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Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(III) with another wide gap iridium(III) complex as sensitizer
Dyes and Pigments 128 (2016) 26e32
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Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(III) with another wide gap iridium(III) complex as sensitizer Yunlong Jiang, Liang Zhou*, Rongzhen Cui, Yanan Li, Xuesen Zhao, Hongjie Zhang** State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2015 Received in revised form 8 January 2016 Accepted 10 January 2016 Available online 15 January 2016
In this work, we report the realization of highly efficient pure red organic co-doped electroluminescent device with tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) and iridium(III) bis(4,6-(difluorophenyl) 0 pyridinato-N,C2 )picolinate (FIrpic) as emitter and sensitizer, respectively. By selecting 4,40 ,400 -Tri(9carbazoyl)triphenylamine and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine as host materials, a series of devices with single or double light-emitting layer(s) were fabricated and investigated. The well-known wide gap blue emitter FIrpic, which possesses low-lying energy levels, was co-doped minutely into electron dominant light-emitting layer. Compared with reference devices, co-doped devices displayed significant improvement of device performances attributed to improved carriers' balance and broadening recombination zone. Finally, the 0.3 wt% co-doped double light-emitting layers device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. Even at the certain brightness of 1000 cd/ m2, EL efficiency as high as 12.06 cd/A can be retained by the same device. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electroluminescence Iridium complex Sensitizer Carrier trapping Energy transfer Recombination zone
1. Introduction Organic light-emitting devices (OLEDs) have attracted great interest throughout the world owing to their potential application in solid-state lighting and in full-color flat panel displays [1e5]. Although the performances of OLEDs have been developed prosperously in recent years, further improvement is still necessary for practical application. Transition metal complexes, which are widely utilized as therapeutic agents or phosphorescent probes for biomolecules [6e13], have been extensively studied as the electroluminescent materials because both singlet and triplet excitons can be harvested and thus 100% internal quantum efficiency was expected [14e16]. For commercial application, especially for full-color display, three primary colors of blue, green and red are basically required [17,18]. Up to now, the major performances of green OLEDs are high enough for industrial application, while red and blue
* Corresponding author. Tel.: þ86 43185262855; fax: þ86 43185698041. ** Corresponding author. Tel.: þ86 43185262127; fax: þ86 43185685653. E-mail addresses: [email protected] (L. Zhou), [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.dyepig.2016.01.011 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
OLEDs remain to be optimized in color purity, efficiency and brightness [19e23]. In the past decades, many groups have devoted their efforts to the design of novel transition metal complexes and the optimization of device structures [24e26]. To realize high performance OLEDs, highly efficient transition metal complexes with matched energy levels, short excited state lifetime, and excellent thermal stability were required. For example, Z. B. Wang et al. have obtained the phosphorescent OLED with external quantum efficiencies above 20% based on a cyclometalated platinum(II) (PtII) complex with a triarylboron group [24]. Chi et al. have realized the orange-red device with the maximum external quantum efficiency, current efficiency and power efficiency of up to 18.3%, 61.0 cd/A and 53.8 lm/W, respectively, by utilizing a novel Os(II) complex Os(pz2py)(PPh2Me)2(CO) (pz2py ¼ 2,6-di(5-trifluoromethylpyrazol-3-yl) pyridine, PPh2Me ¼ diphenylmethylphosphine) as the dopant [25]. F. Dumur et al. utilized heteroleptic cyclometalated iridium(III) (IrIII) complex bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2(acac)) as red dopant and achieved the maximum power efficiency of 7.2 lm/W, the maximum current efficiency of 10.0 cd/A as well as the CIE coordinate of (0.67, 0.33) [26]. Although significant improvement of maximum EL efficiencies has been realized, EL
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efficiencies of most reported devices dropped rapidly attribute to various reasons such as tripletetriplet annihilation, tripletepolaron annihilation, and electric field induced dissociation of excitons [27,28]. The roll-off of efficiency is quite severe in phosphorescent OLEDs due to the relatively long lifetime of triplet excitons, and it detrimentally degrades the device performances for practical applications particularly at high brightness. Therefore, further investigation on how to suppress the roll-off of EL efficiency are most important for the development of red OLEDs based on transition metal complexes. Previously, C.-M. Che et al. have demonstrated an efficient device design strategy to improve the EL performances of red PtII complex by co-doping wide energy gap IrIII complex into electron dominant light-emitting layer (EML) as sensitizer [29]. Experimental results revealed the co-doped IrIII complex molecules function as deeper electron trappers, thus broadening the recombination zone and facilitating the balance of holes and electrons on PtII complex molecules. Due to the matched triplet energies of host material, IrIII complex and PtII complex, the co-doped IrIII complex molecules function as also the energy transfer ladders between host material and PtII complex. Consequently, higher EL performances were obtained by the co-doped devices. Recently, we have significantly enhanced the EL performances of red IrIII complex 0 iridium(III) bis(2-phenylquinoly-N,C2 )dipivaloylmethane (PQ2Ir (dpm)) by utilizing the classical wide band gap trivalent europium complex Eu(TTA)3phen (TTA ¼ thenoyltrifluoroacetone, phen ¼ 1,10-phenanthroline) as sensitizer [30]. Experimental results demonstrated the co-doped Eu(TTA)3phen molecules act as electron trappers and energy transfer ladders, which result in higher EL efficiencies, slower efficiency roll-off, higher brightness, and even higher color purity. Interestingly, wider recombination zone and faster energy transfer of the co-doped devices cause even lower necessary doping concentration of emitter. In this work, we aim to improve the EL performances of red emitter tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) by utilizing the well-known wide gap blue emitter iridium(III) bis(4,60 (difluorophenyl)pyridinato-N,C2 )picolinate (FIrpic) as sensitizer. Based on hole transporting host material 4,40 ,400 -Tri(9-carbazoyl) triphenylamine (TcTa) and electron transporting host material 2,6bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy), a series of single EML or double-EMLs devices were firstly fabricated and compared in order to optimize the doping concentration of Ir(piq)3.
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Single-EML and double-EMLs devices realized the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 20,378 and 24,931 cd/m2, 14.85 and 13.96 cd/A, 13.10 and 12.18 lm/W, 8.9% and 8.7%, respectively. Then, a series of co-doped devices with FIrpic at different co-doping concentrations were fabricated and measured. Experimental results demonstrated that co-doping FIrpic as sensitizer is efficient in delaying the roll-off of EL efficiency thus in realizing higher brightness. Finally, the 0.3 wt% co-doped double-EMLs device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. At the certain brightness of 1000 cd/m2, the same device retained the EL efficiency as high as 12.06 cd/A. 2. Experimental All the organic materials used in this study were obtained commercially and used as received without further purification. ITO coated glass with the sheet resistance of 10 U/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water, dried in an oven and finally treated with oxygen plasma for 10 min at a pressure of 10 Pa to enhance the surface work function of ITO anode (from 4.7 to 5.1 eV) [31]. All organic layers were deposited with the rate of 0.1 nm/s under high vacuum (3.0 105 Pa). The doped and co-doped EMLs were prepared by co-evaporating dopant(s) and host material from two or three individual sources, and the doping concentration was modulated by controlling the evaporation rate of dopant(s). LiF and Al were deposited in another vacuum chamber (8.0 105 Pa) with the rates of 0.01 and 1.0 nm/s, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten 9 mm2 devices on each substrate. Current densityevoltageebrightness (JeVeB) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer. The external quantum efficiency of EL device was calculated based on the photo energy measured by the
Fig. 1. Proposed energy levels diagram of the designed OLEDs in this study (left). Molecular structures of Ir(piq)3 and FIrpic (right).
