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This study proposes an aerodynamically optimized outer shape of a sedan by using an Artificial Neural Network (ANN), which focused on modifying the rear body shapes of the sedan. To determine the optimization variables, the unsteady flow field around the sedan driving at very fast speeds was analyzed by CFD simulation, and fluctuations of the drag coefficient (C D ) and pressure around the car were calculated. After consideration of the baseline result of CFD, 6 local parts from the end of the sedan were chosen as the design variables for optimization. Moreover, an ANN approximation model was established with 64 experimental points generated by the D-optimal methodology. As a result, an aerodynamically optimized shape for the rear end of the sedan in which the aerodynamic performance is improved by about 5.64% when compared to the baseline vehicle is proposed. Finally, it is expected that within the accepted range of shape modifications for a rear body, the aerodynamic performance of a sedan can be enhanced so that the fuel efficiency of the sedan can be improved. The YF SONATA, a sedan manufactured by Hyundai Motors Corporate, played a major role in this research as the baseline vehicle.

This research aims to develop an actively translating rear diffuser device to reduce the aerodynamic drag experienced by passenger cars. One of the features of the device is that it is ordinarily hidden under the rear bumper but slips out backward only under high-speed driving conditions. In this study, a movable arc-shaped semi-diffuser device, round in form, is designed to maintain the streamlined automobile’s rear underbody configuration. The device is installed in the rear bumper section of a passenger car. Seven types of rear diffuser devices whose positions and protrusive lengths and widths are different (with the basic shape being identical) were installed, and Computational Fluid Dynamics (CFD) analyses were performed under moving ground and rotating wheel conditions. The main purpose of this study is to explain the aerodynamic drag reduction mechanism of a passenger car cruising at high speed via an actively translating rear diffuser device. The base pressure of the passenger car is increased by deploying the rear diffuser device, which then prevents the low-pressure air coming through the underbody from directly soaring up to the rear surface of the trunk. At the same time, the device generates a diffusing process that lowers the velocity but raises the pressure of the underbody flow, bringing about aerodynamic drag reduction. Finally, the automobile’s aerodynamic drag is reduced by an average of more than 4%, which helps to improve the constant speed fuel efficiency by approximately 2% at a range of driving speeds exceeding 70 km/h.

A high performance inverted green emission organic light emitting diode with a maximum external quantum efficiency of 20% and a maximum power efficiency of 80 lm/W was realized by properly selecting an electron transporting material to have no energy barrier for electron injection between the n-doped electron transporting layer (n-ETL) and the ETL. Based on the energy levels and the current density–voltage characteristics of electron only devices, we demonstrate that the interface between an n-ETL and an ETL even in homo-junction is as important as the interface between the cathode and the n-ETL for efficient electron injection into an emitting layer.

The solution process is more suitable for large-sized OLED in terms of small material usage and no need for fine mask patterning technique. Among soluble materials, dendrimers are acknowledged as a new class of emissive OLED materials that can carry both the emissive chromophore and the charge transporting moiety simultaneously. Here, we report on a new Ir(III)-based dendrimer and an efficient single emission layer OLED device by spin-coating the dendrimer itself on PEDOT:PSS polymer. This dendrimer carries a pair of carbazole functional groups on the pyridine side and a fluorinated phenyl on the other side of the Ir(ppy) 3 ligand. The latter functional group acts as an electron transporting material and the former as a host and hole-transporting material. The dendrimer device has a green emission at 540 nm with current efficiency as high as 25 Cd/A. Interestingly, we have found that the two neighboring carbazole form the electromer under the applied electrical current and, thus, that the EL device shows new near-IR emission over 720 nm.

Corrugated organic light emitting diodes are fabricated on photonic crystal substrates patterned using UV nanoimprint lithography process at room-temperature and low-pressure. Forty-nine percentage enhancement of light extraction was achieved even with the very low corrugation depth of 50 nm. The power efficiency of the corrugated OLEDs was even further improved by 93% compared to the OLED with a planar structure. Finite difference time domain (FDTD) analyses indicated that the Bragg-reflection by the corrugated metallic mirror plays dominant role in the enhancement of extraction efficiency in the corrugated structure and extraction efficiency can be further improved if corrugation depth is increased to 100 nm.

