IOP Science – Wireless power transmission to inexpensive and disposable smart electronic devices is one of the key issues for the realization of a ubiquitous society where sensor networks such as RFID tags, price tags, smart logos, signage and sensors could be fully interconnected and utilized by DC power of less than 0.3 W. This DC power can be provided by inductively coupled AC from a 13.56 MHz power transmitter through a rectenna, consisting of an antenna, a diode and a capacitor, which would be cheap to integrate with inexpensive smart electronic devices. To integrate the rectenna with a minimum cost, a roll-to-roll (R2R) gravure printing process has been considered to print the rectenna on plastic foils. In this paper, R2R gravure printing systems including printing condition and four different nanoparticle based inks will be reported to print the rectenna (antenna, diode and capacitor) on plastic foils at a printing speed of 8 meter per minute and more than 90% device yield for a wireless power transmission of 0.3 W using a standard 13.56 MHz power transmitter.
This will enable wireless power for less than a penny per device.
Technology Review – it’s a type of near-field communication (NFC). The team envisions using rectennas not as an element of a digital wallet, but more in the vein of QR codes.
(a) Circuit layout and (b) image of the R2R gravure printed rectenna on PET foil with descriptive schemes of each device (printed capacitor, printed diode, printed bottom Ag electrode).
Says study co-author Gyojin Cho: “Our advantage over current technology is lower cost, since we can produce a roll-to-roll printing process with high throughput in an environmentally friendly manner. Furthermore, we can integrate many extra functions without huge extra cost in the printing process. The application of NFC technology with the smartphone will be limitless in the near future. The medical, automotive, military and aerospace industries will benefit greatly.”
In conclusion, six printing units of R2R gravure have been successfully employed to print a rectenna as a wireless power transmission device at 13.56 MHz very cheaply. The constituents of a rectenna, i.e. an antenna, three diodes and three capacitors, were consecutively printed on PET foils at a printing speed of 8 meters per minute with an overlay printing registration accuracy of 50 µm layer by layer. For printing each layer without any intermixing at the interfaces, nanoparticle (silver, ZnO and BaTiO3) based inks were well formulated to meet the wetting on PET and printed layers using solvents orthogonal to each other. The resulting printed antenna, diodes and capacitors performed well to rectify coupled 13.56 MHz AC to 20 V DC at 1.0 MΩ of load with 90% device yield. Therefore, the gravure printed rectenna can provide at least 0.3 W power from the standard 13.56 MHz RFID reader and this method will open up practical RF wireless power transmission as an inexpensive power source.
Ubiquitous Society Enablement
The ubiquitous society will be fully realized when inexpensive RFID-sensor networks can be attached on all items so that a smart phone with a near field communication (NFC) chip can wirelessly and instantaneously communicate with all of the other items. To realize these networks, inexpensive RFID-sensor networks should get their operation DC powers, usually less than 0.3 W, from the NFC chip at 13.56 MHz through the inductive coupling method. Although the distance of the inductively coupled wireless power transmission is limited to a few centimeters, the efficiency of the power transfer is very effective.
For wireless power transmission through an inductive coupling, a rectifier and an antenna, called a rectenna, is an indispensable unit in which an antenna inductively couples AC at 13.56 MHz and then the rectifier converts coupled AC to DC power. Up to the present, usually the antenna has been prepared by etched copper or aluminum films, and the rectifier, consisting of a diode and a capacitor, have been prepared from a Si based photolithography and vapor deposition process. Although the efficiency of wireless power transmission at 13.56 MHz is excellent on these rectennas, the costs of the integration processes nullify the role of the wireless power transmission to integrate with the sensor networks. Therefore, a cheap rectenna to supply at least 0.3 W of DC power to the inexpensive sensor networks from 13.56 MHz AC is highly in demand.
To reduce the costs of producing and integrating the rectenna as the wireless power transmission unit, a roll-to-roll (R2R) gravure has received significant attention as a fabrication tool due to it is high throughput, simple structure, robustness and compatibility to the package printing process. To employ R2R gravure for printing rectenna on plastic foils cheaply, we first need appropriate gravure inks to continuously print antenna, diodes and capacitors on plastic foils. To achieve a practical R2R printing speed of 8 m min−1 with inexpensive poly(ethylene terephtalate) (PET) films, all inks should be cured under 5 s at 150 °C while all printed patterns or layers should not be intermixed and show their required electrical properties with high reliability.
Although many research groups are trying to show a fully R2R gravure printed rectenna, no one has been successful yet because of the following three major difficulties. First, the R2R gravure printed antenna would not provide lower resistance than an etched antenna because the thickness of the gravure printed pattern is usually less than 1 micron. This higher resistance of a gravure printed antenna will make it more difficult to tune the printed rectenna to provide a high Q factor. The Q factor is a measure of the coupled voltage and current of the resonance circuit at the resonance frequency. Second, the high capacitance of a printed capacitor would not be printed without any short due to the lack of high dielectric ink. Third, there are no R2R gravure printable active inks for high frequency ( over 13.56 MHz) performing diodes yet; this is the most important difficulty.
In this paper, the three major difficulties have been overcome by tailoring the interactions between nanoparticles and appropriate polymer binders with selected orthogonal solvents to control the rheology of each ink and avoid intermixing of printed layers. By utilizing those nanoparticle based inks and controlling ink transfer at R2R gravure, we report the first fully R2R gravure printed rectenna with a wireless power transmission unit at 13.56 MHz, which delivers a DC output power of 0.3 W at 13.56 MHz AC input frequency from the commercial 13.56 MHz RFID reader circuit through R2R gravure with six layer printings
A schematic description for a fully R2R gravure printing system.