Efficient Single-Layer Organic Light-Emitting Diodes Based on C545T

Jun 18, 2010 - The doping concentration, as well as nickel oxide (Ni2O3) and ... the emission color varied from green to yellow; the Commission Intern...
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J. Phys. Chem. C 2010, 114, 11931–11935

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Efficient Single-Layer Organic Light-Emitting Diodes Based on C545T-Alq3 System 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: February 9, 2010; ReVised Manuscript ReceiVed: May 28, 2010

We report efficient single-layer organic light-emitting diodes (OLEDs) based on 2,3,6,7-tetrahydro-1,1,7,7,tetramethyl-1H,5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a,1gh] coumarin (C545T) and tris(8-hydroxyquinolinolato)aluminum (Alq3) system. The doping concentration, as well as nickel oxide (Ni2O3) and molybdenum oxide (MoO3)-modified indium tin oxide (ITO) anodes were investigated. With increasing doping concentration from 1 to 23%, the emission color varied from green to yellow; the Commission Internationale de l’Eclairage (CIE) coordinate changed from (0.29, 0.66) to (0.45, 0.53). The green- to yellow-emitting devices showed current efficiency from 6.4 to 4.2 cd/A for ITO/Ni2O3 anode and 12.8 to 5.5 cd/A for ITO/ MoO3 anode at 100 cd/m2, respectively, which are better than 3.8 cd/A obtained in the traditional undoped three-layered device ITO/copper-phthalocyanine (CuPc)/ N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl4,4′-diamine (R-NPD)/Alq3/LiF/Al and comparable to 14.4 cd/A in the doped device ITO/CuPc/R-NPD/ C545T-Alq3 (∼1%)/Alq3/LiF/Al. 1. Introduction Ever since the report of a heterostructure bilayer device by Tang and Vanslyke,1 organic light-emitting diodes (OLEDs) have been extensively studied due to their potential applications in flat-panel or flexible displays and solid state lighting. To improve device performance to commercialize this technology, the design of new electroluminescence materials, as well as device configurations, has received much attention.2-4 During the search for new electroluminescence materials, the development of phosphorescent dopant, such as fac-tris(2-phenylpyridine)iridium (Ir(ppy)3), was an important breakthrough to harvest both singlet and triplet excitons, leading to a potential internal quantum efficiency of 100%.5,6 As for the device configuration, molecules with different functions, such as hole injection layer (HIL), hole transport layer (HTL), hole blocking layer (HBL), electron transport layer (ETL), and so forth, are deposited to form a multilayered structure, thus to balance electrons and holes and to confine excitons in the emission layer.7,8 Consequently, efficiencies that can be reached today with phosphorescent multilayered OLEDs are as high as, or even higher than, those of highly efficient inorganic LEDs.9 There is no doubt that the efficiency of some phosphorescent multilayered OLEDs has already reached and even exceeded the requirements of practical applications. However, phosphorescent dopants are costly and require complicated device structures, so there has been a continuous interest in fabricating fluorescent OLEDs with a much simplified structure, especially single layer devices.10-14 In theory, single-layer structured fluorescent OLEDs are possible since there are organic molecules capable of acting simultaneously as the hole-transporting, electron-transporting, and light-emitting units. However, a material used for efficient single layer OLEDs must not only fulfill a series of basic requirements such as good thin-film properties, high glass transition temperature, and chemical and * To whom correspondence should be addressed. E-mail: (Z. Liu) [email protected]; (Z. Lu) [email protected]. † Present address: Department of Chemistry, University of Southern California, Los Angeles, CA 90089. E-mail: [email protected].

