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We report the effects of dielectric surface properties on the device performance of organic thin film transistors (OTFTs) using polymer/high-k oxide multilayered gate dielectrics. We systematically controlled the surface energy of the gate dielectric from very hydrophobic to very hydrophilic. The modified dielectric surface strongly affected the initial growth mechanism of pentacene and subsequently the performance of the OTFTs. The performance of the OTFTs with a higher surface energy was superior to that of the OTFTs with a lower surface energy.

The effects of tetrafluoromethane (CF4) plasma treatment of indium-tin-oxide (ITO) anode on indium diffusion into a poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS) layer were studied. Auger electron spectroscopy (AES) depth profile showed that 0.2at.% indium was present in the PEDOT:PSS layer when ITO was not plasma treated. The plasma treatment of ITO increased the indium concentration to ∼6at.%. The increase in indium can be explained by an oxygen deficiency in the CF4 plasma treated ITO. The presence of indium in the PEDOT:PSS layer showed a correlation with performance degradation of polymer light-emitting diodes.

We report on the fabrication of OTFTs with organic/inorganic passivation layers to enhance the long-term stability of devices. In order to obtain a flat surface on which an inorganic thin film could be deposited, an organic smoothing layer was employed. The planarization of the rough pentacene surface would induce a very smooth and dense surface for the inorganic layer as well. When the SnO2/PVA film was introduced as a component of the encapsulation, greatly enhanced barrier properties were observed. Our work demonstrated that organic/inorganic hybrid passivation layers are a promising technique for a long-term protection of OTFTs.

The authors have demonstrated efficient polymeric tandem organic light-emitting diodes (OLEDs) with a self-organized interfacial layer, which was formed by differences in chemical surface energy. Hydrophilic poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) was spin coated onto the hydrophobic poly(9,9-dyoctilfluorene) (PFO) surface and a PEDOT:PSS bubble or dome was built as an interfacial layer. The barrier heights of PEDOT:PSS and PFO in the two-unit tandem OLED induced a charge accumulation at the interface in the heterojunction and thereby created exciton recombination at a much higher level than in the one-unit reference. This effect was confirmed in both the hole only and the electron only devices.

The authors have developed a semitransparent, multilayered cathode of indium tin oxide (ITO)/Ag/tungsten oxide (WO3) for transparent organic light-emitting diodes. The device showed a weak negative differential resistance (NDR), until the operating voltage of 8V was reached. NDR was due to the resonant tunneling by both the quantum barrier and quantum well. The silver oxide (Ag2O) on the Ag metal was confirmed by x-ray photoelectron spectroscopy, and the energy levels of Ag2O were quantized due to the quantum size effect and this produced the resonant tunneling channels. The device using ITO∕Ag∕WO3 with a LiF∕Al bilayer was superior to those devices which only used ITO or WO3, mainly because the out coupling was enhanced by employing a WO3 material, which is much more transparent than ITO.

This communication reports on the fabrication of low operating voltage pentacene thin-film transistors with high-k gate dielectrics by ion beam assisted deposition (IBAD). These densely packed dielectric layers by IBAD show a much lower level of leakage current than those created by e-beam evaporation. These results, from the fact that those thin films deposited with low adatom mobility, have an open structure, consisting of spherical grains with pores in between, that acts as a significant path for leakage current. By contrast, our results demonstrate the potential to limit this leakage. The field effect mobility, on/off current ratio, and subthreshold slope obtained from pentacene thin-film transistors (TFTs) were 1.14 cm2/V s, 105, and 0.41 V/dec, respectively. Thus, the high-k gate dielectrics obtained by IBAD show promise in realizing low leakage current, low voltage, and high mobility pentacene TFTs.

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

Pentacene organic thin film transistors (OTFTs) with low-k and high-k hybrid gate dielectrics by CF4 plasma treatment exhibited excellent device performance with field effect mobilities (maximum 1.41cm2∕Vs), a low threshold voltage of +1V, and on/off current ratios of 105 at −5V gate bias. After CF4 plasma treatment, fluorine atoms diffuse into the interior low-k polymer and eliminate ionic impurities which reduce the leakage current density and overall pentacene initial growth on the superhydrophobic surface is significantly improved. It seems apparent that proper surface treatment is desirable for higher quality pentacene film and improving the performance of OTFTs.

