www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Color Tunable Organic Light-Emitting Devices with External Quantum Efficiency over 20% Based on Strongly Luminescent Gold(III) Complexes having Long-Lived Emissive Excited States Gang Cheng, Kaai Tung Chan, Wai-Pong To, and Chi-Ming Che* There is a surge of interest to develop new types of organic light-emitting devices (OLEDs)/polymer light-emitting devices (PLEDs) capable of being manufactured by low-cost technologies for large area display or lighting applications.[1] For high efficiency OLEDs/PLEDs, phosphorescent metal complexes are sought after as light-emitting dopant materials as they can harvest both singlet and triplet excitons via effectively enhancing spin-orbit coupling and promoting an efficient intersystem crossing from their singlet to triplet excited states.[2] Phosphorescent metal complexes are usually dispersed into proper host material(s) to minimize concentration quenching and/or triplettriplet annihilation, as both processes would severely reduce the device efficiency at high luminance.[2–4] In phosphorescent OLEDs/PLEDs, or organic light-emitting devices (OLEDs), voltage dependent electroluminescence (EL) is occasionally encountered due to the voltage dependent energy transfer from host to dopant and/or the shift of exciton recombination zone upon varying driving voltage. This kind of effect has previously been utilized to develop color tunable PLEDs/OLEDs[5–7] that can be used for decorative or color matching purpose.[7a,7d] Since the color span range is a critical parameter for judging the quality of a color tunable PLED/OLED, approaches have been reported to improve this parameter including i) an insulating polymer was used to mix with several substituted polythiophenes that were electroluminescent; this was to diminish the energy transfer from high-band-gap polymer to low-bandgap polymer, and, in the reported case the color of the resulting EL shifted from yellow at bias of 5 V to white at 20 V,[6b] and ii) a hole-blocking or electron-blocking layer was inserted between two adjacent emission layers, leading to shift of color Dr. G. Cheng, K. T. Chan, Dr. W.-P. To, Prof. C.-M. Che State Key Laboratory of Synthetic Chemistry Institute of Molecular Functional Materials and Department of Chemistry The University of Hong Kong Pokfulam Road, Hong Kong SAR, China E-mail: [email protected] Dr. G. Cheng State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University Changchun1 30012, China Dr. G. Cheng, Prof. C.-M. Che HKU Shenzhen Institute of Research and Innovation Shenzhen 518053, China
DOI: 10.1002/adma.201304263 2540
wileyonlinelibrary.com
from white to blue or from yellow to white upon increase in driving voltage.[5e] Nevertheless, the range of color span with the aforementioned approaches is not wide enough when compared to the case of from yellow to blue described in this work. Meanwhile, OLEDs fabricated with sophisticated structure such as the ones with sub-cells having different color independently controlled have been designed.[7] Although the EL color of such OLEDs can be varied over a wide range, sophisticated architecture and expensive technologies, such as fine mask alignment, are needed for the device fabrication. Thus, the realization of inexpensive and simple-structured solution-processable PLEDs with high efficiency and wide color span range would be attractive for color tunable lighting applications. For a good color tunable PLED/OLED, the high-energy emission should be quenched at low voltage while the low-energy one should be minimized at high voltage, and high device efficiency should be maintained at both high and low voltages. To design such color tunable PLED/OLED, the following strategies depicted in Scheme 1 are proposed; i) both high-energy and low-energy emitters should have high emission quantum yields to ensure the device to have high efficiency, ii) the energy transfer between these two emitters should be efficient at low voltage to quench the high energy emission whereas the energy transfer should be blocked at high voltage to minimize the low energy emission, and iii) the efficiency roll-off of the low-energy emitter could be strong at high voltage to further minimize the lowenergy emission component in the EL spectrum. To fulfill these requirements, an efficient low-energy phosphorescent emitter having a long-lived emissive excited state is desirable. Thus, at low voltage, the emissive excited state of the low-energy emitter is partly occupied, resulting in high EL efficiency and allowing for energy transfer from the high-energy emitter; whereas at high voltage, the long-lived emissive excited state is saturated, leading to rapid drop in EL efficiency and blocking of energy transfer from the high-energy emitter. The narrow color span range of the reported simple-structured color tunable PLEDs/ OLEDs[5,6] could be accounted for by the emission lifetimes of the low-energy emitters used in these devices being not long enough and these lifetimes are usually less than 10 µs. Compared to the widely studied phosphorescent emitters of Ir(III) and Pt(II) complexes,[3,4a–4d] luminescent Au(III) complexes are less developed.[8,9] Reports on OLEDs using luminescent Au(III) complexes with external quantum efficiencies (EQEs) of 11.5% (maximum value) and 7.8% (the value at 0.1 mA cm−2) respectively achieved in vacuum-deposited device fabricated with bis-cyclometalated alkynylgold(III) complexes[8c]
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
www.advmat.de www.MaterialsViews.com
and in solution-processed device fabricated with dendrimers containing alkynylgold(III) units[8d] have recently appeared in literature. Nevertheless, the emission quantum yields of the reported Au(III) complexes[8–11] are rarely over 15% in solutions at room temperature prior to our recent findings on strongly luminescent Au(III) complexes supported by C-deprotonated fluorene-C^N^C ligands; our recently reported Au(III) complexes display emission quantum yields of up to 58% in dichloromethane at room temperature.[12] The use of C-deprotonated fluorene-C^N^C ligand changes the lowest triplet excited state of the Au(III) complexes from ligand-to-ligand charge transfer
COMMUNICATION
Scheme 1. Schematic diagram of the strategies proposed for the design of color tunable PLEDs/OLEDs; blue and yellow curves represent highenergy and low-energy components of EL spectra, respectively, at both low and high voltages; blue and yellow circles respectively stand for high-energy and low-energy emitters; the filled yellow circle indicates the low-energy emitter with saturated excited state, whose efficiency is much lower than the one whose excited state is not saturated (open yellow circle). Energy transfer (ET) takes place between the two emitters at low voltage whereas be blocked at high voltage due to the saturation of the low-energy emitter.
