Organic transistors based on pentacene and dibenzothiophene derivatives

Abstract

This thesis is concerned with the fabrication and characterisation of organic thin film transistors. Initially, pentacene thin films were investigated, with results comparable to those found in published literature. Initial studies of pentacene transistors revealed a poor hole mobility of 6.3 x 10(^-3) cm(^2)/V/s. Improvements in the fabrication process (using a more conductive silicon wafer as the gate, and treating the silicon oxide surface with a silanising agent) increased the mobility to around 0.1 cm(^2)/V/s. Better pentacene deposition conditions allowed a polycrystalline structure to form, with dendritic grains of the order of 2-3 pm in size. This increased the mobility of the transistor further, to 0.54 cm(^2)/V/s. Treatment of the silicon oxide surface prior to pentacene deposition was found to affect significantly the hysteresis in the transfer characteristics. Removal of photoresist with acetone and propan-2-ol prior to pentacene deposition resulted in fairly large threshold voltages, with an average shift between the off-on and on-off threshold voltages of 9.7 V. Application of an oxygen plasma prior to deposition resulted in decreased threshold voltages, and a reduced threshold voltage shift of 3.8 V. The hysteresis was attributed to charge trapping on the oxide surface due to organic contamination; the oxygen plasma served to reduce this. X-Ray Photoelectron Spectroscopy measurements confirmed this - following a plasma treatment, the carbon content on the surface was reduced significantly. Incorporation of a layer of gold nanoparticles between the oxide and pentacene was found to provide charge traps - this might be exploited in memory devices. Replacing the silicon oxide with PMMA produced favorable results. Negative threshold voltages with low hysteresis were observed for all the devices. Mobilities of up to 0.21 cm(^2)/V/s were recorded for devices with a 124 nm PMMA layer; a thinner (80 nm) layer of PMMA resulted in reduced mobility, as did a thicker (350 nm) layer. Replacing the silicon oxide with sputtered hafnium oxide produced devices with a large number of defects. Deposited pentacene did not form optimal crystal structures, and the output characteristics of a number of devices showed no significant variation with source-drain voltage. These output characteristics were therefore assumed to be the result of leakage through the oxide. The measured device that showed reasonable output characteristics was found to have a mobility of 0.59 cm(^2)/V/s demonstrating that hafnium oxide could have good potential as a dielectric, if deposited in a manner not resulting in a leaky oxide. Three dibenzothiophene-based molecules, synthesised in the University of Durham, were characterised and thin film transistors fabricated. 3,7-bis(dibenzothiophene-4-yl)-dibenzothiophene-5',S'-dioxide exhibited the characteristics of an air-stable n-type device, with a mobility of 3.5 x 10(^-6)cm(^2)/V/s. A related molecule, 3,7-bis(4-(ethylsulfonyl)phenyl)dibenzo[b,d]jthiophene, incorporating additional electron-deficient groups, did not exhibit any field-effect modified behaviour. A third molecule, that did not contain electron-deficient groups, exhibited p-type behaviour, and transistors showed good output characteristics, but only possessed a mobility of 3.7 x 10(^5) cm(^2)/V/s; the low mobility was attributed to the lack of long-range order in the structure of the deposited film.