Photovoltaics versus rectenna technologies

When electromagnetic waves were experimentally observed, they were generated using antennas and radiating elements. Along the development of radio emission, antenna design became a separate area of expertise where the geometry of those elements configured the characteristics and capabilities of emission and reception of the EM waves. The shape and orientation of those antennas determine the polarization and direction of the emission, and reception. Electromagnetic spectrum was mastered and used in science and technology. Fortunately, the wavelengths associated with the radioelectric and microwave spectra allowed the manufacturing of radiating elements with the available fabrication tools. When increasing the frequency of the electromagnetic radiation, the geometries were shrunk accordingly and new fabrication strategies were used. Actually, an important leap in antenna design and fabrication appeared when using planar antennas written on flat substrates by microlithogra­phy techniques. Millimeter waves and Terahertz still benefit from those fabrication techniques. However, when the optical domain was placed as a feasible goal for antenna design, the use of electron beam lithography, focused ion beam, and related nanometric precision manufac­turing tools were necessary. Even more, those metals traditionally used as materials for antenna fabrication appeared to behave as non-perfect conductors, showing spectral disper­sion and a non-negligible penetration depth.

At the same time that antennas were clearly devoted to the emission and detection of EM wave in the radioelectric, and microwave regimes, light and optical spectrum was covered with other reliable technologies for emission (incandescence lamps, spectral lamps, lasers, etc.) and detection (Golay cells, thermoconductors, photovoltaics, etc.) mainly based for detection in the quantified energy levels of semiconductors. Then, photodetectors improved their performance in responsivity, signal-to-noise ratio, cut-off frequency, size, and biasing requirements.

Then, it is easy to understand that antennas did not find a suitable place to develop as optical detectors. Semiconductor detectors were here to stay, fabrication of optical antennas is difficult and requires high-tech machinery, and metals are non-perfect conductors anymore in the optical regime.

However, some advances were made in using antenna-coupled detectors in the detection of light at higher and higher frequencies, and in its use as frequency mixer or coupled to bolometric devices. Besides, nanoscience has found optical antennas as promising elements to explore materials and media with high spatial resolution. Plasmonic optics has become an emerging field, where the collective oscillation of charges produces exotic phenomenologies that are used for sensing and probing sub-wavelength structures.

Several reports and papers [4,5,6,7] have been published in the past years presenting optical antennas and rectennas as harvesters of electromagnetic radiation in the infrared and visible spectrum. They are based on the principle of rectification of the currents generated in an antenna structure that resonates at the visible frequency. The idea, although appealing, has been somehow over-estimated when promising efficiencies above 80%. However, as we will see in this chapter, some important problems need to be addressed before fabricating an operative device. Unfortunately, the task of rectifying electric fields oscillating at 1014-1015Hz frequencies is formidable, and the efficiency figures obtained so far are well below the announced limit. The bottleneck of the technology remains in the rectification process. At the same time, some important advances have been made to tailor the impedance of optical antennas to properly couple the electromagnetic field and also to transfer the power to the load, i. e., to the rectifier. Then, optical rectennas can be considered as a promising technology with high potential. Based on the current results, more effort needs to be allocated to leap over the rectifying mechanism with novel technologies.

Although it is limited to the solar region of the electromagnetic spectrum, the most mature and standard technology (developed since the mid 70’s) to harvest energy from EM radiation is photovoltaics (PV). According to the National Renewable Energy Laboratory (NREL), conver­sion efficiency of PV technologies has been increasingly evolved during the last 40 years (figure 2). From the most simple variant of the 1st generation represented by the silicon based cells, to the 2nd and 3rd generations corresponding to thin-film and the most sophisticated multijunction cells respectively, a trade-off between efficiency and production cost is defining the market of each variant (table 5).

PV technology

Efficiency (%)

Market Share (%)

1st generation

20

90

2nd generation

5-12

10

3rd generation

40-50

Table 5. Efficiency versus market of the 3 different PV technology generations.

