Stephen William Leeming

Gate insulators influencing electronic transport in organic FETs

New findings are presented relating to the optimal choice of gate insulators in organic field effect transistors (OFET). It was recently found that some organic semiconductors operate better when low-k materials are used in the gate. This is quite contrary to the conventional trend to use high permittivity dielectrics for low voltage operation. Interaction between the insulator and the semiconductor materials plays an important role in carrier transport. On one hand, the insulator is often responsible for the morphology of the semiconductor layer, but on the other hand it can also change the distribution of states by local polarisation effects. Carrier localisation is enhanced by insulators with large permittivities, due to the random dipole field present at the interface. We have investigated this effect on a number of disordered organic semiconductor materials and show here that the use of low-k materials may lead to improvements in mobility, reduced temperature activation and hysteresis. In particular, the behaviour of the threshold voltage is interesting. The differences in the underlying physics compared to the case of FETs based on band-like semiconductors, is also discussed.

Air Stable, Amorphous Organic Films and their Applications to Solution Processable Flexible Electronics

The rapidly expanding field of organic semiconductors for display and low-cost electronic applications requires materials, which not only have high mobility but also benefit from solution processability and environmental stability. In this paper we present a new class of solution coatable organic materials with excellent stability to air and light. Spin-coated FET devices operate at ambient conditions without encapsulation and show p-type field-effect mobilities of 2 x 10 -3 cm 2 V -1 s -1 and on/off ratios greater than 10 4 . Thin films can be deposited from common organic solvents onto a variety of substrates. These films are mechanically robust and can withstand temperatures in excess of 100 °C without significant changes in electrical performance. FET switching and transient characteristics at higher frequencies are also discussed. These types of materials should find applications in many areas of flexible electronics.