High-Brightness, Broad-Spectrum White Organic Electroluminescent

J. Phys. Chem. C 2010, 114, 21723–21727

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High-Brightness, Broad-Spectrum White Organic Electroluminescent Device Obtained by Designing Light-Emitting Layers as also Carrier Transport Layers Liang Zhou, Xiaona Li, Xiyan Li, Jing Feng, Shuyan Song, and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: October 6, 2010

In this study, we demonstrated an efficient energy level alignment technology to design a white organic electroluminescent (EL) device consisting of two light-emitting layers (EMLs) and a hole block layer. By doping green and red dopants into blue-emitting and green-emitting host materials with hole and electron transport abilities, respectively, the two EMLs are designed as also hole and electron transport layers. Interestingly, not only the doping concentrations but also the layer thicknesses strongly influence device performance, especially the EL spectrum. After optimization, broad-spectrum white electroluminescence with maximum current efficiency of 8.02 cd/A and power efficiency of 9.33 lm/W was obtained. With increasing current density, EL efficiency decreases first and then keeps constant at about 6.0 cd/A. Therefore, this device achieves the maximum brightness as high as 35788.0 cd/m2. At the brightness of 20000 cd/m2, this device has the Commission Internationale de l’Eclairage (CIE) coordinates of (0.328, 0.336). With the help of energy levels and EL spectra, injection, transport, distribution, and recombination of holes and electrons in this device are investigated in detail. The presence of the EL efficiency flat zone is attributed to the synergy of improved carrier balance and the broadening of the recombination zone. Introduction White organic light-emitting diodes (WOLEDs) have received significant attention due to their potential application in largearea flat-panel displays and in solid-state lighting.1-4 By definition, white emission requires the mixture of primary (red, green, and blue) or complementary (e.g., blue and yellow or orange) colors.5-8 In a white electroluminescent (EL) device, the EL efficiency of the blue-emitting material is typically relatively low, thus limiting the improvement of EL efficiency and brightness.9-11 Furthermore, blue materials have relatively high energy gaps which are generally accompanied by lowlying highest occupied molecular orbital (HOMO) levels or highlying lowest unoccupied molecular orbital (LUMO) levels.12,13 As a result, most blue EL devices possess relatively high hole injection or electron injection barrier, which causes the unbalanced carrier injections and high operation voltage.14,15 In addition, most blue materials transport only holes or only electrons, which results in a narrow recombination zone, thus causing the undesired roll-off of EL efficiency due to exciton quenching.16,17 Therefore, how to enhance blue emission has thus become the bottleneck of designing efficient and broadspectrum WOLEDs. One efficient strategy generally used to simplify device structure and at the same time to obtain balanced carrier injections and broad spectrum is to design the organic layers with multifunction.18-21 Alternatively, doping method has been demonstrated to be helpful to reduce operation voltage and combine two or more emission colors into one light-emitting layer (EML).22-26 Recently, we have reported the low concentration doping of tris(8-hydroxyquinoline)aluminum (Alq3) into * Address correspondence to this author at the State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, [email protected].

N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as hole injection layer and EML.27 Within this layer, Alq3 functions as not only the green-emitting material but also the electron injection sensitizer, while NPB acts as blueemitting and hole injection material. More interestingly, the doping concentration of Alq3 strongly influences the injection of electrons, thus the distribution of holes and electrons in EML; as a result, high-brightness EL devices with a tunable spectrum from deep blue to blue-green were obtained. It is worthwhile to mention that this is the first report of bright blue or bluegreen EL origin from NPB, and this finding reveals a potential path for the design of WOLED. In this paper, we report a high-brightness, broad-spectrum WOLED with two EMLs consisting of only three organic layers. Alq3-doped NPB film was designed as a hole transport layer and EML1, which provides blue and green emissions.27 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9enyl)-4H-pyran (DCJTB) was doped into Alq3 as electron transport layer and EML2, which provides green and red emissions.28 Between the two EMLs, a thin BCP (2,9-dimethyl4,7-diphenyl-1,10-phenanthroline) layer was inserted as the hole block layer (HBL).29 First, the doping concentrations of Alq3 and DCJTB in EML1 and EML2 were determined to be 0.2% and 0.1%, respectively. Then the thickness of BCP layer was optimized to be 5 nm by monitoring the EL spectra of the devices with increasing the thickness of BCP layer. After the thicknesses of EML1 and EML2 were modulated, a pure white EL device with high brightness and broad spectrum was obtained. On the basis of energy level alignment and EL spectra, injection, transport, distribution, and recombination of holes and electrons in this device were investigated in detail.

