Development of new conjugated polymers for field effect transistors

The use of transistors in our daily lives is ubiquitous, with transistors being found in everything from smart phones, TV’s, e-readers and computers. However, these transistors made from amorphous silicon require a variety of energy intensive batch manufacturing techniques such as material deposition and patterning steps, which take place under high vacuum and at high temperature. Additionally, the fabrication of amorphous silicon transistors is wasteful in terms of materials as several subtractive lithographic patterning and mask steps are required which limit manufacturing throughput. For this reason, each year there is a continued drive to shrink the size of a single transistor, so that more transistors can fit onto a piece of silicon wafer and thus this helps to lower the cost of an individual transistor. Whilst this means that the cost of an individual is very low, the actual cost per unit area is very high. This presents an opportunity for organic semiconductors to be used instead as they can be formulated into inks and cheaply deposited via non energy intensive solution processing techniques such as ink jet and gravure printing. This enables the manufacture of large area, low temperature, high throughput organic field effect transistors (OFETs) which not only enables them to be low cost but also enables the development of flexible circuitry, as the lower temperatures used in the production process will enable plastic substrates to be used. A number of possible applications for these cheap, flexible OFETs have been envisaged ranging from radio frequency identification tags, that could be used in supermarkets instead of bar codes to flexible backplanes in active matrix displays.

            A number of transistor device architectures can be employed depending on the order of material deposition. The most common architectures are Figure 1 a) top contact, bottom gate b) bottom contact, bottom gate c) top contact, top gate and d) bottom contact, top gate. In all these device architectures the underlying principle is the same. OFETs are typically either p-type or n-type depending on the organic semiconductor used. In the case of a p-type device, the charges are holes which are injected and extracted from the semiconductor material via the source and drain electrodes. Upon applying a negative gate voltage holes are injected into the semiconductor and accumulate at the semiconductor/ dielectric interface. Here they form an accumulation layer and are subsequently transported to the drain electrode upon application of an electric field between the source and the drain. By modulating the gate voltage the transistor can be turned ON or OFF.

            The focus of this project is to develop new conjugated polymers with higher charge carrier mobilities and better ambient stability for use in high performance OFET devices.


Figure 1: OFET device architectures a) top contact, bottom gate. b) bottom-contact, bottom-gate. c) top-contact, top-gate. d) bottom-contact, top-gate.

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Published: Mon 09 Feb 2015. Modified: Tue 10 Feb 2015