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Abstract
Phosphorescent white organic light-emitting diodes (WOLEDs) with high efficiency and color rendering index (CRI) are demonstrated based on a macrospirocyclic oligomer matrix (TPA-PO)3, which exhibits good solution processability, stable film morphology, and excellent compatibility with phosphorescent emitters. High-efficiency blue, green, orange, and red OLEDs were demonstrated with maximum current efficiencies of 21.7, 21.9, 13.1, and 6.4 cd A−1, maximum brightness of 22475, 67757, 21232, and 8046 cd m−2, respectively. Furthermore, by simultaneously incorporating blue, green, orange, and red emitters into a single emitting layer, a quaternary doping WOLED with a maximum current efficiency of 18.9 cd A−1 (at 280 cd m−2) was realized, featuring a “warm-white” light emission with correlated color temperatures ranging from 2841 to 3514 K. Importantly, color rendering index values over 90 were obtained at practical brightness of 100 ~1000 cd m−2. Even at a high brightness of ~5000 cd m−2, the CRI was still above 88, which is among the highest CRI values achieved for solution-processed phosphorescent WOLEDs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
White organic light emitting diodes (WOLEDs) have attracted intensive research attention due to their promising applications in liquid crystal displays, solid-state lighting sources, and full-color flat-panel displays [1–4]. Currently, much effort are being actively focused on phosphorescent WOLEDs, which can theoretically achieve 100% internal quantum efficiency originating from the harvest of both singlet and triplet excitons of the phosphorescent emitters [5,6]. To realize white light emission, multiple emitters with different emission wavelengths are typically required for phosphorescent WOLEDs [7,8]. The phosphorescent emitters can be incorporated into different layers to fabricate multiple-emitting layer (multi-EML) WOLEDs by vacuum deposition, which can easily realize high efficiency [9]. As an alternative, they can be co-doped into one single emitting layer (S-EML) [6]. Naturally, the S-EML configuration means a simpler device structure and enables the low-cost fabrication by solution processes, which can conveniently manipulate the weight ratios of the emitting materials [10,11]. Thus, intensive research efforts have been devoted to the development of high-efficiency S-EML WOLEDs by solution processes [12–17].
In addition to device efficiency [18], color rendering index (CRI) is also highly concerned about WOLEDs [19]. CRI is a numerical measure of how true the colors of objects appear when illuminated by the lighting source, ranging from 0 to 100 [20]. Generally, a CRI (≥ 80) is required for in-door lighting application. Much effort has been devoted to obtaining high CRI values for solution-processed phosphorescent WOLEDs based on polymers and/or small molecules host materials. For example, Wong et al. [21] fabricated a ternary doping WOLED by solution process using FIrpic, Ir-3Tz1F, and Ir-G1 as the blue, green, and red emitters and PVK:OXD-7 as mixed host, which exhibited a maximum current efficiency of 33.4 cd A−1 and a CRI up to 80. Ye et al. [22] obtained a CRI of 82 at 3410 cd m−2 in a four-color WOLED by co-doping FIrpic, Ir(ppy)2(acac), Ir(bt)2(acac), and Ir(MPCPPZ)3 as blue, green, orange, and red emitters respectively into PVK:OXD-7 mixed host as a single EML, and the maximum current efficiency was 24.1 cd A−1. Lin et al. [23] reported a quaternary doping S-EML WOLEDs using FIrpic, Ir(mppy)3, PO-01-TB, and Os(btfp)2(pp2b) as the dopants and 26DCzPPy as the host material. The device achieved a high CRI up to 85, with a maximum current efficiency of 24.9 cd A−1. Su et al. [24] developed an S-EML WOLED based on FIrpic, PO-01, and a red-NIR emitter Pt-1 with a PVK hole-transporting layer. The device realized a high CRI up to 87 at 1000 cd m−2, but the current efficiency reduced to 10.9 cd A−1. From these reports, it can be seen that the fabrication of solution-processed S-EML WOLEDs with both high efficiency and high CRI is still a challenge, especially for areas including studio, museum, medical application, and art gallery, where a higher CRI (≥ 90) is crucial [25].
