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J. Phys. Chem. C 2010, 114, 16746–16749
Band Alignment at Anode/Organic Interfaces for Highly Efficient Simplified Blue-Emitting Organic Light-Emitting Diodes Zhiwei Liu,*,† Michael G. Helander, Zhibin Wang, and Zhenghong Lu* Department of Materials Science and Engineering, UniVersity of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4 ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: August 20, 2010
Efficient simplified blue-emitting phosphorescent organic light-emitting diodes (OLEDs) were fabricated with a nickel oxide (Ni2O3) or molybdenum oxide (MoO3) modified indium tin oxide (ITO) anode. The maximum current efficiency of device anode/bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium (FIrpic)/ FIrpic:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi)/LiF/Al was increased from 3.6 cd/A with ITO anode to 22.0 and 23.6 cd/A for Ni2O3 and MoO3 modified ITO anodes, respectively. Moreover, the maximum current and power efficiencies of another bilayer device ITO/MoO3/FIrpic:4,4′-N,N′-dicarbazole-biphenyl (CBP)/ FIrpic:TPBi/LiF/Al were as high as 49 cd/A and 48 lm/W, respectively. Photoelectron spectroscopy measurements were employed to investigate the electronic structure of the anode/organic interfaces. Results show that the band alignment at the oxide/organic interface plays a critical role in these simplified blue devices. 1. Introduction Organic light-emitting diodes (OLEDs) have been extensively studied due to their potential applications in flat-panel or flexible displays and solid state lighting ever since the first efficient OLED was reported.1 In order to meet the need for lower power consumption and higher operational stability in practical application, it is crucial to enhance the carrier injection from the electrode to the organics. For the anode, indium tin oxide (ITO) has been used as the de facto standard anode due to its high transparency and conductivity.2 Thus, numerous efforts have been undertaken on the modification of ITO, including improving the ITO surface with wet,3 plasma,4,5 or UV ozone treatment,6 and modifying the ITO with transition metal oxides.2,7-10 These methods improved the interface between the anode and the organic layers, and significantly enhanced device efficiency and stability. For full-color displays or solid state lighting, the core of primary colors must be established, which involves the development of highly efficient red-, green-, and blue-emitting OLEDs. It is demonstrated that efficient red and green OLEDs have been achieved and meet industry standards for applications.11 On the other hand, the blue component remains a challenge. Therefore, it is desirable to design blue-emitting OLEDs with high efficiency. Moreover, considering that multilayer device structures add to the complexity in material selection/matching and procuring and hence are cumbersome in scaled up mass production, efficient blue-emitting OLEDs with a much simplified structure are more favorable.12 Recently, we have reported simplified blue-emitting devices with the configuration ITO/bis(3,5-difluoro-2-(2-pyridyl)phenyl(2-carboxypyridyl)iridium (FIrpic)/FIrpic:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi)/LiF/Al by using the concept of direct hole injection into dopant, where a neat FIrpic layer * To whom correspondence should be addressed. E-mail: zhiwei.liu@ utoronto.ca (Z. Liu);
[email protected] (Z. Lu). † Present address: Department of Chemistry, University of Southern California, Los Angeles 90089. E-mail:
[email protected].
