Have researchers broken through the “impossible” solar conversion ceiling?

A group of researchers from Japan and Germany may have conjured up a “dream technology” that could one day supercharge solar energy conversion to levels around 130 per cent, compared with the sub-30 per cent levels we see today.

In a paper recently published in the Journal of the American Chemical Society, a research team led by Kyushu University in Japan, in collaboration with Johannes Gutenberg University (JGU) Mainz in Germany, demonstrated a technique capable of generating and harvesting more solar energy than ever before.

The team is hoping to overcome what was believed to be an impossible-to-break “physical ceiling” to solar conversion, the impenetrability for laypeople of the research itself is unsurprising. The premise of the research, however, has to do with the way in which solar power is generated.

Planet Earth is bombarded by enormous amounts of energy from the Sun – a fact which clean energy proponents have long publicised. Solar energy has the potential, like wind energy, to provide virtually all of humanity’s energy needs.

Unfortunately, solar cells currently only capture a fraction of the amount of energy the Sun is pouring down on the planet.

The Kyushu University press release attempts to clarify the problem with an analogy, picturing a relay race among photons from sunlight that strike a semiconductor and pass their energy on to electrons, activating them and driving an electric current.

These photonic runners, however, “vary in ability”, and include lower-energy infrared photos that are unable to excite electrons, while higher-energy photos, like those in blue light, lose their excess as heat. “As a result, solar cells can use only about one-third of the sunlight,” a physical “ceiling” that is known as the Shockley–Queisser limit and one that has long challenged scientists.

In order to break through this limit, scientists have “two main strategies”, according to Yoichi Sasaki, associate professor at Kyushu University’s Faculty of Engineering. “One is to convert lower-energy infrared photons into higher energy visible photons. The other, what we explore here, is to use [singlet fission (SF)] to generate two excitons from a single exciton photon.”

Singlet fission is able to split a high-energy singlet exciton into two lower-energy spin-triplet excitons, a process which theoretically doubles the energy of the original photon.

This process has recently been at the forefront of solar research. Last year, singlet fission was described by one group of researchers from the University of NSW as providing a “scalable pathway to high efficiency silicon photovoltaics”.

But capturing singlet fission-born excitons has remained a challenge for researchers, even though some organic semiconductors like tetracene exhibit the process.

“The energy can be easily ‘stolen’ by a mechanism called Förster resonance energy transfer (FRET) before multiplication occurs,” Sasaki explains. “We therefore needed an energy acceptor that selectively captures the multiplied triplet excitons after fission.”

It is at this point that the researchers from Kyushu University and JGU Mainz turned to metal complexes – molecules whose structures can be flexibly designed – and found that a molybdenum-based “spin-flip” emitter serves as an ideal harvester of excitons after fission.

In these flexibly designed molecules, an electron’s spin is flipped during the absorption or emission of near-infrared light, enabling the system to accept the triplet energy produced in singlet fission.

The researchers, by carefully tuning the energy levels, were able to suppress the wasteful FRET process and allow the multiplied excitons from singlet fission to be selectively extracted.

The end result, when pairing this molybdenum-based complex with tetracene-based materials in solution, allowed the team to harvest energy at quantum yields of around 130 per cent.

This means that approximately 1.3 molybdenum-based metal complexes were excited per photon absorbed, exceeding the conventional 100 per cent limit and indicating that the system generated and harvested more energy carriers than photons received.

The researchers hope that their work can help establish a new design strategy for exciton amplification, though they recognise their current experiments are only at the proof-of-concept stage. The goal is to bring the two types of materials together in the solid state and are aiming for efficient energy transfer and eventual integration into working solar cells.

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