System considerations for RGBW OLED displays

Abstract— The fabrication of full‐color RGBW OLED displays using a white emitter with RGB color filters has been previously described. This paper discusses the effect of several display‐system factors on the important RGBW OLED display performance attributes of power consumption, lifetime, and perceived image quality. These display‐system factors include the spectrum of the white OLED, the white OLED structure, the color‐filter selection, the subpixel aperture ratios, and the pixel arrangement (including sub‐sampling).     

A digitally driven pixel circuit with current compensation for AMOLED microdisplays

A novel digitally driven pixel circuit for active‐matrix organic light‐emitting diode (OLED) microdisplays is proposed and evaluated. This circuit supports both pulse width modulation and pulse density modulation digital drive approaches. Only three transistors and one capacitor are required per pixel for the proposed circuit. A current mirror is used to compensate for the pixel current changes that occur because of the degradation of the OLEDs over time. The compensation current depends on the potential of the common cathode, the properties of the current mirror, and the Width/Length (W/L) ratio of the drive transistor. The proposed digital pixel circuit also has advantages in circuit layout compared with analog pixel circuits.     

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.     

Simple pixel circuits for high resolution and high image quality organic light emitting diode‐on‐silicon microdisplays with wide data voltage range

Two simple pixel circuits are proposed for high resolution and high image quality organic light‐emitting diode‐on‐silicon microdisplays. The proposed pixel circuits achieve high resolution due to simple pixel structure comprising three n‐type MOSFETs and one storage capacitor, which are integrated into a unit subpixel area of 3 × 9 µm² using a 90 nm CMOS process. The proposed pixel circuits improve image quality by compensating for the threshold voltage variation of the driving transistors and extending the data voltage range. To verify the performance of the proposed pixel circuits, the emission currents of 24 pixel circuits are measured. The measured emission current deviation error of the proposed pixel circuits A and B ranges from ?2.59% to +2.78%, and from ?1.86% to +1.84%, respectively, which are improved from the emission current deviation error of the conventional current‐source type pixel circuit when the threshold voltage variation is not compensated for, which ranges from ?14.87% to +14.67%. In addition, the data voltage ranges of the proposed pixel circuits A and B are 1.193 V and 1.792 V, respectively, which are 2.38 and 3.57 times wider than the data voltage range of the conventional current‐source type pixel circuit of 0.501 V.     

Full‐color AMOLED with RGBW pixel pattern

Abstract— A full‐color AMOLED display with an RGBW color filter pattern has been fabricated. Displays with this format require about one‐half the power of analogous RGB displays. RGBW and RGB 2.16‐in.‐diagonal displays with average power consumptions of 180 and 340 mW, respectively, were characterized for a set of standard digital still camera images at a luminance of 100 cd/m². In both cases, a white‐emitting AMOLED was used as the light source, and standard LCD filters were used to provide the R, G, and B emission. The color gamuts of these displays were identical and the higher overall efficiency of the RGBW format results from two factors. First, a large fraction of a typical image is near neutral in color and can be reproduced using the white sub‐pixel. Second, the white sub‐pixel in an RGBW AMOLED display is highly efficient because of the absence of any color filter. The efficiency of these displays can be further enhanced by choosing a white emitter optimized to the target display white point (in this case D65). A two‐emission layer configuration based upon separate yellow and blue‐emitting regions is shown to be well suited for both the RGBW and RGB formats.     

A 13‐bit universal column driver for various displays of OLED and LCD

A universal column driver is implemented in a 0.13‐µm high‐voltage CMOS process for not only TFT‐LCD but also OLED applications. The proposed column driver employs 13‐bit linear DAC to cover all gamma curves of display applications and address‐based configuration for intra‐ panel interface protocol to support both TV and IT applications. Measured results demonstrate the average voltage of output channels (AVO) is under 1mv, which satisfies 1‐LSB resolution at 18.5V of AVDD.     

Recent progress in high‐efficiency phosphorescent OLED technology

Abstract— A key performance attribute for widespread commercialization of OLED technology is achieving maximum power efficiency along with color chromaticity and operational lifetime. Towards this goal, phosphorescent‐OLED (PHOLED) devices have demonstrated potential. Recent PHOLED device results show both excellent device efficiencies and long lifetimes towards the commercialization of low power consumption, full color, passive‐ and active‐matrix (both polysilicon and amorphous‐silicon backplane technologies) OLED displays.     

Flexible active‐matrix OLED displays: Challenges and progress

Abstract— Organic light‐emitting‐device (OLED) devices are very promising candidates for flexible‐display applications because of their organic thin‐film configuration and excellent optical and video performance. Recent progress of flexible‐OLED technologies for high‐performance full‐color active‐matrix OLED (AMOLED) displays will be presented and future challenges will be discussed. Specific focus is placed on technology components, including high‐efficiency phosphorescent OLED technology, substrates and backplanes for flexible displays, transparent compound cathode technology, conformal packaging, and the flexibility testing of these devices. Finally, the latest prototype in collaboration with LG. Phillips LCD, a flexible 4‐in. QVGA full‐color AMOLED built on amorphous‐silicon backplane, will be described.     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

Microcavity white‐emitting OLED devices

Abstract— Microcavity designs for OLED devices with an unpatterned white emitter have the potential to provide greater brightness and larger color gamut than non‐microcavity designs while still enabling lower‐cost large‐format manufacturing. In this paper, such microcavity and non‐microcavity designs are compared. Color filters must still be employed to provide an adequate color gamut. Top‐emitter structures have somewhat greater on‐axis luminance and color gamut, but increased angular change, than bottom‐emitter designs. In a single‐stack bottom‐emitter active‐matrix TFT device using an RGBW format, the use of microcavities is estimated to reduce the average power usage by 35% and the peak power by 58%, while increasing the NTSC ratio for color gamut area by about 10%. Angular luminance and color change is likely to be acceptable, especially for hand‐held applications. Tandem devices employing multiple emitter stacks increase the lifetime of OLED devices but require larger driving voltages; for such devices, microcavity structures are useful although the percentage reduction obtained in power usage is not quite as large. Generally, tandem devices with microcavities have a slightly stronger cavity effect yielding slightly larger color gamut, but also greater angular color and luminance shift. Therefore, microcavity architectures are less appealing for tandem devices.     

