Improved electroluminescence efficiency of organic ...

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We studied the hole mobility of molecularly doped hole transport layer (HTL), ... Keywords: organic light-emitting diodes, electroluminescence, doped HTL, ...
Improved electroluminescence efficiency of organic light-emitting diodes by using molecularly doped hole-transport layer C. H. Lee*, Y. C. Shin, D. S. Kwon, and H. K. Lee School of Electrical Engeering and Nano Systems Institute National Core Research Center, Seoul National Univ., Seoul 151-744, Korea ABSTRACT We studied the hole mobility of molecularly doped hole transport layer (HTL), 4,4’-bis[N-(1-napthyl)-N-phenylamino]-biphenyl (α-NPD), as a function of the doping concentration of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) by employing the time-of-flight photoconductivity (TOF-PC) technique. The hole transport is non-dispersive for α-NPD and the hole mobility of pristine α-NPD is about 10-3 cm2/Vs at room temperature. However, the hole mobility decreases with the BCP doping concentration in α-NPD. We characterized the current-voltage-luminance dependence, the EL quantum efficiency, and transient EL response for the devices of ITO/doped α-NPD/Alq3/LiF/Al. The devices with the BCP doped α-NPD show higher EL efficiency compared with the device with pristine α-NPD. The reduced hole mobility in the BCP doped α-NPD enhances the electron-hole balance, resulting in an increased EL efficiency. Keywords: organic light-emitting diodes, electroluminescence, doped HTL, doping, mobility

1. INTRODUCTION Organic light-emitting diodes (OLEDs) emerge as a new flat-panel display technology with superior display qualities since the first demonstration of efficient light emission by C. W. Tang1. The OLED technology has been developed very rapidly in all aspects of display performances such as efficiency, brightness, and lifetime and undergoes a transition from prototype engineering to commercialization of small-size displays adopted to products such as cell phones and digital cameras.1-4 In order to enhance the electroluminescence (EL) efficiency of OLEDs it is necessary to optimize the device structure so that efficient carrier injection at the electrode interfaces and the electron-hole balance in the emitting layer can be achieved.5,6 The carrier injection is limited not only by the potential energy barrier at the electrode interface but also by the low mobility of organic semiconductors.7 In addition, the mobility of charge carriers affects the balance of electrons and holes in the recombination zone. Therefore, it is very important to understand the charge transport mechanism in organic thin films and elucidate the correlation between the carrier mobility and the EL efficiency of OLEDs. In this work, we studied the carrier mobility of a hole transport layer (HTL) of 4,4'-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), and -NPD doped with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) by employing the time-of-flight photoconductivity (TOF-PC) technique and transient electroluminescence (TEL) measurement. Since the hole mobility of α-NPD can be controlled by doping BCP, we can study the effect of the hole mobility on the EL efficiency of OLEDs by comparing two devices with α-NPD and BCP-doped α-NPD. We found that the BCP-doped αNPD showed lower hole mobility and the EL efficiency is higher for the device with BCP-doped α-NPD as the HTL.

2. EXPERIMENTAL The organic light-emitting devices were fabricated using thermal evaporation of organic materials on ITO glass substrates with a sheet resistance of about 10 Ω/. The ITO substrate was cleaned with sequential ultrasonic cleaning in *

[email protected]; phone 82-2-880-9093; fax 82-2-877-6668 Organic Light-Emitting Materials and Devices IX, edited by Zakya H. Kafafi, Paul A. Lane, Proc. of SPIE Vol. 5937, 593719, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.627072

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organic solvent (isopropyl alcohol, acetone and methanol), and rinsing in de-ionized water. Fig. 1 shows the structure of the devices with the schematic energy level diagram. To enhance the device efficiency, the hole injection layer of 3,4polyethylenedioxythiophene:polystyrenesulfonate (PEDOT-PSS) was deposited by spin coating from solution at 4000 rpm for 30 s, followed by drying at 100 °C for 1 hour in vacuum. Then organic layers were successively deposited on top of the PEDOT:PSS layer under a vacuum of ~2x10-6 Torr. The evaporation rates of organic materials, α-NPD, BCP and Alq3, were about 1~2 Å/s, measured by a quartz crystal oscillator. Especially, to balance electron and hole, the hole blocking material, BCP, were coevaporated with hole transport material, α-NPD. Finally, the electron injection layer, LiF, and the aluminum cathode were also deposited without breaking vacuum. The overlap area of the Al and ITO electrodes is about 2.5 mm2. A1(lOOnm)

