Bidirectional communication in an HF hybrid organic/solution-processed metal-oxide RFID tag

The ambition of printing item-level RFID tags is one of the driving forces behind printed electronics research. Organic RFID tags have been shown, initially using p-type organic semiconductors [1-4]. The introduction of n-type organic semiconductors with reasonable performance made organic CMOS conceivable [5] and organic CMOS RFID tags were shown [6]. However, all currently reported organic RFID tags are based on a tag-talks-first principle: as soon as the tag gets powered from the RF field, its code is transmitted at a data rate determined by an internal ring oscillator. Practical RFID systems will need to be able to read multiple RFID tags within the reach of the reader antenna. Existing anti-collision protocols implemented in organic RFID tags [2,4] are limited to about maximum 4 tags and come at the cost of a slow reading time. In this paper, we for the first time realize a reader-talks-first low-temperature thin-film transistor (TFT) RFID circuit. We use a complementary hybrid organic/oxide technology. As organic transistors with reasonable channel lengths (≥2μm) have a cut-off frequency below 13.56MHz, the base carrier frequency of HF communication, present technologies on foil do not yet allow to extract the circuit clock as a fraction of the base carrier. We solve this by introducing an original uplink (reader-to-tag) scheme, in which a slow clock (compatible with our transistors' speed) is transmitted as amplitude-modulation on the base carrier while data is encoded on this clock by pulse width modulation (PWM).