Superior Efficiency and Low-Efficiency Roll-Off White Organic Light

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Organic Electronic Devices

Superior Efficiency and Low Efficiency Roll-off White Organic Light-Emitting Diodes Based on Multiple Exciplexes as Hosts Matched to Phosphor Emitters Shian Ying, Peiyuan Pang, Shuai Zhang, Qian Sun, Yanfeng Dai, Xianfeng Qiao, Dezhi Yang, Jiangshan Chen, and Dongge Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09429 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Applied Materials & Interfaces

Superior Efficiency and Low Efficiency Roll-off White Organic Light-Emitting Diodes Based on Multiple Exciplexes as Hosts Matched to Phosphor Emitters Shian Ying,† Peiyuan Pang, † Shuai Zhang, † Qian Sun, † Yanfeng Dai, † Xianfeng Qiao,† Dezhi Yang,† Jiangshan Chen,† and Dongge Ma *,† †Center

for Aggregation-Induced Emission, Institute of Polymer Optoelectronic Materials and

Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, People’s Republic of China, E-mail: [email protected]

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ABSTRACT: Based on exciplexes as hosts, the monochromatic organic light-emitting diodes (OLEDs) have achieved high power and external quantum efficiencies. However, the high-quality white OLEDs (WOLEDs) with high color rendering index (CRI) have the unsatisfactory efficiencies at high luminance, particularly in terms of power efficiency (PE), resulting in high energy consumption. Here, a new design concept using multiple exciplexes as hosts to match different phosphors has been demonstrated to develop high-performance WOLEDs. It can be seen that the resulting WOLEDs work at a low turn-on voltage of 2.3 V and exhibit the large forward-viewing PE and external quantum efficiency (EQE) of 79 lm W−1 and 22.5%, respectively, without light outcoupling techniques. Significantly, the PE and EQE still remain 48.0 lm W−1 and 21.4% at 1000 cd m−2, showing extremely low efficiency roll-off. The color rendering index (CRI) is as high as 81. The keys to success are the selection of the different exciplex hosts matched to different phosphors and the reasonable arrangement of emissive layers, which are beneficial to regulate the exciton distribution and reduce the energy losses.

KEYWORDS: exciplex, phosphorescent host, low efficiency roll-off, white organic light-emitting diodes, energy transfer

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1. INTRODUCTION White organic light-emitting diodes (WOLEDs) have aroused extensive attention from academia and industry in lighting and display applications due to their superior characteristics.1-4 As for an ideal white light source, it should emit the entire visible spectrum, which generally need two or more organic dyes to simultaneously emit different color lights.5 For phosphorescent WOLEDs, a significant point to be emphasized is the selection of suitable hosts for different phosphors to prevent back exciton energy transfer from phosphors to hosts.6-7 In terms of the conventional hole-, electron- and bipolar-transporting host materials, 8-11 there are more or less problems, such as the unbalanced hole/electron mobilities, the large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, and the large singlet-triplet splitting, which may lead to the poor chemical (thermal) stability, narrow exciton recombination zone and large operating voltage etc. A recent breakthrough in the exciplexes formed by blending hole-transporting donor and electron-transporting acceptor materials provides a novel host for phosphorescent OLEDs due to the following advantages: (1) Low turn-on voltage.12-16 There exist no energy barriers from transporting layers to emissive layer so that the holes and electrons are directly injected into the HOMO energy level of the exciplex’s donor molecules and the LUMO energy level of the exciplex’s acceptor molecules, respectively. (2) Good bipolar transport properties.17-18 The balanced charge carrier transport and broad exciton recombination zone can be realized by the rational manipulation of donor and acceptor molecules ratio, which is highly beneficial to reduce local triplet exciton density and alleviate exciton quenching. (3) Efficient exciton confinement structure and energy transfer into phosphors.19-20 Becasue the triplet energy level (T1) of the donor and acceptor molecules are higher than that of the exciplex and the T1 3



