OLEDs: A Lighting Revolution?

Significant improvement in OLED performance levels has been made over the past decade. OLED technology has the potential to bring about a new era in lighting.

by Ok-Keun Song and HoKyoon Chung

ORGANIC LIGHT-EMITTING DIODES (OLEDs) are among the most promising sources for the next generation of display and solid-state lighting because of their energy-saving and flexible design aspects. OLEDs' inherent characteristics make it possible for both passive- and active-matrix versions to be successfully commercialized for display applications. And the potential of OLEDs goes beyond displays. An OLED is basically a thin-film-based device on glass or plastic substrates. Its ultra-slimness, transparency, flexibility, and color tunability make it a new and potentially revolutionary source for lighting. It is a flat-area light source that provides advantages over LEDs (which are point sources), including heat management and design flexibility. OLEDs' properties are totally different from those of lighting sources such as LEDs, incandescent light bulbs, and fluorescent lamps, and offer a whole new range of lighting applications that would be impossible to imagine with previous lighting sources.

OLED lighting can be used for general-lighting purposes or for premium-grade applications such as architectural, hotel chandeliers, and pendent lighting. The OLED panels shown in Fig. 1 were introduced at the Light & Building 2010 exhibition in Frankfurt.¹ These concepts are excellent examples of the types of lighting designs that are possible only with OLEDs.

Highly Efficient Materials and Outcoupling Technology

The operational voltage of OLEDs is usually in the range of a few volts, and the internal quantum efficiency is relatively high compared with LEDs,² although these features can introduce other concerns due to the relatively high current densities needed for the inter-connection wiring. In general, however, these properties should make it possible for OLEDs to become an effective light source with high power efficiency. For state-of-the-art OLED devices using phosphorescent materials, it was recently reported that the internal quantum efficiency of a small-molecule OLED is almost close to the theoretical limit of 100%.³ Various efforts have been made to improve the performance of OLED lighting, but key technologies in this area can be classified into three major categories: high-efficiency material, low driving voltage, and effective out-coupling technology.

Highly efficient phosphorescent materials have recently enabled several major OLED players to make considerable progress in power efficiency and reliability. Light-extraction technology is also one of the most effective methods to improve the power efficiency of highly emissive phosphorescent materials. A simple OLED structure has a significant amount of its emitted light trapped inside, due to the refractive-index mismatch between the substrate and organic layers. In a conventional bottom-emission OLED, only about 50% of the generated photons will propagate into the substrate and the remainder will be wave-guided and dissipated in the organic layers due to the refractive mismatch between the organic stack (n = 1.7–1.9) and the substrate (n = 1.5). Finally, only 40% of the photons reaching the substrate will be emitted into the air due to the total internal reflection at the substrate/air interface. As a result, only 20% of all photons formed in an emitting layer can escape from the glass substrate into the air.⁴ From this point of view, the improvement of the light-extraction efficiency is critical to enhancing the power efficiency, lifetime, and brightness.

Performance Results to Date

As shown in Fig. 2, Universal Display Corp. (UDC) has demonstrated the year-on-year improvement of power efficiency and lifetime of its white OLEDs. These impressive results range from warm to cool white, with varied power efficacies of 54–102 lm/W. According to UDC's reports, depending on the specific device designs employed, the color-rendering index (CRI) varies from 70 to 88 and lifetimes vary from 4,000 to 17,000 hours (to 70% of initial luminance at 1000 nits) using UDC's high-efficiency phosphorescent materials and outcoupling technology. Although reliability still needs to be improved in order to satisfy requirements for practical applications, this achievement is significant because the power efficiency of white OLEDs surpasses the power efficiency of two current major indoor lighting technologies: incandescent bulbs with a power efficiency of 15 lm/W and fluorescent lamps with a power efficiency of 60–90 lm/W. According to UDC's projections, a power efficiency of > 110 lm/W at 1000 nits can be achieved within this year. If so, it means that OLED lighting may also be able to compete with LED lighting in terms of energy efficiency. Initially, it may be hard to achieve the total luminance levels required in the same-sized packages as LEDs based on a total luminance of approximately 1000 nits. However, in the future, as total luminace levels approach 4000 nits, the total package sizes might become comparable.

