고성능, 신뢰성 있는 박막트랜지스터 어레이 구현을 위한 유기 반도체 결정의 패터닝 및 배향 기술 연구

Abstract

Organic field-effect transistors (OFETs) have attracted great attentions from academic and industrial fields due to their various applications in display, radio-frequency identification tags, and memory. In particular, the emerging technologies such as flexible and wearable electronics provide a remarkable opportunity to commercialize organic electronic materials and devices, as well as to expect a quantum leap in the organic electronic market. To this end, there has been a considerable progress in the improvement of materials synthesis, device physics, and processing methodologies. For example, high field-effect mobility (μFET) up to 10 ~ 20 cm2/(V·s) has been achieved in the solution-processed polymer semiconductor materials (e.g., thiadiazole, diketopyrrolopyrrole and isoindigo-based copolymers). Furthermore, the development of roll-to-roll and diverse printing processes (e.g., ink-jet, aerosol jet printing) made it possible to fabricate organic devices on large plastic substrates easily and cheaply. In this study, various methods for patterning and aligning organic semiconducting crystals were demonstrated. Patterning and aligning of organic small molecule semiconducting crystals over large areas is an important issue for their commercialization and practical device applications. Prior to the demonstration, basics for OFETs, patterning and aligning issues for organic semiconductors were introduced in chapter 1. In chapter 2, A unified patterning and annealing approach was successfully demonstrated for 5,11-bis(triethylsilylethynyl)-anthradithiophene (TES-ADT) films spun-cast on polymer-treated SiO2 dielectrics. First, rubbery polydimethylsiloxane (μ-PDMS) stamps with microscale periodic grooves were swollen in 1,2-dichloroethane and then softly placed onto amorphous-like TES-ADT films. In this case, the film sides physically contacting the wet stamps were quickly absorbed into the PDMS matrix while the non-contact area formed highly-ordered phases by the solvent-annealing effect. The resulting patterns of TES-ADT contained discernable crystallites, where the grain sizes drastically decreased and their shapes transformed from spherulites to optically featureless ones with a decreasing line width from 100 to 2.5 μm. Unlike ordinary systems containing spherulitic domains, the 2.5 μm line-confined TES-ADT patterns contained layer-stacked crystallites but an optically invisible grain boundary, yielding an unexpectedly high μFET of 2.60 cm2/(V∙s) in OFET, with narrow deviations less than 8 % (averaged from 42 devices). The results suggest that the well -overlapped grains and their smooth connections are key factors to achieve high performance multi-array OFET applications. In chapter 3, uniaxially-aligned and lattice-strained organic single crystal nanowire (NW) array is fabricated via a straightforward solution processing technique called as ‘nanograting-assisted pattern transfer’. 50 nm-wide 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) NWs are grown while the solution is confined between a substrate and a flexible nanograted template. From UV-vis absorption and grazing-incidence wide angle X-ray scattering experiments, NWs are found to be single crystalline and adopt an anisotropic crystal orientation for efficient charge transport. Notably, the crystal growth under nano-confined spaces resulted in lattice-strained packing motif of NWs with close π-π stacking distance. TIPS-PEN NW OFETs yield extremely high μFET ¬up to 9.71 cm2/(V∙s) with narrow μFET distribution. In addition, the nanograting-assisted pattern transfer technique is applied on flexible substrates, demonstrating high-performance flexible OFETs. From these findings, it is expected that the nanograting-assisted pattern transfer technique provides a general method for realization of high μFET OFETs, and also a facile soft lithographic method for nanoscale patterning of well-grown crystallites. In chapter 4, ‘dragging mode’ electrohydrodynamic jet printing is introduced to simultaneously achieve direct writing and aligning of TIPS-PEN crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. OFETs based on the optimally-printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm2/(V∙s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays.