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Title: Charge transport in solution-processable organic field-effect transistors
Author: Jarrett, C. P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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The ease of processing conjugated polymers in conjunction with their novel optical and electrical properties has attracted great interest from both theoretical and applied perspectives. There has been particular interest in the development of organic transistors for mass produced low speed electronics. In this dissertation, metal-insulator-semiconductor field-effect transistors (MISFETs) are used to study the physical mechanisms of charge transport in solution-processable conjugated polymers and oligomers. The effect of dopants upon the electrical characteristics of MISFET devices is investigated using a soluble polythiophene derivative. By carefully controlling the doping level, the relationship between the conductivity and field-effect mobility is explored as a function of the dopant level. Temperature dependent measurements of the conductivity for various doping levels are presented, which suggest variable range hopping as a viable mechanism for charge transport. Combining these observations with magnetic susceptibility measurements allows the relationship between the conductivity and field-effect mobility to be modelled within the framework of variable range hopping. The applicability of this model to other systems is discussed in terms of impurities and molecular order. Recently solution-processable polyacetylene has been produced with mesogenic end-groups attached to the end of the chains. These materials are designed in an attempt to improve the molecular order in the films. Charge transport is investigated using MISFET devices by comparing results from materials with and without mesogenic end-groups. The molecular order is explored using electron microscopy and absorption spectroscopy of field-induced charges. Charge transport in precursor-route conjugated materials is studied and shown to result in improvements in device characteristics. In particular, the recent development of a precursor-route to pentacene is utilised and the device characteristics are studied as a function of the conversion conditions. The temperature dependence of the charge mobility is studied for this system and for other conjugated materials. The measurements are compared with those of inorganic systems and discussed in terms of the physical differences between inorganic materials and conjugated polymers.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available