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Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Fig. 2. EL efficiency-current density characteristics of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations. Insert: Current densityebrightnessevoltage characteristic of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations.
photodiode, the EL spectrum, and the current pass through the device. 3. Results and discussion Fig. 1 depicted the device structure and the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels diagram of the designed OLEDs. In this case, di-[4(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) was chosen as hole transfer layer (HTL) and electron block layer (EBL) due to its high hole mobility (1 102 cm2 V1 s1) and high-lying LUMO level (1.8 eV) [32]. Meanwhile, 1,3,5-Tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB), which posses the high electron mobility (1 103 cm2 V1 s1) and low-lying HOMO level (6.7 eV), was used as hole block/electron transport layer (HBL/ETL) [33]. For single-EML devices, 26DCzPPy was utilized as the host material; for double-EMLs devices, TcTa and 26DCzPPy were chosen as the host materials of EML1 and EML2, respectively. The well-known red IrIII complex Ir(piq)3 was selected as emitter due to its pure red emission characteristic, and the widely used wide gap blue emitter FIrpic was utilized as sensitizer due to its low lying energy levels [34,35]. The molecular structures of Ir(piq)3 and FIrpic were also shown in Fig. 1. For single-EML devices, the stepwise LUMO levels of TmPyPB (2.7 eV) and 26DCzPPy (2.6 eV) are beneficial for the injection
and transport of electrons, while the large HOMO level difference (0.6 eV) between TAPC (5.5 eV) and 26DCzPPy (6.1 eV) causes the accumulation of holes within HTL near the interface of HTL/EML and thus the narrow recombination zone. For double-EMLs devices, the stepwise HOMO levels of TAPC (5.5 eV), TcTa (5.7 eV) and 26DCzPPy (6.1 eV) are advantageous for the injection and transport of holes, while the stepwise LUMO levels of TmPyPB (2.7 eV), 26DCzPPy (2.6 eV) and TcTa (2.4 eV) are advantageous for the injection and transport of electrons [32,33,36]. Therefore, balanced carriers' distribution and wide recombination zone could be expected. In addition, the high-lying LUMO level of TAPC (which is 0.6 eV higher than that of TcTa) and the low-lying HOMO level of TmPyPB (which is 0.6 eV lower than that of 26DCzPPy) would well confine the recombination of holes and electrons within EMLs [37]. Firstly, to determine the optimal doping concentration of Ir(piq)3, three single-EML devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (x wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) and three double-EMLs devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (x wt%):TcTa (10 nm)/Ir(piq)3 (x wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) were fabricated and investigated by controlling x to be 3, 4 and 5, respectively. Fig. 2 depicted the doping concentration dependence of EL efficiency-current density characteristics of these devices, and the inset of Fig. 2 depicted the doping concentration dependence of current densityebrightnessevoltage characteristics of these devices. Higher brightness was realized by the double-EMLs devices, although higher EL efficiencies were realized by the single-EML devices. The key properties of these devices were listed in Table 1, the 4 wt% doped single-EML device obtained the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 20,378 cd/m2, 14.85 cd/A, 13.10 lm/W and 8.9%, respectively, while the 4 wt% doped double-EMLs device obtained the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 24,931 cd/m2, 13.96 cd/A, 12.18 lm/W and 8.7%, respectively. EL spectra of these devices operating at the current density of 10 mA/cm2 were given in Fig. 3. Apart from the characteristic emission (615 nm) of Ir(piq)3, another weak emission peaked at about 390 nm, which originates from 26DCzPPy, was also observed in both 3 wt% doped sing-EML device and 3 wt% doped doubleEMLs device. The emergence of 26DCzPPy emission implies the recombination of holes and electrons on 26DCzPPy molecules. In other words, the doping concentration of 3 wt% is not high enough for Ir(piq)3 molecules to trap all the holes and electrons within EML(s). However, 4 wt% and 5 wt% doped devices displayed the similar EL spectra without discernible 26DCzPPy emission. These results demonstrated that 4 wt% is the optimal doping concentration of Ir(piq)3.
Table 1 Key properties of single-EML and double-EMLs devices with Ir(piq)3 at different concentrations. Device
Vturn-on (V)
Ba (cd/m2)
hc b (EQEc) (cd/A)
hp d (lm/W)
hc e (cd/A) (EQEf) (1000 cd/m2)
CIEx,
S-3% S-4% S-5% D-3% D-4% D-5%
3.7 3.5 3.5 3.8 3.6 3.6
16,352 20,378 19,204 22,040 24,931 23,701
15.97 14.85 12.30 13.94 13.96 12.15
13.19 13.10 9.55 8.71 12.18 8.92
11.86 11.02 10.15 12.64 10.81 10.20
(0.622, (0.647, (0.646, (0.644, (0.654, (0.656,
a b c d e f g
(9.2%) (8.9%) (7.4%) (8.4%) (8.7%) (7.8%)
The data for maximum brightness (B). Maximum current efficiency (hc). External quantum efficiency (EQE). Maximum power efficiency (hp). Current efficiency (hc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
(6.9%) (6.6%) (6.1%) (7.6%) (6.7%) (6.5%)
y
g
0.340) 0.345) 0.345) 0.343) 0.344) 0.341)
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Fig. 3. EL spectra of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations operating at the current density of 10 mA/cm2.
To further improve the EL performances of Ir(piq)3, the widely used wide gap blue emitter FIrpic, which has the low-lying HOMO (6.15 eV) and LUMO (3.47 eV) levels, was selected as sensitizer
Fig. 4. (a) EL efficiencyecurrent density characteristics of single-EML devices with FIrpic at different doping concentrations. Insert: Current densityebrightnessevoltage characteristic of single-EML devices with FIrpic at different doping concentrations. (b) EL spectra of single-EML devices with FIrpic at different doping concentrations operating at the current density of 10 mA/cm2.