This study aims to fabricate solution-processed S-EML WOLEDs with high efficiency and CRI, using a monodisperse oligomer as the matrix material. Monodisperse oligomers are imparted with excellent characteristics including well-defined molecular structures, definite molecular weights, ease of purification and characterization (like small molecules), and excellent film forming ability from solution-processing (like polymers), which have been successfully applied as host materials in solution-processed monochromatic organic light emitting diodes [26–32]. Furthermore, Wang et al. [33] demonstrated high-efficiency phosphorescent WOLEDs based on a monodisperse carbazole oligomer, but the CRI were not reported. Herein, a macrospirocyclic oligomer based on triphenylamine and phosphine oxide was chosen as the matrix material for the fabrication of highly efficient solution-processed phosphorescent WOLEDs. Blue, green, orange, and red monochromatic phosphorescent OLEDs were first demonstrated. Then, based on the high performance of the monochromatic devices, a quaternary doping phosphorescent WOLED was successfully demonstrated with high efficiency and high CRI values, which rationalizes the potential application of the monodisperse oligomer as matrix material in solution-processed phosphorescent WOLEDs.
2. Experimental details
2.1. Materials
The material (TPA-PO)3 was prepared according to our previous study [32]. Other materials and solvents such as FIrpic (bis((3,5-difluorophenyl)-pyridine) iridium picolinate), Ir(ppy)2(acac) (iridium(III) bis(2-phenylpyridinato-C2,N)acetylacetonate), Ir(piq)2(acac) (bis(1-phenyliso-quinoline)(acetylacetonate) iridium (III)), Ir(bt)2(acac) (bis(2-phenyl-benzothiazole-C2,N)(acetylacetonate)iridium(III)), PVK (poly(N-vinylcarbazole)), 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI), 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPb), 4,4’,4”-tri(9-carbazoyl) triphenyl-amine (TCTA), 1,3-bis(5-(4-(tert-butyl) phenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7), di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexan (TAPC), PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate) and chlorobenzene were purchased from commercial sources and used as received, without any purification procedures.
2.2. Characterization and OLEDs fabrication
UV-vis absorption spectra of the samples were obtained on a Lambda 35 spectrophotometer (PerkinElmer, United States of America) at room temperature. Fluorescence spectra were measured at room temperature with an LS-55 fluorescence spectrometer (PerkinElmer, United States of America). Atomic force microscopy (AFM) measurements were carried out on the Nano Surfaces (Bruker Daltonics, Germany).
The general procedure for fabricating the OLEDs are described as follows. The indium tin oxide (ITO) coated glass substrates (with a sheet resistance of 15 Ω/square) were patterned based on a conventional wet-etching process, in which a mixture of HCl (6 N) and HNO3 (0.6 N) was used as the etchant. After the patterning process, the substrates were then rinsed with deionized water and cleaned, followed by ultrasonicating in acetone and ethanol in sequence. Immediately before fabricating the devices, the ITO substrates were treated in a UV-ozone oven for 20 min. PEDOT:PSS was spin-coated on the ITO substrates as the hole injection layer, and the coated substrates were dried in an oven at 120 °C for 30 min to remove the solvent. After the drying process, the PEDOT:PSS-coated substrates were then transferred to a glove box with a nitrogen atmosphere, where other functional layers were deposited in sequence. The materials constituting the emitting layer were dissolved in chlorobenzene and then spin-coated onto the PEDOT:PSS coated substrates, which were then placed on a hot plate and annealed at 120 °C for 30 min. Then, 1,3,5-tris(N-phenylbenz-imidazol-2-yl)-benzene (TPBI, 35 nm) was deposited by vacuum deposition method as the electron transporting layer. Finally, Ca and Ag were deposited as electron injection layer and cathode sequentially at a pressure under 8 × 10−5 Pa, respectively. The thickness values of the films were measured by a Dektak surface profilometer (Bruker Daltanics). The OLEDs were tested under nitrogen protection. The EL spectra, device brightness, and the current-voltage characteristics of the devices were recorded using a combination of a Photo Research PR-655 SpectraScan and a Keithley 2636A system SourceMeter.
3. Results and discussions
The molecular structures and energy level of the materials used are shown in Fig. 1. The macrospirocyclic oligomer (TPA-PO)3 was previously reported by our group [34]. (TPA-PO)3 molecule is composed of triphenylamine (TPA), fluorene, and diphenylphosphine oxide (PO) units. In the molecule, three PO-attached triphenylamine units are connected in series via three sp3-hybridized carbon atoms as the ring, and three fluorene units as the pendants, which endows (TPA-PO)3 with a highly steric three-dimensional structure. (TPA-PO)3 possesses a high glass transition temperature up to 247 °C, which promises excellent amorphous film forming ability. In addition, the compound exhibits a high triplet energy level of 2.81 eV, which is sufficient for application as host material for blue and white phosphorescent OLEDs.
