Last updated Figure 1. Spectral irradiance of wavelengths in the solar spectrum. The red shaded area shows the irradiance at sea level. There is less irradiance at sea level due to absorption of light by the atmosphere.
An optical rectenna is a rectenna (rectifying antenna) that works with visible or infrared light.[1] A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct currentelectricity. While rectennas have long been used for radio waves or microwaves, 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 terahertz for visible light—that only a few types of specialized diodes can switch quickly enough to rectify 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 solar cells. The idea was first proposed by Robert L. Bailey in 1972.[2] As of 2012, only a few optical rectenna devices have been built, demonstrating only that energy conversion is possible.[3] 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. [4] In 2008 it was reported that Idaho National Laboratories designed an optical antenna to absorb wavelengths in the range of 3–15 μm.[5] These wavelengths correspond to photon energies of 0.4 eV down to 0.08 eV. 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 0.4 and 1.6 μm because these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of the solar radiation spectrum[6] (see Figure 1).
History
Robert Bailey, along with James C. Fletcher, received a patent (US 3760257) in 1973 for an "electromagnetic wave energy converter". The patented device was similar to modern day optical rectennas. The patent discusses the use of a diode "type described by [ Ali Javan] in the IEEE Spectrum, October, 1971, page 91", to whit, a 100nm-diameter metal cat's whisker to a metal surface covered with a thin oxide layer. Javan was reported as having rectified 58THz infrared light. In 1974, T. Gustafson and coauthors demonstrated that these types of devices could rectify even visible light to DC current[7] Alvin M. Marks received a patent in 1984 for a device explicitly stating the use of sub-micron antennas for the direct conversion of light power to electrical power.[8] Marks's device showed substantial improvements in efficiency over Bailey's device.[9] In 1996, Guang H. Lin reported resonant light absorption by a fabricated nanostructure and rectification of light with frequencies in the visible range.[9] In 2002, ITN Energy Systems, Inc. published a report on their work on optical antennas coupled with high frequency diodes. ITN set out to build an optical rectenna array with single digit efficiency. Although they were unsuccessful, the issues associated with building a high efficiency optical rectenna were better understood.[6]
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,.[10] Vertical arrays of multiwall carbon nanotubes (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%.[10] 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 Al2O3 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 focussed 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.
Theory
The theory behind optical rectennas is essentially the same as for traditional (radio or microwave) rectennas. Incident light on the antenna causes electrons in the antenna to move back and forth at the same frequency as the incoming light. This is caused by the oscillating electric field of the incoming electromagnetic wave. The movement of electrons is an alternating current (AC) in the antenna circuit. To convert this into direct current (DC), the AC must be rectified, which is typically done with a diode. The resulting DC current can then be used to power an external load. The resonant frequency of antennas (frequency which results in lowest impedance and thus highest efficiency) scales linearly with the physical dimensions of the antenna according to simple microwave antenna theory.[6] The wavelengths in the solar spectrum range from approximately 0.3-2.0 μm.[6] Thus, in order for a rectifying antenna to be an efficient electromagnetic collector in the solar spectrum, it needs to be on the order of hundreds of nm in size.
Figure 3. Image showing the skin effect at high frequencies. The dark region, at the surface, indicates electron flow where the lighter region (interior) indicates little to no electron flow.
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 diodes used in larger scale rectennas cannot operate at THz frequencies without large loss in power.[5] 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.[6] The ideal wavelengths of 0.4–1.6μm correspond to frequencies of approximately 190–750THz, 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) tunneling diodes 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 150 THz.[6]
One of the biggest claimed advantages of optical rectennas is their high theoretical efficiency. When compared to the theoretical efficiency of single junction solar cells (30%), optical rectennas appear to have a significant advantage. However, the two efficiencies are calculated using different assumptions. The assumptions involved in the rectenna calculation are based on the application of the Carnot efficiency of solar collectors. The Carnot efficiency, η, is given by
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. R. L. Bailey 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.[5]
As previously stated, one of the major limitations of optical rectennas is the frequency at which they operate. The high frequency of light in the ideal range of wavelengths makes the use of typical Schottky diodes impractical. Although MIM diodes show promising features for use in optical rectennas, more advances are necessary to operate efficiently at higher frequencies.[11]
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.[11]
After the proof of concept was completed, laboratory-scale silicon wafers were fabricated using standard semiconductor integrated circuit fabrication techniques. E-beam lithography was used to fabricate the arrays of loop antenna metallic structures. The optical antenna consists of three main parts: the ground plane, the optical resonance cavity, and the antenna. The antenna absorbs the electromagnetic wave, the ground plane acts to reflect the light back towards the antenna, and the optical resonance cavity bends and concentrates the light back towards the antenna via the ground plane.[5] This work did not include production of the diode.
