Fluorescent‐based tandem white OLEDs designed for display and solid‐state‐lighting applications

Abstract— Highly efficient tandem white OLEDs based on fluorescent materials were developed for display and solid‐state‐lighting (SSL) applications. In both cases, the white OLED must have high power efficiency and long lifetime, but there are a number of attributes unique to each application that also must be considered. Tandem OLED technology has been demonstrated as an effective approach to increase luminance, extend OLED lifetime, and allow for use of different emitters in the individual stacks for tuning the emission spectrum to achieve desired performance. Here, examples of bottom‐emission tandem white OLEDs based on small‐molecule fluorescent emitters designed for displays and for SSL applications are reported. A two‐stack tandem white OLED designed for display applications achieved 36.5‐cd/A luminance efficiency, 8500K color temperature, and lifetime estimated to exceed 50,000 hours at 1000 cd/m². This performance is expected to meet the specifications for large AMOLED displays. A two‐stack tandem white OLED designed for SSL applications achieved 20‐lm/W power efficiency, 38‐cd/A luminance efficiency, 3500K color temperature, and lifetime estimated to exceed 140,000 hours at 1000 cd/m². With the use of proven light‐extraction techniques, it is estimated that this tandem device will exceed 40 lm/W with more than 500,000‐hour lifetime, performance that should be sufficient for first‐generation lighting products.     

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.     

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.     

Development of 8‐in. oxide‐TFT‐driven flexible AMOLED display using high‐performance red phosphorescent OLED

An 8‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display driven by oxide thin‐film transistors (TFTs) has been developed. In‐Ga‐Zn‐O (IGZO)‐TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200 °C. To form a SiO_x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution‐processable photo‐sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high‐performance phosphorescent OLED was developed as a red‐light‐emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.     

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.     

A 3.0‐in. 308‐ppi WVGA AMOLED with a top‐emission white OLED and color filter

Abstract— A 3.0‐in. 308‐ppi WVGA top‐emission AMOLED display with a white OLED and color filters, driven by LTPS TFTs demonstrating a color gamut of >90% and a Δ(u′,v′) of <0.02 is reported. A white‐emission source with a unique device structure was developed using all fluorescent materials and yielded efficiencies of 8.45% and 16 cd/A at 4000 nits with CIE color coordinates of (0.30, 0.32).     

High‐performance white OLEDs with high color‐rendering index for next‐generation solid‐state lighting

Abstract— Several white‐OLED structures with a high color‐rendering index (CRI) were investigated for lighting applications. A two‐unit fluorescent/phosphorescent hybrid white OLED achieved an excellent CRI of 95, high luminous efficacy of 37 lm/W, and long lifetime of over 40,000 hours at 1000 cd/m². White‐OLED lighting panels of 8 × 8 cm for high‐luminance operation were fabricated, and a stable emission at 3000 cd/m² was confirmed. Quite a small variation in chromaticity in a different directions was achieved by using an optimized optical device structure. With a light‐outcoupling substrate, a higher efficacy of 56 lm/W, high CRI of 91, and longer half‐decay lifetime of over 150,000 hours at 1000 cd/m² was achieved. All‐phosphorescent white OLEDs placed on the light‐outcoupling substrate show a high CRI of 85 and higher efficacy of 65 lm/W with a fairly good half‐decay lifetime of over 30,000 hours. With a further voltage reduction and a high‐index spherical extractor, 128 lm/W at 1000 cd/m² has been achieved.     

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.     

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.     

A pixel structure for simultaneous programming and emission method for shutter‐glasses‐type stereoscopic 3‐D AMOLED displays

Abstract— A pixel structure for shutter‐glasses‐type stereoscopic 3‐D active‐matrix organic light‐emitting‐diode (AMOLED) displays is proposed. The proposed pixel programs data to the pixel during the light‐emission time of an OLED. Because the emission time of the proposed pixel is extended, it is expected that the proposed pixel not only decreases the peak current of the OLED during the emission period but also reduces flicker. Moreover, the aperture ratio of the proposed pixel is 58.69% for a 50‐in. full‐high‐definition (FHD) condition by minimizing the number of thin‐film transistors (TFTs), capacitors, and control signal lines as seven TFTs, two capacitors, two power lines, and four control lines per unit pixel. Simulation results show that the error in the emission current of the proposed pixel is from ?0.82% to +0.90% when the threshold‐voltage variation of the driving TFT is ±1.00 V, and the maximum variation of the emission current is ?1.35% when a voltage drop in the power line is ?0.50 V on a full‐white‐image display.     

PIN‐OLEDs for active‐matrix‐display use

Abstract— Novaled's PIN‐OLED® technology allows for highly efficient, temperature stable, and long‐lived OLEDs suited for a variety of display applications. This paper delivers an overview about Novaled's state of the art, including top‐ and bottom‐emitting structures. It is discussed how PIN‐OLEDs give rise to an increased manufacturing yield. The main focus of this paper is the development of white OLEDs for display use. When the RGBW color‐filter approach is used in combination with white OLEDs, the resulting full‐color OLED display is able to deliver high color quality and remain highly power efficient. For such a case, the manufacturing infrastructure of OLEDs for lighting can be used. We use tandem architectures, bottom‐ and top‐emission architectures, and developed specific high‐temperature stable OLED stacks. The importance of matching color coordinates of the white OLED and the targeted display white color point is of outstanding importance. Results have mainly been achieved under the German‐funded project CARO and the European‐funded project AMAZOLED.     

