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The ability to control the molecular organization of electronically active liquid-crystalline polymer semiconductors on surfaces provides opportunities to develop easy-to-process yet highly ordered supramolecular systems and, in particular, to optimize their electrical and environmental reliability in applications in the field of large-area printed electronics and photovoltaics. Understanding the relationship between liquid-crystalline nanostructure and electrical stability on appropriate molecular surfaces is the key to enhancing the performance of organic field-effect transistors (OFETs) to a degree comparable to that of amorphous silicon (a-Si). Here, we report a novel donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), which contains both electron-donating quaterthiophene and electron-accepting 5,5'-bithiazole units. This copolymer exhibits excellent electrical characteristics such as field-effect mobilities as high as 0.33 cm(2)/V.s and good bias-stress stability comparable to that of amorphous silicon (a-Si). Liquid-crystalline thin films with structural anisotropy form spontaneously through self-organization of individual polymer chains as a result of intermolecular interactions in the liquid-crystalline mesophase. These thin films adopt preferential well-ordered intermolecular pi-pi stacking parallel to the substrate surface. This bottom-up assembly of the liquid-crystalline semiconducting copolymer enables facile fabrication of highly ordered channel layers with remarkable electrical stability.

Certain 4,4'-alkyl substituted 2,2'-bithiazole and bithiazole-thiophene oligomers display an endothermic transition in their DSC trace below their respective melting points. Variable-temperature FTIR, MAS-1H NMR, UV-vis spectra, and XRD all indicate that the thermal transition is due to a crystal-crystal phase transition that we have labeled alpha --beta. FTIR shows a stepwise increase in the concentration of gauche defects at the alpha --beta transition temperature, but MAS NMR spectra show little increase in the side chain motion until the mp is reached. UV-vis spectra demonstrate that the conjugated main chains remain essentially planar through the alpha --beta transition, and significant deviations from planarity occur only at higher temperatures, but well below the mp. The close similarity of this behavior to the phase transitions in long chain n-paraffins and the "side-chain melting" phenomenon in poly(3-alkylthiophenes), P3ATs, suggests that the latter may actually be more accurately described as a crystal-crystal phase transition of the crystalline fraction, driven by side chain disorder.

Barbituric acid derivatives are synthesized as the electro-optic chromophores for photorefractive application. When doped with new chromophores the photorefractive composites which consist of polysiloxane with pendant carbazole as photoconducting host show very large gain coefficient (Γ = 219cm−1 at 100V/μm).

Novel photorefractive poly(glycidyl ether) oligomers of a carbazole-based multifunctional chromophore were synthesized via ring-opening cationic polymerization. This new class of materials showed good solubility in various organic solvents and thermal stability up to 400°C. Optical quality films of excellent transparency at the light wavelength above 430 nm were obtained by spin-coating or casting. Photoconductive sensitivity of the film was as high as 10−13 S cm−1/mW cm−2 at 532 nm. Electrooptic coefficients (r33) of 5–7 pm V−1 were easily achieved by the contact poling of spin-coated films. As evidence of photorefractivity, an asymmetric two-beam coupling gain of 3.2 cm−1 was demonstrated at an electric field of 30 V μm−1 with a low-Tg polymer blend of multifunctional poly(glycidyl ether).

Novel photorefractive chromophores with indole and nitrobenzene as the push and pull groups, respectively, were synthesized and subsequently reacted with methacryloyl and acryloyl chloride to give multifunctional monomers. Polymers were obtained by copolymerization of these monomers with methyl methacrylate (P1, P2) or butyl acrylate (P3). These polymers were very soluble in organic solvents to be fabricated into optically clear polymer films and showed glass transition (Tg) at ca. 150 °C for P1, P2 and at 8 °C for P3, respectively. Large nonlinear optical coefficients (d33 as high as 49 pm/V) and photoconductive sensitivity (on the order of 10-9 S·cm-1/W·cm-2) were identified with high Tg polymers P1 and P2. Preliminary investigation of photorefractivity was carried out with low Tg polymer (P3) by two-beam coupling and four-wave mixing experiments. It was observed that P3 doped with 1% TNF showed two-beam coupling gain of 2.2 cm-1 at 75 V/μm and diffraction efficiency of 0.44% at 60 V/μm. When the photoc...

