Fabrication Of Organic Thin-film Transistors Research Articles

Anisotropic charge-carrier mobilities of liquid crystalline conjugated polymers on photo-aligned PVCN dielectric insulators

We fabricated organic thin-film transistors (OTFTs) with photo-reactive poly(vinylcinnamate) (PVCN) as a dielectric layer and liquid crystalline poly(2,5-bis(3-dodecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) as a semiconducting layer. The PVCN layer plays an important role in the fabrication of OTFTs with solution processable PBTTT, because it provides orientation of the liquid crystalline polymer chains as well as gate insulating characteristics. The electrical properties of PBTTT-OTFTs with the UV-cured PVCN film were investigated for the films prepared with linearly polarized ultraviolet light irradiation along parallel and perpendicular to the electrode channel direction.

Functionalized soluble triethylsilylethynyl anthradithiophenes (TESADTs) for organic electronic devices

Four new solution-processable triethylsilylethynyl anthradithiophene-(TESADT) based organic semiconductors, end-capped with phenyl (DP-; 1), thien-2-yl (DT-; 2), thienothien-2-yl (DTT-; 3), and perfluorophenyl (DFP-; 4) groups have been synthesized, characterized, and incorporated in organic thin-film transistors (OTFTs) and organic photovoltaics (OPVs). For the fabrication of solution-processed OTFT, thin films of all four compounds have been fabricated via a solution process of solution-shearing (SS), droplet-pinned crystallization (DPC), and drop-casting (DC). Among various solution-processing methods, solution-shearing produced TFTs the highest electrical performance. Thin films of compound 1 formed via the SS method exhibited p-channel characteristics, with hole mobilities as high as ∼0.034 cm2V−1s−1. The film morphologies and microstructures of these compounds have been characterized by atomic force microscopy and X-ray diffraction to rationalize device performance trends. Furthermore, DP-TESADT (1) and DFP-TESADT (4) has been employed for the fabrication of OPVs. Especially, pseudo-bilayer polymer solar cells based on an underlayer of poly(3-hexylthiophene) (P3HT) doped with 2.4% DP-TESADT and 9.1% DFP-TESADT and an upper layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) achieved power conversion efficiency up to 2.1%.

Alignment Control of Patterned Organic Semiconductor Crystals in Short-Channel Transistor Using Unidirectional Solvent Evaporation Process

The patterning deposition of highly crystalline organic semiconductor films is a key issue in achieving high-mobility organic thin-film transistor (OTFT) arrays and circuits with practical device dimensions. However, it is generally difficult to obtain uniform crystalline films only on the desired channel regions over a large-area substrate. The alignment control of crystalline domains decreases size variations in the crystalline film. Herein, we present a patterning scheme that can control the alignment direction of solution-grown organic crystals. We employed a simple coating method combining drop casting and lamination coating. By creating a striped pattern of solvent-wettable/solvent-nonwettable regions on a polymer dielectric, we grew nearly uniaxially aligned ribbon-like crystals with a length of 100 $\mu \text{m}$ order within a confined channel region. Bottom-contact short-channel TFTs fabricated on such a stripe pattern showed a high mobility of up to 1.5 cm $^{2}$ /Vs. The proposed method is compatible with the fabrication of OTFTs over an arbitrary large area.

Au-Induced Directional Growth of Inkjet-Printed 6,13-Bis(triisopropylsilylethynyl) Pentacene

An inkjet printing method for directional crystallization was proposed that employs difference in surface energy to induce film growth with higher crystallinity and orderliness. Inkjet printing of 6,13-bis(triisopropyl- silylethynyl) pentacene, a small-molecule semiconductor was performed as a single-line film with the starting point set on a patch of Au film, which had been surface-treated to have a slightly higher dispersive surface energy than the dielectric substrate. A larger dispersive surface energy on the metal resulted in advanced contact-line pinning thereof between the liquid and the substrate and induced film growth along the printing direction. With the influence of the differential surface energy, larger grain size as well as more convergent crystalline microstructure was obtained, which is beneficial to the fabrication of organic thin-film transistors with improved electrical performance and device-to-device uniformity.

