| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 100 === For newly synthesized organic semiconductor, NTETB, this research has identified polymorphous ordering behavior including the growth of orthorhombic crystalline phase at low temperature, and a highly ordered smectic phase above 120 oC, which exhibits a mosaic texture under PLM. The involved structural evolution and corresponding morphological changes has been found fundamental for elucidating thin film morphology and the continuity of crystalline packing, which are critical for the transportation of charge carrier within thin film.
For this calamitic oligothiophene derivative, the crystallization process through slow solvent evaporation is able to result in large crystalline domains with homeotropic molecular alignment. Upon crystallization directly from dissolution state, the crystal packing is established on the scheme of layer stacking, which can be efficiently extended two-dimensionally. This established layer stacking within crystalline phase involves the interdigited packing of studied oligothiophene along the layer normal, yielding a larger stacking periodicity than the molecular length.
While the method of vapor deposition being adopted for thin film fabrication, fast nucleation process during molecular deposition resulted in random stacking of small crystals. Upon the transformation from crystals to smectic phase during heating, the thermally activated coalescence of smectic domains caused significant morphological evolution, which consequently results in large ribbon-like crystals during subsequent cooling process. Therefore, based on this coalescence process of smectic phase, the morphological evolution involved during heating was recognized to favor the formation of continuous crystalline domains within thin film.
However, the slippage of packing plane along the b-axis was recognized to occur frequently within smectic phase, resulting in parallel crystallographic cracks along the b-axis. The following crystallization from prior coalescent smectic domains thus unavoidable includes discontinuities of molecular packing within crystalline domains. Therefore, in spite of lessening boundary effect upon domain coalescence, this crystallization route from prior smectic phase results in missed registries of molecular packing along the a-axis, and thus yields hindered transportation of charge carrier within thin film. Direct crystal growth from dissolution state can be a better option to develop continuous regular lattice packing. In summary, for oligothiophene derivatives to be used as organic semiconductor, this research illustrates both the positive and negative impacts of prior liquid crystalline phase to the formation of crystalline domains within thin film, and thus to semiconducting performance.
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