Can a Hot-Carrier Solar Cell also be an Efficient Up-converter?

The hot-carrier solar cell is a very ambitious device concept, which has a thermodynamic efficiency limit of around 84% when operated at the maximum power point. However, if the same device is instead operated at open circuit, then it becomes an efficient radiator of blackbody radiation at the temperature of the hot carriers. Such a configuration is similar to a thermal photovoltaic converter, but one in which the thermal gradient is maintained by a hot electron-hole gas rather than by a physically hot lattice. In this scheme (see Fig. 1), a low-bandgap hot-carrier material is placed behind a conventional solar cell and absorbs sub-bandgap photons, generating a hot-carrier distribution which re-radiates this energy, some of which can be collected by the solar cell located above. The additional photons have been thermally up-converted by a “hot-carrier radiator.” We will discuss the thermodynamic efficiency limit of a hot-carrier radiator placed behind a conventional single-junction solar cell, and present some experimental results toward developing a proof-of-concept device using strain-balanced quantum wells.