10.1021/jp106823p  2010 American Chemical Society Published on Web 11/19/2010

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Experimental Section Materials. All the organic materials used in this study were obtained commercially and used as received. Fabrication of OLEDs. Indium-tin oxide (ITO) coated glass with a sheet resistance of 15 Ω/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in deionized water, dried in an oven, and finally treated with oxygen plasma for 10 min at a pressure of 10 Pa to enhance the surface work function of ITO anode (from 4.7 to 5.1 eV).30 All the organic layers were deposited at a rate of 0.1 nm/s under high vacuum (e3 × 10-5 Pa). The doped layers were prepared by coevaporating guest and host materials from two individual sources, and the doping concentrations were modulated by controlling the evaporation rates of guest materials. LiF and Al were deposited in another vacuum chamber (e8.0 × 10-5 Pa) with rates of 0.01 and 1 nm/s, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual material were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten 10 mm2 devices on each substrate. Measurement. Current density-voltage-brightness (J-V-B) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. Here, the optical response of the silicon photodiode was calibrated to be 8640 cd/m2 · mV by placing the OLED 3 cm away from the surface of the silicon photodiode. Therefore, brightness (B) can be obtained as

B ) 8640U

(1)

where U is the response voltage detected by the silicon photodiode. The current efficiency (ηL), expressed in cd/A, is the way to characterize the quality of a device and represents the ratio of the brightness (B) to the current density (J) flowing into the diode. The power efficiency (ηP), expressed in lm/W, is the ratio of the optical flux to the electrical input and is given by

ηP ) (Bπ)/(JV) ) ηL(π/V)

Figure 1. Proposed energy levels diagram of the designed WOLED in this paper.

(2)

where V is the working voltage. The EL spectra were measured with a calibrated Hitachi F-4500 fluorescence spectrophotometer. On the basis of the uncorrected EL fluorescence spectra, the Commission Internationale de l’Eclairage (CIE) coordinates were calculated using a test program of the Spectrascan PR650 spectrophotometer. Results and Discussion Figure 1 shows the proposed energy levels diagram of the designed WOLED with a structure of ITO/Alq3:NPB/BCP/ DCJTB:Alq3/LiF(1 nm)/Al(100 nm). Here, Alq3 doped NPB film functions as hole transport layer (HTL) and EML1, which provides blue and green emissions,27 while DCJTB doped Alq3 film functions as electron transport layer (ETL) and EML2, which provides green and red emissions.28 For WOLEDs, it is well-known that the development of blue emission has become the bottleneck of efficiency improvement due to the relatively low luminescent efficiencies of most blue-emitting materials.11 Therefore, the doping concentration of Alq3 in EML1 was first determined to be 0.2%, at which concentration fastest electron injection into EML1 and maximal contribution of blue emission

Figure 2. The brightness-voltage characteristics of these devices with different doping concentrations of DCJTB. Inset: EL efficiency-current density characteristics of these devices with different doping concentrations of DCJTB.