was used as a buffer layer.13,14 However, both the driving voltage and efficiency were not satisfactory, which is attributed to the high hole injection barrier from ITO into the FIrpic layer. There are many metal oxides having a work function higher than that of ITO and which may improve the anode/FIrpic interface. Thus, it is expected that using some of these metal oxides as buffer layers between the ITO anode and FIrpic layer could enhance the electrical properties of simplified blue OLEDs via improvement of hole injection. In this work, we use Ni2O3 or MoO3 modified ITO as the anode in devices with the structure anode/FIrpic/FIrpic:TPBi/ LiF/Al. It is found that the current efficiency was increased from 3.6 cd/A with ITO anode to 22.0 and 23.6 cd/A for Ni2O3 and MoO3 modified ITO anodes, respectively. The improvement in device performance is consistent with a lower hole injection barrier at the anode/organic interfaces measured using photoelectron spectroscopy. To further improve device performance, a double emission layer device structure was adopted in the bilayer device ITO/MoO3/FIrpic:4,4′-N,N′-dicarbazole-biphenyl (CBP)/FIrpic:TPBi/LiF/Al, which shows maximum current and power efficiencies of 49 cd/A and 48 lm/W, respectively. Photoelectron spectroscopy measurements indicate that holes are preferentially injected from the ITO/MoO3 anode into the CBP host, resulting in a much reduced driving voltage and increased efficiency. 2. Experimental Section ITO, Ni, MoO3, FIrpic, TPBi, CBP, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (R-NPD), LiF, and Al were obtained from commercial sources and used as received. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a PHI 5500 Multi-Technique system with monochromatic Al KR (hV ) 1486.7 eV) and He IR (hV ) 21.22 eV).15 For the device fabrication, nickel or molybdenum oxide was thermal vapor deposited on precleaned patterned ITO in a Kurt J. Lesker LUMINOS cluster tool and treated by ex situ oxidation with
10.1021/jp105782w 2010 American Chemical Society Published on Web 09/02/2010
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J. Phys. Chem. C, Vol. 114, No. 39, 2010 16747
Figure 1. Chemical structure and energy level diagram of the molecules used in this work.
ultraviolet (UV) ozone for 30 min. The substrates were then immediately loaded back into the cluster tool. The organic layers were deposited in a dedicated chamber with a base pressure of ∼10-8 Torr. The doping concentration of both FIrpic:CBP and FIrpic:TPBi layers was maintained around 18 wt %. The Al cathode (100 nm) was deposited in a separate chamber under a pressure of ∼10-7 Torr. The luminance-current-voltage (LIV) measurements were performed using a Minolta LS-110 luminance meter and HP4140B picoammeter. The electroluminescent (EL) spectra were recorded using a USB2000-UV-vis Miniature Fiber Optic Spectrometer. All measurements were carried out in ambient atmosphere and at room temperature.
Figure 2. I-V curves for hole only devices with the configuration ITO/metal oxide/R-NPD (200 nm)/Ag (60 nm), where the thicknesses of the deposited metal (Ni) or metal oxide (MoO3) film were varied between 0, 0.5, 1.0, and 2.0 nm.
3. Results and Discussion Figure 1 shows the chemical structure and energy-level diagram of the molecules used in this work. The highest occupied molecule orbital (HOMO) and lowest unoccupied molecule orbital (LUMO) energy levels for FIrpic,16 CBP,17 and TPBi18 are cited from the literature and are in good agreement with our measurements. Both the chemical composition and work function of the metal oxides fabricated in our system were defined.19 XPS results showed that the UV-treated nickel film has a peak of Ni 2p2/3 with a binding energy of 855.4 eV, consistent with the value of Ni2O3. The average composition of MoO3 was calculated from the XPS peak intensities and the corresponding XPS sensitivity factors of the Mo 3d and O 1s core levels. The work functions of Ni2O3 and MoO3 were deduced to be 5.3 and 5.4 eV, respectively, which are higher than that of ITO (∼5.0 eV). In order to optimize the thickness of the metal (Ni) or metal oxide (MoO3) layer, hole only devices were fabricated.19 The devices have a structure of ITO/metal oxide/R-NPD (200 nm)/ Ag (60 nm), where the thickness of the metal or metal oxide layer was varied between 0, 0.5, 1.0, and 2.0 nm (thickness based on a calibrated quartz crystal microbalance). Due to the high energy barrier between the LUMO of R-NPD and the work function of the Ag cathode, any observed current can be exclusively attributed to holes flowing from the anode to the cathode. Figure 2 shows current density-voltage (I-V) curves of these devices. It was found that the current density of the devices with metal oxide layer increased a lot compared to the device without the metal oxide layer, indicating better hole injection from the metal oxide modified ITO to the R-NPD layer. Moreover, the I-V curves showed that the device with 1 nm metal or metal oxide have the highest current density, indicating that Ni2O3 obtained from 1.0 nm nickel and 1.0 nm MoO3 are sufficient for hole injection. After defining the optimal thickness of Ni2O3 and MoO3, two simplified blue-emitting devices were fabricated with a configuration of ITO/Ni2O3 or MoO3/FIrpic (10 nm)/FIrpic:TPBi (100
Figure 3. Electroluminescence spectra of devices with the configuration anode/FIrpic (10 nm)/FIrpic:TPBi (100 nm)/LiF (1 nm)/Al (100 nm), where the anodes are ITO, ITO/Ni2O3, and ITO/MoO3, respectively.