3.1 Charge transport and photophysical properties
In our previous study, it was found that (TPA-PO)3 exhibited relatively stronger electron-transporting ability with regard to hole transport. The single carrier transport properties of (TPA-PO)3:TCTA mixed films were investigated by their hole-only and electron-only devices. The hole-only device structure was ITO/PEDOT:PSS (25 nm)/mixed host (60 nm)/TAPC (35 nm)/Ag (100 nm), and the electron-only device structure was ITO/Ca (15 nm)/mixed host (60 nm)/TmPyPb (35 nm)/Ca (10 nm)/Ag (100 nm). In the hole-only device, TAPC was used to prevent the injection of electrons. In the hole-only device, TAPC was used to prevent electron injection from the cathode. In the electron-only device, Ca at the anode was used to prevent hole-injection, and TmPyPb was used to facilitate electron transport. The weight ratios of (TPA-PO)3:TCTA were set as 3:7, 5:5, and 7:3. The current density-voltage characteristics of the devices are shown in Fig. 2. For comparison, the single carrier devices data for (TPA-PO)3 are also presented in Fig. 2. For the hole-only devices, the increase in the content of TCTA results in the increase of current density, which clear demonstrates that TCTA facilitates hole transport of the host material. For the electron-only devices, the current density increases as the (TPA-PO)3 increases, which reveals the electron-transporting property of (TPA-PO)3. Among the single-carrier devices, devices with (TPA-PO)3:TCTA ratio of 5:5 show relatively balanced hole- and electron densities.
Fig. 2 Current density-voltage characteristics of (a) hole-only and (b) electron-only devices based on (TPA-PO)3:TCTA mixed films.
The photophysical properties of the mixed-host films and the corresponding phosphorescent emitter doped films were investigated by their UV-vis absorption and photoluminescence (PL) spectra, as presented in Fig. 3. (TPA-PO)3:TCTA mixed films with weight ratios of 3:7, 5:5, 7:3 (in accord with those of the single carrier devices) were prepared by spin-coating. For comparison, the corresponding UV-vis and PL spectra of (TPA-PO)3 and TCTA are also shown in Fig. 3. It is observed that the absorption spectra of these (TPA-PO)3:TCTA mixed films show no new absorption peaks other than those attributed to (TPA-PO)3 and TCTA, indicating that no new ground states form in the mixed films. In addition, the PL spectra of the mixed films also reveal no new emission peaks at long wavelengths that are attributable to exciplex, indicating the absence of exciplex formation between (TPA-PO)3 and TCTA. Typically, the formation of exciplex in mixed films will result in a PL peak that is largely red-shifted compared with the emissions of the pristine materials [34,35]. It is noted that the emission peak attributable to (TPA-PO)3 gradually redshifts from 394 nm to 408 nm as (TPA-PO)3 in the mixed films increases from 30 wt% (3:7) to 70 wt% (7:3). Such a prominent redshift has also been reported for other organic mixed films [29,36], which can be attributed to the solid-state solvation effect [37].
Fig. 3 (a) UV-vis absorption and (b) PL spectra of (TPA-PO)3:TCTA mixed films with different ratios; (c) comparison of UV-vis absorption spectra of FIrpic, Ir(ppy)2(acac), Ir(bt)2(acac), Ir(piq)2(acac) emitters and PL spectra of (TPA-PO)3 and TCTA films; (d) PL spectra of solution-processed (TPA-PO)3:TCTA mixed film doped with FIrpic, Ir(ppy)2(acac), Ir(bt)2(acac), and Ir(piq)2(acac) emitters. Note: PO represents (TPA-PO)3 in the legend of the figures.
The PL spectra of (TPA-PO)3 and TCTA exhibit a prominent overlap with the absorption spectra of the blue (FIrpic), green (Ir(ppy)2(acac)), orange (Ir(bt)2(acac)), and red (Ir(piq)2(acac)) phosphorescent emitters as shown in Fig. 3(c), which is expected to facilitates Förster energy transfer from the host to the emitter. The PL spectra of the (TPA-PO)3:TCTA mixed films doped with the phosphorescent emitters exhibit the emission from the emitters exclusively, as shown in Fig. 3(d), signifying effective energy transfer from the host to the emitter. Based on the above charge transport and photophysical properties of mixed films, (TPA-PO)3:TCTA with a weight ratio of 5:5 was selected as the host for fabricating the organic light emitting diodes.