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of exciplex is also higher than that of phosphors, therefore, the utilization of exciplex structures could effectively confine excitons in emissive layer (EML) and transfer exciton energies to phosphors from exciplexes. Based on the exciplexes as hosts, the blue, green and red phosphorescent OLEDs with high power efficiencies and external quantum efficiencies approaching the theoretical limitation have been reported.20-23 However, the high-quality WOLEDs with color rendering index (CRI) over 80 and low efficiency roll-off have still certain difficulty to realize. Liu and co-workers reported a single-EML hybrid WOLED with a CRI of 76 by doping green and red phosphors into 4,4′-bis(9-carbazolyl)-2,2′dimethylbiphenyl (CDBP): ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T) exciplex host.24 The maximum PE and EQE were 84.1 lm W–1 and 25.5%, and decreased to 24.2 lm W–1 and 14.8% at the luminance of 1000 cd m−2. Wu et al. developed a three primary colors-based hybrid WOLED with a high CRI of 86 by using an exciplex sandwich emissive layer structure.25 The maximum PE and EQE were 75.5 lm W–1 and 29.4%, and decreased to 26.6 lm W–1 and 17.6% at 1000 cd m−2, showing pronounced efficiency roll-off. Recently, Zhao et al. reported a multiple-EMLs phosphorescent WOLED with a CRI of 79.2 employing the new exciplex host formed by 3,3′-di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,6-bis(3,5-di(pyridin-4-yl) phenyl)-2phenylpyrimidine (B4PyPPM).17 The maximum PE and EQE were 84.1 lm W–1 and 25.5%, and still reduced to 30.8 lm W–1 and 21.4% at 1000 cd m−2. Obviously, the efficiency roll-off at high luminance in exciplex-based WOLEDs needs further improvement. In this work, we proposed a novel strategy to develop high efficiency and low efficiency roll-off WOLEDs based on exciplexes as hosts by the precise choice of the suitable exciplexes for each phosphor and the rational manipulation of different EMLs. The emission mechanism has been 4


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analyzed by transient electroluminescence characteristics in detail. It can be seen that the efficiency and efficiency roll-off are greatly improved. The maximum forward-viewing PE, CE, and EQE of 79 lm W−1, 61.5 cd A−1, and 22.5% are achieved, respectively, without using light out-coupling technique. At 1000 and 5000 cd m−2, the EQEs can also keep 21.4% and 19.5%, respectively. 2. EXPERIMENTAL SECTION 2.1 Materials Indium tin oxide (ITO) glass substrates with a sheet resistance of 15 Ω per square were used as the anode. 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) was served as the holeinjecting layer. 1-bis[4-[N,N-di(4-tolyl)amino]- phenyl]-cyclohexane (TAPC) with high hole mobility of 6 × 10−3 cm2 V−1 s−1 was used as the hole-transporting layer (HTL). 4,4,4-tris(Ncarbazolyl)triphenylamine (TCTA) was used as the hole-transporting/exciton blocking to reduce hole injection barrier and confine excitons in EMLs. mCBP and N,N’-di-(1-naphthalenyl)-N,N’-diphenyl[1,1’:4’,1’’:4’’,1’’’-quaterphenyl]-4,4’’’-diamine (4P-NPB) were served as the hole-transporting donor of exciplexes. PO-T2T was served as the electron-transporting acceptor of exciplexes. Due to the high electron mobility, PO-T2T was also acted as the electron-transporting layer (ETL). 8hydroxyquinolatolithium (Liq) and lithium fluoride (LiF) were served as the electron-injecting layers. Aluminum (Al) was acted as the cathode. The material number of YDD01 and RD071 were separately served as the orange and red phosphorescent dopants. Iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C2’]picolinate

(FIrpic),

bis(2-phenylpyridine)iridium(III)

(Ir(ppy)2acac)

iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2)acetylacetonate

and

(Ir(tptpy)2acac)

were served as the blue, green and yellow phosphorescent dopants, respectively. All the materials were used as received without further purification. 5



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2.2 OLED Fabrication and Measurements ITO-coated glass substrates were cleaned by ultrasonic cleaning in detergent and deionized water, then dried at 120 °C for 1 h in an oven and treated with oxygen plasma for 6 min, and finally subjected to a deposition chamber. The OLEDs were fabricated through vacuum deposition under the pressure of about 1 × 10−4 Pa. HAT-CN was deposited at a rate of 0.5 Å s−1, and other organic materials were deposited at a rate of 1-1.5 Å s−1. Liq and LiF were deposited at 0.2 Å s−1, and the cathode (Al) was deposited at 5-10 Å s−1 through a shadow mask. The device effective emitting areas were 4 × 4 cm2. The current density–luminance–voltage characteristics were evaluated under a constant source of Keithley 2400 source meter and LS110 luminance meter. The electroluminescence (EL) spectra, Commission Internationale de L’Eclairage (CIE) coordinates were carried out by using a Spectrascan PR650 photometer. The EQEs were calculated from the EL spectra, luminance and current density, assuming Lambertian distribution. The time-resolved EL measurements were performed by applying voltage pulses to the devices corresponding to 10 mA cm−2 for red and green emissions with a pulse width of 20 µs and a period of 100 µs using Agilent 8114A pulse generator, and recording the emission using an optical spectrometer (7IMS3021A) connected with a photomultiplier tube. 2.3 Photoluminescence (PL) Characterization The UV−vis absorption spectra of films and phosphorescent dopants in dichloromethane were performed by Shimadzu UV-2600 spectrophotometer. The PL spectra were carried out in air with an Edinburgh Instruments FLS980 spectrometer. All the films were prepared on quartz substrates by vacuum thermal deposition. The PL quantum yields were recorded by using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus-QY. 3. RESULTS AND DISCUSSION 6


