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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 (MW) polymers is hampered by chain folding or entanglements, which limit their macromolecular orientation. Interestingly, utilizing high-molecular-weight PBTTT (MW > 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 SiO2 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 SiO2 surfaces, respectively, after annealing them beyond liquid crystalline phase transition temperature, as revealed by in-plane and out-of-plane XRD results. On bare SiO2, optimum μ up to 0.03 cm2 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 cm2 V-1 s-1 on SAM-treated SiO2 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.

Improved film's crystallinity and facile charge transport has been well demonstrated by orienting conjugated polymers using solution based thin film fabrication followed by thermal annealing, need for huge halogenated solvents and high temperature annealing are stumbling block for industrially viable large area thin film fabrication. Herein, we report the fabrication of oriented thin films regioregular P3HT by solvent-free friction transfer (FT) technique and enhancement its further enhancement by adding NIR sensitive squaraine dyes. Apart from assisting the molecular orientation of P3HT, it has been demonstrated that dye molecules are also get oriented and a synergistic match between the alkyl chains of the polymer and dye molecule leads to promotion for the extent of the molecular orientation in the dye molecules.

Solution processable conjugated polymers (CPs) possess unique molecular structures in terms of extended π-conjugation in combination with quasi 1-dimensionality, imparting them with an inherent tendency for molecular self-assembly and ultimately determining their optoelectronic properties. Short-range molecular ordering and long-range macroscopic orientation are two different but interdependent phenomena, which should be optimized depending on the nature of electronic devices (planar versus vertical). Judicious control of the macromolecular orientations and conformation-assisted morphologies of CPs are key for determining and controlling the anisotropic charge transport in organic field effect transistors (OFETs). The present review focuses on different solution-based approaches for thin film fabrication developed in the last two decades aimed towards controlling the film morphologies and macroscopic orientations of CPs. It summarizes the information for understanding the various techniques utilized for controlling the molecular ordering and macroscopic orientation in CP thin films in order to correlate device performance and anisotropic charge transport in OFETs. Systematic organization of topics and critical analysis of progress and improvisations are included, along with current state-of-the-art approaches for each technique being discussed in detail. Apart from commentary on the pros and cons of various techniques, this review summarizes the future directions necessary for further development in the area of next-generation flexible electronics.

Harnessing the full potential of solution processable conjugated polymers (CPs) as active semiconductor elements lies in the facile thin film fabrication along with amicable control of molecular self-assembly and orientation. Probing the nature and uniformity of thin films are inevitable for fabrication of devices with high reproducibility. Herein, a new method for the fast and facile profiling of thickness and molecular orientation of large area thin films is being reported. Thin films of PBTTT-C14 fabricated by three different methods like floating film transfer method (FTM), friction transfer and spin coating were subjected to profiling of thickness and molecular orientation using 2D positional mapping system followed by fabrication of organic thin film transistors. In order to prove applicability of the mapping system on other CPs, oriented films of PQT-C12 were also prepared by FTM and optical anisotropy estimated by the mapping system (23.0) and conventional spectrophotometer (22.4) validates the performance of our positional mapping system. Spin-coated thin films of PBTTT-C14 subjected to positional profiling of the film uniformity revealed that films are non-uniform and there was a gradual increase in the thickness from center to the periphery.

Facile fabrication of large area [20 cm (L) × 2 cm (W)] uniform and oriented thin films of a variety of conjugated polymers (CPs) with minimal material wastage has been successfully demonstrated using our newly developed ribbon-shaped floating film transfer method (FTM). Under identical film fabrication conditions of FTM like hydrophilic liquid substrate consisted of ethylene glycol and glycerol (3:1), the temperature of 60 °C and polymer concentration 1% (w/w) in chloroform, order of molecular orientation was found to be PQT-C12 > F8T2 > NR-P3HT > PBTTT-C14 > PTB7. Depending on the nature of polymeric backbone, CPs exhibited different optical anisotropies in their oriented thin films, which was explained in terms of nature and rigidity of polymeric chains in the light of persistent length. Organic field effect transistors fabricated using these CPs exhibited clear p-type behaviour and anisotropic charge transport. Amongst various CPs used, PQT-C12 not only exhibited the highest optical anisotropy (DR = 5.1) but also highest anisotropic charge transport (μ||/μ⊥ = 7.1). Ribbon-shaped thin films of PQT-C12 prepared by slider based FTM exhibited best device performance with charge carrier mobility of 5.0 × 10−2 cm2/V and ON/OFF ratio of 106, when films were oriented parallel to the channel direction. Interestingly, a mobility of parallel oriented thin films prepared by FTM was about two orders of magnitude higher compared to the OFETs fabricated using conventional spin coating (6.7× 10−4 cm2/V).

