Hybrid white organic light-emitting devices based on phosphorescent iridium-benzotriazole orange-red and fluorescent blue emitters

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Hybrid white organic light-emitting devices based on phosphorescent

iridium–benzotriazole orange–red and ﬂuorescent blue emitters

Zhen-Yuan Xia

a,n

, Jian-Hua Su

a

, Chi-Sheng Chang

b

, Chin H. Chen

b

a

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

b_{Display Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu, Taiwan 300, China}

a r t i c l e

i n f o

Article history: Received 2 July 2012 Received in revised form 26 August 2012

Accepted 13 September 2012 Available online 24 September 2012 Keywords:

Benzotriazole–iridium complex Hybrid

White OLED

a b s t r a c t

We demonstrate that high color purity or efﬁciency hybrid white organic light-emitting devices (OLEDs) can be generated by integrating a phosphorescent orange–red emitter, bis[4-(2H-benzotriazol-2-yl)-N,N-diphenyl-aniline-N1_,C3_{] iridium acetylacetonate, Ir(TBT)}

2(acac) with ﬂuorescent blue emitters in two different emissive layers. The device based on deep blue ﬂuorescent material diphenyl-[4-(2-[1,10_;40_,100_{]terphenyl-4-yl-vinyl)-phenyl]-amine BpSAB and Ir(TBT)}

2(acac) shows pure white color with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33,0.30). When using sky-blue ﬂuorescent dopant N,N0_-(4,40_-(1E,10_E)-2,20 -(1,4-phenylene)bis(ethene-2,1-diyl)bis(4,1-phenylene))-bis(2-ethyl-6-methyl-N-phenylaniline) (BUBD-1) and orange–red phosphor with a color-tuning phos-phorescent material fac-tris(2-phenylpyridine) iridium (Ir(ppy)3), it exhibits peak luminance yield and power efﬁciency of 17.4 cd/A and 10.7 lm/W, respectively with yellow-white color and CIE color rendering index (CRI) value of 73.

1. Introduction

White organic light-emitting diodes (white OLEDs) have attracted much attention in recent years, owing to their potential use of backlight system in full color displays and solid-state lightings with low-cost and large-area alternatives [1]. White ﬂuorescent OLEDs can be generated by many approaches such as using vertical red–green–blue (RGB) small-molecular organic emitters stacked with different layers comprising three primary colors or two complementary colors[2–6], polymers combining R, G, B dopants in a single layer[7], utilization of broad exciplex or excimer emission[8,9], and the application of microcavity[10]. However, many of these devices suffered from poor color purity and low power efﬁciency.

The efficiency of OLEDs can be greatly improved by incorpor-ating a phosphorescent dopant in an appropriate host material as the emitting layer, because it captures the triplet state energy which is lost in conventional fluorescent OLEDs. As robust deep blue phosphorescence material is still not available, most high-efficiency white OLEDs are based on the combination of blue fluorescence and orange/red phosphorescence emitter in a single device architecture, which have been called hybrid white OLEDs [11–13].

Lately, we reported an orange–red emitting iridium (III) com-plex based on benzotriazole–iridium derivative bis[4-(2H-benzo-triazol-2-yl)-N,N-diphenylaniline-N1_,C3_]iridium _{acetylacetonate}

([Ir(TBT)2(acac)])[14], which can be used as an efﬁcient dopant

in monochromatic orange–red OLEDs. The low driving voltage and high efﬁciency properties make this material a good candi-date for the application in hybrid white OLEDs.

In this work, we integrated this orange–red phosphor with two fluorescent blue emitters as the emissive layers. High color purity or efficiency hybrid white OLEDs were achieved in the two type devices A and B1. Device A based on deep blue fluorescent material BpSAB and Ir(TBT)2(acac) shows pure white color with

the CIE coordinates of (0.33,0.30) at 10 mA/cm2_{. However, the}

maximum luminance yield of device A is only 4.8 cd/A. While using the sky blue ﬂuorescent dye BUBD-1 and co-doping a green phosphorescent Ir(ppy)3into orange–red phosphor Ir(TBT)2(acac)

layer, device B1 exhibits greatly improved luminance yield of 17.4 cd/A with a red-shifted color (CIEx,y: 0.47, 0.45).

2. Experimental details

The chemical structures of key materials and the device conﬁguration of monochromatic PHOLED are shown in Fig. 1. The synthesis and characterization of Ir(TBT)2(acac) have been

published elsewhere [14,15]. Other materials were obtained commercially and used without further puriﬁcation.

Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/jlumin

Journal of Luminescence

n

Corresponding author. Tel./fax: þ86 21 64252288. E-mail address: [email protected] (Z.-Y. Xia).

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Prior to the deposition of organic materials, indium–tin-oxide (ITO)/glass was cleaned with a routine cleaning procedure and pretreated with oxygen plasma. Devices were fabricated under about 104_{Pa base vacuum in a thin-ﬁlm evaporation coater}

following a routine protocol. The current–voltage–luminance characteristics were measured with a diode array rapid scan system using a Photo Research PR650 spectrophotometer and a computer-controlled, programmable, direct-current (DC) source.