Lithography method
Idaho National Labs used the following steps to fabricate their optical antenna arrays. A metallic ground plane was deposited on a bare silicon wafer, followed by a sputter deposited amorphous silicon layer. The depth of the deposited layer was about a quarter of a wavelength. A thin manganese film along with a gold frequency selective surface (to filter wanted frequency) was deposited to act as the antenna. Resist was applied and patterned via electron beam lithography. The gold film was selectively etched and the resist was removed.
Roll-to-roll manufacturing
In moving up to a greater production scale, laboratory processing steps such as the use of electron beam lithography are slow and expensive. Therefore, a roll-to-roll manufacturing method was devised using a new manufacturing technique based on a master pattern. This master pattern mechanically stamps the precision pattern onto an inexpensive flexible substrate and thereby creates the metallic loop elements seen in the laboratory processing steps. The master template fabricated by Idaho National Laboratories consists of approximately 10 billion antenna elements on an 8-inch round silicon wafer. Using this semi-automated process, Idaho National Labs has produced a number of 4-inch square coupons. These coupons were combined to form a broad flexible sheet of antenna arrays. This work did not include production of the diode component.
Atomic layer deposition
Researchers at the University of Connecticut are using a technique called selective area atomic layer deposition that is capable of producing them reliably and at industrial scales.[12] Research is ongoing to tune them to the optimal frequencies for visible and infrared light.
Economics of optical antennas
Optical antennas (by itself, omitting the crucial diode and other components) are cheaper than photovoltaics (if efficiency is ignored). While materials and processing of photovoltaics are expensive (currently the cost for complete photovoltaic modules is in the order of 430 USD / m2 in 2011 and declining.[13]), Steven Novack estimates the current cost of the antenna material itself as around 5 - 11 USD / m2 in 2008.[14] With proper processing techniques and different material selection, he estimates that the overall cost of processing, once properly scaled up, will not cost much more. His prototype was a 30 x 61 cm of plastic, which contained only 0.60 USD of gold in 2008, with the possibility of downgrading to a material such as aluminum, copper, or silver.[15] The prototype used a silicon substrate due to familiar processing techniques, but any substrate could theoretically be used as long as the ground plane material adheres properly.
Future research and goals
In an interview on National Public Radio's Talk of the Nation, Dr. Novack claimed that optical rectennas could one day be used to power cars, charge cell phones, and even cool homes. Novack claimed the last of these will work by both absorbing the infrared heat available in the room and producing electricity which could be used to further cool the room. (Other scientists have disputed this, saying it would violate the second law of thermodynamics.[16][17])
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.[18] 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:
Low work function: A large work function (WF) difference between the MWCNT is needed to maximize diode asymmetry, which lowers the turn-on voltage required to induce a photoresponse. The WF of carbon nanotubes is 5 eV and the WF of the calcium top layer is 2.9 eV, giving a total work function difference of 2.1 eV for the MIM diode.
High transparency: Ideally, the top electrode layers should be transparent to allow incoming light to reach the MWCNT antennae.
Low electrical resistance: Improving device conductivity increases the rectified power output. But there are other impacts of resistance on device performance. Ideal impedance matching between the antenna and diode enhances rectified power. Lowering structure resistances also increases the diode cutoff frequency, which in turn enhances the effective bandwidth of rectified frequencies of light. The current attempt to use calcium in the top layer results in high resistance due to the calcium oxidizing rapidly.
Researchers currently hope to create a rectifier which can convert around 50% of the antenna's absorption into energy.[14] 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.
The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high frequency these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications.
A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays. It produces an electrical current when it absorbs photons. This can be used for detection and measurement applications, or for the generation of electrical power in solar cells. Photodiodes are used in a wide range of applications throughout the electromagnetic spectrum from visible light photocells to gamma ray spectrometers.
In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote an electron from the valence band to the conduction band. The resulting conduction-band electron are free to move within the crystal lattice and serve as charge carriers to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move within the solid because there are no available states. If the electrons are not free to move within the crystal lattice, then there is no generated current due to no net charge carrier mobility. However, if some electrons transfer from the valence band to the conduction band, then current can flow. Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances having large band gaps are generally insulators, those with small band gaps are semiconductor, and conductors either have very small band gaps or none, because the valence and conduction bands overlap to form a continuous band.
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
A rectenna is a special type of receiving antenna that is used for converting electromagnetic energy into direct current (DC) electricity. They are used in wireless power transmission systems that transmit power by radio waves. A simple rectenna element consists of a dipole antenna with a diode connected across the dipole elements. The diode rectifies the AC induced in the antenna by the microwaves, to produce DC power, which powers a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power losses due to conduction and switching. Large rectennas consist of arrays of many power receiving elements such as dipole antennas.
