[IWFPE 2015] What Will Replace ITO?

2015 IWFPE (International Workshop on Flexible & Printable Electronics) was held at Le Win Hotel in Jeonju, South Korea (November 4-6). During the workshop, many OLED display related presentations commented on ITO’s replacement material.

 

Dr. Jennifer Colegrove, CEO of US research company Touch Display Research, discussed hot trends of 2015-2016. Dr. Colegrove included high resolution, transparent display, wearable device, and flexible display in the hot trends. Of these she pointed out ITO replacement material regarding touch panel. She mentioning metal mesh, silver nanowire, CNT, and graphene as materials that could replace ITO. Dr. Colegrove added that the material has to be flexible in order to be applicable to flexible display and needs to have high efficiency.

 

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Hanwha Techwin’s Dr. Seungmin Cho announced that graphene, which has higher uniformity compared to ITO, will be the material of future. With lower resistance than ITO, graphene shows good characteristics, but Dr. Cho explained that high cost and particles produced are issues that need to be solved. He also commented that China selected graphene related national projects and is striving to develop the technology.

 

Dr. Hyunkoo Lee of ETRI (Electronics and Telecommunications Research Institute) presented that the multi-layered graphene that ETRI, Sungkyunkwan University, and KAIST (Korea Advanced Institute of Science and Technology) co-developed will become the material that can replace ITO. He also introduced the results that as the transmittance is particularly high, it is suitable for transparent display, and compared to silver nanowire material in high resolution top emission structure, the display’s brightness is higher.

 

ITO replacement development is an issue for the future of display industry’s progress. It is estimated that research development on graphene as one of the ITO replacement electrodes will be actively carried out.

 

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[IWFPE 2015] AUO’s Bendable AMOLED

During the IWFPE 2015 (November 4-6) held in Jeonju, South Korea, AUO gave a presentation on bendable AMOLED’s concept and technology first revealed in August.

 

AUO’s R&D manager Terence Lai reported that the bendable AMOELD can actualize new user interface through applying display’s bending characteristics and this can be the innovation that could change the existing touch interface.

 

AUO’s bendable AMOLED is 5inch with 295 PPI using the LTPS TFT and hybrid encapsulation with top emission structure. It has 02.mm thickness and through bending sensor of file type, it can process diverse functions by detecting different bending directions.

 

Current flexible AMOLED trend is moving from curved to foldable, jumping past bendable. Key panel companies and research laboratories are focusing on developing foldable.

 

It is anticipated that the AUO’s bendable AMOLED panel will provide new direction for the flexible AMOLED development that is heading toward foldable. Development of applications suitable to bendable and functions that utilizes bending characteristics are expected to be key issues.

 

AUO's 5" Bendable AMOLED Prototype

AUO’s 5″ Bendable AMOLED Prototype

Attention Focused on Silver Nanowire as Key Material for Display

At IPEC 2015 (International Printed Electronic Conference), held on September 1, Professor Sang-Ho Kim of Kongju National University announced that silver nanowire technology is in initial stages of commercialization and will become display market’s key material.

 

Kim reported that when the bending radius of flexible display is reduced, 2 key issues occur with silver nanowire used as TSP (touch screen panel) material. First, the wiring that are crossed when bending is loosened as can be seen in figure 1. Due to this effect the bending stability decreases.

Fig. 1, Source: Professor Sang-Ho Kim, IPEC 2015

Fig. 1, Source: Professor Sang-Ho Kim, IPEC 2015

 

 

Kim explained that this effect can be solved by welding the two wires as shown in figure 2 using thermal annealing technology, laser process, and IPL photo-sintering technology.

 

Fig 2, Source: Professor Sang-Ho Kim, IPEC 2015

Fig 2, Source: Professor Sang-Ho Kim, IPEC 2015

 

 

Another issue is a decrease in contact stability between nanowires at stress points when bending radius is reduced as shown in figure 3.

