Rectenna (rectifying antenna) behavior of carbon nanotubes and gold nanowires
Concept of optical rectenna 
Solar cells have been investigated for the potential of replacing the traditional energy sources. Conventional photovoltaic (PV) effect includes inducing separated electron-hole pairs (excitons) by illumination, and building a loop with the load for power supply. These photovoltaic cells are greatly limited by their poor efficiency (20~40%) due to high heat loss and recombination rate of the excitons. Rectenna (rectifying antenna), on the other hand, captures light with resonant antennae, and extracts the energy by utilizing a rectifying barrier to convert AC to DC signal. It has efficiency of over 90% in microwave region  , and therefore, proves a very promising concept for future PV technologies. However, one of the critical obstacles is the size of the antennae. The materials need to be at nanometer length scale to work at high frequency (~10 15 Hz) as in optical regime. To date, no optical rectennae with efficiency close to the theoretical value has been reported. Several group have been working on this novel device working at optical frequency, including coupling optical antennae to fast switching tunneling diode  , using multi-wall carbon nanotubes (MWNTs) as optical rectennae  . From previous research, we have fabricated photocell that combines an array of CNT antennaes (photocathode) and with physisorbed surfactant molecules which acts as rectifying barrier  . Currently, we are focusing on the photovoltaic cell assembled by gold nanowires.
Photocurrent generation and energy potential diagram
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