Recent past has witnessed huge scientific efforts aiming toward enhancing in-plane charge transport by unidirectional orientation of conjugated polymer (CP) backbones adopting various techniques. However, in most of the existing methods, excess amounts of toxic halogenated solvents and preaggregation in solution are inevitable, which are the main bottlenecks toward large-scale fabrication. Solvent-assisted friction transfer (SAFT) is being reported as a novel method and improvisation over conventional friction transfer to expand its versatility. In this method, application of a small amount of the solvent (∼3 μL) during drawing not only leads to the entirely changed film morphology and molecular orientation but also addresses the existing substrate compatibility issues. Utilizing poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]-thiophene] as a representative CP under SAFT technique, films with extended backbone and edge-on orientation was successfully fabricated, which was confirmed by various characterization tools such as X-ray diffraction, polarized absorption, and polarized Raman spectroscopies. Further, anisotropic charge transport in these films was investigated by fabricating organic field-effect transistors and the role of contact resistance was also studied. Slight solvent use, compatibility with various substrates, and film fabrication with controlled orientation, and after validation of its generality on different CPs, SAFT can be expected to open new avenues in the area of printed electronics.

Dynamic casting of floating-film and transferring method for preparing oriented multilayer films of conjugated polymers is reported. This method is based on dynamic casting of a floating-film on liquid substrate to obtain oriented thin-film followed by its transfer on a desired solid substrate. The uniqueness in this method lies in the isolation of casting and coating procedures of oriented films, which enables us to fabricate the oriented multilayer with minimum interlayer interference. It provides a key-technology to build up the organic multi-layered architecture while preserving oriented morphologies. Several types of multilayer films have been prepared and investigated in detail in terms of their film characteristics. The layer-by-layer coating of oriented films demonstrated by this method is found to be a unique feature which overcomes cumbersome procedures in the conventional orientation methods. The coating procedure demonstrated in this study provides a facile methodology to construct anisotropic architectures.

New and simple polyaromatic compounds containing two thiophene rings were prepared via photo-cyclization and their structural and photophysical properties were evaluated via 1 H NMR spectroscopy and X-ray crystallographic analysis. On the basis of X-ray analysis, it was determined that the molecular structure of the compound was highly strained and that they contain two hetero [4] helicene moieties. The compounds were investigated as active layer in p-type organic field-effect transistors ( p -OFET) in top contact type devices. Notably, the compound containing two thiophene components exhibited very stable p-type semiconducting behavior in moist air.

Dynamic casting of conjugated polymer films on liquid-substrate is a unique method which provides thin floating-film and can be easily transferred on a desired substrate by stamping. The important feature in this procedure is associated with the formation of thin polymeric film during compression and solidification controlled by viscous drag of liquid substrate and solvent evaporation of the polymer, respectively. Lyotropic liquid-crystalline (LC) characteristics of conjugated polymer possibly assist to orient the polymer chain in one direction. It is found that this method produce highly oriented thin films (dichroic ratio > 5) of thiophene-based conjugated polymers such as PBTTT-C14, PQT-C12 and non-regiocontrolled poly(3-hexylthiophene) NR-P3HT. On the other hand, weak orientation intensity in regioregular poly(3-hexylthiophene) RR-P3HT was found. The mechanism for this diverse orientation in thiophene-based conjugated polymers is discussed in correlation with the backbone chemical structure and lyotropic LC phase transition during the floating-film formation.

A novel film preparation method utilizing polymer suspension, entitled adsorbing deposition in suspensions (ADS), has been proposed. The poly(3-hexylthiophene) (P3HT) toluene solution forms P3HT nanofibrils dispersed suspension by aging. P3HT nanofibrils are highly crystallized with sharp vibronic absorption spectra. By the ADS method, only P3HT nanofibrils in suspension can be deposited on the substrate surface without any disordered fraction from the dissolved P3HT in suspension. Formed ADS film contains only the nanostructured conjugated polymer. Fabricated polymer thin-film transistor (TFT) utilizing ADS P3HT film shows good TFT performances with low off current, narrow subthreshold swing and large on/off current ratio.

