Role of the Charge Generation Layer in Tandem Organic Light

Dec 21, 2010 - In the tandem organic light-emitting diodes (tandem OLEDs), the charge generation layer (CGL) plays an essential role in the performanc...
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J. Phys. Chem. C 2011, 115, 582–588

Role of the Charge Generation Layer in Tandem Organic Light-Emitting Diodes Investigated by Time-Resolved Electroluminescence Spectroscopy Yun-Min Cheng, Hsin-Hung Lu, Tzu-Hao Jen, and Show-An Chen* Chemical Engineering Department, National Tsing-Hua UniVersity, HsinChu, 30041, Taiwan, ROC ReceiVed: October 4, 2010; ReVised Manuscript ReceiVed: December 2, 2010

In the tandem organic light-emitting diodes (tandem OLEDs), the charge generation layer (CGL) plays an essential role in the performance, which is composed of an n-doped layer and a p-doped layer for injection of electrons and holes, respectively. However, the working mechanism of the CGL is still not thoroughly clear so far. By using time-resolved electroluminescence (TREL) measurements, we found that, at the initial stage of device operation, electrons and holes are supplied from the CGL to start the OLED units for electroluminescence. At the subsequent stage, the consumed electrons and holes in the CGL are refilled by the electrons and holes injected from the cathode and anode, respectively; then, the bipolar currents reach a steady state gradually. The time required for charge refilling in the CGL decreases with charge fluxes supplied from the electrodes. Such understanding on the working mechanism of the CGL could inspire design concepts for tandem OLEDs. 1. Introduction The organic light emitting diode (OLED) has been attracting extensive attention because of its potential applications in nextgeneration displays and solid-state lighting.1,2 In order to achieve high performance, an OLED must be operated at a relatively high current density, thus resulting in a reduced lifetime. Therefore, it is important to improve the performance of an OLED while operating at the lowest possible current density. The tandem-type OLED has been reported able to resolve this issue effectively.3-15 The first study of a tandem-type OLED was reported by Kido group; they inserted intermediate connectors between electroluminescence (EL) units that led to a generation of holes and electrons upon applying an electric field across the electrodes, and thus, they termed the intermediate connectors the charge generation layer (CGL).3 One specific character of the tandem OLED is that its brightness, current efficiency, and driving voltage are found to scale linearly with the number of EL units in the tandem structure.4 To date, the most widely used CGL consists of two layers; one is an n-doped layer based on an electron-transport material doped with a low work function metal, such as tris(8-hydrooxyquinoline) aluminum(III) (Alq3) doped with Li,4 Alq3 doped with Mg,5 4,7diphenyl-1,10-phenanthroline (BPhen) doped with Li,6-8 Bphen doped with Cs2CO3,9,10 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) doped with Li,11 and BCP doped with Cs2CO3,12 and the other is a p-doped layer based on a hole-transport material doped with an electron acceptor, such as 4,4′-bis-(1naphthyl-N-phenylamino)-biphenyl (NPB) doped with FeCl3,4 NPB doped with 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ),13,14 4,4′,4′′-tris(N-3-methylphenyl-Nphenylamino)triphenylamine (m-MTDATA) doped with F4TCNQ,5 N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD) doped with F4-TCNQ,15 and NPB doped with WO3.10 There are only a few studies about the working mechanism of CGL reported in the literature. Tsutsui et al. proposed that charges were generated in CGL (Alq3:Mg/TPD:F4-TCNQ), as * To whom correspondence should be addressed. E-mail: sachen@ che.nthu.edu.tw.

