Efficient white organic light-emitting devices based on phosphorescent iridium complexes

Efficient white organic light-emitting devices based on phosphorescent iridium complexes

Organic Electronics 11 (2010) 1511–1515 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orge...

611KB Sizes 0 Downloads 79 Views

Organic Electronics 11 (2010) 1511–1515

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Efficient white organic light-emitting devices based on phosphorescent iridium complexes S.L. Lai a, S.L. Tao a,b,*, M.Y. Chan c, T.W. Ng a, M.F. Lo a, C.S. Lee a,**, X.H. Zhang d, S.T. Lee a a

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China c Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China d Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, China b

a r t i c l e

i n f o

Article history: Received 15 March 2010 Received in revised form 12 May 2010 Accepted 20 June 2010 Available online 8 July 2010 Keywords: Organic light-emitting diodes Phosphorescence Electroluminescence

a b s t r a c t Mixing yellow and blue emissions is a commonly used approach for achieving white emission in organic light-emitting devices (OLEDs). However, as there are relatively few efficient yellow phosphors, their performance is often the bottleneck for the efficiency of phosphorescent white OLEDs (WOLEDs) based on ‘‘yellow plus blue” approach. Here, a yellow phosphorescent iridium complex, bis[2-(2-naphthyl)pyridine] (acetylacetonate) iridium(III), has been designed, synthesized and applied in WOLEDs by combining blue emission from iridium-bis-(4,6,-difluorophenyl-pyridinato-N,C2)-picolinate. Without using optical out-coupling or p–i–n confinement structure, the optimized WOLEDs demonstrate a maximum forward viewing power efficiency of 29.2 lm/W (37.2 cd/A and 12.6%) with CIE coordinates of (0.32, 0.45). Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction White organic light-emitting devices (WOLEDs) have demonstrated great potential in applications as backlighting for LCD displays and large area ambient lighting sources [1–12]. WOLEDs can be broadly classified into two categories depending on whether fluorescent or phosphorescent emitting materials are being used. In general, WOLEDs based on fluorescent materials possess lower efficiency but smaller efficiency roll-off with increasing current density, longer operational lifetime, and good color quality (close to the ideal Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.33) for pure white light). Furthermore, the development of fluorescent mate-

* Corresponding author at: Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China. ** Corresponding author. E-mail addresses: [email protected] (S.L. Tao), [email protected] (C.S. Lee). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.06.011

rials is more mature and the device structures based on them are comparatively simple. However, the highest forward viewing power efficiencies (gp) reported in the scientific literatures are only 9 and 18 lm/W for fluorescent WOLEDs based on non-doped [5] (exciplex emission basis) and doped [7] emissive layers (EML). These values are still far below the gp for fluorescent lamps (60 lm/W), which is a benchmark of the present lighting sources. On the contrary, the utilization of phosphors can theoretically lead to four time increase in efficiency, allowing a 100% internal quantum yield (only 25% for fluorescence), by using both the singlet and the triplet excitons for light emission. Phosphorescent WOLEDs have recently attained a gp up to 40 lm/W based on the strategies of three emitters red–green–blue (RGB) phosphorescent emission in combination with a p–i–n device structure (i.e. using both p- and n-types doped transporting layers) [1]. By using suitable optical out-coupling approaches, the gp can be further raised to 110 lm/W. Such a device was fabricated on a high-refractive-index glass (n = 1.78) tailored with a pattern of pyramids by cutting 90° grooves, and the luminance

1512

S.L. Lai et al. / Organic Electronics 11 (2010) 1511–1515

was measured in an integrating sphere (the total gp measured in an integrating sphere roughly equals to the forward viewing gp multiplied by 1.7) [12]. Another common approach for making WOLEDs involves the mixing of blue and yellow (B/Y) emissions. The main advantages of such devices are their much simpler structures and more stable colors. However, unlike fluorescent materials, the development for yellow phosphorescent materials still lags behind the materials emitting with monochromatic primary colors RGB. For example, maximal gp for each monochromatic device has been demonstrated to be 42.7 lm/W for red (irid0 ium(III) bis-(2-phenylquinolyl-N,C2 )acetylacetonate, PQIr), 68.3 lm/W for green (tris-(phenylpyridine)iridium, Ir(ppy)3), and 22.5 lm/W for blue (iridium(III) bis-(40 ,60 -difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate, FIr6) [8], respectively. On the contrary, there are only few yellow phosphorescent materials reported for achieving WOLEDs, in which the maximal gp for monochromatic device is only 12 lm/W (bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl1H-benzoimidazol-N,C3) iridium(acetylacetonate), (fbi)2Ir(acac)) [9,13], whereas the chemical structures and device performance [10,11,14,15] have not been fully disclosed in other yellow-emitting OLEDs. In order to achieve white light with better color balance, the relatively low gp for yellow phosphorescent materials unavoidably sacrifices the excessive blue emission, resulting in poor efficiency for WOLEDs. Recently, the highly efficient (gp = 29 lm/W) yellow OLED based on yellow phosphor dopant p-PF-py has been reported by Tsuzuki et al. [16]. Nevertheless, it is desirable to develop more high performance yellow emitters and explore their applications for white OLED. In this work, we report phosphorescent WOLEDs using a commercially available blue, iridium-bis-(4,6,-difluorophenyl-pyridinato-N,C2)-picolinate (FIrpic) and a self-designed and synthesized yellow bis[2-(2-naphthyl)pyridine] (acetylacetonate) iridium(III) (Ir(npy)2acac) phosphors. Without using light out-coupling or p–i–n confinement structure, the WOLEDs attain a maximal forward viewing gp of 29.2 lm/W, and a balanced white light with CIE coordinates of (0.32, 0.45).

