Solution-processed Organic Field-effect Transistors Research Articles

Development of high-performance n-type organic thin-film transistors using a small-molecule polymer blend

We report the fabrication of high-performance solution-processed organic field-effect transistors (OFETs) using small-molecule polymer blends as the active layer. N,N-dialkyl-substituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis (dicarboximide) derivative (PDI-RCN2) and polystyrene (PS) or poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) are used as the small molecules and polymer binders, respectively, that comprise the semiconducting layer. Both the n-type PDI-RCN2 and PS or F8BT-blended OFETs have high field-effect mobility (μFET) of 0.4cm2/Vs, with average μFET values of 0.25 and 0.16cm2/Vs for PDI-RCN2/F8BT and PDI-RCN2/PS, respectively. Moreover, OFETs with blended active layers exhibit significantly improved device-to-device uniformity and reproducibility, as evidenced by the μFET standard deviations of 0.23, 0.1, and 0.12 for PDI-RCN2, PDI-RCN2/PS, and PDI-RCN2/F8BT, respectively.

Two-dimensional benzodithiophene and benzothiadiazole based solution-processed small molecular organic field-effect transistors & solar cells

Benzodithiophene (BDT) and benzothiadiazole (BT) based small molecules have been synthesized through palladium-catalyzed Stille coupling reactions. The molecules with and without the conjugated thiophene side-chain were named as DRTBTTBDTT (2D) and DRTBTTBDT (1D), respectively. These molecules exhibited an optical band gap of around 1.8 eV. Solution-processed thin films of 1D and 2D showed that the conjugated thiophene side-chain oriented to produce a uniform 2D structure, which led to a relatively high hole mobility of 0.016 cm2 V−1 s−1, and this was one of the highest values for solution-processed small-molecule donors in organic solar cells (OSCs). With proper engineering of energy levels, the organic solar cells based on DRTBTTBDTT (2D) as donors exhibited a high Voc of around 0.9 V.

Electronic structure of positive and negative polarons in functionalized dithienylthiazolo[5,4-d]thiazoles: a combined EPR and DFT study

2,5-Dithienylthiazolo[5,4-d]thiazole (DTTzTz) derivatives have high potential for solution-processed organic field-effect transistors and solar cells, both as electron acceptors and donors. Here, the electronic structure of positive and negative radicals (polarons) of two functionalized DTTzTz materials is studied using multi-frequency and multi-resonance electron paramagnetic resonance (EPR) in combination with density functional theory (DFT). It is shown that the negative and positive DTTzTz polarons can be distinguished on the basis of their characteristic EPR parameters. The chemically induced polarons are compared to light-generated states observed in a blend of one of the DTTzTz derivatives with a donor polymer. The study gives in-depth information about the spread of the electron or hole in the DTTzTz molecules.

Enhancing the Performance of Solution‐Processed n‐Type Organic Field‐Effect Transistors by Blending with Molecular “Aligners”

A novel approach to enhancing the performance of solution-processed n-type organic field-effect transistors by using trace amounts of molecular "aligners" to manipulate the assembly of "matrix" molecules in thin films is demonstrated. The device performance is one order of magnitude higher in 1wt% blended thin films than that in neat films, which correlates to an induced change of preferred orientation of the in-plane π-stacking molecules upon blending.

Droplet Manipulation by an External Electric Field for Crystalline Film Growth

Combining droplet manipulation by the application of an electric field with inkjet printing is proposed as a unique technique to control the surface wettability of substrates for solution-processed organic field-effect transistors (FETs). With the use of this technique, uniform thin films of 2,7-dioctyl[1]benzothieno[2,3,-b][1]benzothiopene (C8-BTBT) could be fabricated on the channels of FET substrates without self-assembled monolayer treatment. High-speed camera observation revealed that the crystals formed at the solid/liquid interface. The coverage of the crystals on the channels depended on the ac frequency of the external electric field applied during film formation, leading to a wide variation in the carrier transport of the films. The highest hole mobility of 0.03 cm(2) V(-1) s(-1) was obtained when the coverage was maximized with an ac frequency of 1 kHz.

High-performance solution-processed organic transistors with electroless-plated electrodes

Electroless-plated gold and platinum films are used as source and drain electrodes in high-performance solution-processed organic field-effect transistors (OFETs), representing a promising large-area, near-room-temperature and vacuum-free technique to form low-resistance metal-to-semiconductor interfaces in ambient atmosphere. Developing non-displacement conditions using a Pt-colloidal catalyst for soft electroless plating, the electrodes are deposited on crystallized thin films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) without significant damage to the semiconductor material. The top-contact OFETs show remarkable performance, with a mobility of 6.0cm2V−1s−1. The method represents a practical fabrication technique to mass-produce circuitry arrays of nearly best-performing OFETs for the printed electronics industry.

