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A fast meniscus-guided coating for ultrahighly oriented thin films of a typical donor-acceptor π-conjugated polymer, poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole

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.

An ordered arrangement of electron-accepting molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), in three-dimensionally (3D) oriented poly(3-hexylthiophene) (P3HT) film was clarified. The 3D oriented P3HT thin films prepared by the friction-transfer technique were doped with F4TCNQ by dipping into an acetonitrile solution. The presence of F4TCNQ anions in the 3D oriented P3HT thin films was investigated by polarized ultraviolet/visible/near-infrared absorption spectroscopy, grazing incidence X-ray diffractometry, polarized Fourier transform infrared spectroscopy (FT-IR), and infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS). The F4TCNQ-doped 3D oriented P3HT films showed anisotropic properties in all characterizations. In particular, the anisotropic molecular vibrations from polarized FT-IR and pMAIRS have clearly revealed orientations of polymeric chains and molecular dopant molecules. Considering the results from several independent techniques indicated that F4TCNQ anions in the 3D oriented P3HT were orderly arranged in a 3D manner with respect to the 3D oriented P3HT such that their molecular long-axis parallel to the P3HT backbone, with in-plane molecular orientation. Additionally, the direction of the optical transition moment of the F4TCNQ anion was found to be parallel to the molecular short-axis.

An effective approach for tuning the electronic characteristics of polymer-based organic field-effect transistors (OFETs) consisted of poly(3-hexylthiophene) (P3HT) doped with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and depleting layer formed by aluminum (Al) coating is being reported. Doping of P3HT was employed to accumulate and tune the carrier concentration while implementation of depletion layer led to reduce the channel conductance. It has been demonstrated that on-currents in the OFETs were enhanced with the extent of TCNQ doping from 0% to 20% in the P3HT. Upon introduction of an ultra-thin Al led to the formation of depletion region resulting in to further decrease in the off-state current along with the improvement in the subthreshold characteristics. These two procedures provide counter-effects in terms of the on-state characteristics by doping and the off-state characteristics by depletion, which promotes switching performance in OFETs indicating the potentiality of doping technology for organic 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.

Considering the advantages of floating film transfer method (FTM), we have investigated the optical and electronic characteristics of PBTTT-C14 thin-films prepared by the static and the dynamic casting on liquid substrate. It has been demonstrated that judicious selection of solvents during FTM switches the casting mode from the static casting (S-FTM) using high boiling point solvent to the dynamic casting (D-FTM) from low boiling point solvent. Although both of the methods provide the edge-on oriented structure of PBTTT-C14 by XRD, the structural and the optical analyses reveal relatively extended π-conjugation length in parallel D-FTM film as compared to that of S-FTM. A high field-effect mobility (μ) of 0.11 cm2/V.s was exhibited by OFETs fabricated by parallel D-FTM film even without any high temperature post-annealing up to the liquid crystalline phase transition. This observed value of μ for parallel D-FTM is 4.7 and 12.8 times higher than the isotropic S-FTM and the perpendicular D-FTM films, respectively.

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.

Large area (>20 cm × 2 cm)-oriented thin films of PQT-C12 with varying molecular weight and polydispersity index (PDI) were fabricated by the ribbon-shaped floating film transfer method aiming toward their application as an active semiconductor element of organic field effect transistors (OFETs). Investigation on the influence of the molecular weight and PDI upon the extent of molecular alignment and anisotropic charge transport was systematically carried out. It has been demonstrated that high molecular weight in combination with low PDI not only leads to a very high optical anisotropy >10 but also high charge carrier anisotropy with a hole mobility of about 0.07 cm2/V·s for OFETs using parallel-oriented PQT-C12 thin films. Such a structure–property correlation is highly beneficial for the development of high performance organic electronic devices by synergistic and amicable tuning of the optoelectronic anisotropies and polymer synthetic variables.

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.

Fabrication of large-area thin films and their swift characterization is highly desired for the commercial aspects of the organic electronic devices. Although a number of techniques have been proposed for the large-area thin film fabrication, the microstructural variation is still an issue. In this work, the facile characterization of distribution in thickness and macromolecular ordering in the drop-casted and spin-coated poly(3-hexylthiophene) thin films have been performed through newly developed 2D positional mapping technique. Through this technique, a comparative analysis of the absorption spectra in terms of peak absorbance and excitonic bandwidth (W) was performed for the whole sample, the lower value of W observed for the drop-casted thin film depicts its molecular ordering. Moreover, the difference in W (≈ 10 meV) was noticed between two points (≈1 mm apart) on the drop-casted film, which corresponds to the difference in microstructural ordering and it is complicated to characterize through the conventional spectroscopic techniques.

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.

A synergistic approach to enhance charge-carrier transport in organic semiconductors along with facile solution processing and high performance is crucial for the advancement of organic electronics. The floating film transfer method (FTM) is used as a facile and cost-effective method for the fabrication of large-scale, uniform, highly oriented poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT C-14) films under ambient conditions. Utilization of such oriented films as the active semiconducting layer in organic field-effect transistors (OFETs) results in highly anisotropic charge-carrier transport. Highly oriented, FTM-processed pBTTT C-14 thin films are characterized by polarized electronic absorption and Raman spectroscopy, atomic force microscopy, out-of-plane X-ray diffraction, and grazing incident X-ray diffraction (GIXD) measurements. The GIXD data indicate an edge-on orientation, which is highly desirable for planar devices such as OFETs. OFETs built using the oriented films show a mobility anisotropy of 10 and the highest mobility is 1.24 cm2 V−1 s−1 along the backbone orientation, which is among the highest value reported for this class of materials using a similar device configuration.

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.

Detailed structures of friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films have been studied using synchrotron-sourced grazing-incidence X-ray diffraction (GIXD). The results of in-plane GIXD were in full agreement with those of electron diffraction in our previous study. We also revealed the structures of liquid-crystalline (LC) and crystalline films that were prepared by rapid and slow cooling from the LC melted state of the friction-transferred films, respectively. For the crystalline films, the results of in-plane GIXD were not in full agreement with those of electron diffraction; additional weak peaks of the odd 00l reflections were observed only in the GIXD profiles. This implies that the twofold screw symmetry along the c-axis of PFO is broken. The rocking profiles for each reflection gave the order parameter of almost one for the crystalline films and 0.67 for the LC films. These results suggest the ability of friction transfer for obtaining single-crystal-like thin films and the importance of carefully designed annealing process for the applications of the films to various optoelectronic devices.

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 report C60 thin-film transistor characteristics of top-contact structure with low work-function source and drain electrodes. The electron mobility of the Mg electrode device is one order of magnitude higher than that of the Ag electrode device. The depth profile obtained by using secondary-ion mass spectroscopy demonstrates that Mg atoms strongly diffuse into C60 film during Mg deposition. These findings indicate that the improved mobility is due to the reduction of the parasitic resistance under source and drain electrodes by the Mg doping effect.

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.