2. Experimental All glass substrates were coated with patterned anode indium tin oxide (sheet resistance 30 X/square), cleaned with Decon 90, rinsed in de-ionized water, dried in an oven, and finally treated in an ultraviolet–ozone cleaner prior to film deposition. The organic materials used have acronyms as follows: NPB: N, N0 -diphenyl-1,10 -biphenyl-4,40 -diamine; mCP: N,N0 -dicarbazolyl-3,5-benzene; UGH2: p-bis-(triphenylsilyly)benzene; and BAlq: aluminum(III) bis(2methyl-8-quinolinato)4-phenylphenolate. WOLEDs with a structure of ITO/NPB (40 nm)/x%Ir(npy)2acac:mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) were fabricated by thermal evaporation in high vacuum chamber (base pressure 106 Torr). The devices have a cascade-type energy band structure with a NPB hole-transporting layer, yellow- and blue-emitting

hosts of mCP and UGH2, respectively, followed by a BAlq electron-transporting layer, and a LiF/Al cathode. Ir(npy)2acac and FIrpic are respectively used as yellow and blue phosphorescent dopant emitters. All films were sequentially deposited at a rate of 0.1–0.2 nm/s without vacuum break. A shadow mask was used to define the cathode and to make four 0.1 cm2 devices on each substrate. Current density– voltage–luminance (J–V–L) characteristics and electroluminescence (EL) spectra were measured simultaneously with a programmable Keithley model 237 power source and a Photoresearch PR650 spectrometer. All measurements were carried out without using light out-coupling enhancement or integrating sphere. Energies of highest occupied molecular orbital (HOMO) values of the materials were measured by using ultraviolet photoelectron spectroscopy, while energies of lowest unoccupied molecular orbital (LUMO) values were estimated by subtracting optical band gaps from the HOMO energies.

3. Results and discussion The inset of Fig. 1 depicts the molecular structure of the iridium complex Ir(npy)2acac. Identity of Ir(npy)2acac was confirmed by 1H-nuclear magnetic resonance, elemental analysis and mass spectrometry. Details of the molecular design and synthetic route for Ir(npy)2acac will be published elsewhere. Fig. 1 shows absorption and PL spectra of Ir(npy)2acac in a dilute dichloromethane solution. The EL spectra obtained from the monochromatic devices with structures of ITO/NPB (40 nm)/6%Ir(npy)2acac:mCP (30 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) for yellow and ITO/NPB (40 nm)/mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) for blue, denoted as Ir(npy)2acac and FIrpic, respectively, are also shown in Fig. 1. It can be seen that the PL and EL peak positions of Ir(npy)2acac show negligible difference (about 1 nm), while FIrpic has an EL peak at 472 nm with a shoulder at 494 nm. In addition, both FIrpic and Ir(npy)2acac devices demonstrate similar maximal forward viewing gp of 25.6 and 21.8 lm/W (device characteristics for Ir(npy)2acac are shown in Fig. 2), respectively. It is worth noting that Ir(npy)2acac exhibits a slightly broader emission spectrum (with a full width at half maximum (FWHM) of 87 nm) than those of (fbi)2Ir(acac) (FWHM = 70 nm) [9,13] and (R-C^N^N)PtCl (FWHM = 80 nm) [10,14]. This would be beneficial for obtaining a better white color. Fig. 3 gives a schematic energy level diagram for the WOLEDs. It can be observed that the WOLED has a cascade-type energy band structure (stepped progression of HOMO or LUMO levels) for both the holes from the anode and the electrons from the cathode to the EML. This would clearly favor efficient carriers transport to the emission regions. Fig. 4 shows normalized EL spectra of the WOLEDs with different concentrations (x%) of Ir(npy)2acac viewed in the normal direction at a luminance of 100 cd/m2. The EL of the WOLEDs is basically the sum of the emissions of FIrpic and Ir(npy)2acac. With increasing Ir(npy)2acac concentration, the relative intensity of the yellow emission (551 nm) increases and the CIE coordinates of WOLEDs shift from

S.L. Lai et al. / Organic Electronics 11 (2010) 1511–1515

1513

Fig. 1. Absorption and PL spectra of Ir(npy)2acac in a dilute dichloromethane solution. EL spectra from the yellow-emitting device: ITO/NPB (40 nm)/ 6%Ir(npy)2acac:mCP (30 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) and the blue-emitting device: ITO/NPB (40 nm)/mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/ BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) are also shown. Inset shows the molecular structure of Ir(npy)2acac.

