Color-tunable and stable-efficiency white organic light-emitting diode fabricated with fluorescent-phosphorescent emission layers

(1)

Color-tunable and stable-ef

ﬁciency white organic light-emitting diode

fabricated with

ﬂuorescent-phosphorescent emission layers

Su-Hua Yang

n

, Po-Jen Shih, Wen-Jie Wu, Yi-Hua Huang

Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 7 December 2012 Received in revised form 20 March 2013 Accepted 29 March 2013 Available online 10 April 2013 Keywords:

Organic light emitting diode Fluorescence

Phosphorescence Tunability

a b s t r a c t

White organic light emitting diodes (OLEDs) were fabricated for color-tunable lighting applications. Fluorescent and phosphorescent hybrid emission layers (EMLs) were used to enhance the luminance and stability of the devices, which have blue-EML/CBP interlayer/green-EML/phosphorescent-sensitized-EML/red-EML structures. The inﬂuence of the composition and structure of the EMLs on the electro-luminescence properties of the devices were investigated from the viewpoint of their emission spectra. The possible exciton harvesting, diffusion, transport, and annihilation processes occurring in the EMLs were also evaluated. A maximum luminance intensity of 7400 cd/m2 _{and a highly stable current}

efﬁciency of 3.2 cd/A were obtained. Good color tunability was achieved for the white OLEDs; the chromatic coordinates linearly shifted from pure white (0.300, 0.398) to cold white (0.261, 0.367) when the applied voltage was varied from 10 to 14 V.

1. Introduction

As photoelectric technologies have been developed, organic light emitting diodes (OLEDs) have made a crucial technology revolution possible for flat-panel displays and lighting sources [1–4]. OLEDs are distinguished by their extremely low thicknesses, high flexibility, low weights, fast responses, and low power consumption. Nowadays, OLEDs are widely used in commercial applications, most commonly in mobile phones, portable media players, and tablet computers. OLEDs have been used extensively not only in displays but also for lighting applications. In general, the power efficiency is about 10–20 lm/W for electric bulbs and 60_{–90 lm/W for fluorescent lamps. Although the efficiency and} lifetime of OLED lamps are comparable with those offluorescent lamps, OLED lamps are moreflexible and environmentally friendly. For OLED indoor lighting, lamps should be bright sometimes for clear visibility, whereas at other times be of a relaxing, muted color. Therefore, color-tunable white OLEDs are desirable so that the emission color can be changed to provide either sunlight-like illumination or a more muted color depending on the specific requirements[5,6]. On the other hand, a color-tunable white OLED is also required for OLED backlighting applications in order to produce balanced lighting performance. For color-tunable white OLEDs, the emission color must be tunable from typical warm white, to bright sunlight, to cold white. These colors correspond to

correlated color temperatures (CCTs) of 2700, 6500, and 7000 K, respectively. Cold white OLEDs are used for the screens of liquid crystal displays.

Jou has demonstrated OLEDs with wide CCT span from 2300 to 8200 K as well as sunlight-style CCT-tunable lamps [7,8]. The Commission Internationale del’Eclairage (CIE) chromaticity of this device can be varied from (0.48, 0.42) to (0.28, 0.29) by changing the voltage from 3 to 9 V. This chromaticity change occurs because of shifts in the carrier recombination zone of the device. Lee used an OLED structure of ﬂuorescent blue/phosphorescent green/ phosphorescent red emission layers (EMLs) to produce RGB white emission with a high brightness of 24,000 cd/m2 _{and a CIE} chromaticity ranging from nearly pure (0.30, 0.35) to warm white (0.35, 0.40)[9]. Choy reported that if a carrier barrier layer was introduced between the EMLs to shift the region in which the color could be tuned, the emission color could be adjusted by changing the voltage[10]. The color tunability of OLEDs was also discussed by Kim and Li[11,12]. Table 1 lists the structures of white OLEDs withﬂuorescent-phosphorescent (FP) hybrid EMLs; all the devices had unique features and showed good performance [8,9,13–17].

Because of the unlimited potential of white OLEDs for solid-state lighting applications, color-tunable white OLEDs were inves-tigated in this study. The EML was deposited with a structure of blue-EML/interlayer/green-EML/phosphorescent sensitized (PS)-EML/red-EML, in which RGB and FP hybrid EMLs were prepared for obtaining a broadband white emission. An interlayer for manipulating exciton diffusion was deposited, and a PS-EML was used to enhance the luminance and stability of the devices. Contents lists available atSciVerse ScienceDirect

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

Journal of Luminescence

n

Corresponding author. Tel.:+886 7 3814526. E-mail address: [email protected] (S.-H. Yang).

(2)

The device showed good efﬁciency stability and white color tunability. The electroluminescence (EL) properties were analyzed and investigated in detail.