photochemical stability but should also have a well matched highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels with the electrodes to reduce the charge injection barrier. In addition, a high and balanced hole and electron mobility is required to confine the exciton formation zone away from the electrodes, as well as a high-luminescence quantum yield to make sure that excitons can be efficiently transformed into luminance, and so forth. One can image the difficulty in searching for such a molecule; thus the performance of fluorescent single layer OLEDs still remains unsatisfactory. For example, a maximum current efficiency of 7.7 cd/A was reported as the best performance among fluorescent single layer OLEDs.15 In contrast to pursuing a new molecule with multifunctional capacities,16 herein we report single layer OLEDs with a configuration of ITO/Ni2O3 or MoO3/C545T-Alq3/LiF/Al, where C545T and Alq3 are 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a,1gh] coumarin and tris(8-hydroxyquinolinolato)aluminum, respectively. Both C545T and Alq3 are well-known green lightemitting dopant and electron-transporting host, respectively, with good film forming and thermal stability. Ni2O3 and MoO3 are selected to align the energy matching between the anode and the organic molecules. It was found that the emission color of this single layer device can be slowly turned from green to yellow by varying the doping concentration. The best greenemitting device is found to be ITO/MoO3 (1 nm)/C545T-Alq3 (∼1%, 100 nm)/LiF (1 nm)/Al (100 nm), showing a current efficiency of 12.8 cd/A even at a brightness of 100 cd/m2, which is comparable to standard multilayered C545T devices.17 With a high-doping concentration of 23%, the best yellow-emitting device also showed a high current efficiency of 5.5 cd/A at 100 cd/m2. 2. Experimental Section All devices were fabricated in a Kurt J. Lesker LUMINOS cluster tool. Commercially patterned indium tin oxide (ITO) anode with a sheet resistance of 15 Ohms/sq was ultrasonically

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Figure 1. The schematic device structure, chemical structure, and energy level diagram of the molecules in this work.

cleaned with a standard regiment of Alconox, acetone, and methanol followed by ultraviolet (UV) ozone treatment. Patterned nickel or molybdenum oxide film was thermal vapor deposited on ITO using a shadow mask and treated by ex situ oxidation with UV ozone for 30 min. The substrate was then loaded into the cluster tool again to complete the other layers. The organic layers and LiF layer were deposited in a dedicated organic chamber with a base pressure of ∼10-8 Torr. The Al or Ag cathode lines (2 mm wide) were deposited orthogonally to the ITO anode lines (1 mm wide) in a separate metallization chamber with a base pressure of ∼10-7 Torr. The chemical composition of nickel or molybdenum oxide and their work functions were measured by X-ray photoelectron spectroscopy (XPS), using a PHI 5500 Multi-Technique system withmonochromaticAlKR (hν)1486.7eV).Theluminance-current density-voltage (L-I-V) was measured using a HP4140B picoammeter and Minolta LS-110 luminance meter. The electroluminescent (EL) spectra were recorded using an USB2000UV-vis Miniature Fiber Optic Spectrometer. All measurements were carried out in ambient atmosphere and at room temperature. 3. Results and Discussion Figure 1 shows the schematic device structure, chemical structure, and energy level diagrams of the molecules used in this work. The work function of ITO and HOMOs for C545T and Alq3 are ∼5.0,18 5.5,19 and 5.720 eV, respectively. On the basis of the energy-level alignment, the hole injection barrier from ITO to HOMOs of C545T and Alq3 are 0.5 and 0.7 eV, respectively. Thus, it is expected that the electrical property of the device could be improved by adopting nickel oxide- or molybdenum oxide-modified ITO as the anode, since these oxides exhibit higher work function than ITO, which should be more favorable for hole injection.13 Prior to fabricating OLEDs with metal oxide-modified anode, we have defined the chemical composition and work function of these oxides fabricated in our system. XPS results showed that the UV-treated nickel film has a peak of Ni 2p2/3 with binding energy of 855.4 eV, consistent with the value of Ni2O3. The average composition, O/Mo atomic ratio, has been calculated from the XPS peak intensities and the corresponding XPS sensitivity factors of the Mo 3d and O 1s core levels. It was found that the molybdenum oxide has an approximate composition of MoO3. As for the work function measurement, 1 nm nickel film or molybdenum oxide was deposited on ITO, treated

Figure 2. He IR secondary electron cuto-off spectra for ITO, ITO/ Ni2O3, and ITO/MoO3.

