Valley Photovoltaics: Experimental Evidence for a Practical Route towards the Realization of the Hot Carrier Solar Cell

Hot carrier solar cells offer the potential to increase the efficiency of single gap solar cells beyond 60%. Here a new and viable route is proposed where photoexcited electrons are scattered to and collected from the L and X valleys. We demonstrate proof-of-principle results for an InGaAs/AlInAs heterojunction solar cell that shows an operating voltage (~1.4 eV) in excess of the InGaAs absorber bandgap (0.75 eV) under 1-sun AM 1.5G. Hot carriers are confirmed in simultaneous monochromatic current–voltage and photoluminescence measurements. Hot carrier operation in this simple commercially mature system is driven by the transfer, storage, and extraction of hot carriers in the satellite valleys of InGaAs. Combining intervalley scattering and the Gunn Effect allows the majority of photoexcited electrons to be harnessed with voltages defined by the upper valley separation, and thus an optimized system would exceed the Shockley-Queisser limit for a single bandgap solar cell. A mismatch in the valley degeneracy across the n+-AlInAs/n-InGaAs interface currently limits the performance, but a clear route to the realization of such a device in traditional III-V technologies is presented.