Paper Info
Title
A novel red organic light-emitting diode with ultrathin DCJTB and Rubrene layers.
Abstract
A novel red organic electroluminescent diode based on an ultrathin 4-(dicyanomethylene)-2- t -butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) layer has been fabricated. The device with the ultrathin DCJTB layer inserted between N , N ′-bis-(1-naphthyl)- N , N ′-diphenyl-1,1′-biph-enyl-4,4′- diamine (NPB) and tri-(8-hydroxyquinoline) aluminum (Alq 3 ) layers has an emission from both DCJTB and Alq 3 . When utilizing the 5,6,11,12-tetraphenylnaphthacene (Rubrene) bilayer as the emitting-assistant layer in the fabrication of the device, the emission from Alq 3 disappears. We attribute the improvement of color purity to the exciton confinement and energy transfer from the Rubrene molecular. The device with 0.1 nm DCJTB layer has a luminance of 1130 cd/m 2 at 20 mA/cm 2 with an EL efficiency of 5.6 cd/A, and the CIE coordinates was [0.617, 0.379]. The maximum brightness of this device reaches 21,525 cd/m 2 . Keywords Organic light-emitting diodes (OLEDs) Exciton Trap-effect 1 Introduction Organic light-emitting diodes (OLEDs) have attracted significant attention, because the organic multilayer thin-film structures enable efficient carrier injection, transport, recombination and exciton formation [1–4] . During the past two decades, comprehensive developments of organic semiconducting materials and device structures have resulted in the gradual improvement of electroluminescence (EL) quantum efficiency, power conversion efficiency and practical operational lifetime, making them commercially-viable products for high-resolution full color flat-panel displays. For full color display applications, it is necessary to demonstrate a set of primary red, green, blue emitters with sufficiently high luminous efficiencies of proper proportion. Such emitters can be obtained using a guest–host doped system utilizing a host matrix dispersed with various RGB guest dopants leading to electroluminescence of desirable hues [2] . Red organic electroluminescent (EL) devices with high efficiency, luminance and stable color purity are necessary for the development of organic full color display. Doping [2] , especially, the emitting-assisted dopant system [4] is usually utilized to obtain red emission. However, the fabrication process of a device based on dopant emitters is difficult to control. Recently, nondoped pure red organic EL devices based on new novel materials have been reported [5] . However, such novel materials are very rare. A widely used red dopant material for OLED displays on the market is 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) [3] . Generally speaking, tri-(8-hydroxyquinoline) aluminum (Alq 3 ) is widely used as the host material in red EL devices. At a low concentration of DCJTB, the red emission from DCJTB is mixed with the green emission from Alq 3 ; thus, the color purity of the device becomes poor. Increasing the concentration of DCJTB can reduce the Alq 3 emission; however, the efficiency of the device will decrease due to the so-called concentration quenching. By dispersing 5% of 5,6,11,12-tetraphenylnaphthacene (Rubrene) as a red emitting assist dopant with 2% DCJTB in Alq 3 , Hamada and coworkers at Sanyo were able to achieve a luminance efficiency of 2.1 cd/A with CIE x , y = [0.64, 0.35] [4] . Subsequently, the Kodak/Sanyo team discovered that by adding 6% of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) as a hole-trapping dopant to the above emitting system simultaneously, the efficiency could further be improved and near saturated red chromaticity coordinates of CIE x , y = [0.65, 0.34] [6] can be obtained. Furthermore, Liu et al. and Kang et al. reported that luminous efficiency of DCJTB could be improved by doping into the mixed emitting layer of Alq 3 and Rubrene [7,8] . To get good emitting OLEDs, ultrathin layer of DCJTB has been introduced, yet the CIE coordinated is not so good as expected [9,10] . In this paper, we report the emitting performance of a pure red organic EL device based on a DCJTB ultrathin layer by using the emitting-assistant Rubrene layer for carrier and exciton confinement. 