with UV ozone for 30 min (which is exactly the same fabrication method in OLEDs), and then loaded into the XPS measurement. The work function of Ni2O3 and MoO3 were deduced to be ∼5.3 and 5.4 eV, respectively, which is higher than that of ITO (it should be noted that ITO is sensitive to the He IR UV photons used to measure work function hence the measured value may vary from 4.7 to 5.2 eV) (Figure 2). Considering that Ni2O3 is not as transparent as ITO, it is also necessary to optimize the thickness of Ni2O3 layer. Herein, four hole-only devices were fabricated on a single substrate to eliminate possible run-to-run variability caused by subtle variations in process conditions with a configuration of ITO/ Ni2O3/N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′diamine (R-NPD)/(200 nm)/Ag (60), where the thickness of original nickel film varied between 0, 0.5, 1.0, and 2.0 nm. I-V curves indicate that the devices with Ni2O3 layer have much higher current density, arising from a better hole injection from Ni2O3 to R-NPD layer. Moreover, the current density saturated when the thickness of nickel film increased from 1.0 to 2.0 nm, indicating that Ni2O3 obtained from 1.0 nm nickel is sufficient for hole injection improvement. Similarly, four hole-only devices using ITO/MoO3 (0, 0.5, 1.0, or 2.0 nm) anode were also fabricated to optimize the thickness of MoO3. It was found that the devices with MoO3 layer have a better hole injection and the optimal thickness for MoO3 is 1.0 nm. After defining the optimal thickness of Ni2O3 and MoO3, three devices with a configuration of anode/Alq3 (100 nm)/ LiF/Al were fabricated, in which the anodes are ITO, ITO/Ni2O3, and ITO/MoO3, respectively. Typical properties of these single layer

Single-Layer OLEDs Based on C545T-Alq3 System

Figure 3. (a) LIV and (b) luminance-efficiency characteristics of devices anode/Alq3 (100 nm)/LiF (1 nm)/Al (100 nm), where the anodes are ITO, ITO/Ni2O3, and ITO/MoO3, respectively.

devices are shown in Figure 3. The maximum luminance of the device with ITO anode was 210 cd/m2 at 15 V and the highest current efficiency reached 0.17 cd/A at 40 cd/m2. The performance is consistent with that of the device ITO/Alq3 (120 nm)/Mg-Ag reported by Hu et al.12 and ITO/Alq3 (80 nm)/ LiF/Al by Chan et al.14 While for the device with the Ni2O3 layer, the maximum luminance was increased to 2600 cd/m2, and the highest current efficiency was also increased to 1.35 cd/A, which is attributed to that the Ni2O3-coated ITO anode has better hole injection ability. The improved device performance was also observed in the device with ITO/MoO3 anode, having maximum current efficiency of 1.51 cd/A. However, it should be noted that the device with ITO/MoO3 anode has lower current density and driving voltage, which may indicate that the MoO3 layer has another function of electron blocking.21 Since it was proved that holes can be more easily injected from Ni2O3 and MoO3 into Alq3 layer, we first used Ni2O3coated ITO as the anode and investigated the effect of doping concentration on the device performance. Figure 4 shows the L-I-V and efficiency characteristics of devices ITO/Ni2O3/ C545T-Alq3 (100 nm)/LiF/Al, where the doping concentration of C545T was varied from 1 to 23%. For comparison, the properties of the single layer device without C545T were also plotted. As can be seen from the L-I-V curves, the device without C545T has the highest current density. This may be attributed to the dopant C545T, which reduces the electron transport ability of the whole layer. With increasing doping concentration, the electron transport ability reduces while hole transport in C545T increases gradually. The competition of the two factors leads to a higher current density in the device with 23% C545T than the device with 14% C545T. These devices also have similar varying regularity in luminance characteristic responding to doping concentration except for the device with

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Figure 4. (a) L-I-V and (b) luminance-efficiency characteristics of devices ITO/Ni2O3 (1 nm)/C545T-Alq3 (100 nm)/LiF (1 nm)/Al (100 nm) with different doping concentration.

