Organic semiconducting polymers have attracted considerable attention in recent decades due to the wide range of potential applications for them, which includes solar cell, organic devices, organic photovoltaics, biological sensors, and particularly organic thin film transistors (OTFTs). OTFTs, as one of field effect transistors (FETs), are considered to be one of the key components for realization of complex devices such as flexible electronic products. The field effect mobility of OTFTs reflects the charge transfer performance of the organic semiconducting layer, which is affected by many of the device manufacturing parameters. The primary purpose of this thesis was to optimize the self-assembly of semiconducting polymer through solution blending and doping techniques to improve solid-state ordering structures and resultant OTFT electrical performances. Semiconducting regioregular (RR) poly(3-hexylthiophene) (P3HT) containing strictly arranged head-to-tail (HT) asymmetric 3-hexylthiophene repeating units was the primary active material. Regiorandom (rr) P3HT was appointed as a soft blended matrix with identical chemical composition but irregular backbone conformation comparing with RR P3HT. 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is used as a p-dopant for semiconducting P3HT due to its high electron affinity (5.2 eV) and excellent conductivity. RR P3HT spontaneously phase separates in RR P3HT/rr P3HT blending system to form highly ordered stretchable films-based soft devices. F4TCNQ-doped RR P3HT (or rr P3HT) system is introduced for controlling the morphologies, crystalline nanostructures and integer charge transfer of self-assemble films-based high performance OTFT applications. In addition, the absorption spectra of the solution, the morphologies and crystalline microstructures of the films were thoroughly investigated. The thin films were also fabricated using solutions that were subjected to an ultrasonication pretreatment. These approaches have been shown to be effective for the manufacture of flexible, high-performance OTFTs.

In Chapter I, the configuration of the OTFTs and typically conjugated polymers are introduced. Especially, the reports and applications of P3HT in recent years are described. At the same time, some problems existing in the work are proposed, such as the rigidity of the film and the lower electrical performance. Finally, the main purpose and overview of this study are presented. In addition, the methods of thin film manufacturing processes are explained in detail.

In Chapter II, high regioregularity RR P3HT are known to have excellent electrical properties. Solution blending of 97% and 55% graded P3HT with varying degrees of regioregularity was performed to spontaneously separate the high-RR P3HT chains from the rr P3HT media, which lead to the development of highly conjugated nanodomains in both the solution and films. In the spun-cast blend films (1:5 ~ 1:9), the rr P3HT matrix offers the possibility of sufficient deformability of the channel layer desired for flexible OTFTs, compared to neat RR P3HT and blends with a deformable polymer. OTFTs including RR P3HT/rr P3HT blend films show excellent charge carrier mobility (μ) values up to 0.13 cm2 V?1 s?1, which exceeded that in an optimal RR P3HT film (0.026 cm2 V?1 s?1) prepared using an ultrasonicated solution. Furthermore, A RR P3HT/rr P3HT film was stretched by ~50% without strain. The RR P3HT/rr P3HT film retained approximately 55% of its μ value, while hole mobility decreased abruptly in RR P3HT films when they were stretched by just 10%. The simple blending method thus afforded deformable, π-conjugated semiconducting film-based OTFTs.

In Chapter III, F4TCNQ-doped P3HT system shows controllable crystal structures, which contain discernable π-conjugated nanodomains in both solution and spun-cast films. The effect of doping RR P3HT is obviously better than rr P3HT due to the difference the highest occupied molecular orbital (HOMO) values affecting charge transfer behavior. RR P3HT solution was doped with F4TCNQ in molar dopant ratios (MDR) of 0 to 0.1 to investigate the effect of dopant loading on the structural ordering and electrical behaviors of the π-overlapped semiconducting polymer for OTFT applications, while the MDR of rr P3HT is ranged from 0 to 0.5. Via properly inducing the integer charge transfer between RR P3HT and F4TCNQ after simple solution blending, the ionized dopants can be allocated in both hexyl side-chain regions and π-conjugated backbones in RR P3HT crystallites with an increase in molar dopant ratio (MDR, [F4TCNQ]/[hexyl-thiophene]). Also, it is found that some dopants can be positioned to the intermolecular backbones of RR P3HT and form a mixed crystalline structure, which has a shorter π-π overlapping distance, in comparison to the pure polymer system. At low-level of MDR = 0.005 (or 0.015) the conjugated crystallinity of the doped RR P3HT self-assembled film has been greatly improved, and the carrier mobility (0.023 cm2 V-1 s-1) of resultant OTFTs has increased by more than 20 times compared to neat RR P3HT (~10-3 cm2 V-1 s-1). The low-doped RR P3HT-based OTFTs have also shown the typical p-type transfer curves with Vth are closed to 0 V and higher ION/IOFF of ~104. Above a certain MDR, 0.03, however, the excessive dopants cause large-sized and less-ordered RR P3HT aggregates, originating from the coulombic interaction between the ionized molecules. In term of amorphous rr P3HT film with almost no electrical properties, through the introduction of the large amount of F4TCNQ dopant over MDR = 0.05, the arrangement of rr P3HT chains in the solution is induced to form a low-order conjugated structure in the casted film and above ~5×10-4 cm2 V-1 s-1 of resultant OTFTs. The simple blending of the dopant and semiconducting polymer solutions provides potential advantages for OTFT applications.