(LLCT) state to ligand centered 3ππ* one. Such a subtle structural modification gives rise to strongly luminescent Au(III) complexes with excited state lifetimes over 200 µs in solution at room temperature besides their high emission quantum yields.[12] The high emission quantum yields along with long emission lifetimes render these newly developed luminescent Au(III) complexes to be good yellow emitter for the design of color tunable PLEDs/OLEDs according to the aforementioned strategies (Scheme 1). Yam and co-workers previously reported a cyclometalated Au(III)-alkynyl complex that gave a color tunable OLED, the mechanism of which was attributed to the shift of exciton recombination zone.[5d] In this work, we report a series of strongly luminescent Au(III) complexes fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph (Scheme 2) with emission quantum yields of 0.44–0.61 and emission lifetimes of 179–437µs in CH2Cl2 (Table 1). PLEDs based on fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, and fbb_Au_C2Ph at different concentrations were fabricated and characterized. A maximum EQE of 9.18% has been achieved for the PLED fabricated with fPh_Au_C2fb having the highest emission quantum yield of 0.61. Together with a blue emitting phosphorescent Ir(III) complex bis[(4,6-difluorophenyl)pyridinato-N,C2]-(picolinato) iridium (FIrpic),[13] a color tunable PLED has been realized with a maximum EQE of 13.16%. The color of this device can be tuned from yellow with Commission Internationale De L´Eclairage (CIE) coordinates of (0.40, 0.50) at 5 V to blue with CIE coordinates of (0.20, 0.35) at 10 V. Via vacuum deposition, both yellow-emitting and color tunable OLEDs respectively with much higher maximum EQEs of 20.31% and 22.02% have been fabricated by using fBu_Au_CN as yellow light-emitting dopant material thereby highlighting the feasibility of having high efficiency Au(III)-OLEDs/PLEDs for practical applications. As depicted in Figure 1a, the UV-visible absorption spectra of the complexes, fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr,
Scheme 2. Chemical structures of the Au(III) complexes used in this work.
Adv. Mater. 2014, 26, 2540–2546
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2541
www.advmat.de
COMMUNICATION
www.MaterialsViews.com Table 1. Photophysical data of the Au(III) complexes studied in this work. UV/Vis absorptiona) λabs[nm] (ε [dm3 mol–1cm–1])
Complex
fPh_Au_tip
Emission
276 (54200), 326 (31700), 348 (23800), 411 (8400), 431 (8100)
HOMOd) [eV]
LUMOd) [eV]
Solutiona) λmax [nm] (τ [µs])
Quantum yieldb)
Kqc) [dm3mol–1 s–1]
539, 578 (179)
0.46
8.48 × 106
–5.49
–2.97
7
fPh_Au_C2fb
282 (48000), 300 (51400), 311 (46200), 322 (48400), 349 (23500), 388 (7800), 409 (7600), 432 (6800)
540, 579 (217)
0.61
1.58 × 10
–5.58
–2.98
fBu_Au_PhAr
277 (53900), 321 (30500), 348 (24500), 402 (9800), 421 (9900)
534, 573 (300)
0.55
4.84 × 106
–5.82
–2.81
107
–5.94
–3.10
–5.72
–2.88
fBu_Au_CN
269 (38000), 301 (29100), 314 (29100), 327 (28200), 346 (23000), 420 (8300), 440 (8800)
548, 585 (437)
0.44
1.32 ×
fbb_Au_C2Ph
280 (37400), 318 (48100), 337 (30500), 352 (31200), 400 (6500), 423 (12000), 446 (12900)
540, 579 (250)
0.45
2.72 × 106
a) Determined in degassed CH2Cl2 (2 × 10−5mol dm−3). b) Emission quantum yield was measured in a degassed dichloromethane solution (2 × 10−5mol dm−3) by the optical dilute method with [Ru(bpy)3][PF6]2 in degassed acetonitrile as the standard (Φr = 0.062). c) Self-quenching rate constant. d) The highest occupied molecular orbital (HOMO) and LUMO levels are estimated from onset potentials using Cp2Fe0/+ value of 4.8 eV below the vacuum level.
Molar Absorptivity (mol-1 dm3 cm-1)
fBu_Au_CN, and fbb_Au_C2Ph in CH2Cl2 at 298 K show similar intense absorption bands at 250–350 nm (ε = 2 – 5 × 104 dm3 mol−1 cm−1) and moderately intense bands at 60000
(a) fPh_Au_tip fPh_Au_C2fb fBu_Au_PhAr fBu_Au_CN fbb_Au_C2Ph
50000 40000 30000 20000 10000 0 250
300
350
400
450
500
Wavelength (nm) Normalized Emission Intensity
(b)
450
500
fPh_Au_tip fPh_Au_C2fb fBu_Au_PhAr fbb_Au_C2Ph fBu_Au_CN
550
600
650
700
750
Wavelength (nm) Figure 1. (a) UV-vis absorption spectra of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph in dichloromethane. (b) Emission spectra of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_ Au_CN, and fbb_Au_C2Ph in degassed dichloromethane.
2542
wileyonlinelibrary.com
370–440 nm (ε = 1 – 2 × 104 dm3 mol−1 cm−1). These absorption bands are assigned to metal-perturbed intraligand transitions localized on the C^N^C ligand. The low-energy absorption at 400–440 nm has substantial LLCT character as revealed by our previous DFT calculations.[12] As depicted in Figures 1a and S1a, the lowest energy absorption bands of the arylgold complex, fBu_Au_PhAr, in CH2Cl2 reveal a blue-shift [about 7–25 nm (430–1330 cm−1)] in energy from that of the other complexes, and there is a similar blue-shift in emission peak maximum (5–14 nm, 174–478 cm−1) as well (Figure 1b). Likewise, the lowest energy absorption band and emission peak maximum of fBu_Au_CN both are red-shifted from that of fBu_Au_PhAr, fPh_Au_tip, and fPh_Au_C2fb. This difference in spectral properties is attributed to the different donor strength (–CN 8V, the emission band from fPh_Au_C2fb is masked by the blue emission of FIrpic. Under this circumstance, the emission color of the device shifts to blue with CIE coordinates of (0.20, 0.35) at 10 V. EQE-current efficiency-luminance characteristics of this PLED are depicted in Figure S6. A maximum EQE of 13.16%, corresponding to current efficiency of 40.00 cd A−1 and power efficiency of 25.20 lm W−1, has been achieved at low luminance. To our best knowledge, the efficiency of this device based on the combined emissions of Au(III) and Ir(III) complexes is the highest among solution-processed colortunable PLEDs.[7] The EQE of this device is more than 50% higher than that of the 4 wt% fPh_Au_C2fb-device (Table 2). This could be attributed to the energy transfer from FIrpic,
Normalized EL intensity (a. u.)
COMMUNICATION
www.MaterialsViews.com
(b) Voltage 5V 6V 7V 8V 9V 10 V
400
500
600
700
Wavelength (nm) Figure 4. (a) Color shift of the PLED with 2 wt% fPh_Au_C2fb and10 wt% FIrpic upon increase in driving voltage from 5 to 10 V and (b) Normalized EL spectra of the PLED with 2 wt% fPh_Au_C2fb and 10 wt% FIrpic at 5–10 V.
