Semiconductor thermionics for next generation solar cells: photon enhanced or pure thermionic?

A semiconductor thermionic device, which utilises thermally excited electrons, is considered as an alternative in solar conversion technology, yet its working mechanism is not clear. Here, the authors reveal that whether such a device operates in the photon enhanced or purely thermionic mode, greatly depends on the material properties and device physics. Semiconductors have been used in solar energy conversion for decades based on the photovoltaic effect. An important challenge of photovoltaics is the undesired heat generated within the device. An alternative approach is thermionics, which uses the thermal excitation of electrons from an emitter to a collector across a vacuum gap. If the emitter is a p-type semiconductor, the photogeneration-induced quasi-Fermi level splitting can reduce the effective barrier for electron emission-a mechanism used by a photon enhanced thermionic emission device. Here, we evaluate the prospects of this alternative solar conversion technology considering different semiconductor materials and thermionic device configurations. We also reveal that whether such a device operates in the photon enhanced or purely thermionic mode, depends on the complex interplay among materials properties, device physics and solar concentration level.