Simple and High Efficiency Phosphorescence Organic Light-Emitting

Publication Date (Web): March 18, 2014 .... Synthesis of carboline-based host materials for forming copper(I) complexes as emitters: A promising strat...
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Simple and High Efficiency Phosphorescence Organic Light-Emitting Diodes with Codeposited Copper(I) Emitter Zhiwei Liu,†,§ Jacky Qiu,‡ Feng Wei,† Jianqiang Wang,∥ Xiaochen Liu,† Michael G. Helander,‡ Sarah Rodney,§ Zhibin Wang,*,‡ Zuqiang Bian,*,† Zhenghong Lu,‡ Mark E. Thompson,*,§ and Chunhui Huang† †

Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4 § Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States ∥ Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China S Supporting Information *

ABSTRACT: Phosphorescent copper(I) complexes show great promise as emitters in organic light-emitting diodes (OLEDs). However, most copper(I) complexes are neither soluble nor stable toward sublimation and, hence, not amenable to the typical methods to fabricate OLEDs. In this work, a compound 3-(carbazol-9-yl)-5-((3-carbazol-9-yl)phenyl)pyridine (CPPyC) was designed as both a good ligand and host matrix. Codeposition of CPPyC and copper iodide (CuI) gives luminescent films with photoluminescent quantum yields (PLQY) as high as 100%. A dimeric copper(I) complex Cu2I2(CPPyC)4 is formed in the thin film, characterized by X-ray absorption spectroscopy. A series of simple, highly efficient green-emitting OLEDs were demonstrated by using the codeposited film as an emissive layer. A device comprised of only CPPyC and CuI gave an external quantum efficiency (EQE) of 12.6% (42.3 cd/A) at 100 cd/m2, while a device with tailored hole and electron transporting layers gave an efficiency of 15.7% (51.6 cd/A) at the same brightness.

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INTRODUCTION Organic light emitting diodes (OLEDs) have been successfully commercialized in displays and are expected to be introduced for solid-state lighting in the near future.1−3 Phosphorescent OLEDs (PHOLEDs) have been considered as the ultimate technology, as they can give internal quantum efficiencies of 100%.3−6 However, most of the commercialized high performance PHOLEDs employ metal complex emitters incorporating quite expensive metals, such as iridium and platinum. Therefore, there have been significant efforts devoted to developing more cost-effective solutions to achieve high quantum efficiency. One approach is to explore metal-free organic molecules that emit from long-lived excited states (as observed for the Ir and Pt based emitters) via thermally activated delayed fluorescence (TADF). Devices with TADF based emitters can have internal quantum efficiencies to 100%.7 Phosphorescent copper(I) complexes are considered as another good alternative emitter for PHOLEDs, considering their low cost and potential for high performance.8−11 Although many copper(I) complexes are reported as efficient phosphorescent materials,12−17 most are neither soluble nor stable toward sublimation and hence not amenable to traditional methods typically used to fabricate OLEDs. Therefore, few devices containing copper based emitters have been reported,18−23 and those that show high efficiency only give high values at low luminance (e.g., ∼1 cd/m2). To solve these problems, we © 2014 American Chemical Society

proposed that in situ codeposition of CuI and a ligand might be a better solution.24 In this codeposition approach a copper source and a ligand are codeposited under vacuum to produce a copper(I) complex in the resulting thin film. In this way, solvent is not used, and the metal complex is not sublimed. The ligand material in this case serves a dual role as both a ligand for forming the emissive copper(I) complex and as a host matrix for the formed emitter. A challenge is to find an organic compound that can act in these two roles. The ligand must react with CuI quickly to form highly luminescent copper(I) complex and must have the optimal electronic properties, such as the triplet and frontier orbital energies, to make it an efficient host material for the emissive layer (EML) of the PHOLED. Carbazole containing materials have been used to make efficient EMLs in PHOLEDs, and we have used them here, substituted to make them amenable to formation of the emissive copper(I) complex. We report a novel ligand 3-(carbazol-9-yl)-5((3-carbazol-9-yl)phenyl)pyridine (CPPyC) with a pyridyl for copper coordination. CPPyC has a triplet energy level of 2.7 eV, promoting for efficient energy transfer to the emissive copper(I) complex formed on codeposition. Consequently, exceptionally high performance was demonstrated in photoluminescence (PL) Received: February 19, 2014 Revised: March 9, 2014 Published: March 18, 2014 2368

