Synthesis, characterizations of new organic semiconductors and their application in organic electronic devices

有機半導體材料的合成, 表徵及其在電子器件中的應用

Student thesis: Doctoral Thesis

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Author(s)

  • Jiaju XU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date15 Feb 2013

Abstract

In recent years, development of new classes of organic semiconductors has become an active area of research due to their potential application in low cost, flexible, large area and light weight electronic devices. Current approaches employed to modify the charge transfer or electronic properties of the materials rely on changing the type of substitution on the materials or controlling the length of the π-conjugation of the molecules. Through controlling the conjugation length of the system or introducing electron-withdrawing or electron-donating functional groups to the primary frameworks, the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO) energy band gaps can be fine-tuned to give necessary and desirable properties required for organic materials to be useful as semiconductors in electronic devices. Based on this context, a series of π-conjugated functional materials including linear phenylene ethynylene molecules, their aza-substituted analogues, phthalocyanines, subphthalocyanines, and acenaphthylene derivatives have been synthesized and characterized. A series of linear phenylene ethynylene molecules as well as their aza-substituted analogues have been successfully synthesized based on palladium-catalyzed Sonogashira coupling reaction. The pure blue emission with a relatively high fluorescence quantum yield ranging from 0.62 to 0.82 measured in chloroform solutions suggested the promising potential of such materials for the application of optoelectronic devices. Self-assembled nanostructures including quasi-cubes, cubes and rods were successfully fabricated from such compounds. All the nanostructures demonstrated distinct luminescence properties from the corresponding solution and bulk solid samples, which made them the promising candidates as semiconductors that are expected to show interesting electronic or optoelectronic properties for the future device application. A series of phthalocyanine derivatives and subphthalocyanine derivatives have been successfully synthesized. Q-band absorptions of the compounds in dilute solutions were observed at 600-800 nm and 500-600 nm for phthalocyanines and subphthalocyanines, respectively. The broadening of absorption bands along with bathochromic shifts were found for the corresponding thin films, particularly, due to the extension of π-conjugated system the absorption maximums at 745 and 736 nm were observed for the samples of CuPEPc and ZnPEPc, respectively, suggesting the promising potential for the application in organic solar cells. Most of the compounds exhibited a relatively high thermal stability with an onset decomposition temperature of > 371°C, which is one of the critical requirements in long-life electronic devices. According to CV, all the compounds exhibited a characteristic of low energy band gap, particularly, the extension of π-conjugated system led to a significant decrease of the LUMO level as well as HOMO-LUMO band gap, making CuPEPc and ZnPEPc the potential candidates for the application in n-type OFETs. The OTFT devices fabricated with thin film samples of H2MePc, CuMePc, CoMePc, ZnMePc and NiMePc all exhibited a p-type FET behaviour with varying μ values. Charge carrier mobility of ~10-4 cm2V-1s-1 observed for H2MePc, CuMePc, and CoMePc combined with their wide UV-Vis absorption bands (extending to 800 nm) and excellent thermal stabilities (stable up to 433-517°C) makes them promising candidates for the application in organic solar cells. Two acenaphthylene derivatives have been successfully synthesized by the palladium-catalyzed Sonogashira coupling reaction. Both compounds exhibited a relatively wide UV-Vis absorption band with onset of > 600 nm in solid state, making them the promising candidates as semiconductors for organic solar cells. The self- assembled nanoparticles with the size in the range of 390-650 nm were fabricated from 1,2-bis(2-phenylethynyl)acenaphthylene, particularly due to the increased effective π-π stacking in nanostructures the absorption onset was extended to ca. 700 nm. Moreover, well-defined nanofibers with the averaged diameter of 230 nm and the length of tens of micrometers were directly fabricated from 1,2-bis(2-(4-(2-phenylethynyl)phenyl)ethynyl)acenaphthylene on SiO2/Si surface by spin-coating method, providing an approach of preparation in situ of nanostructures on a surface that is suited for device fabrication.

    Research areas

  • Organic semiconductors, Organic electronics