- Email: [email protected]
orange phosphorescent OLEDs based on heteroleptic Ir(Ⅲ) complexes with 2-(9,9-diethylfluorene-2-yl)pyridine main ligand and various ancillary ligands
Organic Electronics 54 (2018) 197–203
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient solution-processed yellow/orange phosphorescent OLEDs based on heteroleptic Ir(Ⅲ) complexes with 2-(9,9-diethylfluorene-2-yl)pyridine main ligand and various ancillary ligands
T
Xuejing Liua,b, Bing Yaoa,c, Hailong Wanga,c, Baohua Zhanga,∗, Xingdong Lina,e, Xiaofei Zhaoa, Yanxiang Chenga, Zhiyuan Xiea,∗∗, Wai-Yeung Wongb,d,∗∗∗ a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China Institute of Molecular Functional Materials and Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China c University of Chinese Academy of Sciences, Beijing, 100049, PR China d Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China e School of Science, Changchun University of Science and Technology, Changchun, 130022, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Solution-processed Iridium complex Yellow/orange emission Organic light-emitting diode Ancillary ligands
Efficient solution-processible yellow/orange emissive phosphors are highly required in solution-processed phosphorescent organic light-emitting diodes (s-PhOLEDs). With respect to phenyl pyridine (ppy)-type Ir(III) complexes, 2-(9,9-diethylfluoren-2-yl)pyridine (Flpy)-type Ir(III) complexes have huge potential in yellow/orange s-PhOLEDs due to their extended conjugation and excellent solubility in common organic solvents. Herein, a new series of phosphorescent Ir(III) complexes, i.e. Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac, based on Flpy main ligand and picolinic N-oxide (pic-NeO), acetylacetone (acac), and dibenzoyl methane (dbm) ancillary ligands are synthesized. The photophysical, electrochemical, and electroluminescent (EL) properties of these phosphorescent Ir(III) complexes are investigated in details. By introducing the CF3 group in Flpy ligand and varying the ancillary ligands, the emission spectra of these phosphorescent complexes are tuned in a large range with EL emission peaks from 544 nm to 588 nm. The s-PhOLEDs employing these phosphors show promising EL performance with maximum external quantum efficiency (EQE) from 15.4% to 23.7% and high power efficiency from 61.9 l m W−1 to 80.4 l m W−1. These highly efficient phosphorescent Ir (III) complexes may serve as ideal chromaticity components in solution-processed full-color displays and lighting devices.
1. Introduction Organic light-emitting diodes (OLEDs) possess huge potential application in the field of full-color flat panel displays and solid lighting sources [1–4]. With a great deal of efforts in the past decades, the internal quantum efficiency close to 100% have been achieved in phosphorescent organic light-emitting diodes (PhOLEDs) [5–12]. Compared to vaccum-deposited PhOLEDs, solution-processed PhOLEDs (s-PhOLEDs) hold good potential to fabricate large-area and flexible devices with low-cost high-throughput ink-jet printing method [13,14]. In view
of the urgent requirements on achieving high-efficiency s-PhOLEDs, the design of efficient solution-processible phosphorescent emitters is crucial to make s-PhOLEDs more competitive. Highly efficient yellow/orange phosphorescent emitters are of paramount importance in actual applications of s-PhOLEDs. This color component is not only indispensable for high color-quality RGBY-TV application, but also the key contribution in power-efficient blue/orange or blue/yellow complementary-color white s-PhOLEDs. However, there are only a few reports on such orange/yellow phosphors for highly efficient s-PhOLEDs up to now. A novel wet- and dry-process
∗ Corresponding author. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗ Corresponding author. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗∗ Corresponding author. Institute of Molecular Functional Materials and Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Xie), [email protected] (W.-Y. Wong).
https://doi.org/10.1016/j.orgel.2017.12.050 Received 15 December 2017; Received in revised form 28 December 2017; Accepted 28 December 2017 Available online 30 December 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.
Organic Electronics 54 (2018) 197–203
X. Liu et al.
capability from 544 to 588 nm in the orange/yellow color range. By employing the interfacial exciplex host structure, the s-PhOLEDs based on these heteroleptic Flpy ligand-based iridium phosphors show the satisfactory EL performance, e.g. peak EQE from 15.4% to 23.7% along with high PE from 61.9 to 80.4 l m W−1. It indicates that these highly efficient heteroleptic Flpy ligand-based iridium phosphors may serve as promising chromaticity components in solution-processed full-color displays and lighting devices.
feasible yellow iridium complex, i.e. bis[5-methyl-7-fluoro-5H-benzo(c) (1,5) naphthyridin-6-one]iridium(picolinate) (2-FBNO) was reported by Jou et al., with the resultant yellow s-PhOLEDs achieving an external quantum efficiency (EQE) of 18.9% and a power efficiency (PE) of 64.4 l m W−1 at a luminance of 100 cd m−2 [15]. Ye et al. reported orange s-PhOLEDs with a peak luminous efficiency (LE) of 41.7 cd A−1 and a PE of 12.5 l m W−1 by employing an orange emitter tris[2-(9,9dioctyl-9H-fluoren-2-yl)pyridinato-C3,N]-iridium(III) (Ir(FP)3) doped in a blend co-host of poly(N-vinylcarbazole) (PVK):1,3-bis[(p-tert-butyl) phenyl-1,3,4-oxadiazolyl]benzene (OXD-7) [16]. Yang et al. reported dendritic triphenylamine-based iridium(III) complexes and the resultant orange s-PhOLEDs achieved maximum LE, PE and EQE of 52.4 cd A−1, 21.6 l m W−1 and 21.0%, respectively [17]. Among the yellow/orange emissive phosphors reported [3,15–17], 2-(9,9-diethylfluoren-2-yl)pyridine (Flpy)-type Ir(III) complexes are a promising choice, since they can achieve high photoluminescent quantum yield, short exciton lifetime, high solubility in common solvents and good miscibility with common host for the fabrication of sPhOLEDs. Wong et al. has reported homoleptic/heteroleptic Flpy-based iridium complexes with yellow to orange emission for s-PhOLEDs by introducing diphenylamine/carbazole end-capping group to the main ligand Flpy [18–22]. Both highly efficient monochromatic and complementary-color white s-PhOLEDs were achieved with these solutionprocessible phosphors [21–25]. Especially, the s-PhOLEDs using homoleptic Ir(Flpy-CF3)3 as phosphorescent dopant and interfacial exciplex couple as host achieved an ultra-high EQE of 25.2%, and a PE of 97.2 l m W−1 with the EL emission peak at 570 nm and Commission Internationale de l’Eclairage (CIE) coordinates of (0.52, 0.47) [26]. Inspired by the superior EL performance of the homoleptic Ir(FlpyCF3)3, it is motivated to develop a series of new heteroleptic Flpy ligand-based iridium phosphors. It is expected to tune the emission spectra in a wide range while maintaining its efficient emission property and understand the key factors on determining the EL performance of heteroleptic Flpy-based iridium complexes. Herein, four iridium complexes Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac are synthesized as depicted in Scheme 1. These heteroleptic Flpy-based iridium complexes show a broad color-tuning
2. Experimental 2.1. General information Commercially available reagents were used without further purification unless otherwise tested. All reactions and manipulations were carried out under argon atmosphere with a standard Schlenk technique. The 1H and 13C NMR spectra were recorded with Bruker Advanced 400 MHz NMR spectrometer. Thermal gravimetric analysis (TGA) was carried out on Perkin-Elmer-TGA 7 thermal gravimetric analyzer under nitrogen flow at a heating rate of 10 °C min−1. MALDI-TOF-MS was measured using Bruker autoflexIII smart beam mass spectrometer. 2.2. Physical and electrochemical measurements UV–vis absorption and PL spectra were recorded on Perkin-Elmer Lambda 35 UV–vis spectrometer and Perkin-Elmer LS 50B spectrofluorometer, respectively. PL quantum yield (Φpl) of these Ir complexes in solution was determined by a relative method using fac-Ir(ppy)3 (Φp = .40 in toluene) as a reference. Cyclic voltammetry experiment was performed on an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat with a three-electrode system using a platinum electrode as a counter electrode, a carbon electrode as a working electrode, and an Ag/AgCl as a reference electrode, respectively. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate (nBu4NClO4). The ferrocene/ferrocenium (Fc/Fc+) couple was used as the internal standard. As for the energy level diagram shown in Fig. 3, the HOMO and LUMO energy levels of all used materials were
Scheme 1. Synthesis and chemical structures of Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2dbm, Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2pic-NeO.