Highly efficient deep-blue fluorescent materials based on phenylquinoline–carbazole derivatives (PhQ-CVz, MeO-PhQ-CVz, and CN-PhQ-CVz) are synthesized for organic light-emitting diodes (OLEDs). The materials form high-quality amorphous thin films by thermal evaporation and the energy levels can be easily adjusted by the introduction of different electron-donating and electron-withdrawing groups on carbazoylphenylquinoline. Non-doped deep-blue OLEDs that use PhQ-CVz as the emitter show bright emission (Commission Internationale de L'Eclairage (CIE) coordinates, x = 0.156, y = 0.093) with an external quantum efficiency of 2.45%. Furthermore, the material works as an excellent host material for 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl dopant to get high-performance OLEDs with excellent deep-blue CIE coordinates (x = 0.155, y = 0.157), high power efficiency (5.98 lm W−1), and high external quantum efficiency (5.22%).

A host material containing tetraphenylsilane moiety, 9-(4-triphenylsilanyl-(1,1′,4,1′′)-terphenyl-4′′-yl)-9 H -carbazole (TSTC), was synthesized for green phosphorescent organic light emitting diodes. The tetraphenylsilane moiety was introduced to provide high triplet energy level, thermal and chemical stability, and glassy properties leading to high efficiency and operational stability of the devices. Ir(ppy) 3 based OLEDs using the TSTC host and DTBT (2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine) hole blocking layer (HBL) resulted in the maximum external quantum efficiency of 19.8% and the power efficiency of 59.4 lm/W. High operational stability with a half lifetime of 160,000 h at an initial luminance of 100 cd/m 2 was achieved from an electrophosphorescent device using TSTC host and BAlq HBL.

This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-005- D00251), partially supported by the Strategic Technology Development Program (10030814), and partially supported by the Ministry of Commerce

Organic thin film diodes made by a polymer blend of poly[2-methoxy,5-(2'-ethyl-hexoxy)-1.4-phenylenevinylene] (MEH-PPV) and poly[1.3-propanedioxy-1.4-phenylene-1,2-ethenylene-(2.5-bis(trimethylsilyl)-1,4-phenylene)-1,2-ethenylene-1,4-phenylene] (called the B-polymer) are investigated. The device of sandwich configuration indium-tin oxide (ITO)/polymer-blend/A1 emits orange light under forward bias at + 10 V and the same device acts as a photodiode under reverse bias. To investigate the photodiode characteristics, the 516 nm wavelength with 9.5 mW/cm2 intensity of light is illuminated through the A1 contact side of the device. The I-V characteristic measurement shows the short circuit current and the open circuit voltage of −1.22 × 10−9 A/cm2 and 0.8 V, respectively. The ratio of the photocurrent to the dark current is about 4 × 102 at − 2.5 V reverse bias. The maximum d.c. sensitivity is 1.35 × 10 −5 A/W at 4× 7 V reverse bias voltage with 16 mW/cm2 intensity of the incident light. The results indicate the possibility of making photosensors using this device.

The magnetoresistance (MR) has been measured on polypyrrole doped with tetraethylammonium toluene-4-sulfonate (PPy(TOS), σ RT =25 S/cm) and on polyacetylene ((CH) x , σ RT =2×10 4 S/cm) in the temperature range from 1.3 to 4.2 K up to H =10 T. The data for PPy(TOS) show a positive MR, but for (CH) x samples they show a positive MR with a maximum, the position of which shifts towards lower fields as the temperature decreases. The obtained data are explained in terms of the effect of the magnetic field on the variable range hopping (VRH).

An etch-less ultraviolet nanoimprint lithography (UV-NIL) process is proposed for patterning a photonic crystal (PC) structure onto an organic light-emitting diode (OLED) substrate. In a conventional UV-NIL, anisotropic etching is used to remove the residual layers and to transfer the patterns onto the substrate. The proposed process does not require an etching process. In the process, a stamp with nano-scale PC patterns is pressed on the dispensed resin and UV light is then exposed to cure the resin. After tens of seconds, the stamp is separated from the patterned polymer layer on the substrate. Finally, high-refractive index material is coated onto the layer. The refractive index of the polymer should be very similar to that of glass. The enhancement of the light extraction was assessed by the three-dimensional (3D) finite difference time domain (FDTD) method. The OLED was integrated on a nanoimprinted substrate and the electro-luminance intensity was found to have increased by as much as 50% compared to a conventional device.

