Synthesis and characterization of 1,3,2-diazaboroine derivatives for organic thin-film transistor applications

2,2′-(1,4-Phenylene)bis(2,3-dihydro-1H-naphtho[1,8-de]-1,3,2-diazaboroine) [PND] and 2,2′-(4,4′-biphenylene)bis(2,3-dihydro-1H-naphtho[1,8-de]-1,3,2-diazaboroine) [BND] were synthesized and characterized by using differential scanning calorimetry (DSC) measurements, single crystal X-ray structure analysis, UV–vis absorption and electrochemical measurements, thin-film X-ray diffraction (XRD) and AFM studies. Organic field-effect transistors (OFETs) were fabricated by vacuum deposition with a bottom contact geometry using Au electrodes. Annealing treatment optimizes the organic active layer and increases the charge carrier mobility. Field-effect mobilities of 7.2 × 10^?3 for PND and 4.1 × 10^?4 cm² V^?1 s^?1 for BND were found.     

A flexible capacitor based on conducting polymer electrodes

Flexible electrodes for supercapacitors have been prepared by depositing polypyrrole (PPy) on to a gold-coated PVDF membrane. Specific capacitance values of the order of 380 F g^?1 for PPy/Nafion and 420 F g^?1 for PPy/p-toluenesulfonate were obtained. For the PPy/Nafion electrode, an energy density of 56 Wh kg^?1 and a power density of 15.50 kW kg^?1 were available after 5000 cycles.Using a self-contained fully flexible device comprising polypyrrole as both electrodes and a PVDF membrane as separator, a capacitance of 30 F g^?1 was observed after 5000 cycles.     

The performance improvement in pentacene organic thin film transistors by inserting C60/MoO3 ultrathin layers

A series of bottom-gated top-contact pentacene organic thin film transistors are fabricated. C₆₀ and/or MoO₃ ultrathin layers are inserted between the pentacene and Al source-drain electrodes to reduce the contact resistance. With proper order and thickness of the two layers modification, the injection barrier is greatly lowered down and the field-effect mobility increases from 0.095 cm²/(V s) to 0.65 cm²/(V s). The threshold voltage decreases from ?11.3 V to ?6.4 V. It means that the injection barrier plays an important role in the contact resistance without modification and multiple ultrathin layers modification is an effective method to improve the performance of the OTFTs. Then the output curve of the devices with better modification is simulated by a charge drift model. Taking into account of the contact effect, the field-effect mobility is improved to 1.05 cm²/(V s). It indicates that after modification, the injection barrier is lowered down, but the contact resistance caused by the charge drift in the contact region become the more important role and still affect the performance.     

Synthesis and properties of phenothiazylene vinylene and bithiophene-based copolymers for organic thin film transistors

A series of new phenothiazylene vinylene and thiophene copolymers (P1, P2, and P3) have been synthesized via Yamamoto and Stille coupling reactions. The number-averaged molecular weights (M_n) of P1, P2, and P3 were found to be 12,000, 10,000, and 8200, with polydispersity indices of 3.5, 1.4, and 1.6, respectively. The UV–visible absorption spectra of the polymers contain two strong absorption bands in the ranges 306–325 nm and 430–480 nm, which arise from the absorptions of the phenothiazine segments and the conjugated main chains respectively. Solution-processed field-effect transistors (FETs) fabricated with these polymers exhibit p-type organic thin film transistor characteristics. The field-effect mobilities for P1, P2, and P3 were measured to be 1.8 × 10^?4, 5.7 × 10^?4, and 2.5 × 10^?7 cm² V^?1s^?1, respectively, with the corresponding on/off ratios of 5 × 10², 1 × 10⁴, and 5 × 10².     

N-channel operation of pentacene thin-film transistors with ultrathin polymer gate buffer layer

N-channel operation of pentacene thin-film transistors with ultrathin poly(methyl methacrylate) (PMMA) gate buffer layer and gold source–drain electrode was observed. We prepared pentacene thin-film transistors with an 8-nm thick PMMA buffer layer on SiO₂ gate insulators and obtained electron and hole field-effect mobilities of 5.3 × 10^?2 cm²/(V s) and 0.21 cm²/(V s), respectively, in a vacuum of 0.1 Pa. In spite of using gold electrodes with a high work function, the electron mobility was considerably improved in comparison with previous studies, because the ultrathin PMMA film could decrease electron traps on SiO₂ surfaces, and enhance the electron accumulation by applied gate voltages.     

