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By doping blue, green and red dyes into a bipolar host system, a simple single-EML white organic light-emitting diode (WOLED) with efficiency and color-stability trade-off was achieved by solution process. The resulting WOLED shows high efficiency (i.e., 36.5 cd/A and 15.7% at 1141 cd/m2), reduced efficiency roll-off (i.e., critical current density jc is as high as 140 mA/cm2) and, especially, extremely stable electroluminescence spectra with a slight CIE coordinate variation of (0.404 ± 0.004, 0.436 ± 0.001). The superior performance of the WOLED is attributed to the effective suppression of exciton quenching and charge trapping in the bipolar EML.
©2013 Optical Society of America
1. Introduction
The rapid development of organic light-emitting diodes (OLEDs) in recent years makes them increasingly attractive and competitive in flat-panel display and solid state lighting applications [1–3]. Although the vacuum-deposition processes are far ahead of the solution processes from the commercialization point of view, the solution processes for the fabrication of OLEDs are still fascinating due to their potential advantages for the production of large area devices at low cost [4, 5]. Phosphorescent emitters based on small molecules have always been considered crucial for high efficiency because of their ability of fully harvesting singlet and triplet excitons in an OLED [6–8]. Therefore, an effective strategy to achieve both low-cost and high-performance is the fabrication of phosphorescent OLEDs by solution processing small molecules [9–19].
Moreover, white OLEDs (WOLEDs) based on phosphorescent emitters have been hitherto regarded as the potential candidates for future lighting and display applications [20–23]. By definition, white emission requires the mixture of complementary (i.e., blue and yellow or orange) or primary colors (red, green, and blue). Researchers usually detour the complicated co-evaporation of multiple components to fabricate multiple emitting layer (multiple-EML) WOLEDs [22–24], which made the complex structures and increasing cost. Naturally, a single emitting layer (single-EML) with precise ratios of multiple components is easier to achieve by solution process, which overcomes the difficulties of co-evaporation and accurately doping performed by vacuum-deposition process. Recently, numerous works about the single-EML WOLEDs by solution-processing small molecules have been reported [13–15], but the comprehensive performances of the devices need to be further improved. Hou et al. reported a series of single-EML WOLEDs based on the mixed-host of TBCPF and OXD-7 [13], but their power efficiencies were not more than 15.6 lm/W. Zhang et al. reported a solution-processed single-EML WOLED with high efficiency based on a dendritic host [14], but its color rendering index (CRI) was still too low (less than 65). We recently reported a single-EML WOLED with low driving voltage and high efficiency based on a unipolar mixed-host system [15]. The WOLED exhibits a maximum efficiencies of 37.1 cd/A and 32.1 lm/W, and even at 1000 cd/m2, the efficiencies still reach 34.2 cd/A and 23.3 lm/W and the driving voltage is only 4.6 V. But its color-stability, CRI and efficiency roll-off should be further improved.
In this paper, we incorporated three phosphorescent dyes of iridium(III) [bis(4,6-difuorophenyl)-pyridinato-N,C2′] picolinate (FIrpic), iridium(III) bis(2-phenylpyridinato-N,C2′) (acetylacetonate) Ir(ppy)2(acac) and iridium(III) bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) Ir(MDQ)2(acac) into a bipolar mixed-host system of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (DCzPPy) and 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) to achieve a simple single-EML WOLED by solution process. The resulting WOLED shows high efficiency and reduced efficiency roll-off. Especially, extremely stable spectra are obtained in the multi-doped single-EML WOLED. We have attributed the high efficiency and reduced efficiency roll-off to the effective suppression of triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) by extending the exciton recombination region in the bipolar single-EML [24, 25]. And the good color stability was due to the suppression of charge trapping on the phosphorescent emitters in the multi-doped single-EML.
2. Experimental procedure
As shown in Fig. 1, the device structure used in this study is indium tin oxide (ITO)/ poly (styrene sulfonic acid)-doped poly (3, 4-ethylenedioxythiophene) (PEDOT: PSS) (40 nm)/emissive layer (EML) (40 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) (50 nm)/LiF (1 nm)/Al. PEDOT: PSS was spin-coated onto the pre-cleaned ITO substrates with a sheet resistance of 10 Ω per square, and then baked at 120 °C in a vacuum oven for 30 min to extract residual water. Afterwards, the samples were moved into a glove box under a nitrogen-protected environment (oxygen and water contents less than 1 ppm), and the emissive layers (EMLs) were spin-coated on top of PEDOT: PSS (AI4083 (H.C.Stack)) from chlorobenzene and annealed at 120 °C in a vacuum oven for 30 min to remove residual solvent. Then, the samples were transferred into a thermal evaporator (pressure less than 5 × 10−4 Pa) which is connected to the glove box and thus avoid the exposure of the samples to the atmosphere. TmPyPB, LiF and Al were then deposited sequentially by thermal evaporation. The current-brightness-voltage characteristics were measured by a Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode. The EL spectra were measured by SpectraScan PR650 spectrophotometer. The film thickness was determined by Dektak 6M Profiler (Veeco Metrology Inc.). All these measurements were done in ambient atmosphere at room temperature.
