Transparent Cathode Research Articles

Highly transparent cathodes comprised of rare earth and Au stacked layers for top-emission organic light emitting diodes

We report the transparent cathodes comprised of a low work function rare earth element (RE) and a Au capping layer for top-emission organic light emitting diodes (TOLEDs). The Sm∕Au, Yb∕Au, Gd∕Au, and Dy∕Au cathodes possess a transmittance up to ∼60% when the RE/Au thickness is 5∕10nm. The light extraction efficiencies for the TOLEDs with a silicon anode and a RE/Au cathode are computed to be 9%–13%, markedly higher than that for a similar device but with an Al∕Au cathode. The surface plasmon polaritons at the Yb(orSm)∕Au cathode are found to be much weaker than those at the Al∕Au cathode.

Performance improvement of top-emitting organic light-emitting diodes by an organic capping layer: An experimental study

The optical outcoupling of top-emitting organic light-emitting diodes (OLEDs) can be improved by a thin dielectric capping layer on top of the transparent cathode. We investigate the emission properties of a set of top-emitting OLEDs with the same device structure, but different organic capping layer thicknesses to understand the capping layer effect. We demonstrate that the distribution pattern of the emitted light from top-emitting OLEDs depends strongly on the capping layer thickness, showing not only a maximum current efficiency enhancement by a factor of 1.38 (up to 78cd∕A in forward direction at 1000cd∕m2), but also a quantum efficiency improvement by 35%. This leads to a device efficiency of up to 17.8% and 69lm∕W at a brightness of 1000cd∕m2. We show that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer. The maximum outcoupling efficiency is achieved at a capping layer thickness where the top contact stack (cathode+capping layer) shows low absorption and high reflection.

Transparent polymer cathode for organic photovoltaic devices

We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 10 2–10 3 S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.

Improvement of the output characteristics of magnetrons using the transparent cathode

The output characteristics of magnetrons can be significantly improved by using a cathode that is transparent to the synchronous electromagnetic fields thereby providing improved conditions for fast conversion of the electrons' potential energy into electromagnetic energy. The transparent cathode consists of separate longitudinal metal strips, arranged to form a cylindrical surface, that act as individual electron emitters. Favorable prebunching of electrons to excite the desired operating mode is provided by a suitable choice of the number and azimuthal position of the cathode strips. The strong azimuthal wave field in the cathode region rapidly captures the prebunched electrons into rotating spokes forming the anode current. This process provides faster start-of-oscillations than priming using a solid cathode. The strong wave field in the electron sheath over the cathode, for any sheath thickness, gives the possibility of improving the efficiency by concomitantly increasing the applied voltage and magnetic field. Computer simulations of the A6 magnetron driven both by a solid and transparent cathode demonstrate the advantages of this approach

Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic light-emitting diodes

We report on plasma damage-free sputtering of an indium zinc oxide (IZO) top cathode layer for top-emitting organic light-emitting diodes (TOLEDs) by using a box cathode sputtering (BCS) technique. A sheet resistance of 42.6Ω∕cm and average transmittance above 88% in visible range were obtained even in IZO layers deposited by BCS at room temperature. The TOLED with the IZO top cathode layer shows electrical characteristics and lifetime comparable to a TOLED with only thermally evaporated Mg–Ag cathode. In particular, it is shown that the TOLED with the IZO top cathode film shows very low leakage current density of 1×10−5mAcm2 at reverse bias of −6V. This suggests that there is no plasma damage caused by the bombardment of energetic particles during IZO sputtering using the BCS system.

Top emission organic light emitting diode with transparent cathode, Ba‐Ag double layer

We fabricated top emission organic light emitting diode (TEOLED) with transparent metal cathode Barium and Silver bilayer. Very thin Ba/Ag bilayer was deposited on the organic layer by thermal evaporation. This cathode showed high transmittance over 70% in visible range, and the device with a Ba‐Ag has a low turn on voltage and good electrical properties.

4.3: Transparent ZnO Thin Film Transistor Array for the Application of Transparent AM-OLED Display

We have fabricated 2.15 inch AM-OLED panel composed of 176 × 144 (106 dpi) transparent pixels driven by ZnO-TFT. Transparent ZnO thin film deposited by means of atomic layer deposition at 100°C was used for the active channel. The ZnO TFT had a mobility of 1.13cm2/V.s, a threshold voltage of 0.8V, and an on/off ratio of 106. The fabrication of the ZnO TFT array for AM-OLED display has been carried out with four-mask process. and the integration of the ZnO TFT array and the transparent OLED consisted of a transparent anode and cathode and organic layers led to really transparent display.

