High-Performance White Organic Light-Emitting Diodes with Simplified

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Organic Electronic Devices

High-Performance White Organic Light-Emitting Diodes with Simplified Structure Incorporating Novel Exciplex-Forming Host Qi-Sheng Tian, Lei Zhang, Yun Hu, Shuai Yuan, Qiang Wang, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17737 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Applied Materials & Interfaces

High-Performance White Organic LightEmitting Diodes with Simplified Structure Incorporating Novel Exciplex-Forming Host Qi-Sheng Tian, Lei Zhang, Yun Hu, Shuai Yuan, Qiang Wang, and Liang-Sheng Liao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China. ABSTRACT It is a challenge to engineer white organic-light emitting diodes (WOLEDs) with high efficiency, low operating voltage, good color quality, and low efficiency roll-off, simultaneously. Herein, we employ a novel exciplex to solve this problem, which mixes a bipolar host material 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) with a common

electron-transporting

material

4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-

methylpyrimidine (B4PyMPM) to form the host for a blue emitter iridium (III)bis(4,6(difluorophenyl)-pyridinato-N,C2’) picolinate (FIrpic). The blue OLED with maximum power efficiency (PE) over 48 lm W-1 and Commission International de I’Eclairage chromaticity diagram (CIE = (0.17, 0.36)) was achieved. To obtain white light emission, a complementary orange emission layer is used, which consists of the bis(4-phenylth1 ACS Paragon Plus Environment

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ieno[3,2-c]pyridine)(acetylacetonate)iridium(III) (PO-01) doped into the single host of 26DCzPPy adjacent to the blue emission layer. Benefitted from the exciplex and effective utilization of the excitons by using the optimized multi-functional device structure, the WOLEDs remarkably exhibit maximum external quantum efficiency (EQE), PE, and current efficiency (CE) which are 28.5%, 95.5 lm W-1, and 82.0 cd A-1, respectively. At the luminance of 100 cd m-2, it maintains the efficiencies of 27.2%, 90.2 lm W-1, and 78.4 cd A-1. Furthermore, the WOLEDs have a low threshold voltage about 2.6 V, and remain around 4.0 V at 10000 cd m-2. These results indicate that the exciplex-forming co-host 26DCzPPy: B4PyMPM can provide an effective strategy to fabricate high efficiency WOLEDs for potential applications.

KEYWORDS: exciplex-forming co-host, high efficiency, low efficiency roll-off, low operating voltage, multi-EML WOLEDs, exciton confinement, charge balance.

INTRODUCTION White organic light-emitting diodes (WOLEDs) have been rapidly applied for full-color display panels and solid-state lightings owing to their unique features such as self-emission, high efficiency, and flexibility.1-4 In the past decades, enormous efforts have been made aiming to fabricate WOLEDs with high power efficiency (PE) and high color-stability.4-8 Such as reducing operating voltage, fully utilizing excitons generated in the emitters, and 2 ACS Paragon Plus Environment

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introducing suitable organic materials for charge balance in the devices.9-14 Recently, full color phosphorescent OLEDs, such as blue, green, and red phosphorescent devices, have already achieved external quantum efficiencies (EQEs) of more than 30%.1618 For

achieving high performance WOLEDs two different color systems can be effectively

applied. The first one is a conventionl system by combining blue, green and red colors to form a white color emission, the other one is just to employ blue and yellow emisions. Undeniable, blue emission is essential in designing WOLEDs.11, 13, 18, 19, 21-24, 32 However, the selection of host materials and the neighboring charge-blocking materials for achieving blue emission is restricted due to high triplet energy level (T1) and large energy gap (Eg) of the blue emitters. Fortunately, blue OLEDs by using host materials (single host or exciplex-forming co-host) with high T1 have been reported. For an example, Kido et al. reported a unique bipolar host material 2.6-bis(3-(carbazol-9-yl)phenyl) pyridine (26DCzPPy) with high T1 (2.9 eV) and good carrier-transporting ability, which can be used as the host for FIrpic-based blue OLEDs. The device exhibited an EQE of 24% at 100 cd m-2, but behaved a high operating voltage.25 This high voltage issue is always occurred in single host-based blue OLEDs. Exciplex composed of a hole-transporting material (HTL) and an electron-transporting material (ETL) along with efficient reverse intersystem crossing (RISC) were introduced.26-29 Some FIrpic-based blue OLEDs with superior performance by employing exciplex-forming co-host system have been reported.18, 42

The blue exciplex system can provide a “no-barrier” device structure and good charge

3 ACS Paragon Plus Environment


balance, which are favorable to lower driving voltages and increase efficiencies. However, degraded color gamut in the exciplex-forming co-host system was mentioned rarely. 21, 27 High efficiency is usually accompanied by decreased color quality in the system. Therefore, how to achieve exciplex-based blue OLEDs with high efficiency, low operating voltage and good color gamut is crucial to the fabrication of high performance WOLEDs. In addition to the aforementioned material aspect, the layer structure in the devices also has great effect on the performance of the WOLEDs.6, 10 For instance, in a WOLED with a single-emissive-layer (single-EML) structure, two or more different emitters should have different but precise concentrations in the single-EML in order to produce required white color emission. Such as, Lee and co-workers reported a WOLED by using TADF blue exciplex CDBP: PO-T2T, which have EQE, PE, and CE of 20.8%, 48.7 lm W-1, and 53.2 cd A-1 at 100 cd m-2. Recently, mCP: B4PyMPM exciplex was employed in WOLED with EQEmax of 28.1% and PEmax over 100 lm W-1 .31, 32. However, the process to form the singleEML is complicated and costly in terms of manufacture.2, 19-20 Compared to the singleEML structure, a multi-emissive-layer (multi-EML) structure in WOLEDs is a better choice for high PE and high EQE. In our previous work, a hybrid WOLED with a maximum PE of 41.1 lm W-1 and EQE of 15.8% was reported by using a simplified multi-EML structure.14 Leo et al. developed a multi-EML WOLED with high total external PE of 57.6 lm W-1 and EQE of 20.3% at 100 cd m-2.9 Ma et al. reported a high-performance multiEML WOLED with high efficiencies of 41.7 lm W-1 and EQEmax of 19.0%.6 However, 4 ACS Paragon Plus Environment

