Efficient electrophosphorescence from low-cost copper(I) complex

Optical Materials 29 (2007) 667–671 www.elsevier.com/locate/optmat

Efficient electrophosphorescence from low-cost copper(I) complex Hong Xia, Lin He, Ming Zhang, Ming Zeng, Xiaomeng Wang, Dan Lu, Yuguang Ma

*

Key Lab for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun, 130012, PR China Received 25 April 2005; accepted 29 November 2005 Available online 18 January 2006

Abstract Light-emitting devices, using a high-phosphorescent copper(I) complex [Cu(phen)(POP)]PF6 [POP = Bis-[(2-diphenyl-phosphino)phenyl]ether and phen = 1,10-phenanthroline] as dopant and emitting center have been investigated, in different device architectures involving single layer devices using the blend of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) as host and heterostructure multi layer devices using PVK as host. The maximum luminance of the phosphorescent devices reached 1400 cd/m2 and the highest luminance efficiency exceeded 1 cd/A for single layer devices and higher luminescence efficiency up to 1.8 cd/A for multi layer ones. Efficient electrophosphorescent OLEDs can be developed by using low-cost Cu(I) complex as guest and polymer as host material. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70.a Keywords: Phosphorescence; Polymer; Copper(I) complex; Energy transfer; Charge trapping; Electroluminescence

1. Introduction Extensive research on organic electroluminescence (EL) in recent years has improved both the reliability and the efficiency of LEDs, which has been applied commercially in cell phones. However, new materials with better properties are still in great need to fully realize the advantages that organic and polymer LED technology can potentially offer and to overcome the disadvantages of current materials such as high costs and environment pollution. Heavymetal complexes that enable the otherwise transition from spin-forbidden triplet-state to ground-state (phosphorescence), due to the spin–orbit coupling effect induced by heavy atoms, are attractive for organic light-emitting devices (OLEDs), especially for the improvement of device efficiency [1–13]. It has been demonstrated that efficient electrophosphorescence was obtained from a class of heavy-metal complexes that feature metal-to-ligand*

Corresponding author. Present address: Jilin University, 2699 Qianwei Avenue, Changchun, 130012, PR China. Tel./fax: +86 431 5168480. E-mail address: [email protected] (Y. Ma). 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.015

charge-transfer (MLCT) excited states with relatively short phosphorescence lifetime [2]. Particularly, most of them derive from d6 transition metals ca. Ir(III) [3–7], Ru(II) [8,9], Re(I) [10], and Os(II) [11,12] and few from d8 transition metals ca. Pt(II) [1,13]. The devices based on Ir(ppy)3 (ppy = 2-phenylpyridine) and its derivatives exhibited the highest external quantum efficiency of 19% in all reported EL materials. However, there are still some potential drawbacks of Ir(III)-based devices, including the high costs and the poor resource (the content of metal Ir in the earth’s crust is about 107%). Existing alternatives, such as Ru(II), Re(I) and Os(II) also have the same inherent limitation. Therefore it is worthwhile to develop some alternatives containing cheap metals to circumvent these difficulties. An attractive alternative, Cu(I) coordination compound, is emerging in the form of d10 transition metal compounds. In fact, Cu(I) diimine compounds exhibit MLCT excited state properties that are completely comparable to RuðbpyÞ2þ 3 , a well-known MLCT compound [8,9]. Cu(I) diimine compounds have been found and investigated since the early 1950s [14], however they did not draw the attention of organic EL community due to their low emission

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2. Experimental 2.1. Materials PVK and PBD were purchased from Aldrich and poly(3,4-ethylene dioxythiophene) (PEDOT) from Bayer Chemical Company and used as received. Cu(I) complex [Cu(phen)(POP)]PF6 (inset in Fig. 1) was synthesized by the reaction of [Cu(NCCH3)4]PF6 with bis[2-(diphenylphosphino)phenyl]ether and 1,10-phenanthroline in CH2Cl2 for 3 h at room temperature, and then purified by crystallization as yellow crystals. The structure of [Cu(phen)(POP)]PF6 was confirmed by 1H NMR, IR spectroscopy, elemental analysis, and XRD. 2.2. UV–vis and PL spectra UV–vis absorption spectra were recorded on UV-3100 spectrophotometer. Fluorescence measurements were carried out by RF-5301PC. The films for photoluminescence (PL) experiments were formed on pre-cleaned quartz plate at air atmosphere. Doped PC was dissolved in chloroform at a concentration of 10 mg/ml. 2.3. Preparation of EL devices and testing The LEDs fabricated in this work had two configurations. One was Single layer LEDs with the structure of ITO/PEDOT(100 nm)/40 wt.% PVK–PBD: x wt.% [Cu(phen)(POP)]PF6(80 nm)/LiF(0.5 nm)/Al(200 nm) and the other was heterostructure multi layer LEDs with the