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Figure 1. (left) PL spectra (λex = 340 nm) of a neat CPPyC film and CuI:CPPyC films at room temperature, the CuI:CPPyC are given as molar ratios; (right top) photos of vacuum deposited CPPyC (left) and CuI:CPPyC (right) films under UV light (365 nm); and (right bottom) chemical structure of CPPyC. conditions. Briefly, the commercially patterned indium tin oxide (ITO) anode with a sheet resistance of 15 Ohms/square was ultrasonically cleaned with a standard regiment of Alconox, acetone, and methanol followed by ultraviolet (UV) ozone treatment prior to loading into a dedicated metallization chamber with a base pressure of ∼10−5 Pa. The MoO3 layer was thermal vapor deposited on and treated by ex situ oxidation with UV ozone for 30 min. The organic and LiF layers were deposited in a separate organic ultrahigh vacuum chamber with a base pressure of ∼10−6 Pa. The Al cathode lines (2 mm wide) were deposited orthogonally to the ITO anode lines (1 mm wide) in the metallization chamber with a base pressure of ∼10−7 Torr. The current−voltage and luminance measurements were measured using HP4140B picoammeter and Minolta LS-110 luminance meter, respectively. All measurements were carried out in ambient atmosphere and at room temperature.

and electroluminescence (EL) of the in situ prepared copper(I) complex (PL quantum yield ∼ 100% in codeposited thin films). The external quantum efficiency (EQE) of OLED prepared with this material was as high as ∼16%, at a luminance of 100 cd/m2.

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EXPERIMENTAL METHODS

Synthesis of 9-(5-Bromopyridin-3-yl)carbazole (CPyBr). To a round-bottom flask were added 3,5-dibromopyridine (7.11 g, 30 mmol), carbazole (5.01 g, 30 mmol), K2CO3 (4.14 g, 30 mmol), Cu (∼1 g), and nitrobenzene (50 mL). The mixture was stirred at 180 °C for 24 h. After being cooled to room temperature the mixture was treated with a dry silica gel column using hexane as the eluent to remove nitrobenzene and followed by hexane/dichloromethane (1:1 volume ratio) to obtain CPyBr. Yield: 35%. MS m/z: 323 (M + 1). 1H NMR (500 MHz, CDCl3): δ 8.85 (s, 1H), 8.79 (s, 1H), 8.13−8.15 (d, J = 8.0 Hz, 3H), 7.44−7.47 (t, J = 7.5 Hz, 2H), 7.38−7.40 (d, J = 8.0 Hz, 2H), 7.33−7.36 (d, J = 7.5 Hz, 2H). Synthesis of 3-(Carbazol-9-yl)-5-((3-carbazol-9-yl)phenyl)pyridine (CPPyC). 9-(3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)carbazole25 (1.94 g, 6.0 mmol), CPyBr (2.44 g, 6.6 mmol), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh 3)4) (119 mg, 0.1 mmol), 2 M aqueous potassium carbonate (40 mL), toluene (90 mL), and ethanol (40 mL) were mixed and refluxed for 24 h. After cooling to room temperature, the organic solvent was evaporated, and the residue was extracted with dichloromethane and purified by column chromatography on silica gel with hexane/dichloromethane as the eluent. The product was further purified by sublimation twice at 270 °C and 10−4 Pa to get 2.1 g of white powder. Yield: 72%. MS m/z: 486 (M + 1). 1H NMR (500 MHz, CDCl3): δ 9.02 (s, 1H), 8.93 (s, 1H), 8.22 (s, 1H), 8.15−8.17 (d, J = 9.5 Hz, 4H), 7.89 (s, 1H), 7.74−7.80 (m, 2H), 7.68−7.70 (m, 1H), 7.41−7.48 (m, 8H), 7.29−7.37 (m, 4H). Anal. Calcd. for C35H23N3: C, 86.57; H, 4.77; N, 8.65. Found: C, 86.60; H, 4.75; N, 8.69. Photophysical Measurements. Photoluminescence spectra were measured using a PTI Quanta Master model C-60SE spectrophotometer equipped with a 928 PMT detector. Phosphorescent lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube instrument equipped with a 331 nm LED excitation source. Quantum yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere, and model C10027 photonic multichannel analyzer. X-ray Absorption Fine-Structure Measurements. The X-ray absorption data at the Cu K-edge of codeposited CuI·CPPyC film was measured at room temperature in fluorescence mode with an SDD detector at beamline BL14W of the Shanghai Synchrotron Radiation Facility (SSRF), China. OLEDs Fabrication and Testing. All devices were fabricated in a Kurt J. Lesker LUMINOS cluster tool, in which a total of four different device structures can be fabricated on a single substrate to eliminate possible run-to-run variability caused by subtle variations in process

