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High-Efficiency Nondoped Blue Organic Light-Emitting Devices with Reduced Efficiency Roll-Off Tianyu Zhang,† Jin Wang,† Tong Li,† Mo Liu,† Wenfa Xie,*,† Shiyong Liu,† Dali Liu,† Cheng-Lung Wu,‡ and Chin-Ti Chen*,‡ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, Changchun, 130012, People’s Republic of China, and Institute of Chemistry, Academia Sinica, Taipei, Taiwan, 11529, Republic of China ReceiVed: NoVember 20, 2009; ReVised Manuscript ReceiVed: January 31, 2010
Nondoped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′- spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10- phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m2. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device. Introduction Organic light-emitting devices (OLEDs) have attracted much interest since the first OLED was reported,1 and considerable success in OLEDs has been achieved.2-5 Blue OLED not only is the major constituent for red-green-blue full color displays, but also the indispensable element for the white OLED, which has practical solid-state lighting applications.6,7 OLEDs employing phosphorescent materials are most effective because phosphorescent materials can harvest both singlet and triplet excitons which lead to the potential for achieving 100% internal quantum efficiency.8,9 However, due to the common problem of short operational lifetimes and the high material cost, electrophosphorescence-based OLEDs with acceptable blue color purity are relatively rare. So the combination of blue fluorescent and orange (or green, red) phosphorescent dyes may solve these problems and provide efficient and stable WOLEDs.10,11 2-Diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′-spirobifluorene (DPV) is a very promising nondoped blue fluorescent material for its color purity and high quantum yield.12 Normally, DPV is used as an emitting layer and inserted between a hole transporting layer (HTL) N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1-biphenyl4,4′-diamine (NPB) and an electron transporting layer (ETL) with hole blocking propriety 2,2′,2′′-(1,2,5-phenylene)tris(1phenyl-1H-benzimidazole) (TPBI). The carrier is unbalance because of the difference of the hole and electron mobility in the materials. Currently, Chi et al.13 reported the high-efficiency nondoped blue OLED using the structurally modified version of DPV with reduced hole mobility. However, the color purity of the device is worsened. In this paper, we report that the performances of the blue OLEDs with DPV as emitter are dependent on the charge mobility, charge injection, and energy level characteristics of the HTL and ETL. The roll-off of the efficiency with an * To whom correspondence should be addressed. E-mail: (
[email protected] (W.F.X.) and
[email protected] (C.T.C.). † Jilin University. ‡ Academia Sinica.
increasing brightness is related to the carrier recombination zone and charge balance in the device. The device with 4,4′,4′′-tris(3methylphenylphenylamino)triphenylamine (m-MTDATA) and 4,7-diphenyl-1,10- phenanthroline (Bphen) as HTL and ETL shows high efficiency at low voltage and significantly lower efficiency roll-off. Experimental Section Organic light-emitting devices with the structure of ITO/HTL (30 nm)/DPV (30 nm)/ETL (40 nm)/LiF/Al are fabricated. The devices with NPB as HTL and tris(8-hydroxyquinoline) aluminum (Alq3), TPBI, or Bphen as ETL were named as device A1-3, respectively. The corresponding devices with m-MTDATA as HTL were named as device B1-3, respectively. Prior to the device fabrication, ITO-coated glass substrates were carefully cleaned by scrubbing and sonication. All organic layers were deposited onto the substrate in high vacuum (10-4 Pa) by thermal evaporation with a rate of 0.1-0.2 nm/s. Then, a bilayer cathode of LiF/Al was subsequently vapor-deposited onto the organic films. The layer thickness and the deposition rate of the materials were monitored in situ with an oscillating quartz thickness monitor. The electroluminescent (EL) spectra and Commission Internationale de I’Eclairage (CIE) coordinates of the devices were measured by using a PR650 spectroscan spectrometer. The luminance-voltage and current-voltage characteristics were measured simultaneously with a programmable Keithley 2400 voltage-current source. All measurements were carried out at room temperature under ambient conditions. Results and Discussion Figure 1 shows the normalized EL spectra of the devices. Devices A2, A3, B2, and B3 show the typical DPV EL characteristics with a peak at 472 nm and almost the same spectra [the full width at half-maximum (fwhm) is 72 nm]. These four devices show pure blue emitting with the CIE coordinates of (0.16, 0.22). However, the emission spectra of devices A1 and B1 are different from the others. Device A1 shows the Alq3
10.1021/jp911065a 2010 American Chemical Society Published on Web 02/16/2010
Nondoped Blue Organic Light-Emitting Devices
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Figure 3. Current density versus voltage characteristics for the blue devices.
Figure 1. The normalized EL spectra of devices.
Figure 4. Brightness-voltage characteristics of the blue devices.