Effect of Au deposition rate on the performance of top-contact pentacene organic field-effect transistors

A study of the influence of the deposition rate of top-contact Au source and drain electrodes deposited by electron-beam evaporation on the electrical performance of pentacene organic field-effect transistors (OFETs) is presented. By adjusting the deposition rate of the Au electrodes to minimize metal diffusion into the semiconductor pentacene layer, the source/drain contact resistance could be reduced. At a Au deposition rate of 10 Å/s, high-performance pentacene p-channel OFETs were obtained with a field-effect mobility of 0.9 cm²/Vs and a normalized channel width resistance of 23 kΩ cm in a device with a channel length of 25 μm.     

Preparation and enhanced stability of flexible supercapacitor prepared from Nafion/polyaniline nanofiber

Polyaniline nanofibers were used to produce a flexible supercapacitor electrode with a specific capacitance of 235 F g^?1. Cycle life and energy density were enhanced after encapsulating the polyaniline nanofiber electrode in Nafion. A specific capacitance of 195 F g^?1 was obtained over 10,000 charge–discharge cycles.A fully flexible, stand alone capacitor comprising two polyaniline nanofiber based electrodes and a PVDF separator was constructed. An initial specific capacitance of 125 F g^?1 (100 mV s^?1) was obtained. This decreased to 95 F g^?1 over 10,000 cycles.     

Micrometer-scaled OFET channel patterns fabricated by using PEDOT/PSS microfibers

We have easily fabricated channel patterns of organic field-effect transistors (OFETs), in which channel lengths were 5 μm, by using wet-spun poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) microfibers with diameters of ca. 5 μm. Pentacene-based FETs with a top-contacted configuration, showed a hole mobility of 0.13 cm² V^?1 s^?1 and on/off current ratio of 9.4 × 10⁴. The device also showed large current of ca. 160 μA (V_D = ?50 V; V_G = ?50 V), reflecting shorter channel length of the devices. We have evaluated self-assembled monolayer (SAM) through measurements of water contact angle and by Zisman plot. As a result, critical surface tension of the octadecyltrichlorosilane (OTS)-treated SiO₂ surface is 17 mN m^?1 which is consistent with that of well-known SAM. We also well analyzed the cross-sections of the device by a scanning transmission electron microscopic (STEM) technique. The results indicated that the thicknesses of the pentacene layer in the channel part and Au layer in the source/drain part were ca. 30 and 30 nm, respectively. Furthermore, it is also indicated that the 100-nm grains of the pentacene were well adhered on the surface of the SAM-formed SiO₂ layer.     

Improved mobility of the copper phthalocyanine thin-film transistor

Copper phthalocyanine (CuPc) organic thin-film transistor (OTFT) was fabricated by thermal evaporation deposition on p-SiO₂ dielectric layer. Organic thin-film transistors used in large display areas need the enhancement of transistor performances by increasing the I_on/I_off ratio and the mobility. The output and transfer characteristics of CuPc-OTFT having source/drain interdigitated-finger geometry were investigated. The mobility, I_on/I_off ratio and inverse sub-threshold slope for the CuPc-OTFT were found to be 5.32 × 10^?3 cm² V^?1 s^?1, 1.94 × 10⁴ and 2.5 V/decade, respectively. The interface state density of the transistor was found to be 3.73 × 10¹¹ eV^?1 cm^?2 using the conductance-frequency method. The CuPc film indicated a homogeneous surface having 3.878 nm small roughness values as observed by atomic force microscope (AFM) measurements. The obtained results indicate that we have improved a CuPc-OTFT transistor with high mobility without being of any substrate treatment.     

Organic field-effect transistors with ultrathin gate insulator

We have used a commercially available Mylar film coated with a thin (≈60 nm) layer of aluminium and an ultrathin (≈3.5 nm) SiO₂ layer as flexible substrate for the manufacture of bottom-gate organic field-effect transistors (OFETs). We show that the SiO₂ layer has insulating properties with a breakdown voltage of 1.6 V and a capacitance of ≈1 μF/cm². We have manufactured organic field-effect transistors using this substrate, regioregular poly(3-hexylthiophene) (rrP3HT) as a p-type semiconductor, and gold source and drain contacts. This results in OFETs that operate with voltages on the order 1 V.     

n- and p-Channel field-effect transistors based on diquinoxalinoTTF derivatives

Three new quinoxalinoTTF derivatives with methyl, trifluoromethyl and fluoro groups were synthesized and characterized by UV–vis absorption spectroscopy, differential scanning calorimetry, X-ray single crystal analysis, X-ray diffraction, and field-effect transistor (FET) characteristics. All of them have π-stacking structures in the single crystals. The quinoxalinoTTF derivative with trifluoromethyl groups exhibited an n-type FET, which is a rare example of n-channel FETs based on TTF derivatives. The highest electron mobility is 0.01 cm² V^?1 s. The FET polarity was converted to p-channel from n-channel by replacing the trifluoromethyl groups with methyl groups. The hole mobility is as high as 0.2 cm² V^?1 s. In contrast, the fluoro substituted derivative did not show FET properties due to the poorly ordered molecular arrangement.     