supported by the observed capacitance change for the case using polychloro-p-xylylene (PCPX) as the insulator on both electrodes to block charges injecting into Alq3 and TPD from the electrodes.15 Lee’s group used energy level diagram to explain that formation of Alq- and m-MTDATA+ ions in the CGL (Alq3:Mg/m-MTDATA:F4-TCNQ) might facilitate the injection of electrons and holes into the adjacent emitting layers.5 Liao’s group reported that the charge generation zone was located at the interface of the n- and p-layers in CGL and that, at the same current density, the device applied voltage increased with increasing energy level difference between the highest occupied molecular orbital (HOMO) level of the p-doped layer and the lowest unoccupied molecular orbital (LUMO) level of the n-doped layer.13 However, these reports all claim that charges actually generate in the CGL. It is reasonable to conceive that if the charges in the CGL are consumed continuously during device operation, then there must be a mechanism to refill the consumed charges in the CGL. So far, this issue has not been reported. In this work, we use the time-resolved electroluminescence (TREL) technique to measure the time when two leading fronts of injected carriers (holes and electrons) meet in the device16 to prove that CGL can generate charges to supply to EL units at the initial stage and that then the charges consumed in the CGL need to be refilled by injected charges from the electrodes. All of the chemical structures of used organic materials are shown in Chart 1, including NPB, Alq3, 4-(dicyanomethylene)-2-tbutyl-6-(1,1,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene (TDAF), m-MTDATA, and F4-TCNQ. 2. Experimental Methods An indium tin oxide (ITO) glass substrate was exposed to oxygen plasma at a power of 50 W and a pressure of 193 mTorr (1 Torr ≈ 133 Pa) for 5 min. All layers were deposited in a high-vacuum (below 2 × 10-6 Torr) system by using a resistively heated tungsten basket. All of the organic materials were purchased from Luminescence Technology Corporation

10.1021/jp1095085  2011 American Chemical Society Published on Web 12/21/2010

Role of the Charge Generation Layer in Tandem OLEDs

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CHART 1: Chemical Structures of the Organic Materials Used in This Work, NPB, Alq3, DCJTB, TDAF, m-MTDATA, and F4-TCNQ

TABLE 1: Structures of the Device and Unit

(in Taiwan) and used without further purification. The typical deposition rate for the organic materials was about 1-2 Å/s, and those for the metals were 0.1-0.5, 0.1-0.2, and 3-4 Å/s for calcium, gold, and aluminum, respectively. For the deposition of the Alq3:Mg, m-MTDATA:F4-TCNQ, and Mg:Ag (10:1) alloy, the evaporation rates of two materials were monitored independently by using two thin-film deposition monitors. The deposition rates for Alq3:Mg were 1.8 (Alq3) and 0.2 Å/s (Mg); for m-MTDATA:F4-TCNQ, they were 1.9 (mMTDATA) and 0.1 Å/s (F4-TCNQ); for the Mg:Ag (10:1) alloy, they were 2 (Mg) and 0.2 Å/s (Ag); and for Alq3:DCJTB, they were 1.8 (Alq3) and 0.2 Å/s (DCJTB). EL spectra were measured by using a fluorescence spectrometer (fluoroMAX-3 from Jobin Yvon). All of the measurements of EL spectra were undertaken in an ambient vacuum environment. TREL spectra were obtained by using a pulse generator (AV-1015-B, AVTECH) to supply a voltage pulse to the device in a cryostat under vacuum (5 × 10-6 Torr) at room temperature. A series of luminescence decay curves at specific wavelengths (a decay curve for every 10 nm from 380 to 650 nm was taken, and every decay curve was measured with the same time period of 20 min) were measured by using a 4096 channel time-correlated single-photon counting (TCSPC) system with a microchannel plate photomultiplier tube (Hamamatsu Photonics R3809U-50) and a spectrometer (Edinburgh, Lifespec-ps with TCC900 data acquisition card). The TREL spectra were obtained by using the function TRES in the fluorescence spectrometer software F900. The experimental setup and typical trigger pulse EL responses were shown in our previous work.17 For the case of the luminescence decay curve at a specific wavelength (i.e., 520 nm), the experimental setup is the same as that described above. The electric characteristics and brightness of the devices were measured by using a Keithley power supply (Model 238) and a luminance meter (BM8 from TOPCON), respectively. The film thickness was measured by using a surface profiler (Tencor P-10).