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3.1 Design Concept of WOLEDs. To fabricate good three primary color WOLEDs based on exciplex hosts, the high T1 of exciplexes is typically required.26-29 Due to the large energy gap (> 0.6 eV) between blue exciplex and red phosphor, there will exist serious energy losses, resulting in the poor power efficiency. Here, we utilized the strategy of different exciplex hosts for different phosphors to construct the light-emitting unit in exciplex-based WOLEDs. In our WOLEDs, the high energy state exciplex was selected as the host of blue phosphor, whereas the low energy state exciplex was selected as the host of orange or red phosphors to guarantee the efficient energy transfer and reduce the energy losses between hosts and phosphors. Moreover, the precise management of charge carriers and excitons can be achieved through reasonable arrangement of each EML. Thus, high efficiency and low efficiency roll-off WOLEDs are realized well. In order to achieve the above purpose, we selected PO-T2T as the electron-transporting acceptor due to its deep LUMO energy level (−3.5 eV), low refractive index (~1.72), high electron mobility (4.4 × 10−3 cm2 V−1 s−1), and high T1 (2.99 eV). 20, 30-31 mCBP and 4P-NPB were selected as the holetransporting donors to build high and low energy state exciplexes, respectively, whose molecular structures are shown in Figure 1a. As shown in Figure S1, the PL spectra of mCBP: PO-T2T and 4PNPB: PO-T2T films are broad with peaks of 478 and 542 nm, which are significantly red-shifted relative to those of mCBP, 4P-NPB and PO-T2T. The emissions of mCBP: PO-T2T and 4P-NPB: PO-T2T corresponding to energies are close to the differences between the HOMO energy levels of mCBP (−6.1 eV) and 4P-NPB (−5.73 eV) and the LUMO level of PO-T2T (−3.5 eV), respectively. What’s more, the absorption spectra of mCBP: PO-T2T and 4P-NPB: PO-T2T films have no new absorption compared with the neat mCBP, 4P-NPB and PO-T2T films, meaning the formation of 7



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exciplex in the mixed films, which is in agreement with those of the reported.20, 31 3.2 EL Performances of Monochromatic OLEDs As known, the performance of monochromatic OLEDs has significant effect on that of WOLEDs.14, 32

Before fabricating the WOLEDs, we characterized the EL properties of the high and low energy

state exciplexes (mCBP: PO-T2T and 4P-NPB: PO-T2T) as hosts for blue, green, yellow, orange and red phosphorescent OLEDs. The optimized device configurations are ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ mCBP (5 nm)/ mCBP: PO-T2T: Dopants (1:1:x, 15 nm)/ PO-T2T (45 nm)/ Liq (1.5 nm)/ Al (150 nm) and ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ 4P-NPB (8 nm)/ 4P-NPB: PO-T2T: Dopants (1:1:x, 20 nm)/ PO-T2T (45 nm)/ LiF (1 nm)/ Al (150 nm). The doping concentrations of FIrpic, Ir(ppy)2acac, Ir(tptpy)2acac, YDD01 and RD071 are 10%, 2%, 2%, 2%, and 2%, respectively. Figure 1b and S2 show the chemical structures and energy level diagrams of the organic materials used in OLEDs. The key EL performance parameters of the resulting monochromatic OLEDs based on mCBP: POT2T and 4P-NPB: PO-T2T hosts are summarized in Table 1. As shown in Figure S3 and S4, compared with devices B2 and G2 based on 4P-NPB: PO-T2T host system, devices B1 and G1 with mCBP: PO-T2T as host have a very remarkable performance. The maximum forward-viewing PE, CE and EQE of devices B1 and G1 are 59.9/86.6 lm W

−1,

50.3/72.1 cd A−1, and 25.2/20.5%, respectively.

At the luminance of 1000 cd m−2, the devices still keep the high EQEs of 24.2% and 19.4%, showing low efficiency roll-off. Such a large EL performance gap between mCBP: PO-T2T and 4P-NPB: POT2T as hosts in devices can be attributed to the lower T1 of 4P-NPB: PO-T2T exciplex than that of blue and green dopants, which leads to the serious energy transfer from dopant to exciplex, resulting in the emission from 4P-NPB: PO-T2T. It is consistent with the photoluminescence quantum yields 8


