Investigation and Control of Charge Transport Anisotropy in Highly Oriented Friction-Transferred Polythiophene Thin Films

Abstract

Highly oriented thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno [3,2-b]thiophene] (PBTTT) were prepared by friction-transfer technique followed by their characterization using polarized absorption spectroscopy, angle-dependent polarized Raman spectroscopy, and X-ray diffraction (XRD) techniques. Orientation in high-molecular-weight (M W ) polymers is hampered by chain folding or entanglements, which limit their macromolecular orientation. Interestingly, utilizing high-molecular-weight PBTTT (M W > 50 kDa) and friction-transfer technique, successful fabrication of highly oriented thin films with very high dichroic ratio (∼30) was demonstrated. The role of the substrate's surface energy and its impact on the field-effect mobility (μ) of the oriented thin films were comprehensively investigated. The influence of annealing the thin films as prepared on the bare and self-assembled monolayer (SAM)-treated SiO 2 surfaces exhibiting differential trends of μ was systematically investigated. This was explained by partial and complete conformational transformation of macromolecules on bare and SAM-treated SiO 2 surfaces, respectively, after annealing them beyond liquid crystalline phase transition temperature, as revealed by in-plane and out-of-plane XRD results. On bare SiO 2 , optimum μ up to 0.03 cm 2 V −1 s −1 along the backbone orientation was obtained for the thin films annealed to 120 °C; whereas, it reached up to 0.36 cm 2 V −1 s −1 on SAM-treated SiO 2 after annealing at 200 °C. Finally, a charge transport mechanism was proposed taking evidence from the anisotropic optical and electrical characteristics of the friction-transferred PBTTT films into consideration.

Extreme Orientational Uniformity in Large-Area Floating Films of Semiconducting Polymers for Their Application in Flexible Electronics

“…20,28 Charge transport anisotropy in edge-on oriented films generally lies in one order of magnitude due to the presence of conjugated backbones and π−π staking in-plane of the substrate compared to face-on-oriented films. 25,42,43 In-plane charge transport property of the films was probed by fabricating OFETs in each region. Since the orientation direction in different regions was already known, the OFET's channel direction was tuned considering 0°in parallel to the orientation direction of the center region and was set as a reference in each region, as shown in Figure 6a.…”

“…GIXD measurements were performed with a Rigaku smart lab X-ray diffractometer with Cu Kα radiation (1.5418 Å) operated at 45 kV (200 mA), as reported in the literature. , A UV–vis–NIR spectrophotometer (JASCO V-770DS) equipped with a polarizer was used to measure the polarized absorption spectra. Thickness measurement and surface topography were measured via an atomic force microscope (AFM) using an SPM-9600 Shimadzu, Japan.…”

“…On the other hand, methods based on mechanical shear forces cause mechanical damages to the underlying layers. Generally, the orientation techniques involving mechanical shear force are highly sensitive to the substrates’ hydrophobicity and are not suitable for versatile flexible substrates. − Moreover, precise control of polymer orientation on a large-area substrate is also challenging and rather reported for small-area substrates of 1 cm 2 . Layer-by-layer transfer of large-area prefabricated films not only provides the opportunity to make complex heterostructures but also offers the separate control of the thin-film morphology independent of the substrate’s surface morphology .…”

Conjugated Polymer Mesocrystals with Structural and Optoelectronic Coherence and Anisotropy in Three Dimensions

“…Conjugated polymers are an interesting class of materials due to their versatile and readily tunable semiconducting properties that have made them promising candidate materials for use in flexible electronics, [1][2][3] large-area and/or semitransparent solar cells, [4,5] novel room-temperature thermoelectric generators, [6,7] and soft bioelectronics systems. [8][9][10] While to date, many applications rely on thin, often ultrathin, films of 200 nm or less, increasingly, demand arises for molecularly ordered bulk structures, both for fundamental studies, e.g., to understand the possible anisotropies in properties due to the macromolecular nature of polymer semiconductors, to emerging applications such as thermoelectrics, [11] sensors, [12][13][14][15] and bio-integrated devices. [16,17] As it is generally challenging to grow single crystals of polymeric materials, and polycrystalline/ semicrystalline thin-film structures typically provide only reliable comparisons of properties in maximum two dimensions Semiconducting mesocrystalline bulk polymer specimens that exhibit nearintrinsic properties using channel-die pressing are demonstrated.…”

Solvent-Assisted Friction Transfer Method for Fabricating Large-Area Thin Films of Semiconducting Polymers with Edge-On Oriented Extended Backbones

“…Such high μ in OFETs have been achieved either via vacuum deposition or by producing single crystals via solution processing; however, nonuniformity in the film morphology leading to device-to-device variation is a big challenge. , On the other hand, CPs offer better solution rheology concomitantly high film uniformity over large areas with high reproducibility . Orienting CPs along the channel direction to increase the charge transport in OFETs has been well documented adopting different methods with major improvements in the past 1 decade reported by various research groups. , In addition to this, the crucial role of macromolecular conformation of CPs in thin films and quality of the semiconductor/dielectric interface on charge transport properties of OFETs so far have also been investigated extensively by different groups. , Typically, to demonstrate the state-of-the-art μ of OFETs, the SiO 2 interface is subjected to hydrophobic treatment, which is an effective approach to enhance the μ by few orders of magnitude, and the same has been reported for CPs. − Such increment in μ is generally ascribed to improvement in film crystallinity; − moreover, it is also reported that the charge traps at the dielectric/semiconductor interface decreases upon hydrophobic treatment of the SiO 2 surface …”