device or unit

device or unit structure

device A device B device C device D device E device F

ITO/EL-G/CGL/EL-G/Mg:Ag (150 nm) ITO/EL-G/Mg:Ag (150 nm) ITO/EL-G/CGL/EL-R/Mg:Ag (150 nm) ITO/EL-G/CGL/EL-B/Mg:Ag (150 nm) ITO/EL-B/Mg:Ag (150 nm) ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Ca (5 nm)/Al (80 nm) ITO/NPB (150 nm)/Alq3 (100 nm)/NPB (100 nm)/ Ca (5 nm)/Al (80 nm) ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Au (50 nm) Alq3 doped with 10 vol % Mg (10 nm)/ m-MTDATA doped with 5 vol % F4-TCNQ (20 nm) NPB (60 nm)/Alq3 (40 nm) NPB (60 nm)/Alq3 doped with 10 vol % DCJTB (40 nm) NPB (60 nm)/TDAF (40 nm)

device G device H CGL EL-G EL-R EL-B

3. Results and Discussion TREL Studies on a Tandem OLED. The CGL used here consists of the n-doped layer (Alq3 doped with 10 vol % Mg (10 nm)) and the p-doped layer (m-MTDATA doped with 5 vol % F4-TCNQ (20 nm)).5 The detailed device structures used in this work are listed in Table 1. In order to investigate the working mechanism of CGL, we first confirm that the CGL can work normally, that is, the current efficiency and brightness of the two-unit tandem OLED are higher than those of its corresponding single-unit OLED by a factor of about 2.3-5,7,8,10-14 We fabricate the tandem OLED (device A) and compare the device performance with that of its corresponding single-unit device (device B). As shown in Figure 1a, at the current density of 500 A/m2, the current efficiency and brightness of device A (4 cd/A and 2000 cd/m2) are higher than those of device B by a factor of about 2. This result indicates that this CGL does work as expected and can be used as CGL for the following investigation on its working mechanism by TREL measurements. In addition, we also investigate whether lights are emitted

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Figure 1. (a) Characteristic curves of current efficiency versus current density for devices A and B. The inset illustrates their corresponding curves of brightness versus current density. (b) EL spectrum of device C. The inset shows the EL spectrum of device B. The structures of devices A, B, and C are ITO/NPB (60 nm)/Alq3 (40 nm)/CGL/NPB (60 nm)/Alq3 (40 nm)/Mg:Ag (150 nm), ITO/NPB (60 nm)/Alq3 (40 nm)/Mg:Ag (150 nm), and ITO/NPB (60 nm)/Alq3 (40 nm)/CGL/NPB (60 nm)/Alq3 doped with 10 vol % DCJTB (40 nm)/Mg:Ag (150 nm), respectively.

Figure 2. EL spectra of devices B, D, and E. The EL spectrum of device B is the same as that shown in the inset of Figure 1b. The structures of devices B, D, and E are ITO/NPB (60 nm)/Alq3 (40 nm)/ Mg:Ag (150 nm), ITO/NPB (60 nm)/Alq3 (40 nm)/CGL/NPB (60 nm)/ TDAF (40 nm)/Mg:Ag (150 nm), and ITO/NPB (60 nm)/TDAF (40 nm)/Mg:Ag (150 nm), respectively.

from the two EL units of device A, not just from one EL unit. In order to indentify this, we fabricate device C based on the two EL units with different emission colors (one is green emission from Alq3, and the other is red emission from DCJTBdoped Alq3). As shown in Figure 1b, device C exhibits two emission peaks located at 510 and 625 nm. The former is the characteristic emission peak of Alq3 because the emission peak of Alq3 in device B is at 510 nm (see the inset of Figure 1b). The latter comes from the emission of DCJTB.4 This indicates that lights can be emitted simultaneously from the two EL units of the tandem OLED as this CGL is used, again demonstrating that this CGL is applicable for the following investigations. 3.1. Charge-Supplying BehaWior of the CGL at the Initial Stage. The TREL technique can be used to measure the time when two leading fronts of injected carriers (holes and electrons) meet in the device.16 Therefore, we utilize it to study the working mechanism of the CGL at the initial stage. Because the red emission EL unit in device C is a doped system, its emission mechanism is not as straightforward as the nondoped system. Thus, we fabricate device D consisting of two different EL units (Alq3 is used as a green emitter, and TDAF in the other EL unit is for blue emission) and compare its EL spectrum with those of single-unit device B (green emission) and device E (blue emission). As shown in Figure 2, the EL emission peaks of Alq3 (device B) and TDAF (device E) are located at 510 and 454 nm, respectively. The emission from device D gives a