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of the blue and green thin films (Table S1). The EL performances of yellow, orange and red OLEDs are shown in Figure 2, S5, and S6. The maximum PE, CE and EQE of yellow, orange and red OLEDs (Y1, O1, and R1) based on mCBP: PO-T2T as host are 83.6/73.1/31.3 lm W−1, 69.4/60.5/28.0 cd A−1, and 20.6/22.3/19.9%, respectively. Correspondingly, using 4P-NPB: PO-T2T as host, all the devices exhibit a lower turn-on voltage around 2.0 V. However, device Y2 with Ir(tptpy)2acac as dopant achieves the maximum PE, CE, and EQE of 77.8 lm W−1, 52.5 cd A−1, and 16.6%, respectively, lower than device Y1 with mCBP: POT2T as host. Surprisingly, the efficiencies of orange and red devices O2 and R2 are higher than those of devices O1 and R1. Devices O2 and R2 show the maximum PEs and EQEs of 90.4/40.1 lm W−1 and 22.3/23.7%, respectively. At the high luminance of 1000 cd m−2, the efficiencies still remain as high as 56.7/18.5 lm W−1 and 20.9/20.5%, which are obviously higher than those of devices O1 and R1 (42.9/12.1 lm W−1 and 20.7/17.1% for PEs and EQEs). Obviously, the high energy state mCBP: PO-T2T exciplex can be used as the good host for blue, green, and yellow phosphors, whereas the low energy state 4P-NPB: PO-T2T exciplex is more suitable as the host for orange and red phosphors due to its low T1. What’s more, the monochromatic OLEDs based on single host (mCBP, PO-T2T, and 4P-NPB) have been fabricated to compare with devices with exciplexes as host. As shown in Figure S7 and S8, the monochromatic OLEDs (B1, G1, Y1, O2, and R2) based on exciplex systems have the significantly higher efficiencies (CE, PE, and EQE) and lower efficiency roll-off. This further demonstrates the effectiveness and advantage of exciplexes as phosphorescent hosts. 3.3 Effects of EML Arrangement In multiple-EMLs WOLEDs, the arrangement of EMLs has an important effect on the EL 9



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efficiency and spectrum.33-35 For this purpose, we selected mCBP: PO-T2T as the host of blue phosphor and 4P-NPB: PO-T2T as the host of yellow phosphor to fabricate two types of two-color WOLEDs with different EML structures, i.e. “Y+B” type device W-S1 with the structure of ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ 4P-NPB (5 nm)/ 4P-NPB: PO-T2T: Ir(tptpy)2acac (1:1:2%, 5 nm)/ mCBP: PO-T2T: FIrpic (1:1:10%, 10 nm)/ PO-T2T (45 nm)/ LiF (1 nm)/ Al (150 nm), and “B+Y” type device W-S2 with the structure of ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ mCBP (5 nm)/ mCBP: PO-T2T: FIrpic (1:1:10%, 10 nm)/ 4P-NPB: PO-T2T: Ir(tptpy)2acac (1:1:2%, 5 nm)/ PO-T2T (45 nm)/ LiF (1 nm)/ Al (150 nm), as shown in Figure S9a. It can be seen from Figure S10 that devices W-S1 and W-S2 show the maximum PEs and EQEs of 49.5 lm W−1 and 84.2 lm W−1, and 11.5% and 20.3%, respectively. Importantly, device W-S2 still keeps the high EQE of 18.8% at 1000 cd m−2. The result suggests that the “B+Y” type EML structure is beneficial to develop high performance WOLEDs in this systems. To further clarify the reasons of difference in EL performances between devices W-S1 and W-S2, we have investigated the effects of HOMO/LUMO energy levels and different dopants in exciplex host on charge carrier transport. It can be seen from the energy level diagrams of devices W-S1 and W-S2 in Figure S9b and S9c that the electrons can be directly injected from the LUMO energy level of PO-T2T into the EMLs without any energy barrier, whereas the holes can be injected into the EMLs from the HOMOs of TAPC, TCTA and then 4P-NPB for device W-S1 or mCBP for device W-S2. Very clearly, the difference between devices W-S1 and W-S2 lies in the hole injection in EMLs. In device W-S1, a portion of the injected holes on 4P-NPB recombine with the injected electrons on the LUMO of PO-T2T to form excitons for yellow emission, while the some are injected to the HOMO of mCBP by overcoming energy barrier of 0.37 eV, and finally recombine with the 10