can be obtained according to our previous investigation.27 To optimize the doping concentration of DCJTB in EML2, five devices having the configuration ITO/NPB(50 nm)/DCJTB(x%): Alq3(60 nm)/LiF(1 nm)/Al(100 nm) were fabricated by controlling x to be 0, 0.1, 0.2, 0.4, and 0.6, respectively. As shown in Figure 2, the 0.1% doped device obtained the highest EL efficiency and brightness. Therefore, the doping concentration of DCJTB in EML2 was provisionally determined to be 0.1%. For most multiple EML WOLEDs, a HBL is necessary to balance the distribution of holes and electrons in different EMLs.31-33 In this device, BCP was selected as the hole block material due to its low-lying HOMO level of -6.7 eV.34-36 Here, the BCP layer function as also the ETL because of its excellent electron transport ability and the near LUMO levels of BCP (-3.2 eV) and Alq3 (-3.3 eV).34,36 The presence of HBL limits the injection of holes into EML2, thus causing the accumulation of holes and electrons in EML1 and HBL, respectively. Therefore, the thickness of BCP layer plays the crucial role in determining the relative distribution of holes and electrons. To determine the optimal thickness of BCP layer in this work, four devices with the structure ITO/Alq3(0.2%):NPB(50 nm)/BCP(y nm)/DCJTB(0.1%):Alq3(50 nm)/LiF(1 nm)/Al(100 nm) were fabricated and measured by modulating y to be 2, 4, 6, and 8, respectively. Figure 3 shows the EL spectra of these devices

White Organic Electroluminescent Device

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Figure 3. Normalized EL spectra of these devices with different thicknesses of BCP layer operating at 100 mA/cm2.

Figure 4. Normalized EL spectra of devices A, B, C, D, and E operating at 100 mA/cm2.

TABLE 1: The Key Properties of These Devices with Different Thicknesses of the BCP Layera

TABLE 2: The Key Properties of Devices A, B, C, D, and Ea

device

Von-set (V)

B (cd/m2)

ηc (cd/A)

ηp (lm/W)

CIE coordinates (x, y)

2 nm 4 nm 6 nm 8 nm

2.7 2.7 3.0 3.2

49370.8 44985.5 23206.7 18534.1

8.88 7.47 4.16 3.06

9.53 6.12 3.58 1.68

(0.471, 0.477) (0.427, 0.440) (0.256, 0.292) (0.214, 0.259)

a The data for brightness (B), EL current efficiency (ηc), and EL power efficiency (ηp) are the maximum values of the device.

operating at 100 mA/cm2, three emissions centered at 450, 515, and 590 nm, respectively, are observed. With increasing thickness, 590 nm emission decreases gradually, indicating the decreasing hole injection into EML2, while the 450 and 515 nm emissions increase gradually, implying the increasing electron injection into EML1. Along with the decreasing brightness and efficiency, as shown in Table 1, the CIE coordinates shift from (0.471, 0.477) to (0.214, 0.259) with increasing BCP thickness. The 4 nm device shows obviously comparatively weak blue emission, while the 6 nm device shows comparatively weak red emission. Therefore, the thickness of BCP layer was temporarily designed to be 5 nm. In succession, a device having the structure ITO/Alq3(0.2%): NPB(50 nm)/BCP(5 nm)/DCJTB(0.1%):Alq3(50 nm)/LiF(1 nm)/ Al(100 nm), denoted as device A, was fabricated. Figure 4 shows the EL spectrum of device A operating at 100 mA/cm2, a broad EL spectrum covering the whole visible zone from 390 to 720 nm can be observed; as shown in Table 2, the corresponding CIE coordinates are (0.365, 0.383). To further optimize the emission spectrum, two devices were fabricated by modulating the thicknesses of EMLs. Increasing the thickness of EML1 to 60 nm (device B) shifts the CIE coordinates to (0.342, 0.369), and further decreasing the thickness of EML2 to 45 nm (device C) causes the shift of CIE coordinates to (0.335, 0.351). Figure 5 describes the EL spectra of device C, 450 and 515 nm emissions increase gently with increasing current density, corresponding to the shift of CIE coordinates from (0.375, 0.414) at 0.1 mA/cm2 to (0.335, 0.351) at 100 mA/ cm2. At a brightness of 20000 cd/m2, device C has the CIE coordinates of (0.328, 0.336), which is very close to ideal for white light, i.e., (0.33, 0.33). More importantly, the broadspectrum white emission helps to obtain higher color rendering index (CRI) compared with two-peak white emission.37 To

device

Von-set (V)

B (cd/m2)

ηc (cd/A)

ηp (lm/W)

CIE coordinates (x, y)

A B C D E

2.8 2.8 2.7 3.4 2.8

32658.5 33515.5 35788.0 20085.7 49687.6

6.29 6.69 8.02 4.20 8.36

5.42 5.90 9.33 2.49 7.76

(0.365, 0.383) (0.342, 0.369) (0.335, 0.351) (0.171, 0.191) (0.502, 0.453)

a The data for brightness (B), EL current efficiency (ηc), and EL power efficiency (ηp) are the maximum values of the device.