 

Fig 3, Source: Professor Sang-Ho Kim, IPEC 2015

Fig 3, Source: Professor Sang-Ho Kim, IPEC 2015

 

 

During the presentation, Kim explained that this can be solved through undercoating process. This process involves mixing 2 polymers with different Tg (glass-transition temperature) and layering it as in figure 4, and placing TSP on top.

 

Fig 4, Source: Professor Sang-Ho Kim, IPEC 2015

Fig 4, Source: Professor Sang-Ho Kim, IPEC 2015

 

 

Silver nanowire has benefit of being more flexible and less resistant compared to transparent electrode material, ITO. As such, it was spotlighted as TSP material most suitable for flexible OLED. Nonetheless, silver nanowire has been considered to fall behind ITO in panel mass production unit cost in display market.

 

However, haze effect which happens when sunlight is reflected off the silver nanowire TSP has been solved recently, and new touch technology that requires improved TSP functions, such as post-touch technology, has been developed. Accordingly, products that use silver nanowire are increasing despite the unit cost difference.

 

Kim reported that as TSP sheet resistance can be reduced through undercoating and welding technology and greatly increase bending stability, it is estimated that silver nanowire’s marketability will grow for flexible display.

 

[IMID 2015] UDC Develops OLED Patterning Technology with Less Masks

At present, FMM (Fine Metal Mask) is considered the main method for large area RGB OLED panel production. However, due to shadow effect, mask total pitch fluctuations, and mask slit tolerance issues, there is a yield limitation to this technology. This limitation increases as the resolution becomes higher; minimizing the number of FMM during the OLED panel production has been a key issue in RGB OLED production.

 

In SID 2014, through a paper titled “Novel Two Mask AMOLED Display Architecture”, UDC revealed a technology that can reduce the number of masks used in RGB-FMM method to 2 from previous 3.

 

Source: UDC, SID 2014

Source: UDC, SID 2014

 

As shown above, this technology coats yellow and blue subpixels using 1 mask each. Following this process, green and red color filters are applied above the subpixels. Pixels are formed as illustrated below.

 

Source : UDC, SID 2014

Source : UDC, SID 2014

 

UDC explained that this technology improves lifetime of the display overall and reduces energy consumption as blue voltage can be lowered. Additionally, tact time can be reduced through this technology, and increase the yield.

 

In IMID 2015, UDC presented research of the same title. However, the content of the presentation unveiled more developed research compared to 2014. First of all, in 2014, UDC announced that the panel’s lifetime could be improved by 2 times compared to the RGB method. In IMID 2015, UDC’s announcement changed the figure to 3.3 times increased lifetime.

 

UDC also revealed that the technology can be actualized through printing method, and has the advantage of being able to print 2 rows of pixels at once. UDC announced that this is most suitable when OVJP (Organic Vapor Jet Printing) applies the printing method.

 

A new technology called SPR (Sub-Pixel Rendering) was also announced by UDC in this paper. The 2014 SID paper included a method that did not uses SPR. As shown below in figure 1, 4 subpixels of RGY and B are used per pixel.

 

[Fig. 1], Source: UDC, IMID 2015

[Fig. 1], Source: UDC, IMID 2015

During IMID 2015, UDC presented APR technology applied pixel structure. Figure 2 shows 3 subpixels per pixel.

 

[Fig. 2], Source: UDC, IMID 2015

[Fig. 2], Source: UDC, IMID 2015

In this case, as the pixels can be arranged as shown in figure 3, smaller number of subpixels can be used.

 

[Fig. 3], Source: UDC, IMID 2015

[Fig. 3], Source: UDC, IMID 2015

UDC announced that because APR technology allows the number of subpixels per pixel to be reduced to 3 or less, the number of data lines and TFT per pixel can also be reduced together.

 

UDC explained that this technology can be applied regardless of the display area or resolution, and will be able to be applied to different types of panels.