Effects of Annealing and Residual Solvents on Amorphous P3HT and PBTTT Films

Organic thin film transistors (OTFT) are metal–insulator–semiconductor field-effect transistors in which the semiconductor is a conjugated organic material. They are the subject of intense industrial research because their fabrication process is less expensive when compared with inorganic TFTs. Among the others, the organic material mostly employed in their construction consists of two semiconductor polymers, namely poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). Despite the large amount of experimental efforts in the characterization of the electronic properties of these devices, several questions regarding their morphological arrangement in bulk and at interfaces remain wide open. Here, we report results obtained by classical molecular dynamics simulations of P3HT and PBTTT inspired by OTFT fabrication techniques. In particular, we investigate how the annealing fabrication process and the presence of residual solvent molecules left over after spin coa...

Compact Modeling and Contact Effects in Thin Film Transistors

A compact model for the current-voltage characteristics of organic thin-film transistors (OTFTs), which includes the effects of the contact regions, is proposed. Different physical and morphological aspects of contacts with organic or other emerging materials such as graphene, semiconducting dichalcogenides such as MoS2, or NW devices are described. The electrical behavior of the contacts is studied in OTFTs, and circuit models that describe them are reviewed. Two trends are observed in the current-voltage curves of the contacts of different OTFTs: linear and nonlinear, and different models are used to explain them. A unified model for the contact region that reproduces both trends and gathers the different physical and structural features of the contacts is developed. It is described by a single parameter and introduced in a generic analytical model for TFTs. The variability in OTFT structures, materials, and fabrication approaches gives rise to a strong variability in the values of the parameters of the model. In this regard, a characterization technique to determine the value of the parameters of the model from experimental data is also developed. Different physical tests are proposed to validate the results of the technique. The procedure is applied to recent experimental data for different pentacene-based transistors. The good agreement between the experimental data and our analytical results provides a way to relate the parameters of the model with the physical or geometrical origin of the contact effects in OTFTs.

Self-stratified semiconductor/dielectric polymer blends: vertical phase separation for facile fabrication of organic transistors

Semiconducting/insulating polymer blends are promising materials for use in organic thin film transistor (OTFT) applications. Here, vertically phase-separated poly(3-hexylthiophene) (P3HT)-top and poly(methyl methacrylate) (PMMA)-bottom blend films were developed for the facile fabrication of OTFTs with excellent electrical properties. The microstructures of the blend films could be adjusted simply by altering the film processing conditions, which impacted the electrical properties of the OTFTs based on the blend films. A decrease in the P3HT content of the blend film significantly reduced the interface roughness between the semiconductor (P3HT) and dielectric (PMMA) layers, which reduced charge trapping or scattering, thereby increasing the field-effect mobility. A higher solution concentration tended to increase the drying time during film deposition, which allowed the P3HT molecules to self-organize over a long period of time. This led to an increase in the ordering of the phase-separated P3HT film, which significantly improved the device performance. TFTs based on a P3HT/PMMA (1/39 w/w) film prepared from a 4 wt% blend solution showed the best field-effect performance with a saturated field-effect mobility of 0.022 cm2 V−1 s−1 and an Ion/Ioff current ratio of 2 × 105. The vertically stratified P3HT/PMMA films were also suitable for the fabrication of all-polymer TFT devices on flexible substrates.

