Schematic showing part of a silicon solar cell sensitized to singlet fission. Absorption of a high energy photon by the tetracene layer produces a singlet exciton. This singlet exciton undergoes singlet fission to generate two triplet excitons. These excitons are then transferred into the Si solar cell. Details of the enlarged image (left) show side views of the models used for the interface between Si(111):H and high density (HD) Tc phases as well as low density (LD). A dangling link fault at the interface (right) is also shown. Credit: Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.076201
Physicists at the University of Paderborn used complex computer simulations to develop a new design for solar cells that are significantly more efficient than those previously available. A thin layer of organic material, called tetracene, is responsible for the increase in efficiency. The results have now been published in Physical examination letters.
“The annual energy of solar radiation on Earth amounts to more than a thousand billion kilowatt hours and thus exceeds global energy demand by more than 5,000 times. Photovoltaics, that is to say the production of electricity from from sunlight, therefore offers significant and still largely untapped potential for the supply of clean, renewable energy. Silicon solar cells used for this purpose currently dominate the market, but have efficiency limits,” explains Professor Wolf Gero Schmidt, physicist and dean of the Faculty of Natural Sciences at the University of Paderborn. One reason for this is that part of the energy from shortwave radiation is not converted into electricity, but into unwanted heat.
Schmidt explains: “In order to increase efficiency, the silicon solar cell can be provided with an organic layer, for example made of semiconductor tetracene. Shortwave light is absorbed in this layer and converted into high-energy electronic excitations. -called excites. These excitons decay into the tetracene in two low energy excitations. If these excitations can be successfully transferred to the silicon solar cell, they can be efficiently converted into electricity and increase the overall usable energy efficiency.”
Density of states and band alignment for Tc superlayers on Si(111):H calculated on the HSE and PBE theoretical levels. The energies refer to the Si valence band maximum (VBM). Black and orange denote Tc and Si bound states, respectively. Occupied states are shaded. Credit: Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.076201
Decisive breakthrough for rapid energy transfer
The excitation transfer of tetracene in silicon is studied by Schmidt’s team using complex computer simulations at the Paderborn Parallel Computing Center (PC2), the university’s high-performance computing center. A breakthrough has now been made: in a joint study with Dr. Marvin Krenz and Prof. Dr Uwe Gerstmann, both from the University of Paderborn, the scientists showed that particular defects in the form of unsaturated chemical bonds at the interface between the tetracene film and the solar cell significantly accelerate the exciton transfer.
Schmidt notes: “Such defects occur during hydrogen desorption and cause electronic interface states with fluctuating energy. These fluctuations transport electronic excitations from the tetracene to the silicon like an elevator. »
Such “defects” in solar cells are actually associated with energy losses. This makes the results of the trio of physicists all the more astonishing.
“In the case of the silicon-tetracene interface, defects are essential for rapid energy transfer. The results of our computer simulations are truly surprising. They also provide precise guidance for the design of a new type of cell solar with significantly increased efficiency,” says Schmidt.
More information:
Marvin Krenz et al, Default-assisted exciton transfer across the Tetracene-Si(111):H interface, Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.076201 journals.aps.org/prl/abstract/ … ysRevLett.132.076201
Provided by the University of Paderborn
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