 

HUD and HMD Meet Virtual Reality and Soar

At SID 2015 Review Workshop held in Konkuk University on July 31, Inha University’s Associate Professor Jae-Hyeung Park announced that interest in HUD (head-up display) and HMD (head-mounted display) increased at SID 2015.

 

HUD signifies technology that provides diverse information for the driver beyond the role of a front glass that simply allows the outside to be viewed and provides protection. Through HUD, drivers can grasp vehicular and destination information without having to take eyes off the road. At this juncture, AR (augmented reality) HUD is more than a simple display as it recognize the user’s movements and applies it to the display.

 

In SID 2015 paper, Japan’s Ricoh termed the device that self-intervenes in the vehicular operation as ADAS (Advanced Driving Assistance System) and announced that the information from the device applied to HUD will be able to help the user’s recognition and decision making. It was also added that improved AR can be realized when the contrast range is increased to be equal to reality and reiterated and the distance from the virtual image is 5m.

 

HMD, as a head-mounted device, provides differentiated feelings of immersion compared to other displays. Due to the increased realism from the wide viewing angle, HMD was widely used as an entertainment display, but with the recent increased interest in AR, it is receiving much spotlight as an AR display device.

 

At SID 2015, the West Saxon University of Applied Sciences of Zwickau revealed an HMD device that actualizes AR naturally by changing the distance where image is shown according to the use by adjusting the focal distance. Zhejiang University’s college of Optical Science and Engineering used method of showing hologram to each user’s eyes using the light field concept that realizes the light strength to all directions from all points in 3D and produced improved AR.

 

Park explained that with SID 2015 as a starting point, the AR related HMD and HUD interest and research will increases greatly and that the trend will continue in future.

 

Samsung Electronics released HMD device Samsung Gear VR that can be used by plugging in to Galaxy S6 or Galaxy S6 Edge. Oculus VR and Sony Computer Entertainment each revealed OLED applied VR headset Oculus Rift and Project Morpheus, and scheduled release regular product in Q1 and H1 of 2016 respectively. Korea’s HLB released AproVIEW S2 which used virtual image distance actualization method, a first for HUD.

 

 

Inha University’s Associate Professor Jae-Hyeung Park at SID 2015 Review Workshop

Inha University’s Associate Professor Jae-Hyeung Park at SID 2015 Review Workshop

Encapsulation Technology That Can Greatly Increase OLED Lifetime Unveiled

On July 29, at Chungcheong Display Forum held in Hoseo University in South Korea, an encapsulation technology that can greatly increase OLED lifetime was revealed.

 

Encapsulation technology prevents permeation of oxygen and moisture from affecting OLED panel’s performance and increases lifetime. As it is also the last process that decides OLED panel yield, OLED panel manufacturing companies are focused on finding the most optimal encapsulation technology.

 

The flexible OLED panel that is currently being mass produced uses hybrid structure of encapsulation where gas barrier cover plate is applied to the organic and inorganic stacks of passivation.

 

During this process, because cover plate with gas barrier characteristics is the key factor in deciding encapsulation performance the materials and technology are very important. The level of encapsulation that OLED panel requires is approximately 10-6g/m2day. The unit signifies the amount of permeation for 1m2 area a day. This amount is same as 1 drop of water in an area size of 6 World Cup stadiums.

 

Generally sputtering technology is used to form gas barrier layer. Sputtering technology is an evaporation technique where ions of the target material is coated to the substrate as noble gas, ionized via high voltage, collides against coating material.

 

This type of sputtering technology creates particles and defects during process. As such, in order to be applied to OLED, multilayer is required leading to a decrease in productivity and increase in production cost.

 

However, at the Chungcheong Display Forum, Professor MunPyo Hong of Korea University, revealed that defect that occurs during the existing sputtering process can be reduced by stabilizing target layer through installing reflector which induces neutral beam to be released.