Figure 2. The energy level diagram of the devices.
emission with a peak at 528 nm, and device B1 shows an emission with a peak at 476 and a 92 nm fwhm. We think the Alq3 is emitting in device B1, which leads to the red shift and broadening of the spectrum. To investigate the above phenomena, the energy level diagram and the carrier mobility of the materials are shown in Figure 2 and Table 1. In devices A1-3, the holes have much higher mobility in NPB and DPV than electrons in ETL, and there is almost no energy barrier between the HOMO (highest occupied molecular orbital) of DPV and NPB. Thus, the holes injected from anode will accumulate at the DPV/ETL interface and the electrons injected from cathode will transport in ETL. Besides, the LUMO (lower unoccupied molecular orbital) electron barrier (∼0.2 eV) is lower than the HOMO holes barrier (∼0.7 eV) in devices A2 and A3 at the DPV/ETL interface. So electrons can reach the DPV layer more easily and the light emission would originate from the DPV layer. But, the HOMO holes barrier (∼0.2 eV) is lower than the LUMO electron barrier (∼0.4 eV) in device A1 at the DPV/Alq3 interface. So holes can reach the Alq3 layer more easily and the light emission would originate
Figure 5. Current efficiency-luminance characteristics of the blue devices.
from the Alq3 layer. In devices B2 and B3, there is a 0.4 and ∼0.7 eV HOMO holes barrier at DPV/m-MTDATA and DPV/ ETL interfaces, and there is a 0.7 and ∼0.2 eV LUMO electron barrier at DPV/m-MTDATA and DPV/ETL interfaces. Besides, the electron mobility in the DPV layer is as high as that in the ETL layer. Thus, the holes will accumulate at the m-MTDATA/ DPV and DPV/ETL interfaces, and the electrons can transport in the DPV layer and reach the m-MTDATA/DPV interface.
TABLE 1: The carrier mobility data of the materials m-MTDATA a
electron mobility (cm /vs) hole mobilitya(cm2/vs) ref a
NPB
DPV
Alq3 ×4
2
∼4 × 10-5 14 -1
At a electric field of ∼4.9 × 10 V cm . 5
∼5 × 10-3 15
∼8 × 10 ∼5 × 10-3 13
∼3 × 10 16
TPBI -6
∼5 × 10 17
Bphen -6
∼3 × 10-4 16
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TABLE 2: Performance of the Present and Several Reported Non-doped Blue OLEDs turn-on voltage [V]
max current efficiency [cd/A]
brightness (cd/m2)
emission peak [nm]
CIE coordinates [x, y]
ref
DPVc
3.0a/3.2b
6.0a/6.7b
472
0.16, 0.22
this work
DPVSBF TBDPA DPF PHAA NAA DFDF PPPd
2.7 n.a. 3.3 3.2 2.9 3.7 3.7
5.33 2.3 6.0 4.0 5.5 3.8 6.9
1000 at 4.6 Va 4635 at 6 Va 4110 at 6.4 V n.a. 1000 at ∼6 V 1000 at ∼7 V 1000 at ∼4.8 V 100 at 4.4 V 1300 at 6 V
474 468 470 470 472 474 474
0.16, 0.24 0.16, 0.21 0.15, 0.19 0.14, 0.20 0.14, 0.21 0.16, 0.23 0.14, 0.20
18 19 20 21 21 22 23
emitter
a Device B3. b Device A3. c Maximum power efficiency of device B3 is 4.3 lm/W. d Maximum power efficiency of the PPP blue device is 3.0 lm/W.