Templating and Charge Injection from Copper Electrodes into Solution-Processed Organic Field-Effect Transistors

Solution-processed organic field-effect transistors (OFETs) using chemically modified copper electrodes are reported. The purpose of this study is to shed light on the use of inexpensive copper electrodes in bottom-contact OFETs, which is consistent with the major goal of organic electronics: the realization of low-cost electronics. 6,13-Bis(triisopropylsilylethynyl)pentacene was used for solution-processed hole-transporting molecular films and pentafluorobenzenethiol was used to form self-assembled monolayers (SAMs) on the contact metals. We conducted a comparative study on copper and gold contacts and realized that, under the same performance improvement schemes, via SAM treatment and controlled crystal growth, the copper electrode device experienced a more significant enhancement than the gold electrode device. We attribute the beneficial effects of SAMs to the improved charge injection and transport properties, which are critical double effects from the fluorinated aromatic SAM structure. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that templating property of SAMs promotes the crystallization of TIPS-pentacene films at the metal/organic interface. The presented result indicates that copper can be regarded as a promising candidate for reducing the use of gold in organic-based circuits and systems, where the cost-effective production is an important issue.

Doping of TIPS-pentacene via Focused Ion Beam (FIB) exposure

We report on the influence of Focused Ion Beam (FIB) exposure on TIPS-pentacene layers which are often used in solution-processable organic field-effect transistors (OFETs) and in many cases yield a field-effect mobility in the order of 1cm2/Vs. We exposed TIPS-pentacene layers to a Ga+ ion beam and measured the device characteristics of OFETs. We observed a strong influence of the FIB on J–V characteristics of TIPS-pentacene-based devices and determined an increase in the OFET mobility and on–off ratio and a decrease of the threshold voltage. To further investigate the underlying process we analyzed FIB-exposed and unexposed TIPS-pentacene samples via X-ray Photoelectron Spectroscopy (XPS). Exposed samples show a clear Ga XPS signature and the C1s peak shifts about 400meV towards smaller binding energies which is an indicator for a Fermi energy shift closer to the valence states and hence p-doping of TIPS-pentacene. With Scanning Kelvin Probe Microscopy (SKPM) we could clearly distinguish FIB exposed areas from the unexposed areas. For exposed areas the work function increases about 200meV which is consistent with XPS measurements and again displays that the implanted Ga+ ions serve as p-dopants. Furthermore we took SKPM measurements on operating OFETs and could investigate a dramatic change in local conductance on FIB exposed areas. This demonstrates a novel way of nanopatterning conductive paths in organic semiconductors.

High-mobility and solution-processable organic field-effect transistors based on carbazole-dihexylquaterthiophenes with controllable morphology

An oligomeric semiconductor based on bithiophene-carbazole-bithiophene as called DH4T-Cz were prepared and applied in solution-processable organic field-effect transistors (OFETs). The thermal, optical, and electrochemical properties of DH4T-Cz were fully studied with TGA, DSC, UV–Vis, photoluminescence (PL), and cyclic voltammetry (CV). The thin film morphologies of deposited-DH4T-Cz via different deposition techniques were also investigated with POM and XRD to figure out the relationship of the corresponding deposition process. After fabricating the OFETs, a relative higher hole mobility was observed in 0.5cm2V−1S−1 with dip-coating deposition at specific dipping speed due to the higher ordered molecular packing and uniform microribbon orientation. The dip-coating process of DH4T-Cz could be considered a large-area and continuous fabrication for OFET, and the performance is highly competent to the high-end OFET application.

New Donor–Acceptor–Donor Molecules with Pechmann Dye as the Core Moiety for Solution-Processed Good-Performance Organic Field-Effect Transistors