High‐efficiency phosphorescent OLED technology

Abstract— Universal Display Corp. (UDC), together with its academic partners at Princeton University and the University of Southern California, are developing high‐efficiency electrophosphorescent small‐molecule OLED devices, based on triplet emission. These device systems show good lifetimes, and are well suited for the commercialization of low‐power‐consumption full‐color active‐matrix OLED displays. In this paper we also show how these phosphorescent devices may be driven by low‐cost amorphous‐silicon backplanes, and discuss benefits that could be gained by employing bistable OLED pixels.     

Development of a phosphorescent white OLED with extremely high power efficiency and long lifetime

Abstract— A white OLED device with extremely high power efficiency and long lifetime was developed, in which blue, yellow‐green, and red phosphorescent emitters were used. The performances achieved were 64 lm/W and 10,000 hours of lifetime at an initial luminance of 1000 cd/m² by using a light outcoupling technique. The device also exhibited the good durability important for practical usage. New technologies, such as blue phosphorescent materials and a sophisticated organic layer structure, were applied to the device. Hopefully, these technologies will open the door to the practical use of OLEDs as light sources.     

Flexible OLED display development: Strategy and status

Abstract— In this paper, the current status of flexible OLED (FOLED®) display development will be reviewed, including previous results for passive‐matrix displays on plastic and current progress on active‐matrix displays on steel foil. The displays incorporate high‐efficiency small‐molecule phosphorescence OLED (PHOLE?) technology. The ultimate goal is to develop high‐information‐content high‐performance long‐lived, and large‐area FOLED displays that can be pulled or rolled out from a smaller pen‐like housing. The strategy for achieving this goal will be presented.     

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.     

Improved lifetime of highly flexible OLEDs based on multilayered transparent electrodes with enhanced barrier performance

Abstract— Flexible organic light‐emitting diodes (FOLEDs) showing enhanced barrier properties under repeated mechanical stress are reported. By combining metal‐based multilayer transparent electrodes (MTEs) as highly flexible anodes replacing ITO electrodes and sol‐gel organic‐inorganic hybrimers which function as both planarizing films and barrier layers, the proposed FOLEDs not only exhibit a level of performance comparable to that of ITO‐based reference devices but also show a superior mechanical flexibility with “after‐bending” lifetime close to that of ITO‐based devices.     

Highly efficient single‐unit white OLED device with emission from both singlet and triplet excitons

We succeeded in developing a single‐unit hybrid organic light‐emitting diode (OLED) device with efficient light emission from both a phosphorescent layer and a fluorescent layer. The single‐unit hybrid OLED achieved a power efficiency higher than that of a two‐unit hybrid tandem OLED with phosphorescent and fluorescent layers.     

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.     

AMOLED panel driven by time‐multiplexed clamped inverter circuit to reduce complex control signals

Abstract— A time‐multiplexing technique employing a pulse‐width‐modulation (PWM) charge‐pump scheme for driving active‐matrix organic light‐emitting‐diodes (AMOLEDs) is described. This scheme greatly reduces the number of control lines. The two‐ or four‐phase PWM driving technique not only reduces costs through the simplification of manufacture but also improves the uniformity and lifetime of OLED panels. Experimental results show that the proposed circuit effectively and precisely controls the timing of OLED data‐writing and light emission.     

A novel lens cap designed for the RGB LEDs installed in an ultra‐thin and directly lit backlight unit of large‐sized LCD TVs

Abstract— The objective of this study is to design a novel cone‐shaped lens cap on LEDs in order to achieve high optical efficiency in an ultra‐thin directly lit RGB LED backlight unit (BLU) for large‐sized LCD TVs. The use of the novel lens cap could play the role of a diffuser, a low light‐efficiency component in a BLU, in order to gain higher efficiency and simultaneously provide satisfactory uniformity in light distribution. The novel cone‐shaped lens is coated with aluminum on the outside surface of the cone for mirroring effects to reflect most of the LED emitted light horizontally and then reflect the light at the BLU boundaries, and then, finally to the output plane. In this way, bright spots on the output plane of the BLU can be avoided, leading to increased uniformity. Simulations were conducted to design and optimize varied aspects of the designed lens and BLU, including the cone angle of the proposed lens and the LED spacing (pitch). To further achieve color balance, a known Genetic Algorithm is used to search for the optimal angular placement of each RGB LED, resulting in better color balance. Finally, a prototype BLU for large‐sized 37‐in. LCD TVs with the proposed lens was built to verify the expected performance.     

Wearable 4‐in. QVGA full‐color‐video flexible AMOLEDs for rugged applications

Abstract— Work on the world's first wrist‐worn communications device built on a flexible, low‐power‐consumption full‐color AMOLED using phosphorescent OLEDs is presented. The device offers the wearer the ability to see high‐information‐content video‐rate information in a thin‐and‐rugged‐form‐factor 4‐in. QVGA display, conformed around a human wrist.