Functional organic field effect transistor devices for sensing applications

Abstract

Organic electronics have demonstrated great potential in sensing applications. The performances of these sensors, especially for temperature and visible light sensing, have great potential for further improvement due to the limitations of the intrinsic properties of the organic materials and therefore warrant further research. Organic field effect transistors (OFETs), which are the basic unit of logic gates and integrated circuits, can also be applied for sensing applications because of the strong tunability of the carrier in the channel area. In this thesis, I focus on demonstrating the ability of high performance OFET-based integrated devices to sense temperature and visible light.

In order to detect temperature, organic materials have to be adequately sensitive, which is related to their conductivity activation energy (Ea). Therefore, in this study, the intrinsic Ea of pentacene is first increased from 180 meV to 450 meV with the insertion of a layer of silver nanoparticles (Ag NPs). Furthermore, the thermistor is connected to the gate electrode of an OFET with a poly(vinyl pyridine) (PVPy) dielectric layer, so that the temperature-induced conductivity variation is largely amplified by the OFET, thus resulting in over three orders of magnitude of output current change from 20 ℃ to 70 ℃. The corresponding dynamic range (DR) of this temperature sensor is 10 bits. I have also examined the failure mechanism of the device when operated above 70 oC, and found that the significant decrease in the capacitance of the PVPy dielectric is responsible for the device failure.

To improve the thermal stability and extend the applications of this temperature sensor, the PVPy dielectric layer is replaced with an anodized Al2O3 high-k dielectric layer and dinaphto[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) is used as the active layer for the OFET. Single temperature sensors can normally operate at temperatures as high as 100 ℃. Next, a 16×16 temperature sensor array with a 100% yield rate is formed on a flexible substrate. The device can map out the shape of thermal objects, temperature distribution of electronic circuit boards, and thermal information of human skin. An electrical and heat transfer model is also formulated to optimize the array device.

To create the optical sensor devices, I combine organic materials that have photovoltaic effects with the OFET to improve the responsivity of the optical sensors. By using a polystyrene (PS) electret layer to capture the photo-generated electrons in the OFET, the output current is close to 1 mA due to the photo induced positive shift of the threshold voltage, Vth. The phototransistor can also maintain a low off-current state (~3 pA) in dark conditions, thus resulting in a DR of about 23 bits and large responsivity of around 460 A W-1. The phototransistor in this study also demonstrates optical data storage capacity due to the stable charge trapping property of the PS electret, with a retention time longer than 〖10〗^4 s.