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and minutely co-doped into the electron dominant EML. To optimize the co-doping concentration of FIrpic, two single-EML codoped devices with the structure of ITO/TAPC (30 nm)/FIrpic (y wt %):Ir(piq)3 (4 wt%):26DCzPPy(10 nm)/TmPyPB (60 nm)/LiF (1 nm)/ Al (100 nm) and two double-EMLs co-doped devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (4 wt%):TcTa (10 nm)/FIrpic (y wt%):Ir(piq)3 (4 wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) were fabricated and investigated by controlling y to be 0.2 and 0.3, respectively. As been demonstrated in a previous paper, the co-doped FIrpic molecules function as electron trapper and energy-transfer ladder as a result of its low-lying LUMO level and matched triplet energy level [29]. On the other hand, the HOMO level (6.15 eV) of FIrpic is even 0.05 eV lower than that of 26DCzPPy (6.1 eV); therefore, the transport of holes to FIrpic molecules would be insignificant. As shown in Figs. 4(a) and 5(a), both single-EML and doubleEMLs co-doped devices displayed significantly delayed roll-off of EL efficiencies, thus the enhanced maximum brightness. No significant improvement of maximum current efficiency was observed in co-doped single-EML devices. With increasing co-doping concentration of FIrpic, the maximum current efficiency of doubleEMLs devices decreases firstly and then increases rapidly. The key properties of these co-doped devices were listed in Table 2. The 0.3 wt% double-EMLs co-doped device realized the maximum
Fig. 5. (a) EL efficiencyecurrent density characteristics of double-EMLs devices with FIrpic at different doping concentrations. Inset: Current densityebrightnessevoltage characteristic of double-EMLs devices with FIrpic at different doping concentrations. (b) EL spectra of double-EMLs devices with FIrpic at different doping concentrations operating at the current density of 10 mA/cm2.
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Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Table 2 Key properties of single-EML and double-EMLs devices with FIrpic at different co-doping concentrations. Device
Vturn-on (V)
Ba (cd/m2)
hc b (EQEc) (cd/A)
hp d (lm/W)
hc e (cd/A) (EQEf) (1000 cd/m2)
CIEx,
S-0% S-0.2% S-0.3% D-0% D-0.2% D-0.3%
3.5 3.5 3.6 3.6 3.6 3.6
20,378 22,723 22,218 24,931 25,382 28,031
14.85 13.00 13.89 13.96 13.54 14.89
13.10 8.09 9.59 12.18 9.88 12.99
11.02 11.56 12.35 10.81 11.36 12.06
(0.647, (0.630, (0.616, (0.654, (0.649, (0.644,
a b c d e f g
(8.9%) (7.7%) (8.1%) (8.7%) (8.3%) (9.0%)
(6.6%) (6.8%) (7.2%) (6.7%) (7.0%) (7.3%)
y
g
0.345) 0.345) 0.344) 0.344) 0.344) 0.345)
The data for maximum brightness (B). Maximum current efficiency (hc). External quantum efficiency (EQE). Maximum power efficiency (hp). Current efficiency (hc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. At 1000 cd/m2, this device retained the EL efficiency of 12.06 cd/A. To our best knowledge, EL performances of
this device amongst the best results of the previously reported devices based on Ir(piq)3. As shown in Figs. 4(b) and 5(b), besides the characteristic emission of Ir(piq)3 and 26DCzPPy, another weak emission peak
Fig. 6. (a) Schematic representation of carriers' distribution in single-EML devices with the increasing co-doping concentration of FIrpic. (b) Schematic representation of carriers' distribution in double-EMLs devices with the increasing co-doping concentration of FIrpic. The dot and solid lines within EML represent the HOMO and LUMO levels of dopants and hosts, respectively. Symbols and þ represent electrons and holes, respectively.
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
(472 nm) originate from FIrpic was also observed. Compared with single-EML co-doped devices, double-EMLs co-doped devices displayed relatively weaker FIrpic emission. This phenomenon suggests it is difficult for holes to transfer to FIrpic molecules due to the low-lying HOMO level and low concentration of FIrpic. With increasing co-doping concentration of FIrpic, the relative intensity of FIrpic emission increased slightly. These experimental results demonstrated the efficacy of selecting FIrpic as sensitizer in improving the EL performances of Ir(piq)3. To make clear the EL mechanisms of these co-doped devices, the detailed injection and transport processes of holes and electrons were investigated. Considering the HOMO and LUMO levels of Ir(piq)3 (5.2 and 3.0 eV) are within those of TcTa and 26DCzPPy, carrier trapping was conceived to be the dominant EL mechanism of these devices. For single-EML devices, holes and electrons will be injected from anode and cathode into EML via HTL and ETL, respectively. For double-EMLs devices, holes will be injected from anode into EML1 via HTL, and some holes could be injected into EML2. On the other hand, electrons will be injected from cathode into EML2 via ETL, and some electrons could be injected into EML1. In addition, holes and electrons will be well confined within EML(s) due to the low-lying HOMO level of ETL and the high-lying LUMO level of HTL. The LUMO level (3.47 eV) of FIrpic is 0.77 eV lower than that (2.7 eV) of Ir(piq)3, therefore electrons will be preferentially trapped by FIrpic molecules within co-doped EML. Consequently, as been demonstrated in a previous paper, the co-doped FIrpic molecules will delay the transport of electrons within the electron dominant EML, thus broadens the recombination zone of holes and electrons in some degree. As a result, the decreased exciton density is helpful in suppressing tripletetriplet annihilation, thus in delaying the roll-off of EL efficiencies. To better understand the mechanisms of EL performance improvement in these co-doped devices, distribution of holes and electrons within the EML(s) of these devices were also analyzed. For single-EML devices, as shown in Fig. 6(a), electrons will be the major carriers within EML due to the relatively higher electron mobility of 26DCzPPy and the higher energy barrier between HTL and EML. For double-EMLs devices, as shown in Fig. 6(b), holes and electrons would be the major carriers within EML1 and EML2, respectively, which results the unbalanced carriers' distribution on Ir(piq)3 molecules. For double-EMLs devices, the presence of Ir(piq)3 molecules within EML1 would trap most holes, thus decrease the accumulation of holes within EML1. Therefore, the double-EMLs co-doped devices displayed relatively weaker FIrpic emission due to the lower hole density on 26DCzPPy molecules. In this case, the co-doped FIrpic molecules within EML function as electron trapper and trap some electrons. Therefore, improved balance of holes and electrons on Ir(piq)3 molecules can be realized by optimizing the co-doping concentration of FIrpic. As a result, significant improvement of EL performances has been obtained by the co-doped devices. 4. Conclusions In summary, we have further confirmed the efficacy of selecting FIrpic as sensitizer in improving the EL performances of redemitting materials. By co-doping FIrpic and red-emitting IrIII complex Ir(piq)3 into the electron dominant EML, a series of singleEML or double-EMLs devices were fabricated and investigated. Compared with reference devices, these co-doped devices displayed higher brightness and slower roll-off of efficiencies attributed to improved carriers' balance and broadening recombination zone. By optimizing the doping concentration of FIrpic to be 0.3 wt %, bright pure red EL device with maximum brightness, current efficiency, power efficiency and external quantum efficiency up to