198
Organic Electronics 54 (2018) 197–203
X. Liu et al.
spin-coated on the ITO substrate and was baked in oven at 120 °C for 45 min to form a hole injection layer. The emissive layers consisting of m-MTDATA doped with Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir (Flpy-CF3)2acac or Ir(Flpy-CF3)2dbm (1, 3, 5 or 7 wt.%) were spincoated from fresh chlorobenzene solution on the PEDOT:PSS layer. A structure of TmPyPB (65 nm)/LiF (1 nm)/Al (100 nm) was sequentially deposited under vacuum at a pressure less than 4 × 10−4 Pa. Keithley source measurement units along with a calibrated silicon photodiode were used to measure the current density (J)-luminance (L)-voltage (V) characteristics of the s-PhOLEDs. The EL spectra of the s-PhOLEDs were recorded with USB2000+ VIS-NIR spectrometer. All measurements were conducted at room temperature under ambient conditions. EQEs were calculated from the data of luminance, current density and EL spectrum of the devices by assuming a Lambertian distribution. 2.4. Synthesis of heteroleptic Ir complexes Synthesis of Flpy and Flpy-CF3 ligands is given in Electronic Supporting information (ESI). 2.4.1. Ir(Flpy-CF3)2dbm The cyclometalated Ir(III) μ-chloride bridged dimer was prepared by the modified method of Nonoyama [27]. Flpy-CF3 (0.97 g, 0.26 mmol) and IrCl3.nH2O (0.31 g, 0.11 mmol) were added into a mixture of 2ethoxyethanol and water (20 mL, 3:1 v/v) under an argon atmosphere. The reaction mixture was stirred at 80 °C overnight and a red precipitate was obtained after being cooled to room temperature. The precipitate was filtered and washed with water and methanol, and subsequently dried in vacuo to afford the Ir dimer. A solution of the dimer (0.55 g), dbm (0.32 g, 1.43 mmol) and Na2CO3 (0.30 g, 2.86 mmol) in 2-ethoxyethanol (15 mL) was stirred at reflux under an argon atmosphere for 16 h. After cooling to room temperature, the mixture was poured into water and extracted with ethyl acetate (EA). The organic phase was dried over MgSO4 and concentrated under vacuum. The residue was purified by silia gel chromatography with hexane/CH2Cl2 as an eluent and further purified by recrystallization using CH2Cl2/CH3OH mixture to afford the title complex as a red solid in 35% yield. 1H NMR (400 MHz, DMSO): δ 8.81 (s, 2H), 8.62 (d, J = 8.7 Hz, 2H), 8.41 (dd, J = 8.8, 1.7 Hz, 2H), 8.08 (s, 2H), 7.79 (d, J = 7.4 Hz, 4H), 7.47 (t, J = 7.4 Hz, 2H), 7.41–7.17 (m, 12H), 6.71 (s, 1H), 6.64 (s, 2H), 1.99 (dddd, J = 47.1, 26.9, 13.5, 7.1 Hz, 8H), 0.29 (t, J = 7.3 Hz, 6H), 0.08 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, DMSO): δ 179.35, 171.86, 171.74, 150.71, 147.36, 143.77, 143.06, 142.16, 139.93, 135.48, 130.92, 128.56, 127.71, 127.56, 126.72, 126.42, 123.04, 122.97, 120.92, 119.89, 119.77, 94.90, 54.87, 31.92, 31.48, 8.64, 8.51. MALDI-TOF-MS (m/z): 1148.3. Anal. Calcd. for C61H49F6IrN2O2: C, 63.81; H, 4.30; N, 2.44; Found: C, 63.61; H, 4.43; N, 2.53.
Fig. 1. (a) UV–vis absorption and (b) PL spectra of four heteroleptic iridium(III) complexes in toluene (10−5 M) at room temperature.
Fig. 2. Cyclic voltammograms of four heteroleptic iridium(III) complexes. (a) Oxidation behaviors in CH2Cl2 and (b) reduction behaviors in DMF under a nitrogen atmosphere.
2.4.2. Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2acac Both Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2acac were also prepared by similar procedure. Ir(Flpy-CF3)2pic-NeO: orange solid (30% yield). 1H NMR (400 MHz, DMSO): δ 8.84 (s, 1H), 8.64 (dd, J = 14.4, 8.7 Hz, 2H), 8.42 (t, J = 8.2 Hz, 2H), 8.15 (dd, J = 14.6, 6.5 Hz, 4H), 7.67 (d, J = 3.3 Hz, 2H), 7.61 (s, 1H), 7.31 (ddd, J = 35.3, 16.1, 9.3 Hz, 8H), 6.69 (s, 1H), 6.48 (s, 1H), 2.16–1.85 (m, 8H), 0.29 (d, J = 3.8 Hz, 6H), 0.07 (d, J = 5.0 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 173.09, 172.63, 171.63, 152.09, 151.26, 151.18, 149.67, 148.49, 148.22, 146.32, 146.28, 145.11, 144.97, 144.93, 144.58, 144.04, 143.62, 141.15, 141.03, 140.73, 140.33, 138.24, 134.13, 134.09, 128.66, 128.40, 128.04, 127.58, 126.72, 126.54, 123.23, 123.08, 123.03, 122.87, 120.43, 120.15, 119.93, 118.84, 118.30, 55.52, 55.40, 32.97, 32.90, 32.67, 32.59, 8.76, 8.67, 8.61. MALDI-TOF-MS (m/z): 1062.6. Anal. Calcd. for C52H42F6IrN3O3: C, 58.75; H, 3.98; N, 3.95; Found: C, 58.56; H, 4.12; N, 4.03.
determined according to the linear relationship between the CV and UPS results. m-MTDATA was used as a reference, whose HOMO level is 5.10 eV as determined by UPS. PL lifetimes of Ir complexes were obtained by single exponential fit of transient PL decay curve measured on a LKS80 system (Applied Photophysics, UK) in PL transient mode with excitation at 355 nm in argon-protected dilute sample solution (10−5 M). 2.3. Device fabrication and testing The s-PhOLEDs were fabricated with a structure of ITO/PEDOT:PSS (40 nm)/emissive layer (40 nm)/TmPyPB (65 nm)/LiF (1 nm)/Al (100 nm). The ITO substrates (10 Ω/square) were pre-cleaned with routine procedure and treated by UV-ozone for 30 min. PEDOT:PSS was 199
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Fig. 3. (a) Device configuration of the s-PhOLEDs. (b) Proposed energy level diagram and main EL operating mechanism. (c) The chemical structures of m-MTDATA and TmPyPB as well as the HOMO/LUMO levels of the four phosphors.
complexes is depicted in Scheme 1. The cyclometalated Ir(III) μchloride bridged dimers 1 and 2 were prepared using iridium trichloride hydrate with an excess of main ligands Flpy or Flpy-CF3 [28]. The dimer 1 reacted with pic-NeO to yield Ir(Flpy)2pic-NeO. Similarly, the dimer 2 reacted with different ancillary ligands such as acac, pic-NeO and dbm to yield the corresponding Ir(III) complexes, Ir (Flpy-CF3)2acac, Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm, respectively. All the Ir complexes were purified by column chromatography and/or recrystallization with appropriate methods. The chemical structure and properties of these Ir complexes were characterized by 1H NMR, 13C NMR, MALDI-TOF-MS, elemental analysis, thermal gravimetric analysis (TGA), cyclic voltammetry (CV), ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectroscopy. The thermal properties of these Ir complexes were evaluated using TGA under a nitrogen atmosphere. Their TGA curves are plotted in Fig. S1 and 5% weight loss temperatures (ΔT5%) are summarized in Table 1. The ΔT5% temperatures of the Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2acac, Ir (Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm are 375 °C, 355 °C, 327 °C and 332 °C, respectively. The FlpyeCF3etype Ir(III) complexes display relatively lower ΔT5% than Ir(III) complex based on the Flpy ligand. The Flpy-CF3 based Ir(III) complex with acac ancillary ligand exhibits a higher ΔT5% than the phosphors containing the pic-NeO and dbm ancillary ligands. The thermal stability characteristics of all these phosphors are sufficient for device applications of s-PhOLEDs.
2.4.3. Ir(Flpy-CF3)2acac Orange-red solid (34% yield) 1H NMR (400 MHz, DMSO): δ 8.75 (s, 2H), 8.61 (d, J = 8.7 Hz, 2H), 8.45 (d, J = 8.6 Hz, 2H), 8.01 (s, 2H), 7.43–7.14 (m, 8H), 6.48 (s, 2H), 5.44 (s, 1H), 2.15–1.85 (m, 8H), 1.82 (s, 6H), 0.33 (t, J = 7.3 Hz, 6H), 0.03–(-0.09) (m, 6H). 13C NMR (101 MHz, CDCl3): δ 184.38, 171.77, 149.96, 148.29, 144.20, 144.15, 143.56, 142.24, 140.95, 139.82, 132.45, 132.42, 125.47, 123.14, 121.52, 119.28, 118.85, 116.90, 99.57, 54.15, 31.99, 31.64, 28.81, 27.00, 7.55, 7.28. MALDI-TOF-MS (m/z): 1024.3. Anal. Calcd. for C51H45F6IrN2O2: C, 59.81; H, 4.43; N, 2.74; Found: C, 59.66; H, 4.51; N, 2.75. 2.4.4. Ir(Flpy)2pic-NeO Ir(Flpy)2pic-NeO (yellow solid 32% yield) was prepared using similar procedure as to Ir(Flpy-CF3)2pic-NeO except that Flpy was used instead of Flpy-CF3. 1H NMR (400 MHz, DMSO): δ 8.61 (d, J = 5.0 Hz, 1H), 8.38 (d, J = 8.2 Hz, 2H), 8.32 (d, J = 8.2 Hz, 2H), 8.18–7.86 (m, 6H), 7.66 (d, J = 5.3 Hz, 1H), 7.56 (ddd, J = 9.4, 7.1, 3.1 Hz, 2H), 7.43 (dd, J = 9.6, 3.6 Hz, 1H), 7.33 (dd, J = 10.6, 4.9 Hz, 2H), 7.29–7.12 (m, 6H), 6.61 (s, 1H), 6.49 (s, 1H), 2.14–1.83 (m, 8H), 0.27 (t, J = 7.3 Hz, 6H), 0.13 (td, J = 7.2, 4.1 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 173.09, 169.59, 167.99, 152.28, 151.03, 150.94, 149.19, 148.44, 148.18, 147.98, 146.06, 143.38, 143.19, 142.95, 142.91, 142.81, 141.48, 141.08, 137.40, 136.95, 136.86, 128.20, 127.97, 127.20, 126.74, 126.43, 126.24, 123.41, 123.15, 122.88, 122.73, 121.68, 121.37, 119.79, 119.65, 119.11, 118.89, 118.62, 118.49, 55.40, 55.29, 32.96, 32.94, 32.73, 32.69, 8.68. MALDI-TOF-MS (m/z): 926.6. Anal. Calcd. for C50H44F6IrN3O3: C, 64.77; H, 4.78; N, 4.53; Found: C, 64.71; H, 4.85; N, 4.54.
