Semitransparent Cathodes Research Articles

ZnO/Ag/ZnO multilayers as an n-type transparent conducting electrode for transparent organic light-emitting diodes

Alternative transparent conducting electrodes (TCEs) have gained significant attention in recent decades in an effort to replace costly indium-tin oxide (ITO) in organic optoelectronic devices. For example, state-of-the-art top-emitting and transparent organic light-emitting diodes (OLEDs) in the display industry require a transparent or semitransparent cathode for electron injection. Current OLED display technology employs ultrathin metallic (typically Mg:Ag) layers as semitransparent cathodes. However, recent studies have shown that metal oxide/metal/metal oxide multilayers can perform better optically and offer improved stability compared to semitransparent metal cathodes. Deposition of these multilayers with minimal damage to the underlying organic emissive layer is imperative. Here, we investigate a ZnO/Ag/ZnO TCE multilayer TCE deposited under mild sputtering conditions that are safe for the emissive layer. We demonstrate ∼ 85 % transparent electrodes with a sheet resistance of 1.47 Ω/sq, which is more conductive than the industry standard, ITO. We also integrate the multilayer TCE as the cathode in a transparent OLED device built on ITO, demonstrating the compatibility of this TCE with the existing OLED technology.

Technical status of top-emission organic light-emitting diodes

Top-emission organic light-emitting diodes (TEOLEDs) that contain semi-transparent metal top cathodes and highly reflective anodes have been actively researched for display applications due to their many advantages such as their high aperture ratio, good color purity, and high efficiency. In this research, the technical design of TEOLEDs used in organic light-emitting diode (OLED) displays is reviewed, covering the optical theory with the microcavity effect, optical losses, and the importance of the Purcell effect in the optical calculation. The key methodologies for addressing the high efficiency and low driving voltage of TEOLEDs are also discussed.

Organic light emitting diodes with p-Si anodes and semitransparent Ce/Au cathodes

The Ce (x nm)/Au (15 nm) stacked layers were used as semitransparent cathodes in the top-emission organic light emitting devices (TOLEDs) fabricated on a p-type silicon anodes and substrate, where x varies from 4 to 16. The consequence of the Ce layer thickness on transmittance and the device performance were studied when the organic layers NPB (60 nm)/ALQ (60 nm) were kept unchanged, where NPB was N, N′-bis-(1-naphthl)-diphenyl-1, 1′-biphenyl-4, 4′-diamine, and AlQ is tris-(8-hydroxyquinoline) aluminum. The cathode of Ce (11 nm)/Au (15 nm) has a transparency of 46%, and the TOLED with it achieves the highest luminescence efficiencies: a current efficiency of 0.91 cd/A at 13.7 V and a peak power efficiency of 0.28 lm/W at 9 V. The turn-on voltage is 3.0 V. The Ce/Au cathode is both chemically and electrically stable.

A Study of Semitransparent Cathodes on the Performance of Top-Emitting Polymer Light-Emitting Diodes

This work reports the influence of semitransparent cathodes, which were made of Ca or Al covered with a thin layer of Ag, on the performance of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based top-emitting polymer light-emitting diodes (T-PLEDs). The physical properties of the semitransparent cathodes alter the optical microcavity of T-PLEDs and adjust the output characteristics of the electroluminescence (EL). The bright, red-emissive, MEH-PPV-based T-PLED, with Ca(10 nm)/Ag(20 nm) as the semitransparent cathode, exhibited 3.6 cd/A, 38.7 nm, and (x = 0.67, y = 0.33) for the luminous efficiency, full-width at half-maximum of the EL, and Commission International de I'Eclairage coordinate, respectively. The favorable performance can be attributed to the formation of an appropriate microcavity structure, which results in spatial redistribution and enhances the out-coupling of EL emission in the direction of the surface normal.