3.2 Morphological properties
Atomic force microscopy (AFM) is used to evaluate the film-forming ability, morphological properties of the mixed films as well as their compatibility with phosphorescent dopants. The AFM images of these films are presented in Fig. 4. As can be observed, the mixed films all exhibit a low RMS below 0.5 nm, demonstrating their smooth surface morphologies and uniform film properties. By comparing the RMS values before and after doping of the films, it is found that doping of emitters into the mixed host of (TPA-PO)3:TCTA does not compromise the film morphology. For example, the RMS of (TPA-PO)3:TCTA mixed host film (before annealing) is 0.43 nm, and that of (TPA-PO)3:TCTA:FIrpic:Ir(ppy)2(acac):Ir(bt)2(acac) doped films is 0.44 nm. In addition, no aggregation is observed in the films doped by phosphorescent emitters. The smooth surface and uniform film nature of the films are favored for fabricating highly efficient devices.
Fig. 4 AFM images of the solution-processed films: (a)/(a’), (b)/(b’), (c)/(c’), and (d)/(d’) correspond to PO:TCTA, PO:TCTA:15 wt% FIrpic, PO:TCTA:15 wt% FIrpic:0.3 wt% Ir(ppy)2(acac):0.5 wt% Ir(bt)2(acac), and PO:TCTA:15 wt% FIrpic:0.3 wt% Ir(ppy)2(acac):0.5 wt% Ir(bt)2(acac):0.5 wt% Ir(piq)2(acac) mixed films before/after annealing, respectively. Note: the weight ratio of PO:TCTA is 5:5; the annealing temperature and time are 120 °C, 1 h.
As OLEDs will inevitably produce heat during operation, crystallization and phase separation may occur in the EML, which is detrimental to the device performance. Thus, high morphological stability of the EML are required. To evaluate the morphological stability of the solution-processed films, the AFM images of the films after annealing at 120 °C for 1 h were also measured and presented in Fig. 4. The results indicate that the films, including the mixed-host films and doped films, all exhibit good morphological stability after thermal annealing, and only a slight change in RMS values is observed. For example, the doped film of (TPA-PO)3:TCTA:FIrpic:Ir(ppy)2(acac):Ir(bt)2(acac):Ir(piq)2(acac) shows an RMS value of 0.43 nm (Fig. 4 (d)) before annealing. After annealing, the RMS value is 0.47 nm (Fig. 4(d’)), and no crystallization or phase separation is observed. These results indicate that the film morphology is not compromised by annealing and the mixed host shows excellent compatibility with the phosphorescent emitters, which is desired for the high performance and long lifetime of the phosphorescent OLEDs. The stable morphologies of the mixed films should benefit from the high glass transition temperatures of both (TPA-PO)3 and TCTA (up to 247 and 151 °C, respectively). In addition, (TPA-PO)3 with a bulky molecular structure also serves as effective spacers to separate the emitter molecules and suppress crystallization and phase separation, thus resulting in the stable film morphology and uniform film property.
3.3 Monochromatic phosphorescent OLEDs
Before the fabrication of WOLEDs, monochromatic devices (blue, green, orange, and red) were prepared and investigated to evaluate the utility of the mixed host. The universal device structure was ITO/PEDOT:PSS (25 nm)/(TPA-PO)3:TCTA: x wt% emitters (35 nm)/TPBi (35 nm) /Ca (10 nm)/Ag (100 nm), where x wt% is the concentration of the blue, green, orange, and red emitter, respectively. The weight ratio of (TPA-PO)3:TCTA is set as 5:5. Emitter concentrations have an impact on device performance, e.g. efficiency and electroluminescence spectra, and different emitters have different optimized concentrations. The current density-voltage-brightness (J-V-L) and current efficiency-brightness (CE-L) characteristics of the devices with optimized emitter concentrations are presented in Fig. 5, and the main device parameters of some typical devices are summarized in Table 1.