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electrons on PO-T2T for blue emission. As we see, the holes can be transported in each EML without any blocking in device W-S2. As a result, device W-S2 avoids the exciton degradation caused by the hole aggregation like in device W-S1. Moreover, the formed excitons can be well confined in EMLs due to the high T1 of mCBP and PO-T2T in device W-S2, whereas the low T1 of 4P-NPB may lead to excitons loss in device W-S1. Additionally, the current density-voltage characteristics of singlecarrier devices are shown in Figure S11, the yellow phosphor (Ir(tptpy)2acac) has no effect on electron transport, but plays a strong hole-trapping role in 4P-NPB: PO-T2T host. This will cause more hole accumulation at the interface of EMLs, thus severer exciton quenching in device W-S1. Therefore, device W-S2 emits high efficiency and low efficiency roll-off. 3.4 EL Performances of WOLEDs As mentioned, the exciplexes mCBP: PO-T2T and 4P-NPB: PO-T2T are respectively suitable to be as hosts for different-color phosphors. To fabricate high performance WOLEDs, the blue-orange two-color WOLEDs (W1, W2, W2-1 and W2-2) were first fabricated by adjusting the orange EML (O-EML). The device structures are ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ mCBP (5 nm)/ mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm)/ orange EML / PO-T2T (45 nm)/ LiF (1 nm)/ Al (150 nm), while the blue EML (B-EML) and other device parameters remain the same. The structures of O-EML and EL performances are shown in Figure S12a, b, and 3, all the devices realize low turnon voltages of 2.4, 2.1, 2.2, and 2.3 V, respectively, due to the “barrier-free” architecture for charge carriers injected into EMLs. The maximum PE, CE, and EQE of device W1 with mCBP: PO-T2T as host of YDD01 reach 70.0 lm W−1, 55.1 cd A−1, and 20.4%, respectively. And the correlated color temperature (CCT) is 3166 K and the CIE coordinates is (0.438, 0.430) at 1000 cd m–2 (Figure S13a). However, replacing mCBP: PO-T2T by 4P-NPB: PO-T2T as the host of YDD01, the device W2 11



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shows the maximum forward-viewing efficiencies of 85.0 lm W−1, 60.4 cd A−1, and 21.9%, higher than those of device W1, but its EL spectra emits less blue light due to the efficient energy transfer from B-EML to O-EML (Figure S13b). To increase the blue emission, on the one hand, we reduced the thickness of O-EML to fabricate the white device W2-1. On the other hand, the 2 nm thick buffer layer of mCBP: PO-T2T (1:9) was introduced into the interface between B-EML and O-EML to fabricate the white device W2-2. As shown in Figure 3, S13 and Table 2, devices W2-1 and W2-2 emit good white light, and exhibit excellent maximum forward-viewing PEs and EQEs of 80.9/75.3 lm W−1 and 21.8/21.5% without using light out-coupling technique. More significantly, they still keep 72.5/67.1 lm W−1 and 21.6/21.5% at 100 cd m−2, 49.8/48.7 lm W−1 and 19.8/21.0% at 1000 cd m−2, respectively, exhibiting a remarkable improvement in efficiency roll-off. The results reveal that the concept of our design is quite feasible. To further enhance the CRI and meet the requirement of healthy lighting source, three-color WOLEDs (devices W3 and W4) have been constructed with the EML structures of mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ mCBP: PO-T2T: Ir(ppy)2acac (1:1:1%, 3 nm)/ 4P-NPB: PO-T2T: RD071 (1:1:2%, 3 nm) for device W3 and mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:2%, 4 nm)/ 4P-NPB: PO-T2T: RD071 (1:1:2%, 3 nm) for device W4 (Figure S14a). The EL performances are shown in Figure 4, S15 and Table 2, devices W3 and W4 with low turn-on voltages of 2.3 V realize the PEs, CEs, and EQEs of 50.2/65.3 lm W−1, 45.1/51.5 cd A−1, and 23.3/23.5%, respectively. Compared with device W3 (A CIE coordinates of (0.439, 0.448), and a CRI of 78), device W4 exhibits a good warm white light emission with a high CRI of 82, CCT of 2788 K, and CIE coordinates of (0.456, 0.415) at 1000 cd m−2. Furthermore, device W4 still remains 52.8 lm W−1 and 22.8% at 100 cd m−2, and 36.5 lm W−1 and 21.6% at 1000 cd m−2, which are higher than 12