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obtaining EQE exceeded 25% and PE at around 100 lm W-1 is a bottleneck in developing high-performance WOLEDs without using any out-coupling technologies. Herein, we employed a new exciplex-forming co-host system to fabricate allphosphorescence WOLEDs based on the multi-EML structure. Under systematic design of the devices structure, we selected phosphorescent blue emitter iridium (III)bis(4,6(difluorophenyl)-pyridinato-N, C2’) picolinate (FIrpic) as the blue emitter that was doped into the exciplex-forming co-host combining a bipolar host material 2,6-bis(3-(carbazol-9yl)phenyl)pyridine (26DCzPPy) with an electron-transporting material 4,6-bis[3,5(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PyMPM), while bis(4-phenylth-ieno[3,2c]pyridine)(acetylacetonate)iridium(III) (PO-01) as the complementary color for orange emission that was doped in the adjacent single host of 26DCzPPy. The exciplex can render the PE of blue OLEDs to over 48 lm W-1 and driving voltage to below 4.68 V (measured at 100 cd m-2) with the color of CIE (0.17, 0.36). Based on the blue device with excellent performances, warm WOLED with EQE, PE, and CE of 27.2%, 90.2 lm W-1, and 78.4 cd A-1, respectively, was achieved at 100 cd m-2. Furthermore, extremely low operating voltage of 4.0 V is applied at high brightness of 10000 cd m-2. And the warm WOLEDs show low CE roll-off to 7.2% (ranging from 100 to 1000 cd m-2), which is among the best values in warm WOLEDs. EXPERIMENTAL Materials and Devices: All the materials used in this work were purchased from Lumtec



Company without further purification. All OLEDs were fabricated on the glass with patterned indium-tin-oxide (ITO) layer (15 Ω/square). The ITO substrates were cleaned in turn with isopropyl alcohol and acetone before exposed to ultraviolet-ozone for 15 min. the actual area of each OLED is 0.09 cm2. And the process of fabricating the OLEDs by thermal evaporation, were carried out in vacuum (~ 4.0 × 10−6 Torr). The organic layers were deposited with the rate of 2 ~ 3 Å/s, meanwhile, the deposition rate of Al is 8 Å/s. We encapsulated the devices by using glass sheet including desiccant in ambient air at room temperature. Measurements. The current density-voltage (J-V) curves and other electroluminescence (EL) information of the devices, such as EL spectra, current efficiency (CE) and power efficiency (PE), were tested by the PR 655 photometer combined with a constant current source (Keithley 2400 SourceMeter). UV-vis absorption spectra were obtained from Lambda 750 spectrophotometer. Photoluminescence (PL) spectra were tested by a fluorescence spectrophotometer (Hitachi F-4600). Transient PL decays were measured on a Time-resolved fluorescence spectrometer (PL-TCSPC) of HORIB-FM-2015. RESULTS AND DISCUSSIONS In this study, we have successfully selected two suitable materials, bipolar host material 26DCzPPy as the donor and the common electron-transporting material B4PyMPM as the acceptor, to form a new exciplex-forming co-host for the blue emission. The molecular structures and energy level diagrams of 26DCzPPy and B4PyMPM are shown in Figure


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1a, b. All the HOMO levels of the materials were obtained by performing cyclic voltammetry (CV) in our previous work, which are basically the same as the publically reported data.13, 32 And the LUMO levels were measured (ELUMO = Eg – EHOMO) from the HOMO levels and the electrical band gap (Eg(electrical)) that estimated from the first absorption peak.50, 51 Therefore, the HOMO and LUMO level can be estimated to be -6.1 and -2.5 eV for 26DCzPPy, and -7.1 and -3.2 eV for B4PyMPM, respectively. As shown in Figure 1c, the 26DCzPPY: B4PyMPM co-deposited film exhibits a broad PL spectrum with a peak emission at 436 nm and slight red-shifted compared with 26DCzPPY and B4PyMPM that have an emission peak at 400 nm and 410 nm, respectively. And the EL spectrum is similar to the PL one. These results demonstrate that the emission in the mixed film of 26DCzPPy: B4PyMPM is caused by the interaction between the donor (26DCzPPy) and acceptor (B4PyMPM). And the exciplex formation process can be expressed as follows, which is similar to that of reference.30 Donor (D) + Acceptor (A) + hv → hv exciplex

→

D* + A or D + A* → D+ + A- → (Dδ+ Aδ-)*

+α+D+A

(1)

Where, D* (A*) is an excited state of Donor (Acceptor), (Dδ+ Aδ-)* is an excited state of exciplex caused by electron-hole coupling, hv (hv

exciplex)

is a photon energy of the

excitation (photon energy of the exciplex), α is an exciton banding energy, which is in the range of 0.20 ± 0.15 eV.52 As we know, a suitable driving force (-ΔGcs) is a requirement for achieving exciplex, namely free energy for forming exciplex.30 The driving force for



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the formation of exciplex can be simply described by the modified Equation (2) −∆GCS = Eexciton (EA∗ or ED∗) – Eexciplex

(2)

