CHIN.PHYS.LETT.
Vol. 25, No. 2 (2008) 719
Organic Light-Emitting Diodes with Magnesium Doped CuPc as an Efficient Electron Injection Layer CAO Jun-Song(曹峻松)1 , GUAN Min(关敏)1∗ , CAO Guo-Hua(曹国华)1 , ZENG Yi-Ping(曾一平)1∗∗ , LI Jin-Min(李晋闽)1 , QIN Da-Shan(秦大山)2∗∗∗ 1
Novel Materials Laboratory, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083 2 Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130
(Received 7 December 2007) Bright organic electroluminescent devices are developed using a metal-doped organic layer intervening between the cathode and the emitting layer. The typical device structure is a glass substrate/indium-tin oxide (ITO)/copper phthalocyanine (CuPc)/N,N0 -bis-(1-naphthl)-diphenyl-1,10 -biphenyl-4,40 -diamine (NPB)/Tris(8-quinolinolato) aluminum(Alq3 )/Mg-doped CuPc/Ag. At a driving voltage of 11 V, the device with a layer of Mg-doped CuPc (1:2 in weight) shows a brightness of 4312 cd/m2 and a current efficiency of 2.52 cd/A, while the reference device exhibits 514 cd/m2 and 1.25 cd/A.
PACS: 78. 60. Fi, 85. 60. Jb Organic light emitting diodes (OLEDs) are of considerable interest owing to their vigorous applications in flat panel display and solid-state lighting fields.[1−5] Though great effort has been taken to improvement of characteristics of OLEDs, hole-electron imbalance, i.e., the number of holes arriving at the recombination zone is in orders of magnitude higher than that of electrons arriving at the recombination zone, is still among the key issues to be solved urgently. Developing n-typed doping technique has been shown to be an effective method to tackle this problem, because organic n-doped layers not only possess very high conductivities but also form very thin depletion layers at the contact with metals, compared to those pristine layers.[6−8] Despite the poor thermal stability, bathophenanthroline (Bphen) has been most frequently used as a host material doped with reactive metals, due to its planar molecular structure and high electron mobility.[9,10] In this study, we fabricate magnesium doped organic electron donor copper phthalocyanine (Mg:CuPc), and demonstrate the OLED utilizing 1:2 Mg:CuPc composite as an electron injection layer, and silver as cathode shows increasing performance, as compared to the reference OLED using a conventional Mg/Ag cathode. N0 -bis-(1-naphthl)-diphenyl-1,10 -biphenyl-4,40 diamine (NPB) and tris(8-quinoli nolato) aluminium (Alq3 ) were used as hole transport and emitting/electron transport layers, respectively. CuPc was used as the hole injection layer and the host to Mg. All the organic materials were purified by a home-made sublimation system prior to the usage. ITO substrates with a sheet resistance of 30 Ω/square ∗ Email:
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[email protected] c 2008 Chinese Physical Society and IOP Publishing Ltd ° ∗∗
were cleaned consecutively with de-ionized water, acetone, and ethanol, and finally treated in the UV-ozone for about 15 min. The base vacuum pressure for the device preparation was 2 × 10−5 Pa. Mg doped CuPc (Mg:CuPc) was formed by the co-evaporation of Mg and CuPc from two separate sources. Four OLEDs were fabricated as follows: Device 1: ITO/CuPc 5 nm/NPB 75 nm/Alq3 50 nm/1:2 Mg:CuPc 10 nm/Ag, Device 2: ITO/CuPc 5 nm/NPB 75 nm/Alq3 50 nm/1:1 Mg:CuPc 10 nm/Ag, Device 3: ITO/CuPc 5 nm/NPB 75 nm/Alq3 50 nm/2:1 Mg:CuPc 10 nm/Ag, Device R: ITO/CuPc 5 nm/NPB 75 nm/Alq3 50 nm/Mg 10 nm/Ag. The current–voltage–luminescence (I–V–L) characteristics of the devices were measured by an ST86LA spot photometer and a computer controlled Keithley electrometer 617 under the ambient condition. The emission area of the devices was 0.01 cm2 as determined by the overlap area of the anode and the cathode. The optical absorption spectra and x-ray diffraction (XRD) measurements of organic thin films were obtained using a UV-visible spectrometer (UV3100) and an x-ray diffractometer (D/max-RB). All of the devices gave nearly the same Alq3 emissions. The x-ray diffraction patterns of CuPc and Mg:CuPc composites have been shown in Fig. 1(a). For the CuPc thin film, there is a prominent peak present at 2θ = 6.8◦ , corresponding to diffraction from the (200) plane of the CuPc phase,[11] and a broad peak present at 2θ = 21.0◦ , assigned to the reflection from the quartz substrate. However, the
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Mg:CuPc composites are amorphous. It can be concluded that Mg can effectively destroy CuPc (200) stacks during the vacuum co-deposition.
