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Illumination & Displays

High-color-rendering white organic LEDs based on novel phosphors

A combination of efficient true-blue iridium complexes and improvements to device architecture significantly enhances the color performance of white-emitting organic LEDs.
20 September 2010, SPIE Newsroom. DOI: 10.1117/2.1201009.003081

Intrinsically high-quantum-efficiency, white-emitting organic LEDs (WOLEDs) incorporating phosphorescent emitters have become promising candidates for meeting stringent efficiency requirements in lighting applications. High-quality white-light illumination requires WOLEDs with International Commission on Illumination (CIE) coordinates similar to those of blackbody radiation, a correlated color temperature between 2500 and 6500K, and a color-rendering index (CRI) above 80. Until recently, one of the major bottlenecks in implementing high-CRI phosphorescent WOLEDs was the lack of efficient deep-blue phosphors. Using a series of efficient true-blue iridium complexes that we developed, and incorporating two complementary colors or three primary colors, we were able to achieve high-color-rendering pure-white phosphorescent OLEDs.

Figure 1. (a) Schematic structure of device A. (b) Phosphor chemical structures. (c) Normalized electroluminescence (EL) spectra at different brightness levels of device A. Al: Aluminum used as a cathode. Cs2CO3: Cesium carbonate, electron-injection material. BCP: Electron-transport material. BCP:Os: BCP doped with 2wt% of Os(bptz)2(dppee). CzSi:Ir: Wide-energy-gap material doped with 6wt% of Ir(dfbppy)(fbppz)2. TCTA:Os: Hole-transport material doped with 1wt% of Os(bptz)2(dppee). TCTA, α-NPD: Hole-transport material. ITO: Indium tin oxide used as an anode. Ir(dfbppy)(fbppz)2: Blue iridium complex. Os(bptz)2(dppee): Yellow osmium complex. tBu: tert-Butyl. CF3: Carbon trifluoride. F: Fluorine. Os: Osmium. Ph: Phosphine. P: Phosphorus. N: Nitrogen. cd/m2: Brightness unit. au.: Arbitrary units.

We prepared two-component WOLEDs using our new blue iridium1 Ir(dfbppy) (fbppz)2 and yellow osmium Os(bptz)2(dppee) complexes. The WOLEDs had high CRIs thanks to the large full width at half-maximum (FWHM) of both the blue and yellow emissions. We used a triple-emitting layer structure to stabilize the colors with varied biases (operation voltages). The layers included ITO (indium tin oxide used as an anode)/30nm of α-NPD (hole-transport material)/15nm of TCTA (hole-transport material)/5nm of TCTA:Os(bptz)2(dppee) 1wt% (weight percent)/15nm of CzSi (wide-energy-gap host material):Ir(dfbppy)(fbppz)2 6wt%/5nm of BCP (electron-transport material):Os(bptz)2(dppee) 2wt%/40nm of BCP/2nm of Cs2CO3 (cesium carbonate, electron-injection material)/150nm of Al (aluminum used as a cathode). Figure 1 shows the chemical structures of the phosphors and the device structures. The ionization potential of Os(bptz)2(dppee) (4.8eV) is substantially lower than that of TCTA, CzSi, and Ir(dfbppy)(fbppz)2 (all of which range between 5.8 and 6.0eV). With such an energy-level relationship, the Os complexes behave as effective hole traps in TCTA. The hole-trapping mechanism retarded the shift of the recombination zone toward the BCP interface, stabilizing electroluminescence (EL) colors with biases.2 For brightness from <102 to 103cd/m2 (brightness unit), the device showed nearly pure white EL with CIE coordinates from (0.311, 0.327) to (0.314, 0.332) (see Figure 1). The small color variation of only about (0.003, 0.005) shows the effectiveness of the device strategy and architecture. Also, the CRI of device A at different brightness levels was higher than 80.

To further improve the color-rendering capability, we introduced three emissive dopants—deep-blue phosphor3 Ir(fppz)2(dfbdp), green phosphor Ir(ppy)3, and red phosphor2 Os(bpftz)2(PPh2Me)2—to cover most of the visible region. The phospholuminescence spectra of these blue, green, and red emitters cover most of the visible wavelengths, meaning that, in principle, pure white colorant and high-color rendering should be attainable with appropriate mixing. For OLEDs incorporating deep-blue phosphors, one of the key challenges is effective device architectures. To achieve acceptable efficiency for the deep-blue device, the double emission layers and the high-triplet-energy buffer layers on the two opposite sides of the emitting layers (EMLs) were simultaneously adopted for better confinement of triplet excitons and carriers.3

With this device structure as a template, we implemented multiple-EML WOLEDs with pure white colorant and unprecedentedly high CRI values by selectively incorporating green and red phosphors without buffer layers between the EMLs. The architecture of the three-component WOLED (device B) consisted of four emitting layers and double confining layers, ITO/30nm of α-NPD/20nm of TCTA/3nm of CzSi/16nm of CzSi:Ir(fppz)2(dfbdp) 6wt%/3nm of CzSi:Ir(ppy)3 2wt%/4nm of CzSi:Os(bpftz)2(PPh2Me)2(orange-red osmium complex with monodentate phosphine ligand) 1.2wt%/3nm of UGH2 (wide-energy-gap host material):Ir(fppz)2(dfbdp) 6wt%/2nm of UGH2/40nm of TAZ (electron-transport material)/0.5nm of lithium fluoride/150nm of Al. Figure 2 shows t\h he chemical structures of the phosphors and the device structures. Device B exhibited a pure white emission with CIE coordinates close to the ideal (0.33, 0.33). Furthermore, the EL spectra and colors were stable at brightness levels ranging from 102 to 104cd/m2, with small color variation Δ(x, y)=(0.014, 0.028) (see Figure 2). Such broad (FWHM ~210nm) EL emission effectively covers the visible spectral range and greatly enriches the color-rendering capability. Our experimental results showed the CRI to be as high as 94. Note also that the CRI remained above 90 throughout the wide brightness range of 102–104 cd/m2.

Figure 2. (a) Schematic structure of device B. (b) Phosphor chemical structures. (c) Normalized EL spectra at different brightness levels of device B. Ir(ppy)3: Green iridium complex. Os(bpftz)2(PPh2Me)2: Orange-red osmium complex with monodentate phosphine ligand. LiF: Lithium fluoride, electron-injection material. TAZ: Electron-transport material. UGH2: Wide-energy-gap host material. B: Blue. R: Red. G: Green. wt%: Weight percent.

In summary, we improved the color rendering of WOLEDs using tailor-made phosphors with complementary colors or three primary colors. We also strategically modulated the doping profiles of emissive dopants for efficiency and color stability. In particular, we implemented three-component phosphorescent WOLEDs with very high color performance based on a deep-blue phosphorescent OLED structure with double emitting layers and confining layers. We then selectively replaced some of the deep-blue phosphors with green and orange-red phosphors. The result was a structure with green/orange-red emitting layers sandwiched between two deep-blue emitting layers. Next, we plan to investigate other multifunctional host and transport materials that may further simplify the device architecture.

The authors gratefully acknowledge financial support from National Science Council of Taiwan.

Chih-Hao Chang
Yuan Ze University
Chung-Li, Taiwan

Chih-Hao Chang is an assistant professor in the Department of Photonics Engineering. He received his PhD degree in electro-optical engineering from National Taiwan University in 2009. His research interests include organic optoelectronic and electronic devices, flat-panel displays, and solid-state lighting.

Chung-Chia Chen, Chung-Chih Wu
National Taiwan University
Taipei, Taiwan
Yun Chi
National Tsing Hua University
Hsinchu, Taiwan