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28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively, was realized. Even at the certain brightness of 1000 cd/m2, current efficiency as high as 12.06 cd/A can be retained by the same device. Acknowledgements The authors are grateful to the financial aid from Research Equipment Development Project of Chinese Academy of Sciences (YZ201562), Youth Innovation Promotion Association CAS (2013150), National Natural Science Foundation of China (Grant Nos. 21201161, 21521092, 91122030 and 21210001), National Key Basic Research Program of China (No. 2014CB643802), and Jilin Provincial Science and Technology Development Program of China (20130522125JH). References [1] Kim SH, Jang J, Lee JY. High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer. Appl Phys Lett 2007;90:223505. [2] Kang DM, Kang J-W, Park JW, Jung SO, Lee S-H, Park H-D, et al. Iridium complexes with cyclometalated 2-cycloalkenyl-pyridine ligands as highly efficient emitters for organic light-emitting diodes. Adv Mater 2008;20: 2003e7. €uerle P. The electroluminescence of organic materials. J Mater [3] Mitschke U, Ba Chem 2000;10:1471e507. [4] Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J Appl Phys 2001;90:5048e51. [5] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998;395:151e4. [6] Leung C-H, Chan DS-H, He H-Z, Cheng Z, Yang H, Ma D-L. Survey and summary: luminescent detection of DNA-binding proteins. Nucleic Acids Res 2012;40:941e55. [7] Ma D-L, Chan DS-H, Leung C-H. Group 9 organometallic compounds for therapeutic and bioanalytical applications. Acc Chem Res 2014;47:3614e31. [8] Lu L, Chan DS-H, Kwong DWJ, He H-Z, Leung C-H, Ma D-L. Detection of nicking endonuclease activity using a G-quadruplex-selective luminescent switch-on probe. Chem Sci 2014;5:4561e8. [9] Leung C-H, Zhong H-J, Chan DS-H, Ma D-L. Review: bioactive iridium and rhodium complexes as therapeutic agents. Coord Chem Rev 2013;257: 1764e76. [10] Liu L-J, Lu L, Zhong H-J, He B, Kwong DWJ, Ma D-L, et al. An iridium(III) complex inhibits JMJD2 activities and acts as a potential epigenetic modulator. J Med Chem 2015;58:6697e703. [11] Zhong H-J, Lu L, Leung K-H, Wong CCL, Peng C, Yan S-C, et al. An iridium(III)based irreversible proteineprotein interaction inhibitor of BRD4 as a potent anticancer agent. Chem Sci 2015;6:5400e8. [12] He H-Z, Wang M, Chan DS-H, Leung C-H, Lin X, Lin J-M, et al. A parallel Gquadruplex-selective luminescent probe for the detection of nanomolar calcium(II) ion. Methods 2013;64:212e7. [13] Lu L, He H-Z, Zhong H-J, Liu L-J, Chan DS-H, Leung C-H, et al. Luminescent detection of human serum albumin in aqueous solution using a cyclometallated iridium(III) complex. Sens Actuators B Chem 2014;201:177e84. [14] Seo JH, Lee SJ, Seo BM, Moon SJ, Lee KH, Park JK, et al. White organic lightemitting diodes showing nearly 100% internal quantum efficiency. Org Electron 2010;11:1759e66. [15] Kim S-Y, Kim J-H, Ha Y, Lee S-H, Seo J-H, Kim Y-K. A study on the characteristics of OLEDs using Ir complex for blue phosphorescence. Curr Appl Phys 2007;7:380e3. e J, Gigmes D, Dumur F. Efficient [16] Lepeltier M, Morlet-Savary F, Graff B, Laleve blue green organic light-emitting devices based on a monofluorinated heteroleptic irdium(III) complex. Synth Met 2015;199:139e46. [17] Wang Q, Ding J, Ma D, Cheng Y, Wang L, Jing X, et al. Harvesting exciton via two parallel channels for efficient white organic LEDs with nearly 100% internal quantum efficiency: fabrication and emission-mechanism analysis. Adv Funct Mater 2009;19:84e95. [18] Tsai Y-C, Jou J-H. Long-lifetime, high-efficiency white organic light-emitting diodes with mixed host composing double emission layers. Appl Phys Lett 2006;89:243521. [19] Lee N-J, Jeon JH, In I, Lee J-H, Suh MC. Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode. Dyes Pigments 2014;101:221e8. [20] Yoo SI, Yoon JA, Kim NH, Kim JW, Kang JS, Moon CB, et al. Improvement of efficiency roll-off in blue phosphorescence OLED using double dopants emissive layer. J Lumin 2015;160:346e50. [21] Kim K-S, Jeon Y-M, Kim J-W, Lee C-W, Gong M-S. Blue light-emitting OLED using new spiro[fluorine-7,90 -benzofluorene] host and dopant materials. Org Electron 2008;9:797e804.
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[22] Zhu M, Li Y, Jiang B, Gong S, Wu H, Qin J, et al. Efficient saturated red electrophosphorescence by using solution-processed 1-phenylisoquinoline-based iridium phosphors with peripheral functional encapsulation. Org Electron 2015;26:400e7. [23] Lindla F, Boesing M, Van Gemmern P, Bertram D, Keiper D, Heuken M, et al. Employing exciton transfer molecules to increase the lifetime of phosphorescent red organic light emitting diodes. Appl Phys Lett 2011;98:173304. [24] Wang ZB, Helander MG, Hudson ZM, Qiu J, Wang S, Lu ZH. Pt(II) complex based phosphorescent organic light emitting diodes with external quantum efficiencies above 20%. Appl Phys Lett 2011;98:213301. [25] Liao J-L, Chi Y, Su Y-D, Huang H-X, Chang C-H, Liu S-H, et al. Os(II) metal phosphors bearing tridentate 2,6-di(pyrazol-3-yl)pyridine chelate: synthetic design, characterization and application in OLED fabrication. J Mater Chem C 2014;2:6269e82. I, Bertin D, et al. Red [26] Lepeltier M, Dumur F, Wantz G, Vila N, Mbomekalle phosphorescent organic light-emitting diodes (PhOLEDs) based on a heteroleptic cyclometalated Iridium(III) complex. J Lumin 2013;143:145e9. [27] Jeon WS, Park TJ, Kim SY, Pode R, Jang J, Kwon JH. Low roll-off efficiency green phosphorescent organic light-emitting devices with simple double emissive layer structure. Appl Phys Lett 2008;93:063303. _ [28] Kalinowski J, Stampor W, Me˛ zyk J, Cocchi M, Virgili D, Fattori V, et al. Quenching effects in organic electrophosphorescence. Phys Rev B 2002;66: 235321. [29] Zhou L, Kwong C-L, Kwok C-C, Cheng G, Zhang H, Che C-M. Efficient red electroluminescent devices with sterically hindered phosphorescent platinum(II) schiff base complexes and iridium complex codopant. ChemdAsian J 2014;9:2984e94.
