Within the Holst Centre program on Organic Circuitry, the world's first plastic diode operating in the Ultra-high frequency (UHF) band has been developed and reported at the IEDM conference in San Francisco.
For organic-electronic applications such as RFID-tags, the rectifier is one of the most critical components. The rectifier is responsible for converting the electromagnetic energy of the incoming RF-signal captured by the antenna into a DC operating voltage that powers the transponder chip on the tag. Plastic transponder chips that hold the promise to result in low cost tags have been shown in the past years,. However, the antenna itself forms a very significant part of the cost of an RFID tag. Antennas for the UHF frequency band (operating at 433MHz, 869MHz, 915 MHz) are smaller and lower cost than antennas for HF (13.56 MHz), therefore UHF is the preferred frequency band for ultra-low-cost tags. For the first time, researchers have been able to make a UHF rectifier (operating at 433MHz and 869MHz) with a plastic diode. This is therefore not only a scientific breakthrough, but really opens the door for low-cost plastic electronic tags.
The rectifier presented at IEDM was made using a 160nm thin film of purified pentacene sandwiched between Al and Au electrodes on glass. The diodes have a reverse breakdown voltage exceeding 25V and a charge carrier mobility of 0.15cm2/Vs.
The diodes where integrated with capacitors into rectifiers on a plastic foil (schematic and picture shown in Figure1). These integrated rectifiers operate up to a frequency of 869MHz In Figure 2, the measurement results have been plotted over the full frequency range. The rectified DC voltage at 869 MHz is 4.5V. Further increases of the voltage level will be possible using a more complex rectifier.
The work was done within the framework of the Holst Centre research program on organic circuitry, in close collaboration between IMEC Leuven and TNO Eindhoven, and was co-funded by the European project POLYAPPLY.