The exciplex photon energy (Eexcipex) estimated by the PL peak emission (436 nm) of 26DCzPPy: B4PyMPM is about 2.84 eV that is equal to singlet energy level of exciplex (S1, ex).31, 32 The exciton energy ( ED* or EA*) can be calculated according to the first peak nearby the onset of the absorption spectrum of 26DCzPPy (3.60 eV) or B4PyMPM (4.10 eV) (Figure 1c).50, 51 As a result, the values of −∆GCS can be calculated by Equation (2) to be 0.76 and 1.26 eV, respectively, when estimated with Eexciton of 26DCzPPy and B4PyMPM. These values are greater than 0.45 eV, resulting in the formation of the exciplex. 18, 27, 29, 30 Empirically, the LUMO and HOMO energy offset between donor and accepter should be over 0.4 eV to form the exciplex, which has intermolecular chargetransfer (CT) properties and exhibits TADF characters with small ΔEST that enables efficient reverse intersystem crossing (RISC).34-38 As shown in Figure 1, the HOMO and LUMO energy offset between 26DCzPPy and B4PyMPM are 1.2 and 0.7 eV, respectively, rendering the formation of exciplex. And the energy of the exciplex emission peak (2.84 eV) is similar to the difference (2.9 eV) between the HOMO level of 26DCzPPY (-6.1 eV) 13 and

the LUMO level of B4PyMPM (-3.2 eV). It suggests that holes from the HOMO of

26DCzPPY and electrons from the LUMO of B4PyMPM can form electron-hole coupling which contributes to the photon radiation of exciplex via CT state as demonstrated in Figure 1b.36 Additionally, in order to confirm that the exciplex were formed between 8 ACS Paragon Plus Environment

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26DCzPPy and B4PyMPM, transient PL decay was tested in solid film at 300 K shown in Figure S1 (Supporting Information). The transient PL decay of 26DCzPPy: B4PyMPM, in which a delayed component with decay time of 6.9 μs can be tested along with a prompt component (43 ns), suggesting the exciplex possesses valid RISC owing to the small ΔEST of the exciplex. The exciton lifetime of the 26DCZPPy: B4PyMPM is observable longer than that of 26DCzPPY and B4PyMPM.13, 14, 31, 32 These results support the formation of 26DCZPPy: B4PyMPM exciplex is helpful to produce blue emission for white OLEDs.35 To fabricate blue OLEDs by using exciplex-forming co-host, the T1 of excipelx has to be higher than that of the blue emitter. In our exciplex-forming co-host system, the energy level T1 of 26DCZPPy and B4PyMPM are 2.71 and 2.85 eV, respectively.31, 32, 37 The singlet energy level S1 of 26DCzPPy: B4PyMPM exciplex can be calculated to be 2.84 eV from the exciplex emission peak (436 nm) shown in Figure 1c. The triplet energy can be estimated about 2.70 eV, according to the inherent characteristic of exciplex, which has small ΔEST.35, 37, 39 Because the exciplex has higher singlet and triplet energy than that of phosphorescent emitter FIrpic (~ 2.6 eV),1 excitions generated on the excipelx can be efficiently confined and transfered to FIrpic molecule,

preventing the excitons quenching.

Low operating voltage and high efficiency of the blue phosphorescent OLEDs is expected to achieve by using this exciplex-forming co-host.37 Therefore, we plan to use the exciplex 26DCzPPY: B4PyMPM as co-host and use blue emitter FIrpic as the guest to fabricate blue phosphorescent OLEDs firstly. And then use the resulting optimized device structure



to design WOLEDs. In order to optimize the device structure based on exciplex-forming co-host system, we investigated the effect of electron-blocking layer (EBL) and designed three devices A, B, C by using different EBLs N,N,N-tris(4-(9-carbazolyl)phenyl)amine (TCTA),1 N,N′dicarbazolyl-3,5-benzene (mCP) and 26DCzPPY, respectively. The devices were fabricated with the structure ITO (120 nm)/HAT-CN (5 nm)/TAPC (30 nm)/TCTA, mCP or 26DCzPPY (10 nm)/26DCzPPy: B4PyMPM: FIrpic (1: 1, 15%, 20 nm)/B4PyMPM (45 nm)/Liq (2 nm)/Al (120 nm). In these devices, dipyrazino [2,3-f:2′,3′-h] quinoxaline2,3,6,7,10,11-hexacarbonitrile (HAT-CN) was used as hole-injection layer to enhance the injection of holes from anode ITO to the emitting layer,36 di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane (TAPC) was employed as the hole-transporting layer because of its shallow HOMO level (-5.5 eV) and favorable hole mobility,40 B4PyMPM was selected as hole-blocking layer as well as electron-transporting layer,41 8-hydroxyquinolinolatolithium (Liq) and Al are electron-injection layer and cathode, respectively. The energy levels of materials and device structure are shown in Figure S2 (Supporting Information). The current density-voltage-luminance (J-V-L) performances are exhibited in Figure S1b. The CE, PE characteristics and EL spectra are described in Figure S1c, d, respectively. And the EL performances of these devices are summarized in Table S1. The sole emission of FIrpic can be directly found from EL spectra. In this sense, the efficient excitonic energy transformation among system in terms of exciplex-forming co-


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host can be confirmed.28 The blue-emitting PhOLED exhibited maximum CE of 42 cd A-1 and a PE of 44.7 lm W-1 in device A when we select TCTA as the EBL. Interestingly, the CE of device B based on 26DCzPPy are better than that of TCTA under high brightness. This phenomenon may be caused by the extra exciplex formed at the interface between TCTA and B4PyMPM according to the literature.34 The T1 level (2.4 eV) of exciplex TCTA: B4PyMPM is lower than that of 26DCzPPy: B4PyMPM (2.84 eV) and FIrpic (2.6 eV), leading to the energy transfer from 26DCzPPy: B4PyMPM and FIrpic to exciplex TCTA: B4PyMPM, causing energy loss.42 Nevertheless, device A based on TCTA has lower operating voltage than that of device B and C (mCP as the EBL). From the perspective of energy barrier, the TCTA is found to have a suitable HOMO level (-5.8 eV) which is just like a bridge between TAPC (HOMO ~ -5.5 eV) and 26DCzPPy (HOMO ~ 6.1 eV).37, 43 In which case, the holes from HOMO of TAPC will smoothly transfer to that of 26DCzPPY and then formed excitons under current-driving, as demonstrated from energy levels diagram in Figure 2. If we want to achieve highly-efficient WOLEDs by using this structure, the structure of blue PhOLED should be further fabricated carefully. One of the strategies is to reduce the operating voltage to acquire high-performance WOLEDs.1, 13, 14 Hence, we anticipated that the combination of TCTA with 26DCzPPy may achieve better results based on the above analysis. As a result, we continued to optimize the device structure to investigate the roles of EBLs. We designed four devices with different EBLs: D) 26DCzPPY (10 nm); E) TCTA (5 nm)/ 26DCzPPY (5 nm); F)



