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Illumination & Displays

All-phosphorescent white stacked organic LEDs for solid-state lighting

A novel technology offers power-efficient high-quality illumination with unprecedented thin form factors and low operating temperatures.
27 October 2011, SPIE Newsroom. DOI: 10.1117/2.1201110.003846

The global lighting industry is in transition. Inefficient light sources, such as the incandescent bulb, are being replaced by energy-efficient alternatives. Solid-state lighting in the form of LEDs and organic LEDs (OLEDs) offers a promising solution. Here, we focus on OLED lighting, which is a rapidly accelerating technology that offers power-efficient, high-quality illumination with unprecedented form factors (e.g., minimal thickness, flexibility, and transparency), and low operating temperatures. However to meet the demands of mainstream lighting applications, improvement in OLED device lifetime is still required.

It is customary to report lifetimes for solid-state lighting (SSL) based on the time taken for emission to decay to 70% of initial luminance (LT70). The longest lifetime reported to date for a single-unit warm white phosphorescent organic LED (PHOLEDTM) pixel is LT70 ≈55, 000h at an initial luminance of 1000cd/m2 for a 2mm2 pixel with 72lm/W efficacy and color-rendering index (CRI)=85.1 (Lumen, lm, is a measure of luminous flux. Candela, cd, is a measured of luminous intensity.)

Although the lifetime of single-unit phosphorescent OLEDs is already extremely encouraging, here we describe an approach to further enhance device lifetime by stacking OLEDs so that light from two stacks can be combined without any need to increase the area of the light source. In this stacked OLED (SOLED) configuration, we separate emissive units by a charge-generation layer (CGL) that serves as an anode for one unit and as a cathode for a second unit.2, 3 SOLEDs offer longer operational lifetime than equivalent single-unit OLEDs because the required luminance from each unit is reduced for a given total light output. For example, where we have two units in a SOLED stack, each need only provide 500cd/m2 luminance for a total of 1000cd/m2. Additionally, the CGL recycles electric charge between units. Consequently, lower current is required for the same light output in comparison. This leads to reduced resistive power loss and reduced resistive heating, especially for large-area light panels. Reduced heating also contributes to lower temperature and extended device lifetime.4


Figure 1. Blue/red-green (B/RG) stacked organic LED (SOLED) architecture (device 1: left), red-green-blue/red-green-blue (RGB/RGB) SOLED architecture (device 2: right). BL: Blocking layer. CGL: Charge-generation layer. EML: Emission layer. ETL: Electron transport layer. HIL: Hole (positive charge carrier) injection layer. HTL: Hole transport layer. ITO: Indium tin oxide.

Figure 2. Power efficacy (PE, lm/W) (squares), luminous efficacy (LE, cd/A) (triangles), and EQE (%) (circles) of (a) device 1 and (b) device 2. Data is shown without outcoupling enhancement.
Table 1. Performance of white SOLED structures. Δu′v′: Color change. CIE: International Commission on Illumination. CRI: Color-rendering index. Duv: Difference in chromaticity between light source and a black body with the same correlated color temperature (CCT). cd/A: Candela/ampere. EQE: External quantum efficiency. lm/W: Lumen/watt. LT70: 70% of initial luminance.
At 3000 cd/m2Device 1: B/RGDevice 2: RGB/RGB
NooutcouplingWith outcouplingNooutcouplingWithoutcoupling
1931 CIE (x, y) (0.461, 0.426) (0.446, 0.420)
CRI 82 83
Duv 0.005 0.005
CCT (K) 2800 2990
Voltage (V) 7.3 6.9 8.3 7.9
Luminous efficacy (cd/A) 83 124 84 136
EQE (%) 40 60 38 61
Efficacy (lm/W) 36 56 32 54
Efficacy enhancement 1.00× 1.57× 1.00× 1.68×
Lifetime LT70 (h) 10,300 20,000 8500 19,500
Lifetime acceleration factor 1.6 1.7
Color stability (Δu′v′) 0.020 (68% aging) 0.006 (60% aging)

We examined two white SOLED architectures: blue/red-green (B/RG, device 1) and red-green-blue/red-green-blue (RGB/RGB, device 2) (see Figure 1). Figure 2 shows efficiency for devices 1 and 2 plotted against luminance. Efficiency was measured by providing power to the device using a Keithley 2400 SourceMeter and by measuring luminance for a given voltage and current density using a PR-705 spectrophotometer. Table 1 summarizes 2mm2 white SOLED pixel data at 3000cd/m2. At 1000cd/m2, efficacy=63lm/W for device 1, and 60lm/W for device 2. By comparison, an incandescent bulb would have only 12lm/W efficacy and a compact fluorescent lamp would have 40–60lm/W efficacy. This data includes light-extraction efficacy enhancement achieved using an index-matched light-extraction block. At 3000cd/m2, efficacy=56lm/W for device 1 and 54lm/W for device 2. These high-efficiency SOLED device architectures could readily be transferred to larger-area light panels and luminaires (lamps) to enable energy-saving lighting.


Figure 3. Normalized EL spectra at 3000cd/m2. a.u.: Arbitrary units.

