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Two electrons for one photon fission mechanism to boost solar cell efficiency

Editorial
article image This 2-for-1 singlet fission conversion process means each high-energy photon doubles the excitons being generated.

RESEARCH from the University of California could see more efficient solar panels powered by singlet fission.

The chemists from the University of California investigated the singlet fission process in which a single photon generates a pair of excited states.

Solar cells work by absorbing a photon, which generates an exciton, which then separates into an electron-hole pair. The electrons then become electricity.

But this 2-for-1 singlet fission conversion process means each high-energy photon doubles the excitons being generated, effectively doubling the potential current generated by high-energy sections of the light spectrum and possibly boosting overall solar cell efficiency by as much as 30 percent. This could allow future solar cells to exceed the 32 percent efficiency limit.

Other applications of the research include more energy-efficient lighting and photodetectors with 200 percent efficiency that can be used for night vision.

"Our research got its launch about ten years ago when we started thinking about solar energy and what new types of photophysics this might require," said Christopher Bardeen, a professor of chemistry, whose lab led the research.

"More efficient solar cells would lead to wider use of this clean energy source."

The singlet fission process exploits the fact that the excitons generated by photons come in two varieties. One is a singlet, where the electron spins inside the excitons are paired. The other is a triplet, where two electrons are unpaired.

A singlet exciton has double the energy of a triplet exciton. This means it is possible to split a singlet exciton into two triplet exciton, yielding double the number of excitons -- and thus electrons, per absorbed photon.

"To absorb a photon, the photon energy has to be greater than the bandgap of the semiconductor, so you already miss part of the solar spectrum," Bardeen said.

"But if you absorb a photon with energy higher than the bandgap, it has too much energy, and that excess energy is usually wasted as heat. The trick is to take that high energy exciton and split the energy into two excitons, rather than dissipating it as heat."

The singlet fission process' exact mechanism is unknown but the researchers have found that it does happen quickly and with high efficiency.

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