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Tailoring the Efficiencies and Spectra of White Organic Light-Emitting Diodes with the Interlayers Guohua Xie, Zhensong Zhang, Qin Xue, Shiming Zhang, Yang Luo, Li Zhao, Qingyang Wu, Ping Chen, Baofu Quan, Yi Zhao,* and Shiyong Liu* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ReceiVed: August 4, 2010; ReVised Manuscript ReceiVed: NoVember 25, 2010
Charge carriers balance and triplet excitons confinement are two key factors determining the performance of phosphor-based organic light-emitting diodes (OLEDs). The characteristics of white OLED (WOLED) based on complementary blue and yellow phosphors could be easily manipulated by the insertion of an ultrathin interlayer between the two emitters. Both the electrical and the optical characteristics of WOLEDs are significantly dependent on the selection of the interlayers, which tailor charge carrier transportation and energy transfer. The interlayers ease the perplexity of probing the dynamics of charges and excitons. High efficiencies could also be achieved by the effective prevention of the high energy blue triplet excitons from diffusing into the adjacent low triplet states via Dexter transfer. The device with the nominal n-type interlayer and triplets confinement architecture reaches a power efficiency of 40.0 lm/W at ∼100 cd/m2 and a luminance of 1333 cd/m2 at a low voltage of 4.0 V. Introduction Organic semiconductor devices have achieved increasingly rapid development in recent years.1-6 Especially in the field of organic optoelectronics, organic light-emitting diodes (OLEDs) are turning out to be one of the most promising candidates for the next generation mainstream flat panel displays and solidstate lighting sources,4,7-10 due to the unique merits of high efficiencies and environmental friendliness, which have a positive effect on the reduction of greenhouse gases, like CO2 and NxO, and some other toxic wastes. White OLEDs (WOLEDs) are of great importance for the above-mentioned applications. Although the state-of-the-art WOLEDs have achieved attractive improvements in efficiencies and lifetime, paving the way for commercial production,11,12 some physical issues remain ambiguous or unknown. The dynamics of charges and excitons in organic semiconductor devices are intractable.10,13-17 It is considered that charge carriers balance and triplet excitons manipulation are effective methods to achieve high efficiencies.15,18-24 WOLEDs based on the complementary blue and yellow emitters, which could also achieve high quantum efficiency in both single stack with two emitting layers and asymmetric tandem OLED architectures,25-27 are favorable due to the simplicity of generating white light. In this embodiment, we tailored the characteristics of WOLEDs by simply inserting different interlayers between the blue and yellow emitters. The influences of the interlayers on the dynamics of charges and excitons are investigated. The interlayers are the nominal p-type and n-type transporter and their mixture (namely ambipolar interlayer), respectively. For comparison, the device without an interlayer is also fabricated. The p-type transporter is N,N′-dicarbazolyl-3,5-benzene (mCP), which is also the common host of the two phosphorescent dopants, while the n-type transporter is 7-diphenyl-1,10phenanthroline (BPhen) with high electron mobility and high * Corresponding author. Tel.: +86 431 85168242 8301. E-mail:
[email protected] (Y.Z.);
[email protected] (S.L.).
ionization potential, which can effectively block the holes from flowing toward the cathodes. In this contribution, we demonstrated the efficiencies as well as the spectra of WOLEDs could be easily tailored by the interlayers. Experimental Section Prior to the device fabrication, the patterned ITO-coated glass substrates were scrubbed and sonicated consecutively with acetone, ethanol, and deionized water, respectively. All the organic layers were thermally deposited in a vacuum (∼4.0 × 10-4 Pa) at a rate of 1-2 Å/s monitored in situ with the quartz oscillator. To reduce the ohmic loss, a heavily p-doped layer with MoOx,28 considering the low doping efficiency in amorphous organic matrix with transition-metal-oxide-based acceptors,29,30 was directly deposited onto the ITO substrate for each sample. After the deposition of LiF, the samples were transferred to metal chamber and suffered from a vacuum break due to the change of the shadow masks to determine the active area. Note that efficiency of the blue emitter iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2 · ] (FIrpic) exposed to atmosphere was investigated to be below one-half of that without a vacuum break.31,32 The current-voltage-luminance characteristics were measured with a PR650 spectrascan spectrometer and a Keithley 2400 programmable voltage-current source. All the samples were measured directly after fabrication without encapsulation in ambient atmosphere at room temperature. Results and Discussion A first set of devices, that is, ITO/m-MTDATA:MoOx (10 nm, 15 wt %)/m-MTDATA (35 nm)/NPB (5 nm)/mCP (15 nm)/ mCP:FIrpic (5 nm, 10 wt %)/interlayer (x nm)/mCP:(FBT)2Ir(acac) (5 nm, 6 wt %)/BPhen (32-x nm)/LiF (1 nm)/Al (100 nm), where interlayer denotes none (x ) 0) for device A1, BPhen (x ) 2) for device A2, mCP (x ) 2) for device A3, and mCP:BPhen (x ) 2, 1:1) for device A4, respectively, was fabricated to reveal the roles of the interlayers. Here, m-
10.1021/jp107319e 2011 American Chemical Society Published on Web 12/16/2010
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Figure 1. Luminance-voltage-current density characteristics of devices A1-A4. Figure 3. Current efficiency (a) and power efficiency (b) versus current density characteristics of devices A1-A4.