[30] Zhou L, Li L, Jiang Y, Cui R, Li Y, Zhao X, et al. Rare earth complex as electron trapper and energy transfer ladder for efficient red iridium complex based electroluminescent devices. Appl Mater Interfaces 2015;7: 16046e53. [31] Chan I-M, Cheng W-C, Hong FC. Enhanced performance of organic lightemitting devices by atmospheric plasma treatment of indium tin oxide surfaces. Appl Phys Lett 2002;80:13e5. [32] Lee J, Chopra N, Eom S-H, Zheng Y, Xue J, So F, et al. Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices. Appl Phys Lett 2008;93: 123306. [33] Su S-J, Chiba T, Takeda T, Kido J. Pyridine-containing triphenylbenzene derivatives with high electron mobility for highly efficient phosphorescent OLEDs. Adv Mater 2008;20:2125e30. [34] Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Lee H-E, Adachi C, et al. Highly phosphorescent bis-cyclometalated iridium complexes: synthesis, photophysical characterization, and use in organic light emitting diodes. J Am Chem Soc 2001;123:4305e12. [35] Tsuboyama A, Iwawaki H, Furugori M, Mukaide T, Kamatani J, Igawa S, et al. Homoleptic cyclometalated iridium complexes with highly efficient red phosphorescence and application to organic light-emitting diode. J Am Chem Soc 2003;125:12971e9. [36] Jeon SO, Jang SE, Son HS, Lee JY. External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes. Adv Mater 2011;23: 1436e41. [37] Parker ID. Carrier tunneling and device characteristics in polymer lightemitting diodes. J Appl Phys 1994;75:1656e66.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(III) with another wide gap iridium(III) complex as sensitizer Yunlong Jiang, Liang Zhou*, Rongzhen Cui, Yanan Li, Xuesen Zhao, Hongjie Zhang** State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2015 Received in revised form 8 January 2016 Accepted 10 January 2016 Available online 15 January 2016
In this work, we report the realization of highly efficient pure red organic co-doped electroluminescent device with tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) and iridium(III) bis(4,6-(difluorophenyl) 0 pyridinato-N,C2 )picolinate (FIrpic) as emitter and sensitizer, respectively. By selecting 4,40 ,400 -Tri(9carbazoyl)triphenylamine and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine as host materials, a series of devices with single or double light-emitting layer(s) were fabricated and investigated. The well-known wide gap blue emitter FIrpic, which possesses low-lying energy levels, was co-doped minutely into electron dominant light-emitting layer. Compared with reference devices, co-doped devices displayed significant improvement of device performances attributed to improved carriers' balance and broadening recombination zone. Finally, the 0.3 wt% co-doped double light-emitting layers device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. Even at the certain brightness of 1000 cd/ m2, EL efficiency as high as 12.06 cd/A can be retained by the same device. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electroluminescence Iridium complex Sensitizer Carrier trapping Energy transfer Recombination zone
1. Introduction Organic light-emitting devices (OLEDs) have attracted great interest throughout the world owing to their potential application in solid-state lighting and in full-color flat panel displays [1e5]. Although the performances of OLEDs have been developed prosperously in recent years, further improvement is still necessary for practical application. Transition metal complexes, which are widely utilized as therapeutic agents or phosphorescent probes for biomolecules [6e13], have been extensively studied as the electroluminescent materials because both singlet and triplet excitons can be harvested and thus 100% internal quantum efficiency was expected [14e16]. For commercial application, especially for full-color display, three primary colors of blue, green and red are basically required [17,18]. Up to now, the major performances of green OLEDs are high enough for industrial application, while red and blue
* Corresponding author. Tel.: þ86 43185262855; fax: þ86 43185698041. ** Corresponding author. Tel.: þ86 43185262127; fax: þ86 43185685653. E-mail addresses: [email protected] (L. Zhou), [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.dyepig.2016.01.011 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
OLEDs remain to be optimized in color purity, efficiency and brightness [19e23]. In the past decades, many groups have devoted their efforts to the design of novel transition metal complexes and the optimization of device structures [24e26]. To realize high performance OLEDs, highly efficient transition metal complexes with matched energy levels, short excited state lifetime, and excellent thermal stability were required. For example, Z. B. Wang et al. have obtained the phosphorescent OLED with external quantum efficiencies above 20% based on a cyclometalated platinum(II) (PtII) complex with a triarylboron group [24]. Chi et al. have realized the orange-red device with the maximum external quantum efficiency, current efficiency and power efficiency of up to 18.3%, 61.0 cd/A and 53.8 lm/W, respectively, by utilizing a novel Os(II) complex Os(pz2py)(PPh2Me)2(CO) (pz2py ¼ 2,6-di(5-trifluoromethylpyrazol-3-yl) pyridine, PPh2Me ¼ diphenylmethylphosphine) as the dopant [25]. F. Dumur et al. utilized heteroleptic cyclometalated iridium(III) (IrIII) complex bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2(acac)) as red dopant and achieved the maximum power efficiency of 7.2 lm/W, the maximum current efficiency of 10.0 cd/A as well as the CIE coordinate of (0.67, 0.33) [26]. Although significant improvement of maximum EL efficiencies has been realized, EL
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
efficiencies of most reported devices dropped rapidly attribute to various reasons such as tripletetriplet annihilation, tripletepolaron annihilation, and electric field induced dissociation of excitons [27,28]. The roll-off of efficiency is quite severe in phosphorescent OLEDs due to the relatively long lifetime of triplet excitons, and it detrimentally degrades the device performances for practical applications particularly at high brightness. Therefore, further investigation on how to suppress the roll-off of EL efficiency are most important for the development of red OLEDs based on transition metal complexes. Previously, C.-M. Che et al. have demonstrated an efficient device design strategy to improve the EL performances of red PtII complex by co-doping wide energy gap IrIII complex into electron dominant light-emitting layer (EML) as sensitizer [29]. Experimental results revealed the co-doped IrIII complex molecules function as deeper electron trappers, thus broadening the recombination zone and facilitating the balance of holes and electrons on PtII complex molecules. Due to the matched triplet energies of host material, IrIII complex and PtII complex, the co-doped IrIII complex molecules function as also the energy transfer ladders between host material and PtII complex. Consequently, higher EL performances were obtained by the co-doped devices. Recently, we have significantly enhanced the EL performances of red IrIII complex 0 iridium(III) bis(2-phenylquinoly-N,C2 )dipivaloylmethane (PQ2Ir (dpm)) by utilizing the classical wide band gap trivalent europium complex Eu(TTA)3phen (TTA ¼ thenoyltrifluoroacetone, phen ¼ 1,10-phenanthroline) as sensitizer [30]. Experimental results demonstrated the co-doped Eu(TTA)3phen molecules act as electron trappers and energy transfer ladders, which result in higher EL efficiencies, slower efficiency roll-off, higher brightness, and even higher color purity. Interestingly, wider recombination zone and faster energy transfer of the co-doped devices cause even lower necessary doping concentration of emitter. In this work, we aim to improve the EL performances of red emitter tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) by utilizing the well-known wide gap blue emitter iridium(III) bis(4,60 (difluorophenyl)pyridinato-N,C2 )picolinate (FIrpic) as sensitizer. Based on hole transporting host material 4,40 ,400 -Tri(9-carbazoyl) triphenylamine (TcTa) and electron transporting host material 2,6bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy), a series of single EML or double-EMLs devices were firstly fabricated and compared in order to optimize the doping concentration of Ir(piq)3.
