Liquid crystalline and polymer network organic semiconductors for application in opto-electronic devices

Hu, Guang

Abstract

A series of novel liquid crystalline and photopolymerisable monomer organic semiconductors for plastic electronics applications, such as organic light-emitting diodes (OLEDs) and/or organic photovoltaics (OPVs) were synthesised and evaluated in this thesis. A number of synthetic reactions were carried out to obtain the desired compounds and intermediates under different reaction conditions. Different aryl-aryl cross-coupling reactions such as Suzuki reactions and direct arylations, palladium-catalysed systems [Pd(OAc)₂, Pd(OAc)₂+P(Ph)₃, and Pd(PPh₃)₄] for Suzuki aryl-aryl cross-couplings, and the choices of reaction solvents and base aqueous for N-alkyl substitutions were compared, optimised and analysed in this thesis.

A variety of electron-withdrawing- or electron-donating moieties, including 2,7-disubstituted carbazole, 2,5-disubstituted thiophene rings, 1,4-disubstituted phenylene, 2,7-disubstituted fluorene, dibenzothiophene, benzothiadiazole and thieno[3,4-c]pyrrole-4,6-dione cores, were designed to incorporate in the molecular structure of various aromatic heterocyclic organic semiconductors. The relationship of liquid crystalline mesophases and molecular structures were analysed and established. Linear and co-axial aromatic backbones and short lateral chains contribute to the presence of liquid crystalline mesophases due to a large length-to-breadth ratio. Structural design of different aromatic cores promote the presence of different electroluminescent colours (i.e., organge for compound 53, green for compound 38, and blue for compound 41) and the tuning of device performance. Particularly, the nematic liquid crystalline 42 with a desired glass transition temperature above room temperature (32°C) and high clearing point (161°C) shows lower switch-on voltages (2.4 V) and higher OLED device brightness, current density and efficiency. Furthermore, There are excellent matches between the values of the ionization potential (IP = -5.53 eV) and the electron affinity (EA = -2.89 eV) of electroluminescent liquid crystalline 42 and the HOMO energy level of the hole-transport layer cross-linked OTPD (IP = -5.48 eV) and the LUMO energy level of electron-transporting layer of SPPO13 (EA = -2.91 eV), respectively. This advantageous combination of energy levels within the test OLED results in low charge-injection barriers for electrons and holes, respectively, leading to a high current density and a corresponding high density of excitons in the emissive layer where efficient recombination occurs efficiently with emission of light.

In addition, novel photopolymerisable carbazole-functionalised triazatruxenes incorporating three or more cross-linkable endgroups including non-conjugated dienes and oxetanes at the end of aliphatic flexible spacers, were first reported and synthesised using simple one-step N-position substitution reactions. The cross-linking abilities of C=C double bonds, C≡C triple bonds, non-conjugated dienes and oxetanes attached to the triazatruxene core were compared and investigated. The result proved that non-conjugated dienes attached to more chemically and photochemically stable tertiary amides (90% @400 J/cm² UV for monomer 96) showed a greater tendency to photopolymerise than that attached to common ester bonds (72% @800 J/cm² UV for monomer K1). The photopolymerisable monomers also afford a cheap thin-film fabrication by solution spin-coating in organic optical-electrical devices. Hole test device of photopolymerisable triazatruxene 100 was fabricated by solution spin-coating. The effect of cross-linking and doping on hole-transporting capacity were studied in this work. It was found that cross-linking of photopolymerisable triazatruxenes 100 does not lead to a decrease of current density while photopolymerisable triaryamine monomer with similar star-shape does. Furthermore, the use of highly electronegative p-type dopant results in an increase of current density due to the formation of more free holes in a device by removing electrons from the host material.

Citation

Hu, G. (2015). Liquid crystalline and polymer network organic semiconductors for application in opto-electronic devices. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4217847

• Thesis Type: Thesis
• Deposit Date: Apr 21, 2016
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4217847
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Dec 1, 2015