Combined harvester

Improvements in our understanding of how nature converts the sun's energy into a flow of electrons, plus the odd properties of extremely small crystals, could help a

team make much more efficient — and potentially cheaper — solar cells.

Luke Wilson and his colleagues in the university's Department of Physics and Astronomy are working with the National Centre for Semiconductor Growth, also in Sheffield, to study the use of nanoscale semiconductor crystals known as quantum dots as solar energy collectors.

Quantum dots are single crystals, generally less than 100nm across, of materials with photovoltaic properties. This means they absorb energy at the wavelengths given off by the sun, and convert it into a form which can be harvested. But their response is very different from that of a bulk photovoltaic material, such as those used in conventional solar cells.

When a material absorbs solar energy, it has to get rid of that energy somehow, using a process called relaxation. 'In bulk semiconductors, the material relaxes by generating heat,' explained Wilson; the lattice structure of the material vibrates in a way which warms up the sample.

But because quantum dots are so small, they can't set up that kind of vibration. Instead, they relax by kicking out some of the electrons which bind the atoms in the crystal together, creating what's known as an electron-hole pair (the hole is the space where the electron used to be).

Each photon of energy absorbed can create up to seven pairs, and this is a far more efficient conversion of energy than a bulk material can manage.

However, said Wilson, harvesting that energy is quite a problem. The conventional way to do it would be by charge transfer — taking the electrons ejected from the crystal and using them to displace electrons in another semiconductor material. But this means that the electrons have to physically tunnel from one material into another, which takes a lot of energy — more than the electrons generally have.

So instead, Wilson's team is using the phenomenon by which natural photosynthetic chemicals, such as chlorophyll in leaves, transfer their energy.

'We use energy transfer rather than charge transfer, so we're not relying on the physical movement of charge carriers over the boundary,' said Wilson.

The phenomenon is known as Förster energy transfer, and it works because the properties of the nanocrystal and the energy carrier substance are carefully tuned. When it absorbs solar energy, the nanocrystal emits it at characteristic wavelengths. These match exactly the absorption wavelengths of the energy carrier.

When the two materials are very close together, the energy transfer is very efficient. One analogy that's often used is two tuning forks. If one is set to vibrate and then brought very close to the other, it will gradually stop vibrating, while its partner starts. Eventually, the first will be still, while the second rings very nearly as loudly as the first one had initially.

This energy transfer is efficient enough to create electron-hole pairs in the structure of the carrier material which Wilson chooses to have good conductive properties. 'We've demonstrated the feasibility of the approach using photosynthetic bacteria, but now we want to use nanocrystals as they are much more robust and more efficient at higher light levels,' said Wilson.

For a standard photovoltaic material using a parabolic mirror to focus sunlight on its surface, the maximum projected efficiency is 38 per cent; in practice, said Wilson, efficiencies in the high 20s are more common.

But for the Sheffield team's hybrid system with quantum dots and a gallium arsenide-based carrier material, the maximum efficiency is 58 per cent. 'The real challenge is to maximise the efficiency of the energy transfer process,' said Wilson.

'Efficiencies in the order of 80 per cent have been predicted, and it's close to unity over very short distances,' he added. 'If we can get near unity — which would mean we get all the energy produced in the nanocrystal out — we'd get those very high conversion efficiencies.'

The project lasts until September 2009, at which point Wilson wants to demonstrate that the hybrid nanocrystal/carrier system is technically feasible as a working photovoltaic cell.

Following that, he wants to develop systems which will demonstrate the potential for improved efficiencies.