3. Results and discussion

According to our previous investigation, the optimized doping concentration of Ir(TBT)2(acac) is 20 wt% in single emitting layer

devices. A two-element WOLED was fabricated with the device structure A: ITO/CFx/NPB (400 ˚A)/

a

,

a

-MADN:3% BpSAB (150 ˚A)/

BCP(10 ˚A)/CBP:20% Ir(TBT)2(acac) (40 ˚A)/Bphen(400 ˚A)/Bphen:

Cs2CO3(100 ˚A)/Al(1400 ˚A) as shown inFig. 1(ITO ¼indium tin oxide,

NPB¼4,4-bis[N-(1-naphthyl)-N-phenyl amino]biphenyl, Bphen¼ 4,7-diphenyl-1,10-phenanthroline,

a

,

a

-MADN¼2-methyl-9,10-di(1-naphthyl)anthracene, BpSAB¼diphenyl-[4-(2-[1,10_;40_,100

]terphenyl-4-yl-vinyl)-phenyl]-amine, CBP¼4,40_-N,N0_{-di(carbazolyl)-biphenyl,}

and BCP¼2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline). CFxacts

as the hole injection layer, NPB is the hole-transporting layer (HTL), CBP:Ir(TBT)2(acac) is the orange–red emissive layer, BPhen is the

electron transporting and hole blocking layer (ETL/HBL), BPhen: Cs2CO3 is the n-doping layer, BCP is the exciton blocking layer,

which prevents the triplet excitons of phosphorescent dopants being quenched by ﬂuorescent non-radiative energy transfer,

a

,

a

–MADN is the host for deep blue dopant BpSAB, and the optimal doping concentration is 3%[16].

The electroluminescence spectra of device A at different current densities are shown inFig. 2. Two blue emission peaks at 456 nm and 476 nm are from the ﬂuorescent dopant BpSAB and the orange–red peak at 588 nm comes from the phosphor-escent dye. The relative intensity of blue emission increased slightly under low current densities (from 0.5 mA/cm2 _to

10 mA/cm2_{). At high current density, the intensity of the blue}

peaks was about 50% higher than that of the normalized orange– red one. The color shift could be attributed to the hole blocking effect of BCP insertion layer. In device A, there is a large energy barrier ( 0.6 eV) of hole injection from

a

,

a

-MADN host (HOMO:

5.8 eV) to BCP (HOMO: 6.4 eV) layer. Apparently large amount hole carriers are accumulated around

a

,

a

-MADN/BCP border at high driving voltage; then more excitons are generated around

a

,

a

-MADN host layer for increasing blue ﬂuorescent emission. It is worth mentioning that the CIE value (0.33,0.30) is

Fig. 1. Chemical structures of key materials and the conﬁguration of two hybrid white OLEDs (devices A and B).

Fig. 2. EL spectra of device A (normalization on orange–red peak) at various current densities. Inset: the current density–voltage (J–V) and current efﬁciency characteristics of device A.

Fig. 3. Energy level diagram and the structure of device B1.

Fig. 4. Normalized EL spectra of devices B1 and B2 (DCJTB as the orange–red dopant) at the current density of 10 mA/cm2

. Z.-Y. Xia et al. / Journal of Luminescence 135 (2013) 323–326

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of a relativly pure white color compared with the ideal CIE coordinates (0.33,0.33) (in Figs. 2 and 7). The CIE coordinates present a negligible bluish change (0.31,0.28), while the driving current density was increased to 50 mA/cm2 _{at 5.7 V. The}

luminance yield and external quantum efﬁciency (EQE) reach about 4.8 cd/A and 2.9% respectively under a current density of 2 mA/cm2 _{and 3.8 V (in}_{Fig. 2}_{, inset). This result indicates good}

color stability and low driving voltage properties of device A. But the efﬁciency of device A is relatively poor, due to the low quantum efﬁciency of deep blue emitter and energetically mis-matched BCP interlayer.

To further improve the luminance efﬁciency, we chose sky blue dopant BUBD-1 with bipolar MADN host[17]instead of deep blue one and co-doped a color-tuning phosphor Ir(ppy)3[18]in

the orange–red emitting layer. An un-doped CBP interlayer plays a similar role as the BCP layer in device A, and it also facilitates the holes transport. The device B1 structure was ITO/CFx/

NPB(550 ˚A)/MADN:1%BUBD-1(200 ˚A)/CBP(40 ˚A)/CBP: 8%Ir(ppy)3:

4% Ir(TBT)(40 ˚A)/BCP(30 ˚A)/Alq3(300 ˚A)/Bphen:5% Cs2CO3(100 ˚A)

/Al(1400 ˚A) (BUBD-1¼ N,N0_-(4,40_-(1E,10_E)-2,20_{-(1,4-phenylene)bis}

(ethene-2,1-diyl)bis(4,1-phenylene))bis(2-ethyl-6-methyl-N-phe-nylaniline), Ir(ppy)3¼fac-tris(2-phenylpyridine) iridium, Alq3¼

tris(8-hydroxyquinolinato)-aluminum) (in Fig. 3). The inserted Alq3layer between the hole blocking BCP and the n-doping layers

helps the injection and transport of electrons. A standard device B2 with a ﬂuorescent dye DCJTB (4-(dicyanomethylene)-2-tert-butyl-6-(1,10_,7,70_{-tetramethyljulolidin-4-yl-vinyl)-4H-pyran) 1.2%}

replaced the orange–red phosphor and no change of other layers was made as well.