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The devices were mounted on to a cold finger of cryostat under vacuum. The device performance was studied by measuring the current-voltage-luminance (I-V-L) characteristics and the EL spectra in the temperature range between 15 and 300 K. The I-V-L characteristics were measured with a Keithley 236 source-measure unit and a Keithley 2000 multimeter equipped with a PMT through an ARC 275 monochromator. The external quantum efficiency (QE) of the EL, defined as the ratio of the emitted photons to the injected charges, was calculated from the EL intensity measured by the calibrated Si photodiode. For the transient EL measurement a Hewlett-Packard HP 214B pulse generator was used to apply square-wave voltage pulses between the ITO and Al electrodes. The transient EL response was detected by a fast photomultiplier tube (ARC P2 PMT). The voltage pulse from the generator, the voltage across the current-limiting resistor (500 ohm) in series with the device, and the transient EL signal were simultaneously digitized with a 500 MHz digital storage oscilloscope (Tektronix TDS 644B). The TOF-PC measurements were performed on a 2-µm thick α-NPD film and a 1-µm thick BCP-doped α-NPD film. The transit time of a packet of holes, photoexcited through a transparent ITO electrode by the N2 pulse laser (pulse width of 0.6 ns and wavelength of 337 nm), was measured as a function of the electric field. The drift hole mobility was

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calculated using the relation µ = d 2 / Vτ , where d is the sample thickness, V is the bias voltage, and τ is the transit time.8

3. RESULTS AND DISCUSSIONS 3.1 Hole mobility of the BCP-doped α-NPD measured by TOF-PC technique Fig. 2 shows the typical photocurrent waveforms of α-NPD and 10 % BCP-doped α-NPD film. The electron photocurrents in both samples decrease continuously, suggesting highly dispersive transport with possible deep trapping. In many organic materials oxygen has been proposed as a possible impurity trap.9 However, the hole photocurrent waveform in both films shows a characteristic plateau of the non-dispersive TOF-PC signal. The transit time is defined at the edge of the plateau where the photocurrent starts to decrease steeply as holes discharge at the opposite electrode. The hole mobility of pristine α-NPD is calculated about 8x10-4 cm2/Vs at an electric field of 0.15 MV/cm. However, it decreases by an order of magnitude for the 10 % BCP-doped α-NPD.

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Fig. 3 shows the electric field dependence of the hole mobility in α-NPD and 10 % BCP-doped α-NPD at room temperature. The data are plotted as a function of the square root of the bias electric field. The hole mobility shows an exponential dependence on the square root of the electric field. This dependence is characteristic of an amorphous organic semiconductor in which the carrier transport is usually described in terms of simple Poole-Frenkel model. The charge carriers move via multiple hopping processes and the energy barrier of a carrier in a Coulomb potential is reduced by an applied field. For the simple one-dimensional case, this effect leads to a mobility with a field dependence of exp(βPFE1/2/kT) where βPF=(e3/πεεο)1/2 and E is the electric field.5 The hole mobility of pristine α-NPD is 5x10-4 ~ 8x10-4 cm2/Vs in the electric field of 40 – 400 kV/cm range.. However, the hole mobility of the 10 % BCP-doped α-NPD film is lower by an order of magnitude (from 4x10-5 cm2/Vs at 90 kV/cm to 7x10-4 cm2/Vs at 0.65 MV/cm) and shows much stronger field dependence is in BCPdoped α-NPD. These results indicate that BCP molecules act as hole traps. The energetic and positional disorder increase by doping BCP in α-NPD.

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3.2 Hole mobility of the BCP-doped α-NPD measured by TEL technique Fig. 4 shows the transient EL signals for the device with α-NPD and 10 % BCP doped α-NPD upon the application of rectangular driving voltage pulses (pulse width ~ 10 µs) with various magnitudes. The device with α-NPD shows that the EL emission arises with the time delay, τd, after the onset of the bias voltage pulse. As the bias voltage increases the EL delay time decreases. The delay time τd of the EL onset can be considered as the time for the leading edges of injected electrons and holes to meet and undergo radiative recombination.7,8 If the distance d from the anode at which the leading fronts of injected holes and electrons meet, the hole mobility is related to the EL delay time via the relation of µ = d 2 / Vτ d . From this relation we can obtain the hole mobility of µ~6x10-4 cm2/Vs for the EL time delay of τd~1.0 µs at 12 V for the device with α-NPD. This value appears to be similar to the hole mobility measured by the TOF-PC technique. However, the device with 10 % BCP doped αNPD shows the fixed EL time delay of about 300 ns which is limited by the overall RC time constant of the measurement system. Therefore, it seems to be unclear that the hole mobility can be directly obtained from the EL time delay as described above. The comparison of the transient EL signals for both devices, however, clearly shows that both the EL rise time to the steady-state level and the EL decay time are larger and increase with increasing bias voltage for the device with 10 % BCP doped α-NPD compared with the undoped device. The slower EL rise time can be attributed to the decreased hole mobility for the BCP doped α-NPD, consistent with the TOF-PC result.