Fig. 2. Power efficiency as a function of current density of the yellow OLED. Inset shows J–V–L characteristics of the yellow OLED. Yellow OLED: ITO/NPB (40 nm)/6%Ir(npy)2acac:mCP (30 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm).

(0.24, 0.39) to (0.44, 0.53). This phenomenon may be attributed to the increase in hole trapping sites formed by Ir(npy)2acac inside mCP, which is typically observed in the other yellow iridium phosphors [9]. With an optimum Ir(npy)2acac dopant concentration of 1%, the device shows balanced white emission with CIE coordinates of (0.32, 0.45). The inset of Fig. 4 shows a photograph of the WOLED with 1% Ir(npy)2acac operated at 9 V. Table 1 summarizes key operation parameters of the WOLEDs including external quantum efficiencies (gEQE), gP, and current efficiencies (gC) etc. Upon increasing the concentration of Ir(npy)2acac, gC increases from 34.7 to 39.6 lm/W and then falls off to 36.8 lm/W. Similar trend can be observed for gP. However, gEQE shows a monotonic decrease with increasing dopant concentration. It is mainly due to the fact that gEQE is independent of emission wavelength

whereas gC is sensitive to the photopic response, g(k), of human eye (the dotted line as shown in Fig. 4) [17]. In particular, gEQE and gC can be described by the following equation:

gEQE

R C gc kIðkÞdk R ¼ IðkÞgðkÞdk

ð1Þ

where k is the wavelength, I(k) is the EL intensity measured, g(k) is the photopic response of human eye at a particular wavelength, and C is a constant that depends on the Plank’s constant, the velocity of light, and the electronic charge. Eq. (1) reveals that gEQE is directly proportional R to gC and inversely proportional to I(k)g(k)dk. In other words, the gEQE increases with increasing gC or decreasing g(k). g(k) is described by a spectral shape with a peak value

1514

S.L. Lai et al. / Organic Electronics 11 (2010) 1511–1515

Fig. 3. A schematic energy level diagram of the WOLED.

Fig. 4. Normalized EL spectra of various WOLEDs with different concentrations of Ir(npy)2acac viewed in the normal direction at a luminance of 100 cd/m2. Inset shows a photograph of the WOLED with the optimal concentration of 1% Ir(npy)2acac at 9 V.

Table 1 Characteristics of WOLEDs with the structure: ITO/NPB (40 nm)/x%Ir(npy)2acac:mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm). gEQE, gP, gC are the maximum external quantum, power, and current efficiencies.

a

Device (x)

gP [lm/W]

gC [cd/A]

gEQE [%]

V at J = 10 mA/cm2 (at L = 1 cd/m2) [V]

CIE coordinates [x, y]a

0.5 1.0 2.0 4.0 6.0

27.9 29.2 34.4 30.4 26.9

34.7 37.2 38.3 39.6 36.8

13.5 12.6 12.1 12.3 11.5

9.2 9.3 9.3 9.5 9.5

0.24, 0.32, 0.38, 0.43, 0.44,

(3.9) (3.8) (3.7) (3.6) (3.6)

0.39 0.45 0.50 0.53 0.53

The value taken at L = 100 cd/m2.

of 683 lm/W at k = 555 nm and falls off rapidly towards either the red or blue. Taking the eye’s spectral sensitivity into account, the gEQE of yellow-emitting OLED is usually

smaller than those of red- or blue-emitting devices when all devices have the same gC, and it is also the reason why gEQE decreases when the concentration of yellow

S.L. Lai et al. / Organic Electronics 11 (2010) 1511–1515

1515

Fig. 5. Power and external quantum efficiencies as a function of current densities of the blue and the white OLEDs. Inset shows J–V–L characteristics of the blue and the white OLEDs. Blue OLED: ITO/NPB (40 nm)/mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm). White OLED: ITO/NPB (40 nm)/1%Ir(npy)2acac:mCP (10 nm)/8%FIrpic:UGH2 (20 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm).