A newly developed floating film transfer method (FTM) has been successfully utilized to fabricate oriented thin films of non-regiocontrolled poly (3-hexyl thiophene) (NR-P3HT) followed by the fabrication of organic field effect transistors (OFETs). AFM microstructural investigations demonstrate the facile molecular alignment of NR-P3HT by FTM leading to highly oriented macromolecular assemblies like fibrous domains with considerably enhanced π-conjugation length. FTM thin films of NR-P3HT not only show enhanced optical anisotropy (dichroic ratio >8) but also significantly improved FET characteristics. FTM films in its parallel orientation exhibited a significant improvement (>2 orders) in the FET mobility as compared to its spin-coated device counterparts.

Introduction of alkyl chains is a plausible approach to impart the facile solution processability in regioregular (poly-3-alkylthiophenes) (RR-P3ATS) but drastic fall in the conductivity as a function of increasing alkyl chain length restricted the versatile use of its hexyl substituted derivative (P3HT). In this work, fabrication of large area oriented thin films of RR-P3ATs with varying alkyl chain lengths was successfully demonstrated using ribbon shaped floating film transfer method (FTM). Observed decrease in the molecular orientation with increasing alkyl chain length was explained by enhanced alkyl chain inter-digitation as evidenced by trends of d-spacing estimated from the out-of-plane XRD results. In-plane GIXD revealed edge-on orientation in the thin films of all of the P3ATs prepared by ribbon-shaped FTM, which is highly desired for planer charge carrier transport. Parallel oriented thin film of longest alkyl chain (octadecyl) substituted P3AT poly (3-octadecylthiophene) exhibited a field effect transistor (FET) mobility of 1.9 × 10−2 cm2/V, which is about 4–5 orders of magnitude higher as compared its previously reported values estimated by time-of-flight and FET methods. Thus, fabrication of thin films by FTM leads to versatile choice of RR-P3ATs for solution processing for FETs using common organic solvents without having any detrimental impact of alkyl chain length on the charge carrier mobility.

The exact molecular structure and the crystal packing of the n-type semiconducting material 3′,3′-(1,4-phenylene)bis{2′-(4′′-trifluoromethyl)phenyl}acrylonitrile was determined by a single crystal X-ray diffraction with twin treatment technique. The air-stable product was crystallized from dichloromethane-hexane mixed solution. The solid-state structure is the example of a typical π-π stacking with side intermolecular CN–H short contact networks.

A long-term air-stable n-type organic semiconductor, 2,5-difluoro-1,4-phenylene-bis{2-[4-(trifluoromethyl)phenyl]acrylonitrile}, was synthesized by a high-yield simple procedure of Knoevenagel condensation with aldehyde and acetonitrile derivatives. A fabricated organic thin-film transistor (OTFT) using this compound exhibited good n-channel OTFT properties with a high electron mobility of 0.17 cm(2) V(-1) s(-1) and an on/off current ratio of 10(6) under both vacuum and ambient air operation. After storage in ambient air for 1 year, a stored n-channel OTFT still shows good n-channel OTFT performance with little degradation in ambient air operation.

Considering the pivotal role of interfaces in controlling the performance of organic electronic devices, implications of metal/organic interfacial quality in a Schottky barrier diode (SBD) are investigated. The nature of metal/organic interfaces and the thin film quality of regioregular poly (3-hexylthiophene) based SBDs fabricated in different device architectures are investigated using experimental and theoretical modeling. The importance of oxidized aluminum nanostructures as an interlayer at the Schottky interface for the dramatic enhancement of the rectification ratio (>106 at ±5 V) has been demonstrated, which is attributed to suppressed leakage current due to the oxide layer and the formation of a charge double layer. Furthermore, electrical performances of all the SBDs were modeled in terms of an underlying particular phenomenon solely or with the combination of multiple physical phenomena. The combined modeling equation used in this work fits well for the different device architectures, which validates its generality in order to extract the device parameters.

Large-scale drop-cast films of π-conjugated polymers such as polythiophene derivatives and polyfluorene-based copolymers have been formed on a liquid-substrate with one-step deposition of Floating-film Transfer Method (FTM). Centimeter-scale uniform cast films could be obtained. Relative sharp vibronic structure was found in the Absorption spectra of FTM films as compared to those in spin-coated ones. FTM provides concentrically oriented films of a polymer having thermotropic liquid crystal characteristics. Comparative study of AFM topography with absorption spectra revealed that the origin of the optical anisotropic absorption was in the uniaxial orientation of nano-scale fibrils. This indicates that the oriented FTM films possess extremely large domains extended over 5 orders from micrometer to 10 centimeter scale. The findings represent that FTM is a simple and effective casting method to provide large-scale and uniform drop-cast films.