Thus, the light emission would originate from the DPV layer. In device B1, the electrons also can reach the m-MTDATA/ DPV interface and the DPV layer will emit. Simultaneity, the holes will reach the Alq3 layer because of the low HOMO holes barrier at the DPV/Alq3 interface. This leads to emission from the Alq3 layer. On the basis of the above analysis and the experimental results, we think the excitons will form at the DPV/ ETL interface in devices A1-3 but at the whole DPV layer in devices B1-3. Figure 3 shows the current density versus voltage characteristics for the blue devices. In the devices with the same HTL, the current density of the device with Bphen as ETL is much higher than that of the device with TPBI as ETL because of the high electron mobility in Bphen and the relatively low electron injection barrier from the cathode. In the devices with the same ETL, the current density of the device with m-MTDATA as HTL is higher than that of the device with NPB as HTL at low voltage because of the relatively low holes injection barrier from the anode. At high driving voltage, the current density of the device with NPB as HTL is higher than that of the device with m-MTDATA as HTL due to the higher hole mobility in NPB than tha tin m-MTDATA. Figure 4 shows the brightness-voltage characteristics of the blue devices. As can be seen, the brightness-voltage characteristics of the devices are similar with the current density-voltage characteristics of the devices. In the devices with the same ETL, the brightness of the device with m-MTDATA as HTL is higher than that of the device with NPB as HTL at a low driving voltage. For example, the brightness of devices A2, A3, B2, and B3 is 48.1, 390.5, 198.9, and 1528 cd/m2 at 5 V, respectively. The significant enhancement of the brightness in device B3 should be attributed to the excellent hole injection characteristic of m-MTDATA and the high mobility of Bphen. Current efficiency-luminance characteristics of the blue devices are shown in Figure 5. The maximum current efficiency of devices A2, A3, B2, and B3 is 6.6, 6.7, 5.6, and 6.0 cd/A, respectively. And in the devices with the same ETL, the efficiency roll-off of the device with m-MTDATA as HTL is less than that of the device with NPB as HTL. From the brightness at maximum current efficiency to 10 000 cd/m2, the efficiency roll-off of devices A2, A3, B2, and B3 is 52%, 31%, 18%, and 16%, respectively. We attributed this to the formation of a broader carrier recombination zone and charge-balancing in devices B2 and B3. We have known that the excitons will form at the DPV/ETL interface in devices A2-3 but at the whole DPV layer in devices B2-3. That is to say, the carrier recombination zone is broader in devices B2-3 than that in devices A2-3. This will eliminate the carrier accumulation at the interface and prevent excitions quenching. Furthermore, the improvement of the hole injection from ITO, the relative low
hole mobility in m-MTDATA, and the high electron mobility in Bphen and DPV guarantee relative balance of the electrons and holes in the DPV emitter for device B3. For comparison, the device performance of the current blue DPV device is listed in Table 2 together with other reported blue OLEDs with nondoped emitter layer. The DPV-based devices have outperformed other nondoped blue OLEDs with similar blue color purity in terms of EL efficiency and intensity.18-23 Conclusions In summary, we have demonstrated a high-efficiency nondoped blue organic light-emitting device with reduced efficiency roll-off. The device shows a slight efficiency roll-off and high efficiency and brightness at low voltage. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device. Besides, a broader carrier recombination zone and the reduction of the carrier accumulation will also be advantageous for the stability of the OLEDs.24 Acknowledgment. This work was supported by the National Nature Science Foundation of China (Grant Nos. 60937001, 60606017, 60707016, and 60723002) and the Ministry of Science and Technology of China (Grant No. 2010CB327701). References and Notes (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Tokito, S.; Lijima, T.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 2459. (3) D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004, 18, 1585. (4) Shih, P. I.; Shu, C. F.; Tung, Y. L.; Chi, Y. Appl. Phys. Lett. 2006, 88, 251110. (5) Sun, Y. R.; Giebink, N.; Kanno, C.; Ma, B.; Thompson, M. E.; Forrest, S. R. Nature (London) 2006, 440, 908. (6) Lai, M. Y.; Chen, C. H.; Huang, W. S.; Lin, J. T.; Ke, T. H.; Chen, L. Y.; Tsai, M. H.; Wu, C. C. Angew. Chem., Int. Ed. 2008, 47, 581. (7) Tonzola, C. J.; Kullkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A. AdV. Funct. Mater. 2007, 17, 863. (8) Zheng, T. H.; Choy, W. C. H.; Ho, C. L.; Wong, W. Y. Appl. Phys. Lett. 2009, 95, 133304. (9) Chen, C. H.; Huang, W. S.; Lai, M. Y.; Tsao, W. C.; Lin, J. T.; Wu, Y. H.; Ke, T. H.; Chen, L. Y.; Wu, C. C. AdV. Mater. 2009, 19, 2661. (10) Schwartz, G.; Fehse, K.; Pfeiffer, M.; Walzer, K.; Leo, K. Appl. Phys. Lett. 2006, 89, 083509. (11) Chen, P.; Xie, W.; Li, J.; Guan, T.; Duan, Y.; Zhao, Y.; Liu, S.; Ma, C.; Zhang, L.; Li, B. Appl. Phys. Lett. 2007, 91, 023505. (12) Chiang, C. L.; Tseng, S. M.; Chen, C. T.; Hsu, C. P.; Shu, C. F. AdV. Funct. Mater. 2008, 18, 248. (13) Chi, C. C.; Chiang, C. L.; Liu, S. W.; Yueh, H.; Chen, C. T.; Chen, C. T. J. Mater. Chem. 2009, 19, 5561. (14) Staudigel, J.; Sto¨ssel, M.; Steuber, F.; Simmerer, J. Appl. Phys. Lett. 1999, 75, 217. (15) Tse, S. C.; Kwok, K. C.; So, S. K. Appl. Phys. Lett. 2006, 89, 262102. (16) Naka, S.; Okada, H.; Onnagawa, H.; Tsutsui, T. Appl. Phys. Lett. 2000, 76, 197.
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