Organic EL, mainstream technology in flexible displays

1 Introduction

This article will introduce the development and practical status and prospects of flexible displays with the title of “organic EL, the mainstay of flexible displays”. The organic EL is also called an OLED (Organic Light EmitTIng Diode), and in particular, an active matrix-driven organic EL (AMOLED: Organic Light EmitTIng Diode) using a TFT can realize high-definition full-color display.

Before entering the topic, the author will first compare the device structure of the liquid crystal and organic EL display. Then, we will clarify the mass production of large-scale organic EL TVs; then introduce the organic EL of single-layer structure published in early November 2015; finally explain the topic "The status quo of flexible organic EL", hope that through the author's explanation, let everyone realize the status quo and the final The gap in the target.

2. LCD vs. Organic EL

2.1 Display device construction and direct material cost ratio

The author has witnessed the change of the display market from the long-term mainstream picture tube (CRT) to the liquid crystal. It's not just CRT and LCD. In all kinds of displays, the role of components and materials is ordinary. It is very important. It is one of the biggest factors that determine the pros and cons of products in the development of display technology.

The common point between CRT and liquid crystal is the high number of components and high versatility. CRT manufacturers and LCD panel manufacturers can easily replace component suppliers. After the market matures, new companies are still pouring in. Competition between component manufacturers to improve performance and reduce costs has increased. Through competition, improving performance and reducing costs, the two opposite trends have gone hand in hand. Under the baptism of fierce competition, the advancement of technology is breathtaking. The author believes that the high versatility of parts and the low barriers are the necessary conditions for the main force of the display.

Now, with the organic EL as the center, the development and practical use of a new generation of displays are in full swing. A new generation of displays generally tends to have a low number of components and a simple structure. The simple construction is of course best, but it is undeniable that a small number of parts may result in fewer companies participating in the development and limited competition.

One of the big advantages of liquid crystals is that there are many technical proposals from component manufacturers. A display with good performance does not necessarily become mainstream. It must also look at the development of investment and component industries that can capture the market. Now, a solid liquid crystal has been established, and there are new options for components such as laser sources and quantum dots. Therefore, in the future, its status is estimated to be unshakable in the future.

1 is a device configuration of a liquid crystal and an organic EL (here, TFT-LCD and AMOLED, respectively). The numbers in () in the figure represent the cost ratio. AMOLED is a self-illuminating device that does not require a backlight unit (BLU). The TFT-LCD (transparent type used in general), the backlight unit is a necessity, and the ratio of the total cost is as high as 50%. From the perspective of cost reduction, components such as backlight units are also important items. Because the direct material cost (component cost) accounts for 60 to 70% of the cost structure of the TFT-LCD, it is impossible to cut costs if it is not improved from here. The way to reduce costs by increasing the size of the TFT-LCD substrate has reached its limit.

(b) Device structure of AMOLED

Figure 1: Device construction and cost structure of TFT-LCD and AMOLED

As shown in FIG. 1, the TFT-LCD includes a BLU as a light source of a transmissive display, a liquid crystal and a polarizing plate that switch light with an electric field, and a color filter (CF) that realizes colorization, and the functions of the components are independent. Further, the TFT substrate and the CF substrate are basically produced by different processes. Therefore, inspection and repair can be carried out step by step. This is particularly useful for improving the yield of a large-screen TFT-LCD (a small number of substrates that can be fabricated) using a large substrate.

The AMOLED is fabricated by superposing a nano-scale organic film on a substrate of a TFT array. Therefore, in terms of device configuration, although the functions of each layer of the organic film are independent, they cannot be inspected and repaired during processing. It can be called a "functional integrated device."

2.2 Problems in mass production of organic EL TVs

In the “LCD or OLED?” lecture (Session 3) held by “SID” in 2013, the author gave a special speech on the topic “TFT-LCD as leading role in FPD”. In the speech, I listed five mass production issues of organic EL TV, pointing out that "it is important to solve these problems, to show consumers the difference with LCD TVs, and to be competitive in price."

(a) Luminescent material: low molecular (vacuum evaporation) or polymer (printing), fluorescent or phosphorescent

(b) need to solve problems such as power consumption, life, and residual image

(c) TFT array manufacturing apparatus supporting the 10th generation panel

(d) Sub-coating technology and apparatus for mask evaporation

(e) Packaging technology and devices supporting large panels

The speech has been over for more than two years, and now the company that produces organic EL panels for large-screen TVs is only one Korean LG monitor. I heard that although the company did not achieve significant technological breakthroughs, it improved the yield by changing the device structure. However, the yield is equal to [(finish yield & TImes; operating rate) / duration], and it is estimated that the operating rate of the IGZO film forming and the light-emitting layer evaporation process is not high.

3. Organic EL device construction and manufacturing process

(a) Organic EL (OLED)

(b) Organic EL (OLED)

Figure 2: Past multilayer construction and newly developed single-layer organic EL (OLED)

The current organic EL achieves a theoretical maximum of 100% by using a phosphorescent luminescent material. The device structure is as shown in Fig. 2(a), and a multilayer structure is employed. Each layer was laminated with a nano-scale organic material using a vacuum evaporation method. It constitutes a structure in which the functions of the organic EL are multi-layered.

