Optical characteristics of tandem and microcavity tandem organic light‐emitting devices

Abstract— In pursuit of the further enhancement of the luminance and efficiency of organic light‐emitting devices (OLEDs), it is worthy of exploring what benefits could be obtained by combining two luminance‐enhancement techniques, i.e., microcavity and tandem OLEDs. Furthermore, a deeper understanding of the optics in tandem OLEDs will be useful for the design and optimization of tandem OLEDs. In this paper, the optical characteristics of noncavity and microcavity tandem OLEDs are theoretically and experimentally investigated. By the use of rigorous electromagnetic modeling of OLEDs, the radiation characteristics of tandem OLEDs as a function of device structures are analyzed and correspondingly, the guidelines for optimizing the performance of tandem devices are suggested. By making use of the analytical results, it is shown that with well‐designed microcavity conditions and device structures, a five‐fold enhancement in luminance in the normal direction can be achieved with cavity‐tandem devices having only two emitting units. A very high efficiency of 200 cd/A for a rather broad brightness range of 100–4000 nits is demonstrated with a phosphorescent cavity two‐unit device.     

Composite silver electrode for double‐sided‐emitting stacked organic light‐emitting diode

Abstract— A reflective composite silver electrode is proposed and characterized as the middle electrode of a stacked organic light‐emitting diode (OLED) with double‐sided light emission. The proposed electrode is composed of a thermally evaporated stack of LiF (1 nm)/Al (3 nm)/Ag (70 nm) layers. The LiF/Al and the plasma‐treated Ag of the electrode function well as the respective cathode and anode of the bottom‐ and top‐emitting stacked OLEDs, with both being of the non‐inverted type. Power efficiencies of 10.3 and 12.1 lm/W at 100 cd/m² have been measured for bottom‐ and top‐emitting OLEDs, respectively, using dye doping. The stacked OLED having this bipolar middle electrode can be constructed as a two‐terminal‐only device, allowing for simpler driving schemes in double‐side‐emitting passive‐/active‐matrix OLED displays.     

Passive‐matrix polymer light‐emitting displays

Abstract— Organic light‐emitting device research focuses on the use of small‐molecule and polymer materials to make organic electroluminescent displays, with both passive‐ and active‐matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full‐color passive‐matrix displays. The stability of polymer OLEDs, and the use of ink‐jet printing for direct high‐resolution patterning of the light‐emitting polymers will also be discussed. It will be shown that the performance of light‐emitting polymers is at the brink of being acceptable for practical applications.     

Luminous‐efficiency improvement of solar‐cell‐integrated high‐contrast organic light‐emitting diode by applying distributed Bragg reflector

Abstract— Solar‐cell‐integrated organic light‐emitting diodes (OLEDs) were fabricated with both high contrast ratio and energy‐recycling ability. However, the luminous efficiency of the integrated devices is reduced to 50% of that of conventional top‐emitting OLEDs. A novel structure to recover the luminous efficiency from 50% to near 85% by applying a distributed Bragg reflector (DBR) made of 20 layers of GaN/AlN was demonstrated. It saves about 40% of the electric power than that of a device without a DBR. The contrast ratio remains high compared to that of conventional OLEDs. In this paper, simulations were conducted first to prove our models and assumptions. Then, two types of thin‐film solar cells — CdTe and CIGS solar cells — were used. They had different contrast ratios as well as viewing‐angle properties. Finally, the emission spectrum was calculated to be 11 nm FWHM, which is narrower than that for the emission spectrum of a typical microcavity OLED and has the advantage of having saturated colors.     

Eliminate angular color shift in top‐emitting OLEDs through cavity design

OLEDs suffer from viewing angle dependent spectral shift due to microcavity effects. To address this issue, we introduce a novel top‐emitting OLED with a dielectric spacer that forms multiple cavity modes. The resulting device shows almost no color shift at different viewing angles.     

Performance enhancement of top‐ and bottom‐emitting organic light‐emitting devices using microcavity structures

Abstract— In order to improve the efficiency of top‐ and bottom‐emitting devices, metallic electrodes have been used to create microcavity effects within the OLED structure. Semi‐transparent Ag is used as the anode in bottom‐emitting microcavity structures, whereas various reflective opaque metallic anodes are used for the top emitters. The cathode used in both configurations is MgAg — thick and opaque in the case of the bottom emitter and thin and semi‐transparent in the case of the top emitter. Modeling and experiments show that for the top‐emitting structures, the device efficiency is roughly proportional to the reflectivity of the anode in the low reflectivity range and increases significantly more than predicted by reflectivity alone in the high‐reflectivity range. An ultrathin CF_x or MoO_x hole‐injecting layer allows for the use of many metals as anodes and is an important feature of the device structure. With an Ag anode, both the top‐ and bottom‐emitting microcavity devices are about twice as efficient (on axis) as the analogous nonmicrocavity bottom‐emitting device. Microcavity devices employing a C545T‐doped Alq emitter exhibit efficiencies of 21 cd/A at 6.4 V and 20 mA/cm², with operational stability equivalent to conventional bottom‐emitting structures.     