Sr/Ag semitransparent cathodes for top emission organic light-emitting devices

We have developed a semitransparent cathode for the top emission organic light-emitting devices using a Sr/Ag double layer prepared by the thermal evaporation technique. The Sr (8–10 nm)/Ag (10 nm) cathode shows the transmittance of 55–76% in the visible spectral region and the sheet resistance of about 12 Ω/□. The underlying Sr layer affects the growing characteristics of Ag layer, resulting in high optical transparency. The bis[2-(2′-benzothienyl)-pyridinato-N,C 3′]iridium(acetylacetonate) doped top emission electro-phosphorescent device with a Sr/Ag semitransparent cathode has been fabricated and studied.

Top emitting poly(p-phenylene vinylene) light-emitting diodes on metal sandwiched polyethylene terephthalate substrates

Flexible poly(p-phenylene vinylene) organic light-emitting diodes (OLEDs) with top-emitting architecture were developed on metal-sandwiched polyethylene terephthalate (PET) substrates. Commercial aluminum-laminated PET (Al-PET) was adopted as initial substrates. Different kinds of metal layers (silver and nickel) were then deposited on Al-PET foil to form metal-sandwiched plastic substrates and also to serve as anodes for the top emitting OLEDs. Interface modification was employed to enhance carrier injection from the metal anodes to the polymeric organic stack. Semitransparent top cathodes were used for the flexible top-emitting diodes. The performance of these OLEDs was characterized and compared among different metal anodes. The flexible top-emitting OLEDs on metal-sandwiched PET foil can be bent to a substantial degree without breaking. Moreover, increased contrast ratio of OLEDs can be achieved with specific anodes due to lower reflectance of nickel material.

Colour tunability of polymeric light-emitting diodes with top emission architecture

Polymeric light-emitting diodes (PLEDs) have been fabricated with top emissive architecture and a shift of the light emission wavelength is observed for a single emitting material—phenyl-substituted poly(p-phenylenevinylene) (Ph-PPV). The device is built on a glass substrate and its structure consists of a bi-layer anode (including a high reflective metal thin film and indium tin oxide anode—ITO thin film), polymer stack and semitransparent multi-layer cathodes. Poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT: PSS) and Ph-PPV are employed as the hole transport layer and light-emitting layer, respectively. Changes in the electroluminescence (EL) characteristics of the PLEDs are examined by adjusting the ITO thickness. The EL peaks of the devices with a single emissive layer have a wide variation in the wavelength range, from 547 to 655 nm, due to the different ITO thicknesses.

Microcavity organic light-emitting diodes on silicon

We study resonant-cavity organic light-emitting diodes made on silicon substrates. The device structure is Al/indium–tin–oxide (ITO)/copper phthalocyanine (CuPc)/a triphenylamine derivative (TPD)/tris-(8-hydroxyquinoline) aluminum (Alq3)/cathode, where the cathode is a semitransparent Al layer or a LiF/Al stack. We use a model based on the transfer-matrix method to deduce the wavelength dependence of the ITO refractive index, and to calculate the spectra and the angular emission diagrams of the diodes. Microcavities limit the spectral and spatial distributions of the emitted light in accordance with the model. Current–voltage characteristics of various devices prove that a thin LiF layer improves the injection of electrons in Alq3 from semitransparent aluminum cathodes.

Semitransparent cathodes for organic light emitting devices

We optimize transparent organic light emitting devices (TOLEDs) using compound cathodes consisting of a thermally evaporated metal contact layer capped with indium–tin–oxide (ITO). The ITO is sputtered at rates of up to 1.6 Å/s using a high power radio frequency magnetron process. With a Mg:Ag contact layer, we demonstrate a TOLED with 50% transparency and an operating voltage within 0.3 V of a device with identical organic layers and a conventional Mg:Ag cathode. The operational lifetime of the TOLED is shown to be equal to that of a similar, nontransparent device. We also study the effects of using different contact metals, including Ca, Al and LiF, on the operating characteristics of the TOLEDs. With a thin Ca contact layer, undoped TOLEDs with >80% peak transparency operating at (5.9±0.1) V at a brightness of >100 cd/m2 are demonstrated. These devices have application to transparent, head-up displays and to full color, stacked organic light emitting devices.

