Low driving voltage blue, green, yellow, red and white phosphorescent organic light-emitting diodes (OLEDs) with a common simply double emitting layer (D-EML) structure are investigated. Our OLEDs without any out-coupling schemes as well as n-doping strategies show low driving voltage, e.g. < 2.4 V for onset and < 3 V for 1000 cd/m2, and high efficiency of 32.5 lm/W (13.3%), 58.8 lm/W (14.3%), 55.1 lm/W (14.6%), 24.9 lm/W (13.7%) and 45.1 lm/W (13.5%) for blue, green, yellow, red and white OLED, respectively. This work demonstrates that the low driving voltages and high efficiencies can be simultaneously realized with a common simply D-EML structure.
© 2014 Optical Society of America
Since the pioneering works by Ma  and Forrest  et al., phosphorescent organic light-emitting diodes (PhOLEDs) with their ideal characteristic of the potential 100% internal quantum efficiency have attracted considerable attention due to their bright future of practical applications in flat-panel displays and solid-state lighting [3–7]. The state-of-the-art PhOLEDs have achieved preeminent improvements in efficiencies and lifetimes , laying the foundation for the commercialization. Due to the long excited-state lifetimes of phosphorescent materials, these emitters must be doped into appropriate host materials to prevent exciton quenching by triplet-triplet annihilation, and this doped emission layer is then typically sandwiched between additional hole-transport layer (HTL) and electron-transport layer (ETL). To date, nearly all development strategies for achieving high efficiencies in PhOLEDs have focused on such a multilayer strategy [9–11].
However, the use of multiple organic layers in an OLED greatly increases the cost of the device, presenting a significant obstacle to commercialization. Furthermore, care must be taken to match the frontier molecular orbital (FMO) energy levels of all adjacent layers to avoid exciplex formation and charge accumulation at each interface within the device. These present significant challenges to OLED mass production, making simplified device structures highly desirable. It is relatively arduous to acquire a common phosphorescent host material, which is suitable for all the blue, green, yellow and red emitters to realize high efficiency simultaneously, particularly, the blue phosphorescent emitter has a more strict demand on the hosts because the host must offer a high triplet energy level (T1) for exothermic energy transfer, suitable FMO energy levels for carrier injection, and optimal molecular configuration and components for carrier transportation at the same time . Unfortunately, the blue-favorite host with a wide energy gap may cause a considerable efficiency loss in the yellow or red unit inevitably [13–15].
It is well-known that the double emitting structure can increase the quantum efficiency and decrease the efficiency roll-off because of wider recombination region and better charge balance [16–18]. In this work, we demonstrate that the low driving voltages and high efficiencies can be simultaneously realized for blue, green, yellow, red and white PhOLEDs with a common simply D-EML structure. In which we utilize both a hole-transport 4,4`,4``-tris(Ncarbazolyl)triphenylamine (TcTa) and an electron-transport 2,4,6-tris(3`-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine (TmPPPyTz)  as host materials, meanwhile, TcTa and TmPPPyTz as hole and electron transport layer (HTL and ETL), respectively. With this simple D-EML structure, our PhOLEDs exhibit low driving voltages (<3 V for 1000 cd/m2) and high efficiency (> 13%) for all of the blue, green, yellow, red and white PhOLEDs. In prospect, the efficiency can be 2~3 times increased by out-coupling schemes as well as air-stable n-doping strategies to meet the requirements for solid-state lighting sources [3,20,21].
Prior to the device fabrication, the patterned ITO-coated glass substrates were scrubbed and sonicated consecutively with acetone, ethanol, and de-ionized water, respectively. All the organic layers were thermally deposited in vacuum (~4.0 × 10−4 Pa) at a rate of 1-2 Å/s monitored in situ with the quartz oscillator. After the deposition of Cs2CO3, the samples were transferred to metal chamber under a nitrogen atmosphere. The electroluminescence (EL) spectra are measured by a PR655 spectroscan spectrometer. The luminance–current density–voltage characteristics are recorded simultaneously with the measurement of the EL spectra by combining the spectrometer with a Keithley model 2400 programmable voltage–current source. The external quantum efficiency (EQE) and luminous efficiency (LE) are calculated assuming Lambertian distribution, and then calibrated to the efficiencies obtained at 1000 cd/m2 in the integrating sphere (Jm-3200). All of the organic materials used in this paper were obtained from Lumtec Corp. and used as received without further purification. All measurements are carried out under atmospheric environments at room temperature.
