Highly efficient single-layer organic light-emitting devices using cationic iridium complex as host
Graphical abstract
Highlights
► We report highly efficient single-layer OLEDs based on blended cationic Ir complexes. ► Use narrow band gap cationic Ir complex C1 as guest and wide band gap C2 as host. ► The resulted device exhibit highly enhanced efficiency of 25.7 cd/A. ► The high efficiency is nearly 3 folds of that of pure C1-based device.
Introduction
Organic light-emitting devices (OLEDs) based on phosphorescent Ir complex as guest have received great attention due to continuous advance toward practical applications both in solid-state lighting and color displays [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. For the purpose of low production cost and large area displays, single-layer device structure and solution-based technologies present distinct advantages [15], [16], [17]. Recently, tremendous effort has been made in the development of neutral Ir (III) complexes-based OLEDs, in which Ir (III) complexes have been primarily used as phosphorescent emitters dispersed in a fluorescent host material (such as poly(N-vinylcarbazole) (PVK)) to produce highly efficient organic electroluminescence [18], [19], [20], [21], [22], [23], [24], [25]. The presence of hydrophobic host is indispensable for these devices for transporting charge as well as minimizing phase aggregation of Ir complexes in solid state. However, the introduction of hydrophobic materials also makes the carrier transportation unbalanced, requiring multilayer structure, for example, hole-transporting (HT) layer, emissive layer (EML), and electron-transporting (ET) layer, thus the devices fabrication become complicated.
Compared with neutral Ir complex, ionic Ir complex shows the advantages of minimized carrier injection barrier for electron and hole as well as balanced carrier injection and transportation due to mobile ions contained in emissive layer which can form ohmic contact with each electrode under applied voltage [26]. However, it is still very scarce for ionic Ir complexes-based OLEDs because of absence of suitable host. The usually used polymer or small molecule hosts are hydrophobic and not effective for ionic Ir complexes because of poor compatibility between the hydrophobic hosts and hydrophilic ionic dopants. Thus, ionic Ir complexes are typically fabricated to host-free Light-emitting electrochemical cells (LECs), in which phase aggregation is moderately suppressed by interaction of the Ir complexes because of their intrinsic ionic nature. However, the efficiencies of ionic Ir complex based host-free devices are always lower than those of neutral Ir complexes based on polymer or small molecule host doping system because of moderate concentration quenching [27]. Qiu et al. reported solution-processed OLEDs based on ionic Ir complexes doped in PVK:OXD-7 (1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene) with maximum efficiencies of 5.2 cd/A [28]. The efficiencies of these devices can be further improved by introducing electron-transporting (ET) layer (such as 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI)) [29], [30]. However, the introduction of TPBI layer will confine the device area and increase the fabrication cost.
For further decreasing concentration quenching of ionic Ir complexes, the development of new host materials with good compatibility with the ionic Ir complex emitters is still a highly desirable and pursued task. Actually, ionic Ir complex itself can be acted as matrix to host narrower band gap ionic Ir emitters to further decrease concentration quenching of emitters [5] and the resulted system show better compatibility than that of hydrophobic hosts due to the same ionic nature. Furthermore, phosphorescent nature of ionic Ir complex host also helps to improve the device efficiency.
Recently, Su et al. reported self-doping ionic Ir complexes in LECs based on green-emitting [Ir(dFppy)2(SB)]+(PF6)− as the host and orange-emitting [Ir(ppy)2(SB)]+(PF6)− as the guest with high quantum efficiency of 10.4% [31]. Efficient white [32] and red-emitting [33] LECs were also achieved under this self-doping strategy by blending blue–green- and red-emitting ionic Ir complexes with CIE of (0.35, 0.39) with quantum efficiency of 3.3% and 3.62%, respectively. Recently, near-infrared LECs using ionic Ir complex as host and fluorescent ionic NIR emitting dyes as guest were also reported with quantum efficiency of 1.24% [34].
However, these devices exhibit much lower brightness (<100 cd/m2) and comparatively long turn-on time [35] (>30 min) which greatly obstruct its practical applications.
In this work, we demonstrate highly efficient single-layer, solution-processed OLEDs based on ionic Ir complexes host–guest system by using narrow-band gap [Ir(Meppy)2(pybm)](PF6) (C1) as guest and wide-band gap [Ir(dfppy)2(tzpy-cn)](PF6) (C2) as host. Our previous work [36] had shown that the incorporation of cyanogen group in the side chain of the ancillary ligand significantly improved the device efficiencies. The resulted devices exhibit high brightness and much enhanced EL efficiencies as compared with those of pure C1 or C2-based devices (three times enhancement to C1), giving a peak LE of up to 25.7 cd/A, which is among the highest values reported for ionic Ir complexes-based solid-state light-emitting devices [27], [37], [38], [39], [40], [41], [42], [43], [44].
Section snippets
Synthesis
The Ir dimer was synthesized using literature procedure [45] by reacting of IrCl3⋅3H2O and 2.5 equiv 2-(4-methylphenyl)pyridine in a mixture of 2-ethoxyethanol and water (3/1, v/v) at 110 °C overnight under argon. After being cooled to room temperature, the resulting precipitate was filtered off, then washed with water, methanol and ethyl ether, and finally dried to afford the desired product.
The ionic Ir complexes [Ir(Meppy)2(pybm)](PF6) (C1) and [Ir(dfppy)2(tzpy-cn)](PF6) (C2) were synthesized
Conclusion
Efficiencies enhanced OLEDs based on cationic Ir complexes as emitting layer have been achieved via using wide band gap cationic Ir complex C2 as host and narrow band gap cationic Ir complex C1 as guest. These host–guest system show highly enhanced efficiencies, with luminous efficiency of 25.7 cd/A, external quantum efficiency of 8.6% at C1 concentration of 1 wt.%. The high efficiencies are nearly 3-folds of those of single C1-based devices and 2-folds of those of multilayer host-free devices
General
All reactants and solvents purchased from commercial sources and used as received unless otherwise stated. All reactions were performed under an argon atmosphere. 1H NMR spectra of compounds were collected on a 300 MHz spectrometer at room temperature. Photo-physical characteristics of complexes were collected at room temperature in dichloromethane (DCM) solution of 10−5 M, which were carefully purged with nitrogen prior to measurements. UV–visible absorption spectra were recorded on a HP 8453
Acknowledgement
The authors acknowledge the financial support by the National Nature Science Foundation of China (Nos. U0634003, 21074038 and 10974223).
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