Enhanced electrical properties in carbon nanotube/poly (3-hexylthiophene) nanocomposites formed through non-covalent functionalization

Poly(3-hexylthiophene) (P3HT) has received much attention as a good candidate to replace inorganic semiconductors for flexible electronics due to its solution-processability. However, the low charge mobility of P3HT is an obstacle to its commercialization. To overcome this problem, we propose a new non-covalent functionalization method for carbon nanotubes (CNTs) for use in CNT/P3HT nanocomposites. By using modified pyrene molecules with hydrophobic long alkyl chains, the non-covalently functionalized CNTs can become well dispersed in hydrophobic solutions and organic semiconductor matrices. Fabrication of organic thin-film transistors (OTFTs) from the non-covalently functionalized CNT/organic semiconductor nanocomposites shows that our non-covalent functionalization method significantly reduces damage to CNTs during functionalization when compared with covalent functionalization by treatment with acids. The OTFTs show 15 times enhancement of field effect mobility (1.5 × 10 -2 cm 2 /(V·s)) compared to the mobility of OTFTs made from pure P3HT. This enhancement is achieved by addition of only 0.25 wt% of CNTs to P3HT.

Organic Thin-Film Transistors with Short Channel Length Fabricated by Reverse Offset Printing

We report on the fabrication of organic thin-film transistors (OTFTs) with a reverse-offset-printed Ag metal source/drain (S/D) electrode pattern. The printed electrodes had a channel length of less than 5 μm and resistivity of 3 × 10−6 Ω cm. The OTFTs were fabricated from regioregular poly(3-hexylthiophene) as the semiconductor and poly(methyl methacrylate) as the gate insulator. The transfer and output characteristics of a top-gate OTFT with a channel length of 5 μm were evaluated. Here we discuss in detail the technological challenges encountered with reverse offset printing and the failure modes.

Study of the Characteristics of Organic Thin Film Transistors with Plasma-Polymer Gate Dielectrics

The effects of gate dielectrics material in organic thin film transistors (OTFTs) were investigated. The gate dielectrics were deposited by plasma enhanced chemical vapor deposition (PECVD) with cyclohexane and tetraethylorthosilane (TEOS) respectively used as organic and inorganic precursors. The gate dielectrics (gate insulators) were deposited as either organic plasma-polymer or organic–inorganic hybrid plasma-polymer thin film by using cyclohexane or cyclohexane with TEOS, respectively. Additionally, hydrogen and argon were used as precursor bubbler gases. A polyimide (PI) substrate was used in the fabrication of pentacene OTFTs with a plasma-polymer gate insulator, an Au source–drain (S/D), and Cu gate electrodes. Different gate dielectrics were investigated. The as-grown plasma-polymer thin films were first analyzed using Fourier-transform infrared (FT-IR) spectroscopy. Also, they were analyzed by nano-indentation and capacitance measurements. The electrical properties, such as mobility and threshold voltage of the pentacene field-effect transistors with the plasma-polymer gate-dielectrics were investigated. Transistor with cyclohexane gate dielectric had a higher field-effect mobility, µFET = 0.84 cm2 V-1 s-1, and a smaller threshold voltage, VT= -6.8 V, than the transistor with the hybrid gate-dielectric.

Fabrication of organic thin-film transistors by spray-deposition for low-cost, large-area electronics

We report on high performance organic thin-film transistors fabricated by spray-deposition, an innovative coating technique for organic electronic devices. This method is simple and inexpensive, compatible with room-temperature processing and easily scalable from laboratory-based devices to large-area electronics. Our spray-deposited 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene transistors show very good homogeneity, with mobilities of 0.2 cm 2 / V s , and on/off ratios of 10 7 . This performance is comparable to that of our best spin-coated devices fabricated on the same substrate type, while offering additional advantages such as using 20 times less organic semiconductor and facilitating fast coating of large-areas.

Top‐Gate Organic Thin‐Film Transistors Constructed by a General Lamination Approach