Fig. 2: Recent tests by UDC demonstrate a high efficacy for white OLEDs. Source: Universal Display Corp.

From an applications perspective, it is noteworthy that the performance of OLEDs exceeds Energy Star Category A, which includes a color specification of CRI ~ 80 and an efficiency specification of > 35 lm/W.⁵ The Energy Star lifetime specification of < 25,000 hours should be satisfied soon, considering the improvement speed of OLED performance. Lighting generally requires high brightness and a good Planckian locus, which is the color a blackbody takes in the chromaticity space as the blackbody temperature changes. One of the most popular technologies for achieving higher brightness is a tandem structure with a combination of emitting layers. Figure 3 shows a typical hybrid tandem structure for different color temperatures. This structure generally provides higher brightness and longer lifetime because of the two emitting units, which are connected by intermediate layers (or charge-generation layers). The high brightness of > 10,000 nits and long lifetime of L70 > 50,000 hours at 1000 nits can be achieved by using these hybrid tandem structures at 3000 and 5000 K.⁶ In addition to higher brightness and longer lifetime, another advantage of this structure is that the color temperature of devices can be easily adjusted by simply switching the order of emitting units. These results strongly indicate that a hybrid tandem structure is a key approach to the performance enhancement of OLED lighting.

Fig. 3: A hybrid tandem structure is shown for white OLEDs at 3000 K (left) and 5000 K (right). Source: Samsung Mobile Display.

Novaled has recently shown that with a combination of key technologies such as a long-life tandem white OLED, a low-voltage-operated PIN structure, and good outcoupling technology, it could achieve a power efficiency of ~51 lm/W at 1000 nits with a warm-white color coordinate of (0.45, 0.45). While this efficiency appears much lower than others discussed earlier, it can partially be explained by Novaled's choice of color temperature as well as its selection of different emitting materials that may have other advantages not disclosed currently. As shown in Fig. 4, by collecting the total light in a device with a macroscopic lens, the power efficiency could be improved from 28 to 85 lm/W. The high efficiency of 120 lm/W at 1000 nits was achieved in a green monochromatic emission PIN OLED.⁷

Fig. 4: The power efficiency of a long-life tandem white OLED was dramatically improved by using outcoupling technology and a macroscopic lens. Source: Novaled.

The introduction of a microlens-array film between glass and air is one of the most effective solutions in reducing waveguide mode loss due to refractive-index mismatch at the glass/air interface. In 2009, Kodak reported that a light-extraction efficiency of 92% was achieved through a newly developed external-extraction structure. It was also reported that a dramatically improved light-extraction efficiency of 128% can be easily achieved by introducing an internal extraction structure composed of a high-index coupling layer and scattering layer at the glass/ITO interface.⁸ The Asahi Glass R&D group discovered that it could also achieve a light-extraction efficiency of 80% by using a scattering layer whose matrix was made of high-refractive-index glass.⁹ Without a doubt, the outstanding results of outcoupling technology will be very helpful in shortening time-to-market for OLED lighting.

Creating Market Opportunities

Among the most important factors for OLED applications are new market opportunities. It will take a while for OLED technology to reach the performance levels needed for many conventional lighting applications, but that should not stop more innovative applications and OLED-based solutions from being created now. Recently, several major OLED lighting companies and designers have created products for a new lighting market.

Osram has shown that it is possible to create a new premium-grade product with a completely new concept of OLED lighting, even though the power efficiency is relatively low (~25 lm/W at 1000 nits). Philips has also demonstrated very attractive, design-oriented OLED lighting with its OLED panel technology, Lumiblade, with a power efficiency of 23 lm/W at 1000 nits. Lumiotec in Japan recently announced small-volume production of its OLED lighting panels, whose efficiency is about ~23 lm/W.