Second-order optical nonlinearities of a series of dipolar photoconductive chromophores have been investigated by semiempirical quantum chemical calculations using time-dependent Hartree–Fock formalism in conjunction with PM3 parameter and experimental evaluation of the first molecular hyperpolarizability ( β ). A linear correlation between the measured and calculated β values was observed, and the structure–property relationship discussed in terms of the structural variation. It was noted that the optical nonlinearities of the dipolar photoconductive molecules with carbazole, indole, or indoline as donor units are large enough for the electrooptic and photorefractive applications.

Two kinds of multifunctional monomers, 2-methyl-acrylic acid 6-[3-[2-cyano-2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M1) and 2-methyl-acrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M2) were synthesized to possess both photoconductivity and nonlinear optical (NLO) properties. Photoconducting NLO polymers (P1, P2) were obtained by the copolymerization of these multifunctional monomers with methylmethacrylate. Charge-transporting monomer, 2-methyl-acrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester (M3) was also synthesized and copolymerized with M1 or M2 to give the photoconducting NLO polymers (P3, P4) with enhanced photoconductivity. The successful polymerization was identified by spectroscopic means and gel permeation chromatography analysis. These polymers were very soluble in common solvents and high optical quality films could be easily fabricated by spin coating. Second harmonic generation (SHG) measurement confirmed P1 and P2 possessed high NLO activity. From the preliminary measurement of photoconductivity, it was found that P1 possessed distinct photoconductivity. It was also found that P3 and P4 showed much higher photoconductivity than P1, presumably due to the enhanced charge transporting property of M3 structure.

Novel nonlinear optical (NLO) chromophore, 2-{3-[2-(4-methylsulfonylphenyl)vinyl]carbazol-9-yl}ethanol was synthesized and subsequently reacted with methacryloyl chloride to give a photoconducting NLO monomer (M1). 2-Methylacrylic acid 2-[3-(diphenylhydrazonomethyl)carbazol-9-yl]ethyl ester (M2) was also synthesized as a comonomer to enhance the carrier mobility of the NLO polymer. Photoconducting NLO polymers, P1 and P2 were obtained by the copolymerization of M1 with methyl methacrylate and M2, respectively. These polymers were well soluble in organic solvents and showed glass transition at 177 °C and 196 °C, respectively. Polymer films of P1 and P2 were optically clear, and were transparent at wavelengths longer than 420 nm. The electro-optic coefficient (r 33 ) of poled P1 films was measured to be ∼ 5 pm/V at 632.8 nm. The photoconductive sensitivities of P1 and P2 were 6.2 × 10 -14 S.cm -1 /mW.cm -2 and 5.6 × 10 -11 S.cm -1 / mW.cm -2 .

We developed an active matrix organic light-emitting diodes (AMOLEDs) on a glass using two organic thin-film transistors (OTFTs) and a capacitor in a pixel. OTFTs switching-arrays with 64 scan lines and 64 (RGB) data lines were designed and fabricated to drive OLED arrays. In this study, OTFT devices have bottom contact structures with an ink-jet printed polymer semiconductor and an organic insulator as a gate dielectric. The width and length of the switching OTFT is 500 μm and 10 μm, respectively and the driving OTFT has 900 μm channel width with the same channel length. The characteristics of the OTFTs were examined using test cells around display area. On/off ratio, mobility, on-current of switching OTFT and on-current of driving OTFT were 10 6 , 0.1 cm 2 /V.sec, order of 8μA and over 70 μA, respectively. These properties were enough to drive the AMOLEDs over 60 Hz frame rate. AMOLEDs composed of the OTFT switching arrays and OLEDs made by deposition of small molecule materials were fabricated and driven to make moving images, successfully.

This communication demonstrates a method of transferring unreacted low molecular weight (LMW) siloxane oligomers from freshly prepared "dry" PDMS stamps for patterning organic semiconductors and conducting polymers into functional devices via selective wetting. The semiconductors were patterned onto the modified surfaces via dip-coating with well-resolved feature sizes as small as 1 mum. Functional transistor arrays exhibited field-effect mobilities as high as 0.07 cm2/Vs. The proposed printing method eliminates the need to ink SAMs for fabricating patterns and results in a simple, fast, and highly reproducible method of patterning organic semiconductors from solution. The method herein also produced a flexible transistor composed of patterned PEDOT source-drain electrodes.

