Organic semiconductor thin films based transistor and sensor applications

Organic semiconductor-based devices, such as thin film transistors (TFTs), have recently been envisioned as an attractive alternative to silicon based devices due to their low process temperature, low cost manufacturing, and good mechanical properties. This enables ability to use cheap and flexible substrates including fabric, polymer, and even paper for electronic devices. Potential applications of organic thin film transistors (OTFTs) include flat panel displays, smart cards, radio frequency identification chips (RFIDs), sensors, and electronic paper.; In this research work, pentacene based OTFTs and sensors have been designed and developed on both rigid and flexible substrates. The sensors developed herein employ a TFT and Wheatstone bridge structure to monitor temperature and strain, respectively. In order to solve a variety of issues related to organic material deposition as well as device fabrications, the molecular structure and morphology of pentacene semiconductors that significantly affects device performances has been initially investigated by changing various deposition conditions such as substrate temperature, film thickness, surface roughness, and surface treatment.; Furthermore, direct photolithographic top-contact pentacene TFTs were fabricated by a newly developed lift-off process, which uses a bi-layer of polyvinyl alcohol (PVA) and SU8 to define source and drain contacts without any significant degradation in the organic thin film surface. The direct photolithographic top-contact pentacene TFTs show comparable performance to bottom-contact devices, and providing its potential to be utilized in the development of new device topologies. Pentacene TFTs also were fabricated on flexible substrates, such as Polyethylene naphthalate (PEN) using polymeric gate dielectrics. Solution processable polyvinyl phenol (PVP) and polyvinyl alcohol (PVA) were used as the gate dielectric materials and deposited by spin coating.; For temperature sensor applications based on pentacene TFTs, post-annealing effects on pentacene TFTs were first explored with/without PVA encapsulation over the pentacene film. From the results, it has been found that the PVA encapsulation layer can improve the thermal stability of TFT devices, suppressing the onset of the bulk phase and/or the physical desorption of the pentacene active film. A temperature sensor based on the PVA encapsulated pentacene TFT was designed, developed, and studied its linearity and reliability of the variation in the saturation and subthreshold drain current with temperature.; Organic strain sensors using pentacene thin film as a sensor active layer have been developed to overcome the limitation of inorganic strain sensors on polymeric substrates due to the large stiffness mismatch between the inorganic semiconductor and polymeric substrate. Gateless organic strain sensors were first demonstrated using a pentacene single layer for different sensor sizes. Moreover, gate-induced sensors employing a thin film transistor-like structure and pentacene-carbon nanotube (CNT) composite thin film based strain sensors were developed and demonstrated to reduce the external noise effect and to improve the sensitivity. The temperature and strain sensors based on pentacene TFT and Wheatstone configurations presented herein will open the possibility of developing practical devices for the next generation of sensing systems.