2544
wileyonlinelibrary.com
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
www.advmat.de
2
10
1
10
Emitter
0
10
fBu_Au_CN (7 wt%) fBu_Au_CN (1 wt%): FIrpic (10 wt%) -1
10
0
1
10
10
2
3
10
10
4
10
-2
Luminance (cd m )
Figure 5. External quantum efficiency-luminance characteristics of the yellow-emitting OLED with 7wt% fBu_Au_CN and the color tunable OLED with 1wt% fBu_Au_CN and 10 wt% FIrpic.
since the latter provides an additional energy transfer channel from PVK to fPh_Au_C2fb via FIrpic in addition to the direct energy transfer from PVK to fPh_Au_C2fb, as well as the contribution of FIrpic emission. At high luminance of 1000 cd m−2, EQE drops to 9.78% with a roll-off of 25.7%. To evaluate the practical application of our Au(III) complexes in OLED technology, we also used fBu_Au_CN (Scheme 2) as dopant to fabricate OLEDs by vacuum deposition. The device architecture of ITO/MoO3 (2 nm)/TAPC (50 nm)/EML (10 nm)/TmPyPb (50 nm)/LiF (1.2 nm)/Al (150 nm) was used. For these devices by vacuum deposition, TAPC was used as the hole-transporting layer while TmPyPb as the electron-transporting layer. For the emissive layer (EML), TCTA was used as the host while fBu_Au_CN or a mixture of fBu_Au_CN and FIrpic as emitter for yellow-emitting or color tunable OLEDs, respectively. Figure 5 depicts EQE-luminance characteristics of these yellow-emitting and color tunable OLEDs with fBu_Au_CN as the yellow emitter. At low luminance, high EQE of 20.31%, current efficiency of 56.00 cd A−1, and power efficiency of 62.84 lm W−1 have been achieved with the yellow-emitting OLED having 7 wt% fBu_Au_CN (Table S1). These values are > 2 fold higher than that of the best aforementioned Au(III)-PLED (Table 2). The efficiency of our vacuum deposited fBu_Au_CN-OLED is the highest among those of the reported Au(III)-OLEDs[8–11] and even approaching those of the best OLEDs fabricated with Ir(III) or Pt(II) emitters.[3,4] Like the case in the aforementioned color tunable PLEDs, by introducing the blue emitter FIrpic into the EML, high performance color tunable OLED has been realized with CIE coordinates shifting from (0.43, 0.51) at 3 V to (0.21, 0.42) at 8 V when the doping concentrations for FIrpic and fBu_Au_CN were 10 and 1 wt%, respectively (Figure S7). The maximum EQE of this color tunable OLED is 22.02%, being slightly higher than that of the yellowemitting OLED attributed to the energy transfer from FIrpic to fBu_Au_CN. To our best knowledge, these values are the highest among the reported color tunable OLEDs/PLEDs.[5–7] Upon increasing luminance, efficiency roll-off of the color tunable OLED is much slower than that of the yellow-emitting one (Figure 5) for the same reason used to account for the performance of the color tunable PLED with FIrpic and fPh_ Au_C2fb. Other key performances of both kinds of OLEDs are
Adv. Mater. 2014, 26, 2540–2546
depicted in Figures S8–S9 and Table S1. Maximum current efficiency of 68.82 cd A−1 and power efficiency of 77.23 lm W−1 have been achieved in the color tunable Au(III)-OLED. In summary, a new class of strongly luminescent gold(III) complexes with high emission quantum yields of 0.44–0.61 and long emission lifetimes of 179–437 µs in CH2Cl2 have been developed. Maximum EQE up to 9.18% has been achieved for the yellow-emitting PLED with 10 wt% fPh_Au_C2fb. Taking advantages of the high emission quantum yield and long emission lifetime of fPh_Au_C2fb, a color tunable PLED has been realized by combining FIrpic and fPh_Au_C2fb as blue and yellow emitters, respectively. This color tunable PLED displayed a maximum EQE of 13.16% and wide color span range from yellow color with CIE coordinates of (0.40, 0.50) to blue color with CIE coordinates of (0.20, 0.35) upon the increase in applied voltage from 5 to 10 V. By vacuum deposition technology, high EQEs of 20.31% and 22.02% have been achieved for the respective yellow-emitting and color tunable OLEDs fabricated with fBu_Au_CN. All these findings show that with further development of luminescent Au(III) emitters and optimization of device structure, Au(III)-based color tunable OLEDs/ PLEDs would have practical application in display and lighting devices.
COMMUNICATION
External quantum efficiency (%)
www.MaterialsViews.com
Experimental Section Materials: PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(sty rene sulfonic acid)] (Clevios P AI 4083) was purchased from Heraeus, PVK (polyvinylcarbazole) from Sigma-Aldrich, OXD-7 [(1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene)], TmPyPb [1,3,5-tri[(3-pyridyl)phen-3-yl]benzene], TPBi [2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole)], TAPC (di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane), TCTA (4,4′,4″-Tris(carbazole-9-yl)triphenylamine), and FIrpic from Luminescence Technology Corp. All of these materials were used as received. The synthetic procedures of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph are described in the Supporting Information. All Au(III) complexes were purified by gradient sublimation before use. Fabrication and characterization of PLEDs: PEDOT:PSS were spincoated onto the cleaned ITO-coated glass substrate and baked at 120 °C for 20 min to remove the residual water solvent in a clean room. Blends of PVK: OXD-7: Au(III) complexes or PVK: OXD-7: fPh_Au_C2fb: Firpic were spin-coated from chlorobenzene atop the PEDOT:PSS layer inside a N2filled glove box. The thickness for all EMLs was about 60 nm. Afterwards, all devices were annealed at 110 °C for 10 min inside the glove box and subsequently transferred into a Kurt J. Lesker SPECTROS vacuum deposition system without exposing to air. Finally, TmPyPb (5 nm), TPBi (40 nm), LiF (1.2 nm), and Al (150 nm) were deposited in sequence by thermal evaporation at a pressure of 10−8 mbar. Detailed fabrication of OLEDs is given in the Supporting Information. PL spectra were obtained on a JobinYvon Fluorolog-3 fluorescence spectrophotometer. EL spectra, luminance, and CIE coordination were measured by a Photo Research Inc PR-655. Voltage-current characteristics were measured by a Keithley 2400 source-meter measurement unit. All devices were characterized at room temperature after encapsulation. EQE and power efficiency were calculated by assuming a Lambertian distribution.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2545
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Acknowledgements We gratefully acknowledge the support from the University Grants Committee of the HKSAR, China (Project No. [AoE/P-03/08]), the National Key Basic Research Program of China (No. 2013CB834802), the National Science Foundation of China (No. 61274002), and the Innovation and Technology Commission of the HKSAR Government (GHP/043/10). This work was also supported by Guangdong Special Project of the Introduction of Innovative R&D Teams.