peak also located at 454 nm without green emission from Alq3, which is due to the much lower emission intensity of Alq3 as compared to that of TDAF as can be supported by the examination above in that this CGL is able to function well and allow simultaneous emission of the two individual EL units. The reason for the low intensity of green emission is due to the low hole flux in the Alq3 emitting layer (EML), as explained below. The LUMO/HOMO levels are 2.4/5.5, 3.0/5.8, and 3.0/ 5.8 eV for NPB,18 Alq3,18 and the n-doped layer of CGL.19 Therefore, for the present structure of device D, the electron provided by CGL can inject into the Alq3 EML without any injection barrier because the LUMO levels of Alq3 and the n-doped layer of CGL are the same (3.0 eV). However, the hole will face an injection barrier of 0.3 eV from NPB to the Alq3 EML. In addition, the electron mobility is larger than the hole mobility by a factor of 100 in the Alq3 EML.20 Therefore, it is no doubt that the hole flux is expected to be lower than the electron flux in the Alq3 EML. However, the issue can be overcome by either (1) selecting a material with a HOMO level close to that of the Alq3 EML to replace NPB for a lower hole injection barrier or (2) reducing the injection barrier between the ITO anode and NPB by depositing a CFx thin film on top of the ITO anode to adjust its work function because the work function of the existing ITO anode (5.05 eV)21 can cause a relative large hole injection barrier to NPB. Then, we performed TREL measurements on device D with the applied voltage pulses (50 kHz, pulse width of 150 ns at 48 V) in a cryostat under vacuum (5 × 10-6 Torr) at room temperature according to our previous work.17 A series of luminescence decay curves at the specific wavelength region from 380 to 650 nm (each decay curve for every 10 nm) were taken, and every decay curve was measured with the same time period of 20 min. With these decay curves, we could extract the EL spectra at different time periods, as illustrated in Figure 3. Note that the TREL spectra at the first 150 ns period correspond to the condition of electro-excitation because the pulse width of the applied rectangular voltage is 150 ns. In the early stage of electro-excitation (0-90 ns, Figure 3a), no blue emission from TDAF is observed, but the intensity of green emission (490 nm, from Alq3) increases at 60-70 ns. Note that the peak position of Alq3 emission (490 nm) shown here is different from that (510 nm) of device B (Figures 1b and 2), which could be attributed to the low intensity of green emission from device D resulting in a slight inaccuracy in identifying

Role of the Charge Generation Layer in Tandem OLEDs

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Figure 3. TREL spectra of device D (the device structure is ITO/NPB (60 nm)/Alq3 (40 nm)/CGL/NPB (60 nm)/TDAF (40 nm)/Mg:Ag (150 nm)) at different time periods, (a) 0-90, (b) 90-120, (c) 70-110 (normalized at 490 nm), (d) 120-150, and (e) 140-270 ns. The TREL spectra of device D are measured by applying a rectangular voltage pulse of 48 V. The pulse width is 150 ns, and the frequency is 50 kHz.