In this paper, we report the synthesis and characterization of two new D-A-D molecules (E)-5,5′-bis(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-1,1′-bis(2-ethyl- hexyl)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BTBPD) and (E)-5,5′-bis- (5-(benzo[b]furan-2-yl)thiophen-2-yl)-1,1′-bis(2-ethylhexyl)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BFBPD). They entail bipyrrolylidene-2,2′(1H,1′H)-dione (BPD, known as Pechmann dye) as the electron-accepting core that is flanked by two benzo[b]thiophene moieties and two benzo[b]furan moieties, respectively. Crystal structures of BTBPD and BFBPD provide solid evidence for the intermolecular donor–acceptor (D-A) interactions, which are favorable for improving charge transport performance. Organic field-effect transistors (OFETs) were prepared based on thin films of BTBPD and BFBPD through solution-processed technique. OFETs of BTBPD exhibit relatively high hole mobility up to 1.4 cm2 V–1 s–1 with high on/off ratio up to 106. In comparison, the hole mobility of OFETs with BFBPD (0.14 cm2 V–1 s–1) is relatively low, because of the poor thin-film morphology and low molecular ordering, even after annealing. Thin-film morphological and XRD studies were carried out to understand the variation of hole mobilities after annealing at different temperatures. The present studies clearly demonstrate the potentials of BPD that is planar and polar as the electron-acceptor moiety to build D-A molecules for organic semiconductors with good performance.

Highly stable fluorine-rich polymer treated dielectric surface for the preparation of solution-processed organic field-effect transistors

Highly stable fluorine-rich polymer dielectrics were fabricated using cross-linked poly(3-(hexafluoro-2-hydroxyl) propyl) styrene (PFS), which shows excellent electrical stability, good adhesive surface properties, and good wettability on deposited solution-processed materials. Solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) could be deposited onto the cross-linked PFS dielectrics to yield highly ordered crystalline structured films that did not delaminate. The field-effect mobilities were as high as 0.56 cm2 V−1 s−1, and negligible hysteresis was observed in the organic field-effect transistors (OFETs). The threshold voltage, the ON/OFF ratio, and the subthreshold slope were −0.043 V, ∼107, and −0.3 V per decade, respectively. The OFETs demonstrated excellent device reliability under gate-bias stress conditions due to the presence of highly stable fluorine groups in the cross-linked PFS dielectrics.

Infrared spectroscopy of narrow gap donor-acceptor polymer-based ambipolar transistors

Donor-acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow band gaps, enabling ambipolar charge transport when integrated in solution-processed organic field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in narrow-gap D-A conjugated polymers polybenzobisthiadiazole-dithienopyrrole (PBBTPD) and polybenzobisthiadiazole-dithienocyclopentane (PBBTCD) using infrared (IR) spectroscopy. We observe a significant modification of the absorption edge in both PBBTPD- and PBBTCD-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. Additionally, we register localized vibrational resonances associated with injected charges. Our findings indicate a significant self-doping of holes that is modified by charge injection. Observations of both electron and hole transport with relatively high carrier mobility strongly suggest an inhomogeneous, phase-separated conducting polymer.

N-Alkyldinaphthocarbazoles, Azaheptacenes, for Solution-Processed Organic Field-Effect Transistors

Substituted N-alkyldinaphthocarbazoles were synthesized using a key double Diels-Alder reaction. The angular nature of the dinaphthocarbazole system allows for increased stability of the conjugated system relative to linear analogues. The N-alkyldinaphthocarbazoles were characterized by UV-vis absorption and fluorescence spectroscopy as well as cyclic voltammetry. X-ray structure analysis based on synchrotron X-ray powder diffraction revealed that the N-dodecyl-substituted compound was oriented in an intimate herringbone packing motif, which allowed for p-type mobilities of 0.055 cm(2) V(-1) s(-1) from solution-processed organic field-effect transistors.

Novel Butterfly-Shaped Fused Heteroacenes: Synthesis, Properties, and Device Performance of Solution-Processed Field-Effect Transistors

Two tetrabrominated intermediates obtained by bromination of naphthodithiophene in different solvents were used to construct novel highly π-extended butterfly-shaped heteroarenes 1-6, containing either an 8- or 10-fused ring. The solution-processed organic field-effect transistors based on compound 1 exhibited promising device performance with a hole mobility of 0.072 cm(2) V(-1) s(-1) and a current on/off ratio of 10(6) under ambient atmosphere.

Functionalized Dithienylthiazolo[5,4-d]thiazoles For Solution-Processable Organic Field-Effect Transistors

A series of 5′-aryl-substituted 2,5-bis(3′-hexylthiophen-2′-yl)thiazolo[5,4-d]thiazole derivatives was synthesized and these expanded semiconductors were investigated as active materials for solution-processable organic field-effect transistors. Field-effect mobilities of up to 10−3 cm2 V−1 s−1 were obtained, representing the first reasonable FET behavior for highly soluble thiazolo[5,4-d]thiazole-based small organic compounds suitable for printable electronics. Thermal and electrooptical material properties were studied by thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry, and UV/Vis spectroscopy. Trends in thermal and optical data were supported by (time-dependent) density functional theory calculations. Additional X-ray diffraction, atomic force microscopy, and scanning electron microscopy studies provided insight in the relationship between the molecular structures, film morphologies, and FET performances. The fibrillar microcrystalline structure observed for the best-performing thienyl-substituted material was linked to the high mobility.

