Major retailers are insisting that suppliers soon start using RFID tags, to replace tags with bar codes. But RFID tags that rely on silicon chips are too expensive to be practical, so going to printable organic materials is crucial. Getting there, however, depends on developing readily printable organics that operate in the needed frequency ranges. In a promising first step, researchers in Belgium have developed a type of organic diode that can rectify ac at 50 megahertz—in the range allowed by international conventions for RFID applications.

An RFID tag is in fact a tiny transponder that reacts to an RF signal from a reader by transmitting data stored in its memory. Typically, such a tag contains logic circuitry, a memory, a transmitter, and a power source. Organic semiconductor circuits are sluggish compared with silicon, because the mobility of charge carriers is low. This isn't a problem for the logic circuitry and memory, because the amount of data to be processed is very small (several hundred bytes of memory suffice for the smaller RFIDs). But it is a problem in the diodes that rectify the high-frequency current obtained from the reader in order to power the RFID's circuitry and transmitter.

Up to now, prototype RFID tags using planar organic transistors for rectification have worked up to frequencies of about 18 kilohertz, reports Klaus Dimmler, president of OrganicID in Colorado Springs, Colo. The planar devices are printable but too slow to rectify at the higher frequencies allotted to RFID devices.

Now a team of researchers at IMEC, a nonprofit research institute in Leuven, Belgium, have sandwiched a thin, 160-nanometer layer of the organic semiconductor pentacene between layers of aluminum and gold. The device reaches high frequencies by reducing the distance the charge carriers have to travel, says Paul Heremans, who heads the polymer and molecular electronics division at IMEC.

The charge carriers in the pentacene layer are known as holes—places in the polymer structure where an electron is missing. The hole appears as a positive charge, and it travels through the material by a kind of domino effect. Rectification results from the fact that electrons from the gold electrode jump into the holes in the pentacene layer easily, while electrons from the aluminum electrode do not.

The Belgian team reported in the August issue of Nature Materials that they obtained 8-volt dc when the receiver coil of the RFID was placed in a 50-MHz electromagnetic field. The rectifier should work at frequencies up to 800 MHz.

There is still a way to go, however. Because of its vertical structure, the diode cannot be printed using proven techniques. Either an alternative structure or new production techniques must be devised for this to be a winner.