TCTA (8 nm)/ 26DCzPPY (2 nm); G) TCTA (10 nm). The detailed device structure is shown in Figure 2c. The EL characteristics of these devices are summarized in Table 1. The results demonstrate that the device structure based on TCTA/ 26DCzPPy not only improve the holes transport properties owing to the roles of TCTA discussed above, but also prevent the formation of TCTA/ B4PyMPM exciplex. When the thickness of TCTA layer is 8 nm, notably, low voltage about 3.06 V and high PE 43.1 lm W-1 at 0.2 mA cm-2 and low CE roll-off in device F is clearly found (Table 1). For better understanding of the significance of exciplex 26DCzPPy: B4PyMPM, a control PhOLED using single host 26DCzPPy was fabricated for comparison (Figure S4). Low operating voltage and high PE can be indeed achieved in exciplex-based OLED. These results indicate that the operating voltage and energy loss can be decreased dramatically in this structure. In this system, TCTA possesses a considerable HOMO level (~ -5.8 eV) and good transporting abilities, which moderates the energy barrier between TAPC and 26DCzPPy. It improves the carrier injection and transporting, which can reduce the operating voltage. Compared to device G in which only use TCTA as the EBL, though have lower operating voltage, the efficiencies are not better than device D), E), F) in which 26DCzPPy as a part of EBL. Meanwhile, the blue OLEDs all have good CIE (0.17, 0.36). And the performance comparison of blue PhOLEDs between host 26DCzPPy: B4PyMPM and mCP: B4PyMPM with same device structure (device F) is shown in Figure S5. As is shown, though higher efficiencies were achieved in the mCP: B4PyMPM exciplex-based blue OLED, a stronger green emission


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than that of 26DCzPPy: B4PyMPM was occurred. Accordingly, the 26DCzPPy: B4PyMPM exciplex is a suitable host for blue emission with better color quality. The effects of electron-transporting layer (ETL) also need to be discussed. It is known that employing exciplex-forming co-host system and decreasing energy barrier by introducing materials with suitable frontier molecular orbitals (FOM) levels can reduce the operating voltage to get high power efficiency.23, 35, 37, 44-45 Recently, the p- or n-doping technology is also an attractive method.35, 46 After that, the co-evaporation of 4,7-diphenyl1,10-phenanthroline (Bphen) and LiH were obtained as the n-doped electron-injection layer (EIL). The lithium ions may diffuse into the emission layer causing exciton quenching, and affecting the performances of OLEDs seriously,47 so we need to find a suitable ETL thickness to prevent lithium ions penetrating into the emission region. Therefore, we fabricated three devices with the following structure: ITO/HAT-CN (5 nm)/TAPC (30 nm)/TCTA (8 nm)/26DCzPPy (2 nm)/co-host: FIrpic 15 wt% (20 nm)/B4PyMPM (X nm, X=5, 15, and 25)/Bphen: LiH 0.1 wt% (40-X nm)/Al (120 nm), in which co-host is 26DCzPPy: B4PyMPM (1: 1 wt%). From Figure S3, we can see all of the three devices show low operating voltages and high efficiencies compared to the undoped B4PyMPM layer as EILs in D ~ G devices. As Figure S3d shows, all blue OLEDs show stable values of CIE (0.17, 0.36) that is benefitted from the suitable exciplex forming cohost 26DCzPPy: B4PyMPM. The best performance can be achieved when the thickness of B4PyMPM is 15 nm, in which the threshold voltage measured at 0.2 mA cm-2 is around at



2.7 V, and even at 10000 cd m-2 the operating voltage is as low as 4.68 V shown in Table S2. These indicate that the introduction of n-doped layer adjacent to suitable updoped ETL thinckess can improve the operating voltage of devices. As a result, we can obtain blue PhOLEDs with a maximum PE of 48.9 lm W-1 that is beneficial to the design of highperformance WOLEDs. As shown in the energy levels diagram of the device (Figure 3c), the large energy offset between Bphen (~ -2.9 eV)46 and B4PyMPM (~ -3.2 eV) creates energy transfer barrier for electrons transport between the LUMO of Bphen to B4PyMPM. However, when LiH was doped into Bphen, easier carrier injection and transport from cathode can be achieved.48 The low Ohmic losses at the interface between Bphen and B4PyMPM occurred, which results in electrons inject recombination zone to form excitons easily leading to low driving voltage. Considering the high-efficiency, low driving voltage and good color quality of the blue PhOLEDs based on exciplex-forming co-host, the significant part has been solved for WOLEDs. Base on the elaborate structure discussed above, WOLEDs were fabricated as the following structure: ITO/HAT-CN (5 nm)/TAPC (30 nm)/TCTA (8 nm)/26DCzPPy: PO-01 (X wt%, X=0.5, 1.0, 2.0, 4.0 ) (2 nm)/26DCzPPy: B4PyMPM: FIrpic (1: 1, 15 wt% 20 nm)/B4PyMPM (15 nm)/Bphen: LiH 0.1 wt% (25 nm)/Al (120 nm). In order to achieve better efficiency and spectral quality of WOLEDs, the concentration of orange emitter PO01 were optimized and doped into the 2 nm bipolar host material 26DCzPPy. Four warm WOLEDs with emission wavelength from 400 to 750 nm were achieved (shown in Figure