the exact peak position in the TREL spectrum. In the next period (90-120 ns, Figure 3b), the intensity of green emission continuously increases, and that of the blue emission starts to increase with time. By normalizing the spectra (collecting at the period of 70-110 ns) at 490 nm, we found that the intensity of blue emission starts to increase at about 90-110 ns (Figure 3c), and the intensities of the blue and green emissions continuously increase with time until end of the pulse (120-150 ns, Figure 3d). In the final period (150-270 ns), the intensity decreases with time because no charge is injected into the device. From these results, we can determine that the delay times (i.e., the time when appreciable amounts of electron and hole meet in the EL layers) of green and blue emissions are within 60-70 and 90-100 ns, respectively. If the CGL actually generates electrons and holes and supplies them to the adjacent EL units for emission, the delay time of green emission should be the same as that of single-unit device B under the same applied electric field and film thickness. To confirm this, we use TREL to measure the delay time for device B (monitored at 520 nm). The voltage pulses (1 kHz, pulse width 300 ns at 30 V) are applied to the device in a cryostat under vacuum (5 × 10-6 Torr) at room temperature. Under this condition, we find that the delay time is 122 ns (see Figure 4a).

In addition, the delay times measured at different applied voltages ranging from 9 to 50 V are shown in Figure 4b, in which a linear plot of delay time versus electric field (E, applied voltage divided by total thickness of the organic layers) is observed. At the same applied electric field for device D, 2.1 × 106 V/cm, the delay time of device B is 190 ns (obtained from Figure 4b). Obviously, the delay time of green emission in device D (60-70 ns) is in reasonable agreement with that of device B (190 ns), indicating that the CGL actually generates charges supplied to EL units at the initial stage of device operation. The longer delay time in the single-unit device could be a result of its larger electron injection barrier from the cathode than that from the n-doped layer in device D, where the interfacial contact is mainly Alq3. In order to further demonstrate that the CGL can generate charges supplied to EL units at the first stage, we calculate the time (t) when TDAF starts to emit light in device D if holes are supplied from the ITO anode (not from the p-doped layer of CGL) and then travel through NPB, Alq3, and CGL to the TDAF layer by using the relationship between the layer thickness (d), electric field (E), and hole mobility (µh) according to the equation t ) d/(µhE) (the equation which is used to calculate charge mobility from time-of-flight (TOF) measure-

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Figure 4. (a) TREL plot of device B measured by the application of a rectangular voltage pulse of 30 V. The pulse width is 300 ns, and the frequency is 1 kHz (the monitored wavelength is at 520 nm). (b) Electric field dependence of the delay time of device B. The structure of device B is ITO/NPB (60 nm)/Alq3 (40 nm)/Mg:Ag (150 nm).

Figure 5. (a) TREL plot of device F measured by the application of a rectangular voltage pulse of 33 V (the monitored wavelength is at 520 nm). The pulse width is 30 µs, and the frequency is 1 kHz. (b) TREL plot of device B measured by the application of a rectangular voltage pulse of 33 V (the monitored wavelength is at 520 nm). The pulse width is 10 µs, and the frequency is 1 kHz. The structures of devices F and B are ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Ca (5 nm)/Al (80 nm) and ITO/NPB (60 nm)/Alq3 (40 nm)/Mg:Ag (150 nm), respectively.

ments).22 Therefore, we first evaluate µh of Alq3 based on the delay time (td) obtained from device B via TREL measurement (i.e., 190 ns at the electric field of 2.1 × 106 V/cm), the thickness of Alq3 (40 nm), and the information that electron mobility is larger than the hole mobility by a factor of 100.20 By using the equation td ) d/[(µh + µe)E] (the equation which is used to calculate charge mobility from TREL measurement),23 the electron mobility (µe) of Alq3 is calculated to be 1 × 10-5 cm2/ (V s), and thus µh of Alq3 is about 1 × 10-7 cm2/(V s). In addition, the µh and film thickness of NPB are 2.6 × 10-2 cm2/ (V s)24 (at E ) 2.1 × 106 V/cm) and 60 nm, respectively. Therefore, if CGL does not supply holes to the EL unit (TDAF) in device D (for TREL studies in Figure 3) and the holes come from the ITO anode, the time that it takes for the the hole to be injected into NPB and then travel through Alq3, CGL, and finally to TDAF is (19000 + th) ns (th is the time of hole traveling through CGL) according to the equation t ) d/(µhE). Even if th is taken as null, the time (19000 ns) is still much longer than the delay time of TDAF emission measured by TREL measurement for device D (90-100 ns) by a factor larger than 190. Evidently, holes are supplied from the CGL at the initial stage, and the situation for electrons should also be the same. Therefore, this again demonstrates that the CGL can generate charges supplied to EL units at the initial stage. 3.2. Charge Filling to CGL from the Electrodes during the Initial Stage. Becuase the charges in the CGL are consumed continuously during device operation at the initial stage, there