3.2. Photophysical properties Fig. 1 shows the UV–vis absorption and PL spectra of these Ir complexes in toluene solution at room temperature. As shown in Fig. 1a, all these Ir complexes show two sets of absorption bands, i.e. the weak absorption bands of 448–560 nm assigned to singlet metal-to-ligand charge transfer (1MLCT), triplet metal-to-ligand charge transfer (3MLCT) and 3π-π∗ transitions, and the strong absorption below 448 nm attributed to the spin-allowed 1π-π∗ transition of the Flpy-based ligand [29]. As indicated by the UV–vis absorption data, the Ir(Flpy-CF3)2pic-
3. Results and discussion 3.1. Synthesis and characterization The synthesis of these heteroleptic Flpy ligand-based iridium 200
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Table 1 Photophysical and thermal properties of four heteroleptic iridium(III) complexes. Complex
λabc(log ε) (nm)a
λem (nm)a
ΦPLb
Τ (μs)c
△T5% (oC)d
Ir(Flpy)2pic-NeO Ir(Flpy-CF3)2pic-NeO Ir(Flpy-CF3)2dbm Ir(Flpy-CF3)2acac
317(4.73), 333(4.73), 349(4.69), 385(4.08), 442 (3.86), 466 (3.86) 347 (4.64), 361 (4.64), 396 (4.16), 492(3.84) 356 (4.63), 408 (3.96), 523 (3.73) 336 (4.74), 357 (4.74), 399 (4.24), 519(3.82)
545 571 580 582
0.156 0.261 0.053 0.270
0.927 0.915 0.064 0.694
375 327 332 355
a b c d
Measured in toluene at a concentration of 10−5 M at 298 K. Excited at 380 nm in degassed toluene solution using fac-Ir(ppy)3 as a reference. Measured in degassed toluene solution at 298 K. ΔT5% is the 5% weight-reduction temperature.
electrochemically stable during anodic/cathodic scans to form cation/ anion species [33]. Based on the potentials of the oxidation and reduction couples and the calibrated HOMO level of the well-known mMTDATA by ultra-violet photoelectron spectroscopy (UPS) method (i.e. HOMO: −5.10 eV) [26], the HOMO and LUMO energy levels of these complexes are estimated. During the anodic scan in CH2Cl2, all the complexes show one reversible wave derived from the IrIV/IrIII oxidation in the range of 0.37–0.57 eV. Compared with the onset of anodic potential (Eox onset) of Ir(Flpy)2pic-NeO at ca. 0.37 eV, the Eox onset of the other three iridium complexes containing CF3 group are located in the more positive potential region. It is ascribed to the fact that the CF3 group can accept the electron from the metal center. Hence, it reduces the electron density on the IrIII center, which will definitely restrain its oxidation [34]. Cathodic sweeps in DMF reveal a reversible reduction wave from −1.86 to −2.23 eV for these iridium complexes. The HOMO energy levels of Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2acac, Ir (Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm are calculated to be −5.59 eV, −5.63 eV, −5.69 eV and −5.62 eV, and the LUMO energy levels are −2.99 eV, −3.34 eV, −3.36 eV and −3.32 eV, respectively. The Ir(Flpy)2pic-NeO shows a bandgap of 2.60 eV. With the introduction of CF3 group in the Flpy ligand, the bandgap of the resultant complex Ir(Flpy-CF3)2pic-NeO is lowered to about 2.43 eV. When the ancillary ligand of pic-NeO is replaced with acac or dbm, the bandgap of Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm is further lowered to 2.29 eV and 2.30 eV, respectively. The bandgap trend of these iridium complexes by introducing CF3 group and varying the ancillary ligand group is consistent with their measured PL spectra as shown Fig. 1b.
NeO absorption maxima of the MLCT bands show bathochromic effect compared with Ir(Flpy)2pic-NeO. Furthermore, the bathochromic effect becomes more distinct when the ancillary ligands are changed from pic-NeO to acac and dbm. The bathochromic shift of the 3MLCT absorption band from Ir(Flpy)2pic-NeO to Ir(Flpy-CF3)2pic-NeO is ca. 30 nm, while the shift from Ir(Flpy-CF3)2pic-NeO to Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm is ca. 36 nm. The absorption wavelength maxima of the MLCT bands of Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm are similar. These heteroleptic Flpy ligand-based iridium complexes emit intense phosphorescence from 545 nm to 582 nm in toluene as shown in Fig. 1b and Table 1, with an emission peak at 545 nm for Ir(Flpy)2picNeO, 571 nm for Ir(Flpy-CF3)2pic-NeO, 580 nm for Ir(Flpy-CF3)2dbm, and 582 nm for Ir(Flpy-CF3)2acac, respectively. Similar to their absorption behaviors, introduction of CF3 group into Ir(Flpy)2pic-NeO strongly affects its emission spectrum. Ir(Flpy-CF3)2pic-NeO shows an obvious red-shift in emission compared with the Ir(Flpy)2pic-NeO due to the electron-withdrawing nature of CF3 group in pyridine ring of cyclometalated ligands. The PL spectra of Ir(Flpy-CF3)2acac, Ir(FlpyCF3)2pic-NeO and Ir(Flpy-CF3)2dbm are broad and featureless, implying that the lowest triplet states of these complexes are likely dominated by the 3MLCT excited state. In contrast, the PL spectrum of Ir (Flpy)2pic-NeO displays vibronic progression, indicating that the complex emits phosphorescence predominantly stemming from the ligand-centered 3π-π* state [29,30]. The photoluminescence quantum yield (Φpl) of these complexes in toluene solution were measured with fac-Ir(ppy)3 as a standard (Φpl = 0.40). As presented in Table 1, the Φpl of Ir(Flpy)2pic-NeO, Ir (Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac are in a range of 0.053–0.270, with Ir(Flpy-CF3)2acac being the highest among them. The measured Φpl of the Ir(Flpy-CF3)2dbm complex is merely about 0.053. It is speculated to originate from the adverse triplet quenching from“Ir(Flpy-CF3)2” fragments with higher triplet level to dbm with the lower triplet level [31]. The increased Φpl of Ir(FlpyCF3)2pic-NeO and Ir(Flpy-CF3)2acac may be attributed to the higher rigidity of carbon-fluorine bond than that of carbon-hydrogen bond [32]. The measured PL lifetimes are 0.927 μs, 0.915 μs, 0.694 μs and 0.064 μs for Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm, respectively. The data are summarized in Table 1 and the transient phosphorescence decay spectra of these iridium complexes are presented in Fig. S2. The distinctly short lifetime of Ir(Flpy-CF3)2dbm may be attributed to the triplet quenching effect and thus results in low Φpl as mentioned above, which is similar to the previous report [31].
3.4. Electroluminescence performance The s-PhOLEDs were fabricated using these heteroleptic Ir(III) complexes as phosphorescent dopants. The corresponding device structure, energy level diagram and basic EL operation mechanism of the devices are shown in Fig. 3. The s-PhOLEDs have a structure of indium tin oxide (ITO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (40 nm)/4, 4′, 4”-tris[3-methylphenyl(phenyl) amino]triphenylamine (m-MTDATA):Ir complex (1, 3, 5, 7 wt.%) (40 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) (65 nm)/ LiF (1 nm)/Al(100 nm). Ir complex dopants include Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac, respectively. The detailed device fabrication procedures are included in the experimental section. As depicted in Fig. 3b, an interfacial exciplex couple of m-MTDATA and TmPyPB is used as the host to characterize the EL performance of these new solution-processible iridium phosphors in s-PhOLEDs. Such kind of device structure is advantageous since the superiority of barrier-free charge injection/transport process of interfacial exciplex host can render the resultant s-PhOLEDs simultaneously achieve ultra-low driving voltage and high EQE as well as high PE [26]. All the s-PhOLEDs employing different Ir complexes were optimized by tuning the doping concentration of Ir complex for achieving efficient energy transfer from the excitons formed at the interfacial exciplex to these phosphor dopants, and the key EL parameters of these devices are
3.3. Electrochemical characteristics of heteroleptic iridium complexes The frontier orbitals, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels for these Ir(III) complexes are evaluated with cyclic voltammetry measurements and the redox curves are shown in Fig. 2. All these Ir(III) complexes show a qusi-reversible redox process over the anodic and cathodic range, implying that these Ir(III) complexes are 201
Organic Electronics 54 (2018) 197–203
X. Liu et al.
EL spectra and CIE coordinate points of these four s-PhOLEDs are shown in Fig. 5. It is shown that the EL spectra are basically the same as their respective PL spectra and the CIE coordinates fully cover the wide ranges of yellow/orange display colors. It is noticed that the current densities of these devices are slightly lowered along with gradually increased driving voltages when the doping concentrations of the Ir complexes are increased from 1 wt.% to 7 wt.% as shown in Figs. S3-S6. According to the energy level diagrams shown in Fig. 3, such behaviors are possibly originated from the holescattering effect of the Ir phosphors at high doping concentration in sPhOLEDs under the EL operation [35–38]. Such effect adversely influences the driving voltage and the power efficiency of the devices. As shown in Figs. S3-S6 and Table S1, the optimized device EQE performance for these Ir complexes is typically achieved at low doping concentration range of 1 wt%−3 wt.%, which is advantageous to simultaneously achieve high PE and low driving voltage as shown in Table 2. The s-PhOLEDs using orange Ir(Flpy-CF3)2acac as the dopant show the best EL performance among these s-PhOLEDs based on different Ir complexes. More importantly, the light-emitting efficiencies of the sPhOLEDs based on Ir(Flpy-CF3)2acac show weak dependence on the Ir (Flpy-CF3)2acac concentration as shown in Table S1. With the Ir(FlpyCF3)2acac concentration increasing from 1 wt.% to 7 wt.%, the maximum EQE is slightly decreased from 23.7% to 22.0%. In contrast, the previously reported s-PhOLEDs with the same device structure but using homoleptic Ir(Flpy-CF3)3 as dopant displayed a sharp EQE decrease from 24.8% (1 wt.%) to 12.4% (4 wt.%) [26]. It implies that the heteroleptic configuration may be more advantageous over the homoleptic counterpart for the promising Flpy-CF3 ligand-based iridium complex on restraining the concentration quenching effect. It is important for practical application since high-performance s-PhOLEDs that is insensitive to doping concentration is easy to control and beneficial for mass production with high reproducibility.
Fig. 4. (a) Current efficiency-current density and power efficiency-current density curves and (b) EQE-current density characteristics of these s-PhOLEDs based on these four phosphor dopants.
