The reduction of driving voltage in organic light-emitting devices by inserting step barrier layer

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<ul><li><p>ic</p><p>rsity00, C</p><p>Received in revised form 20 January 2009Accepted 20 February 2009Available online 12 March 2009</p><p>Keywords:OLEDsLow voltageStep barrierTunnel theory</p><p>nyl-1,10-phenanthroline (Bphen)) and 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN). Since the</p><p>lieved to be a fundamental barrier in organic semiconductor de-vices [24], so its important for organic light-emitting devices(OLEDs) to have lower driving voltage by enhancing electron injec-tion. In order to improve injection balance between holes and elec-tron, buffer layers [5] are often used either between anode andhole transport layer (HTL) or between electron transport layer</p><p>2. Experiment</p><p>In order to examine the enhancement of electron injection, elec-tron-only devices were fabricated with the following structures:</p><p>Cell1: ITO/Alq3 (50 nm)/LiF (1 nm)/Al (130 nm).Cell2: ITO/Gaq (20 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (130 nm).Cell3: ITO/BPhen (50 nm)/LiF (1 nm)/Al (130 nm).Cell4: ITO/Gaq (20 nm)/BPhen (30 nm)/LiF (1 nm)/Al (130 nm).</p><p>* Corresponding author. Tel.: +86 0750 3296040; fax: +86 750 3296400.</p><p>Displays 30 (2009) 119122</p><p>Contents lists availab</p><p>pl</p><p>.e lE-mail address: xu_wei_mail@163.com (W. Xu).Since the rst practical low voltage organic light-emittingdiodes (OLEDs) was reported [1] by a simple stacking of a holetransport layer and an electroluminescent/electron transportbifunctional layer, the OLED has become one of the most active re-search areas. To obtain high power efciency and low driving volt-age, its key to make progress in the following aspects: efcientcharge injection at interfaces, improved transport conductivityand low ohmic losses in the transport layers. However, organicmolecules are known for decades to be good conductors for holesbut extremely poor conductors for electrons. This is widely be-</p><p>ication [69] and doping [10,11] are also used.In this paper, we introduce a new concept of step barrier to en-</p><p>hance electron injection and transport. A layer of Gaq was insertedin between Alq3 (or Bphen) and TBADN as step barrier. The LUMO(lowest unoccupied molecular orbital) of Gaq is 2.9 eV [12] whichlies in between that of Alq3 (3.1 eV) (or BPhen (3.0 eV)) [13] andTBADN (2.8 eV) [14], so a layer of step barrier can be formed fromAlq3 (or BPhen) though Gaq to TBADN. As a result, it is made feasi-ble to improve electron injection and transport, so that the drivingvoltage can be decreased and the current efciency of the devicecan be enhanced.1. Introduction0141-9382/$ - see front matter 2009 Elsevier B.V. Adoi:10.1016/j.displa.2009.02.001LUMO (lowest unoccupied molecular orbital) of Gaq (2.9 eV) lies in between that of Alq3 (3.1 eV) (orBphen (3.0 eV)) and TBADN (2.8 eV), step barrier from Alq3 (or BPhen) though Gaq to TBADN can beformed. The experimental results indicate that the JV characteristics of both the electron-only andthe complete devices show the increase of the current density in devices with step barrier comparedwith the devices without step barrier. For electron-only devices, the driving voltage at the current den-sity of 20 mA/cm2 is reduced from 7.9 V to 4.9 V for devices with Alq3, and from 4.2 V to 3.1 V fordevices with BPhen, respectively, owing to the introduction of step barrier. For the complete devices,when Gaq step barrier is introduced, at 20 mA/cm2, the driving voltage is reduced from 7 V to 5.8 Vfor devices with Alq3 and from 6.2 V to 5.1 V for devices with BPhen. It has also been observed thatfor devices with step barrier layer, the luminance at 200 mA/cm2 is increased from 1992 cd/m2 to3281 cd/m2 for device with Alq3, and from 1745 cd/m2 to 2876 cd/m2 for devices with BPhen, respec-tively. The highest luminance reaches 3420 cd/m2 in devices with Alq3 as ETL and 3176 cd/m2 indevices with BPhen as ETL after the introduction of step barrier. The phenomena are explained by usingtunnel theory.</p><p> 2009 Elsevier B.V. All rights reserved.