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RESULTS AND DISCUSSION Figure 1 shows the PL spectra of a series of CuI:CPPyC films made by codepositing CuI and CPPyC from two separate heating sources under high vacuum (i.e., 3 × 10−4 Pa). For comparison, the PL spectrum of a neat CPPyC film is also shown. The spectrum for the neat CPPyC film at room temperature shows fluorescence at 390 nm [biexponential decay, τ = 2.2 (25%) and 4.4 (75%) ns]. The PL spectra from CuI:CPPyC films are dominated by a band centered near 530 nm, which gives a luminescent lifetime in the microsecond regime (Table 1), suggesting the formation of a phosphorescent copper(I) Table 1. Photophysical Data for Neat CPPyC Film and Codeposited CuI:CPPyC Films CuI:CPPyC molar ratio

CuI conc. (wt %)

λem (nm)

PLQY (%)

lifetime (μs)a [percentage (%)]

0:1 1:1 1:3 1:5 1:7 1:10

0 28 12 7 5 4

386 512, 538 532 506, 535 530 532

45 11 71 74 92 ∼100

0.002 [25], 0.004 [75] 1.8 [26], 7.2 [74] 3.8 [36], 10.3 [64] 3.2 [29], 10.2 [71] 4.3 [34], 11.2 [66] 4.7 [29], 12.0 [71]

a Emission lifetimes were measured at 380 nm for CPPyC film and 530 nm for codeposited CuI:CPPyC films with excitation wavelength of 331 nm.

complex. The PLQY of the codeposited CuI:CPPyC film increases as the CPPyC concentration rises (i.e., CuI:CPPyC ratio decreases from 1:1 to 1:10), indicating an efficient energy transfer between CPPyC and the copper(I) complex formed in situ. As a result, a PLQY approaching 100% was achieved when 2369

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unoccupied molecular orbital (LUMO) energy levels for OLED construction. The compound exhibits irreversible oxidation and reduction waves, as do most carbazole derivatives due to the instability of their radical cations.27 The potentials for oxidation and reduction were observed to be 1.0 and −2.1 V with ferrocene as the internal standard, respectively. Using literatures procedures,28,29 the HOMO and LUMO energy levels of the compound were estimated from the electrochemical potentials to be 6.0 and 2.3 eV, respectively. On the basis of aforementioned data, PHOLEDs with a structure of ITO/MoO3 (1 nm)/CBP (35 nm)/CuI:CPPyC (18.5 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al were fabricated, where the EML is comprised of codeposited CuI:CPPyC. The combination of ITO/MoO3/CBP was used for hole injection, because the high work function of MoO3 enables direct injection of holes into CBP.30,31 Moreover, the compound CPPyC has a similar HOMO energy level to that of CBP, suggesting a very small energy barrier at the CBP/CuI:CPPyC interface. To investigate the dependence of the OLED performance parameters on the CuI:CPPyC ratio, five devices with CuI weight doping concentrations of 1 wt % (device 1), 2 wt % (device 2), 4 wt % (device 3), 5 wt % (device 4), and 10 wt % (device 5) were fabricated. Figure 3 and Table 2 show device structure and performance of the five OLEDs. All devices show green emission with a peak wavelength near 530 nm, indicating that emission originates entirely from the in situ formed copper(I) complex Cu2I2(CPPyC)4 in all cases. By comparing the five OLEDs, it is found that the device 3 with 4 wt % CuI shows the best performance, with EQE reaching 15.7% (51.6 cd/A) at 100 cd/m2 and 13.5% (44.4 cd/A) at 1,000 cd/m2. The power efficiency reaches 38.9 lm/W at 100 cd/m2 and 27.3 lm/W at 1000 cd/m2, which is comparable to OLEDs with iridium based phosphors.32−34 To further compare the five OLEDs, it was found that those devices have similar performance over a wide range of CuI doping levels (Figures 3B−D), i.e., the maximum EQEs were 12.7−15.7% at 100 cd/m2 with CuI doping concentration ranging from 1 to 10 wt %, respectively. Considering that CuI reacts with CPPyC to form the dimeric copper(I) complex Cu2I2(CPPyC)4, the luminophor doping concentrations are 6− 61 wt %. Even with a very high doping concentration of 61 wt %, the device 5 showed high EQE up to 12.7% at 100 cd/m2 and 11.1% at 1000 cd/m2. Similar device performance at different doping levels is advantageous for manufacturing, since an accurate doping level requires careful manufacturing control. It is found that the CuI:CPPyC emissive layer is quite stable and promising for OLED applications. We encapsulated the device 3 under nitrogen and monitored its brightness at a constant current density of 20 mA/cm2 (brightness = 5983 cd/m2). The half-life for the device under these conditions was around