www.MaterialsViews.com C O M M U Top-Gate Organic Thin-Film Transistors Constructed by a General Lamination Approach N IC A T By Lei Zhang , Chong-an Di , * Yan Zhao , Yunlong Guo , Xiangnan Sun , Yugeng Wen , Weiyi Zhou , Xiaowei Zhan , * Gui Yu , and Yunqi Liu * IO N Solution-processed organic thin-fi lm transistors (OTFTs) have received increasing interest because they are thought to be promising candidates for low-cost electronics applications. [ 1 , 2 ] Organic materials offer the benefi t that they can be printed on fl exible polymeric substrates at low temperature by means of solutionbased techniques, which results in a dramatic reduction of manufacturing costs. However, very few organic semiconducting materials possess high environmental stability, hindering their practical applications. Bottom-gate OTFTs are a conventional device structure for material-testing purpose. Unfortunately, the oxygen and moisture can easily penetrate into the active layer and degrade the device performances for bottom-gate OTFTs because the organic semiconducting materials are exposed to air. [ 3–5 ] On the contrary, in top-gate OTFTs the active layer is self-encapsulated by the gate dielectric layer and the gate electrode and, therefore, the device stability is signifi cantly improved. [ 6 , 7 ] It has been recently demonstrated by different groups that top-gate OTFTs exhibit higher stability than bottom-gate devices. [ 8 ] As a result, fabrication of high-performance top-gate OTFTs is highly desirable and have received considerable attention. The dielectric/organic semiconductor interface, the most important one in OTFTs, plays a key role in the device performance, because the conductive channel is located on the few molecular layers near the gate dielectric layer. [ 9–11 ] Consequently, deposition of the dielectric layer on the organic semiconductor is a key step and a challenging task in the fabrication of top-gate OTFTs, especially solution processed devices. So far, the use of orthogonal solvents is the most widely used approach for the fabrication of solution-processed top-gate OTFTs. [ 12–14 ] Nonetheless, orthogonal solvents have to be selected carefully to minimize any adverse effects during the deposition of the organic semiconducting layer and the dielectric layer in two successive steps. Despite the signifi cant progress made over the past few years with the use of this approach, some problems remain. Firstly, although it is possible to ensure that the solvent used for the deposition of the insulating layer does not dissolve the

Flexible organic thin-film transistors using single-walled carbon nanotubes as an activated channel

We report on the fabrication of organic thin-film transistors (OTFTs) with a spun cross linked poly-4-vinylphenol (PVP) dielectric on a polyethersulphone (PES) flexible substrate. To improve the electrical performance of OTFTs, we employed a random single-walled carbon nanotubes (SWNTs) network as a carrier transfer underlay without sacrificing the flexibility of the TFTs. The random SWNTs showed that they can act as a semiconducting channel and conduction path to shorten the channel length in our TFTs. The flexible thin-film transistors (TFTs) with a random SWNTs/pentacene bilayer as an active channel exhibited an improved saturation field effect mobility ( µ sat ) of 2.6 × 10 − 1 cm 2/Vs compared to that of TFTs without the SWNTs underlay, while creating only a minor reduction of the current on/off ratio.

유기물과 유무기 혼합 폴리머 게이트 절연체를 사용한 유기 박막 트랜지스터의 특성

In this study, we have been synthesized the dielectric layer using pure organic and organic-inorganic hybrid precursor on flexible substrate for improving of the organic thin film transistors (OTFTs) and, design and fabrication of organic thin-film transistors (OTFTs) using small-molecule organic semiconductors with pentacene as the active layer with record device performance. In this work OTFT test structures fabricated on polymerized substrates were utilized to provide a convenient substrate, gate contact, and gate insulator for the processing and characterization of organic materials and their transistors. By an adhesion development between gate metal and PI substrate, a PI film was treated using <TEX>$O_2$</TEX> and <TEX>$N_2$</TEX> gas. The best peel strength of PI film is 109.07 gf/mm. Also, we have studied the electric characteristics of pentacene field-effect transistors with the polymer gate-dielectrics such as cyclohexane and hybrid (cyclohexane+TEOS). The transistors with cyclohexane gate-dielectric has higher field-effect mobility, <TEX>$\mu_{FET}=0.84\;cm^2/v_s$</TEX>, and smaller threshold voltage, <TEX>$V_T=-6.8\;V$</TEX>, compared with the transistor with hybrid gate-dielectric.

