Floating Film Transfer Method Research Articles

Facile fabrication of large area oriented conjugated polymer films by ribbon-shaped FTM and its implication on anisotropic charge transport

Abstract 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).

Anisotropic charge transport in highly oriented films of semiconducting polymer prepared by ribbon-shaped floating film

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.

Rapid Formation and Macroscopic Self‐Assembly of Liquid‐Crystalline, High‐Mobility, Semiconducting Thienothiophene

AbstractA 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.

Ultra-high mobility in defect-free poly(3-hexylthiophene-2,5-diyl) field-effect transistors through supra-molecular alignment

Abstract Performance enhancement of organic field-effect transistors (OFETs) based on solution-processable conjugated polymers (CPs) holds critical significance for the realization of cost-effective commercial applications such as organic light-emitting diode displays. One of the most critical performance parameters is the charge-carrier field-effect mobility (μFET) that is significantly influenced by the molecular arrangement in a CP. In this article, floating film transfer method (FTM) is utilized for the deposition of a CP––defect-free poly(3-hexylthiophene-2,5-diyl) (DF-P3HT)––which results in the formation of aligned supra-molecular assemblies. When applied as the active layer in OFET devices, μFET reaching as high as 8.0 cm2/V.s (6.3 cm2/V.s on average) is obtained. The value of μFET observed in the current study is the highest value reported so far for P3HT based OFETs (∼5 times higher as compared to when DF-P3HT is deposited using spin coating).

Electrical and ammonia gas sensing properties of poly (3, 3‴- dialkylquaterthiophene) based organic thin film transistors fabricated by floating-film transfer method

Electrical and ammonia gas sensing properties of a poly (3, 3‴- dialkylquaterthiophene) (PQT-12) based organic thin film transistor (OTFT) fabricated by floating-film transfer method (FTM) on a highly doped p-type silicon substrate has been presented possibly for the first time in this paper. The OTFT is also fabricated using conventional spin coating method for the comparison with our FTM coated devices. The field effect mobility, threshold voltage, on/off current ratio and sub-threshold swing have been found as ∼8.77 × 10−3 cm2/V-s, −13.9 V, 843 and −8.5 V/dec, respectively for FTM coated devices. The drain current measured for different ammonia gas concentrations varying from 10 ppm to 80 ppm is observed to be decreased with increased ammonia concentration. A gas response of ∼56.4%, response time and recovery time of ∼45 s and ∼85 s, respectively at 80 ppm has been measured with a detection limit of 404 ppb for FTM coated devices. Significant changes in the values of field effect mobility, threshold voltage and on/off current ratio have been achieved at 80 ppm of ammonia gas with respect to their corresponding values measured prior to exposing the device to the ammonia gas. Both the electrical and ammonia gas sensing properties of FTM coated device are superior than spin coated devices.

Controlling Factors for Orientation of Conjugated Polymer Films in Dynamic Floating-Film Transfer Method

Controlling Factors for Orientation of Conjugated Polymer Films in Dynamic Floating-Film Transfer Method

Air-stable vapor phase sensing of ammonia in sub-threshold regime of poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) based polymer thin-film transistor

In the present study, device geometry having top contact bottom gated poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) (PBTTT) based polymer thin film transistor (PTFT) is reported to measure the changes in electrical responses with the exposure to various concentrations of ammonia vapor at room temperature. PTFT is preferred over other electronic devices due to its multi-parameter characteristics to explore gas sensing capabilities. A facile floating–film transfer method (FTM) is used to prepare an active layer of PTFT by forming optimized thin layer of PBTTT film having a thickness of ∼25nm. Exposure to various concentrations of ammonia vapor, device parameters like I/O characteristics, mobility, threshold voltage, and trap density are found to change significantly. As PTFT is exposed to 100ppm ammonia vapor a 70% reduction in mobility is observed whereas threshold voltage shifted to more negative from −17.62V to −23.64V. Better sensing capabilities with a detection limit of 0.33ppm, fast response and recovery of exposed ammonia vapor are observed, when PTFT is operated in sub-threshold regime rather than the cut-off or saturated regime. All the electrical characterizations for the sensing purpose are carried out in ambient but in a closed chamber. Device shows enormous potential for ammonia sensing for real and environmental applications.

Solvent driven performance in thin floating-films of PBTTT for organic field effect transistor: Role of macroscopic orientation

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.

Macroscopic Domains within an Oriented TQ1 Film Visualized Using 2D Polarization Imaging.