the CuI:CPPyC molar ratio approaches 1:10 (i.e., CuI doping concentration of 4 wt %). To characterize the chemical structure of the in situ synthesized copper(I) complex, a codeposited CuI:CPPyC film (molar ratio = 1:5) was measured by X-ray absorption finestructure (XAFS) including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Investigation of the XANES pre-edge region confirmed the presence of only monovalent Cu in the codeposited film. While based on Fourier transform (FT, Figure 2) and

Figure 2. Non-phase-shift-corrected Fourier transform of the codeposited CuI:CPPyC film with a CuI:CPPyC molar ratio of 1:5. Inset: chemical structure of Cu2I2(CPPyC)4.

EXAFS fit parameters, the combination of one Cu−Cu (∼2.58 Å), two Cu−I (∼ 2.55 Å), and two Cu−N (∼ 2.00 Å) paths around the Cu(I) center gave the best fit to the data in the R range between ∼1 and ∼3.0 Å (the peak at ∼1.7 Å is mainly arise from the first coordination layer of Cu−N, while that at ∼2.4 Å arises from the second coordination layer of Cu−Cu and Cu−I, and the features in the data at R > 3.0 Å are signals from further coordination layer of copper). Thus, the emitting species in the codeposited CuI:CPPyC films is most likely the dimeric complex Cu2I2(CPPyC)4 (Figure 2, inset). Prior to constructing OLEDs with codeposited CuI:CPPyC EML, both the thermal and electrochemical properties of CPPyC were investigated. The thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA measurement reveals its high thermaldecomposition temperature (Td, corresponding to 5% weight loss) of 373 °C. The DSC trace exhibits distinct glass-transition temperature (Tg) of 97 °C, which is significantly higher than 62 °C of a classic host matrix N,N′-dicarbazolyl-4,4′-biphenyl (CBP).26 The electrochemical properties of CPPyC were investigated by cyclic voltammetry and used to deduce the highest occupied molecular orbital (HOMO) and lowest Table 2. Performance of OLEDs 1−8

device CuI conc. (wt %) complex conc. (wt %) luminance (cd/m2)a EQE (%)b PE (lm/W)b CE (cd/A)b a

1

2

3

4

5

6

7

8

1 6 16440 13.6 31.9 45.9

2 12 16990 14.2 33.1 47.2

4 23 23160 15.7 38.9 51.6

5 31 14200 13.6 31.3 44.6

10 61 14310 12.7 30.0 43.0

4 23 8835 14.8 28.8 49.6

4 23 13890 13.4 28.0 44.4

4 23 3550 12.6 21.1 42.3

Luminances for devices 1−5 were recorded at 10 V, while those for devices 6−8 were recorded at 12 V. bFor luminance at 100 cd/m2. 2370

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Figure 3. Structure and performance of OLEDs 1−5. (A) Schematic energy-level diagram of the simplified OLEDs: ITO/MoO3 (1 nm)/CBP (35 nm)/ CuI:CPPyC (x wt %, 18.5 nm)/TPBi (65 nm)/LiF (1 nm)/Al, where x is CuI mass doping concentrations and x = 1, 2, 4, 5, and 10 for device 1, 2, 3, 4, and 5, respectively. (B) Photo of a large area OLED (80 × 75 mm2, the device structure is same to that of 3) powered from the USB port of a laptop. (C) EQE as a function of luminance, and (D) current density as a function of applied voltage of OLEDs 1−5.

