- Email: [email protected]
Low-voltage and high-efficiency white organic light emitting devices with carrier balance
Physica B 405 (2010) 4434–4438
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Low-voltage and high-efficiency white organic light emitting devices with carrier balance Fuxiang Wei n, Y. Huang, L. Fang School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 March 2010 Received in revised form 5 August 2010 Accepted 6 August 2010
White organic light emitting devices with the structure of ITO/m-MTDATA:x%4F-TCNQ/NPB/ TBADN:EBDP:DCJTB/Bphen:Liq/LiF/Al have been demonstrated in this paper. High-mobility m-MTDATA:4F-TCNQ is added into the region between ITO and NBP to increase hole injection and transport. The high-mobility Bphen:Liq layer is added into the region between cathode and emission layers to lower cathode barrier and facilitate carrier injection. In the meanwhile, an effective carrier balance (number of holes is equal to number of electrons) between holes and electrons is considered to be one of the most important factors for improving OLEDs. During the experiment, by modulating the doping concentration of 4F-TCNQ, we can control hole injection and transport to make the carriers reach a high-level balance. The maximum current efficiency and power efficiency of devices were 9.3 cd/A and 4.6 lm/A, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: White organic light emitting devices Carriers Balance Low voltage
1. Introduction White organic light emitting diodes (OLEDs) have attracted wide attention in view of practical applications for displays and lighting [1–4], since Kido et al. [5] reported white OLEDs in the year of 1995. However, the low efficiency of light emission and high drive voltage become a main difficulty, which urgently needs to be solved. Yamada et al. [6] proved that because of the differences in the aspects of injection and transport capabilities for two carriers, the carrier with higher transporting rate will directly pass through the light-emitting layer and reach the electrode to be quenched. Therefore, it will reduce the recombination probability and affect the luminous efficiency. Currently, several methods are widely used to enhance luminous efficiency: increasing effective injection and transport for both electrons and holes in the organic layer; balancing the injection between electrons and holes for guaranteeing effective recombintion of carriers with opposite charges; modifying the interfaces between electrode/organic layers and between organic/organic layers; and enhancing fluorescence quantum efficiency of the light-emitting devices. So far, the way to improve the performance of white OLEDs stressed the importance of increasing injection and transport capabilities, while relatively overlooking the role of modulating the carrier balance [7–10]. Researchers found that introduction of high-conductivity charge injection and transport layers can efficiently reduce the
n
Corresponding author. Tel.: +86 516 83880131. E-mail address: [email protected] (F. Wei).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.010
drive voltage of devices, while increasing the luminous efficiency to some extent. In addition, it is necessary to increase the charge balancing factor, that is, increase the injection balance of the carriers. One of our previous works has confirmed that m-MTDATA:4F-TCNQ can improve hole injection and transport, while Bphen:Liq can improve electron injection and transport [11]. Therefore in here, we added m-MTDATA:4F-TCNQ into the region between ITO and NPB, and Bphen:Liq layer into the region between cathode and light-emitting layer. By regulating the doping concentration of 4F-TCNQ, we can regulate hole injection and transport, and considerably increase the compound efficiency of the devices, and finally obtain low-voltage white OLEDs with high-level carrier balance.
2. Experimental process The white OLEDs were fabricated on glass substrates precoated with indium tin oxide (ITO) with a sheet resistance of 20 O/square. We selected m-MTDATA:x%4F-TCNQ as the hole injection layer, N,N0 -bis-(1-naphenyl)-N,N0 -biphenyl-1,10 -bipheny1-4-40 -diamine (NPB) as the hole transport layer, TBADN:EBDP:DCJTB as the emission layer, Liq (33%):Bphen as the electron transport layer (ETL), metal Al as the cathode, which was evaporated for about 12 and 0.5 nm LiF as the buffer layer for better electron injection. The device structure was ITO/m-MTDATA:x%4F-TCNQ /NPB/TBADN: EBDP:DCJTB/ Bphen:Liq/LiF/Al. The molecular structures of the main organic materials and schematic structure of white devices are shown in Fig. 1. In the devices, the 4F-TCNQ volume
F. Wei et al. / Physica B 405 (2010) 4434–4438
4435
F O N
N
N Li Liq
F
CN
CN
CN
CN F
N
NPB
F 4F-TCNQ
N
Bphen LiF/Al
H3C
CH3 Bphen:Liq
N
N
TBADN:EBDP:DCJTB
N
NPB m-MTDATA:x%4F-TCNQ
N
CH3 ITO glass
m-MTDATA Fig. 1. Molecular structures of main organic materials and schematic structure of devices.