The ultraviolet-ozone effects on organic thin-film transistors with double polymeric dielectric layers

Organic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). The using of double dielectric layers exhibited an improved device performance comparing with the conventional devices with single dielectric layer. By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. The grain size and crystallization of pentacene film grown on PS are apparently improved as the increase of surface energy of PS. As a result, a field-effect mobility of 1.38 cm² V^?1 S^?1 and an on/off current ratio of 10⁴ had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment. The polymeric materials pair consisting of PS and PVA is a promised candidate as the dielectric layers for high-performance OTFTs.     

Perylenetetracarboxylic di-imide-based bottom-contact devices: A study on two kinds of source/drain electrodes,ITO and MoW

High-performance bottom-contact devices based on an air-stable n-type organic semiconductor N,N-bis(4-trifluoromethoxybenzyl)-perylene-3,4,9,10-tetracarboxylic di-imide, were fabricated, and the effects of crystal packing on indium tin oxide and molybdenum–tungsten alloy were shown in two different electric characteristics. The estimated work function of indium tin oxide and molybdenum–tungsten alloy were 4.7 and 5.0 eV. The calculated lowest unoccupied molecular orbital energy level of the organic material was 3.7 eV. Transistors with indium tin oxide bottom electrodes exhibited a high mobility of 3.37 × 10^?2 cm² V^?1 s^?1, an on/off current ratio of 6.5 × 10⁵ and threshold voltage of ?4.0 V.     

Nitric oxide sensing by cytochrome c bonded to a conducting polymer modified glassy carbon electrode

A nitric oxide biosensor based on cytochrome c (an heme protein) covalently immobilized to poly(5-amino-1-naphthol) by using cyanuric chloride as a bridge was developed. The immobilization was studied by cyclic voltammetry and quartz crystal microbalance. The nitric oxide detection as a function of poly(5-amino-1-naphthol) amount was recorded, and the best result was obtained with the electrode prepared by 70 cycles. The sensitivity and detection limit were 0.015 μA cm^?2/μmol L^?1 and 2.85 μmol L^?1, respectively.     

Oliogothiophenes end-capped with arylacetylenes for organic thin film transistors

Phenylethynyl, butylphenylethynyl and pentafluorophenylethynyl end-capped terthiophene oligomers (PATTP, BPATTP and FPATTP) have been synthesized and characterized. Their physical, optical and electrochemical properties varied with the end substituents were investigated. DSC results revealed that three compounds were crystalline in the film state and thermally stable by 200 °C under nitrogen atmosphere. The oligothiophenes have exhibited H-aggregation, which was verified by significant blue-shift in UV–vis absorption spectra in the films, indicating a close side-to-side molecular packing. X-ray diffraction measurement on the films has revealed molecular edge-on substrate growth orientation. Top-contact field-effect transistors were demonstrated by spin-coating or ink-jetting the solution of the oligomers. The preliminary results showed that PATTP and BPATTP can serve as p-type carriers with hole-motility as 3 × 10^?4 and 1 × 10^?2 cm²/(V s), respectively, while pentafluorophenylacetylene afforded FPATTP as n-type carriers with electron-motility around 10^?5 cm²/(V s).     

Polypyrrole and polyaniline prepared with cerium(IV) sulfate oxidant

The conducting polymers, polypyrrole and polyaniline, were synthesized by chemical oxidative polymerization of the corresponding monomers in 0.1 M sulfuric acid using cerium(IV) sulfate as the oxidant at mole ratios of oxidant-to-monomer ranging from 0.5 to 3. The yields of the oxidation products were determined, and the samples were characterized with respect to their elemental composition, molecular structure, and morphology. The conductivity of polypyrrole prepared in 0.1 M sulfuric acid, 10^?1 to 10⁰ S cm^?1, was higher compared with the conductivity of polyaniline prepared under the same conditions, 10^?3 to 10^?1 S cm^?1. The loss of mass after deprotonation with ammonium hydroxide is reported, and discussed in terms of the type of protonation as also reflected by FTIR spectroscopy. The conductivity of polypyrrole bases remained at relatively high level, 10^?5 to 10^?3 S cm^?1, while PANI bases became non-conducting, 10^?12 to 10^?10 S cm^?1. The polymers had a granular morphology in all cases.     