The basic element in PV technology is the photovoltaic/solar panel/module which is composed of photovoltaic cells connected in parallel, when photogenerated current must be enhanced, or placed in series, when the output voltage is the parameter that needs to be maximized (see chapter 10: "Electronics for Power and Energy Management").

The working principle of a photovoltaic cell is based on the photovoltaic effect, which was firstly described by Alexander-Edmond Becquerel in 1839. As it is reviewed in chapter 3 about Solar Energy Harvesting, the photovoltaic effect has the same quantum nature as the photo­electric effect, so both can only be described by considering that the energy of the electromag­netic radiation is quantized in quanta called photons, with an energy hv, as it has been explained before (equation 2). As it is shown in figure 3.a, photovoltaic effect takes place at the core of the cell, which is found at the junction of the two semiconductors that integrates a typical PV cell. When an individual photon interact with an individual electron at the valence band of the semiconductor, the energy of the photon (and the photon itself) can be absorbed by the electron to get promoted to the conduction band, leaving a hole in the valence band. This process called, photo-generation of an electron-hole pair, is only possible if the photon energy is at least equal to the energy of the band-gap (energy distance between the conduction and valence band). The population of photogenerated electrons and holes is then driven by the electrical field in the depletion zone of the PN junction and can eventually contribute to a photovoltage and the corresponding photocurrent, when an electric load is connected to the PV cell. In this case, both the photovoltage and the photocurrent are dc magnitudes and their product gives directly the electrical power converted by the PV cell.

Photovoltaics versus rectenna technologies

Figure 2. Efficiency evolution of the main photovoltaic technologies (from National Renewable Energy Laboratory).

Instead, the radiofrequency rectenna (RFR) technology is based on the combined operation of two basic elements: an electrical rectifier that follows an electromagnetic antenna (rectenna). The operation principle (figure 3.b) does not require quantum mechanics to be explained since, in this case, electrons in the metallic antenna are already in the conduction band, and do not need to be promoted in energy by absorbing photons from the electromagnetic radiation. In this case, the phenomenology is better explained by the interaction between the electrons in the antenna and the electric field of an electromagnetic incident wave. Similarly to PV technology, in rectenna technology matching conditions must be also satisfied. Now, the wavelength of the EM incident wave has to be a multiple of the antenna characteristic length in order to induce a resonant electrical current in the antenna. As opposite to a PV cell, an antenna will generate at its output both an ac voltage and an ac current. For this reason, a rectifier is needed as the first basic electrical component to transform ac values into dc values.

Optical rectenna (OR) technology can be considered as a particular case of rectenna technology where the frequency of the electromagnetic radiation involved is in the optical range. So, from this point of view, RFR technology covers the radiofrequency part of the electromagnetic spectrum and OR the optical part (figure 4). However, as it will be described in a next section of this chapter, OR technology cannot be considered just as an extrapolation of the RF rectenna concept to the optical range, since neither the antenna element, in this case a nanoantenna, nor the rectifier, typically a metal-insulator-metal (MIM) diode, have exactly the same properties of the RF counterparts. New physics such as plasmon resonances have to be taken into accountin the optical antenna (OA), an antenna with characteristic lengths in the nanometer range (nanoantenna) to match the wavelengths of light radiation. Also special structures and materials are needed to achieve response times short enough to rectify signals in the THz range, which are induced in the nanoantenna element by the incident optical radiationWhen used as light detectors, optical antennas involving rectifiers perform quite well in several specifications, especially in those related with their intrinsic electromagnetic nature. Table 6 shows these figures for a few technologies working in the visible and the infrared. We may already see in this table that the responsivity of optical antennas is lower than the rest of technologies. This figure is in accordance with the low efficiency of rectennas that has been observed in actual experiments involving MIM, or Metal-Insulator-Insulator-Metal (MIIM), junctions as transducers. Summarizing this table we may say that optical antennas are point detectors, very fast, work at room temperature, can be integrated with some other elementsand devices (for example with focusing optics), and they present a broad tuneability, and a remarkable selectivity in direction and polarization.

