Characterisation and modelling of stability issues in EGOFET devices

Electrolyte-gated organic field-effect transistors (EGOFETs) have great potential in extremely sensitive biosensors due to their low-voltage operation and flexible design. These devices suffer from stability issues that need to be rectified before large scale manufacture is possible. The cause, as well as the mechanism, of the instability is poorly understood. The objective of this thesis is to shed a light on the dominating degradation mechanisms in poly(3-hexylthiophene-2,5-diyl) (P3HT) devices operating in an aqueous environment, in order to improve their stability. Several P3HT films were manufactured using different protocols. The optical absorption spectra of the films were measured and analysed by applying a weakly interacting Haggregate model. A highly regioregular (>99%) semiconductor spin coated from a 4mg ml−1 chlorobenzene solution was shown to form films with a microstructure optimal for transistors. Films of different thicknesses were characterised. No correlation between film thickness and microstructure was found. To investigate the effects of water on P3HT, identically prepared samples were stored either in ambient air or submerged in water. Before and after storage, thin film parameters were extracted from the absorption spectrum. After exposure to air, the film was found to have undergone massive degradation. The samples stored in water showed little to no degradation, indicating that exposure to water alone does not cause significant microstructural changes in P3HT. P3HT EGOFET device degradation was monitored over a period of a month, registering around 4,000 transfer characteristics. During this time, no consistent shift in threshold voltage could be observed. This is in stark contrast to solid-state OFETs where a threshold voltage shift is the main mechanism of degradation. The observed decrease in current was instead caused by a decrease in mobility and an increase in trap density. It was shown that device degradation is a function of electrical stress rather than exposure to water. Using the Vissenberg-Matters model, the decrease in mobility was modelled as a widening of an exponential trap density of states. Modelling parameters were very similar to earlier reported values, indicating the model is valid also for water-gated devices. The increase in trap density given by the model was in close agreement to a very simple estimation based on the subthreshold swing