</p><p>(ETL) and cathode. In addition, the method of both electrode mod-Article history:Received 20 September 2007</p><p>The electron injection and transport in OLEDs have been improved by using a tris-[8-hydroxyquinoline]gallium (Gaq) layer as step barrier between tris-[8-hydroxyquinoline]aluminum (Alq3) (or 4,7-diphye-The reduction of driving voltage in organstep barrier layer</p><p>Wei Xu a,*, M.A. Khan b</p><p>aDepartment of Mathematics and Physics, Institute of Functional Materials, Wuyi UnivebDepartment of Materials Science, Shanghai University, Jiading Campus, Shanghai 2018</p><p>a r t i c l e i n f o a b s t r a c t</p><p>Dis</p><p>journal homepage: wwwll rights reserved.light-emitting devices by inserting</p><p>, Jiangmen, Guang Dong 529020, Chinahinale at ScienceDirect</p><p>ays</p><p>sevier .com/locate /displa</p></li><li><p>Based on the step barrier approach, the complete devices withthe following structures were fabricated:</p><p>Cell5: ITO/NPB (20 nm)/TBADN (30 nm)/Alq3 (20 nm)/LiF(1 nm)/Al (130 nm).Cell6: ITO/NPB (20 nm)/TBADN (30 nm)/Gaq (10 nm)/Alq3(10 nm)/LiF (1 nm)/Al (130 nm).Cell7: ITO/NPB (20 nm)/TBADN (30 nm)/BPhen (20 nm)/LiF(1 nm)/Al (130 nm).Cell8: ITO/NPB (20 nm)/TBADN (30 nm)/Gaq (10 nm)/BPhen(10 nm)/LiF (1 nm)/Al (130 nm).</p><p>Among all the materials, NPB is used as hole transport layer(HTL), TBADN as emission layer (EML), Alq3 (or BPhen) as elec-tron transport layer (ETL). LiF and Al are used as electron injec-tion layer and cathode, respectively. The molecular structuresand the energy level diagrams of these materials are shown inFig. 1.</p><p>The devices are fabricated according to the following process:After routine chemical cleaning, ITO is further treated by UV</p><p>ozone. The devices are prepared by vapor deposition onto an in-dium tin oxide coated glass substrate with a sheet resistance of20X/square. The active area of the devices was 5 5 mm2. Thethickness of the organic layers is monitored by using quartz-crystalmonitor. The currentvoltage (IV) and luminance characteristicsare measured by using Keithley 2400 Source Meter and MinoltaLS-110 luminance meter.</p><p>3. Results and discussion</p><p>The JV characteristics of electron-only and complete devicesare shown in Figs. 2 and 3. In Fig. 2, the JV curves of the deviceswith step barrier (Cell2 and Cell4) shift to lower voltages. InFig. 3, the JV curves of the devices with step barrier (Cell6 andCell8) also shift to lower voltages. The above phenomena show thatat a given voltage, the value of current density in the devices withstep barrier is higher than that of the devices without step barrier,indicating that driving voltage of device can be reduced by intro-ducing a step barrier layer. In Table 1, the voltage values for thewhole devices at 20 mA/cm2 (V20) and 100 mA/cm2 (V100), andluminance at 200 mA/cm2 (L200) are shown, respectively.</p><p>Fig. 4 shows the current efciencycurrent density characteris-tics of complete devices. The inset is the luminance vs. current den-sity characteristics of complete devices. It is clear from Fig. 4 thatin the devices with step barrier (Cell6 and Cell8), the current ef-ciency is higher than that of the devices without step barrier (Cell5and Cell7). It is also found in the inset that the device luminance isalso enhanced signicantly after introducing step barrier. The per-formance of the complete devices is shown in Table 2 with g20,gMAX, LMAX, L20 and L200 indicating the current efciency at20 mA/cm2, the maximum current efciency, the maximum lumi-nance, and the luminance at 20 mA/cm2 and 200 mA/cm2,respectively.</p><p>From Table 2, we can learn that with the introduction of stepbarrier layer, the driving voltage has been reduced and luminance</p><p>120 W. Xu, M.A. Khan /Displays 30 (2009) 119122Fig. 1. The energy level diagrams and molecular structure of materials used.</p></li><li><p>5C</p><p>Fig. 2. JV characteristics of electron-only devices.</p><p>Fig. 3. JV characteristics of complete devices.</p><p>Table 1The voltage of the devices at 20 mA/cm2 and 100 mA/cm2.</p><p>Device</p><p>Electron-only devices Complete devices</p><p>Cell1 Cell2 Cell3 Cell4 Cell5 Cell6 Cell7 Cell8</p><p>V20 (V) 7.9 4.9 4.2 3.1 7 5.8 6.2 5.1V100(V) 12.3 7.6 7.9 5.3 9.3 7.4 9 7.2</p><p>Fig. 4. Current efciency vs. current density characteristics of complete devices.Inset: luminance vs. current density characteristics of complete devices.