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An optical rectenna is a rectenna (rectifying antenna) that works with visible or infrared light. A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct current electricity. While rectennas have long been used for or , an optical rectenna would operate the same way but with infrared or visible light, turning it into electricity.
While traditional (radio- and microwave) rectennas are fundamentally similar to optical rectennas, it is vastly more challenging in practice to make an optical rectenna. One challenge is that light has such a high frequency—hundreds of Hertz for visible light—that only a few types of specialized diodes can switch quickly enough to rectifier it. Another challenge is that antennas tend to be a similar size to a wavelength, so a very tiny optical antenna requires a challenging nanotechnology fabrication process. A third challenge is that, being very small, an optical antenna typically absorbs very little power, and therefore tend to produce a tiny voltage in the diode, which leads to low diode nonlinearity and hence low efficiency. Due to these and other challenges, optical rectennas have so far been restricted to laboratory demonstrations, typically with intense focused laser light producing a tiny but measurable amount of power.
Nevertheless, it is hoped that arrays of optical rectennas could eventually be an efficient means of converting sunlight into electric power, producing solar power more efficiently than conventional . The idea was first proposed by Robert L. Bailey in 1972. As of 2012, only a few optical rectenna devices have been built, demonstrating only that energy conversion is possible. It is unknown if they will ever be as cost-effective or efficient as conventional photovoltaic cells.
The term nantenna (nano-antenna) is sometimes used to refer to either an optical rectenna, or an optical antenna by itself. In 2008 it was reported that Idaho National Laboratories designed an optical antenna to absorb wavelengths in the range of 3–15 μm. These wavelengths correspond to photon energies of down to . Based on antenna theory, an optical antenna can absorb any wavelength of light efficiently provided that the size of the antenna is optimized for that specific wavelength. Ideally, antennas would be used to absorb light at wavelengths between because these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of the solar radiation spectrum (see Figure 1).
In 2015, Baratunde A. Cola's research team at the Georgia Institute of Technology, developed a solar energy collector that can convert optical light to DC current, an optical rectenna using carbon nanotubes,. Vertical arrays of multiwall (MWCNTs) grown on a metal-coated substrates were coated with insulating aluminum oxide and altogether capped with a metal electrode layer. The small dimensions of the nanotubes act as antennae, capable of capturing optical wavelengths. The MWCNT also doubles as one layer of a metal-insulator-metal (MIM) tunneling diode. Due to the small diameter of MWCNT tips, this combination forms a diode that is capable of rectifying the high frequency optical radiation. The overall achieved conversion efficiency of this device is around 10−5 %. Nonetheless, optical rectenna research is ongoing.
The primary drawback of these carbon nanotube rectenna devices is a lack of air stability. The device structure originally reported by Cola used calcium as a semitransparent top electrode because the low work function of calcium (2.9 eV) relative to MWCNTs (~5 eV) creates the diode asymmetry needed for optical rectification. However, metallic calcium is highly unstable in air and oxidizes rapidly. Measurements had to be made within a glovebox under an inert environment to prevent device breakdown. This limited practical application of the devices.
Cola and his team later solved the challenges with device instability by modifying the diode structure with multiple layers of oxide. In 2018 they reported the first air-stable optical rectenna along with efficiency improvements.
The air-stability of this new generation of rectenna was achieved by tailoring the diode's quantum tunneling barrier. Instead of a single dielectric insulator, they showed that the use of multiple dissimilar oxide layers enhances diode performance by modifying diode tunneling barrier. By using oxides with different electron affinities, the electron tunneling can be engineered to produce an asymmetric diode response regardless of the work function of the two electrodes. By using layers of alumina and HfO2, a double-insulator diode (metal-insulator-insulator-metal (MIIM)) was constructed that improved the diode's asymmetric response more than 10-fold without the need for low work function calcium, and the top metal was subsequently replaced with air-stable silver.
Future efforts will be focused on improving the device efficiency by investigating alternative materials, manipulating the MWCNTs and the insulating layers to encourage conduction at the interface, and reduce resistances within the structure.