Wide-bandgap semiconductors are semiconductor materials which have a larger band gap than conventional semiconductors. Conventional semiconductors like silicon have a bandgap in the range of 0.6 – 1.5 electronvolt (eV), whereas wide-bandgap materials have bandgaps in the range above 2 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of conventional semiconductors and insulators.
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction.
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There are a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by various performance metrics, such as spectral response. Semiconductor-based photodetectors typically use a p–n junction that converts photons into charge. The absorbed photons make electron–hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.
The photovoltaic effect is the generation of voltage and electric current in a material upon exposure to light. It is a physical phenomenon.
Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to electricity via photons. A basic thermophotovoltaic system consists of a hot object emitting thermal radiation and a photovoltaic cell similar to a solar cell but tuned to the spectrum being admitted from the hot object.
Hybrid solar cells combine advantages of both organic and inorganic semiconductors. Hybrid photovoltaics have organic materials that consist of conjugated polymers that absorb light as the donor and transport holes. Inorganic materials in hybrid cells are used as the acceptor and electron transporter in the structure. The hybrid photovoltaic devices have a potential for not only low-cost by roll-to-roll processing but also for scalable solar power conversion.
Third-generation photovoltaic cells are solar cells that are potentially able to overcome the Shockley–Queisser limit of 31–41% power efficiency for single bandgap solar cells. This includes a range of alternatives to cells made of semiconducting p-n junctions and thin film cells. Common third-generation systems include multi-layer ("tandem") cells made of amorphous silicon or gallium arsenide, while more theoretical developments include frequency conversion,, hot-carrier effects and other multiple-carrier ejection techniques.
Multi-junction (MJ) solar cells are solar cells with multiple p–n junctions made of different semiconductor materials. Each material's p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency.
Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. A promising low cost alternative to conventional solar cells made of crystalline silicon, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.
An organic solar cell (OSC) or plastic solar cell is a type of photovoltaic that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Most organic photovoltaic cells are polymer solar cells.
Solid is one of the four fundamental states of matter along with liquid, gas, and plasma. The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas. The atoms in a solid are bound to each other, either in a regular geometric lattice, or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because the molecules in a gas are loosely packed.
In physics, the radiative efficiency limit is the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell. It was first calculated by William Shockley and Hans-Joachim Queisser at Shockley Semiconductor in 1961, giving a maximum efficiency of 30% at 1.1 eV. The limit is one of the most fundamental to solar energy production with photovoltaic cells, and is one of the field's most important contributions.
A plasmonic-enhanced solar cell, commonly referred to simply as plasmonic solar cell, is a type of solar cell that converts light into electricity with the assistance of plasmons, but where the photovoltaic effect occurs in another material.
There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be categorized into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as more efficient energy converters from light energy into electric current or light absorbers and charge carriers.
Sun-free photovoltaics is a photovoltaics technology which does not require sunlight to produce electricity. This technique was developed by research team at Massachusetts Institute of Technology. Photovoltaic cells convert light to electricity most efficiently at specific wavelengths. The surface features of Sun-free photovoltaics is engineered such that it converts heat energy into the specific wavelengths. This increases the efficiency of existing thermophotovoltaic (TPV) systems.
1 2 3 4 Dale K. Kotter; Steven D. Novack; W. Dennis Slafer; Patrick Pinhero (August 2008). Solar Nantenna Electromagnetic Collectors(PDF). 2nd International Conference on Energy Sustainability. INL/CON-08-13925. Archived from the original(PDF) on 11 August 2016. Retrieved 12 June 2016.
↑ Moddel, Garret (2013). "Will Rectenna Solar Cells be Practical?". In Garret Moddel; Sachit Grover (eds.). Rectenna Solar Cells. Springer New York. pp.3–24. doi:10.1007/978-1-4614-3716-1_1. ISBN978-1-4614-3715-4. Quote: "There has been some discussion in the literature of using infrared rectennas to harvest heat radiated from the earth’s surface. This cannot be accomplished with ambient-temperature solar cells due to the second law of thermodynamics" (page 18)
↑ S.J. Byrnes; R. Blanchard; F. Capasso (2014). "Harvesting renewable energy from Earth's mid-infrared emissions"(PDF). PNAS. 111 (11): 3927–3932. Bibcode:2014PNAS..111.3927B. doi:10.1073/pnas.1402036111. PMC3964088. PMID24591604. Quote: "...there have also been occasional suggestions in the literature to use rectennas or other devices to harvest energy from LWIR radiation (20-23). However, these analyses have neglected the thermal fluctuations of the diode, as discussed below and in ref. 12, which leads to the absurd conclusion that a room-temperature device can generate useful power from collecting the ambient radiation from room-temperature objects."
↑ Rectenna Solar Cells, ed. Moddel and Grover, page 10
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