Advanced technologies for UHD curved OLED TV

Ultra‐high definition (UHD) curved organic light‐emitting diode (OLED) TV requires advanced technologies to realize mass production. White, red, green, and blue (WRGB) OLED TV based on stripe WRGB sub‐pixel structure has distinct advantages in luminance of white, faster response time, wider viewing angle, and potential higher resolution. In this paper, we will introduce technological progress for commercializing large‐sized and UHD curved OLED TV. Those technologies including oxide thin film transistors, white OLEDs, compensation circuit, and solid phase encapsulation enable panel size scalability as well as mass production with lifetime reliability.     

Driving large‐sized OLED TVs with an amorphous‐silicon backplane

Abstract— A 20‐in. OLED display driven by an amorphous‐silicon backplane has been demonstrated. It has been widely believed that the characteristics of amorphous‐silicon TFTs are not sufficient to drive OLED display. This paper challenges this hypothesis and proves that amorphous silicon can be applied to large active‐matrix‐driven displays and discusses many possible approaches that lead to good front‐of‐screen quality. Superior‐video‐image‐quality amorphous‐silicon‐driven OLEDs opens a bright future for a new generation of wall‐hanging televisions.     

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm²/V‐sec, an on/off ratio of >10⁸, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.     

LUT‐based compensation model for OLED degradation

Abstract— A compensation model for blue OLED devices, which is based on an OLED degradation model derived from luminance measurements of OLED devices, has been developed; it reduces degradation effects by monitoring the activity of a set of subpixels and adjusting the driving conditions of the blue subpixels of the display accordingly. To evaluate its performance, the compensation model has been embedded in an OLED display controller model, where its implementation is based on a lookup table. Simulations show that even with a reduced number of monitored subpixels, degradation is effectively reduced.     

In‐line manufacturing tool using belt‐source evaporation techniques for large‐sized OLED devices

Abstract— Whether the manufacturing of the large‐sized OLED devices in display and lighting industry succeeds or not will strongly depend on the concept of a thermal evaporation source and the manufacturing tool. The most important factors in OLED‐device manufacturing are the organic material utilization and the TACT time. An in‐line tool for OLED manufacturing using a novel belt‐source evaporation technique is proposed. The belt source maintains the organic film uniformity at 3% and provides high material utilization of over 80%, and the in‐line system can achieve this in 1‐min TACT time.     

15‐in. RGBW panel using two‐stacked white OLED and color filters for large‐sized display applications

Abstract— A 15‐in. HD panel employing two‐stacked WOLEDs and color filters for which the color gamut can be as high as 101.2% (CIE1976) and the power consumption is 5.22 W. The WOLEDs exhibit a current efficiency of 61.3 cd/A and a power efficiency of 30 lm/W at 1000 nits and their CIE coordinate is (0.340, 0.334). A 15‐in. RGBW panel was investigated to verify the electrical and optical performance compared to that of a 15‐in. RGB TV made by using FMM technology. The characteristics of the 15‐in. RGBW panel are comparable to those of the 15‐in. RGB panel. Color filters combined with WOLEDs is a possible patterning technology for large‐sized OLED TV, which surpasses the limits of fine‐metal‐mask technology.     

Quantification model of proper curvature for large‐sized curved TVs

We present a feasible modeling method to estimate the proper curvature for large‐sized curved TVs. We especially focused on two factors – preference and perceived distortion – to be considered to determine the proper curvature. The preference includes a number of advantages that people expect to fulfill the possibilities of a more immersive image and sense of realism from curved display. On the other hand, the perceived distortion means the side effects that people can notice uncomfortable feeling caused by the shape of a curved one. In order to find out how two factors would be different as a number of conditions like curvature, size, and viewing angle change, a series of subjective assessments were conducted. The evaluation results show that both the preference and perceived distortion vary with the conditions considerably. We performed the statistical analysis based on the results and proposed the quantification model of proper curvature, which has higher preference and less perceived distortion, for various‐sized curved TVs.     

