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The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum devices with LiF/Mg:Ag/Ag cathodes were investigated. The devices with Mg:Ag/Ag, LiF/Al and Al-only cathodes were also fabricated in the same run for comparison. Similar to the LiF/Al cathodes, the LiF/Mg:Ag/Ag cathode greatly improved the performance of the device over the Mg:Ag/Ag cathode. A LiF layer of 0.5 nm significantly enhanced the electron injection and resulted in lower turn-on voltage and increased device efficiency. The turn-on voltage of the device with 0.5 nm LiF as buffered layer is as low as 2.8 V. At injection current density of 20 mA/cm2, the current efficiency and power efficiency of the device is 5.56 cd/A and 1.94 lm/W, respectively. Compared to the device with Mg:Ag/Ag only cathode, the turn-on voltage is 1.6 V lower, the current efficiency improved by 75 %, and power efficiency is almost double. The performance of the device is also much improved over the device with LiF/Al cathode.
©2005 Optical Society of America
1. Introduction
The applications of organic electroluminescent materials to organic light-emitting diodes (OLED) have become increasingly important in recent years because of their technological potential to multi-color self-luminous flat-panel displays. The key advantages of OLED are the capability of a wide range of emission colors, low-voltage operation (< 3V), high-efficiency, wide-viewing angle, high-contrast, potential for low-cost manufacturing, large-area multi-color displays and mechanical flexibility. Fueled by great commercial interest, recent years it have seen an explosive growth in the research and development activities leading to OLED production by several major electronics companies. While OLED products have already appeared, challenges still remain to resolve the important issues concerning reliability, chromaticity, contrast, and luminous efficiencies for OLED.
Lithium fluoride is an insulating inorganic material with large band gap energy of 12 eV. LiF was first introduced into OLED by Hung et al. in 1997 [1]. By inserting a very thin layer (0.5 nm or 1.0 nm) LiF between a tris-(8-hydroxyquinoline) aluminum (Alq3) layer and an Al cathode, the electroluminescent (EL) performance of the OLED had been more significantly improved compared to the devices with Al-only and Mg0.9Ag0.1 alloy cathode. Thereafter, many insulating inorganic materials such as MgF2 [2], CaF2 [3], CsF [4], MgO [1], Al2O3 [5], NaCl [6], NaSt [7], were extensive studied, the optimal thickness of the these insulating layers was usually less than 1.0 nm. In the case of LiF/Al cathode, there are a number of mechanisms that were proposed for enhanced electron injection, including tunneling effect [1], band bending at the organic/metal interface [8], lowering of the work function of Al [9], formation of dipoles at Alq3/LiF/Al interface [10], and LiF dissociation with released Li atoms and subsequent reaction with Alq3 to generate Al anions [11]. Most of these researches are focused on modified high work function metals, such as Al [1, 8–14], Ag [15] cathode. Although OLED with Mg:Ag alloy cathode has a poor rectifying property [16], it begins to leak current at about 10 V reverse bias, which can result in large crosstalk in the passive matrix displays and even cause OLED break down, this problem can be solved by connecting a diode in series to the OLED in external control circuit, which can prevent current leakage when the OLED is reversely biased.
In this paper, we used different thickness LiF buffered Mg:Ag alloy as cathode in OLED with structure of indium-tin-oxide (ITO)/N, N’-diphenyl-N, N’-(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine(TPD)/tris-(8-hydroxyquinoline) aluminum(Alq3)/LiF/Mg:Ag/Ag. The devices with Mg:Ag/Ag, LiF/Al and Al-only cathodes were also fabricated in the same run for comparison.
