Singlet Fission Can Boost Power of Si-Based Solar

Singlet fission and triplet fusion are related photophysical processes with potential applications in photovoltaics, photocatalysis, optical and magnetic resonance imaging, and advanced manufacturing. When applied to photovoltaics, singlet fission and triplet fusion can increase the efficiency of solar cells appreciably, according to a research team at the University of New South Wales (UNSW).

The team is investigating how to integrate singlet fission with existing silicon-based solar technology. Singlet fission would enable solar cells to retain more of the energy they receive from the sun, pushing them beyond their current efficiency limits.

The efficiency of a solar panel is represented by the fraction of energy supplied by the sun that can be converted into electricity. Existing solar cells work by absorbing photons that are then used by the system’s electrical conductors to generate energy.

“As part of this process, a lot of this light is lost as heat, which is why solar panels don’t run at full efficiency,” said professor Tim Schmidt, head of UNSW’s School of Chemistry.

Although each color of light produces a different energy level, the amount of energy that is supplied to the cell does not change, regardless of the color of the incoming light. Excess energy is turned into heat. “So, if you absorb a red photon then there’s a bit of heat,” Schmidt said. “With blue photons, there’s lots of heat.”

Introducing singlet fission into a silicon solar panel enables a molecular layer to supply additional current to the panel.

Singlet fission produces two molecular excitations from one photon, causing the photon to split in two. A solar-powered system can use each half of the photon individually to ensure that more of the high-energy portion of the spectrum is used to produce solar power and less energy is lost as heat.

The researchers explored the process of singlet fission using magnetic fields to interrogate the process. Schmidt said that the UNSW team is the first to use magnetic fields to learn how a light particle becomes separated during singlet fission. “The magnetic fields manipulate the wavelengths of emitted light to reveal the way that singlet fission occurs, and that hasn’t been done before,” he said.

According to UNSW professor Tim Schmidt (pictured), his team’s recent findings could lead to the first prototype of a solar cell operating above the theoretical capacity for silicon-based solar cells. Courtesy of the University of New South Wales.

To better understand the dynamics underlying the process of singlet fission, the researchers analyzed the time-resolved emission of a liquid singlet fission chromophore at room temperature. They observed that the chromophore had three spectral components. Two of the components corresponded to the bright singlet and excimer states. A third component became more prominent during triplet fusion.

Through magnetic field experiments, the researchers gained further insight into the spectral components generated by a triplet pair. The researchers showed that the spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. The spectrum could thus be attributed to a strongly coupled triplet pair state, the team surmised.

The experimental results indicate that there is a spectrally observable emissive intermediate in singlet fission, both in concentrated solutions and the solid state, and that this is distinct from the unstructured, red-shifted excimer emission.

“From this firm scientific understanding of singlet fission, we can now make a prototype of an improved silicon solar cell and then work with our industrial partners to commercialize the technology,” professor Ned Ekins-Daukes said.

The team used a single wavelength laser to excite the singlet fission material and an electromagnet to apply the magnetic fields, which reduced the speed of the singlet fission process, making it easier to observe.

Silicon, the material that most photovoltaic solar panels are made from, is inexpensive, but has almost reached its performance limit for producing solar power. “The highest efficiency was set earlier this year by our industrial collaborator, LONGi,” Ekins-Daukes said. “They demonstrated a 27.3 percent efficient silicon solar cell. The absolute limit is 29.4 percent.”

“We’re confident we can get silicon solar cells to an efficiency above 30 percent,” Schmidt said.

The research was published in Nature Chemistry (www.doi.org/10.1038/s41557-024-01591-0).