2. Experimental process

For the preparation of white OLEDs, an indium tin oxide (ITO) glass with a resistivity of 15Ω/sq was used as the substrate. In addition, N,N_{′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine} (NPB), 4,7-diphenyl-1,10-phenanthroline (Bphen), 4,4 ′-bis(carbazol-9-yl)biphenyl (CBP), 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB), 1,4-bis[2-(3-N-ethylcarbazoryl)vinyl]benzene (BCzVB), and tris-(phenylpyridine)-iridine [Ir(ppy)3] were used as organic source materials. Herein, 30-μm-thick NPB and 50-nm-thick Bphen layers were used for the hole-transporting layer (HTL) and electron-transporting layer (ETL), respectively [18]. Bphen not only shows a high conductivity (5.2 10-4_cm2_{/V s) for electrons but also can block holes in the} emission layer [19]. Thus, it increases the recombination rate of electron-hole pairs. Furthermore, CBP was used as the host material in all EMLs[15]. BCzVB[20], Ir(ppy)3[17], and DCJTB were selected as the blue, green, and red dyes, respectively[21].

For device fabrication, the ITO surface was cleaned with an organic solvent, dried with high-purity nitrogen gas, and then subjected to oxygen plasma treatment. All the organic materials were evaporated on the ITO glass layer by layer at a deposition rate of 0.5−0.8 Å/s at 5 10−6_{Torr. The deposition rate of the materials} can be controlled by adjusting the heating current provided to the crucible. The thicknesses of theﬁlms were monitored in situ using a 5 MHz quartz crystal thickness monitor. Finally, a 1-nm-thick LiF electron injection layer and 200-nm-thick Al cathode were depos-ited on top of the organicﬁlm at a deposition rate of 2−3 Å/s. The emission area of the devices was 0.24 cm2_{. To measure the EL} properties of the devices, a Keithley 2410 programmable voltage– current source was used. The luminance and CIE coordinates were measured using a Minolta Chroma Meter model CS-100A, and a Newport OSM-400 spectrophotometer was used to measure the EL spectra at the maximum luminance intensity of the devices. All the measurements were carried out in air at room temperature, and the devices were not encapsulated.

3. Results and discussion

In this study, aﬂuorescent-phosphorescent hybrid white OLED with multiple EMLs was fabricated and its characteristics were evaluated. The device structure and energy level diagram are shown in Fig. 1(a). The emission and absorption spectra of the CBP, BCzVB, Ir(ppy)3, and DCJTB are shown in Fig. 1(b). Good

spectral overlaps were observed between the CBP emission and BCzVB absorption and between the Ir(ppy)3emission and DCJTB absorption. Therefore, effective Förster energy transfer is possible between these molecules [21]. On the other hand, the Förster energy transfer is ineffective between CBP and DCJTB as well as between the CBP and Ir(ppy)3molecules. However, Dexter energy transfer is possible if molecules are close enough to each other[22]. Our experimental results showed that the highest brightness of a blue OLED was obtained when a 20 nm-thick blue EML (B-EML) prepared from CBP:7 wt% BCzVB was deposited. Adjacent to the B-EML was a green CBP:Ir(ppy)3 EML (G-EML), that can efﬁciently utilize singlet and triplet excitons to achieve near 100% quantum efﬁciency [23]. Fig. 2 shows the current density and luminance intensity as a function of the applied voltage for the G-EML doped

Table 1

Structures and features of white OLEDs with FP EMLs.

Device structure Feature Ref.

BCPO:FIrpic/TPBi/TCTA:Ir(ppy)3/TPBi: Ir(ppy)3:PO-01:Ir

(piq)2(acac)

Three EMLs with different host materials; using TPBi as CML and ETL; four blackbody radiation-complementary emitters

[8] CBP:BCzVBi/CBP/CBP:Ir(ppy)3/CBP:Ir(piq)3 Stacked F-blue/P-green/P-red EMLs; undoped CBP as singlet-exciton separator [9]

mCP/mCP:FCNIrpic/TPBiI:Ir(ppy)3:Ir(pq)2(acac)/TPBi Higher triplet energy of undoped mCP and TPBi leading to effective conﬁnement of triplet excitons in

EML

[13] mCP:Ir(ppy)3:DCJTB/ mCP:Ir(ppy)3/mCP/ mCP:FIrpic Using selectively codoped layer to manage energy transfer path and to enable efﬁcient phosphorescent

sensitization

[14]

NPB/NPB:DCJTB/CBP/CBP:Ir(piq)3/TPBi Solving efﬁciency roll-off issue with a FIP EML structure [15]

mCP/mCP:FIrpic:DCJTB/TAZ:Ir(ppy)3/TAZ Reducing population of triplet excitons in the EML and using phosphor doped ETL to recycle excitons [16]

CBP:BCzVBi/CBP/CPB:Ir(ppy)3:DCJTB/CBP/CBP:BCzVBi Employing a phosphor sensitizer combined with blue and redﬂuorescent dopants [17]

CML, carrier modulating layer; ETL, electron-transporting layer; F,ﬂuorescence; P, phosphorescence; FIP, ﬂuorescence-interlayer-phosphorescence.

300 400 500 600 700 800

Normalized Intensity (a.u.)