27
Single-EML and double-EMLs devices realized the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 20,378 and 24,931 cd/m2, 14.85 and 13.96 cd/A, 13.10 and 12.18 lm/W, 8.9% and 8.7%, respectively. Then, a series of co-doped devices with FIrpic at different co-doping concentrations were fabricated and measured. Experimental results demonstrated that co-doping FIrpic as sensitizer is efficient in delaying the roll-off of EL efficiency thus in realizing higher brightness. Finally, the 0.3 wt% co-doped double-EMLs device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. At the certain brightness of 1000 cd/m2, the same device retained the EL efficiency as high as 12.06 cd/A. 2. Experimental All the organic materials used in this study were obtained commercially and used as received without further purification. ITO coated glass with the sheet resistance of 10 U/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water, dried in an oven and finally treated with oxygen plasma for 10 min at a pressure of 10 Pa to enhance the surface work function of ITO anode (from 4.7 to 5.1 eV) [31]. All organic layers were deposited with the rate of 0.1 nm/s under high vacuum (3.0 105 Pa). The doped and co-doped EMLs were prepared by co-evaporating dopant(s) and host material from two or three individual sources, and the doping concentration was modulated by controlling the evaporation rate of dopant(s). LiF and Al were deposited in another vacuum chamber (8.0 105 Pa) with the rates of 0.01 and 1.0 nm/s, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten 9 mm2 devices on each substrate. Current densityevoltageebrightness (JeVeB) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer. The external quantum efficiency of EL device was calculated based on the photo energy measured by the
Fig. 1. Proposed energy levels diagram of the designed OLEDs in this study (left). Molecular structures of Ir(piq)3 and FIrpic (right).
28
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Fig. 2. EL efficiency-current density characteristics of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations. Insert: Current densityebrightnessevoltage characteristic of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations.
photodiode, the EL spectrum, and the current pass through the device. 3. Results and discussion Fig. 1 depicted the device structure and the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels diagram of the designed OLEDs. In this case, di-[4(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) was chosen as hole transfer layer (HTL) and electron block layer (EBL) due to its high hole mobility (1 102 cm2 V1 s1) and high-lying LUMO level (1.8 eV) [32]. Meanwhile, 1,3,5-Tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB), which posses the high electron mobility (1 103 cm2 V1 s1) and low-lying HOMO level (6.7 eV), was used as hole block/electron transport layer (HBL/ETL) [33]. For single-EML devices, 26DCzPPy was utilized as the host material; for double-EMLs devices, TcTa and 26DCzPPy were chosen as the host materials of EML1 and EML2, respectively. The well-known red IrIII complex Ir(piq)3 was selected as emitter due to its pure red emission characteristic, and the widely used wide gap blue emitter FIrpic was utilized as sensitizer due to its low lying energy levels [34,35]. The molecular structures of Ir(piq)3 and FIrpic were also shown in Fig. 1. For single-EML devices, the stepwise LUMO levels of TmPyPB (2.7 eV) and 26DCzPPy (2.6 eV) are beneficial for the injection
and transport of electrons, while the large HOMO level difference (0.6 eV) between TAPC (5.5 eV) and 26DCzPPy (6.1 eV) causes the accumulation of holes within HTL near the interface of HTL/EML and thus the narrow recombination zone. For double-EMLs devices, the stepwise HOMO levels of TAPC (5.5 eV), TcTa (5.7 eV) and 26DCzPPy (6.1 eV) are advantageous for the injection and transport of holes, while the stepwise LUMO levels of TmPyPB (2.7 eV), 26DCzPPy (2.6 eV) and TcTa (2.4 eV) are advantageous for the injection and transport of electrons [32,33,36]. Therefore, balanced carriers' distribution and wide recombination zone could be expected. In addition, the high-lying LUMO level of TAPC (which is 0.6 eV higher than that of TcTa) and the low-lying HOMO level of TmPyPB (which is 0.6 eV lower than that of 26DCzPPy) would well confine the recombination of holes and electrons within EMLs [37]. Firstly, to determine the optimal doping concentration of Ir(piq)3, three single-EML devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (x wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) and three double-EMLs devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (x wt%):TcTa (10 nm)/Ir(piq)3 (x wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) were fabricated and investigated by controlling x to be 3, 4 and 5, respectively. Fig. 2 depicted the doping concentration dependence of EL efficiency-current density characteristics of these devices, and the inset of Fig. 2 depicted the doping concentration dependence of current densityebrightnessevoltage characteristics of these devices. Higher brightness was realized by the double-EMLs devices, although higher EL efficiencies were realized by the single-EML devices. The key properties of these devices were listed in Table 1, the 4 wt% doped single-EML device obtained the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 20,378 cd/m2, 14.85 cd/A, 13.10 lm/W and 8.9%, respectively, while the 4 wt% doped double-EMLs device obtained the maximum brightness, current efficiencies, power efficiencies and external quantum efficiencies of 24,931 cd/m2, 13.96 cd/A, 12.18 lm/W and 8.7%, respectively. EL spectra of these devices operating at the current density of 10 mA/cm2 were given in Fig. 3. Apart from the characteristic emission (615 nm) of Ir(piq)3, another weak emission peaked at about 390 nm, which originates from 26DCzPPy, was also observed in both 3 wt% doped sing-EML device and 3 wt% doped doubleEMLs device. The emergence of 26DCzPPy emission implies the recombination of holes and electrons on 26DCzPPy molecules. In other words, the doping concentration of 3 wt% is not high enough for Ir(piq)3 molecules to trap all the holes and electrons within EML(s). However, 4 wt% and 5 wt% doped devices displayed the similar EL spectra without discernible 26DCzPPy emission. These results demonstrated that 4 wt% is the optimal doping concentration of Ir(piq)3.