Figure 5. Normalized EL spectra of device C operating at 0.1, 1.0, 10, and 100 mA/cm2, respectively.

clarify the origins of these three emission peaks, two other devices were fabricated with the following structures: ITO/ Alq3(0.2%):NPB(60 nm)/BCP(20 nm)/Alq3(25 nm)/LiF(1 nm)/ Al(100 nm) (device D) and ITO/NPB(60 nm)/DCJTB(0.1%): Alq3(45 nm)/LiF(1 nm)/Al(100 nm) (device E). Devices D and E show emission peaks at 450 and 590 nm, respectively; while both devices have a shoulder emission at 515 nm, which corresponds to the emission of Alq3.27,38 The 590 nm emission in device E comes from DCJTB,28,39 while the 450 nm emission in device D has previously been demonstrated to originate from NPB.27 That is to say, the 450, 515, and 590 nm emissions in this white device come from NPB, Alq3, and DCJTB, respectively. Due to doping effect and the combination of EMLs with carrier transport layers, device structure was significantly

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Figure 6. Brightness-voltage characteristics of devices C, D, and E. Inset: EL efficiency-current density characteristics of devices C, D, and E.

TABLE 3: The Key Properties of Device C and Previously Reported WOLEDsa device C ref 11 ref 20 ref 37 ref 40 ref 41

Von-set (V)

B (cd/m2)

ηc (cd/A)

ηp (lm/W)

CIE coordinates (x, y)

2.7 3.0 2.4

35788.0 12320 21500 55800 12930 18100

8.02 7.0

9.33 7.1 8.1 2.24

(0.335, 0.351) (0.29, 0.34) (0.32, 0.33) (0.33, 0.32) (0.339, 0.318) (0.321, 0.357)

5.0

4.06 2.66 10.6

9.5

a

The data for brightness (B), EL current efficiency (ηc), and EL power efficiency (ηp) are the maximum values of the device.

simplified. Consequently, high current density was obtained at low applied voltage without sacrificing the EL efficiency. Therefore, pure white emission with high brightness and low operational voltage was realized. As shown in Figure 6 and Table 2, devices C, D, and E obtained the maximum brightness of 35788.0, 20085.7, and 49687.6 cd/m2, at 12.7, 12.6, and 13.4 V, respectively. Maximal current efficiency of device C is 8.02 cd/A, while the maximal power efficiency is as high as 9.33 lm/W due to the low turn-on voltage (driving voltage at 1 cd/ cm2) of 2.7 V. With increasing current density, as shown in the insert of Figure 6, EL efficiency of device C first decreases gradually and then keeps constant at about 6.0 cd/A; with further increase in the current density, the EL efficiency decreases rapidly. A flat zone in the EL efficiency ranges from 2 to 200 mA/cm2, ensuring relatively high efficiency at high current density and thus high maximum brightness. As shown in Table 3, these results are comparable to or even much higher than some of the high-performance fluorescent WOLEDs reported before.11,20,37,40,41 More importantly, it is worthwhile to mention that NPB is not an efficient blue-emitting material; therefore, both efficiency and brightness can be further enhanced by selecting more efficient blue-emitting materials.27 Furthermore, we have investigated in detail the injection and transport processes of holes and electrons in this white device. As shown in Figure 7, holes inject into EML1 from the anode and penetrate into EML2 across the HBL. Here, the obvious Alq3 emission in device D confirms the possibility of hole transfer from NPB to Alq3 molecules in EML1 (process 1 in Figure 7) of device C.42,43 Then the appearance of DCJTB emission in device C demonstrates that holes can transfer from NPB to DCJTB molecules (process 2) although the presence

Zhou et al.

