Lee, C. H.

Song, W. J.

Sung, Y. J.

Yun, J. Y.

SNU Open Repository and Archive

AbstractWe report on a highly conductive and stable hole transporting layer comprising of N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine ( -NPD) doped with molybdenum oxide (MoO3). Compared to the reference device, the device with MoO3-doped hole transporting material exhibits higher conductivity and thermal stability. The temperature dependence of the current-voltage characteristics are studied for various MoO3 doping concentration. 1. IntroductionThe operating voltage and efficiency of organic light-emitting diodes (OLEDs) rely on the injection and recombination of charge carriers, it is very important to enhance the charge injection efficiency [1, 2]. However, the work function difference between the electrode and organic materials results in a potential energy barrier, limiting the carrier injection at organic/electrode interfaces [3]. One way to improve the carrier injection is the insertion of an appropriate carrier injection layer which leads to effectively lower potential barrier. At the interface of the transparent indium tin oxide (ITO) and the hole-transport layer (HTL), various materials such as copper phthalocyanine (CuPC) [4], or 4,4’, 4”-tris{N, (3-methylphenyl)-N-phenylamino} -triphenylamine) (m-MTDATA) [5], poly(ethylene-dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) [6], and conducting fluorocarbon coatings (CFx) [7] are inserted as a hole-injection layer (HIL).  In addition, p-doped HTLs with various oxidative dopants such as FeCl3 [8], SbCl5 [9] and F4-TCNQ[10] have been reported to lower the operating voltage. Recently, Ikeda et al. reported that molybdenum oxide (MoO3) forms a charge-transfer complex with N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine ( -NPD) and the operating voltage of OLEDs can be reduced by using the MoO3-doped –NPD layer as a HIL [11]. However, the electrical characteristics of the inorganic-oxide doped HTL is not well understood. Therefore, in this work, we studied in detail the temperature dependence of the current-voltage characteristics of the MoO3-doped –NPD films with various MoO3 concentrations in the range of 0-20 % in order to understand the conduction mechanism in the MoO3-doped –NPD films. 2. Results  We have fabricated hole-only devices to investigate the hole conduction properties of the MoO3 doped -NPD layer as the MoO3 doping concentration varies in the range of 0-20 %. The substrate is a pre-patterned indium-tin-oxide (ITO). It was cleaned by ultrasonication in isopropyl alcohol, acetone, and methanol for 10 minutes, respectively and rinsed in de-ionized (DI) water for 5 minutes between several cleaning steps. And ITO substrate was dried in an oven kept at 120 C for more than 30 minutes. After cleaning and drying, the ITO substrate was treated with ultraviolet ozone (UVO) for 4 minutes. Then the organic layers and cathode were all deposited under the high vacuum (< 3 x 10-6 Torr) without breaking vacuum. The active area of the devices, defined by the overlap of the ITO and the AL cathode, was 1.96 mm2.The deposition rates for the organic layers were 0.1~0.2 nm/sec and 0.3~0.4 nm/sec for metal. The current-voltage (I-V) characteristics were measured with a Keithley 236 source-measure unit. Figure 1 shows the device structures. Fig. 1 (a) is the reference device where the  hole transport layer ( –NPD, 100 nm), and aluminum (100 nm) cathode is thermally evaporated sequentially without breaking the vacuum. Fig. 1 (b) shows the device structure with the MoO3-Electrical Properties of the Molybdenum oxide doped Hole transport layer Jin-Young Yun*, Changhee Lee, Won Jun Song, and Yeun joo Sung School of electrical engineering and computer science, Seoul National University, 151-744, Seoul, KoreaPhone: +82-2-880-9559, E-mail: yunjy21@snu.ac.krKeywords : Hole transport layer, MoO3, p-doped, OLED P1-54 / J.