dc.description.abstract | Organic semiconductors have the advantages of low temperature process, low cost, good ductility, and molecular design, which are favorable for large area coating and can be applied on flexible electronic application such as flexible displays, sensors, transistors, and solar cells. Therefore, this dissertation is focusing on the study of the molecular design and blending system of organic materials for electronic application and it will be divided into three parts.
First, the high-performance P-type organic small molecule semiconductors with a core of heteroalkyl-substitution dithiophene (XBT) have been designed for organic field effect transistors (OFETs). The core of SeBT has investigated the effects of introducing different side chain lengths (R = C14, C10, C6) and end-functionalization with two dithienothiophene (DTT), thienothiophene (TT), and thiophene (T) units. Moreover, the core XBT is also explored the impact of the chalcogen heteroatom (–XR, X = Se, S, O), which is developed selenotetradecyl SeBTs, thiotetradecyl SBTs, and tetradecyloxy OBT. Side chain lengths are able to improve the film continuity and tuning the solubility. Increasing the number of aromatic rings can improve the conjugation length. The impacts of the chalcogen heteroatom obtain the noncovalent conformational lock with the sulfur in the aromatic ring. From the single crystal data, the SeBT small molecule obtain a torsion angle of ~ 0° as well as coplanar backbone. When organic field effect transistors are fabricated by a solution sheared process, the highest hole mobility of DDTT-SeBT-C14 (1) can reach 4.01 cm^2 V^–1 s^–1 which is the best hole mobility value reported to date for fused-thiophene-based small molecules. In addition, these results are also investigated by crystallography, thin film morphology, and microstructure analysis.
In second research, the core of dithiooctyl dithienothiophene with end functionalized fused dithienothiophene (DTT) units, named DDTT-DSDTT, was synthesized and characterized for OFETs. The intramolecular lock with S (alkyl)∙∙∙S (thiophene) interaction bring out a better coplanar structure, resulting in better solubility in the anisole as a green solvent. Therefore, utilizing a solution-sheared processing method to fabricate the active layer for OFETs. The DDTT-DSDTT compound exhibits highest mobility of 3.19 cm^2 V^–1 s^–1. Moreover, the small molecule DDTT-DSDTT and insulting polymer poly(α-methylstyrene) (PαMS) blend films could maintain high performance causing by vertical phase separation with PαMS layer between DDTT-DSDTT layer and dielectric layer. The 50 wt% DDTT-DSDTT blend thin film show high mobility of 2.44 cm^2 V^–1 s^–1 which exhibit high crystallinity and better molecular packing. The blend devices show good air stability and also a smaller threshold voltage shift under gate bias stress.
In third research, solution processable organic phototransistors are reported by the heterojunction composition with different weight ratio of N2200 polymer/ quinoidal thienoisoindigo (TII)-containing small molecule as the semiconductor layer under the various incident red light intensities (680 nm). The blend system of phototransistor shows high photoresponse of 4064 AW–1, detectivity of 1.4 × 10^13 Jones, and high mobility of 1.59 cm^2 V^–1 s^–1. The reason that introduction of TIIQ small molecule is able to improve the absorption of red light, photogenerated carriers, and fast charge carriers. These results show the blending strategy of semiconductor offer successfully developing for organic phototransistor. | en_US |