Figure 7. The injection and transport processes of holes and electrons in device C. The symbols “+” and “-” represent hole and electron, respectively. The energy levels of the materials used in this investigation are also presented.

of 5 nm HBL. The Alq3 emission observed in device E means the transfer of holes from NPB to Alq3 molecules in EML2 (process 3); furthermore, partial holes on Alq3 molecules will be trapped by DCJTB molecules (process 4).37 On the other hand, electrons inject into EML2 from the cathode and penetrate into EML1 via the HBL. During this process, electrons will be trapped by DCJTB molecules in EML2 (process 5) and Alq3 molecules in EML1 (process 6) due to the energy level alignment of these used materials.37,44 In this case, the appearance of NPB emission demonstrates the possibility of electron transfer from BCP to NPB molecules (process 7), which has been reported in a previous paper.27 Furthermore, we have proposed upward band bending of the doped Alq3 to interpret the increased electron injection into EML1 after the controlled doping of Alq3. We suggested that Alq3 molecules in EML1 function as the ladder of electron injection from BCP to NPB molecules (process 8), thus causing the enhancement of electron injection into EML1.27 In this paper, we mainly report the design and demonstration of a novel white devices with Alq3 as electron injection sensitizer, whereas the mechanisms of increased electron injection are not the focus of this work and cannot be described clearly in a few sentences. Therefore, further investigation about this issue is still needed, and detailed results and discussions will be published in another paper in future. As discussed above, HBL in device C limits the injection of holes into EML2, thus controlling the distribution of holes and electrons within the two EMLs. In EML1, at relatively low voltage, electrons injected from HBL will be trapped by Alq3 molecules, while holes injected from the anode situate on NPB molecules predominantly.43 Therefore, as shown in Figure 8a, holes and electrons are the minority carriers on Alq3 and NPB molecules, respectively. In EML2, DCJTB will trap not only the electrons injected from cathode but also the holes injected from EML1 because its energy levels are within those of Alq3.37 As a result, holes will be the minority carriers on both DCJTB and Alq3 molecules due to the limit of hole injection from EML1 into EML2. With increasing voltage, holes and electrons have been demonstrated to accumulate in EML1 and HBL, respectively,29 thus accelerating the injections of holes and electrons into EML2 and EML1, respectively, due to the increasing builtin field.42 Therefore, broadening of recombination zone was expected in both EML1 and EML2, which was suggested to be responsible for the decrease of EL efficiency from turn-on to 2 mA/cm2.45 At relatively high voltage, as shown in Figure 8b, enhanced hole accumulation in EML1 causes the increased hole

White Organic Electroluminescent Device

Figure 8. Distribution of holes and electrons in the two EMLs of device C at relatively low (a) and high (b) voltage. The horizontal lines represent the energy levels of Alq3, DCJTB, NPB, and BCP, respectively. The symbols “+” and “-” represent hole and electron, respectively.