As shown in Fig. 4, the blue ﬂuorescent emitter (BUBD-1, 468 nm) and green/red emitter (CBP:Ir(ppy)3 (508 nm):Ir(TBT)

region, (588 nm)) form a broadband white light emission, and the blue/green color emission peaks are not distinct compare with the orange–red emisson peak, which leads to the reddish-white CIE position (0.47,0.45) at 10 mA/cm2_(in_{Fig. 5}_{). When the orange–}

red dopant material was replaced by DCJTB in device B2 (emission peak at 572 nm), a similar spectrum is detected as well. Moreover, apparent voltage dependent variation in emission color index is observed in the EL spectra of device B1 (in Fig. 5). With the increase of the voltage from 5.1 V (1 mA/cm2_{) to 8.7 V (50 mA/}

Fig. 5. Normalized EL spectra of device B1 at various current densities.

Fig. 6. Current density–voltage–luminance characteristics (J–V–L) of device B1. Inset: the current efﬁciency characteristics of device B1.

Fig. 7. CIE coordinates of devices A and B1 and the photographs of their light emissions.

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cm2_{), the intensity of the blue and green emission gradually}

enhances relative to the red one. The weak blue emission at low current density may be explained by the unbalanced exciton recombination zone. Unlike device A, devices B1 and B2 have holes as dominated carriers due to the intrinsic property of hole-preferred CBP transporter[11]. Also, the low-lying LUMO level of CBP interlayer (LUMO: 2.9 eV) hinders electron injection into the blue host MADN (LUMO: 2.5 eV) [19], as shown in the energy level diagram of Fig. 3. Therefore, less excitons are generated at the MADN matrix and the blue BUBD-1 emission decreases. On the other side, an efﬁcient T–T energy transfer from Ir(ppy)3to Ir(TBT)2(acac) in the co-doped phosphor layer greatly

reduces the green emission and enhances the orange–red one. While at higher current densities, more electrons are injected into the blue layer and the more balanced electron–hole pairs result in the enhanced blue emission. Meanwhile, the easier formation of high-energy excitons at high electric ﬁeld causes enhanced green emission and better CIE value (0.41,0.45) [20]. However, the efﬁciency of device B1 is seriously quenched at high drive current density (in the inset ofFig. 6); this phenomenon is also observed in the Ir(TBT)2(acac) based single-emitter device[15]and device

A. We suppose that it is due to the combination of triplet–triplet phosphorescence annihilation [21] and ﬁeld-induced exciton dissociation effects[22].

The performance of our device B1 is comparable with that of DCJTB-doped device B2 at the current density of 10 mA/cm2 _(in

Table 1), and it owns a good color rendering index (CRI) value 73. Device B1 exhibits peak luminance and power efﬁciency of 17.4 cd/A and 10.7 lm/W (at 1 mA/cm2_{), respectively, with a}

maximum luminance of 13715 cd/m2_{(at 200 mA/cm}2_{, in}_{Fig. 6}_),

which is much more improved than those of the two-element hybrid WOLED device A.

4. Conclusion

In summary, an orange–red iridium–benzotriazole phosphor Ir(TBT)2(acac) was used in hybrid white OLEDs. Two emissive

layers WOLEDs by adjusting Ir(TBT)2(acac) with its

complemen-tary color (deep blue) or the other primary colors (green/blue) were fabricated. Near pure CIE value (0.33,0.30) and power consumption white OLEDs were achieved by Ir(TBT)2(acac) and

deep blue ﬂuorescent emitter BpSAB. While using sky blue

ﬂuorescent dye BUBD-1, green phosphor Ir(ppy)3 and Ir(TBT)2

(acac), the maximum luminance yield and efﬁciency are 17.4 cd/A and 10.7 lm/W, respectively, with good CRI value of 73.

Acknowledgements

This work is ﬁnancially supported by NSFC/China (50673025, 20603009), National Basic Research 973 Program (2006CB806200) and a JD research grant of Industry/Academia Cooperation Project provided by e-Ray Optoelectronics Technology Co., Ltd. Taiwan. References

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Table 1

Performance parameters of devices A, B1 and B2 at the current density of 10 mA/cm2

. Device Orange–red dopant Voltage (V) Current efﬁciency

(cd/A)

Power efﬁciency (lm/W)

Luminance (nit)

EQE (%) CIE 1931 CRI

A Ir(TBT) 20% 4.5 4.3 3.0 433 2.7 (0.33,0.30) – B1 Ir(TBT) 4.0% 6.8 12.7 5.8 1266 5.8 (0.47,0.45) 73 B2 DCJTB 1.2% 6.3 16.7 8.3 1665 6.0 (0.41,0.47) 63

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