| Summary: | 博士 === 國立成功大學 === 光電科學與工程研究所 === 99 === Abstract
Concerning my investigation in the past 3-and-half years as a candidate for doctor's degree, I would like to write brief summaries by below five paragraphs.
Part 1. P3HT
The chapter 2 focuses on the microstructural modifications of regioregular poly (3-hexylthiophene) (rr-P3HT) in the small active channel of thin-film transistors (TFTs) during operations. Polarized absorption and microRaman spectroscopy analyses allow us to probe directly the conformation transitions of rr-P3HT chains parallel or perpendicular to the channel by means of exciton bandwidth, interchain electronic coupling, and effective conjugation length. The results of absorption spectra and a joint experimental-theoretical study of Raman spectra show that an external source-to-drain electric field can align rr-P3HT chains parallel to the channel, improving electrical performance after long-term operations, especially charge transport properties. In comparison, the applied external gate field induced an increase in amorphous fraction of the rr-P3HT films. After the analysis, we propose a chain rearrangement model driven by an external electric field to interpret the changes of the effective conjugation length of rr-P3HT, rather than thermal annealing. Our observations provide a thorough explanation for the previously unknown relationships of structure-electronic properties under the extended operations of polymer TFT devices.
In chapter 3, combined quasi-swelling and recrystallization concepts, a promising approach was developed to reform distorted segments into the highly-ordered structure with the superlong effective conjugation length (＞ 90mer at least) for polymer chains, which are impressive results compared with those previously reported in the literature. These highly extended polymeric conjugated chains may have important implications for various optoelectronic devices applications and theoretical studies.
In chapter 4, we demonstrate high-performance air-stable bottom-contact-top-gate polymer thin-film transistors (TFTs) with linear regime field-effect mobility as high as near 0.2 cm2/Vs using conjugated poly(3-hexylthiophene) (P3HT) as the active layer. A thermal-crosslinked poly(vinylphenol) main layer and a poly(vinylidene fluoride) buffer layer were fabricated on the P3HT layer from the solution process to serve as both the gate dielectric and passivation layer. The ?lin is two orders of magnitude greater than that in bottom-contact-bottom-gate configuration using conventional silicon dioxide gate dielectric.
In chapter 5, a series of binary blends of P3HT and insulator polymer, i.e., poly(methyl methacrylate) (PMMA), were prepared and as the active layer of polymeric TFTs. We investigated the correlation of microstructure of the blending films and electrical properties of the TFTs by absorption spectrometer, atomic force microscopy and X-ray diffraction. The result revealed that blending PMMA reduced the crystalline portion of P3HT and produced phase separation morphology of P3HT and PMMA. When appropriate amount of PMMA was added, the devices exhibit better electrical performance that of the device without PMMA. Especially, the optimal TFT with PMMA show enhanced on-current and low-level off-current of only 19 pA. We found that the electrical properties of the P3HT:PMMA blending films-based TFTs could be controlled by adding different concentrations of insulator polymer.
Part 2. Pentacene
In chapter, a promising and simple method to control the crystal polymorphic transformations of insoluble pentacene through solvent treatments is developed to obtain superior films with stable polymorphs and enhanced intermolecular electronic coupling, as proven by X-ray diffraction, Raman and absorption spectroscopy, and quantum chemical calculations. The degree of polymorphic transformations within films can be managed by the selection of appropriate organic solvents according to the magnitude of pentacene-solvent interaction. A reaction pathway that could interpret how a metastable polymorph T (“thin film” phase) transforms into a more stable polymorph B (“bulk” or “single-crystal” phase) is proposed. The hypothesis is based on the terms of crystal structural parameters, including separation distance, tile angle, and herringbone edge-to-face angle. With the aid of quantum chemical calculations, we combine the binding energy of pentacene dimers and pentacene-solvent interaction energy to develop a new quantitative criterion for the selection of appropriate organic solvents for the structural improvement of organic crystal/films rather than damage. The proposed solvent post-treatments concepts could provide opportunities for improved vacuum-evaporated organic crystal/films and further expand potential applications in organic electronics and photonics.
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