This work demonstrates a method for inducing site-specific nucleation and subsequent growth of large oriented organic semiconductor single crystals using micropatterned self-assembled monolayers (SAMs). We demonstrate growth of oriented, patterned, and large organic semiconductor single crystals for potential use in organic electronic devices. The control over multiple parameters in a single system has not yet been reported. The ability to control various aspects of crystal growth in one system provides a powerful technique for the bottom-up fabrication of organic single-crystal semiconductor devices.

The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility.

Bias stress effects in organic thin-film transistors were investigated. A donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), PQTBTz-C12, was used as the active channel material. This substance contains both electron-donating quaterthiophene and electron-accepting 5,5’-bithiazole units. The threshold voltage (VT) shifts induced by direct current (DC) bias stress were studied under different gate-source and drain-source voltages. By fitting ΔVT versus stress time in compliance with a stretched exponential relationship, characteristic charge trapping time constants (τ) and dispersion parameters (β) for the VT shifts were determined for each stress condition. The time constants decrease with increasing gate-drain voltages. It was also observed that the VT shift due to charge trapping can be recovered by releasing the device from bias stress for several hours. The recovery rate from DC OFF bias stress is slightly slower than the recovery fro...

We have designed and synthesized self-assembled one-dimensional nanowires (1D NWs) from the fused thiophene derivative 2,3-bis(5-(3,5-bis(trifluoromethyl)phenyl)thiophen-2-yl)acrylonitrile (SS-TFMBE). This new material is characterized by a strong self-assembling capability that leads to the formation of 1D NWs exhibiting a dramatic increase in fluorescence emission upon formation of NW aggregates. In addition, the semiconducting properties of the 1D NWs were evidenced by means of conducting-probe atomic force microscopy (CP-AFM) and space-charge-limited-current (SCLC) measurements, which show that SS-TFMBE NWs composed of ordered percolated crystals have an effective carrier mobility of up to 3.1 cm2V−1 s−1.

The “push-pull” co-oligomer, 5,5′-bis(4,4′-dibutyl-2,2′-bithiazol-5-yl)-(3,4,3′,4′-bis(ethylenedioxy)-2,2′-dithienyl) (BT2B) was used to construct organic field effect transistors (OFETs) via vacuum evaporation or by solution casting of BT2B crystals. Crystalline OFETs made in either top- or bottom-contact configuration had measured field effect mobilities of (3–4) × 10−3 cm2 V−1 s−1. INDO and EHT molecular orbital calculations show that the HOMO is confined to the central EDOT fragment whereas the LUMO is concentrated on the relatively electron-poor bithiazole rings. This charge confinement, when combined with the molecular offset in the π-stacked crystal structure, leads to relatively small transfer integrals. Even so, the intrinsic hole mobility was computed to be as high as 0.86 cm2 V−1 s−1.

The charge generation properties at all interfaces of a p∕n junction, bilayer photodiode have been investigated by means of the photoaction spectrum (PAS) as a function of applied bias. The organic photodiode was fabricated with a low-glass transition temperature (Tg) polysiloxane with pendant hydrazone groups as the p-type material and a perylene diimide derivative as the n-type material. The PAS under short circuit and reverse bias showed an antibatic response at the high-energy region (3.0–3.5eV), and a symbatic response at the low-energy region (2.0–3.0eV). However, under forward bias, the PAS showed the opposite behavior. These results are interpreted in terms of the band structure of tin-doped indium oxide (ITO) that prevents effective photoinjection of electrons at the polymer/ITO interface and the relative energy levels of the constituent materials.

Humidity dependence of electrical performance of p-channel organic thin-film transistors (OTFTs) with various semiconductor compounds has been investigated. All devices showed decreased current output and mobility as the relative humidity (RH) was increased. The moisture sensitivity of the OTFT saturation current depends on the device geometry (bottom or top contact device) and channel length. The OTFT configuration with a short channel length and bottom contact was most affected by humidity compared to the top contact and larger channel length OTFT structures. The degradation of electrical performance under high RH is attributed to charge trapping at grain boundaries by polar water molecules reducing the rate of charge transport.