must be a mechanism to refill the charges in the CGL. Otherwise, the CGL will be unstable and prone to decompose due to the excessive loss of electron and hole. Therefore, it is reasonable to conceive that there is a charge refilling process for the CGL with the charges supplied from the electrodes. Here, we use TREL and transient current measurements to investigate this process. For this purpose, we fabricate device F with the CGL/NPB layer inserted between the Ca/Al cathode and the emitting layer Alq3. The insertion of the NPB layer is to prevent direct contact of CGL with the cathode. Then, we first perform TREL measurements on device F by applying the voltage pulses (1 kHz, pulse width of 30 µs at 33 V) on the device in a cryostat under vacuum (5 × 10-6 Torr) at room temperature (the monitored wavelength is 520 nm). The TREL plot is shown in Figure 5a, in which the intensity versus time can be divided into two stages. At the initial stage (0-0.8 µs), the intensity does not decay at the very early stage (0-0.5 µs) because the numbers of charges supplied from the CGL are enough to sustain the emission and even to increase the intensity. Then, the intensity starts to decrease at 0.8 µs. At the second stage (0.8-30 µs), the intensity starts to increase at 1.5 µs, indicating that electrons injected from the Ca/Al cathode refill the CGL to further supply adequate electrons into the EL unit. This inference can be further supported by the fact that this phenomenon does not happen to device B without CGL (see Figure 5b). In addition, we examine whether the measured time required for CGL charge refilling (1.5 µs) is reasonable by comparing it with

Role of the Charge Generation Layer in Tandem OLEDs

Figure 6. The time dependence of the current density of devices F and G. The structures of devices F and G are ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Ca (5 nm)/Al (80 nm) and ITO/NPB (150 nm)/Alq3 (100 nm)/NPB (100 nm)/Ca (5 nm)/Al (80 nm), respectively.

the value obtained by the following calculation for electron refilling time. For such a process, we fabricate device F; electrons injected from the cathode are required to travel through the NPB layer and the p-doped layer (m-MTDATA:F4-TCNQ layer) for charge refilling, and the time required can be calculated to be 1 µs according to the equation t ) [dNPB/ (µe,NPBE) + dm-MTDATA/(µe,m-MTDATAE)],22 where dNPB ) 100 nm, µe,NPB ) 5 × 10-5 cm2/(V s)18 at E ) 1.2 × 106 V/cm, dm-MTDATA ) 20 nm, and µe,m-MTDATA ) 2 × 10-6 cm2/(V s)25 at E ) 1.2 × 106 V/cm. Note that we take µe of F4-TCNQ:mMTDATA as the value for µe,m-MTDATA in the calculation because the dopant F4-TCNQ is only 5 vol % and will not affect the electron mobility much. This value (1 µs) is very close to that obtained from TREL measurements (1.5 µs). Therefore, the result again demonstrates that the charges in the CGL need to be refilled by the charges injected from electrodes. On the other hand, if the present charge refilling mechanism for CGL is correct, the profile of current density versus time must resemble the TREL plot, as shown in Figure 5a. Thus, we measure the profiles of current density versus time for device F and device G (the structure is the same as device F without CGL included) by applying the voltage pulse (1 kHz, pulse width of 30 µs at 33 V) on these two devices in a cryostat under vacuum (5 × 10-6 Torr) at room temperature. As shown in Figure 6, the two sharp cell-current signal peaks existing at the beginning and end of the applied voltage pulse can be attributed to the charging and discharging of the device, respectively. This is because the OLED device possesses the characteristic of a capacitor. Obviously, the current density of

