Effect of Ultrathin Magnesium Layer on the Performance of Organic Light-Emitting Diodes

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1 Available online at Energy Procedia 12 (2011) ICSGCE 2011: September 2011, Chengdu, China Effect of Ultrathin Magnesium Layer on the Performance of Organic Light-Emitting Diodes Wan Wang *, Hongjuan Zeng, Qing Li, Shuangjiang Yu The State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information and School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu Abstract Organic light-emitting diodes (OLEDs) were fabricated with a structure of indium-tin-oxide (ITO)/polystyrene (PS): N,N -bis-(3-naphthyl)-n,n -biphenyl-(1,1 -biphenyl)-4,4 -diamine(npb)/tris-(8-hydro-xyquinoline)- aluminum(alq3)/magnesium(mg)/tris-(8-hydroxyquinoline)-aluminum(alq3)/mg:ag.by insert- ing an ultrathin Mg layer within an Alq3 layer, the electrical and luminescent properties were optimized. As a result, an optimized film thickness about 5nm of Mg layer was obtained. By analyzing the optical and electro transporting properties of OLEDs, electron mobility and trap characteristic energy were extracted based on space charge limited conduction (SCLC) and trapped charge limited conduction (TCLC) theories 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC) Keywords: Electron mobility; organic light- emitting diodes(oleds); trap characteristic energy; ultrathin metallic layer Introduction 1. Introduction Organic light emitting diodes (OLEDs) have been attracting lots of attention because of its potential application for the next generation of flat-panel displays, illuminations source as well as backlight sources [1]-[4]. OLEDs are devices with high efficiency and low cost. Thus, they have good performances on saving energy consumption and decreasing price at the daily lighting. The mechanism of electroluminescent (EL) phenomena includes the injection of holes from anode and electrons from cathode at forward bias, which is followed by the recombination of hole-electron pairs to form excitons, and then a light emission process through radiative decay of excitons [5]. In conventional organic materials, the transport mobility of electrons is lower than that of holes for * Corresponding author. Tel.: address: wangwan1129@gmail.com Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC). Open access under CC BY-NC-ND license. doi: /j.egypro

2 526 Wan Wang et al. / Energy Procedia 12 (2011) about two orders of magnitude, resulting in the unbalance of electrons and holes in emitting layer [6]. It is common to balance the transport of electrons and holes by doping host material and dopant via coevaporation [7]. However, co-evaporation method is unacceptable in large-scale manufacturing of devices for commercialization due to the time consumption and poor reproducibility. In this work, an ultra thin Mg layer was inserted into the organic material, instead of co-evaporating technique. The optical and electrical properties of the device were characterized. The electron mobility and the trap characteristic energy were calculated based on the space charge limited conduction (SCLC) and trapped charge limited conduction (TCLC) theories. And the impact of ultra thin Mg layer with different thickness on device performance was studied and discussed from the view of carrier transportation. 2. Experimental The molecular structures of organic materials are shown in Figure 1. Indium-tin-oxide (ITO) coated glass substrates with a sheet resistance of 10 Ω/sq were cleaned consecutively in ultrasonic baths containing detergent, acetone, ethanol, deionized water for 15 min each and finally dried in a N 2 flow. The substrates were treated by O 2 plasma for 5 min before loading into vacuum chamber. Organic materials and Mg ultra thin layer were deposited onto ITO substrate successively at a rate of 1-2 Å/s at a pressure of Pa. Mg:Ag cathode was then deposited by simultaneous vacuum deposition at a rate of ~10 Å/s under a pressure of Pa. The device configurations are as follows: A: ITO/NPB (20 nm)/alq 3 (80 nm)/mg:ag B: ITO/NPB (20 nm)/ Alq 3 (70 nm)/mg (0.5 nm)/alq 3 (10 nm)/mg:ag C: ITO/NPB (20 nm)/alq 3 (70 nm)/mg (1 nm)/alq 3 (10 nm)/mg:ag D: ITO/NPB (20 nm)/alq 3 (70 nm)/ Mg (2 nm)/alq 3 (10 nm)/mg:ag NPB Alq 3 Fig.1. Molecular structures of organic materials of NPB and Alq3. The evaporation rate and thickness of the thin films were in situ monitored using a quartz crystal oscillator mounted to the substrate holder. The active emissive area for all devices is 6 6 mm 2. Current density-voltage-luminance curves were measured with a Keithley 4200 semiconductor characteriza- tion system. All the measurements were performed at room temperature under ambient circumstance. 3. Results and Discussion The characteristics of luminance-bias voltage (L-V) for the devices with different thicknesses of ultrathin Mg layers are shown in Figure 2. From Figure 2, the turn on voltage (defined as the bias voltage at L =1 cd/m 2 ) of devices A, B, C and D are 3.6, 4, 4 and 4.2V, respectively. This may be attributed to the enhanced energy barrier and discontinuous transport between Alq 3 and Mg, when the thickness of ultrathin Mg layer is increased. The luminance of the devices increases gradually with driving voltage, but their increasing magnitudes are different. The maximum luminance of devices A, B, C and D are cd/m 2 at a bias voltage of 14.6 V, cd/m 2 at 15.8 V, cd/m 2 at 16.4 V, and cd/m 2

