A fundamental limit to the performance of conventional photovoltaic devices is that photogenerated electrons and holes in semiconductors lose part of their energy via phonon emission, which ?cools? the carriers to the lattice temperature It has been suggested that carrier energy filters could be used to separate high-energy (hot) electron-hole pairs before they cool, to thereby boost device performance, and to specifically increase the open-circuit voltage (VOC) by up to hundreds of millivolts. However, this has so far not been realized, primarily because sufficiently fast charge separation of hot, photogenerated electron-hole pairs is difficult to achieve in the planar devices used so far.
In the presentation I will establish the conditions under which hot carrier solar cells can be operated reversibly, that is, at maximum energy conversion efficiency. Under specific conditions, the energy conversion efficiency of a hot carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. I will report first experiments in InAs/InP heterostructure nanowires, demonstrating an open circuit voltage that exceeds that achievable with traditional photovoltaic devices based on a single bandgap.