Ambipolar Organic Transistors for Opto-Electronic Applications

Abstract

This thesis is concerned with the development and study of ambipolar organic field-effect transistors (OFETs), with a specific focus on devices and structures that are relevant to opto-electronic processes. After giving an outline of the relevant theory and experimental methods, the thesis is divided into three experimental chapters. In the chapter on organic phototransistors (OPTs), the fabrication of low-voltage bilayer OPTs is carried out using self-assembled monolayer gate dielectrics. By combining two low-voltage OPTs, the output voltage of a so-called photo-inverter was observed to be modulated over a range of 1.5 V with an incident optical power density of 0 to 1.2 mWcm-2. The characteristics of a high-voltage polymer:fullerene blend photo-inverter device are modelled using variable-range hopping and simple circuit models, suggesting that the device operation is dominated by changes in the threshold voltage. In the second experimental chapter the relationship between the morphology of polymer:fullerene blends and the characteristics of OFETs is studied. It is shown that the thermally-induced clustering of fullerenes is manifest as a reduction in electron mobility, due to a reduction in percolating pathways. It is additionally found that higher molecularweight fullerenes require greater annealing temperature and/or times for this process to occur. The final experimental chapter is concerned with polymer:fullerene diffusion processes and the study of such phenomena using bilayer OFETs. A model based upon the diffusion equation and percolation theory is employed to quantify this process. Again, higher molecularweight fullerenes are observed to require greater annealing temperature for similar phenomena to be observed.