500
400 Current density (mA/cm2)
percentages varied from 0% to 3%, which are used to control the injection efficiency of the hole, so as to obtain high-efficiency white OLEDs with high carrier balance. The organic layers and cathode LiF/Al were sequentially deposited by conventional vacuum vapor deposition in a chamber without breaking the vacuum. The pressure of the chamber was kept at 8 10 4 Pa. Electroluminescence (EL) spectrum and the Commission Internationale de I’Eclairage (CIE) coordinates were measured by a PR650 Spectroscanner, and luminance–current– voltage (L–I–V) characteristics were measured by a Keithley 2400 Source Meter. All the data were obtained in the unsealed condition and measured under room temperature.
H0 H0.2 H2
300
H3 200
100
3. Results and discussion 0
For the study of hole injection and transport capability of m-MTDATA:4F-TCNQ films, a series of hole-only devices were fabricated. These hole-only devices have the following structures: device H0:ITO/m-MTDATA (80 nm)/Al(120 nm) device H0.2:ITO/m-MTDATA:0.2 wt% 4F-TCNQ(80 nm)/Al(120 nm) device H2:ITO/m-MTDATA:2 wt%4F-TCNQ(80 nm)/Al(120 nm) device H3:ITO/m-MTDATA:3 wt%4F-TCNQ(80 nm)/Al(120 nm) A series of electron-only devices were also fabricated in order to obtain data on the electron transport capability of Bphen:Liq films. The structures of the electron-only devices are as follows: device E0:ITO/BCP(5 nm)/Bphen(80 nm)/Al(120 nm) device E0.2:ITO/BCP(5 nm)/Bphen:20 wt%Liq(80 nm)/Al(120 nm) device E0.33:ITO/BCP(5 nm)/Bphen:33 wt%Liq (80 nm)Al(120 nm) device E0.5:ITO/BCP(5 nm)/Bphen:50 wt%Liq (80 nm)/Al(120 nm) Fig. 2 shows the current density versus voltage characteristics at various doping ratios of F4-TCNQ to m-MTDATA for the holeonly devices. In the hole-only devices, a dramatic increase in device current was observed when 4F-TCNQ was doped into an m-MTDATA layer. Compared with the device with an undoped m-MTDATA layer, the slight doping strikingly decreased the onset voltage. The J–V characteristics are strongly dependent on the
0
2
4
6 Voltage (v)
8
10
12
Fig. 2. Current versus voltage characteristics of hole-only devices.
doping ratio of the hole transport layer. At the same voltage, the current density increased as doping rate increased. Fig. 3 shows the current density versus voltage characteristics at various doping ratios of Liq to Bphen for the electron-only devices. In the electron-only devices, the Liq volume percentages varied from 20% to 50%, which is used to investigate the effect of Liq/Bphen ratio on electron injection and transport characteristics of the Liq:Bphen layer. A thin layer of BCP was used to prevent holes from entering the ETL because of its highly occupied molecular orbital (HOMO) level (6.7 eV). A dramatic increase in current was observed when Liq was doped into Bphen. The highest current density was observed for the devices with the Liq volume percentage of 33%, which is due to the highest conductivity, or mobility, of the electrons in the Liq:Bphen layer.