</p><p>W. Xu, M.A. Khan /Displayhas been enhanced signicantly. So its an effective method to in-sert a thin lm, forming a step barrier between ETL and EML to in-crease the minority carrier injection. The LUMO of the selectedmaterials should lie in between that of ETL material and that ofEML. The mechanism behind step barrier approach can be ex-plained on the basis of tunnel theory. According to this theory[15,16]:</p><p>J q3F2m08p/m</p><p>exp 42m1=2/3=23hqF</p><p>" #</p><p>where / is the height of the barrier, F is the eld intensity and q andm0 are electric charge and mass of the electron. h is Planck constantand m* is the effective mass of carriers. The relationship betweenthe current and the injection barrier is mainly dependent on theexponential term. So the above equation can be expressed as:</p><p>J / expA0/3=2as far as electron injection is concerned, the injection barrier be-tween BPhen and TBADN is / = 0.2 eV, so the injection current,J / expAo0:23=2, but we can see from Fig. 1 that the barrier be-tween BPhen and Gaq is 0.1 eV and also between TBADN and Gaq is0.1 eV. Therefore, the electron injection can be separated into twolevels of barriers and each of them is 0.1 eV. So the injection currentcan be expressed as, J / exp A00:13=2 0:13=2</p><p>n o. It can be</p><p>proved that J0:2 &lt; J0:1 J0:1. Similarly, the injection barrier betweenAlq3 and TBADN is / = 0.3 eV, so the injection current isJ / expA00:33=2 and electron injection is separated into two lev-els of barriers and each of them is 0.2 eV and 0.1 eV, respectively,i.e., J / exp A00:13=2 0:23=2</p><p>n o. We can also prove that</p><p>J0:3 &lt; J0:2 J0:1. Accordingly, it is easier for the injection of electrondue to the contribution of step barrier (ETL-inserted layer-EML).The Gaq is no other than one of the materials we want to make astep barrier. It can be understood very clearly from Fig. 1 that theLUMO of Gaq (2.9 eV) lies in between the LUMO of TBADN(2.8 eV) and that of Alq3 (3.1 eV) (or Bphen (3.0 eV)). Thereforethe Gaq thin lm can be deposited to form step barrier. Conse-quently, the injection of electron is improved, leading to lowing ofvoltage and enhancement of luminance, as shown in Figs. 3 and 4.</p><p>4. Conclusion</p><p>The performances of devices are enhanced greatly by insertingstep barrier between TBADN and Alq3 (or BPhen). The LUMOs ofGaq, TBADN, Alq3 and Bphen are 2.9 eV, 2.8 eV, 3.1 eV and 3.0 eV,</p><p>Table 2The performances of the complete devices.</p><p>Device g20 (cd/A) gMAX (cd/A) L20 (cd/m2) L200 (cd/m2) LMAX (cd/m2)</p><p>Cell5 2.7 3.0 509 1992 2059Cell6 3.5 4.1 771 3281 3420Cell7 3.9 4.3 590 1745 1791Cell8 4.8 5.9 831 2876 3176</p><p>s 30 (2009) 119122 121respectively. At 20 mA/cm2, the lowest voltage reaches 3.1 V inelectron-only device (Cell4) and 5.1 V in complete device (Cell8);and the luminance at 200 mA/cm2 increased from1992 cd/m2 to3281 cd/m2 in devices with Alq3 and from 1745 to 2876 cd/m2 indevices with BPhen due to the introduction of step barrier. Simi-larly, the highest luminance reaches 3420 cd/m2 in devices withAlq3 as ETL and 3176 cd/m2 in devices with BPhen as ETL afterthe introduction of step barrier. It is obvious that step barrier strat-egy is an effective way to increase electron injection, resulting inthe reduction of the voltage and enhancement of efciency of theOLEDs. The fact that the step barrier can increase the electroninjection is explained by tunnel theory.</p></li><li><p>Acknowledgements</p><p>This work is supported by the National Natural Science Founda-tion of China (Nos. 90201034, 60477014, and 60577041) 973(2002CB, 613400) and 973 Project of China (No. 2002CB613400).</p><p>References</p><p>[1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913915.[2] Y. Yang, Q. Pei, J. Appl. Phys. Lett. 81 (1997) 32943298.[3] S. Hiroyuki, H. Sato shi, J. Appl. Phys. Lett. 79 (1996) 88168822.[4] Xu Hong-guang, Meng Rui-ping, Xu Chun-xiang, et al., J. Optoelectron. Laser 14</p><p>(2003) 905908 (in Chinese).[5] H.T. Lu, M. Yoloyama, Solid State Electron. 47 (2003) 14091412.</p><p>[6] J. Lee, Y. Park, D.Y. Kim, et al., Appl. Phys. Lett. 82 (2) (2003) 173175.[7] Wang Jing, Lu Lin, Jiang Wen-long, et al., Chin. Phys. Lett. 22 (3) (2005) 727</p><p>729.[8] L.S. Huan, C.W. Tang, M.G. Mason, et al., Appl. Phys. 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