Received: August 24, 2013 Revised: November 17, 2013 Published online: February 5, 2014
[1] a) J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539–541; b) X. Gong, W. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses, A. J. Heeger, Adv. Mater. 2004, 16, 615–619; c) C.-L. Ho, M.-F. Lin, W.-Y. Wong, W.-K. Wong, C. H. Chen, Appl. Phys. Lett. 2008, 92, 083301; d) H. Wu, L. Ying, W. Yang, Y. Cao, Chem. Soc. Rev. 2009, 38, 3391–3400; e) X. Chen, J.-L. Liao, Y. Liang, M. O. Ahmed, H.-E. Tseng, S.-A. Chen, J. Am. Chem. Soc. 2003, 125, 636–637; f) S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 2011, 23, 4956–4959; g) H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, Adv. Mater. 2009, 21, 4181–4184; h) 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. 2012, 24, 1873–1877; i) C. Cebrián, M. Mauro, D. Kourkoulos, P. Mercandelli, D. Hertel, K. Meerholz, C. A. Strassert, L. De Cola, Adv. Mater. 2013, 25, 437–442. [2] a) Y. Ma, H. Zhang, J. Shen, C.-M. Che, Synth. Met. 1998, 94, 245– 248; b) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151–154. [3] a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4–6; b) H. Sasabe, H. Nakanishi, Y. Watanabe, S. Yano, M. Hirasawa, Y.-J. Pu, J. Kido, Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201301069. [4] a) K. Li, G. Cheng, C. Ma, X. Guan, W.-M. Kwok, Y. Chen, W. Lu, C.-M. Che, Chem. Sci. 2013, 4, 2630–2644; b) Y.-L. Tung, S.-W. Lee, Y. Chi, L.-S. Chen, C.-F. Shu, F.-I. Wu, A. J. Carty, P.-T. Chou, S.-M. Peng, G.-H. Lee, Adv. Mater. 2005, 17, 1059–1064; c) G. Cheng, P.-K. Chow, S. C. F. Kui, C.-C. Kwok, C.-M. Che, Adv. Mater. 2013, accepted, DOI: 10.1002/adma. 201302408; d) G. Cheng, Y. Chen, C. Yang, W. Lu, C.-M. Che, Chem. Asian J. 2013, 8, 1754–1759; e) S. Welter, K. Brunner, J. W. Hofstraat, L. De Cola, Nature 2003, 421, 54–57; f) S. Bernhard, X. Gao, G. G. Malliaras, H. D. Abruña, Adv. Mater. 2002, 14, 433–436; g) B.-S. Du, J.-L. Liao, M.-H. Huang, C.-H. Lin, H.-W. Lin, Y. Chi, H.-A. Pan, G.-L. Fan, K.-T. Wong, G.-H. Lee, P.-T. Chou, Adv. Funct. Mater . 2012, 22, 3491–3499.
2546
wileyonlinelibrary.com
[5] a) J.-H. Jou, M.-H. Wu, S.-M. Shen, H.-C. Wang, S.-Z. Chen, S.-H. Chen, C.-R. Lin, Y.-L. Hsieh, Appl. Phys. Lett. 2009, 95, 013307; b) J.-H. Jou, P.-W. Chen, Y.-L. Chen, Y.-C. Jou, J.-R. Tseng, R.-Z. Wu, C.-Y. Hsieh, Y.-C. Hsieh, P. Joers, S.-H. Chen, Y.-S. Wang, F.-C. Tung, C.-C. Chen, C.-C. Wang, Org. Electron. 2013, 14, 47–54; c) S.-H. Yang, P.-J. Shih, W.-J. Wu, Y.-H. Huang, J. Lumin. 2013, 142, 86–91; d) K. M.-C. Wong, X. Zhu, L.-L. Hung, N. Zhu, V. W.-W. Yam, H.-S. Kwok, Chem. Commun. 2005, 2906–2908; e) W. C. H. Choy, J. H. Niu, X.-W. Chen, W. L. Li, P. C. Chui, Appl. Phys. A 2007, 89, 667–671. [6] a) M. Berggren, O. Inganäs, G. Gustafsson, J. Rasmusson, M. R. Andersson, T. Hjertberg, O. Wennerström, Nature 1994, 372, 444–446; b) M. Granström, O. Inganäs, Appl. Phys. Lett. 1996, 68, 147–149; c) Y. Yang, Q. Pei, Appl. Phys. Lett. 1996, 68, 2708–2710. [7] a) P. E. Burrows, S. R. Forrest, S. P. Sibley, M. E. Thompson, Appl. Phys. Lett. 1996, 69, 2959–2961; b) Z. Shen, P. E. Burrows, V. Bulovic´, S. R. Forrest, M. E. Thompson, Science 1997, 276, 2009– 2011; c) G. Parthasarathy, G. Gu, S. R. Forrest, Adv. Mater. 1999, 11, 907–910; d) Y. Jiang, J. Lian, S. Chen, H.-S. Kwok, Org. Electron. 2013, 14, 2001–2006. [8] a) V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung, N. Zhu, Angew. Chem. 2005, 117, 3167–3170; Angew. Chem. Int. Ed. 2005, 44, 3107–3110; b) K. M.-C. Wong, L.-L. Hung, W. H. Lam, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2007, 129, 4350–4365; c) V. K.-M. Au, K. M.-C. Wong, D. P.-K. Tsang, M.-Y. Chan, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2010, 132, 14273–14278; d) M.-C. Tang, D. P.-K. Tsang, M. M.-Y. Chan, K. M.-C. Wong, V. W.-W. Yam, Angew. Chem. 2013, 125, 464–467; Angew. Chem. Int. Ed. 2013, 52, 446–449. [9] a) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-W. Yam, Chem. Eur. J. 2011, 17, 130–142; b) V. K.-M. Au, W. H. Lam, W.-T. Wong, V. W.-W. Yam, Inorg. Chem. 2012, 51, 7537–7545. [10] a) J. A. Garg, O. Blacque, K. Venkatesan, Inorg. Chem. 2011, 50, 5430–5441; b) W.-P. To, G. S.-M. Tong, W. Lu, C. Ma, J. Liu, A. L.-F. Chow, C.-M. Che, Angew. Chem. 2012, 124, 2708–2711; Angew. Chem. Int. Ed. 2012, 51, 2654–2657; c) D. A. Smith, D.-A. Ros¸ca, M. Bochmann, Organometallics 2012, 31, 5998–6000; d) D.-A. Ros¸ca, D. A. Smith, M. Bochmann, Chem. Commun. 2012, 48, 7247–7249. [11] a) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2009, 131, 9076–9085; b) J. A. Garg, O. Blacque, T. Fox, K. Venkatesan, Inorg. Chem. 2010, 49, 11463–11472; c) C. Bronner, O. S. Wenger, Dalton Trans. 2011, 40, 12409–12420; d) V. K.-M. Au, N. Zhu, V. W.-W. Yam, Inorg. Chem. 2013, 52, 558–567; e) A. Herbst, C. Bronner, P. Dechambenoit, O. S. Wenger, Organometallics 2013, 32, 1807–1814. [12] W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok, X. Guan, K.-H. Low, C.-M. Che, Angew. Chem. 2013, 125, 6780–6784; Angew. Chem. Int. Ed. 2013, 52, 6648–6652. [13] a) C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 2082– 2084; b) X. H. Yang, F. Jaiser, S. Klinger, D. Neher, Appl. Phys. Lett. 2006, 88, 021107; c) Y. You, S. Y. Park, J. Am. Chem. Soc. 2005, 127, 12438–12439.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
COMMUNICATION
www.MaterialsViews.com
