The new material converts high-energy photons into electricity without wasting their energy as heat, and also converts low-energy photons to electricity—photons that ordinarily wouldn’t be absorbed by the material.
A similar effect is achieved in commercial multi-junction solar cells, which are made by essentially stacking three solar cells on top of each other, each optimized for a different color of light. But combining these three solar cells is expensive and complex, since each layer has to be closely matched to the other layers.
The prototype solar cell is still relatively inefficient. Part of the problem is that many of the electrons that have absorbed some energy from low-energy photons don’t hold onto that energy for long enough to absorb energy from another photon. These electrons never make it out of material, and the energy is lost as heat. The researchers are working with two companies, Rose Street Labs Energy and Sumika Electronic Materials, to overcome this issue. One option, for example, is doping the material with phosphorous atoms to change their electrical properties.
That will be challenging, says Andrew Norman, a research scientist at the National Renewable Energy Laboratory. Norman has also worked on this type of solar cell, although one made from very different materials. Norman says the new work is interesting, particularly because of the high voltage levels that the cell produces, but he notes that it has proved difficult to commercialize this type of cell. “You have to wonder why, in 50 years, no one has been successful,” he says.