Figure 2. EL spectra of devices A1-A4 at a current density of 20 mA/cm2.
MTDATA is 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine, NPB is N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)-benzidine, and (F-BT)2Ir(acac) is bis(2-(2-fluorphenyl)-1,3benzothiozolato-N,C2), respectively. The luminance-current density-voltage characteristics are plotted in Figure 1. The relative low current density of device A3 with mCP interlayer may be ascribed to the low electron mobility as compared to that of BPhen,33 as the devices tend to be strongly dependent on the electron flow. Thus, it is understandable that device A3 requires a higher driving voltage to produce the equivalent luminance, as a result of imbalance of the charge carriers in the emitting layers. In contrast, 2 nm BPhen interlayer in device A2 not only blocks the holes, but also facilitates the electron injection. Figure 2 shows the spectra of the devices at a current density of 20 mA/cm2. The spectra of the devices without an interlayer (device A1) and with 2 nm mCP (device A3) are almost identical, and the yellow emission is dominant. Interestingly, a more balanced white emission in those with 2 nm BPhen (device A2) and mixed interlayer (device A4) is observed. The reason for the discrimination of the above behaviors could be attributed to the degree of carriers balance and the hindrance of direct energy transfer from high to low energy emitter between the two separate emitting layers (EMLs), which could be modulated by the interlayers. In devices A1 and A3, holes can easily inject into the yellow region and subsequently are trapped by the dopants as the common green and red Ir-based dopants tends to be trapping sites in the wide band gap matrix.23,24,34-36 Simultaneously, a negative space charge barrier is created.37 This can also impede the electrons from traveling
to the blue region. In addition, there is no barrier for holes, and the excess of hole injection in this type of hole-dominant devices worsens the carriers balance.38,39 Thus, less electrons reach the blue region, and the blue emission is suppressed. In contrast, in devices A2 and A4, respectively, the BPhen and mCP:BPhen interlayers can effectively block the holes at the interfaces due to the low position of highest occupied molecular orbital (HOMO) of BPhen. Fewer holes are injected and trapped in the yellow emitting layer in devices A2 and A4. Holes accumulated at the interface may assist the injection of the electrons.40,41 The balanced injection of the charge carriers in the EML could also be confirmed once again in the luminancevoltage-current density characteristics (Figure 1), the current efficiency/power efficiency-current density characteristics (Figure 3a and b), and the external quantum efficiency-luminance characteristics (Figure S1). The performances of the device with 2 nm BPhen interlayer are superior to those of the other devices. Unlike the usually observed maximum efficiency in phosphorescent devices at very low injection,4,14,20,42,43 the above devices exhibit rapid increased efficiencies at low current density (see Figure 3a and b). The current efficiencies roll off gradually after reaching maxima as the current density increases due to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA).44 It is probably caused by the imbalanced injection from the electrodes at low bias and the severe electrode quenching with a relative thin (30 nm) electronic transporting layer (ETL).45-47 Regarded as these particular reasons, we increased the ETL to 40 nm. To investigate how the pristine mCP layer between the hole transporting layer (HTL) and EML influences the confinement of the triplet excitons, another set of devices was fabricated as follows: ITO/m-MTDATA:MoOx (10 nm, 15 wt %)/m-MTDATA (25 nm)/NPB (5 nm)/mCP (y nm)/mCP:FIrpic (5 nm, 10 wt %)/BPhen (2 nm)/mCP:(FBT)2Ir(acac) (5 nm, 6 wt %)/BPhen (40 nm)/LiF (1 nm)/Al (100 nm), y ) 0, 5, 10, and 15 for devices B1, B2, B3, and B4, respectively. Because of the intrinsic long exciton lifetime, the triplet excitons possess long diffusion length, which plays a major role in influencing the performances. The triplet excitons in blue EML tend to diffuse into the HTL with low triplet level (T1),48 following a downward energy transfer and nonradiative decay of triplets in fluorescent NPB. This accounts for the deteriorated performances. As shown in Figure 4, the current density reduces as the thickness of mCP increases, as does the luminance, except for in device B1, which is accompanied by
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Figure 4. Luminance-voltage-current density characteristics of devices B1-B4.
Figure 5. EL spectra of devices B1-B4 at a current density of 20 mA/cm2.
