Date of Degree

2-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor(s)

Shi Jin

Subject Categories

Chemistry | Mechanics of Materials

Abstract

Organic semiconducting materials have been under intensive investigation in the recent decades for potential applications in various electronic or optoelectronic devices such as light emitting diodes, photovoltaic cells and field effect transistors. Compared to inorganic counterparts, organic charge transport materials are attractive for their abilities of forming thin-films, large area manufacturing, compatibility with flexible substrate, light weight and potential low fabrication cost. The charge transport property of the organic active layer is one of the key factors to the electronic or optoelectronic performance of devices. Research projects presented in this thesis focused on improving charge carrier mobility of organic charge transport materials as it is a property determined by the hierarchical structure of the material. Strong effort has been made to the design of advanced molecular structures and controlling self-assembly behaviors. Chapter 1 introduces the general background of charge transport materials, including: the nature of charge transport in organic semiconducting materials, three widely used methods for charge carrier mobility measurements and the current development of organic charge transport materials. Advantages and drawbacks in applications were analyzed with ordered and disordered organic systems. A more thorough review was given to the engineering and the application of the discotic columnar liquid crystalline (DCLC) phase. Chapter 2 describes a DCLC phase with a novel hierarchical structure in which each supra-molecular column features a bundled-stack structure. The molecular design rationale was explained and the thermal behavior and phase structure were characterized. Charge carrier mobility of compound 1 was measured to be 0.05 cm2V-1s-1 with pulse radiolysis time-resolved microwave conductivity. The incorporation of the bundled stack structure may potentially be a fundamental solution towards enhancing the organic semiconductor's electronic performance. Chapter 3 introduces three chain functionalized perylene tetracarboxylic monoimide diester derivatives (PEIs) with monotropic DCLC phases. The intra-column rotation angle was determined to be 60 o between neighboring PEI molecules, which is a substantial improvement of the transfer integral compared to the perylene tetracarboxylic diimides with a 90 o rotation angle. The rotation angle was further tuned by incorporating branched aliphatic substitution to the PEI core as described in chapter 4. By reducing the length of the alkyl swallow tail, the rotation angle changes from 60 o to 72 o which is even more favorable to the electronic coupling between neighboring PEI units. Through those studies, we have shown that the engineering of DCLC phase may lead to substantial improvements on charge transport properties of organic semiconducting materials.Â

 
 

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