nm)/LiF (1 nm)/Al (100 nm). For comparison, a reference device with ITO as the anode was also fabricated on the same substrate to eliminate possible run-to-run variability caused by subtle variations in process conditions. Figure 3 shows the electroluminescence spectra of the three devices at 10 V. The devices all have an identical emission band around 500 nm, which is the same as the device ITO/FIrpic:TPBi/LiF/Al we have fabricated, indicating that the emission originates from the FIrpic:TPBi layer.14 However, the emission spectra observed here are different from those reported for FIrpic-based devices, having an emission peak around 470 nm with a subpeak around 500 nm.11 This may be explained by the low triplet energy level of TPBi, which consequently results in a favored energy transfer to the lower triplet state of FIrpic. The L-I-V and efficiency curves of the three devices are shown in Figure 4. The devices with ITO/Ni2O3 and ITO/MoO3 anodes have higher current density, lower driving voltage, as well as significantly improved efficiency. For example, the turnon voltages at 1 cd/m2 are 8.0, 3.6, and 3.4 V for the devices with ITO, ITO/Ni2O3, and ITO/MoO3 anodes, respectively. The maximum current and power efficiencies for the devices with ITO/Ni2O3 and ITO/MoO3 anodes were 22.0 and 25.5 cd/A and 19.2 and 23.6 lm/W, respectively, which are much higher than the reference device, having highest efficiencies of only 3.6 cd/A and 0.8 lm/W.20 The performance improvement might be attributed to the increase of the anode’s work function and reduction of the energy barrier for hole injection into the FIrpic layer. To test this hypothesis, XPS and UPS measurements were employed to investigate the electronic structure of the anode/
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Liu et al. TABLE 1: Performance of Devices with Structure of Anode/FIrpic (10 nm)/FIrpic:CBP (50 nm)/ FIrpic:TPBi (50 nm)/LiF/Al, Where the Anodes Are ITO, ITO/Ni2O3, and ITO/MoO3, Respectively 1 cd/m2
1000 cd/m2
anode
Va
ηcb
ηpc
V
ηc
ηp
ITO ITO/Ni2O3 ITO/MoO3
7.3 5.0 6.0
2.9 33.0 25.9
1.3 20.6 13.6
12.7 11.7 10.7
17.7 23.7 31.4
4.4 6.4 9.2
a
Voltage (V). b Current efficiency (cd/A). c Power efficiency (lm/
W).
Figure 4. (a) L-I-V and (b) efficiency curves of devices with the structure anode/FIrpic (10 nm)/FIrpic:TPBi (100 nm)/LiF/Al; the anodes are ITO, ITO/Ni2O3, and ITO/MoO3, respectively.
Figure 5. (a) He IR UPS valence band spectra for anode/FIrpic, where the anodes are ITO, ITO/Ni2O3, and ITO/MoO3, respectively. (b) Valence band spectra for ITO/MoO3/FIrpic and ITO/MoO3/CBP.
organic interfaces. Figure 5a shows the He IR valence band spectra measured in situ after the deposition of FIrpic film (3 nm) on different anodes. The anodes modified with Ni2O3 and MoO3 have a significantly lowered hole injection barrier from anode to FIrpic, which is in accordance with the performance improvement in the devices with Ni2O3 and MoO3 modified anode. Here, the injection barrier height is estimated as the offset of the leading edge of the HOMO derived peak (the first peak at ∼1.5 eV for the ITO/MoO3 anode) to the Fermi level (i.e., 0 eV binding energy). The performance of the devices with ITO/Ni2O3 and ITO/ MoO3 anode is indeed improved. However, the efficiency of the two devices dropped quickly with increasing luminance; for example, the roll-off current and power efficiencies were 64 and 89% at a luminance of 1000 cd/m2 for the device with the ITO/Ni2O3 anode, while these were 52 and 85% for the device with an ITO/MoO3 anode, respectively. It was demonstrated that the problematic efficiency roll-off in blue phosphorescent OLEDs can be effectively suppressed by introducing a double emission layer concept.21 Herein, another two devices with a
Figure 6. Electroluminescence spectra of the devices ITO/MoO3/FIrpic (0 or 10 nm)/FIrpic:CBP (50 nm)/FIrpic:TPBi (50 nm)/LiF/Al.
