Article pubs.acs.org/cm
Near-Infrared Polymer Light-Emitting Diodes with High Efficiency and Low Efficiency Roll-off by Using Solution-Processed Iridium(III) Phosphors Xiaosong Cao,† Jingsheng Miao,‡ Minrong Zhu,† Cheng Zhong,† Chuluo Yang,*,† Hongbin Wu,*,‡ Jingui Qin,† and Yong Cao‡ †
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *
ABSTRACT: Two deep-red/near-infrared (NIR) iridium phosphors, (fldpqx)2Ir(acac) and (thdpqx)2Ir(acac), were rationally designed and synthesized considering the emission wavelength, emission efficiency, and solubility. In optimized solutionprocessed phosphorescent polymer light-emitting devices, the (fldpqx)2Ir(acac)-based device achieved a maximum external quantum efficiency of 5.7% with the emission peak at 690 nm, while the (thdpqx)2Ir(acac)-based device achieved a maximum external quantum efficiency of 3.4% with the emission peak at 702 nm, which are comparable to the highest values ever reported for solution-processable NIR emitters. Moreover, these devices merely experienced low efficiency roll-off at high current densities.
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up to 14.5% at ∼700 nm, which is the highest value for NIR devices to date.11d Pt−porphyrin complexes reported by Schanze et al. also exhibited intense electro-phosphorescence with high EQEs of up to 9.2% for OLEDs and 3.0% for PLEDs.11g However, without exception, all these Pt emitters unfolded via gradient efficiency roll-off at high current densities, which could be attributed to the triplet−triplet exciton annihilation aggravated by long excited-state lifetimes that commonly occurs in square-planar Pt(II) phosphors. In contrast, d6 octahedral iridium(III) complexes usually possess relatively short triplet lifetimes.10 Moreover, iridium complexes present good features of high quantum yields, tunable emission color, and splendid thermal and electrochemical stability.12 Recently, Qiu et al. reported an NIR iridium phosphor based on phenyl-benzoquinoline (pbq) derivatives with a short lifetime of 0.57 μs and a maximum EQE of 1.1%, which remained constant over a wide range of current density from 140 to 1000 mA cm−2.10b More recently, their group further developed two solution-processed cationic iridium emitters with negligible efficiency roll-off. Through replacement with an sp2-hybridized N atom opposite the chelating N atom in the phenylbenzoquinoline ligand, the emission peak extends to 855 nm.10e Although progress has been made, much work remains
INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs) and phosphorescent polymer light-emitting diodes (PhPLEDs) based on transition metal complexes as luminescent materials have drawn great interest because the strong spin−orbit coupling (SOC) and fast intersystem crossing could lead to a harvest of both singlet and triplet excitons in the emitting layer and achievement of unity internal quantum efficiency theoretically.1,2 Though tremendous investigations of devices that covered the whole visible part of the electromagnetic spectrum have been conducted,3 near-infrared (NIR) phosphorescent emitters have been rarely reported.4 For potential applications in night-visionreadable displays and sensors,5,6 achieving high efficiency is undoubtedly the foremost premise for NIR devices. However, this category of emitters suffers from intrinsic obstacles: the luminescence quantum yields tend to decrease with an increasing emission wavelength in accordance with the energy gap law;7 the extension of the conjugated system could easily result in close molecular packing and consequently lead to severe triplet−triplet annihilation (TTA).8 Until now, phosphorescent transition metal complexes using osmium, iridium, or platinum as the metal center have been applied to develop NIR OLEDs.9−11 Therefore, the most efficient NIR phosphorescent devices were based on Pt emitters.11 For example, in a device with a sophisticated configuration, Pt emitters based on N^C^N ligands realized an unusually high maximum external quantum efficiency (EQE) of © XXXX American Chemical Society
Received: September 11, 2014 Revised: December 3, 2014
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dx.doi.org/10.1021/cm503361j | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consists of a platinum working electrode with a 2 mm diameter, a platinum wire counter electrode, and a Ag/AgCl wire reference electrode. Cyclic voltammograms were obtained at a scan rate of 0.1 V s−1. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. At the end of each experiment, the ferrocene/ ferricenium (Fc/Fc+) couple was used as the internal standard. The highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) energy levels (electronvolts) of the two compounds are calculated according to the formula −[4.8 eV + Eox/red (vs EFc/Fc+)].16 The geometrical and electronic properties were determined with the Gaussian 09 program package.17 The calculation was optimized by means of the B3LYP (Becke three-parameter hybrid functional with Lee−Yang−Perdew correlation functionals) with the 631G(d) atomic basis set.18 Then the electronic structures were calculated at the τ-HCTHhyb/6-311++G(d,p) level of theory.19 Molecular orbitals were visualized using Gaussview. Device Fabrication and Measurement. Patterned indium tin oxide (ITO)-coated glass substrates with a sheet resistance of 15−20 Ω square−1 underwent a wet-cleaning course in an ultrasonic bath, beginning with acetone, followed by detergent, deionized water, and 2propanol. After oxygen plasma treatment, a 40 nm thick anode holeinjection layer of PEDOT:PSS (Heraeus Clevios P VP AI 4083) film was spin-cast on the ITO substrate and dried by being baked in a vacuum oven at 80 °C overnight. The 80 nm emitting layer was prepared by spincoating from a chlorobenzene solution on top of the PEDOT:PSS layer and then annealed at 120 °C for 20 min to remove the solvent residue. Finally, a cathode composed of a CsF (1.5 nm) and Al (100 nm) layer was evaporated with a shadow mask at a base pressure of 3 × 10−4 Pa. The thickness of the evaporated cathode was monitored by a quartz crystal thickness/ratio monitor (model STM-100/MF, Sycon). The overlapping area between the cathode and anode defined a pixel size of 19 mm2. Except for the spin coating of the PEDOT:PSS layer, all the fabrication processes were conducted inside a controlled atmosphere of a nitrogen drybox (Vacuum Atmosphere Co.) containing