Fabrication of large area and uniform thin films with preferred edge-on orientation is highly demanded for flexible organic electronic devices. In this work, utilization of a newly designed slider in the floating film transfer method to assist unidirectional film spreading led to the fabrication of uniform and macroscopically edge-on oriented ribbon-shaped floating films (15 cm × 2 cm) having very large optical anisotropy (>20). The results pertaining to the in-plane grazing angle incident X-ray diffraction (GIXD) and φ-scan measurements revealed the well-stacked polymer backbone conformation with edge-on orientation without any inter-mixing of face-on oriented counterparts. Optical anisotropy dependent charge transport in organic field effect transistors using anisotropic poly(3,3‴-didodecyl-quaterthiophene) films has also been shown, and the results are supported by polarized electronic absorption and GIXD measurements.

Oriented thin films of regioregular-poly(3-butylthiophene) (P3BT) were prepared by friction transfer method. Molecular orientation of P3BT in the friction-transferred films was investigated with grazing-incidence X-ray diffraction (GIXD), polarized ultraviolet–visible (UV) spectra, and infrared (IR) spectra. Polarized UV spectra showed the polymer backbones were aligned in the friction direction. GIXD measurements suggest that a axis in P3BT film is normal to the substrate, that is, π-stacking direction tends to be parallel to the substrate. This orientation is different from those of friction-transferred poly(3-hexylthiophene) and poly(3-dodecylthiophene).

Multilayered oriented polyfluorene (PF) films were obtained by applying thermal treatment procedure to a multilayered PF film constructed with fluorene derivatives layer formed on top of a highly oriented friction-transferred crystalline poly(9,9-dioctylfluorene) (PF8) film. The orientations in the multilayered PF films were investigated by polarized photoluminescence (PL) spectroscopy and grazing incident X-ray diffraction (GIXD) analysis. The results of the multilayered PF film constructed with spin-coated PF8 on friction-transferred PF8 indicate that the rearrangement of the upper PF8 layer is induced from the orientation of lower PF8 layer by thermal treatment at the nematic phase temperature. Polarized green emission from the multilayered oriented PF film was demonstrated using the blend of PF8 and poly(9,9-dioctylfluorene-co-benzothiadiazol) (F8BT) as green light emitter for upper layer. By this method, the polarized emission color can be tuned using polymer blends for upper layer similar to the liquid-crystalline polymer arrangement without using different materials as an underlying layer such as the rubbed polyimide.

Effects of substituted side-chains of poly(2,5-dialkoxy-1,4-phenylenevinylene) derivatives (PPV) on polymer orientation have been studied using polarized optical spectroscopy and synchrotron-sourced grazing incidence X-ray diffraction (GIXD) analysis. The results of polarized optical spectroscopy indicate that the symmetrically substituted PPV is suitable for homogeneous oriented film preparation by friction-transfer technique. The formed film of poly(2,5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (BEHPPV) exhibited large light emission dichroism of up to 42.0. The results of synchrotron-sourced GIXD for BEHPPV film indicated that the BEHPPV molecules crystallized in not a different manner from that of other PPV.

The relationship between molecular structure, molecular packing in bulk, film structure, and electrical characteristics were investigated for the unsubstituted oligothiophene pentamer and hexamer, both of which have terminal thiophene units linking at the β-position (βnT; n = 5, 6). To reveal the molecular geometry and molecular packing of βnTs, X-ray single-crystal analysis was carried out. For the electrical characterization, organic field-effect transistors (OFETs) based on the βnT thin films were fabricated at various substrate temperatures. Film structure was investigated by X-ray diffraction and atomic force microscopy. Highest field-effect mobility of OFET was 0.050 cm2 V-1 s-1 for β6T at a substrate temperature of 120 °C and 0.020 cm2 V-1 s-1 for β5T at a substrate temperature of 90 °C, which are slightly smaller than those of α-oligothiophene isomers with the same number of thiophene rings. β6T OFET characteristics depend on the film morphology such as grain size, while the dependence of β5T OFET...

Poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOPPV) was found to form a highly oriented film by a friction-transfer technique. Structural investigation of friction-transferred DOPPV was studied by means of polarized ultraviolet-visible (UV-vis) absorption spectroscopy, polarized photoluminescence (PL) spectroscopy, and synchrotron-sourced grazing incident X-ray diffraction (GIXD) analysis. The polarized UV-vis absorption and PL spectra indicate clear axial alignment. DOPPV backbones in friction-transferred film are highly aligned along the drawing direction of the friction-transfer. Further information of the molecular arrangement in friction-transferred DOPPV film was investigated by both the out-of-plane and the in-plane GIXD analyses with synchrotron source. The DOPPV molecules in friction-transferred films were perfectly arranged three-dimensionally: the backbones aligned along the drawing direction of friction-transfer, the alkyl side chains lay in the film plane, and the planar backbones were arranged parallel to the film surface. Additionally, two neighboring DOPPV molecules along the direction of inter-backbones separation by alkyl side chains were found to be shifted with respect to one another by the mean distance of half of a monomeric repeat.

We investigate the correlation of the number of thiophene units with the structural order and carrier mobility of the films through studies on thin-film transistors (TFTs) based on alpha-quinquethiophene (5T), alpha-sexithiophene (6T), and alpha-septithiophene (7T). The X-ray diffraction (XRD) data of the nT films deposited at low substrate temperatures present obviously different structural orders depending on the parity of the number of thiophene units. Although even-numbered nT films present well-ordered structures and large carrier mobilities, odd-numbered nT films present two different crystalline polymorphs and vastly low carrier mobilities reflecting the coexistence of two crystalline polymorphs. However, the XRD data of both even- and odd-numbered nT films deposited at high substrate temperatures indicate that the nT molecules form single well-ordered structures. Those ordered TFTs exhibit large carrier mobilities accompanying an increase in the number of thiophene units, 0.05, 0.08, and 0.13 cm2 V(-1) s(-1) for 5T, 6T, and 7T, respectively. The parity of the number of thiophene units affects the structural order intrinsically in grown thin films, and affects carrier mobilities extrinsically in their TFTs.

A liquid-crystalline polymer, poly(9,9-dioctylfluorene) (PFO), was found to form highly oriented films by a friction-transfer technique. The polarized UV−vis absorption and photoluminescence spectra of the films formed showed strong dichroism with a dichroic ratio of approximately 10 in the drawing direction of friction transfer. Subsequent thermal treatments of the friction-transferred PFO films were specifically effective for the improvement of the physical properties, structure, and morphology of the films. By rapid and slow cooling from the liquid-crystalline melted states of the friction-transferred films, liquid-crystalline and crystalline films were prepared, respectively. Both thermally treated films showed enhanced optical anisotropy with a dichroic ratio of approximately 25 in photoluminescence spectra. The electron diffraction pattern of the crystalline film showed a large number of sharp diffraction spots as seen in a single crystal. The liquid-crystalline polymer was found to form a single-cr...

The regioregular highly head-to-tail-coupled poly(3-alkylthiophene) (HT-P3AT) films were prepared by a friction-transfer technique. Polarized UV−vis absorption spectroscopy and grazing incidence X-ray diffraction (GIXD) were used to study the polymer molecular arrangement in the friction-transferred HT-P3AT films, and scanning electron microscopy (SEM) was used to observe the film surface morphology. The polarized UV−vis absorption spectra show a large dichroism regarding the drawing direction of friction-transfer. The order parameter is evaluated close to unity, which is the ideal orientation state. GIXD measurements show that alkyl side chains lie in the film plane, and the polymer backbones are well-ordered along the drawing direction of friction-transfer within 10° for poly(3-hexylthiophene) and 13° for poly(3-dodecylthiophene) in the film plane. The polymer backbones form a layered structure with the stacking of thiophene rings normal to the film surface. The polymer molecules in the friction-transfe...

We have developed a new method to obtain Ribbon-shaped floating films via dynamic casting of floating-film and transfer method (dynamic-FTM). Dynamic-FTM is a unique method to prepare oriented thin-film of conjugated polymers (CPs) which is quick and easy. This method has several advantages as compared to the other conventional casting procedure to prepare oriented CP films. In the conventional dynamic FTM appearance of large scale circular orientation poses difficulty not only for practical applications but also hinders the detailed analysis of the orientation mechanism. In this present work, pros and cons of this newly proposed ribbon-shaped floating-film have been discussed in detail from those of the conventional floating-film prepared by dynamic-FTM., 12th International Conference on Nanomolecular Electronics (ICNME-2016), December 14-16, 2016, Kobe International Conference Center, Kobe, Japan

We demonstrated the polymer field-effect transistors (FETs) utilizing regioregular poly(3-alkylthiophene)s (P3AT) films prepared by a drawing method. The P3AT film exhibited large optical dichroic ratio, which originated in the polymer backbones aligned to the drawing direction. In-plane anisotropy and enhancement of FET characteristics have been observed that are caused by molecular alignment. In the case of poly(3-dodecylthiophene), the hole mobility along the drawing direction was enhanced by a factor of 25 compared with that of spin-coated film.