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those of device W3. To illustrate the feasibility of the above EML combination by matching exciplex hosts for different phosphors, as shown in Figure S14, the white devices W4-1 and W4-2 have been fabricated. Here, the EMLs are mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ 4P-NPB: PO-T2T: Ir(tptpy)2acac (1:1:4%, 3 nm)/ 4P-NPB: PO-T2T: RD071 (1:1:2%, 3 nm) for device W4-1, and mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:1%, 3 nm)/ mCBP: PO-T2T: RD071 (1:1:2%, 5 nm) for device W4-2. At the high luminance of 1000 cd m−2, the PEs and EQEs of devices W4-1 and W4-2 are 26.2/32.6 lm W‒1 and 21.2/20.8%, respectively. Though the EL spectra of device W42 also shows a warm white light with a high CRI of 85 and CIE coordinates of (0.465, 0.416), their efficiencies are clearly lower than those of device W4, proving the importance in the selection of the matched exciplex hosts for the realization of high-quality WOLEDs. Furthermore, the blue-green-yellow-red four-color WOLED (device W5) with the EML of mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ mCBP: PO-T2T: Ir(ppy)2acac (1:1:1%, 2 nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:2%, 4 nm)/ 4P-NPB: PO-T2T: RD071 (1:1:2%, 3 nm) was fabricated. As shown in Figure 5 and S15c, device W5 shows a turn-on voltage of 2.3 V and the high maximum forwardviewing efficiencies of 79 lm W−1, 61.5 cd A−1, and 22.5%. As illumination sources are typically characterized by their total emitted power, the corresponding total PE and EQE of device W5 are predicted to be around 134.3‒158.0 lm W−1 and 38.3%‒45.0%. Significantly, the high efficiencies of 48.0 lm W−1 (21.4%) and 33.0 lm W−1 (19.5%) are obtained at 1000 and 5000 cd m−2 with operating voltages of 3.7 and 4.7 V, respectively, showing remarkably reduced efficiency roll-off. As we see, device W5 shows a CRI as high as 81, and its spectra have a mild shift from (0.449, 0.478) to (0.399, 0.432) as the luminance increases from 1000 to 20000 cd m−2, which should be ascribed to the 13



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variation of exciton recombination zone due to the rising electron mobility of PO-T2T with increasing voltage. On the basis of our design strategy, the selection of different electron acceptors to construct excellent exciplex hosts would be helpful to stabilize the exciton distribution, thus further improve the spectral stability. 3.5 Working Principle of WOLEDs To demonstrate the essential difference in efficiency improvement by multiple exciplex hosts, the working mechanism of two-color and three-color WOLEDs are further demonstrated. According to the previously reported work, the host-guest energy transfer process is the main emission mechanism for the blue emission in FIrpic doped mCBP: PO-T2T.36 The current density‒voltage characteristics of the single carrier devices based on different phosphors doped mCBP: PO-T2T and 4P-NPB: PO-T2T hosts are shown in Figure S11 and S16. The current densities have a significant shift down in single-hole devices, but a slight change in single-electron devices after doping RD071 and Ir(tptpy)2acac in 4P-NPB: PO-T2T, illustrating that RD071 and Ir(tptpy)2acac are served as deep hole trapping sites, thus blocking the holes transport, but they do not affect the transport of electrons. RD071 and Ir(tptpy)2acac in mCBP: PO-T2T have the same effect on the charge carrier transport as that in 4P-NPB: PO-T2T, which also have been confirmed.33 When doping orange phosphor (YDD01), there is no influence on the electron and hole transport in 4P-NPB: PO-T2T, but a negligible effect on the electron transport as well as strong hole-trapping effect in mCBP: PO-T2T. As shown in Figure S17, there are the large overlaps between the absorption spectra of blue/green/yellow/orange/red phosphors and the PL spectrum of mCBP: PO-T2T exciplex, and between the PL spectrum of 4P-NPB: PO-T2T exciplex and the absorption spectra of orange/red 14


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phosphors, ensuring the enough Förster energy transfer from exciplexes to phosphors.37-38 The transient EL decay characteristics of two-color and three-color WOLEDs were also performed to investigate the emission mechanisms of phosphors. As shown in Figure 6 and S18, there are no transient overshoots or spikes after turning off the voltage pulse, indicating that the charge carrier accumulation or trapping in phosphors can be negligible.24, 39 This means that the energy transfer processes are the dominant emission mechanism of phosphors in our WOLEDs. The transient EL decay properties of two-color WOLEDs with different EMLs at the wavelengths of blue (472 nm) and orange (580 nm) are shown in Figure 6a and 6b. Compared to device E1 with the EML of mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm), the decay lifetime in device E2 (with the same EML as device W1) at wavelength of 472 nm is slightly increased, whereas the lifetime at 472 nm in device E3 is clearly reduced, illustrating that there is not the energy transfer from FIrpic to YDD01 in device W1 (E1), but the direct energy transfer process from FIrpic to 4P-NPB or 4P-NPB: PO-T2T exciplex exists in device E3. It is interesting to note that device E4 (with the same EML as W2) has the same transient decay property as device E3, which indicates that the direct energy transfer process from FIrpic to YDD01 does not exist in device W2 (E4). As shown in the transient EL decay properties of three-color WOLEDs recorded at wavelengths of blue (472 nm), yellow (560 nm), and red (612 nm) (Figure 6c, 6d, and S18), the decay lifetimes observed at 472 nm do not show much variation in devices E5-8 due to the long distance. However, the decay lifetime of the yellow emission in device E8 (with the same EML as W4) is shorter than that in devices E6 and E7. This means that there exists energy transfer from yellow EML to red EML in device W4 (E8). At the wavelength of 580 and 612 nm, the shorter decay lifetime in WOLEDs with multiple-exciplex hosts than that with single-exciplex host could be helpful to decrease the local triplet exciton density and improve the 15