Another approach was presented in 2022 by Proietti Zaccaria Remo and collaborators at the Italian Institute of Technology, based on a top-down solution where the antenna and the rectifier were merged together in a single plasmonic-based solution. The proposed rectenna was tested at 1064 nm, both in single and matrix format, achieving an efficiency of 10−3 %. Furthermore, both experimental and theoretical analyses were conducted at 780 nm, with positive results suggesting that plasmonic-based rectennas could potentially serve as a viable approach for achieving a high-performing rectenna in the visible range. Although this solution represents a significant step forward, several challenges remain that prevent optical rectennas from reaching the 1% efficiency threshold required for practical applications. Key obstacles include achieving optical resonance regardless of the incident radiation conditions (e.g., angle of incidence) and improving the rectifier performance.
Because of simplifications used in typical rectifying antenna theory, there are several complications that arise when discussing optical rectennas. At frequencies above infrared, almost all of the current is carried near the surface of the wire which reduces the effective cross sectional area of the wire, leading to an increase in resistance. This effect is also known as the "skin effect". From a purely device perspective, the I-V characteristics would appear to no longer be ohmic, even though Ohm's law, in its generalized vector form, is still valid.
Another complication of scaling down is that used in larger scale rectennas cannot operate at THz frequencies without large loss in power. The large loss in power is a result of the junction capacitance (also known as parasitic capacitance) found in p-n junction diodes and Schottky diodes, which can only operate effectively at frequencies less than 5 THz. The ideal wavelengths of 0.4–1.6 μm correspond to frequencies of approximately 190–750 THz, which is much larger than the capabilities of typical diodes. Therefore, alternative diodes need to be used for efficient power conversion. In current optical rectenna devices, metal-insulator-metal (MIM) Tunnel diode are used. Unlike Schottky diodes, MIM diodes are not affected by parasitic capacitances because they work on the basis of electron tunneling. Because of this, MIM diodes have been shown to operate effectively at frequencies around .
where Tcold is the temperature of the cooler body and Thot is the temperature of the warmer body. In order for there to be an efficient energy conversion, the temperature difference between the two bodies must be significant. claims that rectennas are not limited by Carnot efficiency, whereas photovoltaics are. However, he does not provide any argument for this claim. Furthermore, when the same assumptions used to obtain the 85% theoretical efficiency for rectennas are applied to single junction solar cells, the theoretical efficiency of single junction solar cells is also greater than 85%.
The most apparent advantage optical rectennas have over semiconductor photovoltaics is that rectenna arrays can be designed to absorb any frequency of light. The resonant frequency of an optical antenna can be selected by varying its length. This is an advantage over semiconductor photovoltaics, because in order to absorb different wavelengths of light, different band gaps are needed. In order to vary the band gap, the semiconductor must be alloyed or a different semiconductor must be used altogether.
Another disadvantage is that current optical rectennas are produced using electron beam (e-beam) lithography. This process is slow and relatively expensive because parallel processing is not possible with e-beam lithography. Typically, e-beam lithography is used only for research purposes when extremely fine resolutions are needed for minimum feature size (typically, on the order of nanometers). However, photolithographic techniques have advanced to where it is possible to have minimum feature sizes on the order of tens of nanometers, making it possible to produce rectennas by means of photolithography.
Improving the diode is an important challenge. There are two challenging requirements: Speed and nonlinearity. First, the diode must have sufficient speed to rectify visible light. Second, unless the incoming light is extremely intense, the diode needs to be extremely nonlinear (much higher forward current than reverse current), in order to avoid "reverse-bias leakage". An assessment for solar energy collection found that, to get high efficiency, the diode would need a (dark) current much lower than 1μA at 1V reverse bias. Rectenna Solar Cells, ed. Moddel and Grover, page 10 This assessment assumed (optimistically) that the antenna was a directional antenna array pointing directly at the sun; a rectenna that collects light from the whole sky, like a typical silicon solar cell does, would need the reverse-bias current to be even lower still, by orders of magnitude. (The diode simultaneously needs a high forward-bias current, related to impedance-matching to the antenna.)
There are special diodes for high speed (e.g., the metal-insulator-metal tunnel diodes discussed above), and there are special diodes for high nonlinearity, but it is quite difficult to find a diode that is outstanding in both respects at once.
To improve the efficiency of carbon nanotube-based rectenna:
Researchers currently hope to create a rectifier which can convert around 50% of the antenna's absorption into energy. Another focus of research will be how to properly upscale the process to mass-market production. New materials will need to be chosen and tested that will easily comply with a roll-to-roll manufacturing process. Future goals will be to attempt to manufacture devices on pliable substrates to create flexible solar cells.
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