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.     

Flexible AMOLED displays on stainless‐steel foil

Abstract— The world's thinnest flexible full‐color 5.6‐in. active‐matrix organic‐light‐emitting‐diode (AMOLED) display with a top‐emission mode on stainless‐steel foil was demonstrated. The stress in the stainless‐steel foil during the thermal process was investigated to minimize substrate bending. The p‐channel poly‐Si TFTs on stainless‐steel foil exhibited a field‐effectmobility of 71.2 cm²/N‐sec, threshold voltage of ?2.7 V, off current of 6.7 × 10¹³ A/μm, and a subthreshold slope of 0.63 V/dec. These TFT performances made it possible to integrate a scan driver circuit on the panel. A top‐emission EL structure was used as the display element, and thin‐film encapsulation was performed to realize a thin and flexible display. The full‐color flexible AMOLED display on stainless‐steel foil is promising for mobile applications because of its thin, light, rugged, and flexible properties.     

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.     

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.     

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.     

Compact and efficient RGB to RGBW data conversion method and its application in OLED microdisplays

In many electronic information displays, a colour pixel comprises three spatially distinct sub‐pixels containing red, green and blue (RGB) colour filters. The option of adding a fourth white (W) sub‐pixel that allows light to pass through unfiltered can significantly improve the optical efficiency of the pixel that, in turn, increases the power efficiency of the display. Such a display is called an RGBW display, and the required transformation of data format from incoming RGB to pixel RGBW is termed as “RGB to RGBW conversion.” This paper reports a method of RGB to RGBW conversion that is highly compact and efficient in terms of system resources while retaining image quality. It processes incoming data through a new colour space conversion algorithm in order to reduce the average power consumption with no noticeable visual artefacts. We explain the method and demonstrate its cost‐effective and power‐effective implementation for the specific case of an organic light emitting diode microdisplay.     

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.     

World's first large size 77‐inch transparent flexible OLED display

Large flexible organic light‐emitting diode (OLED) display provides various electronic applications such as curved, bendable, rollable, and commercial display, because of its thinness, light weight, and design freedom. In this work, the process flow and key technologies to fabricate the world's first large size 77‐inch transparent flexible OLED display are introduced. “White OLED on TFT + color filter” method is used to fabricate the aforementioned display. On both thin‐film transistor and color filter substrates, transparent polyimide (PI) was used as plastic substrate with multi‐barrier. In case of a transparent flexible display, the multi‐barrier is required for the additional consideration to overcome the decrease of transmittance due to the difference in refractive index of the conventional multi‐barrier. We developed the special multi‐barrier to increase transparency with superior water vapor transition rate characteristic. The optimized amorphous indium gallium zinc oxide thin‐film transistors were employed on the multi‐barrier, and it shows the highly uniform electrical performance and reliability on plastic substrate. Also, the typical panel failure mechanism during laser lift‐off process caused by a particle in PI is studied, and a sacrificial layer was suggested between PI and a carrier glass to reduce the panel failure. Finally, we successfully realized the world's first 77‐inch transparent flexible OLED display with ultra‐high‐definition resolution, which can be rolled up to a radius of 80 mm with a transmittance of 40%.     

Improvement in image quality of a 5.8‐in. OTFT‐driven flexible AMOLED display

Abstract— The image quality of an OTFT‐driven flexible AMOLED display has been improved by enhancing the performance of OTFTs and OLEDs. To reduce the operating voltage of OTFTs on a plastic film, Ta²O⁵ with a high dielectric constant was used as a gate insulator. The organic semiconductor layer of the OTFT was successfully patterned by a polymer separator, which is an isolating wall structure using an organic material. The OTFT performance, such as its current on/off ratio, carrier mobility, and spatial uniformity on the backplane, was enhanced. A highly efficient phosphorescent OLED was used as a light‐emission device. A very thin molybdenum oxide film was introduced as a carrier‐injection layer on a pixel electrode to reduce the operating voltage of the OLED. After an OTFT‐driven flexible AMOLED display was fabricated, the luminance and uniformity on the display was improved. The fabricated display also showed clear moving images, even when it was bent at a low operating voltage.     

Truly wearable display comprised of a flexible battery,flexible display panel,and flexible printed circuit

In an effort to create a truly flexible and wearable display having a flexible battery as well as a flexible organic light‐emitting diode panel and a flexible printed circuit, a flexible lithium‐ion battery has been developed, and a prototype wrist‐wearable or arm‐wearable display has been fabricated. Owing to improvements in the internal structure and exterior of the lithium‐ion battery, no remarkable changes in charge and discharge curves and the internal state of the electrodes were observed even after conducting a 10,000‐cycle bending test. Therefore, this flexible lithium‐ion battery prototype demonstrated remarkable bending resistance. Thus, we succeeded in fabricating a truly flexible and wearable display comprised of a flexible organic light‐emitting diode panel, a flexible printed circuit, and a flexible battery.     