Orientation characteristics of non-regiocontrolled poly (3-hexylthiophene) (NR-P3HT) films prepared by dynamic casting of floating film and transferring method (FTM) has been investigated. The film was first cast on liquid-substrate to obtain as a floating-film followed by its transfer on solid-substrate such as white-glass or Si-wafer in order to evaluate their optoelectronic characteristics. As a possible key-factor to generate the orientation of conjugated polymer in this method we focused on the components of liquid-substrate in this study. The orientation dependence upon various liquid-substrates reveals that dichroic ratio strongly changes with liquid-substrates. Pictures of floating-film show the change in size of floating-parts depending upon the liquid-substrate, representing the expansion length of casting solution upon the viscosity. These findings have indicated that spreading speed of polymer solution and solvent evaporation speed controls the size of floating-film leading to change in the orientation intensity. The multilayer coatings of oriented NR-P3HT films were used for polarized FTIR analysis exhibiting clear dichroism. The obtained dichroic characteristics were well corresponded with in-plane, out-of-plane and non-oriented vibronic modes of P3HT., India-Japan Expert Group Meeting on Biomolecular Electronics & Organic Nanotechnology for Environment Preservation (IJEGMBE 2015), December 23-26, 2015, Fukuoka, Japan

Organic field-effect transistors (OFETs) were prepared from an epitaxially grown film fabricated by a wet-transferring process. 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) was grown by thermal evaporation on the (001) surface of potassium bromide (KBr) single crystals. When the film was grown at room temperature, the planar molecules were aligned orthogonally on the crystal surfaces along the [110] direction with edge-on orientation to the surface normal direction. The epitaxy film was transferred to on SiO2/Si surface immediately after removing the KBr on the water surface to product the OFETs. The calculated mu(FET) of the OFET for the wet-transferred vertically aligned film were 1.3x10(-4) and 2.2x10(-4) cm(2) V-1 s(-1) at the linear and saturation regions, respectively, at V-g=-50 V at an I-ON/I-OFF (on/off ratios of source-drain current) of 10(4)similar to10(5). (C) 2003 American Institute of Physics.

Synthesis and electron transport characteristics of p-type organic semiconductor (III) are reported.

Introduction of branched alkyl chains at the terminal positions of sexithiophene drastically improves the solubility of the compound, while still maintaining satisfactory charge carrier mobility. F...

Ordering and alignment of π-conjugated polymer chains are highly desirable for high performance and long life organic electronic devices. We report here self assembly of ordered and aligned solution processible Poly(3,3″′-dialkylquaterthiophene) (PQT-12) polymer at macroscopic level using Floating Film Transfer Method (FTM). PQT-12 polymer film is formed over solution of ethylene glycol and glycerol at different temperatures viz. 22, 26, 33, and 38 °C. PQT-12 films formed by FTM technique are further characterized for optical and morphological properties. UV-vis absorption (for polarize and unpolarize light) and surface topography/phase imaging are carried out by using UV-vis spectrometer and atomic force microscope (AFM), respectively. UV-vis spectra show the polymer chains alignment perpendicular to the film propagation direction and it is well supported by AFM images. The effect of temperature on ordering and alignment of PQT-12 shows 33 °C as an optimum temperature for alignment of polymer chains (a little compromise in ordering). The heating of polymer films at 110 °C for 2 h in ambient causes significant changes in UV-vis absorbance spectra, optical anisotropy, and AFM topography/phase imaging. Our studies provide better understanding of ordering and alignment of PQT-12 chains and also disordering on heating. This work further provides a facile and user-friendly technique for the long range ordered self assembly of PQT-12, which shows enormous potential for various electronic applications.