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

The authors report a promising metal oxide-doped hole transporting layer (HTL) of rhenium oxide (ReO3)-doped N,N′-diphenyl-N,N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB). The tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes with ReO3-doped NPB HTL exhibit driving voltage of 5.2–5.4V and power efficiency of 2.2–2.3lm∕W at 20mA∕cm2, which is significantly improved compared to those (7.1V and 2.0lm∕W, respectively) obtained from the devices with undoped NPB. Furthermore, the device with ReO3-doped NPB layer reveals the prolonged lifetime than that with undoped NPB. Details of ReO3 doping effects are described based on the UV-Vis absorption spectra and characteristics of hole-only devices.

The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes (PhOLEDs) is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone. Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers (D-EMLs) by confining the exciton formation near the interface between the emitting layers. The external quantum efficiency was maintained almost constant up to 22mA∕cm2 (10000cd∕m2) by adopting the D-EMLs in Ir(ppy)3 based PhOLEDs, resulting in high external quantum efficiency (ηext=13.1%) at high luminance.

One of the important factors for high efficiency phosphorescent organic light-emitting devices is to confine triplet excitons within the emitting layer. We synthesized and characterized a new hole blocking material containing silane and triazine moieties, 2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine (DTBT). Electrophosphorescent devices fabricated using the material as the hole-blocking layer and N,N′-dicarbazolyl-4,4′-biphenyl (CBP) doped with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the emitting layer showed a maximum external quantum efficiency (ηext) of 17.5% with a maximum power efficiency (ηp) of 47.8 lm W−1, which are much higher than those of devices using bathcuproine (BCP) (ηext = 14.5%, ηp = 40.0 lm W−1) and 4-biphenyloxolate aluminium(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) (ηext = 8.1%, ηp = 14.2 lm W−1) as hole-blocking layers.

This work was supported by a Korea Research Foundation grant funded by the Korean Government (MOEHRD: Basic Research Promotion Fund)(KRF-2006-003-D00243) and the Ministry of Commerce, Industry and Energy.

The spectral responses of steady state photoconductivity of a polymer blend, poly[2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene](MEH-PPV) and poly[1,3-propanedioxy-1,4-phenylene-1,2-ethenyllene-(2,5-bis(trimethylsilyl)-1,4-phenylene)-1,2-etheny lene-I,4-phenylene](called B-polymer) are investigated. Two peaks of photocurrent(PC) at 2.16 eV and 3.01 eV are observed under continuous illumination. The peak at 3.01 eV is rather broad and has a strong polarity dependence on the applied voltage, which could be due to the rectified microjunction formation in the polymer blend. The incident beam intensity dependences of the two peaks in PC are I pc ∝ I α with α = 1.05. It indicates that the monomolecular recombination kinetics is important in this polymer blend. The observed spectral response of PC is discussed in comparison with the optical absorption as well as the photoluminescence spectra reported earlier in the same polymer blend.

The steady state photoconductivities of poly(2-styryl-1,4-phenylene vinylene) (PSPV) and its oxidized form poly(2-methoxy-5-styryl 1, 4-phenyIene vinylene) (PMSPV) are studied. PSPV contains styryl radical in PPV backbone chain. The spectral response of the photocurrent of PSPV shows a peak at 2.7 eV with no additional peak structure near 2.4 eV. PMSPV made by the substitution of methoxy radical to PSPV shows somewhat unusual feature. The spectral response of PMSPV does not show any peak near the absorption edge and the magnitude of the photocurrent is decreased compared to that of the PSPV. The results are discussed in terms of the different interchain distances of the two polymers as well as the existence of a conjugated double bond in the styryl radical in these PPV derivatives.

We report the photoinduced absorption of the oxygen free C 60 thin film. The thin films of oxygen free C 60 are produced by evaporation of the C 60 powder onto a sapphire substrate. At energies between 1.65 eV and 2.25 eV, two peaks are observed; one at about 1.92 eV and the other one at about 2.25 eV which is broader than the former peak. In the case of the oxygenated film, there exists an another peak at about 1.75 eV in addition to the above two peaks. From the time resolved study of the peak at 1.92 eV, we can conclude that the lifetime of this peak is 40μs in the case of the oxygenated film. In the case of the oxygen free film, however, we can not find a clear decaying behavior up to 30μs.