-Y. YunIMID '07 DIGEST 691.doped –NPD (the thickness=100 nm). Fig. 2 shows the current density-voltage (J-V) characteristics of the MoO3 doped NPD device compared to the reference device. The MoO3 doped NPD device exhibits symmetric J-V characteristics with several orders of magnitude higher current-density under both forward and reverse bias, compared to the undoped NPD device. The current increases with the MoO3 doping concentration and, therefore, the conductivity can be controlled by the doping ratio. The result indicates that doping MoO3 into the hole transporting layer effectively reduces the hole injection barrier and forms an ohmic contact.Fig. 3 shows the temperature dependence of the current-density at a forward bias of 5 V in an Arrhenius plot The MoO3-doped NPD devices show very stable I-V characteristics up to 400 K while the undoped devices show unstable I-V characteristics above about 330 K. Therefore, the MoO3-doped NPD film has high thermal stability. The devices with MoO3-doped NPD films exhibit a thermally-activated temperature dependence at high temperature (T>100 K) and a very weak temperature dependence below about 100 K. Thus, the conduction mechanism in the MoO3 doped NPD can be described as a multiple trapping conduction at high T>100 K and tunneling through the trap at T<100 K. The activation energy Ea was calculated by fitting the data at T>100 K to the Arrhenius equation TkE BaeJ /  (1) Ea decreases as the MoO3 doping concentration increases. Therefore, the MoO3-doped NPD 102103Current Density (mA/cm2 )10x10-38641/T (K-1)         <5V> 20% 10% 5% 2%0.060.050.040.030.020.010.00Ea (eV)252015105Doping concentration (%)(a)(b)Current Density (mA/cm2 )Ea (eV)Current Density (mA/cm2 )Ea (eV)Figure 3. (a) Current-density vs. 1/T characteristics at 5V (b)  activation energy vs. doping concentration-NPD:MoO3(100nm, x%)-NPD (100nm)ITOITOLiF(0.5nm)/ Al (100nm)Al (100nm)Al (100nm)PEDOT:PSSPEDOT:PSS(a) (b)Figure 1. The device structure of reference (a), and  MoO3 doped -NPD device (b) , (x%=2,5,10,20%) 10-710-610-510-410-310-210-1100101102103Current Density(mA/cm2 )86420-2-4Voltage (V) a-NPD MoO3 2% MoO3 5% MoO3 10% MoO3 20%Figure 2. The current density-voltage characteristicP1-54 / J.-Y. YunIMID '07 DIGEST692 .films with high MoO3 doping concentration are an excellent hole injection layer with high electrical conductivity. 3. SummaryWe studied the electrical conduction mechanism of the MoO3-doped NPD films. The MoO3 acts as a p-type dopant in NPD. Doping MoO3 into a hole transport material increases carrier concentration and lowers the thermal activation energy for multiple trapping transport of holes. Therefore, MoO3-dopedHTLs can significantly reduce the operating voltage and enhance the stability of OLEDs. 4. Acknowledgements This work was supported by “Samsung SDI – Seoul National University Display Innovation Program”. 5. References 1. T. M. Brown and F. Cacialli, J.Polym.Sci.Part B: Polym.Phys. 41, 2649 (2003). 2. B. D. Chin, M. C. Suh, S. T. Lee, H. K. Chung, and C. H. Lee, Appl.Phys.Lett. 84, 1777 (2004). 3. K. C. Kao and W. Hwang, Electrical Transport in Solids (Pergamon Press, Oxford, 1981). 4. C. W. Tang, S. A. VanSlyke, and C. H. Chen, Appl. Phys. Lett. 69, 2160 (1996). 5. Y. Shirota, Y. Kuwabara, H. Inada, X. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett. 65, 807 (1994). 6. S. A. Carter, M. Angelopoulos, S. Karg, P. J. Brock, J. C. Scott, Appl Phys Lett 70, 2067 (1997). 7. S. W. Tong, C. S. Lee, Y. Lifshitz, D. Q. Gao, and S. T. Lee, Appl. Phys. Lett. 84, 4032 (2004). 8. D. B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, and B. Francois, Appl. Phys. Lett. 67, 1659 (1995). 9. C. Ganzorig and M. Fujihira, Appl. Phys. Lett. 77,4211 (2000). 10. J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, S. Liu, and K. Leo, Appl. Phys. Lett. 80, 139 (2002). 11. H. Ikeda, J. Sakata, T. Aoyama, T. Kawakami, Y. Iwaki, S. Seo, R. Nomura, S. Yamazaki, SID Symposium Digest 37, 923 (2006). P1-54 / J.-Y. YunIMID '07 DIGEST 693.

Electrical Properties of the Molybdenum oxide doped Hole transport layer

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Electrical Properties of the Molybdenum oxide doped Hole transport layer

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