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4). The blue emission (472 nm) and orange emission (560 nm) are assigned to FIrpic and PO-01, respectively. As the concentration of PO-01 increases, the intensity of blue emission of FIrpic is decreased, because more excitons could be transferred to the PO-01 via Dexter energy transfer from FIrpic. Furthermore, the excitons generated on 26DCzPPy and interface-exciplex can also transfer to PO-01 so that all the excitons could be used for emission.31, 32, 49 When the doping concentration of PO-01 is controlled at 0.5 wt%, the spectral quality of WOLED is unstable. In this kind of device structure, excitons generated on host cannot be efficiently harvested by PO-01 due to the low guest doping concentration. And as the concentration of PO-01 is greater than 1.0 wt%, better spectral quality can be achieved in devices W2-W4 which have small shift of CIE and high efficiencies. The highest EQE of 24.4% and PE of 78.8 lm W-1 were obtained in device W2 with a good CIE (0.41, 0.46) at 1000 cd m-2. Device W3 exhibited ideal CIE (0.45, 0.47) at 1000 cd cm-2 and high efficiencies of 81.0 lm W-1, 65.8 cd A-1, and 25.3% for PE, CE, and EQE, respectively. And the device exhibits low efficiency roll-off with 80.0 lm W-1, 70.0 cd A-1, and 25.0% at 100 cd cm-2, and even as high as 65.0 lm W-1, 64.0 cd A-1, and 22.5% at 1000 cd cm-2, respectively. The maximum PE, CE, and EQE of 95.5 lm W-1,82.0 cd A-1, and 28.5% were achieved at 0.1 mA cm-2, respectively, when the doping concentration of PO-01 is 4 wt% in device W4 (shown in Table 2). At the same time, the efficiencies maintain 90.2 lm W-1, 78.4 cd A-1, and 27.2 % at 100 cd cm-2 as well as 74 lm W-1, 73 cd A-1, and 25.3 % at 1000 cd cm-2, respectively. The low efficiency roll-off is attributed to the proposed device



structure so that excitons can be confined for emission in the stable recombination zone. As we anticipated, one of the reasons that obtain the highly efficient WOLEDs is the low driving voltage. For example, the four warm WOEDs have low voltage about 2.7 V at 0.05 mA cm-2, and ultralow driving voltage remains 4.0 V in device W4 at 10000 cd m-2. CONCLUSIONS This study reports an effective approach to enhance the efficiency of exciplex-based WOLEDs with multi-EML structure by fully utilizing the excitons, reducing the operating voltage, and balancing the charge transport. A novel exciplex system 26DCzPPy: B4PyMPM is introduced as host for high efficiency sky-blue phosphorescent and white OLEDs. Both multi-emissive all-phosphorescent WOLEDs (W1-W4) showed high EQE exceed 20% and low operating voltage about 2.6 V for onset, yet all bellow 4.4 V at 10000 cd m-2. And the PE, CE, and EQE of 89.0 lm W-1, 79.0 cd A-1, and 27.3% were achieved at 100 cd cm-2, respectively, as well as 74 lm W-1, 73 cd A-1, and 25.3% at 1000 cd cm-2. These results indicate that the simple well-designed multi-emissive structure with 26DCzPPy/ 26DCzPPy: B4PyMPM as hosts can provide a reliable method to fabricate high efficiency WOLEDs. Supporting Information PL decay curve of 26DCzPPY: B4PyMPM film and more properties of the device performance based on different structures were shown in the supporting information. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 61575136 and 51773141). This work was also funded by the Collaborative Innovation Centre of Suzhou Nano Science and Technology (Nano-CIC), by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), by the “111” Project of The State Administration of Foreign Experts Affairs of China, and by Yunnan Provincial Research Funds on College-Enterprise Collaboration (No. 2015IB016). RRFERENCES (1)

Su, S. J.; Gonmori, E.; Sasabe, H.; Kido, J. Highly Efficient Organic Blue-and White-Light-Emitting Devices Having a Carrier- and Exciton-Confining Structure for Reduced Efficiency Roll-Off. Adv. Mater. 2008, 20, 4189-4194.

(2)

Kido, J.; Kimura, M.; Nagai, K. Multilayer White Light-Emitting Organic Electroluminescent Device. Science. 1995, 267, 1332-1334.

(3)

Schwartz, G.; Reineke, S.; Rosenow, T. C.; Walzer, K.; Leo, K. Triplet Harvesting in Hybrid White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2009, 19, 13191333.

(4)

D'Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for SolidState Lighting. Adv. Mater. 2010, 16, 1585-1595. 17 ACS Paragon Plus Environment


(5)

Page 18 of 31

Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature. 2009, 459.

(6)

Sun, N.; Wang, Q.; Zhao, Y.; Chen, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D. HighPerformance Hybrid White Organic Light-Emitting Devices Without Interlayer Between Fluorescent and Phosphorescent Emissive Regions. Adv. Mater. 2014, 26, 1617-1621.

(7)

Wu, Z.; Luo, J.; Sun, N.; Zhu, L.; Sun, H.; Yu, L.; Yang, D.; Qiao, X.; Chen, J.; Yang, C. High-Performance Hybrid White Organic Light-Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll-Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter. Adv. Funct. Mater. 2016, 26, 3306-3313.

(8)

Wang, Y. K.; Sun, Q.; Wu, S. F.; Yuan, Y.; Li, Q.; Jiang, Z. Q.; Fung, M. K.; Liao, L. S. Thermally Activated Delayed Fluorescence Material as Host with Novel SpiroBased Skeleton for High Power Efficiency and Low Roll-Off Blue and White Phosphorescent Devices. Adv. Funct. Mater. 2016, 26, 7929-7936.

(9)

Schwartz, G.; Pfeiffer, M.; Reineke, S.; Walzer, K.; Leo, K. Harvesting Triplet Excitons from Fluorescent Blue Emitters in White Organic Light-Emitting Diodes. Adv. Mater. 2007, 19, 3672-3676.