Solution-processed ambipolar vertical organic field effect transistor

We report on a solution-processed ambipolar patterned-electrode vertical organic field effect transistor (PE-VOFET) based on the P(NDI2OD-T2) polymer. The Schottky barrier-based VOFET operation uniquely facilitates an ambipolar transport using a single anode-cathode-electrode and a single semiconductor material. Pin-hole free sub-100 nanometer channel length devices are obtained with no high resolution patterning owing to both the polymer’s smooth morphology and the underlining patterned-electrode’s flatness. The VOFET exhibits n-type on/off ratio >103, current density >50 [mAcm−2] under VDS = 5 V, as well as p-type operation. Prone to design and optimization, the ambipolar PE-VOFET is a promising platform for organic complementary circuit technology.

Synthesis and Characterization of Anthracene Derivative for Organic Field-Effect Transistor Fabrication

Here we report on the synthesis and characterization of anthracene derivative for solution processable organic field-effect transistors. The transistor devices with bottom-contact geometry provided a maximum field-effect mobility of 3.74 x 10(-4) cm2 V(-1) s(-1) as well as current on/off ratio of 5.05 x 10(4) and low threshold voltage. Structural information in the solid state is obtained by thermal analysis and two-dimensional wide angle X-ray scattering (2D-WAXS). From the 2D-WAXS, it is clear that the planes of anthracene rings and benzene ring of the molecule are different in solid state. We assume similar arrangement in the thin-film which limit the effective hopping and thus charge mobility.

Naphtho[2,1-b:6,5-b′]difuran: A Versatile Motif Available for Solution-Processed Single-Crystal Organic Field-Effect Transistors with High Hole Mobility

We here report naphtho[2,1-b:6,5-b']difuran derivatives as new p-type semiconductors that achieve hole mobilities of up to 3.6 cm(2) V(-1) s(-1) along with high I(on)/I(off) ratios in solution-processed single-crystal organic field-effect transistors. These features originate from the dense crystal packing and the resulting large intermolecular π-orbital overlap as well as from the small reorganization energy, all of which originate from the small radius of an oxygen atom.

Donor-acceptor copolymer based on dioctylporphyrin: Synthesis and application in organic field-effect transistors

A novel alternating D–A copolymer, PPor–BT, with dioctylporphyrin (Por) as a donor unit and 5,6-bis(octyloxy)benzo-2,1,3-thiadiazole (BT) as an acceptor unit, was designed and synthesized by Pd-catalyzed Sonogashira-coupling reaction. The copolymer showed good solubility and film-forming ability. PPor–BT exhibited a broad absorption band from 350 to 950 nm with two peaks centered at 456 and 818 nm corresponding to the Soret band and Q-bands absorption of porphyrin segments, respectively. The employment of electron deficient BT unit to construct donor-acceptor structure observably broadened the absorption spectrum and enhanced the Q-band absorption of the porphyrin-based polymer. The HOMO and LUMO energy levels of the polymer are −5.06 eV and −3.63 eV, respectively. The solution-processed organic field-effect transistors (OFETs) were fabricated with bottom gate/top-contact geometry. The mobility of PPor–BT based OFEFs reached 4.3 × 10−5 cm2 V−1 s−1 with an on/off current ratio of 104. This mobility is among the highest values for porphyrin-based polymers.

Solution-processed low leakage organic field-effect transistors with self-pattern registration based on patterned dielectric barrier

We demonstrated a new type of a solution-processed organic field-effect transistor (OFET) in a bottom-gate, top-contact geometry where low leakage current and self-pattern registration were achieved using a patterned dielectric barrier (PDB). The PDB of a hydrophobic fluorinated-polymer was produced on the top of a polymeric gate insulator of poly(4-vinylphenyl) by transfer-printing. The PDB enables to effectively screen out the vertical charge flow generated from the gate electrode, and thus the vertical leakage current between the gate and the drain was reduced by two orders of the magnitude compared to the leakage current in a conventional OFET without the PDB. Moreover, the PDB defines spontaneously an active channel pattern from a solution of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN) by means of the selective wettability and the geometrical confinement.