Color Tunable Organic Light-Emitting Devices with External Quantum Efficiency over 20% Based on Strongly Luminescent Gold(III) Complexes having Long-Lived Emissive Excited States Gang Cheng, Kaai Tung Chan, Wai-Pong To, and Chi-Ming Che* There is a surge of interest to develop new types of organic light-emitting devices (OLEDs)/polymer light-emitting devices (PLEDs) capable of being manufactured by low-cost technologies for large area display or lighting applications.[1] For high efficiency OLEDs/PLEDs, phosphorescent metal complexes are sought after as light-emitting dopant materials as they can harvest both singlet and triplet excitons via effectively enhancing spin-orbit coupling and promoting an efficient intersystem crossing from their singlet to triplet excited states.[2] Phosphorescent metal complexes are usually dispersed into proper host material(s) to minimize concentration quenching and/or triplettriplet annihilation, as both processes would severely reduce the device efficiency at high luminance.[2–4] In phosphorescent OLEDs/PLEDs, or organic light-emitting devices (OLEDs), voltage dependent electroluminescence (EL) is occasionally encountered due to the voltage dependent energy transfer from host to dopant and/or the shift of exciton recombination zone upon varying driving voltage. This kind of effect has previously been utilized to develop color tunable PLEDs/OLEDs[5–7] that can be used for decorative or color matching purpose.[7a,7d] Since the color span range is a critical parameter for judging the quality of a color tunable PLED/OLED, approaches have been reported to improve this parameter including i) an insulating polymer was used to mix with several substituted polythiophenes that were electroluminescent; this was to diminish the energy transfer from high-band-gap polymer to low-bandgap polymer, and, in the reported case the color of the resulting EL shifted from yellow at bias of 5 V to white at 20 V,[6b] and ii) a hole-blocking or electron-blocking layer was inserted between two adjacent emission layers, leading to shift of color Dr. G. Cheng, K. T. Chan, Dr. W.-P. To, Prof. C.-M. Che State Key Laboratory of Synthetic Chemistry Institute of Molecular Functional Materials and Department of Chemistry The University of Hong Kong Pokfulam Road, Hong Kong SAR, China E-mail: [email protected] Dr. G. Cheng State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University Changchun1 30012, China Dr. G. Cheng, Prof. C.-M. Che HKU Shenzhen Institute of Research and Innovation Shenzhen 518053, China
DOI: 10.1002/adma.201304263 2540
wileyonlinelibrary.com
from white to blue or from yellow to white upon increase in driving voltage.[5e] Nevertheless, the range of color span with the aforementioned approaches is not wide enough when compared to the case of from yellow to blue described in this work. Meanwhile, OLEDs fabricated with sophisticated structure such as the ones with sub-cells having different color independently controlled have been designed.[7] Although the EL color of such OLEDs can be varied over a wide range, sophisticated architecture and expensive technologies, such as fine mask alignment, are needed for the device fabrication. Thus, the realization of inexpensive and simple-structured solution-processable PLEDs with high efficiency and wide color span range would be attractive for color tunable lighting applications. For a good color tunable PLED/OLED, the high-energy emission should be quenched at low voltage while the low-energy one should be minimized at high voltage, and high device efficiency should be maintained at both high and low voltages. To design such color tunable PLED/OLED, the following strategies depicted in Scheme 1 are proposed; i) both high-energy and low-energy emitters should have high emission quantum yields to ensure the device to have high efficiency, ii) the energy transfer between these two emitters should be efficient at low voltage to quench the high energy emission whereas the energy transfer should be blocked at high voltage to minimize the low energy emission, and iii) the efficiency roll-off of the low-energy emitter could be strong at high voltage to further minimize the lowenergy emission component in the EL spectrum. To fulfill these requirements, an efficient low-energy phosphorescent emitter having a long-lived emissive excited state is desirable. Thus, at low voltage, the emissive excited state of the low-energy emitter is partly occupied, resulting in high EL efficiency and allowing for energy transfer from the high-energy emitter; whereas at high voltage, the long-lived emissive excited state is saturated, leading to rapid drop in EL efficiency and blocking of energy transfer from the high-energy emitter. The narrow color span range of the reported simple-structured color tunable PLEDs/ OLEDs[5,6] could be accounted for by the emission lifetimes of the low-energy emitters used in these devices being not long enough and these lifetimes are usually less than 10 µs. Compared to the widely studied phosphorescent emitters of Ir(III) and Pt(II) complexes,[3,4a–4d] luminescent Au(III) complexes are less developed.[8,9] Reports on OLEDs using luminescent Au(III) complexes with external quantum efficiencies (EQEs) of 11.5% (maximum value) and 7.8% (the value at 0.1 mA cm−2) respectively achieved in vacuum-deposited device fabricated with bis-cyclometalated alkynylgold(III) complexes[8c]
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
www.advmat.de www.MaterialsViews.com
and in solution-processed device fabricated with dendrimers containing alkynylgold(III) units[8d] have recently appeared in literature. Nevertheless, the emission quantum yields of the reported Au(III) complexes[8–11] are rarely over 15% in solutions at room temperature prior to our recent findings on strongly luminescent Au(III) complexes supported by C-deprotonated fluorene-C^N^C ligands; our recently reported Au(III) complexes display emission quantum yields of up to 58% in dichloromethane at room temperature.[12] The use of C-deprotonated fluorene-C^N^C ligand changes the lowest triplet excited state of the Au(III) complexes from ligand-to-ligand charge transfer
COMMUNICATION
Scheme 1. Schematic diagram of the strategies proposed for the design of color tunable PLEDs/OLEDs; blue and yellow curves represent highenergy and low-energy components of EL spectra, respectively, at both low and high voltages; blue and yellow circles respectively stand for high-energy and low-energy emitters; the filled yellow circle indicates the low-energy emitter with saturated excited state, whose efficiency is much lower than the one whose excited state is not saturated (open yellow circle). Energy transfer (ET) takes place between the two emitters at low voltage whereas be blocked at high voltage due to the saturation of the low-energy emitter.
