Highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs) with a multiple quantum well (MQW) structure were investigated. A peak external quantum efficiency (EQE) of 20.31%, current efficiency of 40.31 cd/A and power efficiency of 30.14 lm/W were achieved in the optimized device with two quantum wells (QWs). The obtained efficiencies are much higher than those of the control devices without QWs. More importantly, the MQW devices exhibit low efficiency roll-off. At a high luminance of 5000 cd/m2, the EQE still keeps at a high value of 18.86% in the optimized MQW device, and the efficiency roll-off is only 7.14%, which is lower than that of 30.78% in the control device (reduced from 16.05% to 11.11%). Meanwhile, the maximum power efficiency of the optimized MQW device was also exhibited more than 54.80% improvement compared to the control device. The high efficiency and low efficiency roll-off are attributed to the effective confinement of charge carriers and excitons by the state-of-the-art MQWs.
©2012 Optical Society of America
Organic light-emitting diodes (OLEDs) have attracted considerable attention since the first presentation of thin film devices based on small organic molecules system by Tang and VanSlyke  in 1987. Tremendous effort has been devoted to developing novel materials and device architectures, which are used to satisfy potential applicability in flat-panel display [2, 3] and the next generation lighting . One of the most essential factors for OLED industrial applications is to realize high efficiency at high brightness to reduce energy consumption. In order to achieve this goal, various approaches were proposed, such as employing the bipolar host materials [5–7] to balance charge carriers, inserting the blocking layers to restrict excitons and charge carriers in the emitting layers (EMLs) [8–12]. Another efficient approach is to confine charge carriers and excitons by multilayer quantum wells, which are widely used in the inorganic LEDs to achieve higher efficiency [13–17]. But in OLEDs, only a few about the effective multiple quantum well (MQW) structures have been reported. Qiu et al. improved the efficiency about four times by adopting the organic MQW structure in the fluorescent devices, they found that the MQW structure can efficiently control the hole carriers transport and achieve a better electron–hole balance [18, 19]. Jiang et al. presented the fluorescent quantum well devices with tris(8-hydroxyquinoline) aluminum (Alq3) as a barrier potential and electron transport layer, showing higher brightness and efficiency than those of the common hetero-structure devices . Lee et al. demonstrated the green phosphorescent devices with a triplet MQW structure, in which a narrow triplet band gap host material was sandwiched between the wide triplet band gap host materials, and the current efficiency was improved from 11 cd/A to 47 cd/A . But to the best of our knowledge, reports on high efficiency blue phosphorescent OLEDs based on the MQW architecture are still quite rare.
Here we fabricated efficient blue phosphorescent OLEDs by adopting a triplet MQW structure. Bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate iridium(III) (FIrpic) doped N,N'-dicarbazolyl-3,5-benzene (mCP) is used as the EML and mCP is also functionalized as the potential electron well and hole barrier, while 4,4',4”-tri(N-carbazolyl)-triphenylamine (TCTA) acts as the potential electron barrier and hole well layer. In the MQW devices, charge carriers are effectively confined in each EML, which can improve the recombination efficiency and decrease the current leakage. Furthermore, the triplet levels of TCTA and mCP are higher than that of FIrpic, which enable consumption of all the triplet excitons contributing to the emission. The confinement of charge carriers and excitons by the MQWs leads to high efficiency and low efficiency roll-off in the blue phosphorescent OLEDs. The maximum EQE of 20.31% was obtained in the optimized MQW device, which exhibited more than 30.69% improvement compared to the control device. Even at the luminance of 5000 cd/m2, the EQE still retained at 18.86%, which was one of the best values that have been reported for blue phosphorescent OLEDs. Furthermore, the maximum power efficiency of the optimized MQW device showed nearly 54.80% improvement compared to the control device (from 19.47 lm/W to 30.14 lm/W).
2. Experimental procedure
The configuration of blue PhOLED with MQW structure is indium tin oxide (ITO)/MoO3 (10 nm)/N,N’-bis-(1-naphthl)-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB) (40 nm)/[TCTA (5 nm)/EML (10 nm)]n/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) (40 nm)/LiF (1 nm)/Al, and the corresponding energy-level diagram is shown in Fig. 1 . Here, EML consists of FIrpic and mCP, and the concentration was optimized at 7 wt%, n is well number, ranging from 1 to 4, and the corresponding device defined as device 1, 2, 3 and 4. NPB is the hole transporting layer, and TmPyPB acts as the excitons/hole blocking and electron transporting layer. NPB, TCTA TmPyPB, FIrpic and mCP were purchased from Luminescence Technology Crop. (Lumtec) and without further purified. MoO3 (99.95%), LiF (99.98%) and Al slugs (99.99%) were purchased from Sigma-Aldrich, and used as received. Meanwhile, a control device with the structure of ITO/MoO3 (10 nm)/NPB (40 nm)/mCP (5 nm)/mCP: FIrpic (7 wt%, 30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (Device 5) is also fabricated.
