Elsevier

Organic Electronics

Volume 10, Issue 2, April 2009, Pages 247-255
Organic Electronics

Highly efficient, orange–red organic light-emitting diodes using a series of green-emission iridium complexes as hosts

https://doi.org/10.1016/j.orgel.2008.11.013Get rights and content

Abstract

We investigated highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on an orange–red emission iridium complex as the guest and five green emission iridium complexes as the host material, respectively. For comparison, a device using a common fluorescent host CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) has also been fabricated. Results show that the steric hindrance and exciton transporting property of the iridium complex host are found to be critical to this kind of doping system, a proper steric hindrance and improved exciton transporting ability result in reducing of triplet–triplet annihilation, thus improving of the device performance. In addition, all devices using iridium complexes as host have better performance than that of CBP, which arised from the fact that those green emission iridium complexes have a lower triplet excited energy befitting for energy confinement and a higher highest occupied molecular orbital (HOMO) level for hole injection.

Introduction

Recently, electrophosphorescent materials such as iridium, platinum, ruthenium, and osmium complexes, which use both singlet and triplet excitons, have received a great deal of attention due to their potential application in flat-panel displays [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The heavy metals such as iridium in the complex forms are known to induce intersystem crossing by strong spin-orbit coupling, leading to mixing of the singlet and triplet excited states. Thus, iridium complexes were known to have high photoluminescence efficiency with a relatively short excited state lifetime, which minimizes annihilation of triplet emissive states. Up to now, high brightness and efficiency of iridium complexes based OLEDs were achieved by using a host:guest system to improve energy transfer and avoid triplet–triplet annihilation. Thus many host materials have been investigated, such as 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) [6], [14], [15], 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) [16], 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) [17], and 9-{4-[5-(4-tert-butylphenyl)-[1,3,4]oxadiazol-2-yl]-benzyl}-9H-carbazole (t-CmOxa) [18]. However, most employed hosts were fluorescent materials, while the host:guest based OLEDs using phosphorescent materials as hosts are rare but their performances seem to be amazing [19], [20].

In this study, we report a series of green-emission iridium complexes used as the host for an orange–red iridium complex guest for highly efficient OLEDs. In particular, we describe the electroluminescent (EL) properties of OLEDs based on five iridium complexes Ir(ppy)2(acac) [bis(2-phenylpyridinato-N,C2) iridium (acetylacetonate)] [21], Ir(ppy)2(dmd) [bis(2-phenylpyridinato-N,C2) iridium (5,5-dimethylhexane-2,4-diketonate)], Ir(ppy)2(tmd) [bis(2-phenylpyridinato-N,C2) iridium (2,2,6,6-tetramethylheptane-3,5-diketonate)] [6], Ir(ppy)2(CBDK) [bis(2-phenylpyridinato-N,C2) iridium (1-(9H-carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] [22], and Ir(ppy)2(FBDK) [bis(2-phenylpyridinato-N,C2) iridium (1-(9-methyl-fluoren-9-yl)-6,6-dimethylheptane-3,5-diketonate)] used as hosts for the guest Ir(DBQ)2(acac) [bis(dibenzo[f,h]quinoxalinato-N,C2) iridium (acetylacetonate)] [2], and compare these results with the device using CBP as the host. The choice of these OLEDs provided us an opportunity for tracing the effect of the host: considering Ir(ppy)2(acac) as a reference, we investigated the effect of structural change of the iridium complex on the performance of this class of host:dopant system, such as exciton transporting ability and molecular steric hindrance; while considering CBP as a reference, we traced the differences between fluorescent and phosphorescent hosts.

Section snippets

General information

1H NMR spectra were recorded on an ARX-400 NMR spectrometer, chemical shift data for each signal were reported in ppm units with tetramethylsilane (TMS) as internal reference, where δ (TMS) = 0. Elemental analyses were performed on a VARIO EL instrument. The UV–vis absorption spectra were measured with Shimadzu UV–3100 spectrometer. The photoluminescence (PL) spectra were recorded on an Edinburgh Analytical Instruments FLS920 spectrometer after removing the oxygen by bubbling of nitrogen through

X-ray crystal structure

Single crystals of Ir(ppy)2(dmd), Ir(ppy)2(tmd), and Ir(ppy)2(FBDK) were all grown from an ethanol/chloroform solution by slow evaporation at room temperature and characterized by X-ray diffraction analysis to establish their exact configuration [32]. Details of crystallographic data are given in Table 1.

The ORTEP diagram only for Ir(ppy)2(FBDK) is shown in Fig. 1 as an example. The Ir(ppy)2(LX) (LX = dmd, tmd, or FBDK) molecule consists of two phenylpyridine fragments as cyclometalated ligands

Conclusion

In summary, phosphorescent OLEDs using an orange–red emitting iridium complex Ir(DBQ)2(acac) as the guest and five green-emitting iridium complexes Ir(ppy)2(acac), Ir(ppy)2(dmd), Ir(ppy)2(tmd), Ir(ppy)2(CBDK), or Ir(ppy)2(FBDK) as the host, respectively, were demonstrated. Results show that these devices have better performance than that of the device based on CBP hosts. Moreover, the steric hindrance and exciton transporting property of the host are found to be the most important factors to

Acknowledgements

We thank the National Basic Research Program (2006CB601103), and the National Natural Science Foundation of China (20021101, 20423005, 50772003, 20671006) for financial support.

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