3. Result and discussion
Due to the high triplet energy level (T1 = 2.71 eV), DCzPPy can be used as the bipolar host material for the blue phosphor FIrpic. But its highest occupied molecular orbital (HOMO) level (−6.05 eV) is extremely much lower compared to PEDOT:PSS, which makes a rather large barrier of 0.95 eV for hole-injection. To overcome this issue, another host material TAPC with moderate HOMO level was introduced into the EML. As shown in Fig. 1(a), the energy barrier between PEDOT: PSS and TAPC is only 0.2 eV, and the holes prefer to inject into TAPC instead of DCzPPy. But on the other hand, the hole-transporting TAPC is a unipolar material, and its percentage in the DCzPPy-based EMLs will influence the device performance. To optimize the ratio of the two hosts, a series of blue devices with the structure of ITO/PEDOT:PSS/DCzPPy:TAPC: FIrpic/TmPyPB/LiF/Al were studied first, where the weight ratios of TAPC in the EML were 0%, 10%, 20%, 30% and 40% and the concentration of FIrpic was fixed at 10%. Table 1 summarizes the performance of the blue OLEDs. As shown (Table 1), the turn-on voltage (Von) (defined as the voltage at 1 cd/m2) is 4.6 V for the device without TAPC, which decreases to 3.6 V when 20, 30 and 40% of TAPC were added in the EMLs. The best performance was achieved in the device with the TAPC concentration of 20%. The maximum forward viewing current and power efficiencies of the optimized device reached 33.7 cd/A and 22.8 lm/W, respectively.
Table 1. Comparison of device characteristics of the solution-processed blue OLEDs with different concentrations of TAPC.
Encouraged by the above results, a single-EML WOLED was fabricated by incorporating blue, green and red phosphors into the DCzPPy:TAPC with 20 wt.% TAPC. The device performance of the solution-processed single-EML WOLED with 10% FIrpic, 0.8% Ir(ppy)2(acac) and 0.5% Ir(MDQ)2(acac) in the EML is illustrated in Fig. 2. As depicted in Fig. 2(b), the maximum forward viewing current and power efficiency reach 36.5 cd/A and 23.4 lm/W at a luminance of 1140 cd/m2 and 292 cd/m2, respectively. As given in Fig. 2(c), the external quantum efficiency (EQE) of the WOLED reaches a maximum value of 15.7% at 3.1 mA/cm2. Notably, the device exhibits reduced efficiency roll-off and the critical current density jc (defined as the current density where the EQE drops to half of its initial value) is as high as 140 mA/cm2, which is much higher than the counterparts reported previously [31, 32]. Most importantly, the white emission is extremely stable with respect to the driving voltage. As shown in Fig. 2(d), the EL spectra exhibit three main peaks of 472, 504 and 600 nm, coming from the emission of FIrpic, Ir(ppy)2(acac) and Ir(MDQ)2(acac), respectively. The change in Commission Internationale de l’Eclairage (CIE) coordinates is less than (0.007, 0.001) as the brightness increases from 100 cd/m2 to 10000 cd/m2. Additionally, the color rendition index (CRI) is as high as 80 at the brightness of 1000 cd/m2, meeting the demand of solid state lighting. The resulting WOLED with good efficiency/efficiency roll-off/CRI/color-stability trade-off is superior to the solution-processed small molecular WOLEDs reported before [33, 34], and even comparable to the WOLEDs fabricated by vacuum-deposition process [22].
Fig. 2 Device characteristics in the solution-processed single-EML WOLED: a) current density-brightness-voltage, b) power efficiency-current efficiency-brightness, c) external quantum efficiency-current density and d) electroluminescence (EL) spectra.
The low efficiency roll-off and stable spectra in the WOLED should originate from the bipolar characteristics of the used host in the EML. To illustrate the role of the bipolar host in the performance, a similar WOLED with the unipolar host of TAPC: TCTA was fabricated for comparison [15]. The EQE and spectra of the WOLED with the unipolar host are given in Fig. 3. It possesses relatively higher efficiency roll-off compared to the WOLED with bipolar host, and its jc is as low as 53.7 mA/cm2. And obvious variation of spectra was found in the range of brightness from 100 to 10000 cd/m2, the corresponding CIE coordinate changes from (0.382, 0.411) to (0.315, 0.399) and CRI decreases from 75 to 70. This clearly demonstrated that the bipolar host should be preferred than the unipolar one to accomplish the trade-off of efficiency roll-off and color-stability in the single-emitting layer WOLEDs.