Fig. 5 Performance of the blue, green, orange, and red phosphorescent OLEDs: (a) current density-voltage-brightness and (b) current efficiency-brightness characteristics. Inset: EL spectra of representative devices at varying driving voltages, with main peak at 472, 524, 564, and 632 nm for blue, green, orange, and red phosphorescent OLEDs, respectively.
Table 1. Performance of the Solution-Processed Blue, Green, Orange, and Red OLEDs.
Blue OLEDs with 15 wt% to 25 wt% FIrpic as dopant designated as B1 to B3 were fabricated. As a result, the devices all exhibited current efficiencies over 20 cd A−1. Among them, device B2 showed a turn-on voltage (Von), maximum current efficiency (CEmax), maximum power efficiency (PEmax), maximum brightness (Lmax) of 4.8 V, 21.7 cd A−1 (at 512 cd m−2), 22475 cd m−2, respectively. As for green OLEDs, the concentration of Ir(ppy)2(acac) green emitter was set as 14 wt%, 16 wt%, and 18 wt%, corresponding to device G1-G3, respectively. Among them, device G2 with 16 wt% Ir(ppy)2(acac) showed the optimal device performance, with Von, CEmax, PEmax, and Lmax of 3.7 V, 21.9 cd A−1, 15.0 lm W−1, and 67757 cd m−2, respectively. For the orange and red OLEDs, CEmax and Lmax are 13.1 and 6.4 cd A−1, 21232 and 8046 cd m−2, respectively. The representative electroluminescence (EL) spectra of the monochromatic OLEDs are presented in the inset in Fig. 5. On varying driving voltages, the emission maxima and spectral profiles are constant, signifying very stable electroluminescence.
3.4 Phosphorescent WOLEDs
For WOLEDs, devices based on binary (blue/orange), ternary (blue/green/orange) phosphorescent emitters were first fabricated and investigated. The basic device structure was ITO/PEDOT:PSS (25 nm)/(TPA-PO)3:TCTA: emitters (35 nm)/TPBi (35 nm) /Ca (10 nm)/Ag (100 nm). The J-V-L and CE-L characteristics, as well as the EL spectra of the two devices are presented in Fig. 6, and the performance are summarized in Table 2. WB and WT exhibit comparable performance, as indicated by their similar J-V-L and CE-L characteristics. The Von, CEmax and Lmax for the binary (WB)/ternary (WT) doping WOLEDs are 3.8/3.7 V, 22.1/22.5 cd A−1, and 34013/43807 cd m−2, respectively. However, the maximum CRI of device WB and WT are only 61 and 65, respectively.
Fig. 6 Performance of the binary and ternary doping WOLEDs: (a) current density-voltage-brightness and (b) current efficiency-brightness characteristics; EL spectra of (c) binary and (d) ternary doping WOLEDs. Device structure: ITO/PEDOT:PSS (25 nm)/(TPA-PO)3:TCTA:15 wt% FIrpic:0.3wt% Ir(ppy)2(acac): x Ir(bt)2(acac) (35 nm)/TPBi (35 nm) /Ca (10 nm)/Ag (100 nm). Note: x = 0 for device WB, and x = 0.5 wt% for device WT.
Table 2. Performance of the Solution-Processed Phosphorescent WOLEDs.
To realize high CRI values (i.e. CRI ≥ 80) for lighting applications [25], quaternary doping WOLEDs simultaneously integrating blue/green/orange/red emitters into the single-layer EML were attempted. The devices adopted the same basic configuration as those of the binary and ternary doping WOLEDs as described above. The weight ratios of blue:green:orange emitters are set as FIrpic:Ir(ppy)2(acac):Ir(bt)2(acac) = 15 wt%:0.3 wt%:0.5 wt%, and adjusting the weight ratio of the red emitter Ir(piq)2(acac) leads to EL spectra with different profiles, CIEx,y coordinates, CCT, and CRI values. The concentration of Ir(piq)2(acac) was set as 0.3 wt%, 0.4 wt%, 0.5 wt% and 0.6 wt%, corresponding to device WQ-1, WQ-2, WQ-3, and WQ-4, respectively. The performance of the devices are summarized in Table 2. Among the four devices, WQ-3 with 0.5 wt% Ir(piq)2(acac) exhibits the best performance. The J-V-L and CE-L-PE characteristics, EL spectra as well as the emission photograph of device WQ-3 are shown in Fig. 7.
