High External Quantum Efficiency in van der Waals Heterostructures for Ultrathin Photovoltaics

High external radiative efficiency and high external quantum efficiency are prerequisites for an efficient photovoltaic cell. In transition metal dichalcogenides (TMDCs), previous work has demonstrated that near-unity external radiative efficiency is possible through superacid passivation. Yet, near-unity external quantum efficiency has remained elusive. In this work, we experimentally demonstrate that high external quantum efficiencies (\textgreater 50{\%}) are possible in vertical van der Waals heterostructures consisting of graphene, tungsten diselenide, and molybdenum disulfide, on metallic substrates. We achieve near-unity absorption in ultrathin (\textless 15 nm) transition metal dichalcogenides by employing non-trivial phase shifts at the TMDC/metal interface. We show that the use of both graphene and a PN junction geometry leads to an enhancement in the internal quantum efficiency, a measure of the carrier collection efficiency. Moreover, the internal quantum efficiency is shown to exhibit exciton resonances with peak efficiencies \textgreater 70{\%}. In summary, our results presented here will serve as design considerations and principles towards achieving near-unity external quantum efficiency in van der Waals materials.