Study of the dc saddle-field discharge: Application to methane

The electrical characteristics of a dc saddle-field discharge were studied for methane. The dc saddle-field configuration consists of a semitransparent anode sandwiched between two semitransparent cathodes, and an electrically isolated substrate holder plate positioned behind each of the cathodes. In comparison to the standard dc discharge, the dc saddle-field discharge has a lower breakdown voltage and a lower pressure at the minimum breakdown voltage. This is mainly due to the anode semitransparency which increases the ionization efficiency of the discharge. The electric field between the substrate holder and the semitransparent cathode can be adjusted. This is easily achieved by self-biasing through the adjustment of the substrate–cathode resistance. Increasing the substrate–cathode resistance increases the voltage breakdown at low pressure, and the pressure at the minimum breakdown voltage. When the discharge is used for thin film deposition in the abnormal glow regime, the density of ions reaching the substrate can be controlled independently of ion energy. This added degree of control over ion current density and ion energy should prove very useful in the growth of thin films.

THE III-V PHOTOCATHODE: A MAJOR DETECTOR DEVELOPMENT

Description of new (III-V) photocathodes which show improvements in sensitivity of as much as ten to a hundred times over conventional cathodes in the near infrared and useful improvements at shorter wavelengths. The development stems from a combination of basic knowledge of the photoemission process, gained in the 1950s, and the advancing understanding of the technology of III-V materials, in the 1960s. The superior performance of these cathodes is due to the fact that the vacuum level at the surface lies below the bottom of the conduction band in the bulk of the material. Consequently, the threshold of response is set by the III-V bandgap. The bandgap (and the threshold of response) can be varied by alloying different III-V materials together. A reduction in thermionic emission is realized with these cathodes. At present no semitransparent III-V cathodes with comparably interesting performance are available. The problems hindering further improvements, as well as the problems of placing these cathodes in practical multipliers and image tubes, are discussed briefly.

Spectrally Selective Photodetectors for the Middle and Vacuum Ultraviolet

A number of “solar blind” photosurfaces have been developed in recent years. Measurements made on phototubes having photocathodes of rubidium telluride, cesium telluride, cesium iodide, and copper iodide are described. Cathode quantum efficiencies for semitransparent cathodes of rubidium or cesium telluride range from 10−1 to 10−2 electrons/quantum in the middle and vacuum ultraviolet, with long wavelength responses of less than 10−4 electrons/quantum beyond about 3500 A. By combining “solar blind” cathodes with windows of LiF, CaF2, or fused silica, detectors with relatively flat quantum yields can be produced, marked by high sensitivities in specific ultraviolet spectral regions and by very low sensitivities at all longer wavelengths.

PERFORMANCE DATA FOR SEVERAL INFRARED-SENSITIVE MULTIPLIER PHOTOTUBES

Infrared photometry has been practiced for many years by photoelectric methods ; however, the necessity in the past for employing diode photocells has restricted photometry in the infrared to those observers versed in the highly specialized electronic techniques required. The picture has changed within the last few years by the gradual appearance of good photomultipliers with the infrared-sensitive CsO-Ag cathode. Good infrared multipliers were first made in the laboratory of Prof. A. Lallemand at the Paris Observatory, who has made these multipliers available to astronomers throughout the world. More recently, perfected infrared-sensitive multipliers have been placed on the market in the United States by the Farnsworth Electronics Company (their 16 M 1 ), the DuMont Laboratories, Inc. (their K 1613), and most recently, by the Radio Corporation of America (their 7102). All of these multipliers are of end-on configuration with semi-transparent cathodes. Several infrared multipliers made by Prof. Lallemand have been in use at the Lick Observatory for three years for multicolor photometry, and our experience was recently extended by thoroughly testing a sample of the DuMont K 1613. Inasmuch as no performance data on infrared-sensitive multipliers are available, it seems timely to present the results of some of our experience. Infrared-sensitive multipliers are practically useless unless they are refrigerated, because of the large thermal emission from the cathode. This thermal emission can be so large that it alone can load the anode with its full allowable output current, leaving nothing for the light to be measured. Furthermore, the shot noise from the large dark current is so great that unrefrigerated multipliers are not useful for measuring light in the brightness range commonly found in astronomical photometry. The tests reported upon here were made on multipliers encased in well-built, her-