3. Results and discussion
Figure 1 shows the schematic diagram of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) positions of some materials used in this paper. The values are obtained from the literatures [14,19,22,23]. It is known that TcTa have very good hole transporting characteristics (μ~10−4 cm2/Vs) with a T1 of 2.79 eV and TmPPPyTz have very good electron transporting characteristics (μ~10−4 cm2/Vs) with a T1 of 2.75 eV, so we believe that TcTa and TmPPPyTz can make a good candidate for host as well as transporting layer use in full color PhOLEDs.
At first, a primary structure of ITO/TcTa:MoOx (40 nm, 15 wt.%)/TcTa (10 nm)/TcTa:dopant (5 nm)/TmPPPyTz:dopant (5 nm)/TmPPPyTz (40 nm)/Cs2CO3 (1 nm)/Al, where the dopant represented 10 wt.% bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] Iridium(III) (FIrpic), 6 wt.% tris(2-phenylpyridine) Iridium(III) (Ir(ppy)3), 6 wt.% bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2) Iridium ((F-BT)2Ir(acac)) and 8 wt.% Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)) for blue, green, yellow and red OLED, respectively, was fabricated. Figure 2 shows the Luminance-Voltage-Current density characteristics of the blue, green, yellow and red OLED. All of the OLEDs with the common structure showed low turn-on voltage (the bias required to give a measurable luminance of 1 cd/m2) < 2.4 V, and reaches a luminance of 1000 cd/m2 < 3 V. Indicating that with the modification of TcTa:MoOx  hole injection layer (HIL) and Cs2CO3  electron injection layer (EIL) our OLEDs exhibit almost no injection barrier for both hole and electron injection, meanwhile, without the mismatch of FMO energy levels of adjacent HTL or ETL within these devices (shown in Fig. 3), our simply structure achieved ultralow driving voltage is under prediction.
Along with the ultralow driving voltages, due to the balanced injection and transport of hole and electron in our simply D-EML device structure (shown in Fig. 3), our OLEDs exhibit high efficiencies (shown in Fig. 4). The maximum efficiencies of the blue device reached 26.9 cd/A for current efficiency (CE), 32.5 lm/W for power efficiency (PE) and 13.3% for external quantum efficiency (EQE). We also achieved green, yellow and red PhOLED with the common primary structure, the green device achieved 48.7 cd/A, 58.8 lm/W and 14.3% for CE, PE and EQE, respectively. The yellow device exhibited 45.6 cd/A, 55.1 lm/W and 14.6% for CE, PE and EQE, respectively. And the red device reached 20.1 cd/A, 24.9 lm/W and 13.7% for CE, PE and EQE, respectively. The low operating voltage made their PE higher than CE, which is unusual for PhOLEDs and reveal their advantage in energy conservation. It is noted that all of the OLEDs with a common simply D-EML structure which can effectively extend the recombination zone  exhibited very low efficiency roll-off, from the maximum efficiency value to that of 1000 cd/m2, their efficiency roll-offs were only 10.3% CE, 23.0% PE and 10.1% EQE for the blue device, 2.0% CE, 9.0% PE and 2.0% EQE for the green device, 3.6% CE, 10.8% PE and 3.4% EQE for the yellow device, and 5.5% CE, 18.1% PE and 5.5% EQE for the red device. Our OLEDs with a common simply double light-emitting structure showed low driving voltage, high efficiency and low efficiency roll-off, which could be comparable with those classic host materials for blue, green, yellow and red phosphorescent emitters.
The excellent performance of the monochromic PhOLEDs inspired us to further investigate the performance of the white PhOLEDs with the similar simply D-EML, namely W1, W2, W3 and W4 with the primary configuration of ITO/TcTa:MoOx (40 nm 15 wt.%)/TcTa (10 nm)/TcTa:(F-BT)2Ir(acac) (5 nm 6 wt.%)/interlayer (x nm)/TmPPPyTz:FIrpic (5 nm 10 wt.%)/TmPPPyTz (40-x nm)/Cs2CO3 (1 nm)/Al, here the interlayer represents none (x = 0) for W1, 2 nm TcTa for W2, 2 nm TmPPPyTz for W3 and 2 nm TcTa:TmPPPyTz (1:1) for W4, respectively. The white emission was achieved through adjusting the interlayer. Figure 5 shows the Luminance-Voltage-Current density characteristics of the white PhOLEDs, the substantially equal current density of these white devices was ascribed to the similar carrier transport ability of TcTa and TmPPPyTz. Without the mismatch of FMO energy levels of adjacent functional layers, all of the WOLEDs exhibit extremely low driving voltage, such as < 2.4 V for onset and < 3 V for 1000 cd/m2. The spectra of device W1 without any interlayer and W3 with 2 nm TmPPPyTz interlayer are almost identical with the yellow portion taking precedence. Interestingly, a more balanced white emission in the devices with 2 nm TcTa interlayer (device W2) and 2 nm mixed TcTa:TmPPPyTz interlayer (device W4) is observed. The reason for the above discrimination behaviors could be attributed to the degree of carriers balance and the hindrance of energy transfer from blue to yellow emitter, which could be modulated by the interlayers. The devices without or with TmPPPyTz interlayer, electrons and holes can easily transport to the interface of TcTa/TmPPPyTz and subsequently form excitons, then a majority of the electronic excitation energy is effectively transferred to yellow emitter accompanied with few electronic excitation energy is transferred to blue emitter. While the devices with TcTa or maxed TcTa:TmPPPyTz interlayer more electrons can be blocked within the blue emission layer due to the excellent electron blocking ability of TcTa, and holes can transport through TcTa directly resonant hole injection onto FIrpic molecules, with more excitons utilized by blue emitter a balanced emission from yellow emitter (F-BT)2Ir(acac) and blue emitter FIrpic comes true.