(10)

Wu, Z. B.; Yu, L.; Zhao, F. C.; Qiao, X. F.; Chen, J. S.; Ni, F.; Yang, C. L.; Ahamad, T.; Alshehri, S. M.; Ma, D. G. Precise Exciton Allocation for Highly

Efficient

White Organic Light-Emitting Diodes with Low Efficiency Roll-Off Based on Blue Thermally Activated Delayed Fluorescent Exciplex Emission. Adv. Opt. Mater. 2017, DOI: 10.1002/adom.201700415 (11)

Higuchi, T.; Nakanotani, H.; Adachi, C. High-Efficiency White Organic LightEmitting Diodes Based on a Blue Thermally Activated Delayed Fluorescent Emitter Combined with Green and Red Fluorescent Emitters. Adv. Mater. 2015, 27, 201918 ACS Paragon Plus Environment

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2023. (12)

Liu, Y.; Liang, F.; Cui, L. S.; Shi, X. B.; Wang, Z. K.; Liao, L. S. Simplified Hybrid White Organic Light-Emitting Diodes with a Mixed Fluorescent Blue Emitting Layer for Exciton Managing and Lifetime Improving. Adv. Opt. Mater. 2016, DOI: 10.1002/adom.201600410.

(13)

Liu, X. K.; Chen, W.; Thachoth Chandran, H.; Qing, J.; Chen, Z.; Zhang, X. H.; Lee, C. S. High-Performance, Simplified Fluorescence and Phosphorescence Hybrid White Organic Light-Emitting Devices Allowing Complete Triplet Harvesting. ACS Appl. Mater. Interfaces. 2016, 8, 26135-26142.

(14)

Cui, L. S.; Liu, Y.; Li, Q.;Jiang, Z. Q.; Liao, L. S.　A Rational Molecular Design on Choosing Suitable Spacer for Better Host Materials in Highly Efficient Blue and White Phosphorescent Organic Light-Emitting Diodes.Org. Electron. 2014, 15, 1368-1377.

(15)

Kim, K. H.; Lee, S.; Moon, C. K.; Kim, S. Y.; Park, Y. S.; Lee, J. H.; Woo Lee, J.; Huh, J.; You, Y.; Kim, J. J. Phosphorescent Dye-Based Supramolecules for HighEfficiency Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, 4769.

(16)

Lee, J. H.; Shin, H.; Kim, J. M.; Kim, K. H.; Kim, J. J. Exciplex-Forming Co-HostBased Red Phosphorescent Organic Light-Emitting Diodes with Long Operational Stability and High Efficiency. ACS. Appl. Mater. Interfaces. 2017, 9, 3277-3281.

(17)

Seino, Y.; Inomata, S.; Sasabe, H.; Pu, Y. J.; Kido, J. High-Performance Green OLEDs Using Thermally Activated Delayed Fluorescence with a Power Efficiency of over 100 lm W(-1). Adv. Mater. 2016, 28, 2638-2643.

(18)

Shin, H.; Lee, J. H.; Moon, C. K.; Huh, J. S.; Sim, B.; Kim, J. J. Sky-Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer. Adv. Mater. 2016, 28, 4920-4925.

(19)

Zhang, D.; Duan, L.; Li, Y.; Zhang, D.; Qiu, Y. Highly Efficient and Color-Stable Hybrid Warm White Organic Light-Emitting Diodes using a Blue Material With 19 ACS Paragon Plus Environment


Thermally Activated Delayed Fluorescence. J. Mater. Chem. C. 2014, 2, 8191-8197. (20)

Li, X. L.; Ouyang, X.; Liu, M.; Ge, Z.; Peng, J.; Cao, Y.; Su, S. J. Highly Efficient Single- and Multi-Emission-Layer Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes with ∼20% External Quantum Efficiency. J. Mater. Chem. C. 2015, 3, 9233-9239.

(21)

Lee, J. H.; Cheng, S. H.; Yoo, S. J.; Shin, H.; Chang, J. H.; Wu, C. I.; Wong, K. T.; Kim, J. J. An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage. Adv. Funct. Mater. 2015, 25, 361-366.

(22)

Song, W.; Lee, I.; Lee, J. Y. Host Engineering for High Quantum Efficiency Blue and White Fluorescent Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 43584363.

(23)

Sasabe, H.; Pu, Y. J.; Nakayama, K.; Kido, J. M-Terphenyl-Modified Carbazole Host Material for Highly Efficient Blue and Green PHOLEDS. Chem. Commun. 2009, 43, 6655-6657.

(24)

Burrows, P. E.; Padmaperuma, A. B.; Sapochak, L. S.; Djurovich, P.; Thompson, M. E. Ultraviolet Electroluminescence and Blue-Green Phosphorescence using an Organic Diphosphine Oxide Charge Transporting Layer. Appl. Phys. Lett. 2006, 88, 183503.

(25)

Su, S. J.; Sasabe, H.; Takeda, T.; Kido, J. Pyridine-Containing Bipolar Host Materials for Highly Efficient Blue Phosphorescent OLEDs. Chem. Mater. 2008, 20, 1691-1693.

(26)

Xu, T.; Zhang, Y. X.; Bo, W.; Huang, C. C.; Murtaza, I.; Hong, M.; Liao, L. S. Highly Simplified Reddish Orange Phosphorescent Organic Light-Emitting Diodes Incorporating a Novel Carrier- and Exciton-Confining Spiro-exciplex-forming Host for Reduced Efficiency Roll-off. ACS Appl. Mater. Interfaces. 2017, 9, 2701-2710.

(27)

Seino, Y.; Sasabe, H.; Pu, Y. J.; Kido, J. High-performance Blue Phosphorescent OLEDs using Energy Transfer From Exciplex. Adv. Mater. 2014, 26, 1612-1616. 20 ACS Paragon Plus Environment

Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28)

Zhou, D. Y.; Siboni, H. Z.; Wang, Q.; Liao, L. S.; Aziz, H. Host to Guest Energy Transfer Mechanism in Phosphorescent and Fluorescent Organic Light-Emitting Devices Utilizing Exciplex-Forming Hosts. J.Phys. Chem. C. 2014, 118, 2400624012.