All the devices are fabricated by vacuum deposition. The ITO substrate with a sheet resistance of 20 Ω/□ was cleaned with the cleaner and deionized water under the ultrasound for 15 minutes respectively. Then the ITO was dried in an oven for 3 hours. Before loaded into a vacuum deposition chamber the ITO was treated with UV-ozone for 5 min. The EL devices were fabricated by successive deposition of organic materials and electrode materials onto the ITO-coated glass substrate at high vacuum (10−6 Torr) with a rate of 1.0-1.2 Å/s. The electroluminance (EL) spectra, luminance, Commission International de I’Eclairage coordinates (CIEx,y) and the current-voltage-luminance characteristics of the devices were measured with a rapid scan system using a Photo Research PR655 spectrophotometer and a Keithley 2400 digital source. All the date of EL characteristics were measured at room temperature under an ambient atmosphere.
3. Result and discussion
To investigate the effect of the quantum well (QW) number on the device performance, the blue phosphorescent OLEDs of ITO/MoO3/NPB/[TCTA/mCP: FIrpic]n/TmPyPB/LiF/Al were fabricated. Figure 2 shows the characteristics of current density-voltage-brightness (J-V-B), external quantum efficiency (EQE)-brightness, power efficiency-brightness and EL spectrum in the MQW OLEDs with various well numbers (n) from 1 to 4. As shown in Fig. 2(a), both the current density and the brightness dramatically decrease with the QW number at the same drive voltage, and the turn-on voltage (Von) (defined as the voltage at 1 cd/m2) increase with the QW number. The lowered current density and enhanced Von are mostly due to the increase of the device thickness. It is clearly seen from Fig. 2(b) that the EQEs of the multiple QW devices are higher than that of the single QW device. The device with two QWs exhibits the best efficiency, and the efficiency reduces with further increasing the number of wells. Device 2 (i.e. n = 2) offers a peak EQE of 20.31% with low efficiency roll-off. Meanwhile, as shown in Fig. 2(c), the power efficiency of devices with different MQW were also exhibit the same changing trend, and the best power efficiency (30.14 lm/W) was obtained when n = 2. The decrease of efficiency with the well number could result from the excess interfaces, which lead to the opposite effect such as unbalance of charge transport and increasing the drive voltage. In contrast, the device with one QW (Device 1) exhibits the lowest EQE, inferring that the few QWs could not able to play a good role on charge confinement, which induce leakage current and unbalance of charge carriers.
The electroluminescent (EL) spectra of the MQW devices are shown in Fig. 2(d), all of the devices only exhibit a FIrpic emission that is centered at 472 nm with a vibrational peak at 496 nm, which indicates that the efficient energy transfer from the host to FIrpic is efficient. But the EL intensity at the vibrational peak obviously increases with the number of QWs. This could originate from the variation of optical lengths in the four devices because the total thickness of the emitting layer is different for the four devices . The CIEx,y are calculated to (0.14, 0.29), (0.14, 0.33), (0.15, 0.37) and (0.15, 0.39) at 100 cd/m2, respectively, in Device 1, 2, 3 and 4.
