Steven Tierney

Low-Voltage Ring Oscillators Based on Polyelectrolyte-Gated Polymer Thin-Film Transistors

There has been a remarkable progress in the development of organic electronic materials since the discovery of conducting polymers more than three decades ago. Many of these materials can be processed from solution, in the form as inks. This allows for using traditional high-volume printing techniques for manufacturing of organic electronic devices on various flexible surfaces at low cost. Many of the envisioned applications will use printed batteries, organic solar cells or electromagnetic coupling for powering. This requires that the included devices are power efficient and can operate at low voltages. This thesis is focused on organic thin-film transistors that employ electrolytes as gate insulators. The high capacitance of the electrolyte layers allows the transistors to operate at very low voltages, at only 1 V. Polyanion-gated p-channel transistors and polycation-gated n-channel transistors are demonstrated. The mobile ions in the respective polyelectrolyte are attracted towards the gate electrode during transistor operation, while the polymer ions create a stable interface with the charged semiconductor channel. This suppresses electrochemical doping of the semiconductor bulk, which enables the transistors to fully operate in the field-effect mode. As a result, the transistors display relatively fast switching (≤ 100 µs). Interestingly, the switching speed of the transistors saturates as the channel length is reduced. This deviation from the downscaling rule is explained by that the ionic relaxation in the electrolyte limits the channel formation rather than the electronic transport in the semiconductor. Moreover, both unipolar and complementary integrated circuits based on polyelectrolyte-gated transistors are demonstrated. The complementary circuits operate at supply voltages down to 0.2 V, have a static power consumption of less than 2.5 nW per gate and display signal propagation delays down to 0.26 ms per stage. Hence, polyelectrolyte-gated circuits hold great promise for printed electronics applications driven by low-voltage and low-capacity power sources.

Polymerisable liquid crystalline organic semiconductors and their fabrication in organic field effect transistors

The performance of the semiconducting component in organic field effect transistors (OFETs) is a key parameter in the advancement of organic electronic devices. New semiconductors are required, which can be solution processed, possess high mobility and current modulation, and are stable in ambient conditions. This work provides the first demonstration of working field effect transistor devices fabricated from novel solution processible, polymerisable, small molecule liquid crystalline semiconductors, referred to as reactive mesogens. The design, synthesis, and performance of these materials in transistor devices are reported. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology was employed to control the liquid crystalline morphology, in an attempt to optimise organisation and packing. Alignment of the molecules in large homeotropic domains was achieved through surface treatment techniques, and the highly ordered mesophase was preserved by polymerisation of the reactive end groups, creating a crosslinked network.

Solution-processed small molecule transistors with low operating voltages and high grain-boundary anisotropy

We present a new soluble pentacene derivative with ethyl substitutions in the 1,13,14,22 backbone positions to modulate the solubility and film forming properties of this material compared to triisopropylsilylethynyl (TIPS) pentacene. This permits reproducible production of molecularly highly ordered structures that feature average transistor mobilities in excess of 1 cm2 V−1 s−1 depending on crystal orientation by careful selection of casting conditions.

Designing solution-processable air-stable liquid crystalline crosslinkable semiconductors

Organic electronics technology, in which at least the semiconducting component of the integrated circuit is an organic material, offers the potential for fabrication of electronic products by low-cost printing technologies, such as ink jet, gravure offset lithography and flexography. The products will typically be of lower performance than those using the present state of the art single crystal or polysilicon transistors, but comparable to amorphous silicon. A range of prototypes are under development, including rollable electrophoretic displays, active matrix liquid crystal (LC) displays, flexible organic light emitting diode displays, low frequency radio frequency identification tag and other low performance electronics. Organic semiconductors that offer both electrical performance and stability with respect to storage and operation under ambient conditions are required. This work describes the development of reactive mesogen semiconductors, which form large crosslinked LC domains on polymerization within mesophases. These crosslinked domains offer mechanical stability and are inert to solvent exposure in further processing steps. Reactive mesogens containing conjugated aromatic cores, designed to facilitate charge transport and provide good oxidative stability, were prepared and their liquid crystalline properties evaluated. The organization and alignment of the mesogens, both before and after crosslinking, were probed by grazing incidence wide-angle X-ray scattering of thin films. Both time-of-flight and field effect transistor devices were prepared and their electrical characterization reported.