4. Conclusions
summarized in Table S1. The optimized EL performance of these sPhOLEDs employing different Ir complexes is plotted in Fig. 4. Among the four heteroleptic Ir complexes, device D (Ir(Flpy-CF3)2acac, 3 wt. %) shows the best EL performance with the maximum LE/PE/EQE of 62.1 cd A−1/80.4 l m W−1/23.7%, respectively, along with the EL emission peak at 584 nm and CIE coordinates of (0.56, 0.43). Such EL performance is among the best results ever reported for yellow/orange color s-PhOLEDs. As for the optimized s-PhOLEDs employing other Ir phosphors, the sequence of overall EL performance is device B, i.e. Ir (Flpy-CF3)2pic-NeO, 1 wt.% (18.1%/55.3 cd A−1/71.9 l m W−1) > device A, i.e. Ir(Flpy)2pic-NeO, 3 wt. % (15.4%/55.4 cd A−1/ 61.9 l m W−1) > device C, i.e. Ir(Flpy-CF3)2dbm, 1 wt. % (10.3%/ 30.2 cd A−1/35.9 l m W−1). This sequence follows the order of ΦPL of these heteroleptic Ir complexes as shown in Table 1. The corresponding
Four heteroleptic Ir(III) complexes, i.e. Ir(Flpy)2pic-NeO, Ir(FlpyCF3)2acac, Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm, employing the Flpy main ligand without or with CF3 substitution and various ancillary ligands (pic-NeO, dbm, or acac), have been synthesized and characterized. The emissive color of these heteroleptic Flpy-based Ir complexes can be tuned from yellow to orange-red by introducing CF3 group and selecting different ancillary ligands. The s-PhOLEDs based on Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(FlpyCF3)2acac dopants show power efficiencies of 61.9, 71.9, 35.9 and 80.4 l m W−1, respectively, along with the corresponding CIE coordinates of (0.43, 0.54), (0.51, 0.48), (0.54, 0.44) and (0.56, 0.43). The heteroleptic Ir(Flpy-CF3)2acac exhibits red-shifted emission with a peak at 584 nm compared to the typical homoleptic Ir(Flpy-CF3)3 with an emission peak at 570 nm. The s-PhOLEDs based on Ir(Flpy-CF3)2acac Fig. 5. The EL spectra (left) and CIE coordinate points in the CIE1931 graph (right) of s-PhOLEDs based on these four phosphor dopants.
202
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Table 2 Summarized EL performance of the optimized s-PhOLEDs employing different iridium(III) complexes. Phosphor
Vona [V]
Lmaxb [cd m−2]
EQEb [%]
LEb [cd A−1]
PEb [lm W−1]
λmax. [nm]
CIE [x, y]
Ir(Flpy)2pic-NeO Ir(Flpy-CF3)2pic-NeO Ir(Flpy-CF3)2dbm Ir(Flpy-CF3)2acac
2.6/3.2 2.2/2.6 2.4/2.6 2.2/2.6
10930 9670 7324 23005
15.4 18.1 10.3 23.7
55.4 55.3 30.2 62.1
61.9 71.9 35.9 80.4
544 568 580 584
0.43,0.54 0.51,0.48 0.54,0.44 0.56,0.43
a b
at a luminance of 1 and 100 cd m−2, respectively. Maximum value.
give an EQE of 23.7%, close to 25.2% of the previously reported sPhOLEDs using homoleptic Ir(Flpy-CF3)3, along with a distinctly weak concentration quenching effect. These new Ir(III) complexes may find applications as ideal chromaticity components in solution-processed full-color displays and lighting devices.
[9] [10] [11] [12]
Acknowledgements
[13]
X.-J. Liu and B. Yao contributed equally to this work. B.-H. Zhang and Z.-Y. Xie acknowledge financial support from the National Key Basic Research and Development Program of China (973 program, Nos. 2015CB655001, 2014CB643504) founded by MOST and the National Natural Science Foundation of China (Nos. 51473162, 51773195, 51325303, 21334006). The financial support of the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200) is also acknowledged. W.-Y.W. acknowledges the financial support from Hong Kong Research Grants Council (HKBU 12304715), Areas of Excellence Scheme, University Grants Committee of HKSAR (AoE/P03/08), the Hong Kong Polytechnic University (1-ZE1C), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.
[14]
[23]
Appendix A. Supplementary data
[24]
[15]
[16] [17] [18] [19] [20] [21] [22]
[25] [26]
Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.orgel.2017.12.050.
[27] [28]
References
[29] [1] [2] [3] [4]
[5]
[6]
[7]
[8]
P.L. Burn, S.C. Lo, I.D.W. Samuel, Adv. Mater. 19 (2007) 1675. J.-H. Zhao, Y.-X. Hu, H.-Y. Lu, Y.-L. Lü, X. Li, Org. Electron. 41 (2017) 56. C. Fan, C. Yang, Chem. Soc. Rev. 43 (2014) 6439. X. Yang, G. Zhou, W.-Y. Wong, Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic lightemitting devices, Chem. Soc. Rev. 44 (2015) 8484–8575. B. Liu, F. Dang, Z. Feng, Z. Tian, J. Zhao, Y. Wu, X. Yang, G. Zhou, Z. Wu, W.Y. Wong, Novel iridium(iii) complexes bearing dimesitylboron groups with nearly 100% phosphorescent quantum yields for highly efficient organic light-emitting diodes, J. Mater. Chem. C 5 (2017) 7871–7883. H. Guo, J. Zhao, Z. Tian, Y. Wu, B. Liu, F. Dang, X. Yang, G. Zhou, Z. Wu, W.Y. Wong, Homoleptic thiazole-based IrIII phosphorescent complexes for achieving both high EL efficiencies and an optimized trade-off among the key parameters of solution-processed WOLEDs, J. Mater. Chem. C 5 (2017) 208–219. X. Yang, Z. Feng, J. Zhao, J.-S. Dang, B. Liu, K. Zhang, G. Zhou, Pyrimidine-based mononuclear and dinuclear iridium(III) complexes for high performance organic light-emitting diodes, ACS Appl. Mater. Interfaces 8 (2016) 33874–33887. X. Xu, H. Guo, J. Zhao, B. Liu, X. Yang, G. Zhou, Z. Wu, Asymmetric tris-heteroleptic IridiumIII complexes containing a 9-Phenyl-9-phosphafluorene oxide moiety with
[30] [31] [32] [33]
[34] [35] [36] [37] [38]
203
enhanced charge carrier injection/transporting properties for highly efficient solution-processed organic light-emitting diodes, Chem. Mater. 28 (2016) 8556–8569. R.N. Bera, N. Cumpstey, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 17 (2007) 1149. M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. M.A. Baldo, D.F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Funct. Mater. 16 (2006) 575. F. So, B. Krummacher, M.K. Mathai, D. Poplavskyy, S.A. Choulis, V.-E. Choong, J. Appl. Phys. 102 (2007) 091101. J.-H. Jou, Y.-X. Lin, S.-H. Peng, C.-J. Li, Y.-M. Yang, C.-L. Chin, J.-J. Shyue, S.S. Sun, M. Lee, C.-T. Chen, M.-C. Liu, C.-C. Chen, G.-Y. Chen, J.-H. Wu, C.-H. Li, C.F. Sung, M.-J. Lee, J.-P. Hu, Adv. Funct. Mater. 24 (2014) 555. T. Ye, S. Shao, J. Chen, L. Wang, D. Ma, ACS Appl. Mater. Interfaces 3 (2011) 410. M. Zhu, J. Zou, S. Hu, C. g. Li, C. Yang, H. Wu, J. Qin, Y. Cao, J. Mater. Chem. 22 (2012) 361. C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin, Adv. Funct. Mater. 18 (2008) 928. W.Y. Wong, G.J. Zhou, X.M. Yu, H.S. Kwok, B.Z. Tang, Adv. Funct. Mater. 16 (2006) 838. C.L. Ho, W.Y. Wong, G.J. Zhou, B. Yao, Z. Xie, L. Wang, Adv. Funct. Mater. 17 (2007) 2925. H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, 21 (2009) 4181. B. Zhang, G. Tan, C.S. Lam, B. Yao, C.L. Ho, L. Liu, Z. Xie, W.Y. Wong, J. Ding, L. Wang, Adv. Mater. 24 (2012) 1873. J. Zou, H. Wu, C.-S. Lam, C. Wang, J. Zhu, C. Zhong, S. Hu, C.-L. Ho, G.-J. Zhou, H. Wu, W.C.H. Choy, J. Peng, Y. Cao, W.-Y. Wong, Adv. Mater. 23 (2011) 2976. B. Zhang, L. Liu, G. Tan, B. Yao, C.-L. Ho, S. Wang, J. Ding, Z. Xie, W.-Y. Wong, L. Wang, J. Mater. Chem. C 1 (2013) 4933. L. Ying, C.L. Ho, H. Wu, Y. Cao, W.Y. Wong, Adv. Mater. 26 (2014) 2459. S. Wang, X. Wang, B. Yao, B. Zhang, J. Ding, Z. Xie, L. Wang, Sci. Rep. 5 (2015) 12487. M. Nonoyama, J. Organomet. Chem. 86 (1975) 263. X.-M. Yu, G.-J. Zhou, C.-S. Lam, W.-Y. Wong, X.-L. Zhu, J.-X. Sun, M. Wong, H.S. Kwok, J. Organomet. Chem. 693 (2008) 1518. A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971. G.-J. Zhou, Q. Wang, W.-Y. Wong, D. Ma, L. Wang, Z. Lin, J. Mater. Chem. 19 (2009) 1872. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. P. Tao, W.-L. Li, J. Zhang, S. Guo, Q. Zhao, H. Wang, B. Wei, S.-J. Liu, X.-H. Zhou, Q. Yu, B.-S. Xu, W. Huang, Adv. Funct. Mater. 26 (2016) 881. S.-J. Lee, J.-S. Park, M. Song, I.A. Shin, Y.-I. Kim, J.W. Lee, J.-W. Kang, Y.-S. Gal, S. Kang, J.Y. Lee, S.-H. Jung, H.-S. Kim, M.-Y. Chae, S.-H. Jin, Adv. Funct. Mater. 19 (2009) 2205. J. Zhao, Y. Yu, X.L. Yang, X.G. Yan, H.M. Zhang, X.B. Xu, G.J. Zhou, Z.X. Wu, Y.X. Ren, W.Y. Wong, ACS Appl. Mater. Interfaces 7 (2015) 24703. X. Liu, B. Yao, Z. Zhang, X. Zhao, B. Zhang, W.-Y. Wong, Y. Cheng, Z. Xie, J. Mater. Chem. C 4 (2016) 5787. K.K. Tsung, S.K. So, Appl. Phys. Lett. 92 (2008) 103315. Y.-Y. Noh, C.-L. Lee, J.-J. Kim, K. Yase, J. Chem. Phys. 118 (2003) 2853. L. Liu, B. Zhang, Z. Xie, J. Ding, L. Wang, Org. Electron. 14 (2013) 55.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient solution-processed yellow/orange phosphorescent OLEDs based on heteroleptic Ir(Ⅲ) complexes with 2-(9,9-diethylfluorene-2-yl)pyridine main ligand and various ancillary ligands
T
Xuejing Liua,b, Bing Yaoa,c, Hailong Wanga,c, Baohua Zhanga,∗, Xingdong Lina,e, Xiaofei Zhaoa, Yanxiang Chenga, Zhiyuan Xiea,∗∗, Wai-Yeung Wongb,d,∗∗∗ a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China Institute of Molecular Functional Materials and Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China c University of Chinese Academy of Sciences, Beijing, 100049, PR China d Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China e School of Science, Changchun University of Science and Technology, Changchun, 130022, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Solution-processed Iridium complex Yellow/orange emission Organic light-emitting diode Ancillary ligands
Efficient solution-processible yellow/orange emissive phosphors are highly required in solution-processed phosphorescent organic light-emitting diodes (s-PhOLEDs). With respect to phenyl pyridine (ppy)-type Ir(III) complexes, 2-(9,9-diethylfluoren-2-yl)pyridine (Flpy)-type Ir(III) complexes have huge potential in yellow/orange s-PhOLEDs due to their extended conjugation and excellent solubility in common organic solvents. Herein, a new series of phosphorescent Ir(III) complexes, i.e. Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac, based on Flpy main ligand and picolinic N-oxide (pic-NeO), acetylacetone (acac), and dibenzoyl methane (dbm) ancillary ligands are synthesized. The photophysical, electrochemical, and electroluminescent (EL) properties of these phosphorescent Ir(III) complexes are investigated in details. By introducing the CF3 group in Flpy ligand and varying the ancillary ligands, the emission spectra of these phosphorescent complexes are tuned in a large range with EL emission peaks from 544 nm to 588 nm. The s-PhOLEDs employing these phosphors show promising EL performance with maximum external quantum efficiency (EQE) from 15.4% to 23.7% and high power efficiency from 61.9 l m W−1 to 80.4 l m W−1. These highly efficient phosphorescent Ir (III) complexes may serve as ideal chromaticity components in solution-processed full-color displays and lighting devices.