High‐resolution OLED display with remarkably low power consumption using blue/yellow tandem structure and RGBY subpixels

A top‐emission organic light‐emitting diode (OLED) with a microcavity structure combined with a blue/yellow tandem structure was developed. A high‐resolution active‐matrix OLED display with the world's lowest level of power consumption using the tandem OLED with red, green, blue, and yellow subpixels was fabricated.     

Red‐phosphorescent OLEDs employing bis(8‐quinolinolato)phenolato‐aluminum(III) complexes as emission‐layer hosts

Abstract— We have used bis(8‐quinolinolato)phenolato‐aluminum complexes as emission‐layer hosts in red‐phosphorescent OLED devices. This enabled high‐efficiency long‐lived OLED devices with a simple device structure that does not require a hole‐blocking layer. Devices with a red‐phosphorescent dopant introduced into a noble bis(8‐quinolinolato)phenolato‐aluminum complex exhibited a high efficiency of 12 cd/A at CIE color coordinates (0.65, 035) and a long operating lifetime of 30,000 hours or more at an initial luminance of 700 cd/m². Moreover, triplet‐triplet annihilation was reduced in the devices because of the wide emission zone enabled by the complex and the short phosphorescent lifetime of the red‐phosphorescent dopant. We have successfully incorporated these red‐phosphorescent devices into commercial OLED displays.     

Novel pixel design for high‐resolution AMOLED displays with a shadow mask

Abstract— The bottlenecks in achieving high resolution for active‐matrix OLED (AMOLED) displays based on currently available manufacturing processes were evaluated and compared. The use of a shadow mask has proven to be viable for mass production, but the fabrication of high‐precision shadow masks becomes very difficult when the resolution exceeds 180 ppi. The latest breakthrough in increasing display resolution is presented. Without an increase in cost, the limitations of the conventional shadow‐mask process using novel subpixel designs have been successfully overcome. A high resolution reaching of 270 ppi has been successfully demonstrated on a 3‐in. VGA‐format AMOLED display, fabricated by using a shadow mask with a much lower resolution of 135 ppi. This innovative pixel design opens up the possibilities of manufacturing high‐resolution displays using a relatively low‐resolution shadow mask.     

Development of 55‐in. 8K AMOLED TV based on coplanar oxide thin‐film transistors and inkjet printing process

In this paper, we presented 55‐in. 8K4K AMOLED TV employing coplanar oxide thin‐film transistor (TFT) backplane, top emissive inkjet‐printing organic light‐emitting diode (OLED) device, gate driver on array (GOA), and compensation technologies. It is so far the largest prototype AMOLED TV fabricated by using inkjet printing process with 8K resolution. It shows the stunning display quality, thanks to the high resolution and fast refresh frequency. It proves that the inkjet printing process is not only cost competitive but also can deliver premium display.     

Letter: Novel approaches to realize high‐resolution AMOLED display

Two different approaches to realize high‐resolution active‐matrix organic light‐emitting device (AMOLED) display were delivered. By adopting specific organic light emitting diode (OLED) structure with pre‐pattern electrode and the utilization of color filter, we successfully simplify the fabrication process with fine metal mask (FMM)‐free or one‐FMM solutions. Each approach was demonstrated with a 4.4″ panel with 413 ppi pixel density based on real stripe RGB. Both panels possessed low power consumption, low reflectivity, and superior NTSC performance. Because the utilization of FMM was avoided or reduced, higher production yield, higher throughput, and lower cost could be achieved. Therefore, these two approaches are very promising for mass production of high‐resolution AMOLED display.     

White stacked OLED with 38 lm/W and 100,000‐hour lifetime at 1000 cd/m2 for display and lighting applications

Abstract— The three critical parameters in determining the commercial success of organic light‐emitting diodes (OLEDs), both in display and lighting applications, are power efficiency, lifetime, and price competitiveness. PIN technology is widely considered as the preferred way to maximize power efficiency and lifetime. Here, a high‐efficiency and long‐lifetime white‐light‐emitting diode, which has been realized by stacking a blue‐fluorescent emission unit together with green‐ and red‐phosphorescent emission units, is reported. Proprietary materials have been used in transport layers of each emission unit, which significantly improves the power efficiency and stability. The power efficiency at 1000 cd/m² is 38 lm/W with CIE color coordinates of (0.43, 0.44) and a color‐rendering index (CRI) of 90. An extrapolated lifetime at an initial luminance of 1000 cd/m² is above 100,000 hours, which fulfils the specifications for most applications. The emission color can also be easily tuned towards the equal‐energy white for display applications by selecting emitting materials and varying the transport‐layer cavities.     

Dual‐plate OLED display (DOD) based on amorphous‐silicon (a‐Si) TFT backplane

Abstract— An improved AMOLED with an a‐Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual‐plate OLED display (DOD). While a top‐emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large‐sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large‐sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.     

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

Abstract— A flexible phosphorescent color active‐matrix organic light‐emitting‐diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink‐jet printing. A mixed solvent system with a high‐viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin‐film‐transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 10⁶ and a mobility of 0.1 cm²/V‐sec. Color moving images were successfully shown on the fabricated display.