Figure 4. Schematic device structures and performances of devices 6−8 and 3. (A) Device structures of OLEDs 6−8 and 3. (B) External quantum efficiency (EQE) as a function of luminance, and (C) current density as a function of applied voltage of OLEDs 6−8 and 3.

12 h. While this is well below the lifetimes reported for optimized OLEDs, it is much longer than that for an analogous device with the CuI:CPPyC emissive layer replaced with a

layer of bis(2-phenylpyridine)(acetylacetonato)iridium(III) [Ir(ppy)2(acac)]:CBP. When this Ir(ppy)2(acac)-based device was driven under the same conditions, a half-life around 3 h was observed. 2371

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Chemistry of Materials Since the compound CPPyC has a HOMO energy level almost identical to that of CBP and similar LUMO energy level to that of TPBi, which are suitable for hole and electron injection from ITO/MoO3 anode and LiF/Al cathode, respectively, the aforementioned OLEDs were simplified by replacing TPBi by CPPyC (device 6), CBP by CPPyC (device 7), and both TPBi and CBP by CPPyC (device 8). Figure 4 compares the EQEluminance and current density−applied voltage curves of the devices 6−8 with the device 3. Both devices 6 and 7 have lower current density than that of the device 3, suggesting that the compound CPPyC has weaker charge injection/transportation ability to the CBP/TPBi combination. However, the simplest device 8 showed maximum EQE up to 16.8%, similar to the other three OLEDs, indicating a balanced hole−electron combination within the EML. It should be noted that the simplest device 8 lacks interface as compared to the other three OLEDs. As a result, the hole/electron combination area may be shifting as the current is increased, which leads to an increase in the efficiency roll-off as shown in Figure 4B.

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REFERENCES

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CONCLUSIONS In summary, a pyridine and carbazole containing compound CPPyC was designed as both a good ligand and host matrix. Codeposition of CPPyC and CuI gives luminescent films with PLQY as high as 100%. A dimeric copper(I) complex Cu2I2(CPPyC)4 is formed in the thin film, characterized by X-ray absorption spectroscopy. By using the codeposited film as an EML, a series of simple, highly efficient green-emitting OLEDs were demonstrated. The best OLED showed a maximum EQE as high as ∼15.7%, at a luminance of 100 cd/m2. We are extending the codeposition method to fabricate simple and high efficiency copper(I) complex OLED with emission colors other than green. This is realizable since the emission of the in situ synthesized copper(I) halide complex is arising from triplet metal to ligand charge transfer (3MLCT) and/or halide to ligand charge transfer (3XLCT) excited states,13,35 which could be well tuned by altering the electronic structure of the ligand.8,15,23,36 ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedure of CPPyC, photoluminescence spectra of neat CPPyC film at 298 and 77 K, transient lifetime curves of neat CPPyC film and codeposited CuI:CPPyC films, thermogravimetric analysis, differential scanning calorimetry, cyclic voltammogram, and density functional theory calculation for CPPyC, Cu K-edge and EXAFS least-squares fitting results for the codeposited CuI:CPPyC film, and additional device performance for OLEDs 1−8 including device stability, electroluminescent spectra, luminance−voltage, current efficiency− luminance, and power efficiency−luminance curves. This material is free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We greatly acknowledge the financial support by The National Basic Research Program of China (No. 2006CB601103), The National Natural Science Foundation of China (NNSFC, Nos. 20971006, 90922004, 21201011, 91127001), The Specialized Research Fund for the Doctoral Program of Higher Education (20120001120116), and the Universal Display Corporation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.B.W.). *E-mail: [email protected] (Z.Q.B.). *E-mail: [email protected] (M.E.T.). Notes

The authors declare the following competing financial interest(s): One of the authors (M.E.T.) has a financial interest in the Universal Display Corporation. 2372

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