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Time (µs) Fig. 4 Transient EL signals for the device with a-NPD (top) and 10 % BCP doped a-NPD (bottom). 3.3 Current-Voltage-Luminance Characteristics We characterized the current-voltage-luminance (I-V-L) dependence and the luminous efficiency for the devices with different BCP doping concentration (0, 10, 25, and 50 %) in α-NPD. Fig. 5 shows the I-V and L-V characteristics for the devices with various BCP doping ratios in α-NPD at room temperature. The current decreases significantly at low bias voltages. It implies that BCP acts as traps for the hole transport in α-NPD, as consistent with the TOF-PC and transient EL measurements described above. However, the current and luminance at high bias voltages are higher for the devices with the BCP doped α-NPD compared that with α-NPD alone. This result can be attributed to the combined effect of the improved electron injection and the electron-hole balance. Since BCP acts as hole traps, the trapped hole concentration is higher for the BCP doped α-NPD, resulting in an increased internal electric field near the cathodic interface. Therefore, the electron injection at high field can be enhanced, which leads to the improved electron-hole balance in the light emitting layer.

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Fig. 6 shows that the luminous efficiency for the devices with different BCP doping concentrations (0, 10, 25, and 50 %) in α-NPD. Clearly, the luminous efficiency is increased for the devices with the BCP doped α-NPD as the hole transport layer. The maximum luminous efficiency is about 6.8 cd/A at 50 mA/cm2 for the device of ITO/PEDOT:PSS/α-NPD:BCP (1:1 weight ratio)/Alq3/LiF/Al. The enhanced EL efficiency is due to the improved electron-hole balance.

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The performances of the devices with different BCP doping concentrations are summarized in Table 1. The luminous efficiency (cd/A), external quantum efficiency (Q.E), and maximum luminance (cd/m2) increases with the BCP doping. However, the driving voltage increases at high doping concentration of BCP due to the decreased hole mobility. Therefore an optimum BCP concentration seems to be around 25-50 % when we consider both the efficiency and driving voltage. Table 1. Performance of the EL device with different BCP doping ratios

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Efficiency at 100 cd/m2 cd/A Q.E. (%) 4.2 1.2 5.8 1.7 5.4 1.6 5.9 1.8

Maximum luminance (cd/m2) 15,157 31,860 30,801 35,664

Fig. 7 shows the normalized EL spectra of the devices with different BCP doping ratios at a constant current density of about 50 mA/cm2. Since the EL spectra are the same for all devices with and without BCP doping, the electron-hole recombination zone is Alq3 layer and not varied with the BCP doping in α-NPD.

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4. CONCLUSIONS We studied the carrier transport property of α-NPD and BCP-doped α-NPD by employing the TOF-PC and transient EL measurement. The hole transport is non-dispersive for α-NPD with the hole mobility of about 10-3 cm2/Vs at room temperature. The hole mobility decreases with the BCP doping concentration in α-NPD, indicating BCP acts as a hole

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trap. The devices with the BCP doped α-NPD show lower current density at low bias voltages, consistent with the reduced hole mobility. However, the current and luminance at high bias voltages are higher for the devices with the BCP doped α-NPD compared that with α-NPD alone. The results are attributed to the combined effect of the improved electron injection and the electron-hole balance. The higher trapped hole concentration for the BCP doped α-NPD results in an increased internal electric field for the electron injection which improves the electron-hole balance in the light emitting layer.

ACKNOWLEDGMENTS This work was supported by the Nano-Systems Institute (NSI-NCRC) program sponsored by the Korea Science and Engineering Foundation (KOSEF), Korea and the BK21 program.

REFERENCES 1. 2.

C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51, pp. 913-915, 1987. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, “Light-emitting diodes based on conjugated polymers,” Nature 347, pp. 539-541, 1990. 3. J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, “Organic electroluminescent devices,” Science 273, pp. 884-888, 1996. 4. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund and W. R. Salaneck, “Electroluminescence in conjugated polymers,” Nature 397, pp. 121-128, 1999. 5. E. W. Forsythe, M. A. Abkowitz, and Yongli Gao, “Tuning the carrier injection efficiency for organic lightemitting diodes,” J. Phys. Chem. B, 104, 3948 (2000). 6. C. F. Qie, L. D. Wang, H. Y. Chen, M. Wong, and H. S. Kwok, “Room-temperature ultraviolet emission from an organic light-emitting diode,” Appl. Phys. Lett. 79, 2276 (2001). 7. Y. Shen, D. B. Jacobs, G. G. Malliaras, G. Koley, M. G. Spencer, and A. Ioannidis, “Modification of indium tin oxide for improved hole injection in organic light emitting diodes,” Adv. Mater. 13, 1234 (2001) 8.

P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for Imaging Systems, (Marcel Dekker, New York, 1993). 9. H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, “Carrier deep-trapping mobility-lifetime products in poly(pphenylene vinylene,” Appl. Phys. Lett. 65, 2030 (1994). 10. A. J. Pal, R. Osterbacka, K. –M. Kallman, and H. Stubb, “Transient electroluminescence: mobility and response time in quinquethiophene langmuir-blodgett films,” Appl. Phys. Lett. 71, 228 (1997). 11. V. R. Nikitenko and H. Bassler, “Tunneling controlled electroluminescence switching in bilayer light emitting diodes,” J. Appl. Phys. 85, 6515 (1999).

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