Table 1 Key properties of single-EML and double-EMLs devices with Ir(piq)3 at different concentrations. Device
Vturn-on (V)
Ba (cd/m2)
hc b (EQEc) (cd/A)
hp d (lm/W)
hc e (cd/A) (EQEf) (1000 cd/m2)
CIEx,
S-3% S-4% S-5% D-3% D-4% D-5%
3.7 3.5 3.5 3.8 3.6 3.6
16,352 20,378 19,204 22,040 24,931 23,701
15.97 14.85 12.30 13.94 13.96 12.15
13.19 13.10 9.55 8.71 12.18 8.92
11.86 11.02 10.15 12.64 10.81 10.20
(0.622, (0.647, (0.646, (0.644, (0.654, (0.656,
a b c d e f g
(9.2%) (8.9%) (7.4%) (8.4%) (8.7%) (7.8%)
The data for maximum brightness (B). Maximum current efficiency (hc). External quantum efficiency (EQE). Maximum power efficiency (hp). Current efficiency (hc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
(6.9%) (6.6%) (6.1%) (7.6%) (6.7%) (6.5%)
y
g
0.340) 0.345) 0.345) 0.343) 0.344) 0.341)
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Fig. 3. EL spectra of single-EML and double-EMLs devices with Ir(piq)3 at different doping concentrations operating at the current density of 10 mA/cm2.
To further improve the EL performances of Ir(piq)3, the widely used wide gap blue emitter FIrpic, which has the low-lying HOMO (6.15 eV) and LUMO (3.47 eV) levels, was selected as sensitizer
Fig. 4. (a) EL efficiencyecurrent density characteristics of single-EML devices with FIrpic at different doping concentrations. Insert: Current densityebrightnessevoltage characteristic of single-EML devices with FIrpic at different doping concentrations. (b) EL spectra of single-EML devices with FIrpic at different doping concentrations operating at the current density of 10 mA/cm2.
29
and minutely co-doped into the electron dominant EML. To optimize the co-doping concentration of FIrpic, two single-EML codoped devices with the structure of ITO/TAPC (30 nm)/FIrpic (y wt %):Ir(piq)3 (4 wt%):26DCzPPy(10 nm)/TmPyPB (60 nm)/LiF (1 nm)/ Al (100 nm) and two double-EMLs co-doped devices with the structure of ITO/TAPC (30 nm)/Ir(piq)3 (4 wt%):TcTa (10 nm)/FIrpic (y wt%):Ir(piq)3 (4 wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm) were fabricated and investigated by controlling y to be 0.2 and 0.3, respectively. As been demonstrated in a previous paper, the co-doped FIrpic molecules function as electron trapper and energy-transfer ladder as a result of its low-lying LUMO level and matched triplet energy level [29]. On the other hand, the HOMO level (6.15 eV) of FIrpic is even 0.05 eV lower than that of 26DCzPPy (6.1 eV); therefore, the transport of holes to FIrpic molecules would be insignificant. As shown in Figs. 4(a) and 5(a), both single-EML and doubleEMLs co-doped devices displayed significantly delayed roll-off of EL efficiencies, thus the enhanced maximum brightness. No significant improvement of maximum current efficiency was observed in co-doped single-EML devices. With increasing co-doping concentration of FIrpic, the maximum current efficiency of doubleEMLs devices decreases firstly and then increases rapidly. The key properties of these co-doped devices were listed in Table 2. The 0.3 wt% double-EMLs co-doped device realized the maximum
Fig. 5. (a) EL efficiencyecurrent density characteristics of double-EMLs devices with FIrpic at different doping concentrations. Inset: Current densityebrightnessevoltage characteristic of double-EMLs devices with FIrpic at different doping concentrations. (b) EL spectra of double-EMLs devices with FIrpic at different doping concentrations operating at the current density of 10 mA/cm2.
30
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
Table 2 Key properties of single-EML and double-EMLs devices with FIrpic at different co-doping concentrations. Device
Vturn-on (V)
Ba (cd/m2)
hc b (EQEc) (cd/A)
hp d (lm/W)
hc e (cd/A) (EQEf) (1000 cd/m2)
CIEx,
S-0% S-0.2% S-0.3% D-0% D-0.2% D-0.3%
3.5 3.5 3.6 3.6 3.6 3.6
20,378 22,723 22,218 24,931 25,382 28,031
14.85 13.00 13.89 13.96 13.54 14.89
13.10 8.09 9.59 12.18 9.88 12.99
11.02 11.56 12.35 10.81 11.36 12.06
(0.647, (0.630, (0.616, (0.654, (0.649, (0.644,
a b c d e f g
(8.9%) (7.7%) (8.1%) (8.7%) (8.3%) (9.0%)
(6.6%) (6.8%) (7.2%) (6.7%) (7.0%) (7.3%)
y
g
0.345) 0.345) 0.344) 0.344) 0.344) 0.345)
The data for maximum brightness (B). Maximum current efficiency (hc). External quantum efficiency (EQE). Maximum power efficiency (hp). Current efficiency (hc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. At 1000 cd/m2, this device retained the EL efficiency of 12.06 cd/A. To our best knowledge, EL performances of
this device amongst the best results of the previously reported devices based on Ir(piq)3. As shown in Figs. 4(b) and 5(b), besides the characteristic emission of Ir(piq)3 and 26DCzPPy, another weak emission peak
Fig. 6. (a) Schematic representation of carriers' distribution in single-EML devices with the increasing co-doping concentration of FIrpic. (b) Schematic representation of carriers' distribution in double-EMLs devices with the increasing co-doping concentration of FIrpic. The dot and solid lines within EML represent the HOMO and LUMO levels of dopants and hosts, respectively. Symbols and þ represent electrons and holes, respectively.