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F. Wei et al. / Physica B 405 (2010) 4434–4438
450 Device E0
Current Density (mA/cm2)
400
Device E0.2
350
Device E0.33
300
Device E0.5
250 200 150 100 50 0 1
2
3
4
5
6 7 8 Voltage (V)
9
10
11
12
4F-TCNQ was doped into m-MTDATA. At the same voltage, current density is significantly enhanced when the doping concentration of 4F-TCNQ increases. When the concentration is higher, the amount of holes will be higher; therefore the total current in the devices will be stronger. The injection and transport of holes can be adjusted easily by modulating the doping concentration of 4F-TCNQ. Typical luminance–voltage characteristics of the devices are plotted in Fig. 5. At the same driving voltage, the luminance of the devices is improved dramatically after introducing the highconductivity hole transport layer. When the driving voltage was 10 V, the luminance for the devices WHB0, WHB0.2, WHB2, and WHB3 were 8214, 12,560, 21,365, and 22,865 cd/m2, respectively. The devices WHB2 and WHB3 are very close in luminance at the same drive voltage, indicating that there is an optimal value for the 4F-TCNQ doping concentrations. Further increment of concentration can only increase the current density of the devices,
40000 Fig. 3. Current versus voltage characteristics of electron-only devices.
35000 WHB0
Device
OLED structure
WHB0 WHB0.2
ITO/m-MTDATA /NPB/TBADN:EBDP:DCJTB/Bphen:Liq/LiF/Al ITO/m-MTDATA:0.2%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al ITO/m-MTDATA:2%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al ITO/m-MTDATA:3%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al
WHB2 WHB3
Lumiance (cd/m2)
Table 1 Structures of organic white light emitting devices.
30000
WHB2
25000
WHB0.2 WHB3
20000 15000 10000 5000 0
Current density (mA/cm2)
400
4
6
8
WHB0 WHB2
300
10 12 Voltage (V)
14
16
18
Fig. 5. Luminance variation with driving voltage of devices.
WHB0.2 WHB3 200
WHB0
WHB0.2
WHB2
WHB3
9
100
4
6
8
10 12 Voltage (V)
14
16
18
Fig. 4. Current density variation with voltage of devices.
Therefore, the following study introduced high-mobility carrier injection and transport layers in the devices: m-MTDATA:4F-TCNQ is added into the region between ITO and NBP to increase hole injection, while Bphen:33 wt%Liq layer is added into the region between cathode and emission layer to increase electron injection, reduce drive voltage, and enhance device efficiency. In the meanwhile, by modulating the doping concentration of 4F-TCNQ to regulate hole injection and transport, white OLEDs with high-level carrier balance have been achieved in this paper. The device structures are listed in Table 1. Fig. 4 shows the current density–voltage relation curves for the white devices. A dramatic increase in current was observed when
7 Power efficiency (lm/W)
0
Current efficiency (cd/A)
8
6 5 4 3 2 1
4
3
2
1 0
0
100 200 300 400 Current density (mA/cm2)
-1 0
100
300 200 Current density (mA/cm2)
400
Fig. 6. Luminance efficiency–current density relationship of devices. Inset: power efficiency–current density relationship of devices.