(29)

Goushi, K.; Kou, Y.; Sato, K.; Adachi, C. Organic Light-Emitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nat. Photonics. 2012, 6, 253-258.

(30)

Liu, X. K.; Chen, Z.; Zheng, C. J.; Liu, C. L.; Lee, C. S.; Li, F.; Ou, X. M.; Zhang, X. H. Prediction and Design of Efficient Exciplex Emitters for High-Efficiency, Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 2378-2383.

(31)

Liu, X. K.; Chen, Z.; Qing, J.; Zhang, W. J.; Wu, B.; Hoi, Lam. T.; Zhu, F.; Zhang, X. H.; Lee, C. S. Remanagement of Singlet and Triplet Excitons in SingleEmissiveLayer Hybrid White Organic Light-Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex. Adv. Mater. 2015, 27, 7079-7085.

(32)

Wu, S.-F.; Li, S.-H.; Wang, Y.-K.; Huang, C.-C.; Sun, Q.; Liang, J.-J.; Liao, L.-S.; Fung, M.-K. White Organic LED with a Luminous Efficacy Exceeding 100 lm W−1 without Light Out-Coupling Enhancement Techniques. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm.201701314.

(33)

Stewart, D. J.; Dalton, M. J.; Swiger, R. N.; Cooper, T. M.; Haley, J. E.; Tan, L. S. Exciplex Formation in Blended Spin-Cast Films of Fluorene-Linked Dyes and Bisphthalimide Quenchers. J. Phys. Chem. A. 2013, 117, 3909-3917.

(34)

Kim, K.-H.; Yoo, S.-J.; Kim, J.-J. Boosting Triplet Harvest by Reducing Nonradiative Transition of Exciplex toward Fluorescent Organic Light-Emitting Diodes with 100% Internal Quantum Efficiency. Chem. Mater. 2016, 28, 1936-1941.

(35)

Lee, J.-H.; Cheng, S.-H.; Yoo, S.-J.; Shin, H.; Chang, J.-H.; Wu, C.-I.; Wong, K.T.; Kim, J.-J. An Exciplex Forming Host for Highly Efficient Blue Organic Light 21 ACS Paragon Plus Environment


Emitting Diodes with Low Driving Voltage. Adv. Funct. Mater. 2015, 25, 361-366. (36)

Hajime Nakanotani; Taro Furukawa; Kei Morimoto; Adach, C. Long-Range Coupling of Electron-Hole Pairs in Spatially Separated Organic Donor-Acceptor Layers. Sci. Adv. 2016, DOI: 10.1126/sciadv.1501470.

(37)

Udagawa, K.; Sasabe, H.; Igarashi, F.; Kido, J. Simultaneous Realization of High EQE of 30%, Low Drive Voltage, and Low Efficiency Roll-Off at High Brightness in Blue Phosphorescent OLEDs. Adv. Opt. Mater. 2016, 4, 86-90.

(38)

Jou, J.-H.; Chen, C.-H.; Tseng, J.-R.; Peng, S.-H.; Chen, P.-W.; Chiang, C.-I.; Jou, Y.-C.; Hong, J. H.; Wang, C.-C.; Chen, C.-C.; Tung, F.-C.; Chen, S.-H.; Wang, Y.S.; Chin, C.-L. Using Light-Emitting Dyes as a Co-Host to Markedly Improve Efficiency Roll-Off in Phosphorescent Yellow Organic Light Emitting Diodes. J. Mater. Chem. C. 2013, 1, 394-400.

(39)

Li, J.; Nomura, H.; Miyazaki, H.; Adachi, C. Highly Efficient Exciplex Organic Light-Emitting Diodes Incorporating a Heptazine Derivative as an Electron Acceptor. Chem. Commun. 2014, 50, 6174-6176.

(40)

Kalinowski, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Williams, J. A. G. Mixing of Excimer and Exciplex Emission: A New Way to Improve White Light Emitting Organic Electrophosphorescent Diodes. Adv. Mater. 2010, 19, 4000-4005.

(41)

Kim, H.-G.; Kim, K.-H.; Moon, C.-K.; Kim, J.-J. Harnessing Triplet Excited States by Fluorescent Dopant Utilizing Codoped Phosphorescent Dopant in Exciplex Host for Efficient Fluorescent Organic Light Emitting Diodes. Adv. Opt. Mater. 2017, 5, 1600749.

(42)

Seino, Y.; Sasabe, H.; Pu, Y. J.; Kido, J. High-Performance Blue Phosphorescent OLEDs using Energy Transfer from Exciplex. Adv. Mater. 2014, 26, 1612-1616.

(43)

Zhao, B.; Zhang, T.; Chu, B.; Li, W.; Su, Z.; Luo, Y.; Li, R.; Yan, X.; Jin, F.; Gao, Y. Highly Efficient Tandem Full Exciplex Orange and Warm White OLEDs Based on Thermally Activated Delayed Fluorescence Mechanism. Org. Electron. 2015, 22 ACS Paragon Plus Environment

Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17, 15-21. (44)

Hung, W. Y.; Chiang, P. Y.; Lin, S. W.; Tang, W. C.; Chen, Y. T.; Liu, S. H.; Chou, P. T.; Hung, Y. T.; Wong, K. T. Balance the Carrier Mobility To Achieve High Performance Exciplex OLED Using a Triazine-Based Acceptor. ACS. Appl. Mater. Interfaces. 2016, 8, 4811-1818.

(45)

Zhang, L.; Zhang, Y. X.; Hu, Y.; Shi, X. B.; Jiang, Z. Q.; Wang, Z. K.; Liao, L. S. Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes Employing a Host Material with Small Bandgap. ACS. Appl. Mater. Interfaces 2016, 8, 1618691.

