| In this thesis, the molecular packing motifs of our newly designed fused thiophenes, benzo[d,d]thieno[3,2-b;4,5-b]dithiophene (BTDT) derivatives, were studied by utilizing grazing incidence wide angle X-ray scattering (GIWAXS). Considering the potential of fused thiophene molecules as an environmentally stable, high performance semiconductor building block, it must be an important groundwork to investigate their thin film structures in relation to molecular structures, single crystal structures, and organic thin-film transistors (OTFT) performances.;OTFT device performance is not only determined by semiconductor materials, but also influenced by the interfacial properties. Since there are three major components in TFT structures---electrodes, semiconductors, and dielectrics, two types of major interfaces exist. One is the semiconductor-electrode interface, and the other is the semiconductor-dielectric interface. Both of these interfaces have critical roles for TFT operation. For example, the semiconductor-electrode interface determines the charge injection barrier. Before charge carriers go through the electrode (source)-semiconductor-electrode (drain) pathways, the energy gaps between the work function of the electrodes and the HOMO energy of the semiconductor materials must be overcome for hole injection, or the energy gap between the metal work function of the electrodes and the LUMO energy of the semiconductor materials must be overcome for electron injection. These charge injection barriers are largely determined by the energetic structure of the semiconductor material and work function of the electrode. However, the size of energy gap can be modified by introducing an organic self-assembled monolayer (SAM) on the surface of metal electrode. In addition, the structure of semiconductor films, especially within several monolayers right above the electrode, is greatly influenced by the SAM, and it changes charge injection property of OTFT devices. In this thesis, the role of a thiol SAM on top of the gold electrode is investigated in terms of semiconductor film structure and OTFT performance in the bottom-contact/ bottom-gate TFT structure by using one of the most successful small molecule based n-type organic semiconductors, &agr;,&ohgr;-diperfluorohexylquarterthiophene (DFH-4T) and N,N' bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarb-oximide) (PDI-8CN2). The study of semiconductor film morphogy shows that the semiconductor molecules at the gold/SAM/semiconductor interface are aligned normal to the substrate, facilitating charge transport at the interfacial region. As a result, contact resistance was minimized, and the OTFT device performance was improved.;When it comes to semiconductor-dielectric interface, it is important because the charge transport layer of the OTFTs is formed within several monolayers of semiconductor films right above the gate dielectric. The physical and chemical nature of the dielectric surface significantly influences charge flow. For example, the surface of a SiO2 dielectric contains a large number of SiOH functional groups in air. After depositing semiconductor material on top of the SiO2 surface, those SiOH functional groups play a role of charge traps. One of the most effective ways of circumventing this problem is to introduce organic self-assembled monolayers (SAMs) on the SiO 2 dielectric surface. The SAMs in the semiconductor-dielectric interface not only minimize the charge traps but also improve the crystallinity of top semiconductor layers. Furthermore, the improvement of the semiconductor film microstructure depends on the structure of the SAM. When the SAM is disorganized, the size and density of crystalline domains in the semiconductor film decline. Meanwhile, the domain size and population density of crystalline domains expand when the SAM is tightly packed and vertically aligned. In this thesis, a humidity control method of fabricating high quality octadecyltrichlorosilane (OTS) SAM on SiO2/Si substrate was introduced. The crystalline nature of OTS SAM and its molecular orientation were investigated using grazing incidence X-ray diffraction (GIXRD) and sum frequency generation (SFG). It turned out that our OTS SAM has in-plane crystallinity and the alkyl chains inside SAM are vertically aligned. The tightly packed SAM induces a large number of big crystalline domains with better surface coverage in the semiconductor film with submonolayer thickness. This film microstructure ensures complete charge transport pathways within several monolayers of semiconductor films. As a result, the OTFT performance of P-BTDT fabricated on our OTS SAMs exhibits high mobility of 0.25 cm2 V-1s -1 while OTFT devices fabricated on conventional OTS SAMs produce mobility of 0.085 cm2 V-1s-1 . (Abstract shortened by UMI.)... |