Device‐dependent angular luminance enhancement and optical responses of organic light‐emitting devices with a microlens‐array film

Abstract— By taking the organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of organic light‐emitting devices (OLEDs) with a microlens‐array film (MAF) can be further evaluated by the ray‐tracing approach. First, the OLEDs of different Alq³ thickness are fabricated and their angular luminance measurements are compared to simulation results. Second, mode analyses for different layers are performed to estimate the enhancement potential of the MAF‐attached devices. Finally, by decreasing the Alq³ thickness, increasing the viewing angle, and attaching the MAF, the EL spectral peak shifts of the OLEDs seem irregular, but the spectral blue shifts induced by the optical structures are all explained by the optical responses (EL spectra divided by the intrinsic PL spectrum). In conclusion, the organic emitters with higher off‐axis‐angle luminous intensity cause lower out‐coupling efficiency but gain higher enhancement after the MAF is attached. With the choices of apodizations and microstructures, the tailored or customized angular radiation patterns can be also made possible.     

Electrical characterization and modelling of top‐emitting PIN‐OLEDs

Abstract— Positively doped, intrinsic, negatively doped organic light‐emitting diodes (PIN‐OLEDs) have been shown to exhibit high efficiency and a long lifetime compared to conventional small‐molecule OLEDs (SM‐OLEDs). The improved performance of PIN‐OLEDs makes them attractive for use in display applications. Knowledge of the electrical load exhibited by these devices is used to develop an equivalent electrical‐circuit model. Such models are used by circuit designers to assist with the precise design of active‐matrix‐display driver circuits used in such applications. In this paper, the development of a SPICE model for a top‐emitting PIN‐OLED stack is reported.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

Organic light‐emitting diodes with enhanced out‐coupling efficiency using high‐refractive‐index glass frit

We have fabricated a novel type of substrate for organic light‐emitting diodes (OLEDs) to improve the light out‐coupling efficiency. It was fabricated by forming an excellent flat layer using a high‐refractive‐index B₂O₃‐SiO₂‐Bi₂O₃ frit glass on the light diffusive glass substrate. By using this substrate, we have sufficiently reduced the total internal reflection of OLEDs, and we successfully obtained more than 1.9 times higher light out‐coupling efficiency without spectral changes and viewing angle dependency. Furthermore, we have also successfully demonstrated 50 × 50 mm large‐area white OLEDs with this novel substrate.     

A novel electron‐transport material for organic light‐emitting devices

Abstract— A nanocrystalline electron‐transport material [ET68] was introduced into organic light‐emitting devices (OLEDs). By integrating a p‐doped transport system and phosphorescent emitters, a very bright and stable device could be obtained. Furthermore, 40% saving in power consumption can be achieved when the efficient pixels with ET68 were applied to AMOLEDs.     

White‐emitting OLED devices in an RGBW format with microelement white subpixels

Abstract— OLED devices with an RGBW pixel format using an unpatterned white emitter have the potential to provide very good efficiency and color gamut while enabling lower‐cost and large‐format manufacturing. However, the white subpixel often has unacceptably large color shifts with viewing angle. Furthermore, for some architectures such as top‐emitting microcavity devices, it can even be difficult to produce a white subpixel with good on‐axis color. In this paper, we describe the use of a white subpixel made up of a combination of differently tuned microelements and demonstrate how such an approach can overcome these problems. By carefully tuning the color and areas of each of the microelements in the white subpixel, we can trade off between better on‐axis color, less color change with angle, and higher efficiency. Furthermore, it was demonstrated that an RGBW top‐emitter microcavity device with a microelement white subpixel can achieve an increase in both power efficiency and color gamut relative to a conventional RGBW bottom‐emitter non‐microcavity device.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

Organic light‐emitting devices integrated with internal scattering layers for enhancing optical out‐coupling

