Belgian research organisation IMEC has produced 50MHz polymer rectifier diodes, jumping another hurdle on the way to all-plastic RFID tags.

“Some people doubted this was possible,” researcher Soeren Steudel told Electronics Weekly.

RFID needs fast rectifiers to generate on-tag DC from the applied reading field.

The IMEC diodes are a 150nm thick layer of polycrystalline pentacene deposited on gold, with an aluminium top contact. “Because of the work functions, you can inject holes from the gold, but not from the aluminium,” said Steudel.

Diodes made like this lack performance because they are injection-limited, so IMEC borrowed from OLED research and added a 20nm layer of polymer PEDOT:PSS to improve injection from the gold. “This is an absolutely standard configuration for organic LEDS,” said Steudel.

Where the structure differs from OLEDs is in the details. “Current densities are several orders of magnitude higher than in OLEDs,” said Steudel. “Instead of light-emitting materials, we have ones with very high mobility.”

The result is a diode that can operate at 2,100A/cm². Hole mobility is estimated at 0.15cm²/Vs.

Whereas silicon diodes drop 0.6V when conducting, the IMEC devices loose around 10V at 500A/cm² (10x170µm diodes). A 36Vp-p input is converted to 11V at the capacitor.

This is not an insurmountable issue, according to Steudel, as even the best polymer circuits need 10V to operate and solution-processed devices may need as much as 50V. This compares with silicon RFID chips which operate at 3V. “Voltage is not a problem on tags. It is just how many [aerial] windings you have,” he said. “The issue is power. If you use more power, you have to get close [to the reader].”

IMEC is using photolithography and vacuum-processed polymers for its proof-of-concept work. Cheaper printable polymers are not expected to perform as well.

As for the rest of the circuitry required on a tag, the only other fast component is the modulation transistor which replies to the tag reader. “At 13.56MHz the modulation transistor probably won’t be a problem. At 800MHz it could be,” said Steudel.

www.imec.be