(LLCT) state to ligand centered 3ππ* one. Such a subtle structural modification gives rise to strongly luminescent Au(III) complexes with excited state lifetimes over 200 µs in solution at room temperature besides their high emission quantum yields.[12] The high emission quantum yields along with long emission lifetimes render these newly developed luminescent Au(III) complexes to be good yellow emitter for the design of color tunable PLEDs/OLEDs according to the aforementioned strategies (Scheme 1). Yam and co-workers previously reported a cyclometalated Au(III)-alkynyl complex that gave a color tunable OLED, the mechanism of which was attributed to the shift of exciton recombination zone.[5d] In this work, we report a series of strongly luminescent Au(III) complexes fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph (Scheme 2) with emission quantum yields of 0.44–0.61 and emission lifetimes of 179–437µs in CH2Cl2 (Table 1). PLEDs based on fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, and fbb_Au_C2Ph at different concentrations were fabricated and characterized. A maximum EQE of 9.18% has been achieved for the PLED fabricated with fPh_Au_C2fb having the highest emission quantum yield of 0.61. Together with a blue emitting phosphorescent Ir(III) complex bis[(4,6-difluorophenyl)pyridinato-N,C2]-(picolinato) iridium (FIrpic),[13] a color tunable PLED has been realized with a maximum EQE of 13.16%. The color of this device can be tuned from yellow with Commission Internationale De L´Eclairage (CIE) coordinates of (0.40, 0.50) at 5 V to blue with CIE coordinates of (0.20, 0.35) at 10 V. Via vacuum deposition, both yellow-emitting and color tunable OLEDs respectively with much higher maximum EQEs of 20.31% and 22.02% have been fabricated by using fBu_Au_CN as yellow light-emitting dopant material thereby highlighting the feasibility of having high efficiency Au(III)-OLEDs/PLEDs for practical applications. As depicted in Figure 1a, the UV-visible absorption spectra of the complexes, fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr,
Scheme 2. Chemical structures of the Au(III) complexes used in this work.
Adv. Mater. 2014, 26, 2540–2546
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2541
www.advmat.de
COMMUNICATION
www.MaterialsViews.com Table 1. Photophysical data of the Au(III) complexes studied in this work. UV/Vis absorptiona) λabs[nm] (ε [dm3 mol–1cm–1])
Complex
fPh_Au_tip
Emission
276 (54200), 326 (31700), 348 (23800), 411 (8400), 431 (8100)
HOMOd) [eV]
LUMOd) [eV]
Solutiona) λmax [nm] (τ [µs])
Quantum yieldb)
Kqc) [dm3mol–1 s–1]
539, 578 (179)
0.46
8.48 × 106
–5.49
–2.97
7
fPh_Au_C2fb
282 (48000), 300 (51400), 311 (46200), 322 (48400), 349 (23500), 388 (7800), 409 (7600), 432 (6800)
540, 579 (217)
0.61
1.58 × 10
–5.58
–2.98
fBu_Au_PhAr
277 (53900), 321 (30500), 348 (24500), 402 (9800), 421 (9900)
534, 573 (300)
0.55
4.84 × 106
–5.82
–2.81
107
–5.94
–3.10
–5.72
–2.88
fBu_Au_CN
269 (38000), 301 (29100), 314 (29100), 327 (28200), 346 (23000), 420 (8300), 440 (8800)
548, 585 (437)
0.44
1.32 ×
fbb_Au_C2Ph
280 (37400), 318 (48100), 337 (30500), 352 (31200), 400 (6500), 423 (12000), 446 (12900)
540, 579 (250)
0.45
2.72 × 106
a) Determined in degassed CH2Cl2 (2 × 10−5mol dm−3). b) Emission quantum yield was measured in a degassed dichloromethane solution (2 × 10−5mol dm−3) by the optical dilute method with [Ru(bpy)3][PF6]2 in degassed acetonitrile as the standard (Φr = 0.062). c) Self-quenching rate constant. d) The highest occupied molecular orbital (HOMO) and LUMO levels are estimated from onset potentials using Cp2Fe0/+ value of 4.8 eV below the vacuum level.
Molar Absorptivity (mol-1 dm3 cm-1)
fBu_Au_CN, and fbb_Au_C2Ph in CH2Cl2 at 298 K show similar intense absorption bands at 250–350 nm (ε = 2 – 5 × 104 dm3 mol−1 cm−1) and moderately intense bands at 60000
(a) fPh_Au_tip fPh_Au_C2fb fBu_Au_PhAr fBu_Au_CN fbb_Au_C2Ph
50000 40000 30000 20000 10000 0 250
300
350
400
450
500
Wavelength (nm) Normalized Emission Intensity
(b)
450
500
fPh_Au_tip fPh_Au_C2fb fBu_Au_PhAr fbb_Au_C2Ph fBu_Au_CN
550
600
650
700
750
Wavelength (nm) Figure 1. (a) UV-vis absorption spectra of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph in dichloromethane. (b) Emission spectra of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_ Au_CN, and fbb_Au_C2Ph in degassed dichloromethane.
2542
wileyonlinelibrary.com
370–440 nm (ε = 1 – 2 × 104 dm3 mol−1 cm−1). These absorption bands are assigned to metal-perturbed intraligand transitions localized on the C^N^C ligand. The low-energy absorption at 400–440 nm has substantial LLCT character as revealed by our previous DFT calculations.[12] As depicted in Figures 1a and S1a, the lowest energy absorption bands of the arylgold complex, fBu_Au_PhAr, in CH2Cl2 reveal a blue-shift [about 7–25 nm (430–1330 cm−1)] in energy from that of the other complexes, and there is a similar blue-shift in emission peak maximum (5–14 nm, 174–478 cm−1) as well (Figure 1b). Likewise, the lowest energy absorption band and emission peak maximum of fBu_Au_CN both are red-shifted from that of fBu_Au_PhAr, fPh_Au_tip, and fPh_Au_C2fb. This difference in spectral properties is attributed to the different donor strength (–CN 8V, the emission band from fPh_Au_C2fb is masked by the blue emission of FIrpic. Under this circumstance, the emission color of the device shifts to blue with CIE coordinates of (0.20, 0.35) at 10 V. EQE-current efficiency-luminance characteristics of this PLED are depicted in Figure S6. A maximum EQE of 13.16%, corresponding to current efficiency of 40.00 cd A−1 and power efficiency of 25.20 lm W−1, has been achieved at low luminance. To our best knowledge, the efficiency of this device based on the combined emissions of Au(III) and Ir(III) complexes is the highest among solution-processed colortunable PLEDs.[7] The EQE of this device is more than 50% higher than that of the 4 wt% fPh_Au_C2fb-device (Table 2). This could be attributed to the energy transfer from FIrpic,
Normalized EL intensity (a. u.)
COMMUNICATION
www.MaterialsViews.com
(b) Voltage 5V 6V 7V 8V 9V 10 V
400
500
600
700
Wavelength (nm) Figure 4. (a) Color shift of the PLED with 2 wt% fPh_Au_C2fb and10 wt% FIrpic upon increase in driving voltage from 5 to 10 V and (b) Normalized EL spectra of the PLED with 2 wt% fPh_Au_C2fb and 10 wt% FIrpic at 5–10 V.
