Highly efficient, orange–red organic light-emitting diodes using a series of green-emission iridium complexes as hosts
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.
References (38)
- et al.
Org. Electron.
(2006) - et al.
Org. Electron.
(2008) - et al.
Nature
(1998) - et al.
Adv. Mater.
(2003) - et al.
J. Mater. Chem.
(2004) - et al.
J. Am. Chem. Soc.
(2003) - et al.
Chem. Mater.
(2004) - et al.
J. Am. Chem. Soc.
(2001) - et al.
Chem. Commun.
(2002) - et al.
Adv. Mater.
(2002)
J. Am. Chem. Soc.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
J. Appl. Phys.
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2020, PolyhedronCitation Excerpt :First, the cyclometallating ligand is reacted with IrCl3·x H2O following the example of Nonoyama [52] or Sprouse [53] to form a chloro-bridged dimeric iridium compound, e.g. [(ppy)2IrCl]2 [53,54]. Next, the ancillary ligand is added by reacting the iridium dimer with acetylacetone in refluxing 2-ethoxyethanol and base (usually Na2CO3) [23,47,55,56]. In a variation, Baranoff [45] has used tetrabutylammonium hydroxide as base in refluxing dichloromethane.
“Iridium effect” in cyclometalated iridium complexes for p-type dye sensitized solar cells
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