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efficiency (<1%) at room temperature [15–19]. We have reported EL devices based on a high phosphorescence tetranuclear Cu(I) complex Cu4 (C„Cph)4L2 (L = 1,8bis(diphenylphosphino)-3,6-dioxaoctane), but due to the poor energy collection of phosphor in devices, only low efficiency (<0.1%) was obtained [20]. The structure of Cu4(C„Cph)4L2 was too complex to be put into practical application, and further photochemical and device study on Cu4(C„Cph)4L2 showed that an unexpected decomposition occurred upon photo irradiation and in EL process. Another tetrameric copper(I)-amide cluster [CuN(Si(CH3)3)2]4 was also applied as the emitting center of OLEDs with the maximum external quantum efficiency of 0.2% [21]. Recently, a simple mixed-ligand Cu(I) 2,9-dimethyl-1,10-phenanthroline complex [Cu(dmp)(POP)]BF4 [POP = Bis[(2-diphenylphosphino)phenyl]ether] is reported to exhibit good stability, unusually high efficient and long-lived PL in solution [22] (kmax = 570 nm, with the efficiency of ca. 15% and the lifetime of ca. 15 ls), which is comparable to other electrophosphorescent metal complexes such as Ir(ppy)3. We believe [Cu(dmp)(POP)]BF6 and its derivatives have great potentials to be candidates for current electrophosphorescence materials. For this aim, the electroluminescence properties of Cubased OLEDs of different architectures are investigated.

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Fig. 1. Normalized spectra of absorption of [Cu(phen)(POP)]PF6 in solution (CH2Cl2) and photoluminescence of [Cu(phen)(POP)]PF6: polycarbonate (PC) blend film excited at 350 nm, and photoluminescence spectra of PVK and PVK–PBD in thin film excited at 325 nm. Inset: the molecular structure of [Cu(phen)(POP)]+. Anal. Calcd for C48H36CuF6N2OP3 (i.e. [Cu(phen)(POP)]PF6): C, 62.17; H, 3.91; N, 3.02. Found: C, 62.08; H, 3.99; N, 2.92. Crystal data for [Cu(phen)(POP)]PF6 are deposited at the Cambridge Crystallographic Data Centre (CCDC) and the deposition number is CCDC 224679.

structure of ITO/PEDOT(100 nm)/PVK: x wt.% [Cu(phen) (POP)]PF6(80 nm)/PBD(50 nm)/Alq3(45 nm)/LiF(0.5 nm)/ Al(200 nm). Indium–tin-oxide (ITO)-coated glass with a sheet resistance of <50 X/h was used as substrate. The substrate was pre-patterned by photolithography to give an effective device size of 4 mm2. Pre-treatment of ITO includes a routine chemical cleaning using detergent and alcohol in sequence, followed by oxygen plasma cleaning. The poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was coated from a water dispersion yielding a 100 nm thick layer after drying (ca. 105 °C; 5 min). Active layers were spin-coated from chloroform solution containing x wt.% dyes in 10 mg/ml PVK or PVK–PBD blend on PEDOT:PSS to give a film with the thickness of 80–100 nm. The cathode LiF (0.5 nm) and Al (200 nm) was deposited by thermo-evaporation. The electroluminescence was recorded by a PR65-spectrometer. Current–voltage and light intensity measurements were made at room temperature and ambient condition. 3. Results and discussion The absorption and photoluminescence (PL) spectra for [Cu(phen)(POP)]PF6 are given in Fig. 1. The ligand p–p* absorption band is centered at 285 nm and the MLCT singlet absorption band is centered at 385 nm. The diluted chloroform solution of [Cu(phen)(POP)]PF6 shows weak luminescence as excitation at both p–p* and 1MLCT absorption band. However, the crystal of [Cu(phen)(POP)]PF6 and a doped polycarbonate (PC) film shows a bright yellow light emission with the maximum emission