P-198: High Transparency and Low Operation Voltage Top Emission Organic Light Emitting Devices with Thin Film Encapsulation

We have fabricated 3V turn on AMOLED displays with a 70 % transparent cathode consisting of ITO film at 40 A/s deposition rate. The stack of organic and inorganic films plus over-coated UV glue prepared in low temperature was optimized for protection of top-emitting displays, proven by Ca test at 60 °C/90% RH.

Radio frequency magnetron sputter-deposited indium tin oxide for use as a cathode in transparent organic light-emitting diode

Indium tin oxide (ITO) films were prepared by radio frequency magnetron sputtering at room temperature, for use as a cathode in a transparent organic light-emitting diode (TOLED). To minimize damage to the TOLED by the ITO sputtering process, the target-to-substrate distance was increased to 20 cm. An ITO film deposited at the optimum oxygen partial pressure exhibited an electrical resistivity as low as 4.06 × 10 − 4 Ω cm and a high optical transmittance of 91% in the visible range. The film was used as a transparent cathode for a TOLED with structure of an ITO coated glass substrate / Naphthylphenyldiamide (60 nm) / Tris–(8-hydroxyquinoline) aluminum (60 nm) / LiF (1 nm) / Al (2 nm) / Ag (8 nm) / ITO cathode (100 nm). A maximum luminance of 37,000 cd/m 2 was obtained. The device performance was comparable to a conventional OLED.

Novel transparent Yb-based cathodes for top-emitting organic light emitting devices with high performance

We have studied three kinds of transparent low-work-function Yb-based cathodes for the top-emitting organic light emitting devices (TEOLEDs) with a structure of ITO/NPB/Alq(3)/cathodes and compared them with each other. For the Yb/Au cathodes, a series of Yb layers with various thicknesses have been tested and it is found that the Yb layer with a thickness of 4 nm is the optimum one. The Yb:Au (19 nm) and Yb:Ag (19 nm) co-evaporation cathodes possess very high transmittance but relative poor electron injection; whilst the Yb (4 nm)/Au (15 nm) cathode possess a little lower transmittance but much improved electron injection and the TEOLED with this cathode has the highest power efficiency among the TEOLEDs with the three kinds of Yb-based cathodes mentioned above. (c) 2005 Elsevier B.V. All rights reserved.

Transparent organic light-emitting devices with LiF/Mg:Ag cathode

Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/ N, N'-diphenyl-N, N'-(3- methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) (90 nm)/ tris-(8- hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turnon voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.

49.3: A 200-dpi Transparent a-Si TFT Active-Matrix Phosphorescent OLED Display

We have fabricated a 120×160 high-resolution (200dpi) a-Si TFT active-matrix transparent phosphorescent OLED (PHOLED™) display with novel pixel architecture to maximize transparency and aperture ratio and also ensure comparable light emission from both sides of the display. The a-Si backplane was selected as the technology that would most easily enable the pathway toward achieving high-resolution flexible transparent AMOLEDs (T-AMOLEDs) on polymeric substrates. A-Si TFTs are preferred for fabrication on polymeric substrates since lower process temperatures can be used in comparison to poly-Si TFT processes. As a TOLED generally emits less light from a transparent cathode than anode, a standard 2T pixel was designed with both an opaque, reflective anode region on top of the TFTs as well as a conventional transparent ITO anode to equal the emission from both contacts. This design achieves a total pixel aperture ratio of 64% with a display transparency of 23% in the off-state.

51.3: 2.1‐inch QCIF+ Dual Emission AMOLED Display having Transparent Cathode Electrode

AbstractWe have developed 2.1‐inch Dual Emission AMOLED Display having a transparent cathode electrode. An image can be seen from both the top and bottom sides of the display with the same brightness. By combining with optical films, the display has high contrast of 400 under room lighting.