2. Experimental details
The OLEDs were fabricated on lithographically patterned indium-tin-oxide (ITO) coated glass substrate. The ITO layer was about 60 nm thick with a sheet resistance of about 50 Ω/square. The routine cleaning procedure included ultra-sonication in acetone, ethanol, rinsing in de-ionized water and isopropyl alcohol, and finally irradiated in an oxygen plasma chamber. After the oxygen plasma treatment, the ITO substrates were transferred to the main chamber under high vacuum for devices fabrication. The main chamber is equipped with ten sources, each of which is heated by a tantalum heater. The opening and closing of shutters control the deposition sequences. The deposition rate and thickness is measured by a crystal sensor, quartz oscillator combined with frequency meter. In order to obtain large-area uniformity and abrupt interface, the chamber is equipped with three sets of shutters, i.e. besides the shutters for each crucibles, there are also a big shutter between the crucibles and substrates, and a small shutter under each substrate. The thickness/rate crystal sensor is installed in the center of the substrate holders, which is designed in a planetary rotation, and the rotation rate can be adjusted. Four samples with same/different structure can be fabricated during every run. The device has a multilayer structure of ITO/TPD (60 nm)/Alq3 (60 nm)/Cathode, where TPD is the hole-transporting layer, and Alq3 is the emissive as well as electron-transporting layer. The organic films, layer by layer, were deposited on the ITO substrate surface. After deposition of the organic layers, the top cathode was prepared by sequential deposition of a 0.5 or 1 nm LiF layer and a 200 nm Al or Mg:Ag (10:1 mass ratio) overlayer without breaking the vacuum. The four different cathode, such as Al, LiF/Al, Mg:Ag/Ag and LiF/Mg:Ag/Ag can be easy realized by opening/closing the shutter under each substrate when it is necessary. The chamber pressure was below 2×10-4 Pa during deposition of the organic materials and the metals. EL spectra of the fabricated devices were measured with a PR650 Spectra Scan spectrometer. Luminance - current density - voltage (L-I-V) characteristics were recorded simultaneously with the measurements of the EL spectra by attaching the spectrometer to a programmable Keithley 236 voltage-current source. All measurements were carried out at room temperature under ambient atmosphere.
3. Results and discussions
Current-voltage (I-V) and luminance-voltage (L-V) characteristics of the devices having LiF/Al cathode with different thickness of LiF layer and Al only cathode are shown in Figs. 1 (a) and (b), respectively. It can be seen that both I-V and L-V curves are shifted towards the lower voltage region compared with the device without LiF. At the same driving voltage, the device with a 0.5 nm LiF layer has higher current injection and luminance compared to the devices with Al only cathode or with 1 nm LiF layer. The efficiency of device with 0.5 nm LiF layer is also improved over the device with Al only cathode and the device with 1 nm LiF layer. For instance, at injection current density of 20 mA/cm2, the current efficiency and power efficiency of the devices with 0.5 nm LiF layer is as high as 4.65 cd/A and 1.63 lm/W, respectively. The Al only cathode device exhibits a lowest current efficiency and power efficiency, 0.83 cd/A and 0.19 lm/W, respectively. By increasing LiF layer to 1 nm, the current efficiency and power efficiency is decreased to 3.52 cd/A and 0.92 lm/W, respectively.
Fig. 1. Current density (a) luminance (b) versus voltage characteristics of the devices with Al cathode and different thicknesses of LiF layer.
Figure 2 shows I-V (a) and L-V (b) characteristics of the devices having LiF/Mg:Ag/Ag cathode with different thickness of LiF layer and Mg:Ag/Ag cathode. Both I-V and L-V behaviors are in accordance with the devices with LiF/Al cathode. At the same driving voltage, the device with a 0.5 nm LiF layer has higher current injection and luminance compared to the devices with Mg:Ag/Ag cathode or with 1 nm LiF layer. The efficiency of device with 0.5 nm LiF layer is much improved over the device with Mg:Ag/Ag cathode and the device with 1 nm LiF layer. At injection current density of 20 mA/cm2, the current efficiency and power efficiency of the devices with 0.5 nm LiF layer is as high as 5.56 cd/A and 1.94 lm/W, respectively. The Mg:Ag/Ag cathode device exhibits the current efficiency and power efficiency of 3.17 cd/A and 0.95 lm/W, respectively. By increasing LiF layer to 1 nm, the current efficiency and power efficiency is also decreased to 3.58 cd/A and 1.12 lm/W, respectively. The improved efficiency of the devices with LiF buffered layer is due to higher current density injection at lower drive voltage.
Fig. 2. Current density (a) luminance (b) versus voltage characteristics of the devices with Mg:Ag/Ag cathode and different thicknesses of LiF layer.