F. Wei et al. / Physica B 405 (2010) 4434–4438
6
5 9 4 8 3
7
Power efficiency (lm/W)
Current efficiency (cd/A)
10
2 0
0.2%
2%
1.5
Intensity / arb.units
but not the luminance of devices. In such a case, the number of holes is more than that of electrons. Therefore, the superfluous holes will directly pass through the emission layer and reach the electrode to be quenched. Fig. 6 provides the current efficiency–current density relation curves of the devices (the inset is the power efficiency–current density relation). At low current densities, the current efficiency of all the four devices rose sharply to the maximum value, and then remained almost unchanged at higher current densities. This current independent efficiency response is favorable in the field of passive matrix OLED displays and lighting, where high luminance values are required. Fig. 7 shows the tendencies of current efficiency (at current density of 200 mA/cm2) and power efficiency (at current density of 4 mA/cm2) with change in 4F-TCNQ doping concentration levels. As shown, the efficiencies of devices increased gradually along with the doping concentration, but decreased after reaching a maximal value. The current efficiencies for the devices WHB0, WHB0.2, WHB2, and WHB3 were 8.1, 9.3, 8.33, and 7.75 cd/A, respectively. The current efficiency reached the maximum of 9.3 cd/A for the device WHB0.2, increased by 15% compared with that of WHB0. The power efficiencies for all the devices have similar changing tendencies. The 4F-TCNQ volume percentages varying from 0% to 3% can reduce drive voltage and control the injection efficiency of the hole. Power efficiencies for the devices WHB0.2, WHB2, and WHB3 have been markedly enhanced compared to that of WHB0. There is an optimal value for the 4F-TCNQ doping concentration at which the holes and electrons can reach high-level equilibrium, so as to improve the lightemitting efficiency of the devices. However, when the doping concentrations are too high or too low, the amount of electron and hole carriers will depart from the equilibrium, leading to lower efficiency of the devices. In white devices, the TBADN layer is simultaneously doped with blue dye EBDP and red dye DCJTB. White emission was obtained through an incomplete energy transfer of TBAND to EBDP and DCJTB in the same layer. The EL spectrums for different devices were almost the same; so in this paper we select only the typical representative (WHB2) to display, which has a fairly high current efficiency and luminance efficiency. The EL spectra of device WHB2 are shown in Fig. 8. The emission peaks of EBDP are
4437
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm) Fig. 8. EL spectra of white device WHB2.
located at 460 nm and a shoulder peak at 490 nm. It can be seen that in addition to the original blue peaks, another peak at 560 nm appears, which is ascribed to the red light emission from DCJTB. This indicates that energy is transferred from TBADN and EBDP to DCJTB. By varying the DCJTB concentration, the resulting color can be turned from blue to white. We have found that the optimal concentrations in TBADN for EBDP and DCJTB were both about 3%, providing high EL efficiency and good white color purity [12]. The CIE coordinates were 0.33 and 0.42. There is practically no EL color shift with varying drive currents of device WHB2. The CIEx,y color coordinates only shift from (0.333, 0.424) at 1 to (0.331, 0.420) at 100 mA/cm2 with Dx, Dy¼ 7[0.002,0.004]. 4. Conclusion By introducing high-mobility m-MTDATA:4F-TCNQ into the region between ITO and NBP, and introducing the high-mobility Bphen:Liq layer into the region between cathode and emission layer, low-voltage and high-efficiency white OLEDs have been achieved in this paper. By modulating the doping concentration levels of 4F-TCNQ we can obtain the optimum value for the 4F-TCNQ doping concentration, at which the holes and electrons can reach a high degree of equilibrium, so as to increase the efficiency of the devices. However, when the doping concentration levels are too high or too low, the amount of the electron and hole carriers will depart from the equilibrium, affecting the whole efficiency of the devices. The maximum current efficiency and power efficiency of devices achieved are 9.3 cd/A and 4.6 lm/W, respectively.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no.90202034) and Youth Foundation of China University of Mining and Technology (2007A051).
3%
wt% concentration of 4F-TCNQ Fig. 7. Relationship between efficiencies and 4F-TCNQ concentration of the devices. The arrow for each polyline points to the corresponding scales at both sides. On the X-axis, 0%, 0.2%, 2%, and 3% correspond to different devices WHB0, WHB0.2, WHB2, and WHB3, respectively.
References [1] C.-H. Chang, H.-C. Cheng, Y.-J. Lu, K.-C. Tien, H.-W. Lin, C.-L. Lin, C.-J. Yang, C.-C. Wu, Org. Electron. 11 (2010) 247. [2] 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.
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S. Sepeai, M.M. Salleh, M. Yahaya, A.A. Umar, Thin Solid Films 517 (2009) 4679. J. Lee, J.-I. Lee, H.Y. Chu, Synth. Met. 159 (2009) 991. J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. T. Yamada, F. Rohlfing, T. Tsutsui, Jpn. J. Appl. Phys. 39 (2000) 1382. Y.L. Meng, W.F. Xie, N. Zhang, N. Zhang, S. Chen, J. Li, W. Hu, Y. Zhao, J. Hou, S. Liu, Microelectron. J. 39 (2008) 1. [8] C.H. Tsai, C.H. Liao, M.T. Lee, C.H. Chen, Appl. Phys. Lett. 87 (2005) 243505.
