Solar cells are sensitive only to photons with wavelengths that correspond to the energy gap of the material from which they are made. When photons with this wavelength reach the cell, they excite electrons into the conduction band of the material, where they are registered as an electrical current. But higher-energy photons cannot contribute to this current, and can reduce the efficiency of the cell by heating it up.

Now Green and colleagues say that high-energy photons could be harnessed and converted into current using a ‘down-converter’ – a device that splits high-energy photons into two lower-energy photons. When a photon reaches the down-converter, it excites an electron into a higher energy level. But the electron returns to its ground state via an intermediate energy level, and emits a lower-energy photon at each stage.

By tuning a down-converter to emit photons with a wavelength corresponding to the energy gap of a solar cell, one high-energy photon could be split into two ‘useful’ photons. According to Green and co-workers, this could increase the efficiency of solar cells from the current maximum of about 30% to almost 40%.

The team calculated that the highest efficiency could be reached by placing a down-converter on the ‘back’ of the solar cell – that is, the opposite side to which the sunlight falls. This would allow high-energy photons to pass through the cell to the down-converter, while still allowing the cell to capture low-energy photons.

The drawback of this set-up is that most solar cells are made from semiconductors, which would not allow high-energy photons to pass through. However, the researchers say that this arrangement would be suitable for ‘dye-sensitized’ solar cells.

But the most promising arrangement would see the down-converter connected to the ‘front’ of a solar cell. Although this would block out low-energy photons, it would still boost the overall efficiency of the cell to 38.6%. Most importantly, say the researchers, this set-up would allow existing semiconductor solar cells to be fitted with down-converters.

According to Green and colleagues, the down-converters should be simple to make from materials such as aluminium arsenide or gallium phosphide, using established ‘epitaxial’ manufacturing techniques.