Designing AMOLEDs for Mobile Displays

Active-matrix OLEDs are attractive displays for mobile applications, and designers are rapidly overcoming the technology's remaining weaknesses.

by Ho Kyoon Chung and Seong Taek Lee

THE DISPLAY MARKET can be divided into several categories – mobile, monitor/ notebook PC, and TV – according to their application and display size. All product designers would probably like their displays to be thin, light, and rugged, having excellent luminance, contrast, color gamut, viewing angle, pixel density (often called resolution), response time, and power consumption. But users of mobile displays prioritize these requirements very differently than users of fixed displays designed into products such as monitors and TVs.

High on the prioritized mobile requirements list are thinness, light weight, low power consumption, and high resolution. In addition, the movement toward advanced mobile devices with multifunctionality requires greater display performance. As the functions of mobile devices are expanded to include broadcasting and Internet service, higher resolution and luminance are required (Fig. 1).

High-resolution mobile displays having pixel densities in the range of 200 pixels per inch (ppi) are already commanding a significant share of the mobile-display market, and prototype liquid-crystal displays (LCDs) with 300 and 400 ppi have been demonstrated. These displays are expected to become commercial products in the near future with luminances of over 150 cd/m². Even at this high luminance level, low power consumption is essential for mobile applications.

AMOLED Displays for Mobile Applications

Many companies are very interested in developing active-matrix organic light-emitting diode (AMOLED) displays for mobile applications. Among the reasons are that AMOLED displays have an advantage over thin-film-transistor LCDs (TFT-LCDs) in terms of thickness and weight because OLEDs do not require the use of a backlight or color filters. But meeting the requirements for low power consumption and high resolution is challenging, and further improvements are required.

The power consumption of an AMOLED display greatly depends on the driving voltage, effective light extraction, and the efficiency of the light-emitting materials. For conventional fluorescent materials, the light-emitting efficiency, i.e., the internal quantum efficiency, is theoretically limited to 25% because only the singlet excitons contribute to the radiative decay. For phosphorescent materials, both singlet and triplet excitons canbe utilized to emit light. Thus, phosphorescent OLEDs can have an internal quantum efficiency of 100%. These phosphorescent materials, particularly when combined with a top-emitting structure, can substantially reduce the power consumption of an AMOLED device (Table 1).

Fig. 1: As consumers demand more advanced functions, such as broadcast television and Internet service, in their mobile systems, product designers are demanding higher resolution and greater luminance from their display suppliers.

But there is still work to be done on phos-phorescent materials. Blue-light-emitting phosphorescent materials are not yet ready for production because of their insufficient lifetimes. Thus, a conventional fluorescent blue-light-emitting material was used to calculate the values for the 2.2-in. quarter-common-intermediate-format (QCIF) AMOLEDs listed in Table 1.

The calculated power consumption of the 2.2-in. QCIF AMOLED for a bottom-emitting structure at 100 cd/m² full white is 669 mW for fluorescent materials and 456 mW for phosphorescent light-emitting materials. The actual power required for a video image is expected to be about one-third of these values.

The use of phosphorescent red- and green-light-emitting materials result in a power savings of about 30%. As shown in the table, most of the power is consumed by the relatively inefficient red- and blue-light-emitting materials; further improvements in efficiency are required.

Using highly efficient phosphorescent light-emitting materials will reduce power consumption, and a great deal of materials development is being performed by several key players. A further reduction in power consumption can be achieved by using a top-emitting device structure, which produces better color purity and efficiency by optimizing optical resonance phenomena within the OLED's multiple layers, which is referred to as the micro-cavity effect.

Improving the OLED Structure

A conventional OLED consists of several organic and inorganic thin layers. Because of the use of these multi-layered structures, the light-extraction efficiency – the so-called external quantum efficiency – is very low and only about 20% of the internally emitted light can be utilized in a display because most of the light is guided away through the thin layers. Several approaches have been proposed to improve the external quantum efficiency, including the use of a 1-D/2-D grating structure and an ordered micro-lens array. These techniques enhance the external quantum efficiency by 50–70%. To further improve the efficiency, we have developed an OLED with a photonic crystal structure, which features a periodic modulation of the index of refraction at the interface between the substrate and the light-emitting layers. This minimizes the loss from the optical guiding effect (Fig. 2).

Calculations indicate that the external efficiency can be improved by a factor of four. A preliminary result from test coupons shows a 70% luminance improvement for green. Further optimization of the structure is needed to achieve the theoretical limit.

