Highly Efficient Red Electrophosphorescent Devices Based on Iridium Isoquinoline Complexes: Remarkable External Quantum Efficiency Over a Wide Range of Current

Abstract

Organic light-emitting devices (OLEDs) hold great potential for application in flat-panel displays due to rapid progress in material design and device fabrication in recent years. [1±4] Red, green, and blue emissions of matching efficiencies are required for a full-color display. Greater success has been achieved in the development of a green emitter than that of red one in both fluorescent and phosphorescent displays. For example, devices based on green fluorescent dopant C545T [5] can reach a luminance efficiency > 10 cd A ±1 . In contrast, the best red OLEDs reported [6±10] can only reach~3 cd A ±1 and external quantum efficiencies less than 1 %. OLEDs based on phosphorescent materials [11] can significantly improve electroluminescence performance because both singlet and triplet excitons can be harvested for light emission. Theoretically, the internal quantum efficiency of phosphorescent emitters can approach 100 %. Unfortunately, most of phosphorescent emitters have a long lifetime (> 10 ls), which results in saturation of emissive sites at increasing currents. The first red phosphorescent material was reported for complexes of the type EuL 3 , where L denotes a ligand.[12] The long lifetime of the electronically excited europium(III) (350 ls) leads to dominant triplet±triplet (T±T) annihilation at high currents, and the maximum efficiency is only 1.4 %. Although platinum(II) octaethylporphyrin (PtOEP) doped into 4,4,N,N¢-dicarbazolebiphenyl (CBP) shows an external quantum efficiency g ext = 5.6 %, for a red phosphor, [4,13] the occurrence of T±T annihilation is inevitable at high currents (g ext = 0.5 % at 100 mA cm ±2 ) due to its relatively long phosphorescent lifetime (50±80 ls). Recently, a more efficient red phosphor has been reported, Ir(btp) 2 (acac) [14] (btp = 2-(2¢-benzo[4.5-a]thienyl)pyridinato; acac = acetylacetonate), that achieved an efficiency of g ext = 7.0 % at low current. The relatively short phosphorescent lifetime (4 ls) significantly improves the external efficiency at high currents (g ext = 2.5 % at 100 mA cm ±2 ). A short phosphorescence lifetime is crucial for the performance of a phosphorescent material, particularly its maximum brightness and efficiency at high currents. In this communication, we wish to report a series of efficient red phosphorescent emitters based on iridium 1-(phenyl)isoquinoline complexes which show strong electroluminescence (EL) brightness and efficiency, even at high currents. Scheme 1 outlines the synthetic protocol for red phosphorescent complexes: Ir(piq) 2 (acac), Ir(piq-F) 2 (acac), and Ir(piq-F) 3 , with piq and piq-F representing the 1-(phenyl)isoquinoline and 2-(4¢-fluorophenyl)isoquinoline ligands, respectively. The para-fluoro substituent of Ir(piq-F) 2 (acac) and Ir(piq-F) 3 is used to tune emission wavelength without loss of performance efficiency.[15] 1-(Phenyl)isoquinoline ligand was prepared from the reaction of isoquinoline with the corresponding phenyl magnesium bromide. The complexes Ir-(piq) 2 (acac) and Ir(piq-F) 2 (acac) were ...

Organic Semiconductor Electroluminescent Materials

“…Ligands therefore play an important role in color emission and quantum efficiency. Ir(piq) 3 [154] and (piq) 2 Ir(acac) [155] have been reported by two different research groups about the same time, respectively. The common feature of these two Ir complexes is the cyclometalate ligand 1-phenylisoquinoline (piq).…”

“…Liu's group [155] substituted a fluorine on 1-(phenyl)isoquinoline ligand; the obtained Ir(piq-F) 2 (acac) has a slightly blue-shifted emission with CIEx, y of (0.61, 0.32) although the EQE remains 8.67 % at a current density of 20 mA/cm 2 . Yang et al [160] developed Ir(m-piq) 2 (acac) by adding a methyl group on position 5 of 1-(phenyl)isoquioline, whose EL emission is around 623 nm.…”

Optical and electrochemical properties and the calculated structure of pentacoordinate aluminum 8-hydroxyquinoline

“…AlQ 3 is layered with hole transport materials between anode and cathode layers, and can be applied in flat panel or flexible displays by wide-range film deposition [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Although AlQ 3 has good electric and photophysical properties, its quantum yield (ca.…”

Recent advances in white organic light-emitting diodes employing a single-emissive material

“…Remarkably, this emitter exhibited efficient emission across all the tested concentrations, achieving a peak EQE of 26.3% for low-dopant concentrations of 2% and 24.1% for dopant concentrations as high as 18%. Furthermore, devices of 40 in a stable structure yielded moderate device operational lifetimes of 36.5 h to 50% of initial luminance at a high brightness of 2775 cd∕m 2 . This demonstrates that the tetradentate…”

“…Through rapid development in material design and novel device architectures, OLEDs emitting across the visible spectrum have been demonstrated. [2][3][4][5][6][7][8][9] In particular, the development of phosphorescent transition metal complexes has yielded impressive efficiencies due to their ability to harvest 100% of electrogenerated excitons, i.e., both singlet and triplet excitons. 10,11 In fact, devices employing both Ir and Pt complexes have essentially demonstrated 100% electron to photon conversion efficiencies.…”