density on Alq3 molecules, while the improved hole injection into EML2 causes the increased hole densities on both DCJTB and Alq3 molecules in EML2. Similarly, improved electron injection into EML1 causes the increased electron density on NPB molecules. With increasing voltage, however, the densities of electrons on Alq3 molecules in EML1 and on DCJTB molecules in EML2 tend to saturation due to the low doping levels.29,43 On the basis of these discussions, we can conclude that holes and electrons on NPB, Alq3, and DCJTB molecules tend to balance with increasing voltage, which compensates the effect of broader recombination zone on EL efficiency. As a result, roll-off of EL efficiency was retarded, which results in the flat zone of EL efficiency shown in the insert of Figure 6. Conclusions In summary, we have demonstrated an efficient energy level alignment technology to design WOLED consisting of three organic layers. Alq3 doped NPB film was designed as HTL and EML1, while DCJTB doped Alq3 film was selected as ETL and EML2. To balance the distribution of holes and electrons within the two EMLs, a thin BCP layer was inserted between them as HBL. After the doping concentrations and thicknesses were optimized, pure white device with maximum brightness of 35788.0 cd/m2, current efficiency of 8.02 cd/A, and power efficiency of 9.33 lm/W was obtained. With the help of energy levels and EL spectra, injection, transport, and distribution of holes and electrons in this device were investigated. Finally, the interesting phenomenon of EL efficiency flat zone from 2 to 200 mA/cm2 was attributed to the synergy of improved carrier balance and the broadening of recombination zone. Acknowledgment. We are grateful for the financial aid from the National Natural Science Foundation of China (Grant No. 21071140) and National Natural Science Foundation for Creative Research Group (Grant No. 20921002). We would like to thank Professor Dongge Ma for valuable discussions. References and Notes (1) Duggal, R.; Shiang, J. J.; Heller, C. M.; Foust, D. F. Appl. Phys. Lett. 2002, 80, 3470. (2) Shen, Z.; Burrows, P. E.; Bulovic´, V.; Forrest, S. R.; Thompson, M. E. Science 1997, 276, 2009. (3) Wong, W.-Y.; Ho, C.-L. Coord. Chem. ReV. 2009, 253, 1709. (4) Zhou, G. J.; Wang, Q.; Ho, C.-L.; Wong, W.-Y.; Ma, D. G.; Wang, L. X. Chem. Commun. 2009, 3574. (5) Wang, Q.; Ding, J.; Ma, D.; Cheng, Y.; Wang, L.; Jing, X.; Wang, F. AdV. Funct. Mater. 2009, 19, 84.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21727 (6) Ho, C.-L.; Lin, M.-F.; Wong, W.-Y.; Wong, W.-K.; Chen, C. H. Appl. Phys. Lett. 2008, 92, 083301. (7) Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Lin, Z. Y. AdV. Funct. Mater. 2008, 18, 928. (8) Wang, Q.; Ho, C.-L.; Zhao, Y. B.; Ma, D. G.; Wong, W.-Y.; Wang, L. X. Org. Electron. 2010, 11, 238. (9) Xie, Z.; Xie, W.; Li, F.; Liu, L.; Wang, H.; Ma, Y. J. Phys. Chem. C 2008, 112, 9066. (10) Lee, M. T.; Chen, H. H.; Liao, C. H.; Tsai, C. H.; Chen, C. H. Appl. Phys. Lett. 2004, 85, 3301. (11) Tao, S. L.; Zhou, Y. C.; Lee, C.-S.; Lee, S.-T.; Huang, D.; Zhang, X. H. J. Phys. Chem. C 2008, 112, 14603. (12) Gebeyehu, D.; Walzer, K.; He, G.; Pfeiffer, M.; Leo, K.; Brandt, J.; Gerhard, A.; Sto¨βel, P.; Vestweber, H. Synth. Met. 2005, 148, 205. (13) Lee, J.; Lee, J.-I.; Song, K.