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Illumination & Displays

New molecules for efficient multi-color organic LEDs

Universal host materials can be used to produce high-performance devices with simplified structures.
10 August 2012, SPIE Newsroom. DOI: 10.1117/2.1201207.004283

Phosphorescent organic light-emitting devices (PhOLEDs) have drawn broad research interest both from academia and industry because of their potential use in full-color flat-panel displays and solid-state lighting.1 By dispersing transition-metal-based phosphors (the guest) homogeneously within a suitable organic matrix (the host), devices can achieve 100% internal quantum efficiency. Recent advances have shown that hybrids involving bipolar host materials are particularly promising. The development of a universal host material that can work with red, green, and blue (RGB) phosphors and with white OLEDs (WOLEDs) could pave the way towards the relatively simple and more cost-effective manufacture of full-color OLEDs with comparable output efficiencies.

Until now, reported examples of universal host materials for PhOLEDs were limited.2 Based on such work, an ideal universal host should have three properties. First, a high triplet energy is needed: it must be suitable for blue, green, and red phosphors and for preventing reverse energy transfers. Second, appropriate highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) levels (relative to neighboring functional layers) are required to allow charge carriers to be injected smoothly into the emitting layer. Finally, the host must have bipolar transporting characteristics that produce a broad exciton-formation zone and consequently reduce the efficiency roll-off.

Scheme 1. The molecular structure of CPhBzIm and CNBzIm.

Figure 1. Schematic representing the energy transfer, lighting processes, and device performance in CPhBzIm host-dopant systems.

Figure 2. Shown are the energy transfer, lighting processes, and device performance for CNBzIm host-dopant systems.

A new bipolar molecule CPhBzIm (Scheme 1),3 is composed of two N-phenylbenzimidazole units linked to the C3 and C6 positions of N-phenylcarbazole. Although CPhBzIm exhibits a relatively low triplet energy and so is not suitable for confining blue phosphors, the extended π-conjugation through C3 and C6 leads to deep-blue emission with a high quantum yield that can be used to generate blue electroluminescence efficiently. CPhBzIm displays high morphological and thermal stability that together with the high quantum yield makes it an efficient emitter in non-doped deep-blue OLEDs. The material can also function as an excellent host for the yellow-green emitting dopant Ir(pbi)2(acac) and can thus be used in dual roles as blue emitter and bipolar host within one doped single-emissive-layer two-color-based WOLED. This device has an external quantum efficiency (EQE) of 7% with CIE coordinates (0.31, 0.33; see Figure 1).

The CPhBzIm was modified by changing the linking topology that connects the electron-accepting N-phenylbenzimidazole to the electron-donating carbazole core from a C- to a N-bridge. This molecular design leads to a new benzimidazole-carbazole hybrid compound, CNBzIm (Scheme 14), with reduced π-conjugation and thus an increased triplet energy. The altered molecule maintains a suitable frontier orbitals energy, bipolar charge transport, and morphological stability. The triplet energy of CNBzIm is higher than that of the blue phosphor FIrpic and other long-wavelength phosphorescent dopants. This implies that the triplet excitons on the guest are effectively confined to prevent back energy transfer from the guest to the host.

PhOLEDs that have CNBzIm as a host and are doped with FIrpic, (PPy)2Ir(acac), or Os(bpftz)2(PPh2Me)2 under the same device structure show excellent performance, with EQE as high as 12.7% for blue, 17.8% for green, and 19.1% for red (see Figure 2). A two-color all-phosphor single-emitting-layer WOLED hosted by CNBzIm has a maximum EQE of 15.7%, current efficiency of 35cdA−1, and power efficiency of 36.6lmW−1. The high color stability of this WOLED is due to the broad charge recombination zone in the emitting layer that incorporates the improved bipolar host.

We have developed and applied new universal host materials composed of benzimidazole and carbazole substructures bridged by different linkages (C- or N-connectivity). The physical properties of CPhBzIm and CNBzIm make them suitable for use as hosts doped with various phosphors in highly efficient OLEDs. Our work should benefit those designing other novel universal host materials that can be used to develop cost-effective, high-performance PhOLEDs with simplified device structures.

The authors gratefully acknowledge financial support from the National Science Council of Taiwan (NSC 100-2112-M-019-002-MY3, 98-2119-M-002-007-MY3).

Wen-Yi Hung
National Taiwan Ocean University
Keelung, Taiwan

Wen-Yi Hung is an associate professor in the Institute of Optoelectronic Sciences. His research interests cover the study of organic optoelectronic devices including organic LEDs, organic photovoltaic cells, and the physical processes of organic semiconductors.

Ken-Tsung Wong
National Taiwan University
Taipei, Taiwan

Ken-Tsung Wong is a professor in the Department of Chemistry. His research interests mainly focus on the molecular design and organic synthesis of novel π-conjugated materials for optoelectronic applications.

1. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, M. E. Thompson, S. R. Forrest, Highly efficient phosphorescent emission from organic electroluminescent devices, Nature 395, p. 151-154, 1998.
2. A. Chaskar, H.-F. Chen, K.-T. Wong, Bipolar host materials: a chemical approach for highly efficient electrophosphorescent devices, Adv. Mater. 23, p. 3876-3895, 2011.
3. W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou, K.-T. Wong, A new benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue electrofluorescence, yellow-green electrophosphorescence, and two-color–based white OLEDs, J. Mater. Chem. 20, p. 10113-10119, 2010.
4. W.-Y. Hung, L.-C. Chi, W.-J. Chen, E. Mondal, S.-H. Chou, K.-T. Wong, Y. Chi, A carbazole-phenylbenzimidazole hybrid bipolar universal host for high efficiency RGB and white PhOLEDs with high chromatic-stability, J. Mater. Chem. 21, p. 19249-19256, 2011.