Magnetron Discharge Characteristics for Improvement of an Inertial Electrostatic Confinement Neutron/Proton Source

A magnetron discharge was adopted in the inertial-electrostatic confinement (IEC) fusion device for drastic improvement of fusion reaction rate. With this discharge in the vicinity of the vacuum chamber, a substantial number of ions produced there are expected to have almost full energy corresponding to the applied voltage to the transparent IEC cathode under relatively low pressures compared with the conventional glow discharge. The magnetron discharge is found to occur even for the pressure of 0.07 mTorr (H2) in the present configuration of the experiment, compared with 5 mTorr in the glow discharge.

27.4: Top‐emitting Organic Light‐Emitting Diode using Nanometer Platinum Layers as Hole Injector

AbstractTop‐emitting organic light emitting diodes (Top‐OLED) were made with platinum (Pt) and praseodymium oxide (Pr2O3) as the anode buffer. ITO on thin Al was used as the transparent cathode. Efficient Top‐OLED were obtained with a microcavity design. It was found that this Top‐OLED was highly non‐Lambertian. It emitted 10% more photons than conventional bottom‐emitting‐ OLED after integration over all angles.

DC sputtered indium-tin oxide transparent cathode for organic light-emitting diode

The performance of top-emitting organic light-emitting diodes depends not just on the choice of the transparent cathodes but also on their techniques of formation. Compared to the damage induced by radio frequency sputtering of indium-tin oxide cathode, that induced by DC sputtering was verified to be less severe and relatively independent of the sputtering power. Consequently, a high DC sputtering power of 120 W could be employed to achieve a high deposition rate of 0.1 nm/s. Adequate emission efficiency was maintained, even with a relatively thin 7-nm copper (II) phthalocyanine buffer layer.

Organic optical bistable switch

We demonstrate an organic optical bistable switch by integrating an efficient organic photodetector on top of a transparent electrophosphorescent organic light-emitting diode (TOLED). The bistability is achieved with an external field-effect transistor providing positive feedback. In the “LOW” state, the TOLED is off and the current in the photodetector is solely its dark current. In the “HIGH” state, the TOLED emits light that is directly coupled into the integrated photodetector through the transparent cathode. The photocurrent then is fed back to the TOLED, maintaining it in the HIGH state. The green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host was used as the luminescent material in the TOLED, while alternating thin layers of copper phthalocyanine and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole were used as the active region of the organic photodetector. The circuit has a 3 dB bandwidth of 25 kHz, and can be switched between HIGH and LOW using pulses as narrow as 60 ns. The bistable switch can be both electrically and optically reset, making it a candidate for image-retaining displays (e.g., electronic paper) and other photonic logic applications. The integrated organic device also has broad use as a linear circuit element in applications such as automatic brightness control.

High-efficiency top-emitting organic light-emitting devices

Based on theoretical arguments that top-emitting organic light-emitting devices (TOLEDs) can be more efficient than equivalent bottom-emitting devices, we fabricated TOLEDs comprising reflective anodes and transparent compound cathodes that emit 20.8% more photons in the forward 120° cone than equivalent bottom-emitting OLEDs. Device optimization by tuning the thickness of the top indium–tin–oxide layer according to a microcavity model is also reported.

Applications of Ytterbium in organic light-emitting devices as high performance and transparent electrodes

Potential applications of Ytterbium (Yb) in cathode system for organic optoelectronic/electronic devices were explored in NPB/Alq 3 based bi-layer organic light-emitting devices (OLEDs). When a thin (14.5 nm) Yb layer capped with a thicker (200 nm) Ag layer was used as the cathode, the OLEDs show enhanced electron injection over those using the standard Mg:Ag cathode. Performance of the OLEDs with the Yb/Ag cathode is comparable to that using LiF/Al cathode. Interestingly, we also found that Yb can also be used to prepare a highly transparent cathode by co-evaporating Yb and Ag to form a Yb:Ag alloy electrode. Surface-emitting (or top emission) and transparent (emission from both surfaces) OLEDs with low turn-on voltage (3.75 V) and high efficiency were prepared with the Yb:Ag electrode.

Liquid ammoniates: nonaqueous electrolytes for electrochromism

New conductive materials involving Cu + ions are proposed for electrochromic devices. They are liquid ammoniates which can be used from room temperature up to 80°C, and whose conductivity and p(NH 3) can be modulated according to requirements. The synthesis does not require strong acids or complexing agents such as halide ions. The principle of operation of the electrochromic material is the reduction of Cu + to Cu at the transparent cathode while Cu + is oxidized to Cu 2+ (thus developing a blue color) at the transparent anode. The bleaching is realized by the reverse reactions.