J. Phys. Chem. C, Vol. 115, No. 2, 2011 587 device G almost does not change with time, except the charging and discharging regions, similar to the behavior reported by Barth et al.16 However, the current density of device F decreases initially and then increases, just like the trend of TREL intensity shown in Figure 5a. This behavior can be explained as follows. The electrons generated from the CGL inject into the adjacent emitting layer at the initial stage of the device operation. However, the generated charge number in CGL is limited according the calculation result as reported by Lee’s group.5 Therefore, the continuous consumption of electrons results in the observed decrease of current density. When the electrons injected from the cathode arrive at CGL and refill the CGL, the current density starts to increase, as observed in Figure 6. In other words, the observation proves that the charge refilling mechanism of CGL by charges injected from electrodes as mentioned above is correct. Finally, we investigate the effect of electron injection on the refilling of CGL by comparing the characteristics of current density and brightness versus electric field and TREL plots of the two identical devices, except for the cathodes, device F (Ca/ Al as the cathode) and device H (Au as the cathode). Because the work functions of Ca and Au are 2.87 and 5.1 eV, respectively, and the LUMO level of NPB is 2.4 eV,18 the electron injection barriers of devices F and H are 0.47 and 2.7 eV, respectively. In other words, device F can inject more electrons than device H. This can be manifested by comparing the electric characteristics and brightness of the two devices as follows. As shown in Figure 7a, the current densities of devices F and H are 12.1 and 6.5 mA/cm2, respectively, at the electric field of 7.5 × 105 V/cm. In addition, the brightnesses of devices F and H are 173.4 and 78.1 cd/m2, respectively, at the electric field of 7.5 × 105 V/cm (see Figure 7b). We then perform TREL measurements on the two devices by applying a voltage pulse (1 kHz, pulse width of 30 µs at 33 V) on the devices in a cryostat under vacuum (5 × 10-6 Torr) at room temperature (the monitored wavelength is at 520 nm). As shown in Figure 8, we find that the consumed ratio in device F (13%) is lower than that of device H (33%). This can be attributed to that larger flux of electrons injected from the Ca/Al cathode can refill the CGL fast as compared to the Au cathode, as supported by the fact that the time when the EL intensity starts to increase is shorter for device F (1.5 µs) than that for device H (1.65 µs). 4. Conclusions We adopt Alq3 doped with 10 vol % Mg (10 nm)/mMTDATA doped with 5 vol % F4-TCNQ (20 nm) as the CGL

Figure 7. Electric field dependence of the (a) current density and (b) brightness of devices F and H. The structures of devices F and H are ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Ca (5 nm)/Al (80 nm) and ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Au (50 nm), respectively.

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Figure 8. TREL plots of devices F and H measured by applying rectangular voltage pulses of 33 V. The pulse width is 30 µs, and the frequency is 1 kHz (the monitored wavelength is at 520 nm). The plots are normalized at 0.7 µs. The structures of devices F and H are ITO/ NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Ca (5 nm)/Al (80 nm) and ITO/NPB (150 nm)/Alq3 (100 nm)/CGL/NPB (100 nm)/Au (50 nm), respectively.

in the tandem OLED and investigate its working mechanism mainly by performing TREL measurements. We find that CGL generates charges for supplying to EL units for emission at the initial stage. The consumed electrons in the CGL are refilled by electrons injected from the cathode for a further supply of them into the EL units. The larger electron flux injected from the cathode can refill the CGL faster. It is reasonable to infer that the CGL refilling mechanism is also applicable to the hole. Acknowledgment. The authors thank the National Science Council for financial support through Projects NSC 98-2752E-007-004&-001-PAE and NSC 99-2221-E-007-002-MY3. References and Notes (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913– 915. (2) Forrest, S. R. Org. Electron. 2003, 4, 45–48.