Fig. 7 Performance of WOLED device WQ-3: (a) current density-voltage-brightness, (b) current efficiency-brightness-power efficiency characteristics, (c) EL spectra, and (d) CIE 1931 chromaticity diagram as well as the emission photograph (at 7 V, ~1000 cd m-2). Device structure: ITO/PEDOT:PSS (25 nm)/(TPA-PO)3:TCTA:15 wt% FIrpic:0.3wt% Ir(ppy)2(acac):0.5 wt% Ir(bt)2(acac): 0.5 wt% Ir(piq)2(acac) (35 nm)/TPBi (35 nm) /Ca (10 nm)/Ag (100 nm). Inset: photograph of the emitting device WQ-3.
Device WQ-3 exhibits a relatively low turn-on voltage of 4.1 V, and the CEmax, PEmax, and Lmax are 18.9 cd A−1, 9.6 lm W−1, and 17063 cd m−2, respectively. It is noted that the CEmax is achieved at a practical brightness of 280 cd m−2, and the device shows a very small efficiency roll-off at brightness from 100 cd m−2 to 1000 cd m−2. Indeed, the current efficiency at 1000 cd m−2 remains 17.3 cd A−1, signifying a roll-off of only 8.5%. As can be observed from Fig. 7(c), device WQ-3 exhibits four emission peaks at 476, 508, 556, and 620 nm, corresponding to FIrpic, Ir(ppy)2(acac), Ir(bt)2(acac), and Ir(piq)2(acac), respectively. The CCT ranges from 2800 K to 3600 K at operating voltages from 6 V to 10 V, featuring a physiologically friendly “warm-white” light emission (CCT below 5000 K). The CCT and CIEx,y coordinates at a driving voltage of 6 V (at ~150 cd m−2) are (0.46, 0.44) and 2841 K, respectively. It should be noted that the CCT and CIEx,y coordinates achieved for device WQ-3 are close to those of the tungsten incandescent lamps (CCT = 2856 K, CIEx,y (0.448, 0.408)), which represent the ideal warm-white light sources for human eyes. Importantly, the CRI values are above 90 at the practical brightness of 100 – 1000 cd m−2 for device WQ-3, which are among the highest CRI values for solution-processed S-EML WOLEDs [10,21–24,38,39]. Even at the high brightness of ~5000 cd m−2 (~10 V), the CRI is still above 88. The obtained high CRI and low CCT for the WOLED are expected to be physiologically friendly [24], which demonstrates its potential application in solid-state lighting. These results show that solution-processed phosphorescent WOLEDs with high efficiency and high CRI values have been realized, demonstrating (TPA-PO)3 can serve as an efficient matrix material for WOLEDs.
4. Conclusions
This study is to explore the utilization of a monodisperse oligomer (TPA-PO)3 with a macrospirocyclic structure as the matrix host for the application of phosphorescent WOLEDs with high efficiency and CRI. The (TPA-PO)3 matrix exhibits excellent form-forming ability/morphological stability, and excellent compatibility with phosphorescent dopants. Blue, green, orange, and red monochromatic phosphorescent OLEDs were first demonstrated, achieving a high current efficiency of 21.7, 21.9, 13.1, and 6.4 cd A−1, respectively. Furthermore, single-emitting-layer WOLEDs incorporating two, three, and four phosphorescent emitters were fabricated. Among them, the quaternary doping WOLEDs showed a turn-on voltage of 4.1 V and a maximum current efficiency of 18.9 cd A−1 (at 280 cd m−2), with only 8.5% efficiency roll-off at the brightness of 1000 cd m−2. The device featured a “warm-white” light emission with CCT below 3600 K. Importantly, the WOLED achieved excellent CRI values over 90 at practical brightness of 100 – 1000 cd m−2 (with a maximum of 91 at 150 cd m−2), which are among the highest CRI values for solution-processed WOLEDs. This study rationalizes the promising application of the monodisperse oligomer as host in solution-processed phosphorescent WOLEDs with high efficiency and high CRI.
Funding
National Natural Science Foundation of China (21674046, 61106017, 61136003); Natural Science Foundation of Jiangsu Province (BM2012010, BK20131375); the Synergetic Innovation Centre for Organic Electronics and Information Displays; Program for Changjiang Scholars and Innovative Research Team in University (IRT1252); Fundamental Research Funds for the Central Universities.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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