Wavelengh (nm)

CBP (PL) BCzVB (Abs) BCzVB (PL) Ir(ppy)₃ (Abs) Ir(ppy)₃(PL) DCJTB (Abs) DCJTB (PL) NPB CBP: BCzVB _CBP CBP: Ir(ppy) 3 CBP: Ir(ppy) 3 : DCJTB CBP: DCJTB Bphen 2.2 5.2 2.6 5.9 2.97 5.5 2.7 5.5 LiF/Al ITO 4.8 3.2 3.0 6.4 3.1 5.3

Fig. 1. (a) Energy level diagram of the device, and (b) emission and absorption spectra of CBP, BCzVB, Ir(ppy)3, and DCJTB.

(3)

Acknowledgements

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for ﬁnancially supporting this research under contract No. NSC 101_{–2622-E-151-004-CC3.} References

[1]M. Dong, L. Zhong, IEEE Trans. Mob. Comput. 11 (2012) 724.

[2]K.-Y. Lee, Paul C.-P. Chao, IEEE Trans. Electron Dev. 59 (2012) 1123.

[3]M. Eritt, C. May, K. Leo, M. Toerker, C. Radehaus, Thin Solid Films 518 (2010)

3042.

[4]N.T. Kalyani, S.J. Dhoble, Renew. Sust. Energy Rev. 16 (2012) 2696.

[5]P.E. Burrows, G. Gu, V. Bulović, Z. Shen, S.R. Forrest, M.E. Thompson, IEEE Trans.

Electron Dev. 44 (1997) 1188.

[6]H.M. Zhang, Wallace C.H. Choy, IEEE Photonics Technol. Lett. 20 (2008) 1154.

[7]J.-H. Jou, M.-H. Wu, S.-M. Shen, H.-C. Wang, S.-Z. Chen, S.-H. Chen, C.-R. Lin,

Y.-L. Hsieh, Appl. Phys. Lett. 95 (2009) 013307.

[8]J.-H. Jou, P.-W. Chen, Y.-L. Chen, Y.-C. Jou, J.-R. Tseng, R.-Z. Wu, C.-Y. Hsieh,

Y.-C. Hsieh, P. Joers, S.-H. Chen, Y.-S. Wang, F.-C. Tung, C.-C. Chen, C.-C. Wang, Org. Electron. 14 (2013) 47.

[9]Y.H. Lee, B.-K. Ju, W.S. Jeon, J.H. Kwon, O.O. Park, J.-W. Yu, B.D. Chin, Synth. Met.

159 (2009) 325.

[10]Wallace C.H. Choy, J.H. Niu, X.-W. Chen, W.L. Li, P.C. Chui, Appl. Phys. A 89

(2007) 667.

[11] T.-H. Kim, J.H. Park, O.O. Park, Thin Solid Films 520 (2011) 452.

[12]W.X. Li, J. Hagen, R. Jones, J. Heikenfeld, A.J. Steckl, Solid-State Electron. 51

(2007) 500.

[13] J.H. Seo, S.J. Lee, B.M. Seo, S.J. Moon, K.H. Lee, J.K. Park, S.S. Yoon, and Y.K. Kim, Org. Electron. 11 (2010) 1759.

[14]C.-H. Hsiao, J.-H. Lee, J. Appl. Phys. 106 (2009) 024503.

[15]T. Zheng, Wallace C.H. Choy, Thin Solid Films 519 (2010) 872.

[16]C.-H. Chang, Y.-. Lu, C.-C. Liu, Y.-H. Yeh, C.-C. Wu, IEEE/OSA J. Disp. Technol. 3

(2007) 193.

[17] H. Kanno, Y. Sun, S.R. Forrest, Appl. Phys. Lett. 89 (2006) 143516.

[18]S.-H. Yang, C.-H. Chuang, S.-C. Huang, J. Electrochem. Soc. 158 (2011) J394.

[19]S.-H. Jin, Proc. SPIE 7415 (2009) 1–741507.

[20] H.-P. Lin, F. Zhou, J. Li, X.-W. Zhang, X.-Y. Jiang, Z.-L. Zhang, J.-H. Zhang, Appl.

Phys. B 107 (2012) 473.

[21]X. Zhu, D.-H Lee, H. Chae, S.M Cho, J. Lumin. 132 (2012) 12.

[22] A. Pereira, H. Gallardo, G. Conte, W.G. Quirino, C. Legnani, M. Cremona, Ivan

H. Bechtold, Org. Electron. 13 (2012) 90.

[23] S.-J. Su, C. Cai, J. Takamatsu, and J. Kido, Org. Electron. 13 (2012) 1937. [24] J.C. Deaton, D.Y. Kondakov, M.E. Kondalava, K.P. Klubek, D.L. Comfort, United

States Patent, Hybrid Fluorescent/phosphorescent OLEDs, Pub. No. US 20080284318 A1.

[25] J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, J.-J. Kim,

Appl. Phys. Lett. 90 (2007) 223508.

[26] C. Rothe, S.M. King, A.P. Monkman, Phys. Rev. B: Condens. Matter 72 (2005)

085220.

[27] Y. Luo, H. Aziz, Z.D. Popovic, G. Xu, Appl. Phys. Lett. 89 (2006) 103505.

[28] L.S. Liao, K.P. Klubek, Appl. Phys. Lett. 92 (2008) 223311.