Alternative approach to large‐sized AMOLED HDTV

Abstract— In this work, alternative approaches to existing technologies for the fabrication of large‐sized AMOLEDs, such as non‐laser crystallization methods for poly‐Si TFT fabrication and color patterning using laser‐induced thermal imaging (LITI), is proposed. In particular, it was found that the super grain crystallization (SGS) method resulted in high‐performance TFTs in terms of mobility and off‐current. The feasibility of these techniques for large‐sized AMOLEDs is demonstrated by 17‐in. UXGA AMOLED displays which show good brightness uniformity.     

Towards large‐area full‐color active‐matrix printed polymer OLED television

Abstract— We have developed a new multi‐head polymer OLED ink‐jet‐printing technology to make large‐screen OLED television displays. This printer is used to make a 13‐in.‐diagonal 16:9‐format polymer‐OLED prototype driven by an LTPS active matrix with a pixel circuit which compensates for TFT threshold‐voltage variations. A novel scrolling‐bar addressing scheme is used to reduce motion artifacts and to make sparkling images with a high local peak brightness. The scalability of the polymer‐OLED technology to larger sizes for television applications is discussed.     

Design of large‐area OLED displays utilizing seamless tiled components

Abstract— In recent years, the majority of R&D for large‐area displays has been to serve the production of large monolithic substrate technology such as plasma‐display panels (PDPs) and TFT‐LCD. While the pursuit of large displays for domestic and light‐industrial use benefits from the production of these high‐quality high‐pixel‐count technologies, there is still a need to produce displays in formats other than 4:3 or 16:9 and on a larger scale than currently available in single‐substrate displays. The options that exist for producing tiled displays from emerging technologies is examined and a practical technique for creating large‐area (1.8 × 1.2 m and larger) monochrome or color displays from tiling smaller units is discussed. This presents a cost‐effective approach for arranging small tiles to create a much larger screen and offers a simple way to address the market gap between large monolithic displays and small conventional LED video‐wall displays, in the size range of 1–10‐mm pixels for advertising and industrial use. By examining the requirements for pixel size and pitch against the range of viewing distances commonly associated with the target markets for these displays, it will be shown that complex manufacturing is not always required.     

System design for a wide‐color‐gamut TV‐sized AMOLED display

Abstract— By using current technology, it is possible to design and fabricate performance‐competitive TV‐sized AMOLED displays. In this paper, the system design considerations are described that lead to the selection of the device architecture (including a stacked white OLED‐emitting unit), the backplane technology [an amorphous Si (a‐Si) backplane with compensation for TFT degradation], and module design (for long life and low cost). The resulting AMOLED displays will meet performance and lifetime requirements, and will be manufacturing cost‐competitive for TV applications. A high‐performance 14‐in. AMOLED display was fabricated by using an in‐line OLED deposition machine to demonstrate some of these approaches. The chosen OLED technologies are scalable to larger glass substrate sizes compatible with existing a‐Si backplane fabs.     

Ultra‐high‐resolution 1058‐ppi OLED displays with 2.78‐in size using CAAC‐IGZO FETs with tandem OLED device and single OLED device

We fabricated new 2.78‐in 1058‐ppi organic light‐emitting diode (OLED) displays. The displays used OLED devices with a tandem structure and a single structure and a field effect transistor (FET) using c‐axis aligned crystalline In–Ga–Zn–O (CAAC‐IGZO) for an active layer and employing the 1.5‐µm rule over a glass substrate. Even in the displays with such high resolution exceeding 1000 ppi, crosstalk that was observed in the lower luminance region was suppressed. The displays achieved high color reproducibility and reduced viewing angle dependence.     

Super‐high‐resolution transfer printing for full‐color OLED display patterning

Abstract— A transfer‐printing method for the patterning of thin polymer layers is described. A hard stamp with a raised feature is brought into contact with a spin‐coated organic film under elevated pressure and temperature to break the films. The patterned film is then transfer printed onto the devices. This method is used to print red/green/blue subpixel arrays with a pattern size as small as 12 μm at a resolution of 530 ppi to demonstrate its ability for full‐color organic light‐emitting‐display fabrication. Devices with printed organic layers have similar performance to spin‐coated controls under optimized printing temperature and pressure settings. The critical physical parameters include a soft intermediate plate for the sharp breaking of edge patterns, control of surface energies, and printing at moderate temperature and pressure to achieve intimate contact between the printed layer and the underlying substrate.     