1% C545T. The device with 1% C545T has the lowest driving voltage, even lower than the pure Alq3-based device with highest current density; this may indicate that C545T-doped Alq3 is more electroluminescent than Alq3. As for the efficiency properties of these devices, the doped device has lower power efficiency when the doping concentration is increased. Similar to the results for the C545T-Alq3 system, the doped device achieves maximum power efficiency for mass ratio of C545T-Alq3 of ∼1%, since the aggregate quenching is increased at high concentrations. Differently, the device with 4% C545T has the highest current efficiency of 7.5 cd/A at 100 cd/m2. Though the device has a heavier quenching than the device with only 1% C545T, the much reduced current density in the former device may be the key reason for the high current efficiency. It should be noted that even the doping concentration of the device is increased to 23%, the device still has high current and power efficiencies of 4.2 cd/A and 1.5 lm/W, respectively. Encouraged by the performance of the single layer devices with the ITO/Ni2O3 anode, another three devices with ITO/MoO3 anode were fabricated, in which the doping concentration of C545T varied between 1, 4, and 23%. Figure 5 shows the L-I-V and efficiency characteristics of these three devices. Similar to the above demonstrated devices, the device with lower C545T doping concentration has higher current density and higher efficiencies. The best performance was observed in the device with a doping concentration of 1%, having maximum luminance of 26 750 cd/m2 at 15 V, and current and power efficiencies of 14.3 cd/A and 15.0 lm/W at 1.18 cd/m2, respectively. The device still has current and power efficiencies as high as 12.8 cd/A and 6.6 lm/W even when the luminance was increased to 100 cd/m2, respectively. As the doping concentration increased to 4%, the maximum luminance decreased to 2238 cd/m2 at 15 V and efficiencies to 10.8 cd/A

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Figure 7. Lifetime tends of the single layer device ITO/MoO3 (1 nm)/ C545T-Alq3 (1%, 100 nm)/LiF/Al and standard multilayer device ITO/ CuPc (25 nm)/R-NPD (45 nm)/C545T-Alq3 (1%, 30 nm)/Alq3 (15 nm)/LiF/Al. Devices were tested at 49 mA/cm2 in air; the initial luminance for the single layer and multilayer devices are 3363 and 9223 cd/m2, respectively.

Figure 5. (a) L-I-V and (b) luminance-efficiency characteristics of devices ITO/MoO3/C545T-Alq3 (100 nm)/LiF (1 nm)/Al (100 nm) with different doping concentration.

and CIE chromaticity coordinate of (0.45, 0.53) as the doping concentration increased to 23%. The red-shifted emission spectrum might be assigned to excimer of C545T molecules at high concentration. Since the yellow-emitting devices still have high efficiency, especially for the device with MoO3 modified anode, it is reasonable to realize efficient green to yellow singlelayer OLEDs via changing the doping concentration in the C545T-Alq3 system. To understand the stability of single layer device, Figure 7 shows the accelerated lifetime trends of the single layer device ITO/MoO3 (1 nm)/C545T-Alq3 (1%, 100 nm)/LiF/Al and standard multilayer device ITO/CuPc (25 nm)/R-NPD (45 nm)/ C545T-Alq3 (1%, 30 nm)/Alq3 (15 nm)/LiF/Al, where CuPc is copper phthalocyanine. As can be seen from the figure, the single layer device has a much shorter lifetime. This is understandable since the electron-hole recombination zone in the single layer device may close to electrode, which is in contrast to the fact that the recombination zone of the multilayer device is confined near the junction. 4. Conclusions

Figure 6. EL spectra of devices ITO/Ni2O3 (1 nm)/C545T-Alq3 (100 nm)/LiF (1 nm)/Al (100 nm) with different doping concentration (spectra were measured around 500 cd/cm2).

and 8.9 lm/W at 1.14 cd/m2, respectively. The maximum luminance was decreased to 932 cd/m2 and efficiencies to 6.1 cd/A and 4.9 lm/W with a further doping concentration increased to 23%, respectively. The single layer devices show significant decrease of efficiency with an increase of current density, suggests that the electron-hole recombination zone is strongly dependent on the operational conditions of the devices.12 It was also observed that the emission spectrum of the single layer device is only critical to doping concentration. Figure 6 shows EL spectra of devices ITO/Ni2O3 (1 nm)/C545T-Alq3 (100 nm)/LiF (1 nm)/Al (100 nm) with varied doping concentration. The spectrum has been red shifted when the doping concentration is increased. The device with 1% doped C545T has a green emission with maximum band at 523 nm, and the calculated Commission Internationale de l’Eclairage (CIE) coordinate is (0.29, 0.66). The emission was red shifted gradually to a yellow region with a maximum band at 571 nm,

