Moving one step nearer to the goal of an organic RFID inlay, IMEC, an independent nonprofit research center, has developed an organic rectifier diode capable of powering a passive RFID tag operating at a frequency as high as 50 MHz. The organization claims its design even has the potential to operate in the UHF range.
Passive RFID tags need rectifiers to generate direct current (DC) from the alternating current (AC) radio waves transmitted by an RFID interrogator, or reader. The tag uses DC power to operate the logic circuit on the tag and transmit a signal back to the interrogator. A rectifier that is organic—that is, constructed of polymers, rather than silicon—could potentially be used to power organic tags.
What is most impressive about its rectifier development, according to IMEC, is that the new diode can operate at up to 50 MHz. “Many people considered that 13.56 MHz was in reach, but 50 MHz was not at all obvious,” says Paul Heremans, director of the organic electronic group at IMEC, located in Leuven, Belgium.
Based on laboratory experiments, IMEC calculates its rectifier design could be developed further to reach the 800 MHz UHF frequency range. However, the group says it will concentrate on figuring out how to integrate its design with other components required in an organic 13.56 MHz RFID tag, and to develop a way to mass-produce its organic rectifiers.
IMEC’s rectifier diode consists of a 150nm-thick layer of polycrystalline pentacene deposited on gold, with an aluminum top contact. “In this way, we achieve vertical current flow with no shorting,” says Heremans. Pentacene, an organic molecule composed of carbon and hydrogen, has one of the highest reported charge-carrier mobilities among organic semiconductor materials. (A charge carrier is the mechanism within semiconductor material that conducts an electrical charge , while the mobility is a measure of how well a charge carrier can move under the influence of an electric field.) Although charge-carrier mobility of pentacene ranks high compared with other organic semiconductors, it is about 1,000 times lower than that of silicon.
The new diode’s vertical design contrasts with the “thin-film transistor” (TFT) designs also being considered by researchers for making an organic rectifier. Compared with a vertical organic diode, TFT designs can be easier to integrate with other components on the RFID tag, but higher frequencies should be easier to attain with the vertical approach, because the charge needs to travel a shorter distance than that found in a TFT design.
IMEC has also made rectifiers with the TFT approach in which charge moves in a lateral direction. “This thin-film transistor approach will, I believe, also lead to a rectifier at 13.56 MHz, but such a rectifier will be more difficult to produce because of the longer travel distance for the carriers,” says Heremans.
Having proved in laboratory tests that its rectifier design could convert a 50 MHz signal into enough DC power to run an RFID tag, IMEC will now work on how best to integrate its rectifier with other components of an organic tag. Figuring out how to incorporate the rectifier in an entirely organic tag will take time, according to Heremans. “A lot of work is still needed to integrate the rectifier with other parts of the tag,” he says. Heremans believes that within a year, there will be laboratory demonstrations of integrated organic rectifiers with organic digital electronic circuits operating at 13.56 MHz.
IMEC’s work on the rectifier is part of the pan-European PolyApply research project, which is examining the application of polymer electronics to ambient intelligence. The group defines this as intelligence cheap enough to be embedded in everyday products the same way bar codes are used today. Launched last year, the PolyApply project brings together 20 groups, including several European industrial companies and academic and research institutes, looking to develop polymer semiconductor systems and high-speed organic devices.
In April, OrganicID, a Colorado Springs, Colo., company, announced its success in printing rectifier circuits able to process radio waves at frequencies of 13.56 MHz. Additionally, German company Poly IC announced it had developed a rectifier made with the polymer semiconductor polythiophene that works at 13.56 MHz.