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pected that there is some charge transfer from Mg to CuPc, because the Fermi level of Mg is 3.7 eV, very close to CuPc lowest unoccupied molecular orbit (LUMO) of 3.5 eV. To investigate the electrical properties of Mg:CuPc composites, the double-layer devices with structure of ITO/Mg:CuPc 20 nm/Alq3 80 nm/LiF 1 nm/Al were fabricated. No light is observed from the devices with Mg:CuPc composites of 1:2, 1:1, and 2:1 in weight, fully indicating that the current of the Mg:CuPc/Alq3 bilayer device is electron-dominated, because the device with structure of ITO/CuPc 20 nm/Alq3 80 nm/LiF 1 nm/Al is able to emit strong green light peaking at 520 nm. These results clearly demonstrate that Mg doping changes the conductive property of CuPc more n-typed. The effect of the 1:2 Mg:CuPc thickness on the device current is also studied. As shown in Fig. 2, at a bias of 9 V, when the thickness of Mg:CuPc increases from 40 to 80 nm, the current density of device increases from 1 mA/cm2 to 128 mA/cm2 . Though 1:2 Mg:CuPc is amorphous, its electron conductivity is much higher than that of Alq3 .
Fig. 1. XRD measurements (a) and normalized UVvis absorption spectra (b) for 50-nm CuPc and 50-nm Mg:CuPc thin films of 1:2, 1:1 and 2:1 in weight on quartz glasses. The four Mg: CuPc composites exhibit the same XRD patterns.
The normalized absorption spectra of Mg:CuPc and CuPc thin films are shown in Fig. 1(b). It can be seen that there is no new absorption band present in Mg:CuPc composites, compared to in neat CuPc, demonstrating that there is likely no chemical reaction between Mg and CuPc. For Mg:CuPc composites, the relative intensities of the CuPc dimeric absorption band peaking at 629 nm to the CuPc monomeric absorption band peaking at 693 nm are nearly the same, but lower than that for the neat CuPc thin film. This implies that Mg doping can suppress the tendency of CuPc molecules to aggregate, consistent with the earlier XRD observations. In general, CuPc is considered as a hole transporter. In fact, Hung et al.[12,13] used CuPc to show an electron mobility of 0.01 cm2 V−1 s−1 , as an electron transporter to improve the OLED performance greatly. Though Mg cannot chemically react with the rigid ring of CuPc, it may be ex-
Fig. 2. I–V characteristics of the devices of ITO (anode)/Mg:CuPc (1:2) (x nm)/Alq3 (140-x nm)/LiF (1 nm)/Al (cathode), where x = 40 (squares), 60 (circles) and 80 (triangles), respectively.
Figure 3 compares the performance of devices 1–3 and device R. At a driving voltage of 11 V, device 1 exhibits a brightness of 4312 cd/m2 , a current density of 173 mA/cm2 , and a device efficiency of 2.52 cd/A; in contrast, device R shows 514 cd/m2 , 36 mA/cm2 , and 1.92 cd/A at 14 V. It is obvious that the combination of Mg:CuPc composite and silver cathode may provide enhanced electron injection into Alq3 , compared to a conventional Mg/Ag cathode. The influence of Mg concentration on the device performance has been summarized in Fig. 4. Obviously, the device with 1:2 Mg:CuPc gives the lowest working voltage and the highest current efficiency at a current density of 50 mA/cm2 . The fact that the working voltage of device increases with increasing Mg con-
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centration indicates that at the interface of Mg:CuPc and Alq3 , CuPc is in charge of injecting electrons into Alq3 instead of Mg, mainly because the LUMO level of CuPc is 0.2 eV closer to the LUMO level of Alq3 than the Fermi level of Mg. With increasing Mg concentration, the density of CuPc LUMO decreases, correspondingly at the interface of Mg:CuPc and Alq3 , therefore resulting in the decreased electron injection and the increased device voltage. Electron injection from CuPc into Alq3 more efficient than that from Mg to Alq3 may be responsible for the increasing performance of devices 1–3 over device R. Fig. 4. Driving voltage and current efficiency versus weight concentration of Mg for OLEDs using Mg:CuPc electron injection layers at a current density of 50 mA/cm2 .
The optical, structural, and conductive properties of Mg:CuPc have been studied. An electron injection technique comprising of Mg:CuPc as the electron injection layer and silver as the cathode has been proposed and the resulting device shows increasing performance, as compared to the control device using the Mg/Ag cathode. The authors thank Jianping Li and Hongxin Liu for their technical support and Chengji Li and Chunhui Huang at Peking university for measurements.
References
Fig. 3. I–V (a), L–V (b) and efficiency-current density (c) characteristics of device R (squares) and device 1 (solid circles), device 2 (triangles), device 3 (open circles).
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