Figure 7. (a) Triplet energy levels of some materials used in this embodiment. Blue dot lines: triplet energy level of mCP. (b) Schematic diagram of HOMO-LUMO values of the materials. The values are obtained from the literature.48,50,51 The rectangle with blue dash border denotes the HOMO and LUMO positions of mCP. (c) Charges and excitons in the proposed energy level (not to scale) of the emitting layers with the interlayers, respectively. The switches denote the adoption of the interlayer. Dashed lines: triplet energy levels. Solid lines: HOMO-LUMO positions of mCP (light blue) and BPhen (deep blue), respectively. Figure 6. Current efficiency (a) and power efficiency (b) versus current density characteristics of devices B1-B4.
the notoriously residual singlet emission of NPB (see Figure 5). On account of the high lowest triplet state (T1) of mCP (2.9 eV) restricting the triplet excitons from diffusing to NPB with a small T1 of 2.4 eV,48,49 the current efficiency (Figure 6a) and quantum efficiency (Figure S3) increase as the mCP thickness increases. However, the power efficiency (shown in Figure 6b), compensated with the increased voltage (Shown in Figure 4), does not follow this rule. It is clearly evident that the triplet excitons confinement is responsible for the high efficiency.
To eliminate the residual NPB emission and improve the efficiencies, the NPB/mCP layers were replaced by tris(phenylpyrazole)iridium (Ir(ppz)3) with high triplet level and ultra high energy level of lowest unoccupied molecular orbital (LUMO) (shown in Figure 7a and b). Here, we fabricated the devices with the interlayers once again to reveal that both charges balance and the triplet excitons confinement contribute to the high efficiencies. The structures are ITO/m-MTDATA: MoOx (10 nm, 15 wt %)/m-MTDATA (30 nm)/Ir(ppz)3 (15 nm)/ mCP:FIrpic (5 nm, 10 wt %)/interlayer (z nm)/mCP:(FBT)2Ir(acac) (5 nm, 6 wt %)/BPhen (42 - z nm)/LiF (1 nm)/ Al (100 nm), where interlayer is none (z ) 0) for device C1,
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Figure 8. Luminance-voltage-current density characteristics of devices C1-C4.
Figure 9. EL spectra of devices C1-C4 at a current density of 20 mA/cm2.
BPhen (z ) 2) for device C2, mCP (z ) 2) for device C3, and mCP:BPhen (z ) 2, 1:1) for device C4, respectively. Depicted in Figure 8, the devices with 2 nm BPhen (device C2) and mCP: BPhen (device C4) mixed interlayer have similar electrical performances, while the performance of the device with 2 nm mCP interlayer (device C3) is the worst, mainly caused by the poor charge transportation as mentioned above. In contrast to device A1 without an interlayer possessing dominant yellow emission, device C1 without an interlayer has a balanced white emission (see Figure 9). This is probably due to the accumulated charges and excitons blocked by the high LUMO position and high T1 of Ir(ppz)3. As can be inferred, the pristine mCP layer
between HTL and EML cannot effectively block the electrons from passing through (see Figure 7b). A portion of electrons, dependent on the thickness of mCP, arrive at the NPB layer and recombine with the holes. That is why the emission of NPB is observed in devices A1-A4 (see Figure S2) and B1-B4 (see Figure S4). As (F-BT)2Ir(acac) turns to be an efficient trap due to its narrow band gap as compared to the wide band gap of mCP host, the direct carrier trapping contributes to the dominant yellow emission (shown in Figure 2) in device A1 without an interlayer accompanying the concurrent inevitably Dexter energy transfer from FIrpic to (F-BT)2Ir(acac). Dissimilarly, the major recombination zone shifts toward the HTL/electro blocking layer (EBL) interface, and the dominant blue emission (observed in devices C2 and C4) is compensated by the energy transfer to the yellow region, contributing to the balanced white emission in device C1. In the presence of the interlayers, the energy transfer is partially interrupted. That is the reason why devices C2, C3, and C4 have reduced yellow emission. Device C3 with 2 nm mCP interlayer possesses hole mobility superior to electron mobility; thus it would not block the holes, dissimilar to devices C2 and C4. As the triplet level of mCP is higher than that of FIrpic, the triplets would not be quenched by the mCP interlayer. Although the triplets would diffuse to the yellow region with respect to the intrinsic long diffusion length, the energy transfer rate is likely to slow in the presence of the mCP interlayer. These phenomena are also presented in the explanatory illustration of Figure 7c. Now, it is clear why device C3 has less yellow emission than device C1, while more than devices C2 and C4. However, device C3 exhibits poor efficiencies (shown in Figure 10a and b) that contributed to the imbalance of charge carriers. The imbalance carrier distribution in the EML of device