2544
wileyonlinelibrary.com
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
www.advmat.de
2
10
1
10
Emitter
0
10
fBu_Au_CN (7 wt%) fBu_Au_CN (1 wt%): FIrpic (10 wt%) -1
10
0
1
10
10
2
3
10
10
4
10
-2
Luminance (cd m )
Figure 5. External quantum efficiency-luminance characteristics of the yellow-emitting OLED with 7wt% fBu_Au_CN and the color tunable OLED with 1wt% fBu_Au_CN and 10 wt% FIrpic.
since the latter provides an additional energy transfer channel from PVK to fPh_Au_C2fb via FIrpic in addition to the direct energy transfer from PVK to fPh_Au_C2fb, as well as the contribution of FIrpic emission. At high luminance of 1000 cd m−2, EQE drops to 9.78% with a roll-off of 25.7%. To evaluate the practical application of our Au(III) complexes in OLED technology, we also used fBu_Au_CN (Scheme 2) as dopant to fabricate OLEDs by vacuum deposition. The device architecture of ITO/MoO3 (2 nm)/TAPC (50 nm)/EML (10 nm)/TmPyPb (50 nm)/LiF (1.2 nm)/Al (150 nm) was used. For these devices by vacuum deposition, TAPC was used as the hole-transporting layer while TmPyPb as the electron-transporting layer. For the emissive layer (EML), TCTA was used as the host while fBu_Au_CN or a mixture of fBu_Au_CN and FIrpic as emitter for yellow-emitting or color tunable OLEDs, respectively. Figure 5 depicts EQE-luminance characteristics of these yellow-emitting and color tunable OLEDs with fBu_Au_CN as the yellow emitter. At low luminance, high EQE of 20.31%, current efficiency of 56.00 cd A−1, and power efficiency of 62.84 lm W−1 have been achieved with the yellow-emitting OLED having 7 wt% fBu_Au_CN (Table S1). These values are > 2 fold higher than that of the best aforementioned Au(III)-PLED (Table 2). The efficiency of our vacuum deposited fBu_Au_CN-OLED is the highest among those of the reported Au(III)-OLEDs[8–11] and even approaching those of the best OLEDs fabricated with Ir(III) or Pt(II) emitters.[3,4] Like the case in the aforementioned color tunable PLEDs, by introducing the blue emitter FIrpic into the EML, high performance color tunable OLED has been realized with CIE coordinates shifting from (0.43, 0.51) at 3 V to (0.21, 0.42) at 8 V when the doping concentrations for FIrpic and fBu_Au_CN were 10 and 1 wt%, respectively (Figure S7). The maximum EQE of this color tunable OLED is 22.02%, being slightly higher than that of the yellowemitting OLED attributed to the energy transfer from FIrpic to fBu_Au_CN. To our best knowledge, these values are the highest among the reported color tunable OLEDs/PLEDs.[5–7] Upon increasing luminance, efficiency roll-off of the color tunable OLED is much slower than that of the yellow-emitting one (Figure 5) for the same reason used to account for the performance of the color tunable PLED with FIrpic and fPh_ Au_C2fb. Other key performances of both kinds of OLEDs are
Adv. Mater. 2014, 26, 2540–2546
depicted in Figures S8–S9 and Table S1. Maximum current efficiency of 68.82 cd A−1 and power efficiency of 77.23 lm W−1 have been achieved in the color tunable Au(III)-OLED. In summary, a new class of strongly luminescent gold(III) complexes with high emission quantum yields of 0.44–0.61 and long emission lifetimes of 179–437 µs in CH2Cl2 have been developed. Maximum EQE up to 9.18% has been achieved for the yellow-emitting PLED with 10 wt% fPh_Au_C2fb. Taking advantages of the high emission quantum yield and long emission lifetime of fPh_Au_C2fb, a color tunable PLED has been realized by combining FIrpic and fPh_Au_C2fb as blue and yellow emitters, respectively. This color tunable PLED displayed a maximum EQE of 13.16% and wide color span range from yellow color with CIE coordinates of (0.40, 0.50) to blue color with CIE coordinates of (0.20, 0.35) upon the increase in applied voltage from 5 to 10 V. By vacuum deposition technology, high EQEs of 20.31% and 22.02% have been achieved for the respective yellow-emitting and color tunable OLEDs fabricated with fBu_Au_CN. All these findings show that with further development of luminescent Au(III) emitters and optimization of device structure, Au(III)-based color tunable OLEDs/ PLEDs would have practical application in display and lighting devices.
COMMUNICATION
External quantum efficiency (%)
www.MaterialsViews.com
Experimental Section Materials: PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(sty rene sulfonic acid)] (Clevios P AI 4083) was purchased from Heraeus, PVK (polyvinylcarbazole) from Sigma-Aldrich, OXD-7 [(1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene)], TmPyPb [1,3,5-tri[(3-pyridyl)phen-3-yl]benzene], TPBi [2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole)], TAPC (di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane), TCTA (4,4′,4″-Tris(carbazole-9-yl)triphenylamine), and FIrpic from Luminescence Technology Corp. All of these materials were used as received. The synthetic procedures of fPh_Au_tip, fPh_Au_C2fb, fBu_Au_PhAr, fBu_Au_CN, and fbb_Au_C2Ph are described in the Supporting Information. All Au(III) complexes were purified by gradient sublimation before use. Fabrication and characterization of PLEDs: PEDOT:PSS were spincoated onto the cleaned ITO-coated glass substrate and baked at 120 °C for 20 min to remove the residual water solvent in a clean room. Blends of PVK: OXD-7: Au(III) complexes or PVK: OXD-7: fPh_Au_C2fb: Firpic were spin-coated from chlorobenzene atop the PEDOT:PSS layer inside a N2filled glove box. The thickness for all EMLs was about 60 nm. Afterwards, all devices were annealed at 110 °C for 10 min inside the glove box and subsequently transferred into a Kurt J. Lesker SPECTROS vacuum deposition system without exposing to air. Finally, TmPyPb (5 nm), TPBi (40 nm), LiF (1.2 nm), and Al (150 nm) were deposited in sequence by thermal evaporation at a pressure of 10−8 mbar. Detailed fabrication of OLEDs is given in the Supporting Information. PL spectra were obtained on a JobinYvon Fluorolog-3 fluorescence spectrophotometer. EL spectra, luminance, and CIE coordination were measured by a Photo Research Inc PR-655. Voltage-current characteristics were measured by a Keithley 2400 source-meter measurement unit. All devices were characterized at room temperature after encapsulation. EQE and power efficiency were calculated by assuming a Lambertian distribution.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2545
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Acknowledgements We gratefully acknowledge the support from the University Grants Committee of the HKSAR, China (Project No. [AoE/P-03/08]), the National Key Basic Research Program of China (No. 2013CB834802), the National Science Foundation of China (No. 61274002), and the Innovation and Technology Commission of the HKSAR Government (GHP/043/10). This work was also supported by Guangdong Special Project of the Introduction of Innovative R&D Teams.
