an enumeration: it does not take into account the energy of an electron or a photon. QE is generally reported as a function of wavelength. QE is a useful troubleshooting tool to determine why a device is underperforming,” adds Buonassisi.
“QE values can be quite high (between 60 and 99% for certain wavelengths) and thus can be used by crafty individuals to misrepresent the conversion efficiency of a solar cell device,” he said.
Tricky people! This certainly does not include the media office at Lehigh. They are careful to explain that the new proof of concept involves quantum efficiency, not conversion efficiency. It’s titled “New quantum material promises over 190% quantum efficiency in solar cells.”
Why 190% Quantum Efficiency Solar Cells Matter?
Turning to the Solar Manufacturing trade organization, we learn that external quantum efficiency is not the same as internal quantum efficiency.
“There are two types of quantum efficiency: intrinsic and extrinsic,” they explain. “The external quantum efficiency (EQE) includes the reflection losses of the solar cell. Internal quantum efficiency (IQE) is corrected for optical losses due to reflections at the front of the solar cell.
In a conventional solar cell, the EQE can reach 100%, meaning that each incoming photon produces an electron. Exceeding the 100% threshold is not uncommon in today’s world of atomic-level tailoring and two-dimensional advanced materials, the Lehigh media team notes.
A jump to 190% is something else. To get there, Ekum and his team took a two-dimensional compound of germanium selenide and tin sulfide and filled it with zero-valent copper atoms, referring to the nanoscale form of copper commonly used in environmental remediation.
“This is a promising candidate for the development of next-generation, high-efficiency solar cells that will play an important role in solving global energy needs,” explains Ekuma. “Its fast response and enhanced efficiency show the potential of Cu-intercalated GeSe/SnS as a quantum material for use in powerful photovoltaic applications and offer a way to increase efficiency in solar energy conversion.”
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From Promising Candidates to Solar Cells of the Future
As Professor Ekuma clarified, the new quantum material is not a full-fledged solar cell, but a material. This will be the next step in the R&D process. Meanwhile, full details on the new material are available in 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 its appearance in the new study is not surprising. The tin angle is also related to perovskite solar technology, a new class of photovoltaic material based on the structure of the naturally occurring mineral perovskite.
When it comes to Lehigh University, the school doesn’t get as much media attention as other top schools in the US. However, the facility is an important energy research facility, and it crosses CleanTechnica’s radar fairly regularly.
The latest addition to the school’s research resources is a new solar thermal concentrator installed on the Mountaintop campus.
“The new equipment is the result of a collaboration with Lehigh’s Energy Research Center (ERC; Dr. Romero is director), Department of Mechanical Engineering and Mechanics (MEM), and industry partner Solarflux Energy Technologies,” explains Lehigh.
Solarflux is also a familiar face at CleanTechnica, so stay tuned for more on the Mountaintop setup. As described by Lehigh, the company’s parabolic trough thermal concentration technology is combined 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 new photovoltaic material could lead to next-generation solar cells (courtesy of US Department of Energy).
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