1. Introduction Organic light-emitting diodes (OLEDs) possess huge potential application in the field of full-color flat panel displays and solid lighting sources [1–4]. With a great deal of efforts in the past decades, the internal quantum efficiency close to 100% have been achieved in phosphorescent organic light-emitting diodes (PhOLEDs) [5–12]. Compared to vaccum-deposited PhOLEDs, solution-processed PhOLEDs (s-PhOLEDs) hold good potential to fabricate large-area and flexible devices with low-cost high-throughput ink-jet printing method [13,14]. In view
of the urgent requirements on achieving high-efficiency s-PhOLEDs, the design of efficient solution-processible phosphorescent emitters is crucial to make s-PhOLEDs more competitive. Highly efficient yellow/orange phosphorescent emitters are of paramount importance in actual applications of s-PhOLEDs. This color component is not only indispensable for high color-quality RGBY-TV application, but also the key contribution in power-efficient blue/orange or blue/yellow complementary-color white s-PhOLEDs. However, there are only a few reports on such orange/yellow phosphors for highly efficient s-PhOLEDs up to now. A novel wet- and dry-process
∗ Corresponding author. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗ Corresponding author. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗∗ Corresponding author. Institute of Molecular Functional Materials and Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Xie), [email protected] (W.-Y. Wong).
https://doi.org/10.1016/j.orgel.2017.12.050 Received 15 December 2017; Received in revised form 28 December 2017; Accepted 28 December 2017 Available online 30 December 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.
Organic Electronics 54 (2018) 197–203
X. Liu et al.
capability from 544 to 588 nm in the orange/yellow color range. By employing the interfacial exciplex host structure, the s-PhOLEDs based on these heteroleptic Flpy ligand-based iridium phosphors show the satisfactory EL performance, e.g. peak EQE from 15.4% to 23.7% along with high PE from 61.9 to 80.4 l m W−1. It indicates that these highly efficient heteroleptic Flpy ligand-based iridium phosphors may serve as promising chromaticity components in solution-processed full-color displays and lighting devices.
feasible yellow iridium complex, i.e. bis[5-methyl-7-fluoro-5H-benzo(c) (1,5) naphthyridin-6-one]iridium(picolinate) (2-FBNO) was reported by Jou et al., with the resultant yellow s-PhOLEDs achieving an external quantum efficiency (EQE) of 18.9% and a power efficiency (PE) of 64.4 l m W−1 at a luminance of 100 cd m−2 [15]. Ye et al. reported orange s-PhOLEDs with a peak luminous efficiency (LE) of 41.7 cd A−1 and a PE of 12.5 l m W−1 by employing an orange emitter tris[2-(9,9dioctyl-9H-fluoren-2-yl)pyridinato-C3,N]-iridium(III) (Ir(FP)3) doped in a blend co-host of poly(N-vinylcarbazole) (PVK):1,3-bis[(p-tert-butyl) phenyl-1,3,4-oxadiazolyl]benzene (OXD-7) [16]. Yang et al. reported dendritic triphenylamine-based iridium(III) complexes and the resultant orange s-PhOLEDs achieved maximum LE, PE and EQE of 52.4 cd A−1, 21.6 l m W−1 and 21.0%, respectively [17]. Among the yellow/orange emissive phosphors reported [3,15–17], 2-(9,9-diethylfluoren-2-yl)pyridine (Flpy)-type Ir(III) complexes are a promising choice, since they can achieve high photoluminescent quantum yield, short exciton lifetime, high solubility in common solvents and good miscibility with common host for the fabrication of sPhOLEDs. Wong et al. has reported homoleptic/heteroleptic Flpy-based iridium complexes with yellow to orange emission for s-PhOLEDs by introducing diphenylamine/carbazole end-capping group to the main ligand Flpy [18–22]. Both highly efficient monochromatic and complementary-color white s-PhOLEDs were achieved with these solutionprocessible phosphors [21–25]. Especially, the s-PhOLEDs using homoleptic Ir(Flpy-CF3)3 as phosphorescent dopant and interfacial exciplex couple as host achieved an ultra-high EQE of 25.2%, and a PE of 97.2 l m W−1 with the EL emission peak at 570 nm and Commission Internationale de l’Eclairage (CIE) coordinates of (0.52, 0.47) [26]. Inspired by the superior EL performance of the homoleptic Ir(FlpyCF3)3, it is motivated to develop a series of new heteroleptic Flpy ligand-based iridium phosphors. It is expected to tune the emission spectra in a wide range while maintaining its efficient emission property and understand the key factors on determining the EL performance of heteroleptic Flpy-based iridium complexes. Herein, four iridium complexes Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac are synthesized as depicted in Scheme 1. These heteroleptic Flpy-based iridium complexes show a broad color-tuning
2. Experimental 2.1. General information Commercially available reagents were used without further purification unless otherwise tested. All reactions and manipulations were carried out under argon atmosphere with a standard Schlenk technique. The 1H and 13C NMR spectra were recorded with Bruker Advanced 400 MHz NMR spectrometer. Thermal gravimetric analysis (TGA) was carried out on Perkin-Elmer-TGA 7 thermal gravimetric analyzer under nitrogen flow at a heating rate of 10 °C min−1. MALDI-TOF-MS was measured using Bruker autoflexIII smart beam mass spectrometer. 2.2. Physical and electrochemical measurements UV–vis absorption and PL spectra were recorded on Perkin-Elmer Lambda 35 UV–vis spectrometer and Perkin-Elmer LS 50B spectrofluorometer, respectively. PL quantum yield (Φpl) of these Ir complexes in solution was determined by a relative method using fac-Ir(ppy)3 (Φp = .40 in toluene) as a reference. Cyclic voltammetry experiment was performed on an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat with a three-electrode system using a platinum electrode as a counter electrode, a carbon electrode as a working electrode, and an Ag/AgCl as a reference electrode, respectively. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate (nBu4NClO4). The ferrocene/ferrocenium (Fc/Fc+) couple was used as the internal standard. As for the energy level diagram shown in Fig. 3, the HOMO and LUMO energy levels of all used materials were
Scheme 1. Synthesis and chemical structures of Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2dbm, Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2pic-NeO.
198
Organic Electronics 54 (2018) 197–203
X. Liu et al.
spin-coated on the ITO substrate and was baked in oven at 120 °C for 45 min to form a hole injection layer. The emissive layers consisting of m-MTDATA doped with Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir (Flpy-CF3)2acac or Ir(Flpy-CF3)2dbm (1, 3, 5 or 7 wt.%) were spincoated from fresh chlorobenzene solution on the PEDOT:PSS layer. A structure of TmPyPB (65 nm)/LiF (1 nm)/Al (100 nm) was sequentially deposited under vacuum at a pressure less than 4 × 10−4 Pa. Keithley source measurement units along with a calibrated silicon photodiode were used to measure the current density (J)-luminance (L)-voltage (V) characteristics of the s-PhOLEDs. The EL spectra of the s-PhOLEDs were recorded with USB2000+ VIS-NIR spectrometer. All measurements were conducted at room temperature under ambient conditions. EQEs were calculated from the data of luminance, current density and EL spectrum of the devices by assuming a Lambertian distribution. 2.4. Synthesis of heteroleptic Ir complexes Synthesis of Flpy and Flpy-CF3 ligands is given in Electronic Supporting information (ESI). 2.4.1. Ir(Flpy-CF3)2dbm The cyclometalated Ir(III) μ-chloride bridged dimer was prepared by the modified method of Nonoyama [27]. Flpy-CF3 (0.97 g, 0.26 mmol) and IrCl3.nH2O (0.31 g, 0.11 mmol) were added into a mixture of 2ethoxyethanol and water (20 mL, 3:1 v/v) under an argon atmosphere. The reaction mixture was stirred at 80 °C overnight and a red precipitate was obtained after being cooled to room temperature. The precipitate was filtered and washed with water and methanol, and subsequently dried in vacuo to afford the Ir dimer. A solution of the dimer (0.55 g), dbm (0.32 g, 1.43 mmol) and Na2CO3 (0.30 g, 2.86 mmol) in 2-ethoxyethanol (15 mL) was stirred at reflux under an argon atmosphere for 16 h. After cooling to room temperature, the mixture was poured into water and extracted with ethyl acetate (EA). The organic phase was dried over MgSO4 and concentrated under vacuum. The residue was purified by silia gel chromatography with hexane/CH2Cl2 as an eluent and further purified by recrystallization using CH2Cl2/CH3OH mixture to afford the title complex as a red solid in 35% yield. 1H NMR (400 MHz, DMSO): δ 8.81 (s, 2H), 8.62 (d, J = 8.7 Hz, 2H), 8.41 (dd, J = 8.8, 1.7 Hz, 2H), 8.08 (s, 2H), 7.79 (d, J = 7.4 Hz, 4H), 7.47 (t, J = 7.4 Hz, 2H), 7.41–7.17 (m, 12H), 6.71 (s, 1H), 6.64 (s, 2H), 1.99 (dddd, J = 47.1, 26.9, 13.5, 7.1 Hz, 8H), 0.29 (t, J = 7.3 Hz, 6H), 0.08 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, DMSO): δ 179.35, 171.86, 171.74, 150.71, 147.36, 143.77, 143.06, 142.16, 139.93, 135.48, 130.92, 128.56, 127.71, 127.56, 126.72, 126.42, 123.04, 122.97, 120.92, 119.89, 119.77, 94.90, 54.87, 31.92, 31.48, 8.64, 8.51. MALDI-TOF-MS (m/z): 1148.3. Anal. Calcd. for C61H49F6IrN2O2: C, 63.81; H, 4.30; N, 2.44; Found: C, 63.61; H, 4.43; N, 2.53.