High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds

We report high ON/OFF ratios for organic field-effect transistors (OFETs) based on amorphous thin films of spiro-linked compounds. Bottom-contact OFET structures are fabricated, using 2,2′,7,7′-tetra-(m-tolyl-phenylamino)-9,9′-spirobifluorene (Spiro-TPD) and 2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD) as active materials. The field-effect mobility of holes in Spiro-TPD and Spiro-TAD thin films is 7 × 10⁻⁵ cm²/Vs. We obtained ON/OFF ratios up to 3.6 × 10⁶ with low OFF currents in the pA range. Time-dependent measurements show that the transistor characteristics do not change significantly over 9 months.     

A highly efficient deep blue fluorescent OLED based on diphenylaminofluorenylstyrene-containing emitting materials

We have designed and synthesized five blue emitters based on diphenylaminofluorenylstyrene emitting core groups. Multilayered OLEDs were fabricated using these materials as dopants in a 2-methyl-9,10-di(naphthen-2-yl)anthracene (MADN) host. One of them in particular a deep blue OLED using dopant 9-[4-(2-diphenylamino-9,9-diethylfluoren-7-yl)phenyl]-9-phenylfluorene (3) at 15% doping concentration exhibited a maximum luminance of 4720 cd m^?2 at 9.0 V, a luminous efficiency of 5.3 cd A^?1 at 20 mA cm^?2, a power efficiency of 2.9 lm W^?1 at 20 mA cm^?2, an external quantum efficiency of 4.8% at 20 mA cm^?2, and CIE coordinates (x = 0.15, y = 0.13) at 8.0 V. Furthermore, this deep blue device had very stable CIE coordinates of (x = 0.15, y = 0.13) that did not vary with doping concentration from 5% to 15%.     

Iridium(III) complexes with cyclometalated styrylbenzoimidazole ligands: Synthesis,electrochemistry and as highly efficient emitters for organic light-emitting diodes

Two phosphorescent iridium complexes (psbi)₂Ir(acac) and (ppbi)₂Ir(acac) (psbi = 1-phenyl-2-styryl-1H-benzo[d]imidazole, ppbi = 1-phenyl-2-(1-phenylprop-1-en-2-yl)-1H-benzo[d]imidazole, acac = acetylacetonate) were synthesized, and their photophysical, electrochemical and electroluminescent properties were also studied. Organic light-emitting devices with these two complexes as dopant emitters having the structure ITO/NPB (10 nm)/TCTA (20 nm)/x%Ir:CBP (y nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The device based on (psbi)₂Ir(acac) exhibited a maximum brightness of 56,162 cd m^?2, while the device based on (ppbi)₂Ir(acac) gave a maximum brightness of 31,232 cd m^?2. At high brightness of 1000 cd m^?2 and 10,000 cd m^?2, high current efficiencies of 25.7 cd A^?1 and 20.7 cd A^?1 were achieved, respectively, for the (psbi)₂Ir(acac)-based EL device. For the EL device based on (ppbi)₂Ir(acac), current efficiencies of 20.1 cd A^?1 at 1000 cd m^?2 and 14.2 cd A^?1 at 10,000 cd m^?2 were observed.     

Organic field-effect transistors using liquid crystalline compound synthesized to enhance carrier-transport ability

We report the characteristics of organic field-effect transistors (OFETs) using the liquid crystalline compound synthesized to enhance carrier-transport ability. Oblique evaporation with a tilt angle of 60° and subsequent thermal annealing at 200 °C yielded anisotropic carrier transport due to the anisotropic molecular alignment. The OFETs fabricated with polymethylmethacrylate (PMMA) intermediate layers showed mobility up to 2.6 × 10^?2 cm²/V s. The characteristics of OFETs depended on the deposition rate of liquid crystalline materials. The mobilities increased with decrease in the deposition rate. Polarization-resolved photoluminescence (PL) measurements exhibited that the molecular orientation in liquid crystalline films was improved with decrease in the deposition rate. Therefore it is assumed that the carrier-transport properties were enhanced by improving the molecular orientation in the liquid crystalline films.