Microstructural Controls of Solution-Processable Semiconducting Layer for High-Performance Organic Thin-Film Transistors

Abstract

Solution processable semiconductors have attracted significant attention in recent years due to their superior intermolecular interactions and solution-processability. Because the molecular orientation and film morphology of the soluble semiconductors are decisive determinants of device-performance, controlling self-organization behaviors is essential. Particularly, identifying interfacial phenomena between organic semiconductor and gate dielectric layer is a critical key because charge transport in organic field-effect transistors (OFETs) takes place within several monolayers near the interface. Herein I address the systematical investigations of interfacial characteristics in soluble semiconductors for high performance OFETs. In Chapter 2, the effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device-performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, since slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three dimensional (3D) growth of the TIPS-PEN crystals, since the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm2 V-1 s-1, which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs. In Chapter 3, I demonstrated the effects of microstructural (crystallization and molecular orientation) and morphological alternation (grain boundary) of poly(3-hexylthiophene) (P3HT) films on the field-effect mobility ( μ ) before (as-spun P3HT) and after (melt-crystallized P3HT) melting of P3HT films. Although grazing incidence X-ray scattering shows that melt-crystallized P3HT has a more highly ordered edge-on structure than as-spun P3HT, the melt-crystallized P3HT reveals μ = 0.003 cm2V-1s-1

this is an order of magnitude lower than that of as-spun P3HT (μ = 0.01 cm2V-1s-1). In addition, the interfacial morphologies of the bottom surfaces of P3HT films, which are attached to the gate dielectric, were investigated using a film transfer technique. The melt-crystallized P3HT at this interface consists of well-developed nanowire crystallites with well-defined grain boundaries that act as trap states, as verified by analysis of the temperature-dependence of μ. The remarkable reduction of μ in low molecular weight P3HT film (8 kg/mol) that results from melt-crystallization is due to the increased number of well-defined grain boundaries.In Chapter 4, I demonstrate the roughness effect on device performance in dual gate P3HT transistor, comparing the mobility of bottom-gate and top-gate device, respectively. Reduced channel roughness (0.591 nm) via thermal annealing results in dramatically enhanced field-effect mobility, showing the highest mobility of 0.12 cm2 V-1 s-1 at the top-gate transistor. The –OH functional group of gate dielectric in bottom-gate transistor is caused the reduction of field-effect mobility, which is improved by the modification of the dielectric surface with self-assembled monolayer. The high performance OFETs can be achieved by the controls of channel roughness as well as well-made interface between semiconductor and gate dielectric.In Chapter 5, I investigated air-stability behaviors which are important issue for the commercialization in organic electronics. In general, low HOMO level and dense structure of semiconductor can lead to improved air-stability, a novel polymeric semiconductor, poly(3,6-dihexyl-[2,2’]bi-[thieno[3,2-b]thiophene]) (PDHTT), was synthesized and tested as an active layer in organic thin film transistors (OTFTs). This semiconductor showed considerable potential for use in commercial electronic devices because of its superior characteristics, particularly its good stability. PDHTT-based OTFTs exhibited high stability in air, retaining their initial performance after exposure to 70% relative humidity for 50 days

they were also stable under repeated electrical stress and even after exposure to temperatures as high as 250°C. We attribute the remarkable stability of PDHTT OTFTs to the relatively low highest occupied molecular orbital (5.1 eV) level of the polymer and its highly interdigitated structure in the thin film state.