Figure 3 shows electroluminescence (EL) spectra. Illumination quality is high, with CRI=82 for device 1 and CRI=83 for device 2. CRI and chromaticity (Duv) meet US Energy Star criteria for solid-state lighting luminaires (CRI>75 and Duv<0.006).5 These results show that both SOLED devices emit light with emission color close to the blackbody curve (a plot of emission color of an ideal emitter at different correlated color temperatures) and render colors accurately.

Another important consideration for SSL is that high-quality illumination be maintained throughout the device lifetime. For this reason, it is critical to minimize change in emission color with aging. We measured EL spectra for devices 1 and 2 before and after aging. Here, the CIE (International Commission on Illumination) 1976 (L* , u*, v*) color space chromaticity diagram is used to quantify color change because, on this diagram, distance is approximately proportional to perceived difference in color. The color change for device 1 was Δu′v′ =0.020 after aging to 68% of initial luminance, and that of device 2 was Δu′v′ =0.006 after aging to 60% of initial luminance. The excellent color stability of device 2 meets Energy Star criteria (Δu′v′ <0.007). It is likely that this result is enabled by the equivalence of the two units in the RGB/RGB device architecture, which minimizes differential aging. For device 1, lifetime is LT70 ≈20, 000h from an initial luminance of 3000cd/m2. For device 2, lifetime is LT70 ≈19, 500h at 3000cd/m2. At 1000cd/m2, we expect LT70 ≈130, 000h. All data includes light-extraction enhancement.

In summary, we have described white SOLED lifetimes that are > 2× longer than previously reported for equivalent single-unit all-phosphorescent white OLED pixels.1 They also exceed requirements for mainstream lighting applications. These lifetimes are also thought to be the longest reported to date for all-phosphorescent white OLEDs. This performance provides a clear pathway to commercialization of long-lifetime and high-efficacy OLED lighting. Future work will focus on extending lifetime still further through synthesis of increasingly stable organic materials, and by the development of light-extraction schemes that reduce optical losses in the OLED stack.

The authors thank the US Department of Energy for support under contracts DE-PS02-08ER08-17 and DE-FG02-07ER84809.


Vadim Adamovich, Peter A. Levermore, Xin Xu, Alexey B. Dyatkin, Zeinab Elshenawy, Michael S. Weaver, Julie J. Brown
Universal Display Corporation (UDC)
Ewing, NJ

Vadim Adamovich is the research team leader in PHOLED development. He graduated with a PhD in chemistry from the University of Southern California (2002). His primary focus is research on the cutting edge of PHOLED devices.

Peter Levermore is a research scientist focusing on research and development of highly efficient and stable PHOLEDs for display and lighting applications. He has over 50 journal publications and patent applications in the field of OLEDs.

Xin Xu received her PhD in electrical engineering from Princeton University (2009). She is currently a research scientist working on research and development of PHOLEDs for lighting and display applications. She has authored over 10 publications and seven patent applications on organic electronics.

Alexey Dyatkin is a principal scientist. He graduated from Moscow University and received his PhD in organic chemistry from the Russian Academy of Sciences. After postdoctoral work at Trinity University, TX, and Boehringer Ingelheim Pharmaceuticals, CT, he worked in research and development for Johnson and Johnson Pharmaceutical Research, PA. In 2006, he joined UDC. He has published a number of patents, publications, and reviews in organic and medicinal chemistry.

Zeinab Elshenawy is a research chemist. She received her BS in chemistry from Ian Shames University, Egypt. She worked in agriculture research and development for FMC Corporation for 18 years, and joined UDC in 2006. She has a number of patents, publications, and reviews in organic chemistry.

Michael Weaver is director of PHOLED applications engineering and development. He graduated from Sheffield University with a PhD in physics (1996). From 1996 to 2000 he worked for Sharp developing OLED displays. He joined UDC in 2000. He has published more than 90 papers and has 25 US patents on OLEDs.

Julie Brown is senior vice president and chief technology officer. She received a BS from Cornell University (1983) and worked at Raytheon Company from 1983 to 1984, Bell Laboratories from 1984 to 1986, and Hughes Research Laboratories (now HRL Laboratories) from 1991 to 1998. She received a PhD in electrical engineering from the University of Southern California (1991). She is an IEEE Fellow, a Society for Information Display Fellow, and a member of the New Jersey High-Tech Hall of Fame.


References:
1. P. A. Levermore, A. B. Dyatkin, Z. M. Elshenawy, H. Pang, R. C. Kwong, R. Ma, M. S. Weaver, J. J. Brown, Phosphorescent OLEDs: enabling solid state lighting with lower temperature and longer lifetime, SID Digest 72.2, pp. 1060, 2011.
2. G. Gu, G. Parthasarathy, S. R. Forrest, A metal-free, full-color stacked organic light-emitting device, Appl. Phys. Lett. 74(2), pp. 305-307, 1999.
3. Y.-S. Tyan, Y. Rao, X. Ren, R. Kesel, T. R. Cushman, W. J. Begley, N. Bhandari, Tandem hybrid white OLED devices with improved light extraction, SID Digest 60.1, pp. 895-898, 2009.
4. M. Ishii, Y. Taga, Influence of temperature and drive current on degradation mechanisms in organic light-emitting diodes, Appl. Phys. Lett. 80, pp. 3430, 2002.
5. US Department of Energy, Energy Star Criteria for Solid State Lighting Luminaires, 2008.