Conventional solar cells generate electricity when an incoming photon excites an electron across the band gap; the energy gap between the valence and conduction bands. If the incoming photon does not have enough energy, the electron cannot bridge the gap. If the photon possesses too much energy then the electron is excited high into the conduction band, and the extra energy is lost as heat. One approach to building more efficient photovoltaics is therefore to convert more of the spectrum into useful energy in both the low and high energy regions.
One way to achieve this goal is to lay different semiconductors on top of each other, within the same device. Each semiconductor layer would be optimized for a different wavelength (energy), and so a device with enough layers could make use of the whole spectrum. Dr Nair López explained, “Current high efficiency multijunction solar cells are based on a complex technology requiring growth of a multiplicity of thin films to produce three separate p/n junctions”. This results in an expensive device.
One alternative would be to fabricate a material that had an “intermediate band” between a valence and conduction band that were widely separated in energy. It would then be possible for high energy photons to directly excite electrons into the conduction band, without losing energy as heat, while low energy photons could be utilized through a two-step process. By having just a single p/n junction, but using similar production methods to multijunction solar cells, the cost of constructing the device should be reduced.
Perhaps surprisingly such a solar cell has never been experimentally demonstrated. Until now. Researchers at Lawrence Berkeley National Laboratory have used highly mismatched alloys (HMAs) to construct of such a device [López et al., Phys Rev Lett (2011) 106, 028701]. These alloys, in this case GaNAs based materials, can provide exotic band structures thanks to the combination of the individual semiconductors.
The resulting solar cell was sensitive to a broad spectrum of energies, ranging from ~1.1 – 3.2 eV; covering the entire optical spectrum. However, there is still work to be done, as López told us, “The effort in this area is still in the R&D stage.  There is a need to improve the quality of the light absorbing material and to increase the charge collection efficiency”.

Stewart Bland