Although these techniques show that higher light-extraction efficiencies are definitely possible, manufacturability and cost considerations are still issues in adapting the techniques to actual products.

Reducing Power Consumption

Lowering the driving voltage of an OLED device is another way to reduce power consumption. Recently, a top-emitting PIN structure has demonstrated a significantly higher power efficiency than that of conventional bottom-emitting structures. The use of a PIN structure in OLEDs results in the incorporation of intentionally doped charge-carrier transport layers, thus lowering the operating voltage and achieving higher power efficiencies.

By using PIN technology and a very low driving voltage of about 3 V, designers have produced a device having a luminance of 1000 cd/m² with reasonable lifetime. The doping of the charge-carrier transport layers lowers the charge-injection barrier from the electrodes to the organic layers, thus lowering the driving voltage. Although further research on the doping materials, lifetime, and manufacturability is required, this PIN technology on its own is a substantial advancement and has great potential for power savings.

Making High-Resolution OLEDs

There are two ways to realize a full-color OLED display (Table 2). One way is by generating an RGB pattern of colors by creating the colors directly with red-, green-, and blue OLEDs. The other way is by using color conversion, which can be implemented either with a white OLED and color filters or with a blue OLED and color-changing materials. Both of the color-conversion techniques have advantages in scalability and simplicity in manufacturing, but they have limitations in color purity and device efficiency.

The RGB-diode method is main stream and has many advantages, including high device efficiency, good color purity, andcompatibility with a wide variety of light-emitting materials. The most common method for patterning the color primaries for this approach is to fabricate small-molecule light-emitting devices with a fine metal shadow mask. By using a shadow mask, full-color displays from 2 to 20 in. have been demonstrated. But there are still problems regarding high resolution and scalability to large motherglass formats. The patterning accuracy of this method is about ±15 μm, which is marginal for pixel densities above 200 ppi.

 Samsung SDI

Fig. 2: Improving external quantum efficiency is an important way to increase OLED output or reduce power consumption. A photonic crystal increases the luminance of the OLED display by 42% (right) compared to an OLED using a conventional structure (left).

Ink-jet printing of soluble, organic, light-emitting materials is another patterning method that can be used to fabricate full-color OLEDs, and it has many advantages, including low material consumption, good scalability to large-format motherglass, and the ability to be used in atmospheric-pressure conditions. The patterning accuracy of this printing method is almost the same as that of thermal evaporation with fine metal shadow masks. However, ink-jet printing has a serious limitation when using OLED materials because the process requires soluble materials such as polymers or soluble small molecules, which currently have relatively poor efficiency and lifetime.

Given the difficulties with shadow-mask patterning and ink-jet printing, is there a third approach to OLED fabrication? At Samsung SDI, we have recently developed a 2.6-in. full-color VGA AMOLED with a resolution of 302 ppi, fabricated with a new color-patterning method called Laser-Induced Thermal Imaging (LITI) (Table 3). LITI is the transfer of light-emitting material from a donor film to a substrate using a highly accurate laser exposure system (Fig. 3).

The LITI patterning accuracy is better than ±3.5 μm, and this exceptional accuracy is a distinctive advantage of the LITI transfer technology, thus making it possible to fabricate a display having a 28-μm subpixel pitch with a 40% aperture ratio. The white luminance is 200 cd/m² and the color gamut is greater than 80% of the NTSC standard. To achieve a very high pixel density using an active-matrix circuit, a 2-μm design rule for the LTPS TFT fabrication process was used.

These results demonstrate that a high-resolution low-temperature polysilicon (LTPS) AMOLED display fabricated by using the LITI method is feasible for mobile digital applications.

Summary

The significant advantages of using AMOLED displays for portable devices have been tempered by their high power consumption (which has been higher than comparable AMLCDs) and difficulty in obtaining high pixel densities using thin metal shadow masks (the current mainstream fabrication technology) and ink-jet printing (the up-and-coming fabrication technology).

To reduce power consumption, developers are working diligently on top-emission structures, high-efficiency phosphorescent materials, better optical-output coupling, and low-voltage operation.

Patterning OLED materials for high-resolutions applications has also been a problem, but we believe the LITI patterning method provides an excellent solution. It has been used to fabricate the world's highest-resolution AMOLED, a 2.65-in. VGA device with 302 ppi, using a 2-μm design rule and a top-emission structure. •