Hybrid materials could smash the solar efficiency ceiling

October 9, 2014
When light is absorbed in pentacene, the generated singlet excitons rapidly undergo fission into paris of triplets that can be efficiently transfered onto inorganic nanocrystals.Credit: Maxim Tabachnyk

(Phys.org) —A new method for transferring energy from organic to inorganic semiconductors could boost the efficiency of widely used inorganic solar cells.

Researchers have developed a new method for harvesting the energy carried by known as 'dark' spin-triplet excitons with close to 100% efficiency, clearing the way for hybrid solar cells which could far surpass current efficiency limits.

The team, from the University of Cambridge, have successfully harvested the energy of triplet excitons, an excited electron state whose energy in harvested in solar cells, and transferred it from organic to inorganic semiconductors. To date, this type of energy transfer had only been shown for spin-singlet excitons. The results are published in the journal Nature Materials.

In the natural world, excitons are a key part of photosynthesis: light photons are absorbed by pigments and generate excitons, which then carry the associated energy throughout the plant. The same process is at work in a solar cell.

In conventional semiconductors such as silicon, when one photon is absorbed it leads to the formation of one free electron that can be extracted as current. However, in pentacene, a type of organic semiconductor, the absorption of a photon leads to the formation of two electrons. But these electrons are not free and they are difficult to pin down, as they are bound up within 'dark' triplet exciton states.

Excitons come in two 'flavours': spin-singlet and spin-triplet. Spin-singlet excitons are 'bright' and their energy is relatively straightforward to harvest in solar cells. Triplet-spin excitons, in contrast, are 'dark', and the way in which the electrons spin makes it difficult to harvest the energy they carry.

"The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons," said Maxim Tabachnyk of the University's Cavendish Laboratory, the paper's lead author. "If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells."

Using state-of-art femtosecond laser spectroscopy techniques, the team discovered that triplet excitons could be transferred directly into inorganic semiconductors, with a transfer efficiency of more than 95%. Once transferred to the inorganic material, the electrons from the triplets can be easily extracted.

"Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic , could enable us to further push the efficiency of inorganic solar cells, like those made of silicon," said Dr Akshay Rao, who lead the team behind the work.

The team is now investigating how the discovered transfer of spin-triplet excitons can be extended to other organic/inorganic systems and are developing a cheap organic coating that could be used to boost the power conversion of silicon .

Explore further: Solar energy gets a boost: Work on 'singlet fission' can increase solar cell efficiency by as much as 30 percent

More information: "Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals" Nature Materials (2014) DOI: 10.1038/nmat4093

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7 comments

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holoman
1 / 5 (5) Oct 09, 2014
Why not just use metamaterials and silicon ?
gkam
4 / 5 (8) Oct 09, 2014
Poco a poco, we are replacing dirty fuels.
Scottingham
5 / 5 (5) Oct 09, 2014
Whenever you hear something like 'Why not just do X'....the word 'just' hides sooo much complexity as to render the statement useless.

Just sayin ;-)
AZstrider
1 / 5 (3) Oct 09, 2014
Oh come on, can't you at least give us some idea of what this fantastic efficiency boost might be? The cells on my roof are 20% efficient and with this new tech they would be __%?
EnergySage
1 / 5 (4) Oct 10, 2014
Expanding the efficiency of solar cell technology is one of the main factors of success that needs to happen with solar. The future of this technology does look bright as well as it's financial and economical future in the U.S. and abroad. The reliability of PV systems improves everyday and the cost of going solar are declining at a fast rate. Going solar can reduce your monthly electricity costs and even lower your emissions. Learn more about the financial benefits of solar PV technology. http://bit.ly/1heY7sw
foolspoo
5 / 5 (1) Oct 15, 2014
energy storage is a far bigger issue personalwebsitepushingwannabeSage. efficiency will continue to advance exponentially.
david_king
5 / 5 (1) Nov 07, 2014
If we could just add a little more funding to energy research we might get these ideas online a little quicker. We spend less than 400 million/year on energy research compared to 150 billion on R and D generally. That seems out of whack.

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