Charge injection and transport in organic semiconductors

Abstract

In this thesis I present a comprehensive study of charge transport and injection into organic semiconductors from solution processed metal oxide contacts. Charge transport is investigated using Time of Flight (TOF) transient spectroscopy to measure the mobility in fluorene-bridged triarylamine and polyfluorene copolymers. Dark Injection (DI) transient spectroscopy is used to measure bulk hole mobility in the polymer (poly)triarylamine (PTAA). Temperature dependent mobility experiments are conducted to calculate energetic and positional disorder in PTAA by application of the Gaussian Disorder Model. Solution processed metal oxides are investigated as hole and electron injecting contacts using current density-voltage space charge limited analysis (JV-SCLC) in tandem with the transport measurements. This approach allows the injection efficiency of several metal oxide contacts (including chlorinated-ITO, vanadium pentoxide and caesium carbonate) to be quantified and comparative studies to be conducted. The work is then extended to study the resultant performance of these contacts in Organic Light Emitting Diodes (OLEDs) with a view to better understanding the roles of transport and injection in optoelectronic devices. A novel solution processed metal oxide based Electron Injection Layer (EIL) is presented as a viable alternative to conventional evaporated contacts such as calcium in OLED devices with enhanced performance observed in both conventional and inverted architectures. Building on this work I report on the development of a transient method of charge mobility measurement for organic semiconductors in thin film transistor devices. I present Dark injection Thin Film Transistor spectroscopy (DI-TFT) as a significant new technique for understanding injection and transport effects in TFT’s. This technique is successfully applied to charge transport measurements in triarylamine and polyfluorene copolymers as well as a novel truxenone small molecule organic semiconductor.