Figure 3: White display realized with a single layer structured organic EL (OLED)

A professor at Tohoku University, Isobe Kwanji, and others found a subversive common sense: "By designing a basic material, a single layer of organic EL with a luminous efficiency close to the theoretical limit can be produced" (Fig. 2(b)). The organic material used is an organic substance (hydrocarbon) composed only of two elements of carbon and hydrogen. From the essence of molecular design, the design guidelines for organic EL have been successfully simplified. From the perspective of the elemental strategy of “maximizing element performance”, it is also an important discovery. The research team has confirmed that this new type of hydrocarbon can use phosphorescent materials to achieve the three primary colors of red, green and blue. As shown in Fig. 3, a white light-emitting device was successfully fabricated.

The focus of this discovery is the use of toluene with methyl (CH3) around benzene to produce a more functional, single-layer, high-luminescence organic EL base material. High-functional electronic materials can be produced by molecular design and chemical conversion of natural products (toluene) isolated from trees to form five connected molecules (5Me-[5] CMP). This electronic material uses only carbon and hydrogen. In the near future, it is expected to achieve "a light spray can be made."

4. Current status and prospects of flexible organic EL

Figure 4 is an organic EL display that has been put into mass production or is under development. The display used in the large-screen organic EL television today is a fixed curved shape with a radius R of 4500 mm. Not a flexible display. The "flexible organic EL" used in smartphones that have been put into production is actually a fixed curve or a fixed edge curve (Fixed Edge Curve), and it is also not flexible (in terms of radius R, the fixed curve is 700 mm, and the fixed edge curve is 10 mm). The wearable display is also a fixed curve (R = 5 ~ 10mm), which is also not flexible. The vertical Fold and the Acute Foldable that are currently being developed should be referred to as flexible displays. In terms of the radius R, the folding is 3 mm, and the multi-layer folding is 1 mm or less.

Figure 4: Flexible Organic EL (OLED) that has been put into practical use and under development

Now, organic EL panels for large-screen TVs, only LG monitors have been put into mass production. The TFTs equipped with IGZO are fabricated on a glass substrate. For the panels used in mobile products, there are two Korean Samsung monitors and LG monitors for mass production. There are also types in which a resin substrate is used. In these panels, a polyimide film is coated on a glass substrate, a low temperature polysilicon (LTPS) TFT is formed thereon, and then an organic EL is laminated, and the polyimide film is peeled off from the glass substrate. Mobile products are used differently than TVs, and power consumption must be low, so only LTPS TFTs that can form COMS circuits can be used. These are all TFTs based on inorganic semiconductors, and there is a limit to the radius at which they can be bent.

5. JOLED strategy

The following is a description of the technology of "JOLED" company, which was born on January 5, 2015, bringing together Japan's most advanced organic EL technology (Sony and Panasonic). JOLED will use the following technologies to establish the dominant position of OLED.

5.1 Organic EL Printing Technology (Printed OLED)

This technology uses a printing method that does not require RGB vacuum coating. This method is to form a light-emitting layer (EL layer) by printing a material in the air, "no need for a vacuum environment", "no mask", and the manufacturing process requires less investment and is simple to maintain. Moreover, it is easy to support the enlargement of the panel. Since it is only necessary to apply the required component at the desired position, the loss of the material is small.

However, does the organic EL thus produced have the same characteristics and longevity as the vapor deposition method? How high is the printing method? These are very important topics when considering the application of organic EL. 4K, 8K high-definition panels are being developed or put into practical use by application in medium-sized TFT liquid crystals.

5.2 Oxide semiconductor technology

This technology uses a self-aligned top-gate structure developed by Sony to fabricate TFTs using five masks. The resistance of the electrode portion of the TFT is lowered by the self-developed metal reaction. This technology ensures high stability of the TFT. By combining the self-developed compensation circuit technology, we will fully utilize the performance of organic EL in medium-sized and above sizes, and strive to achieve high-definition, low-cost displays.

However, the vacuum plating (sputtering) of an oxide semiconductor has a problem in terms of the operating rate and the yield of the device compared with the plasma CVD used for the amorphous Si (a-Si) plating. Further, an oxide semiconductor typified by IGZO is a multi-element material (for example, quaternary), and it is not easy to produce on a large-area substrate with a uniform composition and thickness. From the viewpoint of green technology, the author sincerely hopes to develop a coating-type oxide semiconductor TFT technology and put it into practical use.

5.3 flexible panel

JOLED is developing a flexible panel using a technique of forming a film on a glass substrate and transferring it to a film. However, the key to the realization of a flexible display is to use the roll-to-roll (R2R) method instead of a single sheet. What is important is to provide a flexible display that users need, unlike technologies that are available from existing manufacturers.

6. Conclusion

Organic EL has a display characteristic that liquid crystals are difficult to achieve, so it has been regarded as the mainstream of flexible displays. This column compares the device construction and cost components of the two. At the same time, the current flexible organic EL was introduced while clarifying the mass production problems of large organic EL televisions. The production methods currently used by these organic ELs are still far from the green ones. In other words, Japanese manufacturers still have the opportunity to regain their disadvantages. The author introduced the technology of "Inventing Japan" this time, and hope that these new technologies can lead the world's display industry and the society to keep moving forward. (Contributing author: Yu Yuhong, representative of Ukai Display Device Institute) Technology Online

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