Date Published: March 06, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Xiaolong Yang, Haoran Guo, Boao Liu, Jiang Zhao, Guijiang Zhou, Zhaoxin Wu, Wai‐Yeung Wong.
Organic light‐emitting diodes (OLEDs) are one of the most promising technologies for future displays and lighting. Compared with the blue and green OLEDs that have achieved very high efficiencies by using phosphorescent Ir(III) complexes, the red OLEDs still show relatively low efficiencies because of the lack of high‐performance red‐emitting Ir(III) complexes. Here, three highly efficient asymmetric red‐emitting Ir(III) complexes with two different cyclometalating ligands made by incorporating only one electron‐deficient triarylboron group into the nitrogen heterocyclic ring are reported. These complexes show enhanced photoluminescence quantum yields up to 0.96 and improved electron transporting capacity. In addition, the asymmetric structure can help to improve the solubility of Ir(III) complexes, which is crucial for fabricating OLEDs using the solution method. The photoluminescent and oxidation–reduction properties of these Ir(III) complexes are investigated both experimentally and theoretically. Most importantly, a solution‐processed red OLED achieves extremely high external quantum efficiency, current efficiency, and power efficiency with values of 28.5%, 54.4 cd A−1, and 50.1 lm W−1, respectively, with very low efficiency roll‐off. Additionally, the related device has a significantly extended operating lifetime compared with the reference device. These results demonstrate that the asymmetric diarylboron‐based Ir(III) complexes have great potential for fabricating high‐performance red OLEDs.
Organic light‐emitting diodes (OLEDs) have been regarded as the most promising candidate for both displays and solid‐state lighting sources.1, 2, 3 In the development of OLEDs, the most important issue is the research on emitting materials because they have crucial influence on the emission colors and efficiencies.4, 5 Efficient blue, green, orange, and red‐emitting materials are essential to develop high‐performance OLEDs applied to displays and solid‐state lighting sources. Among all the emitting materials developed for OLEDs, phosphorescent transition metal complexes, e.g., Ir(III) and Pt(II) complexes, have drawn great attention due to their capability of harvesting 100% of the both singlet and triplet excitons electrogenerated in devices.6, 7 Using Ir(III) complexes as emitters, blue, green, and orange OLEDs with external quantum efficiencies (EQEs) over 30% have been reported.8, 9, 10, 11, 12 However, even based on elaborately designed bipolar host materials and sophisticated device structures, only several most efficient vacuum‐deposited red OLEDs using Ir(III) complexes as emitters show EQEs around 26%.11, 13, 14, 15, 16 As for the solution‐processed red OLEDs based on Ir(III) complexes, the highest EQEs are less than 22%.17, 18, 19 Compared with the extremely high efficiencies of blue, green, and orange OLEDs, one of the reasons for the inferior efficiencies of these red OLEDs is the low photoluminescence quantum yields (PLQYs) of the emitter. For example, the PLQYs of the widely used red phosphors bis(2‐methyldibenzo‐[f,h]‐quinoxaline)Ir(III)(acetylacetonate) [(MDQ)2Ir(acac)] and bis(1‐phenylisoquinoline)Ir(III)(acetylacetonate)[(piq)2Ir(acac)] are only 0.48 and 0.2, respectively.20, 21 According to “the energy gap law,”22 as the energy gap between the emissive excited state and the ground state decreases, the nonradiative rate constant (knr) increases while the radiative rate constant (kr) decreases. Thus, the red emissive materials with narrow energy gaps tend to display low PLQYs.23 However, because the phosphorescence of Ir(III) complexes mainly result from the metal‐to‐ligand charge transfer (MLCT) and ligand‐centered (LC) transitions, the emission properties including the PLQY of an Ir(III) complex are decided by the organic ligands. It means that there is still a chance to improve the photoluminescence quantum yield (PLQY) of an Ir(III) complex by carefully designing the organic ligands.
The syntheses of these diarylboron‐based Ir(III) complexes were similar to the common methods reported for other heteroleptic Ir(III) complexes except the solvents used here were the mixture of THF and H2O rather than the conventional used mixture of 2‐ethoxyethanol and H2O (see the Supporting Information).30 Adopting the one‐pot method, one asymmetrical complex and two symmetric complexes were formed at the same time with acceptable yields. If the two cyclometalating ligands were heated with IrCl3 one after another, isolated yields of the asymmetric complex could be further improved. The NMR and mass spectral investigations confirmed the asymmetric structures (see Figures S1 and S2 in the Supporting Information) of these Ir(III) complexes. Their thermal properties were evaluated by the thermogravimetric analysis. The decomposition temperatures (Td) of these Ir(III) complexes were in the range from 225 to 267 °C (Table1 and Figure S3, Supporting Information), which are high enough for the device fabrication processes and practical usages.
In summary, we report three red‐emitting asymmetric Ir(III) complexes and demonstrate that the introduction of the diarylboron group into one of the cyclometalating ligands can also significantly improve the PLQYs and lower the LUMO levels of the resultant Ir(III) complexes, which are beneficial to the EL performance. Most importantly, the solution‐processed red OLED using the common unipolar host TCTA and the emitter BPyThIr can achieve extremely high peak EQE, CE, and PE of 28.5%, 54.4 cd A−1, and 50.1 lm W−1, respectively, with very low efficiency roll‐off even at high luminance of 1000 cd m−2. In addition, the lifetime value at 50% initial luminance for the device based on BPyThIr is about 2000 h. These results represent one of the best performances for Ir(III) complex‐based red OLEDs reported so far, indicating that BPyThIr is very promising for practical application in red OLEDs. This work shows the great potential to design diarylboron‐based Ir(III) complexes with an asymmetric molecular structure for highly efficient OLEDs.
The authors declare no conflict of interest.