[9] C.I. Wu, C.T. Lin, Y.H. Chen, M.H. Chen, Y.J. Lu, C.C. Wu, Appl. Phys. Lett. 88 (2006) 152104. [10] H. You, D.G. Ma, J. Phys. D 41 (2008) 155113. [11] M.A. Khan, W. Xu, F. Wei, Y. Bai, X.Y. Jiang, Z.L. Zhang, W.Q. Zhu, Solid State Commun. 144 (2007) 343. [12] F. Wei, X. Zhang, J. Cao, M.A. Khan, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, Displays 28 (2007) 186.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Low-voltage and high-efficiency white organic light emitting devices with carrier balance Fuxiang Wei n, Y. Huang, L. Fang School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 March 2010 Received in revised form 5 August 2010 Accepted 6 August 2010
White organic light emitting devices with the structure of ITO/m-MTDATA:x%4F-TCNQ/NPB/ TBADN:EBDP:DCJTB/Bphen:Liq/LiF/Al have been demonstrated in this paper. High-mobility m-MTDATA:4F-TCNQ is added into the region between ITO and NBP to increase hole injection and transport. The high-mobility Bphen:Liq layer is added into the region between cathode and emission layers to lower cathode barrier and facilitate carrier injection. In the meanwhile, an effective carrier balance (number of holes is equal to number of electrons) between holes and electrons is considered to be one of the most important factors for improving OLEDs. During the experiment, by modulating the doping concentration of 4F-TCNQ, we can control hole injection and transport to make the carriers reach a high-level balance. The maximum current efficiency and power efficiency of devices were 9.3 cd/A and 4.6 lm/A, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: White organic light emitting devices Carriers Balance Low voltage
1. Introduction White organic light emitting diodes (OLEDs) have attracted wide attention in view of practical applications for displays and lighting [1–4], since Kido et al. [5] reported white OLEDs in the year of 1995. However, the low efficiency of light emission and high drive voltage become a main difficulty, which urgently needs to be solved. Yamada et al. [6] proved that because of the differences in the aspects of injection and transport capabilities for two carriers, the carrier with higher transporting rate will directly pass through the light-emitting layer and reach the electrode to be quenched. Therefore, it will reduce the recombination probability and affect the luminous efficiency. Currently, several methods are widely used to enhance luminous efficiency: increasing effective injection and transport for both electrons and holes in the organic layer; balancing the injection between electrons and holes for guaranteeing effective recombintion of carriers with opposite charges; modifying the interfaces between electrode/organic layers and between organic/organic layers; and enhancing fluorescence quantum efficiency of the light-emitting devices. So far, the way to improve the performance of white OLEDs stressed the importance of increasing injection and transport capabilities, while relatively overlooking the role of modulating the carrier balance [7–10]. Researchers found that introduction of high-conductivity charge injection and transport layers can efficiently reduce the
n
Corresponding author. Tel.: +86 516 83880131. E-mail address: [email protected] (F. Wei).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.010
drive voltage of devices, while increasing the luminous efficiency to some extent. In addition, it is necessary to increase the charge balancing factor, that is, increase the injection balance of the carriers. One of our previous works has confirmed that m-MTDATA:4F-TCNQ can improve hole injection and transport, while Bphen:Liq can improve electron injection and transport [11]. Therefore in here, we added m-MTDATA:4F-TCNQ into the region between ITO and NPB, and Bphen:Liq layer into the region between cathode and light-emitting layer. By regulating the doping concentration of 4F-TCNQ, we can regulate hole injection and transport, and considerably increase the compound efficiency of the devices, and finally obtain low-voltage white OLEDs with high-level carrier balance.