H. Xia et al. / Optical Materials 29 (2007) 667–671

at 558 nm upon excitation at 350 nm at room temperature. Inset in Fig. 1 shows the [Cu(phen)(POP)]PF6 molecule structure. Blue-emitting polymer PVK or polymer blend, PVK with PBD, was selected as the host materials. Excitation of PVK and PVK–PBD at 325 nm, a maximum PL at 410 and 425 nm was observed, respectively. A lower energy level was obtained for PVK–PBD, and the emitting species in PVK–PBD blends is from an exciplex [23]. There is a good overlap between the emission spectrum of PVK or PVK–PBD exciplex and the absorption band of singlet MLCT of [Cu(phen)(POP)]PF6. Such a guest–host system meets the requirement for efficient Fo¨rster energy transfer from the singlet-excited state of the host to a singlet MLCT band of the guest, [Cu(phen)(POP)]PF6 (see Fig. 1). Since PVK is mainly a hole transfer material, 40 wt.% PBD doped into PVK may enhance its electronic transport ability, which has been proven to be a suitable host for phosphorescence guest in single layer device configuration [5,6]. Fig. 2 shows the current–voltage–brightness (J–B– V) curves of ITO/PEDOT/2 wt.% [Cu(phen)(POP)]PF6: PVK–PBD-40 wt.%/LiF/Al device. The turn-on voltage is approximate 10 V and the maximum brightness is 1400 cd/m2 (at 19 V). The inset in Fig. 2 shows the luminance efficiency (LE) as a function of voltage for the single layer devices made from varied [Cu(phen)(POP)]PF6 doping concentrations from 2 to 16 wt. %. The highest LE of 1.04 cd/A (at 15 V) was obtained from the device with 2 wt.% [Cu(phen)(POP)]PF6 in PVK–PBD-40 wt.% at a current density of 21.44 mA/cm2 and a brightness of 200 cd/m2. Even at 60 mA/cm2, the devices retain relatively high LE of 0.95 cd/A (at 17 V). The LE decreases with increasing concentration of [Cu(phen)(POP)]PF6, which probably results from aggregation and self quenching of [Cu(phen)(POP)]PF6 emitter. Significantly, in almost all single layer devices the highest LE occurs near the highest brightness voltage instead of the turn-on voltage like most

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other LEDs [1–3], which indicates the suppressed exciton (triplet–triplet) annihilation of [Cu(phen)(POP)]PF6 in PVK–PBD blend. Fig. 3 shows the EL spectra of the optimized single layer device made from PVK–PBD-40 wt.% with 2 wt.% [Cu(phen)(POP)]PF6. The maximum emission band centered at 576 nm with a fresh yellow light, which can be due to the 3MLCT state emission of [Cu(phen)(POP)]PF6. The inset in Fig. 3 shows the PL spectra of thin films of [Cu(phen)(POP)]PF6: PVK with different doping concentrations ranged from 2 wt.% to 8 wt.%. The host emission appeared in PL spectra is absent in EL spectra at both low and high applied voltage and as the changing of [Cu(phen)(POP)]PF6 doping level from 2 wt.% to 16 wt.%. The phenomena that MLCT contribution in total emission will be enhanced in EL devices has been observed in other electrophosphorescence devices, especially in polymer host systems [5,6,24–26], which has been suggested to be due to the presence of a process of charge trapping and then a recombination with opposite charge carriers in metal complexes [27,28]. We noticed that the turn-on voltage of the devices increased with the increasing of doping concentrations of [Cu(phen)(POP)]PF6 (Table 1), which is consistent with the charge trapping mechanism in the Cu-based LEDs. Furthermore, we tested the performances of heterostructure multi layer devices, in which pure PVK was used as host material. The hole and exciton-block material PBD and electron transfer material AlQ were inserted between the cathode and emitting layer. Such structure is also frequently used in polymer-based electrophosphorescence devices to achieve high efficiency [29,2]. Table 1 summarizes the EL performance of the single layer and the multi layer LEDs based on [Cu(phen)(POP)]PF6. The LE of heterostructure multi layer devices with varied doping concentrations as a function of current density is shown in Fig. 4. 0.0080

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Fig. 2. I–L–V curve of the single layer device with the emitting layer of 2 wt.% [Cu(phen)(POP)]PF6 in 40 wt.% PBD–PVK. Inset: the luminance efficiency of single layer devices with varied [Cu(phen)(POP)]PF6 doping concentrations as a function of voltage.

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Fig. 3. The EL spectra of LEDs made from PVK–PBD (40 wt.%) with 2 wt.% [Cu(phen)(POP)]PF6 at different applied voltages. Inset: the PL spectra of thin films of PVK with different [Cu(phen)(POP)]PF6 concentrations.

H. Xia et al. / Optical Materials 29 (2007) 667–671

Table 1 EL characteristics of devices with dopants of [Cu(phen)(POP)]PF6, including doping concentration of [Cu(phen)(POP)]PF6 by wt.% (c) and turn-on voltage (VTurn-on), maximum brightness (Bmax) and maximum luminance efficiency (LEmax) Device structure

Structure a

Structure b

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19 22 21 21

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15 17 21 20

15.5 18 21 16.5

1.85 0.51 0.73 1.54

7 11 16 10

178 164 165 383

Structure a. single layer device: ITO/PEDOT/[Cu(phen)(POP)]PF6:PVK– PBD(40 wt.%)/LiF/Al. Structure b. multi layer devices: ITO/PEDOT/[Cu(phen)(POP)]PF6:PVK/ PBD/Alq3/LiF/Al.