Ir(npy)2acac increases as the gC increases. It is one of the reasons responsible for the intrinsically higher gEQE of three emitters RGB WOLEDs as compared to B/Y emission WOLEDs. Fig. 5 shows gp, gEQE and J–V–L characteristics of the white and the blue OLEDs. The turn-on (defined as the voltage required to obtain L = 1 cd/m2) and operating voltages at 10 mA/cm2 for white OLED are respectively 3.8 and 9.3 V, which are slightly larger than those (3.7 and 8.6 V) of the blue OLED. The higher driving voltage of the WOLED may be due to the charge trapping effect of the Ir(npy)2acac dopants. At a luminance of 1000 cd/m2 (a typical brightness required for lighting source purpose), the CIE coordinates and the gp are (0.27, 0.33) and 6.5 lm/W for the white OLED and (0.16, 0.33) and 7.0 lm/W for the blue OLED, respectively. In addition, it can be seen that the maximal forward viewing gp, gC, and gEQE are 29.2 lm/W (roughly equivalent to a total gp of 50 lm/W) [12], 37.2 cd/A, and 12.6% for the WOLEDs and 25.6 lm/W, 28.5 cd/A, and 14.6% for the blue OLEDs, respectively. The differences in gC and gEQE of the two devices suggest that the yellow Ir(npy)2acac dopant has negative effect on the gEQE while the gC is increased by 30%. Significantly, the forward viewing efficiencies of the present WOLEDs are among the highest values reported for B/Y emission WOLEDs without optical out-coupling [1,8,11] or a p–i–n confinement structure [1].

4. Conclusion In conclusion, we report phosphorescent WOLEDs based on the combination of our self-designed and synthesized yellow Ir(npy)2acac phosphor and the commercial blue FIrpic. Such WOLEDs show a maximal forward viewing gp of 29.2 lm/W and balanced white light with CIE coordinates of (0.32, 0.45) with superior efficiencies.

Acknowledgment This work was supported by the Innovation and Technology Commission (Grant No. ITP/011/08NP) of Hong Kong SAR. The author S.L. Tao thanks for the financial supports from the Fundamental Research Funds for the Central University and Dongguan LITEWELL (OLED) Technology Incorporation. References [1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature (London) 459 (2009) 234. [2] S.J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. (Weinheim, Ger.) 20 (2008) 4189. [3] C.H. Chang, Y.H. Lin, C.C. Chen, C.K. Chang, C.C. Wu, L.S. Chen, W.W. Wu, Y. Chi, Org. Electron. 10 (2009) 1235. [4] Y.J. Lu, C.H. Chang, C.L. Lin, C.C. Wu, H.L. Hsu, L.Y. Chen, Y.T. Lin, R. Nishikawa, Appl. Phys. Lett. 92 (2008) 123303. [5] Q.X. Tong, S.L. Lai, M.Y. Chan, J.X. Tang, H.L. Kwong, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 91 (2007) 023503. [6] S.L. Tao, Y.C. Zhou, C.S. Lee, D. Huang, X.H. Zhang, S.T. Lee, J. Mater. Chem. 18 (2008) 3981. [7] J.H. Jou, C.C. Chen, Y.C. Chung, M.F. Hsu, C.H. Wu, S.M. Shen, M.H. Wu, W.B. Wang, Y.C. Tsai, C.P. Wang, J.J. Shyue, Adv. Funct. Mater. 18 (2008) 121. [8] X. Qi, M. Slootsky, S. Forrest, Appl. Phys. Lett. 93 (2008) 193306. [9] Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, Appl. Phys. Lett. 94 (2009) 103503. [10] B.P. Yan, C.C.C. Cheung, S.C.F. Kui, H.F. Xiang, V.A.L. Roy, S.J. Xu, C.M. Che, Adv. Mater. (Weinheim, Ger.) 19 (2007) 3599. [11] M.T. Lee, J.S. Lin, M.T. Chu, M.R. Tseng, Appl. Phys. Lett. 93 (2008) 133306. [12] B.W. D’Andrade, R.J. Holmes, S.R. Forrest, Adv. Mater. (Weinheim, Ger.) 16 (2004) 624. [13] W.S. Huang, J.T. Lin, C.H. Chien, Y.T. Tao, S.S. Sun, Y.S. Wen, Chem. Mater. 16 (2004) 2480. [14] S.C.F. Kui, I.H.T. Sham, C.C.C. Cheung, C.W. Ma, B. Yan, N. Zhu, C.M. Che, W.F. Fu, Chem. Eur. J. 13 (2007) 417. [15] H. Zhen, C. Luo, W. Yang, W. Song, B. Du, J. Jiang, C. Jiang, Y. Zhang, Y. Cao, Macromolecules 39 (2006) 1693. [16] T. Tsuzuki, N. Shirasawa, T. Suzuki, S. Tokito, Adv. Mater. (Weinheim, Ger.) 15 (2003) 1455. [17] S.R. Forrest, D.D.C. Bradley, M.E. Thompson, Adv. Mater. (Weinheim, Ger.) 15 (2003) 1043.