Organic TFT Circuit Design for Smart Sensor Tags

Improvements in organic electronic material synthesis and device performance have enabled printed organic electronics for systems applications. However, circuit design requires adapting to the performance limitations of current organic thin film transistors (TFT), particularly printed devices. This work focuses on the design and implementation of a multi-sensor smart tag with printed organic TFT circuits on a flexible substrate. The goal is a circuit that self-cycles through multiple sensor outputs connecting them to a single analog-to-digital conversion channel, and designed to minimize the effects of TFT variability. N- and p-type semiconductors are inkjet printed on flexible substrates to form logic gates in conjunction with a high-k PVDF dielectric to reduce voltage operation. Based on device simulation, an astable RC vibrator is used to feed a clock and anti-clock signal into a master-slave flip flop connected as a frequency divider. The output signals from the vibrator and frequency divider show rail-to-rail operation, which address a multiplexer that selects the sensor circuit voltage output. The operation of the vibrator, which only requires a power supply, enables a continuous cycling of the outputs. The performance of the circuit is reasonably insensitive to TFT variation. The output voltage of this circuit is very stable since the operation point of the sensor circuit is independent of the performance of the OTFTs. This system is integrated with humidity, temperature and light sensors to demonstrate performance as a sensor tag.