Visible CCD/ CMOS

MIM Junctions

Avalanche

Photodetectors

Pyroelectric

Detectors

Bolometric

Detectors

Optical

Antennas

Size

102A2

102A2

102A2

101-102 A2

101-102 A2

10-2-100 A2

Polarization

selective,

directivity,

No

No

No

No

No

Yes

tuneability

Cooling

Better

performance

No

Better

performance

No

No

No

Responsivity

1 03-1 04 V/W

0,7-0,9 A/W

0.7 A/W

1 03-1 04 V/W

1 03-1 04 V/W

0.1 V/W

Time response

100 ns

10 ps

9 ps

400 |is

400 |is

1 ps

Table 6. Four different photodetection mechanisms are compared with optical antennas technology.

Nowadays, space in urban areas, including work and home environments is strongly packed with EM radioelectric waves at various bands and spectral regions: besides the ubiquitous presence of radio and TV bands, cell phones and personal communications devices, a myriad of wi-fi stations, Bluetooth gadgets, and remote emitters and detectors produce a non- negligible amount of EM energy flowing around us. Then, from a harvesting point of view this energy could be recycled and properly used by electronic systems with ultra-low power requirements. This strategy may work in those environments with strong RF signals, where signal-to-noise ratio of other operative elements is not compromised. This idea of RF and microwave recycling has been developed some time ago in the form of antenna arrays and half or full wave rectifiers. Optical rectennas can be seen as an evolution and transposition of those designs and devices already working in the microwave region. In this band some designs have demonstrated more than 75% of efficiency when used for power transmission [8]. These figures are reduced when considering broad-band antennas designed to recycle microwave energy from ambient background.

Unfortunately, so far the efficiency number obtained at those frequencies have not been replicated at infrared or visible frequencies. The reasons are mostly derived from the inherent behaviour of materials when frequency increases. Besides, the difficulties of designing THz electronics and oscillators, metals begin to behave as dispersive materials and the currents built on their surface penetrates within the structure.

In order to place the reader in a position to make an educated guess on the different technol­ogies we present here a brief comparison among the photovoltaics, radiofrequency rectifiers, and optical rectennas (optical antennas coupled to rectifiers)

Photovoltaics: Direct conversion of light into electric power using the photovoltaic effect exhibited by semiconductor materials.

• Efficiency: The theoretical limit is around 41% for single junction solar cells, and reaches 87% for multiple junctions.

• Pros: Well established and mature technology. Fabrication issues have been solved due to the intrinsic relation with semiconductor technology.

• Cons: The performance is strongly dependent on temperature, especially for multiple junction cells.

Radiofrequency Rectifiers: Direct conversion of light into electricity using a rectifier working at radio or microwave frequencies.

• Efficiency: The limit is set around 85%. Practical devices have been demonstrated with an efficiency larger than 75%.

• Pros: Well known basic mechanism of rectification. Fabrication can be made using standard photolithography on dielectric substrates.

• Cons: Polarization and spectral selectivity.

Optical Rectennas: Direct conversion of light into electricity using rectifiers working at optical frequencies.

• Efficiency: The theoretical limit is around 85%.

• Pros: Antenna theory and its scaling to optical frequencies is known and antenna-coupled detectors have been demonstrated in the infrared and the visible. Minimum size of about Л2, allowing very high packaging. No dependence with temperature. Metals are used for fabrication with some advances in the use of conducting graphene.

• Cons: The efficiency of working devices is well below 85%. Barrier rectifiers are not able to follow optical frequencies and behave as square law rectifiers. Further advances are needed to have feasible rectifying mechanisms. Nano-fabrication technologies are necessary (nano­imprint could solve large scale fabrication numbers).

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