Third generation photovoltaics

We review recent progress towards increasing solar cell efficiencies beyond the Shockley-Queisser efficiency limit. Four main approaches are highlighted: multi-junction cells, intermediate-band cells, hot carrier cells and spectrum conversion. Multi-junction cells use multiple solar cells that selectively absorb different regions of the solar spectrum. Intermediate-band cells use one junction with multiple bandgaps to increase efficiencies. Hot-carrier cells convert the excess energy of above-bandgap photons into electrical energy. Spectrum conversion solar cells convert the incoming polychromatic sunlight into a narrower distribution of photons suited to the bandgap of the solar cell.