Energetic Nanoparticles Swing Sunlight Into Electricity

Electricity-generating solar cells are one of the most attractive alternatives for creating a long-term sustainable energy system, but thus far solar cells have not been able to compete economically with fossil fuels. Researchers are now looking at how nanotechnology can contribute in bringing down the cost.

Solar cells are constructed of layers that absorb sunlight and convert it to electrical current. Thinner solar cells can yield both cheaper and more plentiful electricity than today's cells, if their capacity to absorb sunlight is optimized.

One way to enhance the absorption of the solar harvesting material in a solar cell is to make use of nanoparticles of noble metal. Carl Hägglund at Chalmers has looked at how this can be done in his recently completed doctoral dissertation.

The particles involved have special optical properties owing to the fact that their electrons oscillate back and forth together at the same rate as the frequency of the light, that is, the color of the light. The particles catch the light as tiny antennas and via the oscillations the energy is passed on as electricity. These oscillations, plasmons, are very forceful at certain so-called plasmon resonance frequencies, which in turn are influenced by the form, size, and surroundings of the particles.

"What we've done is to make use of nanotechnology to produce the particles and we've therefore been able to determine the properties and see how they can enhance the absorption of light of different colors," says Carl Hägglund.

In the context of solar cells, the great challenge is to efficiently convert the energy that is absorbed in the electron oscillation to energy in the form of electricity.

"We show that it is precisely the oscillations of the particles that yield the energy, how it is transmitted to the material and becomes electricity. It might have turned out, for example, that the oscillations simply generated heat instead," says Carl Hägglund.

The efficiency of the best solar cells today is already very high. The possibility of achieving even better solar cells therefore lies in using less material and in lowering production costs.

With solar cells of specially designed nanoparticles of gold, which is what Carl Hägglund has looked at, a layer only a few nanometers thick is required for the particles to be able to absorb light in an efficient way.

The dissertation examines the effect of nanoparticles of noble metal on two different types of solar cells, which can be said to represent two extremes. In one type of solar cell the light is absorbed in molecules on a surface, and in the other type deep inside the material.

The experimental and theoretical results show that the particles can help transmit the light's energy to useful electricity in several different ways and that it's possible to enhance the absorption of solar cells both on the surface and deep inside via different mechanisms.

This work has been carried out within the framework of a materials science research program (PhotoNano) funded by the Swedish Foundation for Strategic Research.