The authors fabricated a no-bias pi cell using a dual alignment layer with an intermediate pretilt angle via a rubbing. In the dual alignment layer system, the competition between crest region favoring the vertical alignment and trough region favoring planar alignment made it possible to achieve various pretilt angles, and adjusted pretilt angle from 90° to 20° with rubbing. In addition, as the intermediate pretilt angle plays a role in eliminating the activation energy and thus allowing formation of the initial bend state in pi cell fabrication, this approach achieved a no-bias pi cell for a liquid crystal display with both low power consumption and fast response.

We report on the fabrication of low-voltage pentacene thin-film transistors (TFTs) with CeO 2 -SiO 2 composite dielectric layers in the thickness range of 20 to 300 nm. The maximum field effect mobility of 0.97 cm 2 /V s and on/off current ratio of 10 4 were achieved under a low operating voltage of -2 V from our pentacene TFTs with 50 nm thin CeO 2 -SiO 2 composite dielectric layer. The capacitance and surface smoothness of the dielectric layer were improved with lowering the dielectric thickness. Pentacene TFTs with thin dielectric layers were thus found to be generally superior to the others with thick dielectric layers in device performance although the dielectric also showed its own thickness limit in enduring the 2 V gate bias. We conclude that there is an optimum dielectric thickness for the most desirable device performance and that our TFTs with the 50 nm thin gate dielectric have demonstrated the performance.

Current-voltage and capacitance-voltage measurements of the organic thin-film transistors (OTFTs) were performed to observe the change of the surface state of gate dielectrics using various surface treatments. The hexamethyldisilazane (HMDS)-treated OTFTs, which has few OH groups on the dielectric surface, showed little hysteresis behavior than that of other surface-treated OTFTs. In particular, when the positive gate bias (for a depletion state) was applied, more hysteresis was observed, indicating that OH groups on a gate dielectric are responsible for the electron trapping in the channel. However, when the surface is terminated with (CH 3 ) 3 -Si, eliminating OH groups by HMDS treatment, much less hysteresis was observed in the positive direction. Therefore, it seems apparent that an OH-free surface is desirable for a more stable operation of OTFTs.

CeO2–SiO2 nanocomposite films were used as the gate dielectrics in organic thin film transistors (OTFTs) with pentacene active semiconductor. CeO2–SiO2 composite films exhibited a high dielectric capacitance of 57nF∕cm2 with exceptionally low leakage current. Good device characteristics were obtained with saturation at low operating voltages (∼2V) and with a field effect mobility of 0.84cm2V−1s−1, a threshold voltage of ∼0.25V, an on/off current ratio of 103, and a subthreshold slope of 0.3V∕decade, whereas the gate leakage current density is considerably lowered. These results should therefore increase the prospects of using OTFTs in low power applications such as portable devices.

We report on the fabrication of flexible pentacene thin-film transistors (TFTs) encapsulated with an ultraviolet (UV)-protecting SnO 2 thin-film that has been prepared by ion-beam-assisted deposition (IBAD). We deposited thermally evaporated SnO 2 on a pentacene TFT as a buffer layer prior to the IBAD process. When a UV light of 254 nm wavelength was continuously illuminated onto our encapsulated device for periods of 5, 30, 60, and 120 min under a vacuum of 1 X 10 -6 Torr, its field mobility gradually degraded from 0.34 to 0.22 cm 2 /V s in 2 h while the other device without encapsulation rapidly degraded (its mobility was reduced to 0.021 cm 2 /V s during the same period). Our X-ray diffraction data indicates that the UV-induced-degradation of device characteristics is directly correlated to the degradation of pentacene crystallinity against UV radiation. We conclude that our IBAD SnO 2 encapsulation is a promising way to protect pentacene TFTs.

