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Solar cells have reached about 130% efficiency in tests with a new molecular system based on molybdenum, surpassing a historical barrier of solar energy and paving the way for more powerful panels, with more energy carriers generated than absorbed photons
Solar cells have taken an important step towards overcoming a historical efficiency barrier with an advance achieved by researchers from Kyushu University in Japan, in collaboration with Johannes Gutenberg University of Mainz in Germany. The new strategy allowed solar cells to achieve energy conversion efficiency of around 130%, by producing more energy carriers than absorbed photons.
The result was published on March 25 in the Journal of the American Chemical Society and is based on the use of a molybdenum-based metal complex with “spin inversion.”
The system was designed to capture the extra energy generated by singlet fission, a process seen as a promising route to enhance the utilization of sunlight in solar cells.
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How solar cells lose part of the energy
Solar cells generate electricity when photons from sunlight hit a semiconductor and transfer energy to electrons, putting these particles in motion and forming an electric current. Despite this, not all the received light can be utilized with the same efficiency.
Low-energy infrared photons cannot activate electrons, while high-energy photons, such as those from blue light, end up wasting the excess energy in the form of heat. Because of this physical limit, modern solar cells can only use about one-third of the incident sunlight.
This restriction is known as the Shockley-Queisser limit and has long represented an obstacle to the advancement of solar technology. The search for ways to circumvent this barrier has guided research aimed at creating more efficient solar cells.
The strategy that raised efficiency to 130%
One of the studied routes to overcome this ceiling is singlet fission. In this process, a single singlet spin exciton, generated after the absorption of a photon, can split into two lower-energy triplet spin excitons, increasing the number of available energy carriers.
Under normal conditions, each photon produces only one exciton. However, with singlet fission, this yield can be increased, opening the possibility of raising the efficiency of solar cells beyond what is currently considered conventional.
Materials such as tetracene have already demonstrated the ability to sustain this process, but the efficient capture of these multiplied excitons remained a challenge. Before the energy multiplication could be harnessed, part of it was lost through a mechanism called Förster resonance energy transfer, known by the acronym FRET.
The role of molybdenum and spin inversion
To reduce these losses, researchers sought an energy acceptor capable of selectively capturing the triplet excitons generated after singlet fission. The solution found was a spin inversion emitter based on molybdenum, designed to absorb or emit light in the near-infrared while an electron changes its spin.
By adjusting the energy levels of the system, the team was able to minimize the losses caused by FRET and extract the multiplied excitons more efficiently. When combined with tetracene-based materials in solution, the arrangement exhibited quantum yields of about 130%.
In practice, this means that approximately 1.3 molybdenum-based metal complexes were activated for each absorbed photon. The result showed that the system managed to produce more energy carriers than the number of incident photons, surpassing the traditional limit of 100%.
Next steps for new solar cells
The work is still in the proof-of-concept phase, but researchers intend to integrate these materials into solid-state systems. The goal is to improve energy transfer and bring the technology closer to practical applications in solar cells.
In addition to the potential impact on more powerful solar cells, the results may also stimulate new research that combines singlet fission and metal complexes in other areas. Among the cited possibilities are LEDs and emerging quantum technologies.
This article was prepared based on information about research published on March 25 in the Journal of the American Chemical Society, conducted by researchers from Kyushu University in Japan, in collaboration with Johannes Gutenberg University of Mainz in Germany.
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