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While further increasing the Cu(I) complex concentration to 10 wt.% causes a much smaller shift of the I–V curves. This observation is similar to what has been reported by Noh et al. [30] and Neher et al. [31]. Most like, the direct hopping of carriers between Cu dyes becomes possible at higher concentrations, without the need for detrapping to the PVK host [31]. Compared with heterostructure multi layer devices, the ones with the configuration of single layer with PVK–PBD blend as host material are more suitable and stable because the latter structure can balance of electron and hole in emitting layer more efficiently. This indicated that the balance of carriers (electrons and holes) in the emissive layer is very important for this kind of Cubased LEDs (Fig. 5).

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Current density (mA/cm ) Fig. 4. The luminous efficiency of heterostructure multi layer devices with varied [Cu(phen)(POP)]PF6 doping concentrations as a function of current density.

A higher efficiency up to 1.8 cd/A can be achieved in this device configuration, but the maximum LE was obtained at low brightness and decreased rapidly as the driving voltage and brightness increasing. The current density versus voltage characteristics of multi layer devices are shown in Fig. 4. The doping concentration of [Cu(phen)(POP)]PF6 was varied between 0.2% and 10% by weight. The characteristics are rectifying in nature with a turn-on voltages, defines as the voltage at a current density of 60 mA/cm2, of 14.5, 16.5, 20.5 and 15 V for doping concentrations of 0.2%, 1%, 5%, 10% by weight, respectively. The increase the device turn-on voltage is consistent with charge trapping on the [Cu(phen)(POP)]PF6 molecules at doping concentrations of 0.2–5%. The build-up of a space charge induces an electric field that opposes the injection of majority carriers, which in turn increases the turn-on voltage.

4. Conclusion We demonstrated that Cu(I) complex [Cu(phen)(POP)]PF6 can be applied as dopant and emitter in semiconductor polymer layer for the fabrication of carrier-injection-type LEDs. Efficient carrier transporting ability for Cu-complex-dopant system is very important for device performance. The devices with PVK–PBD (40 wt.%) blend were more suitable than PVK-based devices likely due to PBD’s electron transport and hole block ability. The EL spectra show the characteristic spectra of [Cu(phen)(POP)]PF6 with a peak at 576 nm and CIE of (0.44, 0.48). Particularly, the highest efficiency was always achieved near the highest brightness voltage instead of near the turn-on voltage where normal devices usually have their highest efficiency. The highest luminous efficiency of 1.04 cd/A and 1.85 cd/A were achieved in optimized multi-doped single layer and heterostructure multi layer devices, respectively. It is promising to develop such phosphorescence materials containing cheap metals to circumvent the problem of high costs of noble metals.

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Acknowledgement The authors acknowledge support from National Science Foundation of China (Grant No. 20125421, 90101026) and Ministry of Science and Technology of China (Grant No. 2002CB6134003). References [1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151. [2] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [3] C. Adachi, M.A. Baldo, S.R. Forrest, Appl. Phys. Lett. 77 (2000) 904. [4] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999) 2095. [5] X. Gong, M.R. Robinson, J.C. Ostrowski, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Mater. 14 (2002) 581. [6] X. Gong, J.C. Ostrowski, G.C. Bazan, D. Moses, A.J. Heeger, Appl. Phys. Lett. 81 (2002) 3711. [7] S.C. Lo, N.A.H. Male, J.P.J. Markham, S.W. Magennis, P.L. Burn, O.V. Salata, I.D.W. Samuel, Adv. Mater. 14 (2002) 975. [8] S. Welter, K. Brunner, J.W. Hofstraat, L. De Cola, Nature 421 (2003) 54. [9] T.J. Meyer, Acc. Chem. Res. 22 (1989) 163. [10] F. Li, M. Zhang, J. Feng, G. Cheng, Z.J. Wu, Y.G. Ma, S.Y. Liu, J.C. Shen, S.T. Lee, Appl. Phys. Lett. 83 (2003) 365. [11] Y.G. Ma, H.Y. Zhang, C.M. Che, J.C. Shen, Synth. Metal. 94 (1998) 245. [12] X.Z. Jiang, A.K.Y. Jen, B. Carlson, L.R. Dalton, Appl. Phys. Lett. 81 (2002) 3125.

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