structure of ITO/Ni2O3 or MoO3/FIrpic (10 nm)/FIrpic:CBP (50 nm)/FIrpic:TPBi (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated. Table 1 summarizes the performance of the two devices, as well as that of the device with ITO anode. The two devices with modified anode have higher maximum efficiency and lower roll-off efficiency, compared to the single emission layer device. However, we noticed that both the double emission layer and single emission layer devices had high driving voltage (a luminance of 1000 cd/m2 was achieved at over 11 V) and low current density (not more than 50 mA/cm2 even at a high applied bias of 15 V). Since the hole injection barrier from ITO to FIrpic is reduced by using a Ni2O3 or MoO3 layer and there will be no problem for electron injection/transport in the doped TPBi layer,13 the low current density may be attributed to weak hole transport ability of the FIrpic layer. Moreover, the turn-on voltage in the double emission layer devices is only suppressed by 1-2 V with a thin metal oxide layer, while it was much suppressed (4-5 V) in the single emission layer devices. This may be attributed to inefficient hole injection from FIrpic to FIrpic: CBP, as well as the fact that the FIrpic:CBP layer has weaker electron transport ability than the FIrpic:TPBi layer, which is not good for hole-electron recombination. Thus, a bilayer device without the pure FIrpic layer ITO/MoO3/FIrpic:CBP (50 nm)/FIrpic:TPBi (50 nm)/LiF/Al is expected to have better electrical properties and hence better performance. Figure 6 shows the electroluminescence spectrum of the device and that of the device with the FIrpic layer. The two devices have almost the same emission spectra, indicating that hole-electron combination occurs in the doped layers. The L-I-V and efficiency curves of the two devices are compared in Figure 7. The current density increased remarkably as the FIrpic layer was eliminated, 219 mA/cm2 compared to 34 mA/cm2 at 15 V. Accordingly, the device without the FIrpic layer has a lower driving voltage and higher efficiency. The maximum current and power efficiencies were 49 cd/A and 48
Band Alignment at Anode/Organic Interfaces
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16749 functions than that of ITO, lowered hole injection barrier from anode to FIrpic, balanced hole and electron injection, and hence improved device performance. Moreover, it is also found that the device configuration can be simplified to ITO/MoO3/FIrpic: CBP/FIrpic:TPBi/LiF/Al. The simplified device exhibits lower driving voltage and higher efficiency, since the hole injection barrier from ITO/MoO3 to CBP is found to be much lower than that to FIrpic. Acknowledgment. We wish to acknowledge Ontario Centres of Excellence and Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support. References and Notes
Figure 7. (a) L-I-V and (b) efficiency curves of the devices ITO/ MoO3/FIrpic (0 or 10 nm)/FIrpic:CBP (50 nm)/FIrpic:TPBi (50 nm)/ LiF/Al.
lm/W at 1 cd/m2, respectively, and these were as high as 40 cd/A and 23 lm/W even when the luminance was increased to 100 cd/m2. To understand the difference of the two devices, valence band spectra of the ITO/MoO3/FIrpic and ITO/MoO3/ CBP interfaces were measured, shown in Figure 5b. The HOMO derived peak of CBP is much closer to the Fermi level (i.e., 0 eV binding energy) than that of FIrpic, indicating holes are preferentially injected into CBP even though it has a higher HOMO energy level (with respect to vacuum). Clearly, band alignment at the anode/organic interface is critical to device performance. 4. Conclusion In summary, we have demonstrated improved blue-emitting OLEDs by coating thin Ni2O3 and MoO3 layers on ITO with device structures of anode/FIrpic/FIrpic:TPBi/LiF/Al and anode/ FIrpic/FIrpic:CBP/FIrpic:TPBi/LiF/Al. XPS and UPS measurements indicate that the Ni2O3 and MoO3 layers have higher work
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