Furthermore, we compare the MQW device with n = 2 (Device 2) and the well-studied control device without QWs (Device 5). The corresponding device performances are shown in Fig. 3 . As shown from the J-V-B curves in Fig. 3(a), the blue phosphorescent OLED with MQW structure shows a higher brightness and lower drive voltage. For example, at current density of 100 mA/cm2, Device 2 exhibits the luminance of 21080 cd/m2, much higher than that of 14400 cd/m2 in the Device 5. Meanwhile, the driving voltage of MQW device 2 is 10.2 V, about 1.3 V lower than that of the control device at the same current density. The enhancement in brightness and decrease in drive voltage can be attributed to the introduction of TCTA in the MQW structure, which could effectively improve hole injection and transport into the EMLs. Meanwhile, the host of mCP could confine electron in the EMLs at a certain extent. Thus, the recombination efficiency of holes and electrons can be improved in the MQW devices. It can be found from the EQE versus brightness characteristics of the MQW Device 2 and the control device, as shown in Fig. 3(b), the EQE of Device 2 is 20.31% at 100 cd/m2 and 19.29% at 1000 cd/m2, respectively. While the control device with traditional structure shows only 15.54% at 100 cd/m2 and 14.34% at 1000 cd/m2. Meanwhile, the efficiency of the device with MQW structure rolls-off more slowly with the brightness compared to the control device 5. For example, at the high luminance of 5000 cd/m2 the EQE of the MQW device is still as high as 18.86%. But for control device 5, the EQE reduces to 11.11% at the same high brightness. The efficiency roll-off of control device is 30.78%, about four times more than that of the MQW device 2 (7.14%). Furthermore, the current efficiency and power efficiency of device 2 is also higher than those of the control device 5. Device 2 exhibits the peak efficiencies of 40.31 cd/A and 30.14 lm/W, while Device 5 only exhibits the peak efficiencies of 30.09 cd/A and 19.47 lm/W. Moreover, the maximum power efficiency of device 2 exhibited more than 54.80% improvement and lower power efficiency roll-off, compared to the control device (device 5). For example, at brightness of 100 cd/m2 the power efficiency of MQW device 2 rolls-off only 10.42%, but for device 5, the value is higher than 18.37%. Here, it should be mentioned that the current efficiency of Device 2 still reaches as high as 30.38 cd/A, even at the ultrahigh brightness of 10000 cd/m2, which is about two times higher than that of Device 5 (16.86 cd/A). In order to undersand the function of multiple quantum well structure, the performance of Device 1, Device 2 and Device 5 are summarized in Table 1 as a comparision.
The excellent performances of the MQW device with n = 2 could be ascribed to the efficient confinement of charges and excitons in the EML, which increase the recombination probability of electrons and holes and enable consumption of all the formed excitons contributing to emitters. As we know, the hole mobility of hole transport materials is usually higher than the electron mobility of electron transport materials . But here, the hole mobility of NPB  is 5.1 × 10−4 cm2 V−1 s−1, and the electron mobility of TmPyPB  is as high as 1 × 10−3 cm2 V−1 s−1, which could result in imbalance of hole and electron. So the excitons often are formed at the interface of EML and hole-blocking layer (HBL) or electron transport layer (ETL). When the current density increase, the excitons recombination zone may shift, which could result in triplet-triplet, triplet-dipolar quenching and the spectral change [26, 27]. In MQW device, the charges and excitons are efficiently confined into EML due to the introduction of TCTA as potential electron barrier and the EML functional as the potential electron well and hole barrier. As shown from energy levels, the highest occupied molecular orbital (HOMO) energy level of mCP  is 6.1 eV, 0.4 eV higher than that of TCTA , which can improve the hole injection from NPB to the EML. Meanwhile, the lowest unoccupied molecular orbital (LUMO) energy level of mCP is 2.9 eV, 0.6 eV higher than that of TCTA and 0.2 eV higher than that of TmPyPB, which can efficiently confine electron in the EML. Therefore, most of carriers could be confined in the EML and improve the recombination ratios. Furthermore, the triplet energy of TCTA , mCP  and TmPyPB are 2.82, 2.9 and 2.78 eV, respectively, all of them are higher than that of the FIrpic emitter, which can suppress triplet excitons diffusing out of the EML. So both charges and triplet excitons can be effectively confined in the EMLs of the MQW device, resulting in high device efficiency and low efficiency roll-off.
In summary, we have demonstrated high efficiency and low efficiency roll-off blue phosphorescent OLEDs based on a MQW structure. The MQW can effectively confine charges and excitons, thus improve the recombination of electrons and holes and reduce the quenching of triplet excitons. The peak EQE of 20.31%, current efficiency of 40.31 cd/A and power efficiency of 30.14 lm/W were obtained in the optimized device with two quantum wells. Furthermore, the EQE reduces slightly from its peak value of 20.31% at 100 cd/m2 to 18.86% at 5000 cd/m2. Even at the ultrahigh brightness of 10000 cd/m2, the EQE still reaches as high as 15.21%. Meanwhile, the maximum power efficiency showed nearly 54.80% improvement compared to the control device, and power efficiency roll-off is only half of the conventional device at the brightness of 100 cd/m2. This study provides a good approach to develop high performance OLEDs.
We thank the National Natural Science Foundation of China (21161160442) and Wuhan Science and Technology Bureau (NO: 01010621227; 51203056) and the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences for financial support.
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