The CE, PE and EQE versus luminance characteristics of the white devices are compared in Fig. 6. Although device W1 with little blue emission is far from white emission, where electrons can easily inject into the yellow region and trapped by the emitters as the common yellow Ir-based emitters tends to be trapping sites in the wide band gap matrix and simultaneously there exist direct energy transfer from high to low energy emitter, it still shows high efficiencies of 42.2 cd/A for CE, 44.2 lm/W for PE and 13.3% for EQE at a brightness of 1485 cd/m2, and device W3 with the similar spectra as device W1 also shows high efficiency of 41.0 cd/A, 43.1 lm/W and 13.1% at 1434 cd/m2. In contrast, device W2 with a TcTa interlayer and device W4 with a mixed TcTa:TmPPPyTz interlayer can effectively block more electrons within the blue region and resist direct energy transfer from high to low energy emitter between the separate emitting layers, and both of them exhibit more balanced white emission. At about 1000 cd/m2, the Commission Internationale deL’Eclairage (CIE) coordinates measured in forward direction of device W2 and W4 are (0.30, 0.44) and (0.34, 0.47), respectively. Device W2 achieves a CE of 35.7 cd/A, PE of 37.4 lm/W and EQE of 13.3% at 1243 cd/m2 accompany with low efficiency roll-off 12.9% calculated from the maximum EQE to that of 5000 cd/m2, and device W4 achieves a CE of 38.9 cd/A, PE of 39.8 lm/W and EQE of 13.7% at 1316 cd/m2 accompany with a roll-off 13.5%. The high efficiency and low efficiency roll-off can be ascribed to the D-EML extends the recombination zone, while the accelerated roll-offs at higher luminance may due to the nonradiative excitons quenching processes, such as triplet-triplet excitons annihilation (TTA) and triplet-polaron annihilation (TPA) processes, and /or field-induced quenching [26,27]. Table 1 summarizes the performance of several representative WOLEDs. The state-of-art performance makes our WOLEDs among the best reported WOLEDs [28–33].
It worth mentioning that the white spectra could be tailored by the control of the ratio of the mixed interlayer which can selectively switch on or off the charge carriers between the two adjacent EMLs (shown in Fig. 7), which could be attributed to the degree of carriers balance and the hindrance of direct energy transfer from high to low energy emitter between the separate emitting layer.
In summary, we firstly investigated the monochromatic blue, green, yellow and red PhOLED with a common simply D-EML structure, in which hole transport material TcTa and electron transport material TmPPPyTz served as host materials, as well as, TcTa and TmPPPyTz serve as HTL and ETL, respectively. With the effective HIL TcTa:MoOx and EIL Cs2CO3, there is nearly no barriers for both holes and electrons injection, meanwhile, without the mismatch of FMO energy levels within general multilayer devices, our simply structure monochromatic PhOLEDs achieved ultralow driving voltage (>1000 cd/m2 at 3 V) and high efficiency (EQE > 13%). We also have demonstrated efficient white PhOLEDs based on complementary blue and yellow emitters with the simply D-EML structure. The electrical and optical characteristics can be easily manipulated by the interlayer inserted between these two EMLs. The effective charge carriers manipulation and triplet excitons confinement make balanced emission from the blue and yellow EML come true. Here, we demonstrated white PhOLEDs with ultralow driving voltage (1316 cd/m2 at 3 V), high efficiency (39.8 lm/W and 13.7% at 1316 cd/m2) and low efficiency roll-off (13.5% calculated from the maximum EQE to that of 5000 cd/m2). The low driving voltages, high efficiencies and improved efficiency stability pave the way for the practical applications of these devices in portable displays and lighting.
This work was supported by the National Basic Research Program and Development Program of China (973 Program) under Grant No.2010CB327701, and the National Science Foundation of China (Grant No. 61275033).
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