The application of molecular alignment to conjugated polymers offers a unique approach for constructing organic semiconductor devices. We made polymer thin film field-effect transistors (FET) by friction-transfer technique, based on regioregular poly(3-alkylthiophene) (P3AT). Friction-transferred films of P3AT were found to align approximately all of the polymer main-chains within a 9-13° range along the friction direction. When introduced in FET devices, the mobility anisotropies of 5-20 for current flow parallel and orthogonal to the friction direction has been observed. In addition, when compared with conventional isotropic FET devices of non-aligned P3AT films, they were found to enhance the mobility by a factor of 2-70.

Oriented layers of conjugated-polymers. poly(p-phenylene vinylene) and pol(3-dodecylthiophene). were fabricated on titania films by friction transfer method. The polymer chains were aligned along the friction direction in the substrate plane. the photoelectric properties of the polymer/titania were measured under the polarized light. Photocurrent was enhanced under the polarized light with the electric vector parallel to the chain direction than orthogonal-polarized light. This device can be used as a polarized light detector.

We investigated electronic effects of an ultrathin LiF interlayer on device properties of copper(II) perfluorophthalocyanine (F 16 CuPc)-based field effect transistors (FETs) with Al source and drain electrodes. We observed FET operation only for the device with LiF/Al bilayer electrodes. We concluded that an effective electronic contact was formed between Al and F 16 CuPc layer by the presence of the LiF layer, which prevented the F 16 CuPc layer from reacting with Al and n-doped with dissociated Li atoms in the vicinity of the source and drain electrodes.

Uniaxially self-oriented centimeter-scale thin films of poly(9,9'-dioctylfluorene) (PFO) could be obtained with a one-step deposition by the floating-film transfer method (FTM) under ambient conditions. β-phase PFO was also generated along with the orientation direction during the film formation by FTM. Polarized β-phase electroluminescence having a maximum dichroic ratio of 7.7 was realized in bilayer polymer light-emitting diodes consisting of a self-oriented PFO film as the emission layer and a poly(9-vinylcarbazole) film as the hole-transporting and electron-blocking layer.

We demonstrated the fabrication of self-aligned poly(9,9'-dioctyl-fluorene-alt-bithiophene) copolymer (F8T2) thin films at ambient temperature with a new solution-process technique named floating film transfer method (FTM). Atomic force microscope topography and polarized absorption spectroscopy showed that the polymer main chains aligned perpendicularly to the film propagation direction during the fabrication process. FTM films presented absorption dichroic ratios slightly below 3. Top-contact/bottom-gate field effect transistors made with FTM films exhibited anisotropic transport properties with a hole mobility along the aligned direction of F8T2 main chains of 2.2×10-3 cm2/(V·s), which was around 2.5 times greater than that along the perpendicular direction. Dichroic and transport anisotropy ratios were further enhanced up to 7–8 by thermal annealing, although the mobility improvement remained limited due to possible trapping effect at domain boundaries.

Photocarrier transport in soluble poly(3-alkylthiophenes)(PATs) has been studied by means of time of flight method. Temperature and field dependencies of mobility were measured for a number of polythiophene derivatives having various alkyl chain lengths from butyl to octadecyl. It has been found that regioregularity has a marked effect on hole mobility in PATs with shorter alkyl group. The results of the mobility in PAT films are discussed in terms of main chain regularity and alkyl chain lengths.

The carrier transport was investigated depending upon the processes of film preparation in regioregular poly(3-alkylthiophene)s (PATs). The relatively non-dispersive characteristics of photocarrier transport were obtained in drop-cast film. In spin-coated film the transient photocurrent was strong dispersive in contrast. The dispersivity was also slightly increased with the alkyl chain length. The X-ray profiles showed the large crystallinity and the absorption spectra showed the red-shift of peaks in drop-cast film. These results indicate that the dispersivity of carrier transport in PATs is strongly affected by the casting processes of PAT film.

Field-effect mobilities and threshold voltages in organic field-effect transistors (OFETs) were studied to elucidate the difference of the OFETs performance between Si/SiO2 and flexible polymer substrates. The polymer OFETs were built on poly(ethylene naphthalate) (PEN) film as the substrate with polyimide (PI) as the gate-insulators. Pentacene and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylicdiimide (PTCDI-C13) were employed as p- and n-type semiconductors. The n-type OFET on polymer substrates exhibited higher electron mobility and smaller threshold voltages compared with those on Si/SiO2. On the contrary, the p-type OFET on the Si/SiO2 substrate showed better transport performance than that on the polymer substrate. The results are discussed taking the difference of surface potential between SiO2 and PI gate-insulator, which possibly modifies OFET characteristics.