2. Experimental process The white OLEDs were fabricated on glass substrates precoated with indium tin oxide (ITO) with a sheet resistance of 20 O/square. We selected m-MTDATA:x%4F-TCNQ as the hole injection layer, N,N0 -bis-(1-naphenyl)-N,N0 -biphenyl-1,10 -bipheny1-4-40 -diamine (NPB) as the hole transport layer, TBADN:EBDP:DCJTB as the emission layer, Liq (33%):Bphen as the electron transport layer (ETL), metal Al as the cathode, which was evaporated for about 12 and 0.5 nm LiF as the buffer layer for better electron injection. The device structure was ITO/m-MTDATA:x%4F-TCNQ /NPB/TBADN: EBDP:DCJTB/ Bphen:Liq/LiF/Al. The molecular structures of the main organic materials and schematic structure of white devices are shown in Fig. 1. In the devices, the 4F-TCNQ volume
F. Wei et al. / Physica B 405 (2010) 4434–4438
4435
F O N
N
N Li Liq
F
CN
CN
CN
CN F
N
NPB
F 4F-TCNQ
N
Bphen LiF/Al
H3C
CH3 Bphen:Liq
N
N
TBADN:EBDP:DCJTB
N
NPB m-MTDATA:x%4F-TCNQ
N
CH3 ITO glass
m-MTDATA Fig. 1. Molecular structures of main organic materials and schematic structure of devices.
500
400 Current density (mA/cm2)
percentages varied from 0% to 3%, which are used to control the injection efficiency of the hole, so as to obtain high-efficiency white OLEDs with high carrier balance. The organic layers and cathode LiF/Al were sequentially deposited by conventional vacuum vapor deposition in a chamber without breaking the vacuum. The pressure of the chamber was kept at 8 10 4 Pa. Electroluminescence (EL) spectrum and the Commission Internationale de I’Eclairage (CIE) coordinates were measured by a PR650 Spectroscanner, and luminance–current– voltage (L–I–V) characteristics were measured by a Keithley 2400 Source Meter. All the data were obtained in the unsealed condition and measured under room temperature.
H0 H0.2 H2
300
H3 200
100
3. Results and discussion 0
For the study of hole injection and transport capability of m-MTDATA:4F-TCNQ films, a series of hole-only devices were fabricated. These hole-only devices have the following structures: device H0:ITO/m-MTDATA (80 nm)/Al(120 nm) device H0.2:ITO/m-MTDATA:0.2 wt% 4F-TCNQ(80 nm)/Al(120 nm) device H2:ITO/m-MTDATA:2 wt%4F-TCNQ(80 nm)/Al(120 nm) device H3:ITO/m-MTDATA:3 wt%4F-TCNQ(80 nm)/Al(120 nm) A series of electron-only devices were also fabricated in order to obtain data on the electron transport capability of Bphen:Liq films. The structures of the electron-only devices are as follows: device E0:ITO/BCP(5 nm)/Bphen(80 nm)/Al(120 nm) device E0.2:ITO/BCP(5 nm)/Bphen:20 wt%Liq(80 nm)/Al(120 nm) device E0.33:ITO/BCP(5 nm)/Bphen:33 wt%Liq (80 nm)Al(120 nm) device E0.5:ITO/BCP(5 nm)/Bphen:50 wt%Liq (80 nm)/Al(120 nm) Fig. 2 shows the current density versus voltage characteristics at various doping ratios of F4-TCNQ to m-MTDATA for the holeonly devices. In the hole-only devices, a dramatic increase in device current was observed when 4F-TCNQ was doped into an m-MTDATA layer. Compared with the device with an undoped m-MTDATA layer, the slight doping strikingly decreased the onset voltage. The J–V characteristics are strongly dependent on the
0
2
4
6 Voltage (v)
8
10
12
Fig. 2. Current versus voltage characteristics of hole-only devices.
doping ratio of the hole transport layer. At the same voltage, the current density increased as doping rate increased. Fig. 3 shows the current density versus voltage characteristics at various doping ratios of Liq to Bphen for the electron-only devices. In the electron-only devices, the Liq volume percentages varied from 20% to 50%, which is used to investigate the effect of Liq/Bphen ratio on electron injection and transport characteristics of the Liq:Bphen layer. A thin layer of BCP was used to prevent holes from entering the ETL because of its highly occupied molecular orbital (HOMO) level (6.7 eV). A dramatic increase in current was observed when Liq was doped into Bphen. The highest current density was observed for the devices with the Liq volume percentage of 33%, which is due to the highest conductivity, or mobility, of the electrons in the Liq:Bphen layer.