C3 could also be demonstrated, Figure S5. The observed efficiency roll-off is obvious at low current density. This is consistent with the result reported by Forrest’s group,18 that loss of charge balance is one of the primary causes of the efficiency roll-off at low current density in phosphorescent devices. The efficiency roll-offs (shown in Figures 10 and S5) between 100 and 5000 cd/m2 are 48.3% for device C1, 31.7% for device C2, 47.1% for device C3, and 41.9% for device C4, respectively, while the accelerated roll-offs at higher luminance may be relative to some other mechanisms, such as severe TTA and TPA. As listed in Table 1, Ir(ppz)3 exhibits not only superior electron blocking ability but also excellent triplet excitons confinement capability, contributing to the low voltage and high efficiencies. The BPhen interlayer could block the holes from directly flowing out in the blue region and could balance the charge carriers. In contrast, the mCP interlayer would slow the
TABLE 1: Summary of the Performances of the Devices EQEc (%) device
EBLa
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4
mCP mCP mCP mCP mCP mCP mCP mCP Ir(ppz)3 Ir(ppz)3 Ir(ppz)3 Ir(ppz)3
a
interlayer BPhen mCP mCP:BPhen BPhen BPhen BPhen BPhen BPhen mCP mCP:BPhen
voltage at 1000 cd/m2 (V)
CIEb (x, y) over 103-104 cd/m2
1 mA/cm2
10 mA/cm2
6.0 5.8 6.2 6.0 5.0 4.7 5.3 5.9 4.1 3.9 4.6 4.0
(0.42 ( 0.01, 0.47 ( 0.01) (0.35 ( 0.02, 0.43 ( 0.01) (0.40 ( 0.02, 0.47 ( 0.01) (0.34 ( 0.02, 0.44 ( 0.01) (0.38 ( 0.01, 0.44 ( 0.01) (0.36 ( 0.01, 0.44 ( 0.01) (0.36 ( 0.01, 0.45 ( 0.01) (0.36 ( 0.01, 0.45 ( 0.01) (0.32 ( 0.00, 0.43 ( 0.00) (0.27 ( 0.01, 0.40 ( 0.01) (0.30 ( 0.02, 0.42 ( 0.01) (0.23 ( 0.00, 0.37 ( 0.00)
10.8 12.1 10.5 11.4 3.2 11.1 12.1 12.3 12.9 16.0 10.8 16.0
9.8 11.3 9.8 10.3 3.1 9.9 10.5 10.6 9.1 12.5 7.7 11.6
Electron blocking layer. b The Commission Internationale de l’Eclairage 1931 coordinates. c External quantum efficiency.
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Xie et al. National Natural Science Foundation of China (Grant nos. 60977024 and 60907013). We gratefully acknowledge Dr. Liying Zhang and Dr. Bin Li for providing (F-BT)2Ir(acac). Supporting Information Available: External quantum efficiency-luminance characteristics and EL spectra of the three sets of devices at different voltages. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 10. Current efficiency (a) and power efficiency (b) versus current density characteristics of devices C1-C4.
process of the high energy triplets from diffusing and being captured by the adjacent low energy triplets. The mixed interlayer functions eclectically with respect to the electrical characteristics. Interestingly, the spectra of device C4 with the mixed interlayer are extremely stable (see Figure S6), nearly independent of the driving voltage. It is worth mentioning that the result could also be tailored by the control of the ratio of the mixed interlayer, while not revealed here. The competition of charge carriers and excitons determines the performance of the devices. The interlayers can selectively switch on or off the charges and excitons traversing between the two adjacent EMLs. Conclusions We have demonstrated efficient WOLEDs based on complementary blue and yellow phosphorescence. The electrical and optical characteristics could be easily manipulated by an interlayer inserted between the two emitters. The efficient host to guest energy transfer contributes the blue emission, while the yellow emission mainly originates from the direct exciton formation on guest emitter. Exemplarily, balanced charge carriers and exciton confinement as well as host-guest energy transfer improve the blue emission, while direct charge trapping and triplet energy transfer play a role in shaping the yellow emission. In this contribution, we investigated the effects of charge carriers balance, electrode quenching, trapping, energy transfer, and triplet excitons confinement on the performances of WOLEDs. The competition of energy transfer and charge carriers balance is revealed by the interlayers. The ease of tailoring the device behaviors by the interlayers sheds light on the dynamics of charges and excitons in WOLEDs. This, no doubt, provides valuable information for further manufacture of efficient devices. Here, we also demonstrated WOLED with low voltage (1333 cd/m2 at 4 V) and high external quantum efficiency (15.0% at ∼1000 cd/m2). In prospect, deliberate encapsulation and outcoupling schemes as well as air-stable n-doping strategies will boom the efficiencies of our WOLEDs to meet the requirements for commercial solid-state lighting sources. Acknowledgment. We acknowledge funding for this research from the National Key Basic Research and Development Program of China under Grant no. 2010CB327701, and the
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