 

According to Hong, this technology is sufficient to achieve the OLED level encapsulation standard of 10-6g/m2day even using a single layer. He revealed that this technology will be able to reduce the production cost and increase the productivity.

 

(a)(a)Device Immediately After Production (b) Device with Insufficient Encapsulation with Insufficient Encapsulation (dark spot and pixel shrinkage occurs after certain amount of time) Source: UBI Research

(a) Device Immediately After Production (b) Device with Insufficient Encapsulation with Insufficient Encapsulation
(dark spot and pixel shrinkage occurs after certain amount of time)
Source: UBI Research

OLED 8K TV, When Would It Be Possible?

The current TV market trends are curved design, large area, and high resolution. LCD and OLED, competing to lead the next generation display market, have both released curved large size premium TV of 55 inch screen or larger. In terms of resolution, UHD grade products are being released following FHD, and displays with higher resolution are being required.

 

Looking at Korea and Japan’s contents roadmap, UHD resolution OLED TV development is essential as Japan is aiming to test 8K contents broadcasting in 2016, and Korea in 2018. Korea began test broadcasting UHD from 2013, and is aiming for regular application in 2016 for satellite/cable channels and 2018 for broadcast channels. Considering active release of UHD TV occurred in 2014, 8K TV’s market release is estimated to be in 2019-2020. It is analyzed that approximately 3 years are left to prepare for 8K TV mass production.

 

At present 8K LCD TV have been revealed by key panel companies through various exhibitions, and its mass production is set for 2016-2017. However, only up to 4K OLED TV have been unveiled, falling behind LCD in terms of resolution.

 

The keys to 8K OLED TV actualization are pixel size reduction and aperture ratio achievement. LCD uses 1 transistor and capacitor per pixel whereas OLED requires 2 or more transistors and 1 capacitor per pixel. This leads to OLED’s difficulty in acquiring adequate aperture ratio and reducing pixel size compared to LCD. The key solutions are developments of top emission structure of OLED panel for TV, instead of bottom emission that produces light through TFT, and emitting materials that can generate sufficient light efficacy from bottom emission produced aperture ratio.

 

OLED demonstrated its strength as display by achieving what LCD took more than 10 years in 2-3 years. Considering this, although approximately only 3 years are left to mass produce 8K display, it is anticipated that OLED is capable of catching up to LCD’s resolution.

 

4K OLED TV by LG Display and Samsung Display

4K OLED TV by LG Display and Samsung Display

New Solution for Next Generation OLED Lighting

Professor Lee Taek Seung of Chungnam National University

Professor Lee Taek Seung of Chungnam National University

 

Professor Taek Seung Lee and Jongho Kim (Chungnam National University’s Department of Advanced Organic Materials and Textile System Engineering), and Professor Jin Sung-Ho and Park Juhyeon (Pusan National University’s Graduate Department of Chemical Materials, and Institute for Plastic Information and Energy Materials) authored a paper titled ‘Synthesis of conjugated, hyperbranched copolymers for tunable multicolor emissions in light-emitting diodes’. For 2015 June issue, Polymer Chemistry, published by the Royal Society of Chemistry, selected it as its back cover.

 

The paper discusses research of polymer material applied to solution process and explains that through polymer structure in the form of hyperbranched red, green, and blue monomers, diverse colors, including white, can be actualized depending on the amount of each monomer. Existing OLED lighting used R/G/B or YG/B stacking structure to produce white OLED, complicating the process. Although a method of producing white by combining R/G/B together is being developed, energy displacement between R/G/B can cause unwanted colors. However, if the R/G/B monomers can be introduced to polymer structure as hyperbranched forms as the paper suggests, the energy displacement can be minimized when the polymer solidifies which makes it easier for the colors to be realized.

 

Professor Lee revealed that hyperbranched polymer materials were used in the research and that as white can be produced from just one polymer material, simple process can be used for the production.