Fig. 3 Device characteristics in the solution-processed single-EML WOLED based on unipolar host: a) external quantum efficiency-current density and b) EL spectra.
It was demonstrated that the EQE roll-off at high brightness in OLEDs was mainly attributed to triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) [25, 35]. Since TTA strongly depends on the triplet exciton density, the thickness of the exciton recombination zone (RZ) within the EML intensely influences the TTA and, consequently, the EQE roll-off. Owing to the good abilities of transporting both holes and electrons, the bipolar host should exhibit more advantages than the unipolar host to suppress the TTA. As shown in Fig. 4(a), the bipolar host can distribute the exciton RZ more uniformly within the EML instead of a narrow layer at the interface between the EML and the electron-transporting layer (ETL) in the unipolar host device, avoiding the partially high triplet excition density. Due to the high electron mobility of DCzPPy [36, 37], electrons can transport more deeply into the EML where they combine with holes, and then the triplet excitons prefer to radiate instead of quenching each other. Additionally, TPQ originates from the carrier accumulation at the interlayer between EML and transport layer, thus the differences of the energy level and the carrier mobility between the EML and adjacent charge transporting layer affect the TPQ as well as the EQE roll-off. Due to the good match of the lowest unoccupied molecular orbital (LUMO) levels between DCzPPy and TmPyPB (Fig. 1(a)), electrons can easily transport into the EML to combine with holes, thus the accumulation of charge carriers at the interface between EML and ETL is significantly reduced, which helpfully restraining TPQ. Since the TTA and TPQ are successfully suppressed by the electronic characteristics of the used DCzPPy, the WOLED with the bipolar host exhibits reduced efficiency roll-off.
Fig. 4 a) Diagrams of EL processes in unipolar host and bipolar host, b) J-V characteristics of device with different Ir(ppy)2(acac) concentrations, c) J-V characteristics of device with different Ir(MDQ)2(acac) concentrations. B, G and R are corresponding to blue dopant FIrpic, green dopant Ir(ppy)2(acac) and red dopant Ir(MDQ)2(acac), respectively.
Otherwise, the unstable spectral behavior in doped-type OLEDs is generally considered to be caused by the direct electron and hole trapping on the dopants, which seems to be seriously affected by the driving voltage [24]. In doped-type OLEDs, host-guest energy transfer and direct charge trapping are two main emission mechanisms. Usually, the two emission processes are competitive in WOLEDs, which also influence the color-stability. Besides the energy transfer processes, the emission of Ir(ppy)2(acac) and Ir(MDQ)2(acac) partially come from the direct exciton formation by trapped electrons and holes on the emitters, which should lead to the significant decrease of current-density with the increase of the doping concentration [20, 21]. However, the charge trapping by Ir(ppy)2(acac) and Ir(MDQ)2(acac) can be successfully suppressed in the bipolar EML due to the broaden recombination zone, where the charge carriers transporting on the host sites can mostly recombine to form excitons and only a few of them can be trapped by the phosphorescent dopants. The suppression of charge trapping is well confirmed by the current density-voltage (J-V) characteristics as shown in Fig. 4(b) and 4(c). The J-V characteristics of the devices with different Ir(ppy)2(acac) and Ir(MDQ)2(acac) concentrations are almost independent on the doping, suggesting that the emission processes of Ir(ppy)2(acac) and Ir(MDQ)2(acac) must be dominated by the energy transfer as depicted in Fig. 4(a), resulting in stable spectrum property.
4. Conclusion
In summary, we have demonstrated a simple three primary-color WOLED based on a multi-doped bipolar single-EML fabricated by solution process. The single-EML WOLED possesses high efficiency, reduced efficiency roll-off and good color-stability. The maximum forward viewing current efficiency and external quantum efficiency are 36.5 cd/A and 15.7% at a luminance of 1140 cd/m2, respectively. The solution-processed single-EML WOLED with superior efficiency/CRI/color-stability trade-off can be comparable to the multilayered devices fabricated by vacuum-evaporation. Importantly, the solution-process approach is more convenient and accurate than the vacuum-evaporation to fabricate the multi-component EMLs by weighting. This should be helpful for the fabrication of low-cost and high-efficiency WOLEDs for lighting applications.
Acknowledgements
This work was supported by the National Basic Research Program of China (973 program No. 2009CB623604 and 2013CB834805), the National Natural Science Foundation of China (21161160442, 61036007), the Science Fund for Creative Research Groups of NSFC (20921061) and the Foundation of Jilin Research Council (2012ZDGG001, 201105028).
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