3 Wan Wang et al. / Energy Procedia 12 (2011) at 16.2 V, respectively. Fig. 2. Luminance-voltage characteristic of OLEDs. Fig 3 shows the characteristics of current density- voltage (J-V) for four devices. At the driving voltage of 15 V, the current densities of devices A, B, C and D are 6150, 7229, 4924 and 3808 A/m 2, respectively. The brightness and current density of device B are obviously higher than devices A, C and D. Fig. 3. Current density-voltage characteristics of devices. Power efficiency of the devices were calculated according to the following equation: L π η = V J where η is power efficiency, L is luminance, J is current density, V is bias voltage. Figure 4 is the power efficiency versus voltage characteristic of devices A, B, C and D. The maximum power efficiencies of devices A, B, C and D are 2.83, 3.24, 2.72 and 2.21 lm/w. As a result, device B with the 0.5 nm ultrathin Mg layer was chosen to be the optimized device. where μ 0 is the electron transport mobility at zero electric field and β is a constant [19]. In this case, the expression for the SCLC can be approximated to: J SCLC 2 9εµ 0V V = exp(0.89 β ) (5) 3 8d d The electron mobilities of four devices shown in Table 1 were calculated by fitting equation (5). The electron mobility of Alq 3 in device A is cm 2 /V s, consistent with that reported in the literature [20]. There is an obvious enhancement in electron mobility in device B, C and D with an ultra-thin Mg layer. As an optimized result, device B with a 5 nm Mg layer has a μ n = cm 2 /Vs. However, as the thickness of ultra thin Mg layer increases, the electron mobility decreases orderly, e.g., cm 2 /V s (1)

4 528 Wan Wang et al. / Energy Procedia 12 (2011) for device C and cm 2 /V s for device D. Fig. 4. OLED power efficiency as a function of current density. Fig. 5. J-V characteristics and fitting curves at 4-8 V based on trapped charge limited model. Table 1: Simulated parameters based on the TCLC at 4-8 V and SCLC at V 4-8V 15-18V Device S Et (ev) S μ 0 (cm 2 /Vs) A B C D The inserted ultrathin Mg layer improves the electron transport mobility and results in obvious improvement in optoelectronic performance of OLEDs. Meanwhile, the depth of trap characteristic energy is increased, leading to no obvious improvement at low bias voltage. As bias voltage increasing, traps within organic material are filled, and the advantage of high electron mobility becomes evident. Device B has a remarkable enhancement in both luminance and power efficiency. When Mg layer is too thick in devices C and D, the transportation of electrons becomes discontinuous, resulting in high turn-on voltage and performance degradation. Fig. 6. J-V characteristics and fitting curves at V based on space charge limited model. In order to gain an insight into the charge transport properties of four devices, J-V characteristics were plotted in logarithm coordinate in Figures 5 and 6. For bias voltage ranging from 4 to 8V, the slops (S)~7 of the J-V curves suggest trapped charge limited conduction (TCLC), in which current is determined by

5 Wan Wang et al. / Energy Procedia 12 (2011) the bulk properties of traps in organic material rather than contact effects[8]-[10]. J is given by: J ε m 2m+ 1 V m+ 1 m m+ 1 TCLC = NLUMOµ n ( ) ( ) 2m+ 1 Nqm t ( + 1) m+ 1 d where N LUMO is the effective density of states in the LUMO band, μ n is the electron transport mobility, d is the thickness of organic layers,m is a constant related to the distribution of traps, m=t t /T, T t is the characteristic temperature of exponential trap distribution, ε is permittivity, and N t is total trap density. From the fitting, the slops of devices A, B, C and D correspond to 6.1, 6.8, 7.2 and 8.0. Characteristic trap energy (E t ) is defined as E t =kt t [11, 12], so the characteristic trap energy of devices A, B, C and D are 0.13, 0.15, 0.16and 0.18 ev, respectively. We could assume that the trap characteristic energy of the materials become deeper with a thicker Mg layer. As bias increasing, the injected electrons increase, filling the limited number of traps in organic film. The conduction behaves as trap filled limited conduction [13]. After the traps are completely filled, the influence on the electron transportation becomes very weak. Then the conduction becomes space charge limited conduction [14]. When the bias voltage becomes higher than 15 V, S~2 indicates the transports of electrons reach space charge limited conduction (SCLC) regime [15-18]. Thus, J is given by: 2 9εµ nv J SCLC = (3) 3 8d The electron transport mobility μ n is assumed to be dependent on the electric field E as µ ( E n ) = µ exp( β E ) (4) 0 4. Conclusion OLEDs with an ultrathin Mg layer of different thickness were fabricated. The measurement of optical and electrical characteristics demonstrated that the device with a 5nm ultrathin Mg layer showed the highest luminance and efficiency performances. By calculating the electron transport mobility based on SCLC theory, it is concluded that the inserted Mg ultra-thin layer increases the electron transport mobility, and improve the balance of electron and whole transportation. This work is a valuable reference and guide for the construction of metal ultra thin layer and the technique improvement used to fabricate novel OLEDs. (2) Acknowledgements This work was supported by the National Science Foundation of China (NSFC) (Grant No and ). References [1] C. W. Tang, S. A. Vanslyke, C. H. Chen, Electroluminescence of doped organic thin films, J. Appl. Phys., vol. 65, no. 9, pp , May [2] S. Denis, Organic Devices: A Review, Microelectronic engineering research conference, [3] B. Qu, Z. Chen, F. Xu, Green light-emitting organic material with narrow FWHM and high electroluminescence, Materials Letters, vol, 60, no. 15, pp , July 2006.

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