 

The patent for this technology has been applied (application number 10-2012-0091350) in Korea. It is anticipated that this will become a key technology for reducing the next generation OLED lighting production cost.

 

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[SID 2015] AUO’s New RGBY Pixel Structure, Can It Be an Answer for Low-Powered OLED?

Display consumes most power out of smartphone battery usage. This means that low-powered display is most important in lengthening the smartphone’s usage time.

 

AMOLED panel is a self-illuminating device driven by each RGB subpixels, and theoretically power consumption should be much lower than LCD’s which requires BLU (back light unit) to be constantly lit. However, as OLED materials’ performance, particularly blue, is not sufficient the power consumption falls short of expectation.

 

In SID 2015, AUO gave a speech on new RGBY pixel structure attempting to solve the power consumption issue. AUO applied PSA (power saving algorithm) and SPR (sub pixel rendering) of RGBY method, not RGBG structure’s pentile method of existing FMM RGB mechanism, and reported that this showed higher definition and lower power consumption compared to the pentile method RGB applied for high resolution. Particularly it was shown that it can be a key technology for low-powered AMOLED panel through HD 4.65inch (317ppi) panel demo; the power consumption of yellow sub pixel (efficiency 80-120 cd/A) was reduced by 16-20% in comparison to existing RGB method.

 

Dr. Meng-Ting Lee of AUO told the audience yellow sub pixel application improves high resolution, high definition, low power consumption, and panel’s lifetime simultaneously and that AUO’s RGBY SPR and PSA technology can become key technology for AMOLED panel for mobile device.

 

AUO’s RGBY Pixel Structure, SID 2015

AUO’s RGBY Pixel Structure, SID 2015

 

Comparison of Power Consumption Between RGB and RGBY

Comparison of Power Consumption Between RGB and RGBY

Seoul National University’s Research Team Lead by Professor Changhee Lee Succeed in Developing High Output QLED Emitting True Ultraviolet Light

South Korean research team lead by Professor Changhee Lee in Seoul University succeeded in making first quantum dots that emit ultraviolet light and used them to produce a flexible, light-emitting diode.

UV light is usually produced by mercury lamps or LEDs made from inorganic materials such as gallium nitride (GaN). However, mercury lamps tend to emit a wide range of visible wavelengths as well as UV, and high-performance gallium nitride LEDs are expensive to make. According to Professor Lee, quantum dots are an attractive alternative which can be made using potentially less expensive solution-based processes.

Quantum dots, made out of a semiconductor material, emits different wavelengths depending on the size and shapes. The smaller the crystal, the shorter the wavelength of the light it emits. The Professor Lee’s team is the first in succeeding in making quantum dots that emit wavelengths shorter than about 400 nm, the high end of the UV spectrum.

In order to produce UV nanocrystals, the team had to figure out how to make quantum dots with light-emitting cores smaller than 3 nm in diameter. To make these, the team utilized cadmium zinc sulfide, which emits high-frequency light, zinc sulfide shell. The quantum dots produced through this method emit true UV radiation at about 377 nm. Professor Lee explained that they “can go to much shorter wavelengths than people generally expected from quantum dots”.

The research group then made a flexible LED with the quantum dots, using a design for a high-efficiency device they developed in 2012. Professor Lee’s team showed that the UV LED could illuminate an anticounterfeiting mark in a bill of paper currency. Franky So, a materials scientist at the University of Florida, says making a device out of the quantum dots that shines brightly enough to reveal the currency mark is a remarkable accomplishment. If their lifetimes can be improved, these potentially low-cost UV LEDs could find uses in counterfeit currency detection, water sterilization, and industrial applications.

The research team headed by Professor Lee includes Seoul National University, Professor Seonghoon Lee and Professor Koonheon Char, and Dong-a University’s Professor Jeonghun Kwak. The research was published in Nano Letters (Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b00392), a monthly peer-reviewed scientific journal, and reported on Chemistry & Engineering (27 May), published by the American Chemical Society.11