Fig. 1. (a) UV–vis absorption and (b) PL spectra of four heteroleptic iridium(III) complexes in toluene (10−5 M) at room temperature.
Fig. 2. Cyclic voltammograms of four heteroleptic iridium(III) complexes. (a) Oxidation behaviors in CH2Cl2 and (b) reduction behaviors in DMF under a nitrogen atmosphere.
2.4.2. Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2acac Both Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2acac were also prepared by similar procedure. Ir(Flpy-CF3)2pic-NeO: orange solid (30% yield). 1H NMR (400 MHz, DMSO): δ 8.84 (s, 1H), 8.64 (dd, J = 14.4, 8.7 Hz, 2H), 8.42 (t, J = 8.2 Hz, 2H), 8.15 (dd, J = 14.6, 6.5 Hz, 4H), 7.67 (d, J = 3.3 Hz, 2H), 7.61 (s, 1H), 7.31 (ddd, J = 35.3, 16.1, 9.3 Hz, 8H), 6.69 (s, 1H), 6.48 (s, 1H), 2.16–1.85 (m, 8H), 0.29 (d, J = 3.8 Hz, 6H), 0.07 (d, J = 5.0 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 173.09, 172.63, 171.63, 152.09, 151.26, 151.18, 149.67, 148.49, 148.22, 146.32, 146.28, 145.11, 144.97, 144.93, 144.58, 144.04, 143.62, 141.15, 141.03, 140.73, 140.33, 138.24, 134.13, 134.09, 128.66, 128.40, 128.04, 127.58, 126.72, 126.54, 123.23, 123.08, 123.03, 122.87, 120.43, 120.15, 119.93, 118.84, 118.30, 55.52, 55.40, 32.97, 32.90, 32.67, 32.59, 8.76, 8.67, 8.61. MALDI-TOF-MS (m/z): 1062.6. Anal. Calcd. for C52H42F6IrN3O3: C, 58.75; H, 3.98; N, 3.95; Found: C, 58.56; H, 4.12; N, 4.03.
determined according to the linear relationship between the CV and UPS results. m-MTDATA was used as a reference, whose HOMO level is 5.10 eV as determined by UPS. PL lifetimes of Ir complexes were obtained by single exponential fit of transient PL decay curve measured on a LKS80 system (Applied Photophysics, UK) in PL transient mode with excitation at 355 nm in argon-protected dilute sample solution (10−5 M). 2.3. Device fabrication and testing The s-PhOLEDs were fabricated with a structure of ITO/PEDOT:PSS (40 nm)/emissive layer (40 nm)/TmPyPB (65 nm)/LiF (1 nm)/Al (100 nm). The ITO substrates (10 Ω/square) were pre-cleaned with routine procedure and treated by UV-ozone for 30 min. PEDOT:PSS was 199
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Fig. 3. (a) Device configuration of the s-PhOLEDs. (b) Proposed energy level diagram and main EL operating mechanism. (c) The chemical structures of m-MTDATA and TmPyPB as well as the HOMO/LUMO levels of the four phosphors.
complexes is depicted in Scheme 1. The cyclometalated Ir(III) μchloride bridged dimers 1 and 2 were prepared using iridium trichloride hydrate with an excess of main ligands Flpy or Flpy-CF3 [28]. The dimer 1 reacted with pic-NeO to yield Ir(Flpy)2pic-NeO. Similarly, the dimer 2 reacted with different ancillary ligands such as acac, pic-NeO and dbm to yield the corresponding Ir(III) complexes, Ir (Flpy-CF3)2acac, Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm, respectively. All the Ir complexes were purified by column chromatography and/or recrystallization with appropriate methods. The chemical structure and properties of these Ir complexes were characterized by 1H NMR, 13C NMR, MALDI-TOF-MS, elemental analysis, thermal gravimetric analysis (TGA), cyclic voltammetry (CV), ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectroscopy. The thermal properties of these Ir complexes were evaluated using TGA under a nitrogen atmosphere. Their TGA curves are plotted in Fig. S1 and 5% weight loss temperatures (ΔT5%) are summarized in Table 1. The ΔT5% temperatures of the Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2acac, Ir (Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm are 375 °C, 355 °C, 327 °C and 332 °C, respectively. The FlpyeCF3etype Ir(III) complexes display relatively lower ΔT5% than Ir(III) complex based on the Flpy ligand. The Flpy-CF3 based Ir(III) complex with acac ancillary ligand exhibits a higher ΔT5% than the phosphors containing the pic-NeO and dbm ancillary ligands. The thermal stability characteristics of all these phosphors are sufficient for device applications of s-PhOLEDs.
2.4.3. Ir(Flpy-CF3)2acac Orange-red solid (34% yield) 1H NMR (400 MHz, DMSO): δ 8.75 (s, 2H), 8.61 (d, J = 8.7 Hz, 2H), 8.45 (d, J = 8.6 Hz, 2H), 8.01 (s, 2H), 7.43–7.14 (m, 8H), 6.48 (s, 2H), 5.44 (s, 1H), 2.15–1.85 (m, 8H), 1.82 (s, 6H), 0.33 (t, J = 7.3 Hz, 6H), 0.03–(-0.09) (m, 6H). 13C NMR (101 MHz, CDCl3): δ 184.38, 171.77, 149.96, 148.29, 144.20, 144.15, 143.56, 142.24, 140.95, 139.82, 132.45, 132.42, 125.47, 123.14, 121.52, 119.28, 118.85, 116.90, 99.57, 54.15, 31.99, 31.64, 28.81, 27.00, 7.55, 7.28. MALDI-TOF-MS (m/z): 1024.3. Anal. Calcd. for C51H45F6IrN2O2: C, 59.81; H, 4.43; N, 2.74; Found: C, 59.66; H, 4.51; N, 2.75. 2.4.4. Ir(Flpy)2pic-NeO Ir(Flpy)2pic-NeO (yellow solid 32% yield) was prepared using similar procedure as to Ir(Flpy-CF3)2pic-NeO except that Flpy was used instead of Flpy-CF3. 1H NMR (400 MHz, DMSO): δ 8.61 (d, J = 5.0 Hz, 1H), 8.38 (d, J = 8.2 Hz, 2H), 8.32 (d, J = 8.2 Hz, 2H), 8.18–7.86 (m, 6H), 7.66 (d, J = 5.3 Hz, 1H), 7.56 (ddd, J = 9.4, 7.1, 3.1 Hz, 2H), 7.43 (dd, J = 9.6, 3.6 Hz, 1H), 7.33 (dd, J = 10.6, 4.9 Hz, 2H), 7.29–7.12 (m, 6H), 6.61 (s, 1H), 6.49 (s, 1H), 2.14–1.83 (m, 8H), 0.27 (t, J = 7.3 Hz, 6H), 0.13 (td, J = 7.2, 4.1 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 173.09, 169.59, 167.99, 152.28, 151.03, 150.94, 149.19, 148.44, 148.18, 147.98, 146.06, 143.38, 143.19, 142.95, 142.91, 142.81, 141.48, 141.08, 137.40, 136.95, 136.86, 128.20, 127.97, 127.20, 126.74, 126.43, 126.24, 123.41, 123.15, 122.88, 122.73, 121.68, 121.37, 119.79, 119.65, 119.11, 118.89, 118.62, 118.49, 55.40, 55.29, 32.96, 32.94, 32.73, 32.69, 8.68. MALDI-TOF-MS (m/z): 926.6. Anal. Calcd. for C50H44F6IrN3O3: C, 64.77; H, 4.78; N, 4.53; Found: C, 64.71; H, 4.85; N, 4.54.