In summary, we have demonstrated efficient single layer OLEDs with configuration of ITO/Ni2O3 or MoO3 (1 nm)/ C545T-Alq3 (100 nm)/LiF (1 nm)/Al (100 nm). The maximum emission band red shifted from 520 to 570 nm, accompanying with CIE coordinate changed from (0.29, 0.66) to (0.45, 0.53) as the doping concentration of C545T varied from ∼1 to 23%. It was found that the green- and yellow-emitting devices showed maximum luminance of 26750 and 2238 cd/m2 at 15 V, and current efficiency of 12.8 and 5.5 cd/A at a brightness of 100 cd/m2, respectively. We attribute the excellent performance of this simplified system to metal oxide-modified ITO anode, which is favorable for hole injection or electron blocking and hence balances charge in the emission layer. Acknowledgment. We wish to acknowledge Ontario Centres of Excellence and Natural Sciences and Engineering Research Council (STPGP-351047-07) of Canada for financial support. References and Notes (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Tang, C. W.; Vanslyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (3) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.

Single-Layer OLEDs Based on C545T-Alq3 System (4) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (5) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (6) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (7) He, G. F.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.; Pudzich, R.; Salbeck, J. Appl. Phys. Lett. 2004, 85, 3911. (8) He, G. F.; Schneider, O.; Qin, D. S.; Zhou, X.; Pfeiffer, M.; Leo, K. J. Appl. Phys. 2004, 95, 5773. (9) Cho, T. Y.; Lin, C. L.; Wu, C. C. Appl. Phys. Lett. 2006, 88, 111106. (10) Lane, P. A.; Kushto, G. P.; Kafafi, Z. H. Appl. Phys. Lett. 2007, 90, 023511. (11) Tse, S. C.; Tsung, K. K.; So, S. K. Appl. Phys. Lett. 2007, 90, 213502. (12) Hu, W. P.; Matsumura, M. Appl. Phys. Lett. 2002, 81, 806. (13) Chan, I. M.; Hsu, T. Y.; Hong, F. C. Appl. Phys. Lett. 2002, 81, 1899. (14) Chan, I. M.; Hong, F. C. Thin Solid Films 2004, 450, 304. (15) Huang, T. H.; Lin, J. T.; Chen, L. Y.; Lin, Y. T.; Wu, C. C. AdV. Mater. 2006, 18, 602. (16) Wang, G.; He, Y. Mater. Lett. 2009, 63, 470.

J. Phys. Chem. C, Vol. 114, No. 27, 2010 11935 (17) To understand the reliability of our single layer OLEDs, as well as to compare with the tradtional fluorescent OLEDs, we fabricated a threelayered fluorescent nondoped device with a configuration of ITO/CuPc (25 nm)/R-NPD (45 nm)/Alq3 (45 nm)/LiF (1 nm)/Al (100 nm). The current efficiency at 100 cd/m2 of the device is 3.8 cd/A. Moreover, we also fabricated a four-layered fluorescent doped device with a configuration of ITO/CuPc (25 nm)/R-NPD (45 nm)/C545T-Alq3 (∼1%, 30 nm)/Alq3 (15 nm)/LiF (1 nm)/Al (100 nm); the device has current efficiency of 14.4 cd/A at 100 cd/m2, which is comparable to the value of 14 cd/A reported by Chwang et al.; see: Chwang, A. B. Appl. Phys. Lett. 2002, 80, 725. (18) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 7, 904. (19) You, H.; Dai, Y. F.; Zhang, Z. Q.; Ma, D. G. J. Appl. Phys. 2007, 101, 026105. (20) Jeon, S. O.; Jeon, Y. M.; Kim, J. W.; Lee, C. W.; Gong, M. S. Org. Electron. 2008, 9, 522. (21) We have fabricated three hole-only devices with configuration of anode/Alq3 (100 nm)/Ag, where the anodes are ITO, ITO/Ni2O3, and ITO/ MoO3, respectively. The I-V curves show that the devices with ITO/MoO3 and ITO/Ni2O3 anodes have identical current density and which are higher than that of the device with ITO anode.

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