We report on the influence of oxygen concentration on structural and electrical properties of the amorphous yttria-stabilized zirconia (YSZ) thin films which are the potential high-k gate dielectric material of organic thin film transistors (OTFTs). The films were prepared by the E-beam evaporation process. To investigate the influence of the oxygen flow rate on the structural and electrical properties of the YSZ films, X-ray diffraction, X-ray photoelectron spectroscopy, current density-electric field, current-voltage were carried out in this work. Oxygen vacancies are expected to be the most predominant type of defect in metal-oxide dielectrics. The leakage current density decreased mainly because of the reduction of oxygen vacancies with increasing oxygen flow rate.

Using ion-beam assisted e-beam evaporation with the ion beam directed at 55° to the normal of the film plane, (200) oriented CeO2films with biaxial texture were deposited on Hastelloy C276 substrates at room temperature. The crystalline quality and in-plane orientation of films was investigated by X-ray diffraction 2θ-scan and Φ-scan, atomic force microscopy (AFM). It was shown that the in-plane and out-of-plane textures of the CeO2films were controlled by the deposition parameters. The orientation of the films was studied as a function of ion-to-atom ratio and film thickness. The ion-to-atom ratio was varied by independently adjusting the deposition rate and the ion current density. Under optimum condition, (200) textured CeO2films have been successfully grown on Hastelloy C276.

To enhance the stability of pentacene field-effect transistors, a highly polarizable amorphous oxide is used as a gas barrier due to the strong chemical interaction between permeating polar water molecules and an oxide. For a more reliable barrier function, in addition to chemical effects, a dense amorphous microstructure of a barrier is also preferred and achieved by ion-beam-assisted deposition (IBAD) at room temperature in this study. Our OTFTs encapsulated with IBAD SnO2 showed a degraded field-effect mobility of 0.5 cm2/(V s) after encapsulation. However, the field-effect mobility was surprisingly sustained up to one month and then gradually degraded down to 0.35 cm2/(V s) after 100 days in air, which was still three times higher than that of an OTFT without an encapsulation layer. The encapsulated OTFTs also exhibited a superior on/off current ratio of more than 105 to that of the unprotected device (∼104), which was reduced from ∼106 before aging. The effects of encapsulation on the electrical properties of OTFTs are discussed in terms of the physical and chemical properties of barrier films.

We investigated the effect of anode rotation on the nucleation and growth of B–C–N nanotubes by the plasma rotating electrode process (PREP). With rotation of the anode and/or increase in metal concentration, the number and length of nanotubes were increased, that is, the yield of B–C–N nanotubes was increased. This result is related to the improvement of discharge characteristics by rotating the anode and/or the enhanced catalytic effect of metal species.

The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by ion-beam-assisted deposition (IBAD) was investigated. When a buffer layer of 100-nm SnO2 film had been thermally deposited to reduce ion-induced damage prior to the IBAD process, our encapsulated organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5cm2∕V that was initially 0.62cm2∕Vs, while the OTFTs without a buffer layer showed a 60% reduction in field-effect mobility after the IBAD process. However, surprisingly, the mobility was sustained up to one month and then gradually degraded down to 0.35cm2∕Vs, which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air. The encapsulated OTFTs also exhibited superior on/off current ratio of over 105 to that of the unprotected devices (∼104), which was reduced from ∼106 before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs...

We report a fabrication of organic light-emitting diodes (OLEDs) based on soluble phenyl-substituted poly-p-phenylene-vinylene (Ph-PPVs) thin films with aluminum cathode prepared by ion-beam-assisted deposition (IBAD). The electrical properties of the aluminum cathode, prepared by IBAD, on Ph-PPV have been investigated and compared to those by thermal evaporation. Energetic particles of Al assisted by an Ar+ ion may damage the organic material generating undesirable leakage current even though a thin Al buffer layer is applied to avoid Ar+-ion-induced damages. Substantial improvements of passivation characteristics were observed in IBAD device because the dense Al cathode inhibits the permeation of H2O and O2 into Ph-PPV film through pinhole defects, and thus retards dark spot growth. These results may be deduced from the highly packed structure that has small contact resistance between Al and Ph-PPV in ion-beam-assisted aluminum devices.