4436
F. Wei et al. / Physica B 405 (2010) 4434–4438
450 Device E0
Current Density (mA/cm2)
400
Device E0.2
350
Device E0.33
300
Device E0.5
250 200 150 100 50 0 1
2
3
4
5
6 7 8 Voltage (V)
9
10
11
12
4F-TCNQ was doped into m-MTDATA. At the same voltage, current density is significantly enhanced when the doping concentration of 4F-TCNQ increases. When the concentration is higher, the amount of holes will be higher; therefore the total current in the devices will be stronger. The injection and transport of holes can be adjusted easily by modulating the doping concentration of 4F-TCNQ. Typical luminance–voltage characteristics of the devices are plotted in Fig. 5. At the same driving voltage, the luminance of the devices is improved dramatically after introducing the highconductivity hole transport layer. When the driving voltage was 10 V, the luminance for the devices WHB0, WHB0.2, WHB2, and WHB3 were 8214, 12,560, 21,365, and 22,865 cd/m2, respectively. The devices WHB2 and WHB3 are very close in luminance at the same drive voltage, indicating that there is an optimal value for the 4F-TCNQ doping concentrations. Further increment of concentration can only increase the current density of the devices,
40000 Fig. 3. Current versus voltage characteristics of electron-only devices.
35000 WHB0
Device
OLED structure
WHB0 WHB0.2
ITO/m-MTDATA /NPB/TBADN:EBDP:DCJTB/Bphen:Liq/LiF/Al ITO/m-MTDATA:0.2%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al ITO/m-MTDATA:2%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al ITO/m-MTDATA:3%4F-TCNQ /NPB/TBADN:EBDP:DCJTB/Bphen: Liq/LiF/Al
WHB2 WHB3
Lumiance (cd/m2)
Table 1 Structures of organic white light emitting devices.
30000
WHB2
25000
WHB0.2 WHB3
20000 15000 10000 5000 0
Current density (mA/cm2)
400
4
6
8
WHB0 WHB2
300
10 12 Voltage (V)
14
16
18
Fig. 5. Luminance variation with driving voltage of devices.
WHB0.2 WHB3 200
WHB0
WHB0.2
WHB2
WHB3
9
100
4
6
8
10 12 Voltage (V)
14
16
18
Fig. 4. Current density variation with voltage of devices.
Therefore, the following study introduced high-mobility carrier injection and transport layers in the devices: m-MTDATA:4F-TCNQ is added into the region between ITO and NBP to increase hole injection, while Bphen:33 wt%Liq layer is added into the region between cathode and emission layer to increase electron injection, reduce drive voltage, and enhance device efficiency. In the meanwhile, by modulating the doping concentration of 4F-TCNQ to regulate hole injection and transport, white OLEDs with high-level carrier balance have been achieved in this paper. The device structures are listed in Table 1. Fig. 4 shows the current density–voltage relation curves for the white devices. A dramatic increase in current was observed when
7 Power efficiency (lm/W)
0
Current efficiency (cd/A)
8
6 5 4 3 2 1
4
3
2
1 0
0
100 200 300 400 Current density (mA/cm2)
-1 0
100
300 200 Current density (mA/cm2)
400
Fig. 6. Luminance efficiency–current density relationship of devices. Inset: power efficiency–current density relationship of devices.
