Scientists are always pushing the boundaries of solar cell efficiency – how much of the available sunshine gets turned into electricity – and a new approach to the technology has resulted in an astonishingly high 130 percent 'quantum yield'.
It's important to note that this is a quantum-level energy return, so we're not talking about a solar panel converting sunlight into electricity at a 130 percent rate. However, the breakthrough is an efficiency improvement in terms of how often a specific event occurs per photon absorbed by the system.
To break through the 100 percent barrier, the new approach splits the energy harvested from a single incoming light photon into two, which then powers two excited states (known as excitons) in the receiving material.
It's a process known as singlet fission, and as the international team behind the research explains, it prevents excess energy from being lost as heat.
That loss is part of the reason that solar cells typically max out at around the 33 percent mark in terms of overall efficiency, a restriction known as the Shockley-Queisser limit.
"We have two main strategies to break through this limit," says chemist Yoichi Sasaki, from Kyushu University in Japan.
"One is to convert lower-energy infrared photons into higher-energy visible photons. The other, what we explore here, is to use singlet fission to generate two excitons from a single exciton photon."
The researchers used an organic molecule called tetracene to act as the splitting material here, through which singlet fission can work. Its properties make it suitable for splitting one high-energy packet into two lower-energy packets through electron excitation.
Singlet fission isn't a completely new concept, though, and is only half of the story here. A major stumbling block in previous experiments had been giving singlet fission enough time to work before the energy was lost or transferred elsewhere.
This is where the metallic element molybdenum comes in, again chosen for its particular properties. By mixing it with tetracene, the team was able to catch the split excitons in the molybdenum compound.
At the tiniest quantum level, the molybdenum acts as what's called a spin-flip emitter. First, it locks in energy, and then it uses a quantum spin-flip to turn the invisible states into light. That gave the team the breakthrough result: 1.3 molybdenum-based metal complexes excited per photon absorbed.
"The energy can be easily 'stolen' by a mechanism called Förster resonance energy transfer (FRET) before multiplication occurs," says Sasaki.
"We therefore needed an energy acceptor that selectively captures the multiplied triplet excitons after fission."
It's worth emphasizing again that these are early lab tests. The next steps are to convert the liquid solution used here into a solid form that can be fitted to a solar panel, reliably and effectively – which the researchers themselves admit will be quite a challenge.
There's also the issue of the molybdenum complexes hanging onto the energy long enough for it to be useful, as well as capturing it in the first place. This "decay process" is something else the study addresses.
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However, those future practical concerns shouldn't take away from the excitement of the research: It clearly sets out a path towards solar panels that can go above and beyond the efficiency limits of today, and there are multiple ways that this proof-of-concept can be tweaked and experimented with going forward.
With solar energy a vital part of reducing our reliance on fossil fuels and slowing down climate change, being able to substantially improve conversion rates on solar panels would potentially be transformative for the energy industry – especially when paired with new energy storage mechanisms.
"This work represents a significant step toward developing exciton/photon amplification materials by combining singlet fission materials with transition-metal complexes, advancing the application of singlet fission beyond conventional limitations," write the researchers in their paper.
The research has been published in the Journal of the American Chemical Society.