Fabrication of Organic Thin-Film Transistors with Roll-Printed Electrodes Using Patterned Polymer Stamp

The roll-printed electrodes of organic thin-film transistors (OTFTs) were fabricated by gravure or flexography printing using patterned poly(dimethylsiloxane) (PDMS) stamp with various channel lengths and low-resistance silver (Ag) paste on plastic substrates. The roll-printed OTFTs used poly(vinyl)(phenol) polymeric dielectrics and bis(triisopropyl-silylethynyl) pentacene organic semiconductors. The roll-printed OTFTs obtained had a field-effect mobility 0.08 (±0.02) cm2/Vs, an on/off current ratio of 104 and a subthreshold slope of 2.53 V/decade. The roll printing using patterned PDMS stamp for Ag paste transferring made it possible to fabricate a printed OTFT with a channel length as small as to 12 μm on plastic substrate.

Soluble pentacene thin-film transistor using a high solvent and heat resistive polymeric dielectric with low-temperature processability and its long-term stability

Solution processable organic thin-film transistors (OTFTs) were fabricated using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) and low-temperature processable polyimide gate dielectric. The TIPS-pentacene OTFT with the dielectric was found to have a field-effect mobility of 0.15 cm 2/Vs, which is comparable to that of OTFT with an inorganic dielectric. The OTFTs with the polyimide dielectric did not show any significant performance degradation as time passed. A field-effect mobility of the OTFTs in 60 days was found to be almost identical to that of pristine OTFT. The combination of TIPS-pentacene and our polyimide gate dielectric can be one of the potential candidates for the fabrication of stable OTFTs for large-area flexible electronics.

Hysteresis Effects by Source/Drain Interdigitated-Finger Geometry in 6,13-Bis(triisopropylsilylethynyl)pentacene Thin-Film Transistors

This work reports on the fabrication of organic thin-film transistors (OTFTs) with a solution-based 6,13-bis(triisopropylsilylethynyl)pentacene by a drop-casting method, and the determination of the electrical properties of OTFTs having different source/drain interdigitated-finger electrodes. The results show that the hysteresis in transfer characteristics of the OTFTs depends on the variation of contact fingers. These hystereses lead to changes in threshold voltage, which originates from charge trapping/detrapping at or near the organic semiconductor/dielectric interface. These related phenomena also influence the device parameters such as the field-effect mobility, the on/off current ratio, and the gate current.

High-Performance n-Type Organic Thin-Film Transistors Based on Solution-Processable Perfluoroalkyl-Substituted C60 Derivatives

We have demonstrated n-type organic thin-film transistors (OTFTs) based on solution-processable perfluoroalkyl-substituted C60 derivatives. C60PC12F25-based OTFTs exhibit high electron mobility of 0.25 cm2/(V s) and can operate in air. We found that introduction of a long-chain perfluoroalkyl group to C60 is effective for fabrication of stable and high performance n-type OTFTs.

Photodefinable organofunctionalized inorganic dielectric for organic thin film transistors

The authors present an organic functionalized inorganic dielectric for use as a gate dielectric of organic thin film transistors (OTFTs). This inorganic-organic hybrid material is not only solution processable but also directly definable by photolithography. The gate dielectric can thus be patterned in the process of forming the dielectric layer. The organic functional moieties incorporated into inorganic siloxane backbone allow photocurability and development in aqueous solvents. Fabrication of pentacene-based OTFTs with the gate dielectric results in a device performance that is better than that of the device fabricated with spin-coatable inorganic dielectric such as spin on glass.

Copper phthalocyanine organic thin-film transistors with calcium fluoride gate insulator

We report the fabrication of organic thin-film transistors (OTFTs) with copper phthalocyanine (CuPc) as the semiconductor and calcium fluoride (CaF2) as the gate dielectric on the glass substrate. The fabricated transistors show a gate voltage dependent carrier field effect mobility that ranges from 0.001 to 0.5 cm2 V−1 s−1. In the devices, the CaF2 dielectric is formed by thermal evaporation; thus OTFTs with a top-gate structure can be fabricated. This provides a convenient way to produce high-performance OTFTs on a large scale and should be useful for the integration of organic displays.