A new current‐mirror pixel circuit employing poly‐Si TFTs for active‐matrix organic light‐emitting‐diode displays

Abstract— A novel active‐matrix organic light‐emitting‐diode (AMOLED) display employing a new current‐mirror pixel circuit, which requires four‐poly‐Si TFTs and one‐capacitor and no additional signal lines, has been proposed and sucessfully fabricated. The experimental results show that a new current mirror can considerably compensate luminance non‐uniformity and scale down a data current more than a conventional current‐mirror circuit in order to reduce the pixel charging time and increase the minimum data current. Compared with a conventional two‐TFT pixel, the luminance non‐uniformity induced by the grain boundaries of poly‐Si TFTs can be decreased considerably from 41% to 9.1%.     

A 13.5‐in. Quad‐FHD flexible CAAC‐OS AMOLED display with long‐life OLED device structure

In this study, the device structure of a white tandem organic light‐emitting diode (OLED) was changed to control the emission area and thereby achieve less luminance decay. A long‐life 13.5‐inch 4 K flexible c‐axis‐aligned crystal oxide semiconductor (CAAC‐OS) active‐matrix OLED with less color shift and high resolution was fabricated using this long‐life white OLED, transfer technology, and a CAAC‐OS field‐effect transistor.     

Fabrication and characteristics of ZnO thin films deposited by RF sputtering on plastic substrates for flexible display

ZnO thin films were successfully grown on flexible plastic substrates using radio-frequency mag-netron sputtering method at room temperature.The effects of the sputtering power on the quality of the ZnO films have been investigated.The results show that thin films were polycrystalline,with wurtzite structure and a strong preferred c-axis orientation (002).The root-mean-square (rms) surface roughness of the ZnO thin films is 22.1 nm.The ZnO thin films fabricated by sputtering with 70 W sputtering power have a high mobility of 34.33 cm 2 /V·s.The ZnO films are shown to be compatible with flexible display on plastic substrates.     

Analyzing and tuning emission characteristics of top‐emitting organic light‐emitting devices

Abstract— Top‐emitting organic light‐emitting devices (OLEDs) have several technical merits for application in active‐matrix OLED displays. Generally, stronger microcavity effects inherent with top‐emitting OLEDs, however, complicate the optimization of device efficiency and other viewing characteristics, such as color and viewing‐angle characteristics. In this paper, using the rigorous classical electromagnetic model based on oscillating electric dipoles embedded in layered structures, the emission characteristics of top‐emitting OLEDs as a function of device structures will be analyzed. From comprehensive analysis, trends in the dependence of ewmission characteristics on device structures were extracted, and, accordingly, a general methodology for optimizing viewing characteristics of top‐emitting OLEDs for display applications will be suggested. The effectiveness of the analysis and the methodology was confirmed by experimental results.     

Oxide TFT with multilayer gate insulator for backplane of AMOLED device

Abstract— An indium gallium zinc oxide (IGZO) film with an amorphous phase was deposited and had a very flat morphology with a RMS value of 0.35 nm. IGZO TFTs were fabricated on a glass substrate by conventional photolithography and wet‐etching processes. IGZO TFTs demonstrated a high mobility of 124 cm²/V‐sec, a high on/off ratio of over 10⁸, a desirable threshold voltage of 0.7 V, and a sub‐threshold swing of 0.43 V/decade. High mobility partially resulted from the fringing‐electric‐field effect that leads to an additional current flow beyond the device edges. Therefore, considering our device geometry, the actual mobility was about 100 cm²/V‐sec, and had a very low dependence on the variation of W/L (channel width and length) and thickness of the active layer. IGZO TFTs were also fabricated on a flexible metal substrate for a conformable display application. TFT devices showed an actual mobility of 72 cm²/V‐sec, a high on/off ratio of ～10⁷, and a sub‐threshold swing of 0.36 V/decade. There was no significant difference before, during, or after bending. Moreover, an IGZO TFT array was fabricated and a top‐emitting OLED device was successfully driven by it. Therefore, the oxide TFT could be a promising candidate as a backplane for OLED devices.     

Static active‐matrix OLED display without pixel refresh enabled by amorphous‐silicon non‐volatile memory

Abstract— An active‐matrix organic light‐emitting‐diode (AMOLED) display which does not require pixel refresh is demonstrated. This was achieved by replacing the thin‐film transistor (TFT) that drives the OLED with a non‐volatile memory TFT, in a 2‐transistor pixel circuit. The threshold voltage of the non‐volatile‐memory TFT can be changed by applying programming voltage pulses to the gate electrode. This approach eliminates the need for storage capacitors, increases the pixel fill factor, and potentially reduces power consumption. Each pixel can be individually programmed or erased using a standard active‐matrix addressing scheme. The programmed image is stored in the display even if power is turned off.     

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.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.