Large-area self-assembly of functional conjugated polymers holds a great potential for practical applications of organic electronic devices. We obtained well-aligned films of poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) using the floating film transfer method. Thereby, a droplet of the TQ1 solution was injected on top of the surface of an immiscible liquid substrate, at the meniscus formed at the edge of a Petri dish, from where the polymer solution and the film spread in one direction. Characterization of the TQ1 film using the recently developed two-dimensional polarization imaging (2D POLIM) revealed large, millimeter-sized domains of oriented polymer chains. The irregular shape of the contact line at the droplet source induced the appearance of disordered stripes perpendicular to the spreading direction. A correlation of polarization parameters measured using 2D POLIM revealed the microstructure of such stripes, providing valuable information for further improvement and possible upscaling of this promising method.

Enhancement of carrier mobility along with anisotropic transport in non-regiocontrolled poly (3-hexylthiophene) films processed by floating film transfer method

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.

High-sensitivity organic phototransistors prepared by floating film transfer method

In this report, the performance characteristics of organic phototransistors (OPTs) prepared by the floating film transfer method (FTM) and spin coating (SC) technique are compared. The FTM OPT shows a 2-order-higher photosensitivity under an electrically induced off-state as compared with the SC OPT. Furthermore, the FTM OPT shows improved gate voltage tunabilities of photosensitivity and responsivity as compared with the SC OPT. These observed results are explained on the basis of the improved thin film morphology of the active poly(3-hexylthiophene-2,5-diyl) (P3HT) layer in the FTM OPT, pointing towards the significance of thin film processing conditions in obtaining high-sensitivity OPTs.

A comparative study of spin coated and floating film transfer method coated poly (3-hexylthiophene)/poly (3-hexylthiophene)-nanofibers based field effect transistors

A comparative study on electrical performance, optical properties, and surface morphology of poly(3-hexylthiophene) (P3HT) and P3HT-nanofibers based “normally on” type p-channel field effect transistors (FETs), fabricated by two different coating techniques has been reported here. Nanofibers are prepared in the laboratory with the approach of self-assembly of P3HT molecules into nanofibers in an appropriate solvent. P3HT (0.3 wt. %) and P3HT-nanofibers (∼0.25 wt. %) are used as semiconductor transport materials for deposition over FETs channel through spin coating as well as through our recently developed floating film transfer method (FTM). FETs fabricated using FTM show superior performance compared to spin coated devices; however, the mobility of FTM films based FETs is comparable to the mobility of spin coated one. The devices based on P3HT-nanofibers (using both the techniques) show much better performance in comparison to P3HT FETs. The best performance among all the fabricated organic field effect transistors are observed for FTM coated P3HT-nanofibers FETs. This improved performance of nanofiber-FETs is due to ordering of fibers and also due to the fact that fibers offer excellent charge transport facility because of point to point transmission. The optical properties and structural morphologies (P3HT and P3HT-nanofibers) are studied using UV-visible absorption spectrophotometer and atomic force microscopy , respectively. Coating techniques and effect of fiber formation for organic conductors give information for fabrication of organic devices with improved performance.

Macroscopic self ordering of solution processible poly(3,3″′-dialkylquaterthiophene) by floating film transfer method

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.

Fabrication of Large-scale Drop-cast Films of π-conjugated Polymers with Floating-film Transfer Method

Fabrication of Large-scale Drop-cast Films of π-conjugated Polymers with Floating-film Transfer Method

Optical and Transport Anisotropy in Poly(9,9'-dioctyl-fluorene-alt-bithiophene) Films Prepared by Floating Film Transfer Method

Optical and Transport Anisotropy in Poly(9,9'-dioctyl-fluorene-<i>alt</i>-bithiophene) Films Prepared by Floating Film Transfer Method

Optical and Transport Anisotropy in Poly(9,9'-dioctyl-fluorene-alt-bithiophene) Films Prepared by Floating Film Transfer Method

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.

Ambipolar Transport in Bilayer Organic Field-Effect Transistor Based on Poly(3-hexylthiophene) and Fullerene Derivatives

Ambipolar characteristics in an organic field-effect transistor (FET) with a bilayer structure consisting of poly(3-hexylthiophene) (P3HT) and a fullerene derivative (PCBM) are reported. P3HT was deposited by a floating film transfer method (FTM) with toluene solution on spin-coated PCBM. The FTM-deposited film was found to show relatively high hole mobility even when cast using toluene solution. Even after coating P3HT on PCBM by FTM, a relatively high n-type transport was obtained. This indicates that FTM employed in this study is a mild way to coat an organic thin film on an organic semiconductor layer in terms of minimizing the effect of carrier transport in the underlayer. The transport characteristics have been discussed in comparison with those of ambipolar FETs prepared by other methods previously reported.