From the results mentioned above, it can be seen that the optimal thickness of LiF is 0.5 nm for both kinds of devices with high work function metal, Al (4.3 eV) and low work function metal, Mg:Ag (3.7 eV). Figure 3 shows I-V (a) and L-V (b) characteristics of the devices having Al, Mg:Ag/Ag cathode with or without 0.5 nm LiF buffered layer. Table 1 summarizes the EL performance of these devices. It can be seen that the device with 0.5 nm LiF buffering Mg:Ag cathode has the highest performance among the four kinds of devices. The device with 0.5 nm LiF buffering Mg:Ag/Ag cathode has the turn-on voltage as low as 2.8 V, the maximum luminance reaches 9498 cd/m2 at a voltage of 12.5 V. At injection current density of 20 mA/cm2, the current efficiency and power efficiency of the device is 5.56 cd/A and 1.94 lm/W, respectively. Compared to the device with Mg:Ag/Ag only cathode, the turn-on voltage is 1.6 V lower, the current efficiency is improved by 75 %, and power efficiency is almost double. The performance of the device is also much improved over the device with LiF/Al and Al only cathodes.
Fig. 3. Current density (a) luminance (b) versus voltage characteristics of the devices with Al, Mg:Ag/Ag cathode with or without o.5 nm LiF buffered layer.
Table 1. Electroluminescence characteristics of the OLEDs with different cathode, including the turn-on voltage (Von), maximum luminance under DC bias (Lmax), current-density (J) and voltage (V) at Lmax, current-efficiency (ηc) and power efficiency (ηP) at 20mA/cm2.
In the standard device with a configuration of ITO/TPD/Alq3/Mg:Ag, the hole injection barrier between ITO and TPD interface is only 0.2 eV, although Mg:Ag (10:1) alloy has a low work function of 3.7 eV. However, there still exists a higher electron injection barrier of 0.6 eV between Alq3 and Mg:Ag cathode. The injection of hole is much easier than electron. So the majority carrier is hole, and the minority one is electron in the device. In addition, the mobility of hole in TPD is two orders higher than that of electron in Alq3, so the hole current is much lager than electron current, the injection of electron and hole is unbalanced, resulting in low luminance efficiency of the device.
In this study, the improved performance of the devices should be due to enhanced electron injection, which can be realized by electron injection barrier reduction, i.e. reduction of cathode work function. It is known that the work function of Li (2.9 eV) is much lower than that of both Al (4.3 eV) and Mg:Ag (3.7 eV). One mechanism that can be used to explain the reduction of work function (ϕ) relates to the possible chemical reaction, Al+3LiF→AlF3+3Li+340.37 kJ/mol, Mg+2LiF→MgF2+2Li+109.62 kJ/mol and Ag+LiF→AgF+Li+412.33 kJ/mol [17], at the LiF/metal interface. The positive enthalpy of formation, 109.62 kJ/mol, 340.37 kJ/mol and 412.33 kJ/mol for LiF reaction with Mg, Al, and Ag, respectively, means the reaction is endothermic. According to Mason et al., with presence of Alq3, reaction between Al, LiF and Alq3 to liberate Li and form Alq3 anion is possible with an overall enthalpy of formation close to zero [18]. With a smaller enthalpy of formation (109.62 kJ/mol), the reaction of Mg with LiF and Alq3 is more thermodynamically favored than Al and Ag (the reaction of Ag with LiF and Alq3 should be prohibited). Thus, Mg is expected to liberate more Li. More and evenly distributed low work function Li is expected for LiF/Mg:Ag electrode. Another possible mechanism is that there is a dipole at the LiF/metal interface, which would be able to shift the work function of the metal cathode to lower energies [19]. In this case, the fact that ϕ(LiF/Mg:Ag) < ϕ (LiF/Al) should simply follow ϕ(Mg) < ϕ(Al), provided the difference of dipole values is not too much different [20].
4. Conclusion
LiF buffered low work function Mg:Ag alloy cathode can greatly improve the performance of the Alq3 based OLEDs. A 0.5 nm LiF layer can significantly enhance the electron injection, resulting in lower turn-on voltage and increased device efficiency. The enhanced electron injection is due to the reduction of metal work function, which is caused by the easier chemical reaction of Mg with LiF and possibly a dipole layer formation at the LiF/Mg:Ag interface.
Acknowledgments
The financial support from Honeywell Foundation is gratefully acknowledged.
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