A novel n-type organic semiconductor, cyano-substituted distyrylbenzene derivative, 1,4-bis2-[4-(trifluoromethyl)phenyl]acrylonitorilebenzene, was synthesized by Knoevenagel condensation with aldehyde and acetonitrile derivatives. Fabricated thin-film transistors (TFTs) exhibited high electron field-effect mobility of 10-2–10-1 cm2 V-1 s-1, on/off current ratio of 6×105. Hysteresis-free n-type transport characteristics observed in this device promises a steady operation of organic logic circuit. Almost same TFT characteristic was observed even after 1 month storage in ambient condition. The findings indicate that the material has a good resistance to atmospheric oxidants.

Uniaxially oriented films of beta-phase poly(9,9-dioctylfluorene) (PFO) were realized by a friction-transfer technique followed by thermal annealing and vapor treatments. Absorption and photoluminescence (PL) spectra show the characteristics of beta-phase: an additional absorption peak at 433 nm and redshifted PL peaks compared with those of the usual nematic (N) phase. We fabricated polarized polymer light-emitting diodes utilizing oriented films of beta-phase PFO. Highly polarized beta-phase emission with an integrated polarization ratio of 51 was observed from the devices. The efficiency of the devices based on beta-phase reached 2.0 cd/A, which is two times higher than that based on N-phase.

Ambipolar organic field effect transistors (OFETs), consisting of a composite of polyhexylthiophene (PHT) and [6,6]-phenyl C61-butylic acid methyl ester (PCBM), was converted into a p- or n-type OFET by insertion of a thin tetracyanoquinodimethane (TCNQ) or tetrathiafluvalene (TTF) buffer layer. The interface in the Au/TCNQ/PHT:PCBM composite transports hole but blocks electron, while the transported carrier was switched to electron with insertion of a TTF layer. The selective transport is probably due to vacuum level matching or temporal doping. High impedance in a complementary metal-oxide-semiconductor inverter was demonstrated with unipolarized ambipolar FETs, resulting in a decrease in the through current.

Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF/Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300 cd/m2 was observed from the PLEDs.

We report the performance of solution-processed n-type organic thin-film transistors (OTFTs) based on C60 derivatives. Long-chain alkyl-substituted C60, C60-fused N-methylpyrrolidine-meta-C12 phenyl (C60MC12), was used as a semiconducting layer. The C60MC12-thin-film transistor shows high electron mobility of 0.067 cm2/V s in saturation regime. From the result of x-ray diffraction analysis, the C60MC12 active layer forms highly ordered crystalline film. We found that self-assemble ability of long alkyl chains plays an important role for fabrication of highly ordered crystalline film, leading to achievement of high electron mobility in solution-processed n-type OTFTs.

π-conjugated low-molecules having the chemical structure of bis(alkyl-thiophene)thienothiophene, abbreviated as, C6-BTTT and C12-BTTT were synthesized and evaluated in their thermal, optical and electronic characteristics. Compounds were synthesized according with Suzuki reaction or Stille reaction. Thermal analysis represents both of C6- and C12-BTTTs did not possess liquid-crystalline phase. Comparison of hole transport characteristics shows a relative high hole transport in C6-BTTT and high air-stability in C12-BTTT, representing a contribution of alkyl-substituent.

Carrier mobilities in poly(3-alkylthiophene) (PAT) films (alkyl = 6, 12 and 18), synthesized by chemical oxidation of 3-alkylthiophenes with FeCl3, have been studied using the time of flight (TOF) method. The response of photocurrents indicates the hole as the carrier sign and the highly dispersive transport particularly in the films substituted with long alkyl chains. Using regioregular poly(3-dodecylthiophene) (PAT12) and indium tin oxide (ITO) in the ITO/PAT12/Al sandwich cell, the depletion layer width at the Al/PAT12 interface was estimated to be 60 nm. It has been proposed that photocarriers are effectively generated at the interface and the high internal field helps in the separation of photocarriers. The drift mobilities in PAT12 and PAT18 are 8 ×10-6 cm2/V.s and 3 ×10-6 cm2/V.s, respectively, at the bias field, E = 1.0 MV/cm. They depend on the field as exp (βE 1/2/kT), indicating the Poole-Frenkel mechanism for the carrier transport. On the other hand, the mobility of PAT6 is 1.1 ×10-4 cm2/V.s at E = 0.1 MV/cm, shows very weak positive field dependence. The mechanisms of photocarrier generation and transport are discussed taking the polymer structure into consideration and are compared with the results of regioregular PAT films.