Molecular orientation of conjugated polymers in organic electronics devices
p-conjugated organic-electronic materials are finding use in an increasing number of applications in today's opto-electronic devices due to their relatively simple manufacturing processes, flexibility, and low cost. Contrary to conventional inorganic materials, which are deposited on a substrate under high temperature and high vacuum conditions, organic materials can be fabricated via wet-processes such as spin casting or printing at room temperature. Such advantages of organic materials make them promising candidates for the applications in organic light emitting diodes (OLED), organic thin film transistors (OTFT), and flexible photovoltaic cells.
Spurred by the advances of polymer synthetic methods, the control of molecular weight and structural purity of polymeric materials have improved. Accordingly, these advances also enable controlled studies to gain a better understanding of the relationships between the macroscopic properties of polymers and their molecular structure. The molecular structure at the surface and at the interfaces of organic-electronic material films is of crucial importance, because the electronic properties of the polymer/conductor interface is strongly dependent upon the molecular structure. Many research groups have reported investigations of different p-conjugated organic materials such as polyacetylenes, polythiophenes, polyfluorenes, poly(p-phenylene)s, and their derivatives. Among these materials, the molecular structure at the film surfaces of polyfluorene and poly(alkylthiophene) are intensively investigated.
For this research, we employed three methods of inducing or affecting orientation: direct rubbing, imprint lithography, and microcontact printing. The preferential orientation of polymer segments induced by rubbing process at the surface of thin polyfluorene films and the corresponding oligofluorene films were investigated by carbon K-edge soft X-ray absorption spectroscopy, resolving the near edge X-ray absorption fine structure (NEXAFS). In this study, direct rubbing of the film surfaces of the conjugated materials and the alignment behavior on a rubbed alignment layer were observed and compared to each other. Also, the molecular structure and the related electrical properties of conjugated organic semiconductors such as poly(3-hexylthiophene) (P3HT) in OTFT devices were investigated. For the device manufacturing, we used and developed the soft mold microcontact printing method to transfer the electrode to the semiconducting polymer surface and established an in-plane anisotropy on the gate dielectric layer surface through the imprint lithography or by the surface rubbing with velour cloth.
Mentor: Eric K. Lin
Materials science and Engineering Laboratory
National Institute of Standards and Technology
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