By using an intermediate material researchers hope to drastically improve the performance of solar cells

New approach increases performance of solar cells

A novel approach to harvesting solar energy could improve efficiency while making it easier to store the energy for later use, researchers say.

MIT researchers have demonstrated a method whereby sunlight is used to heat a high-temperature material whose infrared radiation is then collected by a conventional photovoltaic cell.

The extra step improves the performance of the system because it makes it possible to take advantage of wavelengths of light that go to waste in traditional solar cells as the material they are traditionally made from, silicon, does not absorb them.

Converting the energy of a photon into electricity requires that the photon's energy levels match that of a characteristic of the photovoltaic (PV) material called a bandgap, but silicon's bandgap does not correspond to many wavelengths of light.

To address the limitation, the team inserted a two-layer absorber-emitter device – made of novel materials including carbon nanotubes and photonic crystals – between the sunlight and the PV cell.

The intermediate material collects energy from a broad spectrum of sunlight, heating up in the process, and as it heats up it emits light of a particular wavelength, which in this case is tuned to match the bandgap of the PV cell mounted nearby.

The process, described in the journal Nature Nanotechnology, is not a new concept, but previous experiments have been unable to produce a device with efficiency of greater than 1 per cent.

But graduate student Andrej Lenert, associate professor of mechanical engineering Evelyn Wang and their team, have already produced an initial test device with a measured efficiency of 3.2 per cent. With further work they expect to be able to reach 20 per cent efficiency – enough, they say, for a commercially viable product.

The design of the two-layer absorber-emitter material is key to this improvement. Its outer layer, facing the sunlight, is an array of multiwalled carbon nanotubes, which very efficiently absorbs the light's energy and turns it to heat.

This layer is bonded tightly to a layer of a photonic crystal, which is precisely engineered so that when it is heated by the attached layer of nanotubes, it ‘glows’ with light whose peak intensity is mostly above the bandgap of the adjacent PV, ensuring that most of the energy collected by the absorber is then turned into electricity.

In their experiments, the researchers used simulated sunlight, and found that its peak efficiency came when its intensity was equivalent to a focusing system that concentrates sunlight by a factor of 750. This light heated the absorber-emitter to a temperature of 962°C.

This level of concentration is already much lower than in previous attempts at such systems, which concentrated sunlight by a factor of several thousand. But the MIT researchers say that after further optimisation, it should be possible to get the same kind of enhancement at even lower sunlight concentrations, making the systems easier to operate.

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