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efficiency roll-off (Figure 6b and S18). Therefore, as shown in Figure S19, the main EL processes of WOLEDs based on mCBP: PO-T2T exciplex host can be described as follows: the holes and electrons recombine in mCBP: PO-T2T host to form excitons, then the excitons are, respectively, transferred to different phosphors, and finally realizing the white light emission. All the EMLs do not affect each other (Figure S19a). In the WOLEDs with multiple-exciplex hosts, the excitons can be formed in both mCBP: PO-T2T and 4PNPB: PO-T2T hosts (Figure S19b), then the excitons in mCBP: PO-T2T host are transferred to FIrpic, resulting in the blue light emission. The orange emission in device W2 comes from the direct energy transfer from 4P-NPB: PO-T2T host to YDD01 and the indirect energy transfer from B-EML to 4PNPB or from 4P-NPB: PO-T2T to YDD01. In device W4, the yellow emission then originates from the energy transfer from mCBP: PO-T2T host. And the energy transfer from yellow EML and 4PNPB: PO-T2T host to red phosphor leads to the red light emission. 4. CONCLUSIONS In summary, the low driving voltage, high efficiency, high CRI and low efficiency roll-off WOLEDs have been successfully fabricated by the utilization of the matched multiple-exciplex hosts for different phosphors and the reasonable arrangement of the EMLs. It can be seen that the resulting WOLEDs show not only the low driving voltage and high CRI, but also the high efficiency and low efficiency roll-off. This work offers a new route to develop high quality WOLEDs with good comprehensive performance at high luminance.

16


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Figure 1. (a) Molecular structures of mCBP, 4P-NPB and PO-T2T. (b) HOMO/LUMO energy level diagram of the used organic materials.

17



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Figure 2. (a) EQE via luminance characteristics of the resulting orange phosphorescent OLEDs (O1 and O2) based on mCBP: PO-T2T and 4P-NPB: PO-T2T excipexes, respectively, as hosts. Inset: EL spectra of devices O1 and O2 at the voltage of 4 V. (b) EQE via luminance characteristics of the resulting red phosphorescent OLEDs (R1 and R2) based on mCBP: PO-T2T and 4P-NPB: PO-T2T excipexes, respectively, as hosts. Inset: EL spectra of devices R1 and R2 at the voltage of 4 V.

18


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Figure 3. (a) Power efficiency‒current efficiency‒luminance and (b) EQE‒luminance characteristics of the resulting two-color WOLEDs.

19



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Figure 4. Electroluminescence characteristics of the resulting three-color WOLEDs (devices W3 and W4). (a) PE, CE and EQE‒luminance characteristics of device W3. (b) EL spectra of device W3 at the luminance from 1000 to 20000 cd m‒2. (c) PE, CE and EQE‒luminance characteristics of device W4. (d) EL spectra of device W4 at the luminance from 1000 to 20000 cd m‒2.

20


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Figure 5. (a) PE, CE and EQE‒luminance characteristics of device W5. (b) EL spectra of device W5 at the luminance from 1000 to 20000 cd m‒2.

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Figure 6. Transient EL decay curves of the fabricated OLEDs (E1-8) with different EMLs. (a), (c) Transient EL decay curves of OLEDs measured at 472 nm. (b) Transient EL decay curves of device E1 and E2 measured at 580 nm. (d) Transient EL decay curves of device E6, E7, and E8 measured at 560 nm. The device structures are ITO/ HAT-CN (15 nm)/ TAPC (60 nm)/ TCTA (5 nm)/ mCBP (5 nm)/ EMLs / PO-T2T (45 nm)/LiF (1 nm)/ Al (150 nm). The EML of E1: mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm); The EML of E2: mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm)/ mCBP: PO-T2T: YDD01 (1:1:2%, 6 nm); The EML of E3: mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm)/ 4P-NPB: POT2T (1:1, 6 nm); The EML of E4: mCBP: PO-T2T: FIrpic (1:1:10%, 12 nm)/ 4P-NPB: PO-T2T: YDD01 (1:1:2%, 6 nm); The EML of E5: mCBP: PO-T2T: FIrpic (1:1:10%, 5nm); The EML of E6: mCBP: PO-T2T: FIrpic (1:1:10%, 5nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:2%, 3 nm); The EML of E7: mCBP: PO-T2T: FIrpic (1:1:10%, 5nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:2%, 3 nm)/ 22


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mCBP: PO-T2T: RD071 (1:1:2%, 5 nm); The EML of E8: mCBP: PO-T2T: FIrpic (1:1:10%, 5 nm)/ mCBP: PO-T2T: Ir(tptpy)2acac (1:1:2%, 4 nm)/ 4P-NPB: PO-T2T: RD071 (1:1:2%, 3 nm).

