Optical Power Conversion via Tunneling of Plasmonic Hot Carriers
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2018 American Chemical Society. Optical and photochemical power converters based on resonant absorption in metal nanostructures generally employ a mechanism whereby optically excited "hot" carriers are injected over a Schottky barrier at a semiconductor or molecular interface. This process is inefficient because most of the excited carriers relax and thermalize with the lattice before they can be collected. Here we outline an alternative strategy that can take better advantage of both optically excited and thermalized electrical carriers by leveraging the tunneling transport phenomenon across metal junctions that concentrate and absorb light preferentially on one side of a nanoscale gap. We have developed a general description for electron transport within a parabolic conduction band approximation accounting for both thermal (Fermi-Dirac) and nonthermal contributions to the steady-state electronic energy distribution that results from optical excitation. A nonzero current density is predicted when the excited-state distribution of carriers is dissimilar on opposite sides of a tunnel junction, with electrons emitted from the electrode that absorbs more light. An increase of the short-circuit photocurrent and the associated open-circuit voltage at elevated temperatures indicates a cooperative interaction between thermal and nonthermal excitation mechanisms. We also use full wave optical simulations (FDTD method) to demonstrate a simple device design for obtaining optical power conversion efficiency that is competitive with conventional photovoltaic devices.