3.2. Photophysical properties Fig. 1 shows the UV–vis absorption and PL spectra of these Ir complexes in toluene solution at room temperature. As shown in Fig. 1a, all these Ir complexes show two sets of absorption bands, i.e. the weak absorption bands of 448–560 nm assigned to singlet metal-to-ligand charge transfer (1MLCT), triplet metal-to-ligand charge transfer (3MLCT) and 3π-π∗ transitions, and the strong absorption below 448 nm attributed to the spin-allowed 1π-π∗ transition of the Flpy-based ligand [29]. As indicated by the UV–vis absorption data, the Ir(Flpy-CF3)2pic-
3. Results and discussion 3.1. Synthesis and characterization The synthesis of these heteroleptic Flpy ligand-based iridium 200
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Table 1 Photophysical and thermal properties of four heteroleptic iridium(III) complexes. Complex
λabc(log ε) (nm)a
λem (nm)a
ΦPLb
Τ (μs)c
△T5% (oC)d
Ir(Flpy)2pic-NeO Ir(Flpy-CF3)2pic-NeO Ir(Flpy-CF3)2dbm Ir(Flpy-CF3)2acac
317(4.73), 333(4.73), 349(4.69), 385(4.08), 442 (3.86), 466 (3.86) 347 (4.64), 361 (4.64), 396 (4.16), 492(3.84) 356 (4.63), 408 (3.96), 523 (3.73) 336 (4.74), 357 (4.74), 399 (4.24), 519(3.82)
545 571 580 582
0.156 0.261 0.053 0.270
0.927 0.915 0.064 0.694
375 327 332 355
a b c d
Measured in toluene at a concentration of 10−5 M at 298 K. Excited at 380 nm in degassed toluene solution using fac-Ir(ppy)3 as a reference. Measured in degassed toluene solution at 298 K. ΔT5% is the 5% weight-reduction temperature.
electrochemically stable during anodic/cathodic scans to form cation/ anion species [33]. Based on the potentials of the oxidation and reduction couples and the calibrated HOMO level of the well-known mMTDATA by ultra-violet photoelectron spectroscopy (UPS) method (i.e. HOMO: −5.10 eV) [26], the HOMO and LUMO energy levels of these complexes are estimated. During the anodic scan in CH2Cl2, all the complexes show one reversible wave derived from the IrIV/IrIII oxidation in the range of 0.37–0.57 eV. Compared with the onset of anodic potential (Eox onset) of Ir(Flpy)2pic-NeO at ca. 0.37 eV, the Eox onset of the other three iridium complexes containing CF3 group are located in the more positive potential region. It is ascribed to the fact that the CF3 group can accept the electron from the metal center. Hence, it reduces the electron density on the IrIII center, which will definitely restrain its oxidation [34]. Cathodic sweeps in DMF reveal a reversible reduction wave from −1.86 to −2.23 eV for these iridium complexes. The HOMO energy levels of Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2acac, Ir (Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm are calculated to be −5.59 eV, −5.63 eV, −5.69 eV and −5.62 eV, and the LUMO energy levels are −2.99 eV, −3.34 eV, −3.36 eV and −3.32 eV, respectively. The Ir(Flpy)2pic-NeO shows a bandgap of 2.60 eV. With the introduction of CF3 group in the Flpy ligand, the bandgap of the resultant complex Ir(Flpy-CF3)2pic-NeO is lowered to about 2.43 eV. When the ancillary ligand of pic-NeO is replaced with acac or dbm, the bandgap of Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm is further lowered to 2.29 eV and 2.30 eV, respectively. The bandgap trend of these iridium complexes by introducing CF3 group and varying the ancillary ligand group is consistent with their measured PL spectra as shown Fig. 1b.
NeO absorption maxima of the MLCT bands show bathochromic effect compared with Ir(Flpy)2pic-NeO. Furthermore, the bathochromic effect becomes more distinct when the ancillary ligands are changed from pic-NeO to acac and dbm. The bathochromic shift of the 3MLCT absorption band from Ir(Flpy)2pic-NeO to Ir(Flpy-CF3)2pic-NeO is ca. 30 nm, while the shift from Ir(Flpy-CF3)2pic-NeO to Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm is ca. 36 nm. The absorption wavelength maxima of the MLCT bands of Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm are similar. These heteroleptic Flpy ligand-based iridium complexes emit intense phosphorescence from 545 nm to 582 nm in toluene as shown in Fig. 1b and Table 1, with an emission peak at 545 nm for Ir(Flpy)2picNeO, 571 nm for Ir(Flpy-CF3)2pic-NeO, 580 nm for Ir(Flpy-CF3)2dbm, and 582 nm for Ir(Flpy-CF3)2acac, respectively. Similar to their absorption behaviors, introduction of CF3 group into Ir(Flpy)2pic-NeO strongly affects its emission spectrum. Ir(Flpy-CF3)2pic-NeO shows an obvious red-shift in emission compared with the Ir(Flpy)2pic-NeO due to the electron-withdrawing nature of CF3 group in pyridine ring of cyclometalated ligands. The PL spectra of Ir(Flpy-CF3)2acac, Ir(FlpyCF3)2pic-NeO and Ir(Flpy-CF3)2dbm are broad and featureless, implying that the lowest triplet states of these complexes are likely dominated by the 3MLCT excited state. In contrast, the PL spectrum of Ir (Flpy)2pic-NeO displays vibronic progression, indicating that the complex emits phosphorescence predominantly stemming from the ligand-centered 3π-π* state [29,30]. The photoluminescence quantum yield (Φpl) of these complexes in toluene solution were measured with fac-Ir(ppy)3 as a standard (Φpl = 0.40). As presented in Table 1, the Φpl of Ir(Flpy)2pic-NeO, Ir (Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac are in a range of 0.053–0.270, with Ir(Flpy-CF3)2acac being the highest among them. The measured Φpl of the Ir(Flpy-CF3)2dbm complex is merely about 0.053. It is speculated to originate from the adverse triplet quenching from“Ir(Flpy-CF3)2” fragments with higher triplet level to dbm with the lower triplet level [31]. The increased Φpl of Ir(FlpyCF3)2pic-NeO and Ir(Flpy-CF3)2acac may be attributed to the higher rigidity of carbon-fluorine bond than that of carbon-hydrogen bond [32]. The measured PL lifetimes are 0.927 μs, 0.915 μs, 0.694 μs and 0.064 μs for Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2acac and Ir(Flpy-CF3)2dbm, respectively. The data are summarized in Table 1 and the transient phosphorescence decay spectra of these iridium complexes are presented in Fig. S2. The distinctly short lifetime of Ir(Flpy-CF3)2dbm may be attributed to the triplet quenching effect and thus results in low Φpl as mentioned above, which is similar to the previous report [31].
3.4. Electroluminescence performance The s-PhOLEDs were fabricated using these heteroleptic Ir(III) complexes as phosphorescent dopants. The corresponding device structure, energy level diagram and basic EL operation mechanism of the devices are shown in Fig. 3. The s-PhOLEDs have a structure of indium tin oxide (ITO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (40 nm)/4, 4′, 4”-tris[3-methylphenyl(phenyl) amino]triphenylamine (m-MTDATA):Ir complex (1, 3, 5, 7 wt.%) (40 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) (65 nm)/ LiF (1 nm)/Al(100 nm). Ir complex dopants include Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac, respectively. The detailed device fabrication procedures are included in the experimental section. As depicted in Fig. 3b, an interfacial exciplex couple of m-MTDATA and TmPyPB is used as the host to characterize the EL performance of these new solution-processible iridium phosphors in s-PhOLEDs. Such kind of device structure is advantageous since the superiority of barrier-free charge injection/transport process of interfacial exciplex host can render the resultant s-PhOLEDs simultaneously achieve ultra-low driving voltage and high EQE as well as high PE [26]. All the s-PhOLEDs employing different Ir complexes were optimized by tuning the doping concentration of Ir complex for achieving efficient energy transfer from the excitons formed at the interfacial exciplex to these phosphor dopants, and the key EL parameters of these devices are
3.3. Electrochemical characteristics of heteroleptic iridium complexes The frontier orbitals, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels for these Ir(III) complexes are evaluated with cyclic voltammetry measurements and the redox curves are shown in Fig. 2. All these Ir(III) complexes show a qusi-reversible redox process over the anodic and cathodic range, implying that these Ir(III) complexes are 201
Organic Electronics 54 (2018) 197–203
X. Liu et al.
EL spectra and CIE coordinate points of these four s-PhOLEDs are shown in Fig. 5. It is shown that the EL spectra are basically the same as their respective PL spectra and the CIE coordinates fully cover the wide ranges of yellow/orange display colors. It is noticed that the current densities of these devices are slightly lowered along with gradually increased driving voltages when the doping concentrations of the Ir complexes are increased from 1 wt.% to 7 wt.% as shown in Figs. S3-S6. According to the energy level diagrams shown in Fig. 3, such behaviors are possibly originated from the holescattering effect of the Ir phosphors at high doping concentration in sPhOLEDs under the EL operation [35–38]. Such effect adversely influences the driving voltage and the power efficiency of the devices. As shown in Figs. S3-S6 and Table S1, the optimized device EQE performance for these Ir complexes is typically achieved at low doping concentration range of 1 wt%−3 wt.%, which is advantageous to simultaneously achieve high PE and low driving voltage as shown in Table 2. The s-PhOLEDs using orange Ir(Flpy-CF3)2acac as the dopant show the best EL performance among these s-PhOLEDs based on different Ir complexes. More importantly, the light-emitting efficiencies of the sPhOLEDs based on Ir(Flpy-CF3)2acac show weak dependence on the Ir (Flpy-CF3)2acac concentration as shown in Table S1. With the Ir(FlpyCF3)2acac concentration increasing from 1 wt.% to 7 wt.%, the maximum EQE is slightly decreased from 23.7% to 22.0%. In contrast, the previously reported s-PhOLEDs with the same device structure but using homoleptic Ir(Flpy-CF3)3 as dopant displayed a sharp EQE decrease from 24.8% (1 wt.%) to 12.4% (4 wt.%) [26]. It implies that the heteroleptic configuration may be more advantageous over the homoleptic counterpart for the promising Flpy-CF3 ligand-based iridium complex on restraining the concentration quenching effect. It is important for practical application since high-performance s-PhOLEDs that is insensitive to doping concentration is easy to control and beneficial for mass production with high reproducibility.
Fig. 4. (a) Current efficiency-current density and power efficiency-current density curves and (b) EQE-current density characteristics of these s-PhOLEDs based on these four phosphor dopants.