F. Wei et al. / Physica B 405 (2010) 4434–4438
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but not the luminance of devices. In such a case, the number of holes is more than that of electrons. Therefore, the superfluous holes will directly pass through the emission layer and reach the electrode to be quenched. Fig. 6 provides the current efficiency–current density relation curves of the devices (the inset is the power efficiency–current density relation). At low current densities, the current efficiency of all the four devices rose sharply to the maximum value, and then remained almost unchanged at higher current densities. This current independent efficiency response is favorable in the field of passive matrix OLED displays and lighting, where high luminance values are required. Fig. 7 shows the tendencies of current efficiency (at current density of 200 mA/cm2) and power efficiency (at current density of 4 mA/cm2) with change in 4F-TCNQ doping concentration levels. As shown, the efficiencies of devices increased gradually along with the doping concentration, but decreased after reaching a maximal value. The current efficiencies for the devices WHB0, WHB0.2, WHB2, and WHB3 were 8.1, 9.3, 8.33, and 7.75 cd/A, respectively. The current efficiency reached the maximum of 9.3 cd/A for the device WHB0.2, increased by 15% compared with that of WHB0. The power efficiencies for all the devices have similar changing tendencies. The 4F-TCNQ volume percentages varying from 0% to 3% can reduce drive voltage and control the injection efficiency of the hole. Power efficiencies for the devices WHB0.2, WHB2, and WHB3 have been markedly enhanced compared to that of WHB0. There is an optimal value for the 4F-TCNQ doping concentration at which the holes and electrons can reach high-level equilibrium, so as to improve the lightemitting efficiency of the devices. However, when the doping concentrations are too high or too low, the amount of electron and hole carriers will depart from the equilibrium, leading to lower efficiency of the devices. In white devices, the TBADN layer is simultaneously doped with blue dye EBDP and red dye DCJTB. White emission was obtained through an incomplete energy transfer of TBAND to EBDP and DCJTB in the same layer. The EL spectrums for different devices were almost the same; so in this paper we select only the typical representative (WHB2) to display, which has a fairly high current efficiency and luminance efficiency. The EL spectra of device WHB2 are shown in Fig. 8. The emission peaks of EBDP are
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located at 460 nm and a shoulder peak at 490 nm. It can be seen that in addition to the original blue peaks, another peak at 560 nm appears, which is ascribed to the red light emission from DCJTB. This indicates that energy is transferred from TBADN and EBDP to DCJTB. By varying the DCJTB concentration, the resulting color can be turned from blue to white. We have found that the optimal concentrations in TBADN for EBDP and DCJTB were both about 3%, providing high EL efficiency and good white color purity [12]. The CIE coordinates were 0.33 and 0.42. There is practically no EL color shift with varying drive currents of device WHB2. The CIEx,y color coordinates only shift from (0.333, 0.424) at 1 to (0.331, 0.420) at 100 mA/cm2 with Dx, Dy¼ 7[0.002,0.004]. 4. Conclusion By introducing high-mobility m-MTDATA:4F-TCNQ into the region between ITO and NBP, and introducing the high-mobility Bphen:Liq layer into the region between cathode and emission layer, low-voltage and high-efficiency white OLEDs have been achieved in this paper. By modulating the doping concentration levels of 4F-TCNQ we can obtain the optimum value for the 4F-TCNQ doping concentration, at which the holes and electrons can reach a high degree of equilibrium, so as to increase the efficiency of the devices. However, when the doping concentration levels are too high or too low, the amount of the electron and hole carriers will depart from the equilibrium, affecting the whole efficiency of the devices. The maximum current efficiency and power efficiency of devices achieved are 9.3 cd/A and 4.6 lm/W, respectively.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no.90202034) and Youth Foundation of China University of Mining and Technology (2007A051).
3%
wt% concentration of 4F-TCNQ Fig. 7. Relationship between efficiencies and 4F-TCNQ concentration of the devices. The arrow for each polyline points to the corresponding scales at both sides. On the X-axis, 0%, 0.2%, 2%, and 3% correspond to different devices WHB0, WHB0.2, WHB2, and WHB3, respectively.
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