-I.; Lee, S. J.; Chu, H. Y. Appl. Phys. Lett. 2008, 92, 203305. (14) He, F.; Tian, L. L.; Xie, W. J.; Li, M.; Gao, Q.; Hanif, M.; Zhang, Y. F.; Cheng, G.; Yang, B.; Ma, Y. G.; Liu, S. Y.; Shen, J. C. J. Phys. Chem. C 2008, 112, 12024. (15) Tong, Q.-X.; Lai, S.-L.; Chan, M.-Y.; Zhou, Y.-C.; Kwong, H.-L.; Lee, C.-S.; Lee, S.-T. Chem. Mater. 2008, 20, 6310. (16) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967. (17) He, G. F.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.; Pudzich, R.; Salbeck, J. Appl. Phys. Lett. 2004, 85, 3911. (18) Shi, J.; Tang, C. W. Appl. Phys. Lett. 2002, 80, 3201. (19) Kalinowski, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Williams, J. A. G. AdV. Mater. 2007, 19, 4000. (20) Li, L.; Yu, J. S.; Tang, X. Q.; Wang, T.; Li, W.; Jiang, Y. D. J. Lumin. 2008, 128, 1783. (21) Wong, W.-Y.; Ho, C.-L. J. Mater. Chem. 2009, 19, 4457. (22) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (23) Shaheen, S. E.; Kippelen, B.; Peyghambarian, N.; Wang, J. F.; Anderson, J. D.; Mash, E. A.; Lee, P. A.; Armstrong, N. R.; Kawabe, Y. J. Appl. Phys. 1999, 85, 7939. (24) Chwang, A. B.; Kwong, R. C.; Brown, J. J. Appl. Phys. Lett. 2002, 80, 725. (25) Wu, H. B.; Zhou, G. J.; Zou, J. H.; Ho, C.-L.; Wong, W.-Y.; Yang, W.; Peng, J. B.; Cao, Y. AdV. Mater. 2009, 21, 4181. (26) Li, A. Y.; Li, Y. Y.; Cai, W. Z.; Zhou, G. J.; Chen, Z.; Wu, H. B.; Wong, W.-Y.; Yang, W.; Peng, J. B.; Cao, Y. Org. Electron. 2010, 11, 529. (27) Zhou, L.; Tang, J. K.; Guo, Z. Y.; Feng, J.; Li, X. N.; Li, X. Y.; Deng, R. P.; Zhang, H. J. J. Lumin. 2010, 130, 2265. (28) Young, R. H.; Tang, C. W.; Marchetti, A. P. Appl. Phys. Lett. 2002, 80, 874. (29) Zhou, L.; Zhang, H. J.; Deng, R. P.; Guo, Z. Y.; Feng, J.; Li, Z. F. J. Phys. Chem. C 2008, 112, 15065. (30) Chan, I.-M.; Cheng, W.-C.; Hong, F. C. Appl. Phys. Lett. 2002, 80, 13. (31) Guo, F. W.; Ma, D. G.; Wang, L. X.; Jing, X. B.; Wang, F. S. Semicond. Sci. Technol. 2005, 20, 310. (32) Kim, C. H.; Shinar, J. Appl. Phys. Lett. 2002, 80, 2201. (33) Huang, C. C.; Meng, H. F.; Ho, G. H.; Chen, C. H.; Hsu, C. S.; Huang, J. H.; Horng, S. F.; Chen, B. X.; Chen, L. C. Appl. Phys. Lett. 2004, 84, 1195. (34) Baldo, M. A.; Forrest, S. R. Phys. ReV. B 2000, 62, 10958. (35) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967. (36) Adachi, C.; Baldo, M. A.; Forrest, S. R. J. Appl. Phys. 2000, 87, 8049. (37) Chang, M.-Y.; Wang, C.-H.; Lin, S.-C.; Chen, Y.-F. J. Appl. Phys. 2009, 105, 064318. (38) Qin, D. S.; Li, D. C.; Wang, Y.; Zhang, J. D.; Xie, Z. Y.; Wang, G.; Wang, L. X.; Yan, D. H. Appl. Phys. Lett. 2001, 78, 437. (39) Yang, S. H.; Liu, M. H.; Su, Y. K. J. Appl. Phys. 2006, 100, 083111. (40) Ding, G.-Y.; Jiang, W.-L.; Wang, J.; Wang, L.-Z.; Wang, G.-D.; Cong, L.; Ouyang, X.-H.; Zeng, H.-P. Semicond. Sci. Technol. 2009, 24, 025016. (41) Jou, J.-H.; Wang, C.-J.; Lin, Y.-P.; Chung, Y.-C.; Chiang, P.-H.; Wu, M.-H.; Wang, C.-P.; Lai, C.-L.; Chang, C. Appl. Phys. Lett. 2008, 92, 223504. (42) Parker, I. D. J. Appl. Phys. 1993, 75, 1656. (43) Zhou, L.; Zhang, H. J.; Deng, R. P.; Li, Z. F.; Yu, J. B.; Guo, Z. Y. J. Appl. Phys. 2007, 102, 064504. (44) Nu¨esch, F.; Berner, D.; Tutisˇ, E.; Schaer, M.; Ma, C.; Wang, X.; Zhang, B.; Zuppiroli, L. AdV. Funct. Mater. 2005, 15, 323. (45) Kalinowski, J.; Palilis, L. C.; Kim, W. H.; Kafafi, Z. H. J. Appl. Phys. 2003, 94, 7764.

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