Abstract— In this work, studies on organic light‐emitting devices (OLEDs) incorporating nanoparticle‐based nanocomposite scattering layers as the internal extraction structures (i.e., between the substrate and the OLED structure) were concluded. By adjusting the nanoparticle sizes and concentrations, the optical properties (optical scattering and integrated transmittance) of the nanocomposite scattering layers can be widely tuned. With appropriate compositions, nanoparticle‐based nanocomposite scattering layers with strong optical scattering, high integrated transmittance, and yet enough flatness for device integration can be obtained. The use of such nanocomposite internal scattering layers provides a convenient and effective approach for simultaneously achieving large efficiency enhancement (1.96× for quantum efficiency and 2.04× for cd/A efficiency) and improving viewing characteristics (more stable colors and emission patterns over angles). The fabrication of nanoparticle‐based nanocomposite scattering films is based on solution‐processing, which is relatively simple and convenient. These features may make it highly attractive for various OLED applications and mass production.     

Novel front‐light system using fine‐pitch patterned OLED

Abstract— A novel front‐light system that uses an organic light‐emitting‐diode (OLED) light source patterned with a fine pitch has been developed. The front‐light system has the following characteristics: (1) excellent uniformity within the light‐emitting area; (2) emittance that is consistent at all viewing angles; (3) no light leakage at any viewing angle from the side of the observer. This system can be adopted for reflective LCDs, electrophoretic displays (EPDs), microelectromechanical systems (MEMS), and other applications.     

Active‐matrix technology for high‐efficiency OLED displays

Organic light‐emitting device (OLED) technology has recently been shown to demonstrate excellent performance and cost characteristics for use in numerous flat‐panel‐display (FPD) applications. Universal Display Corp. (UDC), together with its academic partners at Princeton University and the University of Southern California, are developing high‐efficiency electrophosphorescent OLEDs, based on triplet emission. These material systems show good lifetimes, and are well suited for the commercialization of low‐power‐consumption full‐color active‐matrix OLED displays. Their very high conversion efficiencies may even allow them to be driven by amorphous‐silicon backplanes, and in this paper we consider design guidelines for an amorphous‐silicon pixel to minimize display non‐uniformities due to threshold voltage variations.     

Displacement‐current analysis of organic light‐emitting diodes

Abstract— Organic light‐emitting diodes (OLEDs) having multiple organic layers were fabricated to analyze the physical phenomena occurring in an OLED according to the amplitude of the applied voltage. The staircase voltage with both an increasing period and a constant period was designed and applied to an OLED. The displacement current began to change at a voltage where the conduction current began to change, and partly originated from the formation of space charge due to the low mobility of the majority carrier. The displacement current was shown to be constant at low voltage and decreased after showing a maximum value as the applied voltage increased. The exact voltage for the injection of two types of carriers and light emission could be obtained from the variation in the displacement current.     

Light‐emitting polymers for full‐color display applications

The latest developments in light‐emitting‐polymer (LEP) technology at CDT continue to show steady progress. Device performance for blue, green, and red systems as well as a high‐performance yellow system in terms of device efficiency and stability will be described. Some of the issues associated with the commercialization of LEP technology including the development of direct‐patterning techniques enabling full‐color passive‐ and active‐matrix display will be discussed.     

Quantum dot light emitting devices for photomedical applications

While OLEDs have struggled to find a niche lighting application that can fully take advantage of their unique form factors as thin, flexible, lightweight and uniformly large‐area luminaire, photomedical researchers have been in search of low‐cost, effective illumination devices with such form factors that could facilitate widespread clinical applications of photodynamic therapy (PDT) or photobiomodulation (PBM). Although existing OLEDs with either fluorescent or phosphorescent emitters cannot achieve the required high power density at the right wavelength windows for photomedicine, the recently developed ultrabright and efficient deep red quantum dot light emitting devices (QLEDs) can nicely fit into this niche. Here, we report for the first time the in‐vitro study to demonstrate that this QLED‐based photomedical approach could increase cell metabolism over control systems for PBM and kill cancerous cells efficiently for PDT. The perspective of developing wavelength‐specific, flexible QLEDs for two critical photomedical fields (wound repair and cancer treatment) will be presented with their potential impacts summarized. The work promises to generate flexible QLED‐based light sources that could enable the widespread use and clinical acceptance of photomedical strategies including PDT and PBM.     

Review Paper: Progress on efficient cathodes for organic light‐emitting diodes

Abstract— Balanced charge injection is essential for highly efficient and stable OLEDs. Various cathode materials, such as elemental metals, metal alloys, and metal compounds, have been adopted to facilitate electron injection. Currently, composite cathodes utilizing an electron‐injection layer and an air‐stable metal, such as LiF/Al, is the most common choice. This article will review the progress of efficient vacuum‐evaporated cathodes for OLEDs and their mechanisms.