Wei-Yang Chou

Increasing the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) by ultraviolet irradiation

The effects of ultraviolet (UV) irradiation on the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) have been investigated in this study. Spectroscopic methods [x-ray photoelectron spectroscopy and Raman spectroscopy (532nm excitation)] and electrical conductivity measurements were used to characterize the electrical conducting polymer PEDOT:PSS prepared by UV irradiation. The authors found that UV irradiation could lead to an increase in the work function and the enhancement of electrical conductivity of PEDOT:PSS, resulting from a decrease in the number of the charge-trapping-related defects and the formation of linear or expanded-coil conformation.

Influence of molecular structure and microstructure on device performance of polycrystalline pentacene thin-film transistors

The authors have fabricated the pentacene thin films on polymethylmethacrylate (PMMA) and on silicon dioxide dielectric surfaces featuring similar surface energy and surface roughness. On both surfaces the pentacene films displayed high crystal quality from x-ray diffraction scans, although the film on PMMA had significantly smaller grain size. The pentacene transistors with PMMA exhibited excellent electrical characteristics, including high mobility of above 1.1cm2∕Vs, on/off ratio above 106, and sharp subthreshold slope below 1V∕decade. The analysis of molecular microstructure of the pentacene films provided a reasonable explanation for the high performance using resonance micro-Raman spectroscopy.

Electric field-induced structural changes in pentacene-based organic thin-film transistors studied by in situ micro-Raman spectroscopy

We have investigated the electric field-induced microscopic structural changes in polycrystalline pentacene-based organic transistors by using in situ micro-Raman spectroscopy. Extra vibrational modes resulting from molecular coupling effect in pentacene film were studied. The herringbone packing of pentacene molecules in solid film is affected by external field and the process is proven to be partially irreversible. In the meantime, in-phase coupling of the C-H bending mode was found to be highly related to the carrier transport of pentacene film. Obtained results suggest that optimal intermolecular π-orbital overlap of pentacene molecules is still a critical factor impacting the carrier transportation for pentacene film featuring polycrystalline morphology.

High Luminescence Polarized Polymer Light-Emitting Diodes Fabricated Using Aligned Polyfluorene

We have successfully constructed a polarized polymer light-emitting diodes (PLEDs) by employing poly(N-vinylcarbazole) (PVK) as an extra hole-transporting alignment layer that prevents the damage of the hole-injected layer of poly(3,4 ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT–PSS) by mechanical rubbing. The maximum luminance efficiency 0.31 cd A-1 of blue polarized PLED fabricated with the aligned polufluorene (PF) layer is obtained. The brightness approaches 1000 cd m-2 at a relatively low biased voltage. By using a PVK alignment layer, a dichroic ratio of 25.7 (at a 451 nm emission wavelength) was observed by finding the optimum thickness of PF that was sufficiently highly polarized to be used as an efficient light source in liquid crystal displays. The spectrum of photon emission is narrow, characterized by its full width at half maximum value.

Changes in surface roughness and work function of indium-tin-oxide due to KrF excimer laser irradiation

In this study, from the observed x-ray photoelectron spectroscopy and atomic force microscopy results, and Kelvin probe measurements, it is suggested that the induced indium-tin-oxide (ITO) surface chemical changes by KrF excimer laser irradiation had strong effects on the surface work function (SWF) and surface roughness of ITO. During the laser irradiation, the incorporation of O22− peroxo species or the dissolution of oxygen species near the ITO surface leads to the reduction of the surface roughness and an increase in the SWF. In addition, it is worth noting that the laser-irradiated ITO sample has an excellent stability in the SWF.

Microcell Liquid Crystal Film for High-Contrast Flexible Display Applications

We have successfully fabricated high-contrast microcell liquid crystal displays (LCDs) using microstructure replication technology in thin plastic film substrates. These novel flexible LCDs comprise microcell replication, alignment treatments, and cell assembly. The cell gap of such flexible LCDs was maintained well by cross-type spacers formed by photolithography, despite bending: moreover, the deviation in cell gap under various bending states was less than 0.1 µm. The structure, fabrication technique, and basic display characteristics of the flexible LCD are also described. Finally, this device shows high mechanical stability against external bending, a high contrast ratio of 500:1, and an acceptable response time. Compared with photopolymerisation-induced phase separation, the micro-cell LCD has a higher contrast ratio, a more reliable fabrication process, and a strong potential as a high-performance flexible LCD.

Enhanced efficiency in polymer light-emitting diodes due to the improvement of charge-injection balance

The authors report the enhancement of efficiency of polymer light-emitting diodes (PLEDs) in the study. According to the experimental results, we find that PLEDs, fabricated on irradiated indium-tin-oxide surfaces by KrF excimer laser, with an organic layer between the cathode and the emitting layer may lead to the improvement of charge-injection balance and prevention of cathode metal quenching, resulting in a remarkable increase in external quantum efficiency.

Tuning surface properties in photosensitive polyimide. Material design for high performance organic thin-film transistors

A photosensitive polyimide (PSPI) is synthesized to serve as the gate dielectric in organic thin-film transistors (OTFTs). A method to control the charge density within the channel of the OTFT using the PSPI on a silicon dioxide gate dielectric is proposed. The surface energy, surface carriers, and capacitance of gate dielectrics can be tuned by varying irradiation doses of UV light on the PSPI surface to create ultrahigh performance pentacene-based OTFTs with an average field-effect mobility (μ) of above 6.0 cm2/Vs. The correlation between the surface energy of the PSPI films and the μ value of OTFT devices is investigated. Moreover, the impact of gate-dielectric properties on the structural properties of pentacene films is closely related to the electrical characteristics of OTFTs. The results of contact angle, X-ray diffraction, electrostatic force microscopy, and micro-Raman spectroscopy measurements indicate that the surface energy and doping carriers coming from the PSPI film play important roles in the carrier transport of pentacene film for OTFT applications.

Innovative Plasma Alignment Method in Flexible Liquid Crystal Display Films

Liquid crystals (LCs) in flexible cell were aligned on polyimide (PI) surface treated by a paperlike plasma beam. The beam had energy of 450 eV and incident angle of 70° with respect to normal flexible indium-zinc oxide-coated substrate. The pretilt angle and anchoring energy of LCs on plasma-aligned PI surface were almost identical to the angle and energy of the rubbing PI. However, optical performance of the plasma-aligned cell was superior to the rubbing cell under various bending conditions. Furthermore, such noncontact alignment technology is suitable for large area and roll-to-roll fabrication of flexible LC films.

Epitaxial pentacene films grown on the surface of ion-beam-processed gate dielectric layer

The following research describes the process of fabrication of pentacene films with submicron thickness, deposited by thermal evaporation in high vacuum. The films were fabricated with the aforementioned conditions and their characteristics were analyzed using x-ray diffraction, scanning electron microscopy, polarized Raman spectroscopy, and photoluminescence. Organic thin-film transistors (OTFTs) were fabricated on an indium tin oxide coated glass substrate, using an active layer of ordered pentacene molecules, which were grown at room temperature. Pentacene film was aligned using the ion-beam aligned method, which is typically employed to align liquid crystals. Electrical measurements taken on a thin-film transistor indicated an increase in the saturation current by a factor of 15. Pentacene-based OTFTs with argon ion-beam-processed gate dielectric layers of silicon dioxide, in which the direction of the ion beam was perpendicular to the current flow, exhibited a mobility that was up to an order of magn...