Received: August 24, 2013 Revised: November 17, 2013 Published online: February 5, 2014
[1] a) J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539–541; b) X. Gong, W. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses, A. J. Heeger, Adv. Mater. 2004, 16, 615–619; c) C.-L. Ho, M.-F. Lin, W.-Y. Wong, W.-K. Wong, C. H. Chen, Appl. Phys. Lett. 2008, 92, 083301; d) H. Wu, L. Ying, W. Yang, Y. Cao, Chem. Soc. Rev. 2009, 38, 3391–3400; e) X. Chen, J.-L. Liao, Y. Liang, M. O. Ahmed, H.-E. Tseng, S.-A. Chen, J. Am. Chem. Soc. 2003, 125, 636–637; f) S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 2011, 23, 4956–4959; g) H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, Adv. Mater. 2009, 21, 4181–4184; h) 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. 2012, 24, 1873–1877; i) C. Cebrián, M. Mauro, D. Kourkoulos, P. Mercandelli, D. Hertel, K. Meerholz, C. A. Strassert, L. De Cola, Adv. Mater. 2013, 25, 437–442. [2] a) Y. Ma, H. Zhang, J. Shen, C.-M. Che, Synth. Met. 1998, 94, 245– 248; b) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151–154. [3] a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4–6; b) H. Sasabe, H. Nakanishi, Y. Watanabe, S. Yano, M. Hirasawa, Y.-J. Pu, J. Kido, Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201301069. [4] a) K. Li, G. Cheng, C. Ma, X. Guan, W.-M. Kwok, Y. Chen, W. Lu, C.-M. Che, Chem. Sci. 2013, 4, 2630–2644; b) Y.-L. Tung, S.-W. Lee, Y. Chi, L.-S. Chen, C.-F. Shu, F.-I. Wu, A. J. Carty, P.-T. Chou, S.-M. Peng, G.-H. Lee, Adv. Mater. 2005, 17, 1059–1064; c) G. Cheng, P.-K. Chow, S. C. F. Kui, C.-C. Kwok, C.-M. Che, Adv. Mater. 2013, accepted, DOI: 10.1002/adma. 201302408; d) G. Cheng, Y. Chen, C. Yang, W. Lu, C.-M. Che, Chem. Asian J. 2013, 8, 1754–1759; e) S. Welter, K. Brunner, J. W. Hofstraat, L. De Cola, Nature 2003, 421, 54–57; f) S. Bernhard, X. Gao, G. G. Malliaras, H. D. Abruña, Adv. Mater. 2002, 14, 433–436; g) B.-S. Du, J.-L. Liao, M.-H. Huang, C.-H. Lin, H.-W. Lin, Y. Chi, H.-A. Pan, G.-L. Fan, K.-T. Wong, G.-H. Lee, P.-T. Chou, Adv. Funct. Mater . 2012, 22, 3491–3499.
2546
wileyonlinelibrary.com
[5] a) J.-H. Jou, M.-H. Wu, S.-M. Shen, H.-C. Wang, S.-Z. Chen, S.-H. Chen, C.-R. Lin, Y.-L. Hsieh, Appl. Phys. Lett. 2009, 95, 013307; b) J.-H. Jou, P.-W. Chen, Y.-L. Chen, Y.-C. Jou, J.-R. Tseng, R.-Z. Wu, C.-Y. Hsieh, Y.-C. Hsieh, P. Joers, S.-H. Chen, Y.-S. Wang, F.-C. Tung, C.-C. Chen, C.-C. Wang, Org. Electron. 2013, 14, 47–54; c) S.-H. Yang, P.-J. Shih, W.-J. Wu, Y.-H. Huang, J. Lumin. 2013, 142, 86–91; d) K. M.-C. Wong, X. Zhu, L.-L. Hung, N. Zhu, V. W.-W. Yam, H.-S. Kwok, Chem. Commun. 2005, 2906–2908; e) W. C. H. Choy, J. H. Niu, X.-W. Chen, W. L. Li, P. C. Chui, Appl. Phys. A 2007, 89, 667–671. [6] a) M. Berggren, O. Inganäs, G. Gustafsson, J. Rasmusson, M. R. Andersson, T. Hjertberg, O. Wennerström, Nature 1994, 372, 444–446; b) M. Granström, O. Inganäs, Appl. Phys. Lett. 1996, 68, 147–149; c) Y. Yang, Q. Pei, Appl. Phys. Lett. 1996, 68, 2708–2710. [7] a) P. E. Burrows, S. R. Forrest, S. P. Sibley, M. E. Thompson, Appl. Phys. Lett. 1996, 69, 2959–2961; b) Z. Shen, P. E. Burrows, V. Bulovic´, S. R. Forrest, M. E. Thompson, Science 1997, 276, 2009– 2011; c) G. Parthasarathy, G. Gu, S. R. Forrest, Adv. Mater. 1999, 11, 907–910; d) Y. Jiang, J. Lian, S. Chen, H.-S. Kwok, Org. Electron. 2013, 14, 2001–2006. [8] a) V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung, N. Zhu, Angew. Chem. 2005, 117, 3167–3170; Angew. Chem. Int. Ed. 2005, 44, 3107–3110; b) K. M.-C. Wong, L.-L. Hung, W. H. Lam, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2007, 129, 4350–4365; c) V. K.-M. Au, K. M.-C. Wong, D. P.-K. Tsang, M.-Y. Chan, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2010, 132, 14273–14278; d) M.-C. Tang, D. P.-K. Tsang, M. M.-Y. Chan, K. M.-C. Wong, V. W.-W. Yam, Angew. Chem. 2013, 125, 464–467; Angew. Chem. Int. Ed. 2013, 52, 446–449. [9] a) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-W. Yam, Chem. Eur. J. 2011, 17, 130–142; b) V. K.-M. Au, W. H. Lam, W.-T. Wong, V. W.-W. Yam, Inorg. Chem. 2012, 51, 7537–7545. [10] a) J. A. Garg, O. Blacque, K. Venkatesan, Inorg. Chem. 2011, 50, 5430–5441; b) W.-P. To, G. S.-M. Tong, W. Lu, C. Ma, J. Liu, A. L.-F. Chow, C.-M. Che, Angew. Chem. 2012, 124, 2708–2711; Angew. Chem. Int. Ed. 2012, 51, 2654–2657; c) D. A. Smith, D.-A. Ros¸ca, M. Bochmann, Organometallics 2012, 31, 5998–6000; d) D.-A. Ros¸ca, D. A. Smith, M. Bochmann, Chem. Commun. 2012, 48, 7247–7249. [11] a) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2009, 131, 9076–9085; b) J. A. Garg, O. Blacque, T. Fox, K. Venkatesan, Inorg. Chem. 2010, 49, 11463–11472; c) C. Bronner, O. S. Wenger, Dalton Trans. 2011, 40, 12409–12420; d) V. K.-M. Au, N. Zhu, V. W.-W. Yam, Inorg. Chem. 2013, 52, 558–567; e) A. Herbst, C. Bronner, P. Dechambenoit, O. S. Wenger, Organometallics 2013, 32, 1807–1814. [12] W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok, X. Guan, K.-H. Low, C.-M. Che, Angew. Chem. 2013, 125, 6780–6784; Angew. Chem. Int. Ed. 2013, 52, 6648–6652. [13] a) C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 2082– 2084; b) X. H. Yang, F. Jaiser, S. Klinger, D. Neher, Appl. Phys. Lett. 2006, 88, 021107; c) Y. You, S. Y. Park, J. Am. Chem. Soc. 2005, 127, 12438–12439.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2540–2546