RGB‐to‐RGBW conversion with current limiting for OLED displays

Abstract— Organic‐light‐emitting‐diode (OLED) displays employing white‐light‐emitting OLEDs in combination with RGBW color filters can demand high peak currents to present images with bright, highly saturated colors. Image‐processing methods that take advantage of a very highly efficient white subpixel in addition to filtered RGB subpixels to reduce the peak current and power of these displays are described. The image‐quality impact of these algorithms are explored to develop a final image‐processing algorithm.     

Requirements for large‐sized high‐resolution TFT‐LCDs

Abstract— A 22‐in. prototype TFT‐LCD with a resolution of 200 pixels per inch and wide‐viewing‐angle capability has been developed and its requirements in terms of screen quality and technology will be discussed. An in‐plane‐switching mode with dual‐domain structure, post‐spacers, and high‐resolution process were implemented to achieve superior front‐of‐screen quality. And, also, in order to improve reliability and productivity, we developed a new injection method for liquid crystals which enabled us to eliminate injection holes.     

An OTFT‐driven rollable OLED display

Abstract— An 80‐μm‐thick rollable AMOLED display driven by an OTFT is reported. The display was developed so as to be rollable in one direction with an integrated OTFT gate driver circuit. It was successfully operated by an originally developed organic semiconductor, a peri‐xanthenoxanthene derivative. The display retained its initial electrical properties and picture quality even after being subjected to 1000 cycles of a roll‐up‐and‐release test with a radius of 4 mm.     

Anode‐voltage‐control circuit for compensation of luminance deterioration

Abstract— An external driving circuit that has realized long lifetime, power‐consumption control, and peak luminance for organic light‐emitting diode (OLED) displays have been developed. This circuit realizes an effective method for constant‐anode‐voltage (CV) driving refered to as clamped inverter (CI) driving. The feature of CV driving is to achieve low‐power consumption compared with constant‐anode‐current (CC) driving and to control the power consumption and peak luminance according to the image because display luminance can be easily changed by controlling the anode voltage. On the other hand, CV driving has the problem that luminance deterioration appears to be serious compared with that of CC driving because the current of the OLED element decreases according to usage time. To cope with this, a lifetime compensation circuit that has increased the anode voltage so that it compensates for the luminance deterioration has been developed. This circuit can compensate not only the decrease in current but also the decrease in luminance at a constant current that CC driving cannot. However, increasing the anode voltage causes an increase in stress on the OLED element. The influence of stress on OLED lifetime was verified. As a result, it was confirmed that this circuit can extend the lifetime by 32% even if the anode voltage is increased, causing stress on the OLED structure.     

New pixel driving circuit using self‐discharging compensation method for high‐resolution OLED micro displays on a silicon backplane

A new 4T2C pixel circuit formed on a silicon substrate is proposed to realize a high‐resolution 7.8‐μm pixel pitch AMOLED microdisplay. In order to achieve high luminance uniformity, the pixel circuit compensates its Vth variation of the MOSFET for the driving transistor internally by using self‐discharging method. Also presented are 0.5‐in Quad‐VGA and 1.25‐in wide Quad‐XGA microdisplays with the proposed pixel circuit.     

A new method for measuring ultra‐low water‐vapor permeation for OLED displays

Abstract— It is well known that proper encapsulation is crucial for the lifetime of organic light‐emit‐ting‐diode (OLED) displays. With the development of increasingly better barrier coatings and perimeter seals, it has now become very desirable to be able to precisely measure the rate of water‐vapor and oxygen permeation through barrier coatings and perimeter sealing. This paper demonstrates a new permeation‐measurement method that uses tritium‐containing water (HTO) as a tracer material. The theoretical detection limit of this direct method is 2.4 × 10^?8 g/(m²‐day).     

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.