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Solar cells are inefficient, it’s a sad fact. With todays technology they boast about a 10-15% efficiency, compare that to todays gas engine at roughly 20-25% and you can see it’s not quite up to par. Well that could all change very soon thanks to a new method for transferring energy from organic to inorganic semiconductors. This could boost the efficiency of widely used inorganic solar cells to as close as 100% efficiency as they can get.

In the new paper, researchers have developed a method for harvesting the energy carried by particles known as ‘dark’ spin-triplet excitons with close to 100% efficiency, thus clearing the way for hybrid solar cells which could far surpass the current paltry efficiency limits.

The team has successfully harvested the energy of triplet excitons — an excited electron state whose energy is 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.

So let’s talk excitons, 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. This is basically the same process 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. This is a big part of why solar cells are seemingly so inefficient.

“The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” said Maxim Tabachnyk, 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, meaning all that energy that was wasted in these triplets can now be harvested as electricity.

“Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic semiconductors, 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.

The team is now investigating how the discovered energy 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 efficiency of silicon solar cells. Even if this boosts solar cell efficiency by 20 or 30% (making them roughly 40-50% efficient) this would be a huge improvement over what is currently used, at a (ideally) much cheaper production cost.

Of course there still is one very large problem, even if solar cells were 100% efficient (impossible due to the laws of thermodynamics I know), we would only be able to use them when the sun is out. To truly be a solar planet our battery technology needs to improve so that we can store all that energy to use when we need it. Right now the power we use is made as we are using it, it is not stored and that is a whole other problem that needs to be solved… and hopefully soon.

Tabachnyk M, Ehrler B, Gélinas S, Böhm ML, Walker BJ, Musselman KP, Greenham NC, Friend RH, & Rao A (2014). Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals. Nature materials PMID: 25282509

2 responses

  1. That is great news. I hope organic semiconductors will make the world more green. I hope there is another breakthrough in device performance degradation and organic solar cells jumps above 20% net performance.


    October 13, 2014 at 3:26 am

    • I agree, its all very exciting! Personally I’m keeping my fingers crossed for a big battery breakthrough. We need to find better ways to store energy to make solar or wind a real viable option no matter how efficient they can be.


      October 13, 2014 at 10:02 am

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