2 Experimental The effect of position and thickness of the thin DCJTB layer on the EL device performance has been investigated in article [9] . To determine the interaction between the ultrathin layers and get a red emitting device with good color purity, three kinds of devices have been fabricated in this study. And the devices properties were compared with each other. The devices reported here have the configurations of A. ITO/NPB (47 nm)/DCJTB (0.1 nm)/NPB (3 nm)/Alq 3 (40 nm)/LiF (0.3 nm)/Al (150 nm); B. ITO/NPB (49 nm)/DCJTB (0.1 nm)/NPB (1 nm)/Rubrene (0.05 nm)/Alq 3 (40 nm)/LiF (0.3 nm)/Al (150 nm); C. ITO/NPB (48 nm)/Rubrene (0.05 nm)/NPB (1 nm)/DCJTB (0.1 nm)/NPB (1 nm)/Rubrene (0.05 nm)/Alq 3 (40 nm)/LiF (0.3 nm)/Al (150 nm). The hole transport layer (HTL) and electron transport layer (ETL) NPB and Alq 3 were selected, respectively. An ultrathin DCJTB layer was used as light-emitting layer. The devices were fabricated on patterned indium tin oxide (ITO) coated glass plates (with a sheet resistance of 20 Ω/square). And the ITO glass plates were cleaned by sonication, in a detergent solution and then in deionized water, followed by UV ozone treatment. The organic layers, LiF layer and the Al cathode were sequentially deposited by conventional vacuum vapor deposition in the chamber under the vacuum of approximately 7 × 10 −4 Pa. The rate of deposition was typically 0.2–0.3 nm/s for NPB and Alq 3 . The deposition rate for DCJTB, Rubrene and LiF was 0.01 nm/s. A quartz-crystal oscillator thickness monitor monitored the thickness and deposition rates of the organic layers. EL spectra were taken with a charge-coupled device (CCD)-spectrometer (LPS-045, Labsphere). The current–voltage–luminescence ( J–V–L ) characteristics were measured simultaneously by a Keithley 2410 programmable voltage–current source and the PR 650 spectroscan spectrometer. All the measurements were carried out under ambient conditions without encapsulation. 3 Results and discussion The effect of position and thickness of the thin DCJTB layer on the EL device performance have been invested in article [9] . The EL spectrum of Device A at 5 V is shown in Fig. 1 a. The peak emission of DCJTB locates at 627 nm. The emission from Alq 3 can also be found in this figure. To get a pure red emitting device with the thin DCJTB layer, the emission from Alq 3 should be depressed. Xie and Liu [10] used the Alq 3 (5 nm)/TPBI (45 nm) bilayer as a hole/exciton blocking and electron transporting layer to make a device, in which emission from Alq 3 disappears. They attributed it to the excitons confinement effect by the Alq 3 /TPBI bilayer. Rubrene is a common yellow dopant and it has been used as the emitting-assistant dopant and co-host to improve the DCJTB emission. In the co-dopping system, Rubrene can assist the energy transfer from Alq 3 to DCJTB more efficiently. In this article, the Rubrene sub-monolayer was adoped as the emitting-assistant layer to improve the emission of DCJTB and the color purity of the device. The spectra of the device were optimized by inserting a 0.05-nm-thick Rubrene sub-monolayer into NPB layer, which is shown in Device B. Fig. 1 a shows the EL spectra of Device B at different voltage. For comparison, the EL spectrum of Device A is also shown in this figure. As can be seen, the adoption of the Rubrene ultrathin layer can effectively suppress the emission of Alq 3 . Fig. 1 b shows the CIE coordinates of Device B at different voltage. When the current density increased from 6 mA/cm 2 to 450 mA/cm 2 , only a slight shifting can be observed in the CIE coordinates from [0.649, 0.350] to [0.619, 0.375]. It shows the CIE coordinates of Device B are stable. We think it is because the inserted Rubrene layer can trap holes, and then cations of Rubrene capture electrons [11] . In Device B, there is 1 nm NPB layer between DCJTB and Rubrene layer. The 1-nm-NPB layer can decrease quenching of the excitons formed on Rubrene and DCJTB molecules. Since the critical energy transfer distance is about 3 nm [12] , energy transfer can also take place between Rubrene and DCJTB molecules. Thus the DCJTB emission in this device is improved. Considering this, we can improve the energy transfer from Rubrene to DCJTB by inserting another Rubrene layer at the other side of DCJTB layer. The device has a structure as Device C. Fig. 2 a shows the normalized EL spectra of Device C at different voltage, EL spectrum of Device A and photoluminescent (PL) emission of Rubrene are also shown in this figure. The Alq 3 emission almost disappears in the spectra of Device C. And from the CIE coordinates ( Fig. 2 b) of Device C, we find that the color coordinates of the device located at the red region. When the current density increased from 10 mA/cm 2 to 420 mA/cm 2 , we observed a shifting of the CIE coordinates from [0.617, 0.379] to [0.601, 0.398]. When the thickness of DCJTB and Rubrene layer is improved, the CIE coordinates of the devices is hardly changed, yet the current efficiency will be decreased, the data was not shown here. We think it may come from the concentration quenching of the DCJTB and Rubrene molecular. From the EL spectra of Device B and C, we can get the decreased emission from the Alq 3 by using the Rubrene ultrathin