Cheng et al. (3) Kido, J.; Matsumoto, T.; Nakada, T.; Endo, J.; Mori, K.; Kawamura, N.; Yokoi, A. SID Int. Symp. Digest Tech. Papers 2003, 34, 964–965. (4) Liao, L. S.; Klubek, K. P.; Tang, C. W. Appl. Phys. Lett. 2004, 84, 167–169. (5) Fung, M. K.; Lau, K. M.; Lai, S. L.; Law, C. W.; Chan, M. Y.; Lee, C. S.; Lee, S. T. J. Appl. Phys. 2008, 104, 034509/1–034509/5. (6) Qi, X.; Slootsky, M.; Forrest, S. Appl. Phys. Lett. 2008, 93, 193306/ 1–193306/3. (7) Kanno, H.; Giebink, N. C.; Sun, Y.; Forrest, S. R. Appl. Phys. Lett. 2006, 89, 023503/1–023503/3. (8) Kanno, H.; Holmes, R. J.; Sun, Y.; Kena-Cohen, S.; Forrest, S. R. AdV. Mater. 2006, 18, 339–342. (9) Hamwi, S.; Meyer, J.; Kro¨ger, M.; Winkler, T.; Witte, M.; Riedl, T.; Kahn, A.; Kowalsky, W. AdV. Funct. Mater. 2010, 20, 1762–1766. (10) Ho, M. H.; Chen, T. M.; Yeh, P. C.; Hwang, S. W.; Chen, C. H. Appl. Phys. Lett. 2007, 91, 233507/1–233507/3. (11) Wang, Q.; Ding, J.; Zhang, Z.; Ma, D.; Cheng, Y.; Wang, L.; Wang, F. J. Appl. Phys. 2009, 105, 076101/1–076101/3. (12) Lee, T. W.; Noh, T.; Choi, B. K.; Kim, M. S.; Shin, D. W.; Kido, J. Appl. Phys. Lett. 2008, 92, 043301/1–043301/3. (13) Liao, L. S.; Klubek, K. P. Appl. Phys. Lett. 2008, 92, 223311/1– 223311/3. (14) Liao, L. S.; Slusarek, W. K.; Hatwar, T. K.; Ricks, M. L.; Comfort, D. L. AdV. Mater. 2008, 20, 324–329. (15) Terai, M.; Tsutsui, T. Appl. Phys. Lett. 2007, 90, 083502/1– 083502/3. (16) Barth, S.; Mu¨ller, P.; Riel, H.; Seidler, P. F.; Rieβ, W.; Vestweber, H.; Ba¨ssler, H. J. Appl. Phys. 2001, 89, 3711–3719. (17) Liao, J. L.; Chen, X.; Liu, C. Y.; Chen, S. A.; Su, C. H.; Su, A. C. J. Phys. Chem. B 2007, 111, 10379–10385. (18) Liu, S. W.; Wang, J. K. Proc. SPIE 2006, 6333, 63331R/1– 63331R/8. (19) Law, C. W.; Lau, K. M.; Fung, M. K.; Chan, M. Y.; Wong, F. L.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2006, 89, 133511/1–133511/3. (20) Tse, S. C.; So, S. K.; Yeung, M. Y.; Lo, C. F.; Wen, S. W.; Chen, C. H. Jpn. J. Appl. Phys. 2006, 45, 555–557. (21) Hsiao, C. C.; Chang, C. H.; Lu, H. H.; Chen, S. A. Org. Electron. 2007, 8, 343. (22) Liu, C. Y.; Chen, S. A. Macromol. Rapid Commun. 2007, 28, 1743– 1760. (23) Savvateev, V.; Yakimov, A.; Davidov, D. AdV. Mater. 1999, 11, 519–531. (24) Chen, B.; Lee, C. S.; Lee, S. T.; Webb, P.; Chan, Y. C.; Gambing, W.; Tian, H.; Zhu, W. Jpn. J. Appl. Phys. 2000, 39, 1190–1192. (25) Staudigel, J.; Sto¨ssel, M.; Steuber, F.; Simmerer, J. J. Appl. Phys. 1999, 86, 3895–3910.

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