Y. Jiang et al. / Dyes and Pigments 128 (2016) 26e32
(472 nm) originate from FIrpic was also observed. Compared with single-EML co-doped devices, double-EMLs co-doped devices displayed relatively weaker FIrpic emission. This phenomenon suggests it is difficult for holes to transfer to FIrpic molecules due to the low-lying HOMO level and low concentration of FIrpic. With increasing co-doping concentration of FIrpic, the relative intensity of FIrpic emission increased slightly. These experimental results demonstrated the efficacy of selecting FIrpic as sensitizer in improving the EL performances of Ir(piq)3. To make clear the EL mechanisms of these co-doped devices, the detailed injection and transport processes of holes and electrons were investigated. Considering the HOMO and LUMO levels of Ir(piq)3 (5.2 and 3.0 eV) are within those of TcTa and 26DCzPPy, carrier trapping was conceived to be the dominant EL mechanism of these devices. For single-EML devices, holes and electrons will be injected from anode and cathode into EML via HTL and ETL, respectively. For double-EMLs devices, holes will be injected from anode into EML1 via HTL, and some holes could be injected into EML2. On the other hand, electrons will be injected from cathode into EML2 via ETL, and some electrons could be injected into EML1. In addition, holes and electrons will be well confined within EML(s) due to the low-lying HOMO level of ETL and the high-lying LUMO level of HTL. The LUMO level (3.47 eV) of FIrpic is 0.77 eV lower than that (2.7 eV) of Ir(piq)3, therefore electrons will be preferentially trapped by FIrpic molecules within co-doped EML. Consequently, as been demonstrated in a previous paper, the co-doped FIrpic molecules will delay the transport of electrons within the electron dominant EML, thus broadens the recombination zone of holes and electrons in some degree. As a result, the decreased exciton density is helpful in suppressing tripletetriplet annihilation, thus in delaying the roll-off of EL efficiencies. To better understand the mechanisms of EL performance improvement in these co-doped devices, distribution of holes and electrons within the EML(s) of these devices were also analyzed. For single-EML devices, as shown in Fig. 6(a), electrons will be the major carriers within EML due to the relatively higher electron mobility of 26DCzPPy and the higher energy barrier between HTL and EML. For double-EMLs devices, as shown in Fig. 6(b), holes and electrons would be the major carriers within EML1 and EML2, respectively, which results the unbalanced carriers' distribution on Ir(piq)3 molecules. For double-EMLs devices, the presence of Ir(piq)3 molecules within EML1 would trap most holes, thus decrease the accumulation of holes within EML1. Therefore, the double-EMLs co-doped devices displayed relatively weaker FIrpic emission due to the lower hole density on 26DCzPPy molecules. In this case, the co-doped FIrpic molecules within EML function as electron trapper and trap some electrons. Therefore, improved balance of holes and electrons on Ir(piq)3 molecules can be realized by optimizing the co-doping concentration of FIrpic. As a result, significant improvement of EL performances has been obtained by the co-doped devices. 4. Conclusions In summary, we have further confirmed the efficacy of selecting FIrpic as sensitizer in improving the EL performances of redemitting materials. By co-doping FIrpic and red-emitting IrIII complex Ir(piq)3 into the electron dominant EML, a series of singleEML or double-EMLs devices were fabricated and investigated. Compared with reference devices, these co-doped devices displayed higher brightness and slower roll-off of efficiencies attributed to improved carriers' balance and broadening recombination zone. By optimizing the doping concentration of FIrpic to be 0.3 wt %, bright pure red EL device with maximum brightness, current efficiency, power efficiency and external quantum efficiency up to
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28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively, was realized. Even at the certain brightness of 1000 cd/m2, current efficiency as high as 12.06 cd/A can be retained by the same device. Acknowledgements The authors are grateful to the financial aid from Research Equipment Development Project of Chinese Academy of Sciences (YZ201562), Youth Innovation Promotion Association CAS (2013150), National Natural Science Foundation of China (Grant Nos. 21201161, 21521092, 91122030 and 21210001), National Key Basic Research Program of China (No. 2014CB643802), and Jilin Provincial Science and Technology Development Program of China (20130522125JH). References [1] Kim SH, Jang J, Lee JY. High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer. Appl Phys Lett 2007;90:223505. [2] Kang DM, Kang J-W, Park JW, Jung SO, Lee S-H, Park H-D, et al. Iridium complexes with cyclometalated 2-cycloalkenyl-pyridine ligands as highly efficient emitters for organic light-emitting diodes. Adv Mater 2008;20: 2003e7. €uerle P. The electroluminescence of organic materials. J Mater [3] Mitschke U, Ba Chem 2000;10:1471e507. [4] Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J Appl Phys 2001;90:5048e51. [5] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998;395:151e4. [6] Leung C-H, Chan DS-H, He H-Z, Cheng Z, Yang H, Ma D-L. Survey and summary: luminescent detection of DNA-binding proteins. Nucleic Acids Res 2012;40:941e55. [7] Ma D-L, Chan DS-H, Leung C-H. Group 9 organometallic compounds for therapeutic and bioanalytical applications. Acc Chem Res 2014;47:3614e31. [8] Lu L, Chan DS-H, Kwong DWJ, He H-Z, Leung C-H, Ma D-L. Detection of nicking endonuclease activity using a G-quadruplex-selective luminescent switch-on probe. Chem Sci 2014;5:4561e8. [9] Leung C-H, Zhong H-J, Chan DS-H, Ma D-L. Review: bioactive iridium and rhodium complexes as therapeutic agents. Coord Chem Rev 2013;257: 1764e76. [10] Liu L-J, Lu L, Zhong H-J, He B, Kwong DWJ, Ma D-L, et al. An iridium(III) complex inhibits JMJD2 activities and acts as a potential epigenetic modulator. J Med Chem 2015;58:6697e703. [11] Zhong H-J, Lu L, Leung K-H, Wong CCL, Peng C, Yan S-C, et al. An iridium(III)based irreversible proteineprotein interaction inhibitor of BRD4 as a potent anticancer agent. Chem Sci 2015;6:5400e8. [12] He H-Z, Wang M, Chan DS-H, Leung C-H, Lin X, Lin J-M, et al. A parallel Gquadruplex-selective luminescent probe for the detection of nanomolar calcium(II) ion. Methods 2013;64:212e7. [13] Lu L, He H-Z, Zhong H-J, Liu L-J, Chan DS-H, Leung C-H, et al. Luminescent detection of human serum albumin in aqueous solution using a cyclometallated iridium(III) complex. Sens Actuators B Chem 2014;201:177e84. [14] Seo JH, Lee SJ, Seo BM, Moon SJ, Lee KH, Park JK, et al. White organic lightemitting diodes showing nearly 100% internal quantum efficiency. Org Electron 2010;11:1759e66. [15] Kim S-Y, Kim J-H, Ha Y, Lee S-H, Seo J-H, Kim Y-K. A study on the characteristics of OLEDs using Ir complex for blue phosphorescence. Curr Appl Phys 2007;7:380e3. e J, Gigmes D, Dumur F. Efficient [16] Lepeltier M, Morlet-Savary F, Graff B, Laleve blue green organic light-emitting devices based on a monofluorinated heteroleptic irdium(III) complex. Synth Met 2015;199:139e46. [17] Wang Q, Ding J, Ma D, Cheng Y, Wang L, Jing X, et al. Harvesting exciton via two parallel channels for efficient white organic LEDs with nearly 100% internal quantum efficiency: fabrication and emission-mechanism analysis. Adv Funct Mater 2009;19:84e95. [18] Tsai Y-C, Jou J-H. Long-lifetime, high-efficiency white organic light-emitting diodes with mixed host composing double emission layers. Appl Phys Lett 2006;89:243521. [19] Lee N-J, Jeon JH, In I, Lee J-H, Suh MC. Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode. Dyes Pigments 2014;101:221e8. [20] Yoo SI, Yoon JA, Kim NH, Kim JW, Kang JS, Moon CB, et al. Improvement of efficiency roll-off in blue phosphorescence OLED using double dopants emissive layer. J Lumin 2015;160:346e50. [21] Kim K-S, Jeon Y-M, Kim J-W, Lee C-W, Gong M-S. Blue light-emitting OLED using new spiro[fluorine-7,90 -benzofluorene] host and dopant materials. Org Electron 2008;9:797e804.
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