According to DisplaySearch, the OLED lighting market will start to pick up in 2011, and the major OLED players such as Philips, Osram, GE, Konica Minolta, Ledon, and Comedd will gear up for mass production. The total market, consisting of decorative flexible and rigid general lighting, rigid healthcare lighting, and flexible signage lighting, is expected to be $391 million and $862 million for 2013 and 2014, respectively.¹⁰ By 2018, it will reach $6.3 billion. Over 100 companies and universities are currently working to create new applications for OLED lighting.

The Challenges of Mass Manufacturing Technology

Outstanding progress has been made, as mentioned above, to create a new category of OLED lighting, but many challenges still exist before OLED lighting can be completely commercialized. Improvements need to be made in the fabrication process, in device performance, and in cost reduction.

With respect to performance, it is noteworthy that the performances of products in small volumes have thus far been relatively inferior to those in the laboratory. Laboratory results are usually obtained from very small test cells, which are relatively free from the effect of internal heat generation. OLED panels, which are larger than the test cells, more easily generate internal heat and are very sensitive to the thermal environment, and phosphorescent materials are even more sensitive. Mass production requires relatively high processing temperatures for reasonable yields. For that reason, materials that show excellent performances as test cells often cannot be used for mass production due to thermal decay. This phenomenon indicates that manufacturing technology for mass production is clearly one of the most important challenges. The remaining issues involve cost and are directly related to materials utilization rate, facility investment, and product yields. Finding a collective solution for cost reduction in these areas ultimately will be crucial for the commercial success of OLED lighting.

With regard to fabrication, the thermal evaporation process is well-established and currently the most popular method. A roll-to-roll process for OLED fabrication has been under development at companies such as GE and Konica Minolta for some time in order to lower the manufacturing cost, but there remain many more unsolved technological challenges to take OLEDs into roll-to-roll mass production. The materials development and encapsulation technology will most likely be the most challenging process element to resolve.

A Promising Future for OLED Lighting

In summary, remarkable progress in OLED performance has been made and outstanding new concepts of OLED lighting have been successfully introduced. Although many challenges confront its commercial success, OLED lighting looks promising. The lighting market/applications will need to change somewhat before the technology becomes wide-spread. The technology needs to improve, and the market needs to adapt in order to exploit OLED's unique features, as well as its limitations. When the technical challenges are overcome, OLEDs' enhanced performance combined with their unique inherent characteristics will be able to inspire a revolution in lighting.

References

¹Light & Building, Frankfurt, Germany (2010).

²2009 DOE Solid-State Lighting R&D Multi-Year Program Plan.

³L. Xiao, S. J. Su, Y. Agata, H. Lan, and J. Kido, Advanced Materials 21 (12), 1271 (2009).

⁴G. Gaertner and H. Greiner, Proc. SPIE 6999, 69992T-1 (2008).

⁵Energy Star Program requirements for SSL Luminaries – Version 1.0, Second draft as of April 9, 2007.

⁶M. W. Lee, O. K. Song, Y. M. Koo, Y. H. Lee, H. K. Chung, and S. S. Kim, SID Symposium Digest 41, 1800–1803 (2010).

⁷Printed Electronics World, Oct. 29 (2008).

⁸Y. Tyan, Y. Rao, X. Ren, R. Kesel, T. R. Cushman, W. J, Begley, and N. Bhandari, SID Symposium Digest 40, 895 (2009).

⁹N. Nakamura, N. Fukumito, F. Sinapi, N. Wada, Y. Aoki, and K. Maeda, SID Symposium Digest 40, 603 (2009).

¹⁰DisplaySearch 2009, OLED Lighting in 2009 and Beyond. •