layer. Yet, it should pointed that there is an obvious down shift of the EL peak of Device C to Device B (from 627 nm to 611 nm), which implies that the respective fractions of the EL contribution from DCJTB, Alq 3 , and Rubrene are different between Device B and C. Thus, the emission of Rubrene should account for the shift of the peak in some degree. From Fig. 2 b, the peak emission of Rubrene locates at 558 nm, while the intensity of it is nearly zero at the position of 700 nm. In the normalized EL emission of Device C, the intensity at 558 nm is about 0.2. So, we can suppose the contribution from Rubrene is little. Fig. 3 shows the schematic energy level diagram of Device C. As can be seen that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Rubrene and DCJTB just locate between the values of NPB and Alq 3 . Therefore, Rubrene and DCJTB act as trap for both electrons and holes in the diode. After carriers being injected from the electrode and transporting, they will be trapped and recombine at the Rubrene and DCJTB molecules. And the energy transfer from Rubrene molecule to DCJTB will enhance the DCJTB emission [13] , thus the luminance of the device is improved. Because NPB is an efficient hole-transport material, few holes can tunnel cross the barrier at the Rubrene/Alq 3 interface and recombine near the interface in Alq 3 . Fig. 4 shows the J – V – L characteristics (a) and EL efficiency vs current density characteristics (b) of Device B and C, respectively. The brightness of Device B and C reaches 11,570 and 21,525 cd/m 2 at 15 V, respectively. The current efficiency of Device C is 4.1 cd/A, which is about two times of Device B, 2.1 cd/A at the current density of 100 mA/cm 2 . The brightness and current efficiency are higher than that of the device with the only one DCJTB ultrathin layer. We think the improvement of the efficiency is due to the energy transfer from Rubrene to DCJTB. The current efficiency of Device C is nearly two times than that of the device as shown in Ref. [4] at the same CIE coordinate of [0.64, 0.35]. It should be noted that Device B and C does not require the co-evaporation process, which is necessary for the fabrication of doped devices. This is advantageous from the viewpoint of the fabrication of EL device, because the deposition process of organic layers can be simplified. The simple device structures make them easy to adapt for low-cost mass production processes. 4 Conclusion In summary, a red organic electroluminescent devices based on ultrathin DCJTB and Rubrene layers were fabricated. Utilizing Rubrene as the emitting-assistant layer, the emission from Alq 3 was eliminated. With a 0.1 nm thick DCJTB layer, a luminance of 1130 cd/m 2 was achieved at 20 mA/cm 2 with an EL efficiency of 5.6 cd/A, and the CIE coordinates were (0.617, 0.379). The maximum brightness can reach 21,525 cd/m 2 . Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 61007021 and 60977027), Scientific Research Common Program of Beijing Municipal Commission of Education (Grant No. KM201010011009 and KM200910011010) and the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (Grant No. PHR201007122). Reference [1] C.W. Tang S.A. Vanslyke Appl. Phys. Lett. 51 1987 913 [2] C.W. Tang S.A. Vanslyke C.H. Chen J. Appl. Phys. 65 1989 3610 [3] C.H. Chen C.W. Tang J. Shi K.P. Klubek Macromol. Symp. 125 1997 49 [4] Y. Hamada H. Kanno T.T. sujioka H. Takahashi T. Usuki Appl. Phys. Lett. 75 1999 1682 [5] H.C. Yeh S.J. Yeh C.T. Chen Chem. Commun. 20 2003 2632 [6] T.K. Hatwar, G. Rajeswaran, J. Shi, Y. Hamada, H. Kanno, H. Takahashi, in: Proceeding of the 10th International Workshop on Inorganic and Organic Electroluminescence (EL’00), Hamamatsu, Japan, 2000, p. 31. [7] C.Y. Liu, C.H. Iou, C.H. Chen, in: Proceedings of the 9th International Display Workshops (IDW’02), Hiroshima, Japan, 2002, p. 1135. [8] H.Y. Kang G.W. Kang K.M. Park I.S. Yoo C. Lee Mater. Sci. Eng. C 24 2004 229 [9] D.H. Xu Z.B. Deng X.F. Li Z. Chen Z.Y. Lv Phys. E 40 2008 2999 [10] W.F. Xie S.Y. Liu Semicond. Sci. Technol. 21 2006 316 [11] M. Matusumura T. Furukawa Jpn. J. Appl. Phys. 40 2001 3211 [12] R.S. Deshpande V. Bulovic S.R. Forrest Appl. Phys. Lett. 75 1999 888 [13] C.J. Huang T.H. Meen S.L. Wu C.C. Kang Displays 30 2009 146
Year
DOI
Venue
2011
10.1016/j.displa.2011.01.002
Displays
Keywords
Field
DocType
Organic light-emitting diodes (OLEDs),Exciton,Trap-effect
Rubrene,OLED,Exciton,Optics,Chromaticity,Organic electronics,Engineering,Light-emitting diode,Optoelectronics,Electroluminescence,Bilayer
Journal
Volume
Issue
ISSN
32
2
0141-9382
Citations 
PageRank 
References 
0
0.34
1
Authors
5
Name
Order
Citations
PageRank
Denghui Xu175.52
Xiong Li200.34
Hailang Ju300.34
Yaohui Zhu400.34
Zhenbo Deng576.20