4. Conclusions
summarized in Table S1. The optimized EL performance of these sPhOLEDs employing different Ir complexes is plotted in Fig. 4. Among the four heteroleptic Ir complexes, device D (Ir(Flpy-CF3)2acac, 3 wt. %) shows the best EL performance with the maximum LE/PE/EQE of 62.1 cd A−1/80.4 l m W−1/23.7%, respectively, along with the EL emission peak at 584 nm and CIE coordinates of (0.56, 0.43). Such EL performance is among the best results ever reported for yellow/orange color s-PhOLEDs. As for the optimized s-PhOLEDs employing other Ir phosphors, the sequence of overall EL performance is device B, i.e. Ir (Flpy-CF3)2pic-NeO, 1 wt.% (18.1%/55.3 cd A−1/71.9 l m W−1) > device A, i.e. Ir(Flpy)2pic-NeO, 3 wt. % (15.4%/55.4 cd A−1/ 61.9 l m W−1) > device C, i.e. Ir(Flpy-CF3)2dbm, 1 wt. % (10.3%/ 30.2 cd A−1/35.9 l m W−1). This sequence follows the order of ΦPL of these heteroleptic Ir complexes as shown in Table 1. The corresponding
Four heteroleptic Ir(III) complexes, i.e. Ir(Flpy)2pic-NeO, Ir(FlpyCF3)2acac, Ir(Flpy-CF3)2pic-NeO and Ir(Flpy-CF3)2dbm, employing the Flpy main ligand without or with CF3 substitution and various ancillary ligands (pic-NeO, dbm, or acac), have been synthesized and characterized. The emissive color of these heteroleptic Flpy-based Ir complexes can be tuned from yellow to orange-red by introducing CF3 group and selecting different ancillary ligands. The s-PhOLEDs based on Ir(Flpy)2pic-NeO, Ir(Flpy-CF3)2pic-NeO, Ir(Flpy-CF3)2dbm and Ir(FlpyCF3)2acac dopants show power efficiencies of 61.9, 71.9, 35.9 and 80.4 l m W−1, respectively, along with the corresponding CIE coordinates of (0.43, 0.54), (0.51, 0.48), (0.54, 0.44) and (0.56, 0.43). The heteroleptic Ir(Flpy-CF3)2acac exhibits red-shifted emission with a peak at 584 nm compared to the typical homoleptic Ir(Flpy-CF3)3 with an emission peak at 570 nm. The s-PhOLEDs based on Ir(Flpy-CF3)2acac Fig. 5. The EL spectra (left) and CIE coordinate points in the CIE1931 graph (right) of s-PhOLEDs based on these four phosphor dopants.
202
Organic Electronics 54 (2018) 197–203
X. Liu et al.
Table 2 Summarized EL performance of the optimized s-PhOLEDs employing different iridium(III) complexes. Phosphor
Vona [V]
Lmaxb [cd m−2]
EQEb [%]
LEb [cd A−1]
PEb [lm W−1]
λmax. [nm]
CIE [x, y]
Ir(Flpy)2pic-NeO Ir(Flpy-CF3)2pic-NeO Ir(Flpy-CF3)2dbm Ir(Flpy-CF3)2acac
2.6/3.2 2.2/2.6 2.4/2.6 2.2/2.6
10930 9670 7324 23005
15.4 18.1 10.3 23.7
55.4 55.3 30.2 62.1
61.9 71.9 35.9 80.4
544 568 580 584
0.43,0.54 0.51,0.48 0.54,0.44 0.56,0.43
a b
at a luminance of 1 and 100 cd m−2, respectively. Maximum value.
give an EQE of 23.7%, close to 25.2% of the previously reported sPhOLEDs using homoleptic Ir(Flpy-CF3)3, along with a distinctly weak concentration quenching effect. These new Ir(III) complexes may find applications as ideal chromaticity components in solution-processed full-color displays and lighting devices.
[9] [10] [11] [12]
Acknowledgements
[13]
X.-J. Liu and B. Yao contributed equally to this work. B.-H. Zhang and Z.-Y. Xie acknowledge financial support from the National Key Basic Research and Development Program of China (973 program, Nos. 2015CB655001, 2014CB643504) founded by MOST and the National Natural Science Foundation of China (Nos. 51473162, 51773195, 51325303, 21334006). The financial support of the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200) is also acknowledged. W.-Y.W. acknowledges the financial support from Hong Kong Research Grants Council (HKBU 12304715), Areas of Excellence Scheme, University Grants Committee of HKSAR (AoE/P03/08), the Hong Kong Polytechnic University (1-ZE1C), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.
[14]
[23]
Appendix A. Supplementary data
[24]
[15]
[16] [17] [18] [19] [20] [21] [22]
[25] [26]
Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.orgel.2017.12.050.
[27] [28]
References
[29] [1] [2] [3] [4]
[5]
[6]
[7]
[8]
P.L. Burn, S.C. Lo, I.D.W. Samuel, Adv. Mater. 19 (2007) 1675. J.-H. Zhao, Y.-X. Hu, H.-Y. Lu, Y.-L. Lü, X. Li, Org. Electron. 41 (2017) 56. C. Fan, C. Yang, Chem. Soc. Rev. 43 (2014) 6439. X. Yang, G. Zhou, W.-Y. Wong, Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic lightemitting devices, Chem. Soc. Rev. 44 (2015) 8484–8575. B. Liu, F. Dang, Z. Feng, Z. Tian, J. Zhao, Y. Wu, X. Yang, G. Zhou, Z. Wu, W.Y. Wong, Novel iridium(iii) complexes bearing dimesitylboron groups with nearly 100% phosphorescent quantum yields for highly efficient organic light-emitting diodes, J. Mater. Chem. C 5 (2017) 7871–7883. H. Guo, J. Zhao, Z. Tian, Y. Wu, B. Liu, F. Dang, X. Yang, G. Zhou, Z. Wu, W.Y. Wong, Homoleptic thiazole-based IrIII phosphorescent complexes for achieving both high EL efficiencies and an optimized trade-off among the key parameters of solution-processed WOLEDs, J. Mater. Chem. C 5 (2017) 208–219. X. Yang, Z. Feng, J. Zhao, J.-S. Dang, B. Liu, K. Zhang, G. Zhou, Pyrimidine-based mononuclear and dinuclear iridium(III) complexes for high performance organic light-emitting diodes, ACS Appl. Mater. Interfaces 8 (2016) 33874–33887. X. Xu, H. Guo, J. Zhao, B. Liu, X. Yang, G. Zhou, Z. Wu, Asymmetric tris-heteroleptic IridiumIII complexes containing a 9-Phenyl-9-phosphafluorene oxide moiety with
[30] [31] [32] [33]
[34] [35] [36] [37] [38]
203
enhanced charge carrier injection/transporting properties for highly efficient solution-processed organic light-emitting diodes, Chem. Mater. 28 (2016) 8556–8569. R.N. Bera, N. Cumpstey, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 17 (2007) 1149. M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. M.A. Baldo, D.F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Funct. Mater. 16 (2006) 575. F. So, B. Krummacher, M.K. Mathai, D. Poplavskyy, S.A. Choulis, V.-E. Choong, J. Appl. Phys. 102 (2007) 091101. J.-H. Jou, Y.-X. Lin, S.-H. Peng, C.-J. Li, Y.-M. Yang, C.-L. Chin, J.-J. Shyue, S.S. Sun, M. Lee, C.-T. Chen, M.-C. Liu, C.-C. Chen, G.-Y. Chen, J.-H. Wu, C.-H. Li, C.F. Sung, M.-J. Lee, J.-P. Hu, Adv. Funct. Mater. 24 (2014) 555. T. Ye, S. Shao, J. Chen, L. Wang, D. Ma, ACS Appl. Mater. Interfaces 3 (2011) 410. M. Zhu, J. Zou, S. Hu, C. g. Li, C. Yang, H. Wu, J. Qin, Y. Cao, J. Mater. Chem. 22 (2012) 361. C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin, Adv. Funct. Mater. 18 (2008) 928. W.Y. Wong, G.J. Zhou, X.M. Yu, H.S. Kwok, B.Z. Tang, Adv. Funct. Mater. 16 (2006) 838. C.L. Ho, W.Y. Wong, G.J. Zhou, B. Yao, Z. Xie, L. Wang, Adv. Funct. Mater. 17 (2007) 2925. H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, 21 (2009) 4181. B. Zhang, G. Tan, C.S. Lam, B. Yao, C.L. Ho, L. Liu, Z. Xie, W.Y. Wong, J. Ding, L. Wang, Adv. Mater. 24 (2012) 1873. J. Zou, H. Wu, C.-S. Lam, C. Wang, J. Zhu, C. Zhong, S. Hu, C.-L. Ho, G.-J. Zhou, H. Wu, W.C.H. Choy, J. Peng, Y. Cao, W.-Y. Wong, Adv. Mater. 23 (2011) 2976. B. Zhang, L. Liu, G. Tan, B. Yao, C.-L. Ho, S. Wang, J. Ding, Z. Xie, W.-Y. Wong, L. Wang, J. Mater. Chem. C 1 (2013) 4933. L. Ying, C.L. Ho, H. Wu, Y. Cao, W.Y. Wong, Adv. Mater. 26 (2014) 2459. S. Wang, X. Wang, B. Yao, B. Zhang, J. Ding, Z. Xie, L. Wang, Sci. Rep. 5 (2015) 12487. M. Nonoyama, J. Organomet. Chem. 86 (1975) 263. X.-M. Yu, G.-J. Zhou, C.-S. Lam, W.-Y. Wong, X.-L. Zhu, J.-X. Sun, M. Wong, H.S. Kwok, J. Organomet. Chem. 693 (2008) 1518. A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971. G.-J. Zhou, Q. Wang, W.-Y. Wong, D. Ma, L. Wang, Z. Lin, J. Mater. Chem. 19 (2009) 1872. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. P. Tao, W.-L. Li, J. Zhang, S. Guo, Q. Zhao, H. Wang, B. Wei, S.-J. Liu, X.-H. Zhou, Q. Yu, B.-S. Xu, W. Huang, Adv. Funct. Mater. 26 (2016) 881. S.-J. Lee, J.-S. Park, M. Song, I.A. Shin, Y.-I. Kim, J.W. Lee, J.-W. Kang, Y.-S. Gal, S. Kang, J.Y. Lee, S.-H. Jung, H.-S. Kim, M.-Y. Chae, S.-H. Jin, Adv. Funct. Mater. 19 (2009) 2205. J. Zhao, Y. Yu, X.L. Yang, X.G. Yan, H.M. Zhang, X.B. Xu, G.J. Zhou, Z.X. Wu, Y.X. Ren, W.Y. Wong, ACS Appl. Mater. Interfaces 7 (2015) 24703. X. Liu, B. Yao, Z. Zhang, X. Zhao, B. Zhang, W.-Y. Wong, Y. Cheng, Z. Xie, J. Mater. Chem. C 4 (2016) 5787. K.K. Tsung, S.K. So, Appl. Phys. Lett. 92 (2008) 103315. Y.-Y. Noh, C.-L. Lee, J.-J. Kim, K. Yase, J. Chem. Phys. 118 (2003) 2853. L. Liu, B. Zhang, Z. Xie, J. Ding, L. Wang, Org. Electron. 14 (2013) 55.