(46)

Fung, M. K.; Li, Y. Q.; Liao, L. S. Tandem Organic Light-Emitting Diodes. Adv. Mater. 2016, 28, 10381-10408.

(47)

He, G.; Schneider, O.; Qin, D.; Zhou, X.; Pfeiffer, M.; Leo, K. Very High-Efficiency and Low Voltage Phosphorescent Organic Light-Emitting Diodes Based on a P-i-N Junction. J. Appl. Phys. 2004, 95, 5773-5777.

(48)

Yang, J. P.; Xiao, Y.; Deng, Y. H.; Duhm, S.; Ueno, N.; Lee, S. T.; Li, Y. Q.; Tang, J. X. Electric-Field-Assisted Charge Generation and Separation Process in Transition Metal Oxide-Based Interconnectors for Tandem Organic Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22, 600–608.

(49)

Zhang, D.; Duan, L.; Li, Y.; Zhang, D.; Qiu, Y. Highly Efficient and Color-Stable Hybrid Warm White Organic Light-Emitting Diodes using a Blue Material with Thermally Activated Delayed Fluorescence. J. Mater. Chem. C. 2014, 2, 8191-8197.

(50)

Liao, L, S.; Fung, M, K.; Lee, C, S.; Lee, S, T.; M. Inbasekaran, E. P. Woo; W. W. Wu. Electronic Structure and Energy Band Gap of Poly (9,9-dioctylfluoren) Investigated by Photoelectron Spectroscopy. Appl. Phys. Lett. 2000, 76, 3582.

(51)

Santos F. Alvarado ; Walter Rieß. Energy Level Alignment and Exciton Binding Energies Using Model Interfaces between Metals and Evaporable Organic Electroluminescent Materials. Mat. Res. Soc. 2001, 665, C512 23 ACS Paragon Plus Environment


(52)

He, S, J.; Wang, D, K.; Jiang, N.; John S. Tse.; Lu Z, H. Tunable Excitonic Processes at Organic Heterojunctions. Adv. Mater. 2016, 28, 649-654.


Page 24 of 31

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Figures and Tables

Figure 1. a) Molecular structures of 26DCzPPy and B4PyMPM, b) Energy level scheme of 26DCzPPY and B4PyMPM c) UV-vis absorption and PL spectra of the 26DCzPPy: B4PyMPM, 26DCzPPy and B4PyMPM, all of the solid films were measured at 370 nm excitation.



Figure 2. EL characteristics of the devices with structures of ITO/HAT-CN (5 nm)/TAPC (30 nm)/TCTA (10-X nm)/26DCzPPy (X nm)/26DCzPPy: B4PyMPM: FIrpic (1: 1, 15%, 20 nm)/B4PyMPM (45 nm)/Liq (2 nm)/Al (120 nm) a) Current density-voltage-luminance characteristics. b) Current efficiency-luminance-power efficiency characteristics. c) Energy level diagram of the devices. d) EL spectra at 5 mA cm-2.


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Table 1. EL performances of four different devices. Device

Voltage (V) a)

(V) b)

CE c)

PE c)

Roll-off d)

Emission peak

CIE e)

(cd/A)

(lm/W)

(%)

(nm)

(x, y)

D

3.51

3.57

40.5

36.2

4.9

472

(0.17,0.36)

E

3.19

3.23

42.5

41.8

4.1

472

(0.17,0.35)

F

3.06

3.12

42.0

43.1

3.2

472

(0.17,0.35)

G

2.98

3.00

42.5

44.7

17.2

472

(0.17,0.36)

a), b)

Voltages measured corresponding to onset and 100 cd m-2, respectively. c) The maximum efficiencies at 0.2 mA cm-2, d) CE roll-off at the luminance from 100 cd m-2 to 1000 cd m-2. e) Commission International de I’Eclairage coordinates (CIE) tested at 100 cd m-2.



Figure 3. a) Current density-voltage-luminance characteristics. b) Current efficiencyluminance-power efficiency characteristics. c) Energy diagram and structure of the devices. d) EQE-Luminance characteristics.


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Figure 4. Normalized EL spectra at various luminance of 500, 1500, 2000 and 5000 cd m-2 and the corresponding CIE values of WOLEDs of a) W1, b) W2, c) W3, and d) W4.



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Table 2. EL performance of four different WOLEDs. WOLEDs

Voltage

PE (lm/W)

CE (cd/A)

EQE (%)

Maximum/At 100 cd m-2

Roll-off

CIE e)

(%) d)

(x, y)

(V) a)

(V) b)

(V) c)

W1

2.66

3.14

4.45

70.7/67.6

60.0/55.3

21.9/21.7

2.9

(0.36,0.44)

W2

2.66

3.12

4.34

78.8/75.9

68.0/67.7

24.4/24.1

12.6

(0.41,0.46)

W3

2.65

3.11

4.19

81.0/80.3

70.0/69.8

25.3/25.1

7.9

(0.45,0.47)

W4

2.69

3.13

4.02

95.5/90.2

82.0/78.4

28.5/27.2

7.2

(0.45,0.48)

a), b), c)

Voltages were measured corresponding to onset, 1000 cd m-2 and 10000 cd m-2, respectively. d) CE roll-off at the luminance from 100 cd m-2 to 1000 cd m-2. e) Commission International de I’Eclairage coordinates (CIE) tested at 100 cd m-2.


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Table of Contents Graphic

The high-efficiency white organic light-emitting diodes (WOLEDs) by employing a new exciplex-forming host 26DCzPPy: B4PyMPM were fabricated. The multi-emissive layers (multi-EMLs) all-phosphorescent WOLEDs achieved peak power efficiency (PE) and external quantum efficiency (EQE) of 95.5 lm W-1 and 28.5%, respectively.


High-Performance White Organic Light-Emitting Diodes with Simplified

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