Liquid crystalline organic semiconductors for application in opto-electronic devices

Bao, Weixiao

Abstract

This thesis collates and describes the research work carried out during my PhD programme. This work of research is mainly based on the synthesis and evaluations of novel liquid crystalline materials for use in plastic electronic applications, such as Organic Light Emitting Diode (OLED) and Organic Photovoltaic (OPV) devices. It also focuses on the study of the relationship between chemical structures and the mesomorphic behaviour, the liquid crystalline transition temperatures and energy levels of these new compounds.

Optical Polarising Microscopy (OPM) and Differential Scanning Calorimetry (DSC) were used to identify the mesomophic behaviour and transition temperatures of all the new liquid crystals. A combination of UV-vis absorption and Cyclic Voltammetry (CV) was used to characterize the Highest Occupied Molecular Orbitals (HOMO), Lowest Occupied Molecular Orbitals (LUMO) energy levels and, therefore, the band gap (Eg) of the final compounds.

Another primary focus of this work was the study and application of various aryl-aryl, cross-coupling reaction methods, including the Stille and the Suzuki reactions and a new direct arylation method. The reaction conditions were optimised during the research work for different compounds. The direct arylation method was found to be a successful and efficient way to synthesise new thiophene derivatives using aryl halides via C-H bond activation by palladium catalysts, but unfortunately not for all substrates.

A series of fluorene- and carbazole-based materials have been developed for use as hole-transporting materials and electroluminescent materials in plastic electronic applications. Some of these materials exhibit nematic phases, which is beneficial due to the lower viscosity present in the nematic phase compared to that of the smectic phases. Some of them are found to form a glass above room temperature without any observable liquid crystal phases, despite significant supercooling below the melting point. The transition temperatures and mesomorphic behaviours of these novel materials were determined. The materials are expected to exhibit appropriate ionization potential (IP) and energy levels (HOMO, LUMO and Eg).

A number of compounds incorporating a thiophene-based central core have been synthesized and evaluated as hole-transporting materials in OLEDs or/and as electron-donors in OPVs hopefully with the required appropriate energy levels. A compound with four 2,5-disubstituted thiophene rings in the molecular core shows promising properties for use as an electron-donor material with PCBM as an electron-acceptor in test OPV cells fabricated by members in the Organic Semiconductor Group in the Department of Physics at the University of Hull.

Several liquid crystals based on thiazolo[5,4-d]thiazole central cores were synthesised by various aryl-aryl cross-coupling reaction pathways. The thiazolo-thiazole compounds based on five-membered rings only exhibit a nematic phase, while some thiazolo-thiazole compounds incorporating six-membered rings exhibit both a smectic phase and a nematic phase. However, this class of compounds exhibit relatively high melting points and liquid crystalline transition temperatures, as well as a very poor solubility, which indicates that they were not able to be used as organic semiconductors in plastic electronics applications.

A class of oxadiazole homologues and a small number of isoxazole derivatives were successfully synthesized and purified. They were expected to possess relatively high electron affinity (EA) and strong fluorescence as potential materials for use as electron-transporting layers and/or emissive layers in OLEDs. The mesomorphic behaviour of these compounds is interesting and includes an unidentified SmX phase and a banana phase. The relationships between the chemical structures and mesomorphic behaviours of these materials were established.

Citation

Bao, W. (2014). Liquid crystalline organic semiconductors for application in opto-electronic devices. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4216495

• Thesis Type: Thesis
• Deposit Date: May 8, 2015
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4216495
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Sep 1, 2014