23



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Table 1. Summary of the key EL parameters for monochromatic OLEDs. Device

a

Vona

CEb max/1000

PEb max/1000

EQEb max/1000

CIEc

[cd A-1]

[lm W-1]

[%]

(x, y)

[V]

B1

2.5

50.3/48.4

59.9/39.0

25.2/24.3

(0.147, 0.328)

B2

2.2

1.4/0.7

2.0/0.4

0.6/0.2

(0.414, 0.521)

G1

2.5

72.1/68.3

86.6/51.0

20.5/19.4

(0.311, 0.624)

G2

2.2

3.6/1.0

5.2/0.6

1.3/0.3

(0.416, 0.543)

Y1

2.5

69.4/65.4

83.6/49.8

20.6/19.4

(0.456, 0.536)

Y2

2.0

52.5/46.5

77.8/52.2

16.6/14.7

(0.491, 0.504)

O1

2.5

60.5/57.4

73.1/42.9

22.3/20.7

(0.524, 0.469)

O2

2.0

61.0/57.8

90.4/56.7

22.3/20.9

(0.528, 0.468)

R1

2.7

28.0/24.7

31.3/12.1

19.9/17.1

(0.641, 0.356)

R2

2.0

25.5/22.9

40.1/18.5

23.7/20.5

(0.663, 0.336)

At the luminance of 1 cd m-2;

b

The maximum efficiencies and values taken at 1000 cd m-2;

Measured at the voltage of 4 V.

24


c

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Table 2. Summary of key EL parameters of the resulting WOLEDs. Von a

CE max/100/1000 b

PE max/100/1000 b

EQE max/100/1000 b

CCT c CIE(x, y) c

Device [V]

a

c

[cd

A-1]

[lm

W-1]

[%]

[K]

W1

2.4

55.1/53.1/48.0

71.0/55.5/39.7

20.4/20.0/18.5

3166

(0.438, 0.430)

W2

2.1

60.4/59.8/52.1

85.0/72.2/49.1

21.9/21.4/18.9

2232

(0.527, 0.467)

W2-1

2.2

62.3/61.9/53.8

80.9/72.5/49.8

21.8/21.6/19.8

3003

(0.454, 0.442)

W2-2

2.3

59.8/59.7/55.8

75.3/67.1/48.7

21.5/21.5/21.0

3223

(0.441, 0.445)

W3

2.3

45.1/44.8/43.9

50.2/46.9/35.0

23.3/22.8/21.5

3283

(0.439, 0.448)

W4

2.3

51.5/50.4/46.3

65.3/52.8/36.5

23.5/22.8/21.6

2788

(0.456, 0.415)

W4-1

2.2

34.4/34.4/33.4

45.8/37.0/26.2

24.1/23.0/21.2

-

(0.599, 0.384)

W4-2

2.3

50.6/48.0/43.5

66.0/47.1/32.5

22.4/21.9/20.8

2690

(0.465, 0.416)

W5

2.3

61.5/60.5/55.0

79.0/66.8/48.0

22.8/22.5/21.4

3431

(0.438, 0.468)

At a luminance of 1 cd m-2; b The maximum efficiencies and values taken at 100 and 1000 cd m-2;

Measured at 1000 cd m-2.

25



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available: UV–vis absorption and photoluminescence (PL) spectra of films. Chemical structures of the organic materials used in this study. EL performances of the monochromatic based on mCBP: PO-T2T and 4P-NPB: PO-T2T hosts. EL performances and proposed energy level diagrams of two-color (blue and yellow) WOLEDs. Current density–voltage characteristics of hole-only and electron-only devices based on mCBP: PO-T2T and 4P-NPB: POT2T hosts. Device and EML configurations and current density–luminance–voltage characteristics of two-color (blue and orange) WOLEDs. EL spectra of the resulting two-color (blue and orange) WOLEDs. EL performances of three-color and four-color WOLEDs. UV-vis absorption spectra of phosphors and PL spectra of mCBP: PO-T2T and 4P-NPB: PO-T2T films. Transient EL decay curves of the fabricated devices at the wavelength of 612 nm. Schematic illustration diagrams of the emission mechanism in WOLEDs based on single-exciplex and multiple-exciplex as hosts.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Shian Ying: 0000-0003-0781-240X Xianfeng Qiao: 0000-0001-8633-8771 Dongge Ma: 0000-0002-5371-8759 Author Contributions 26


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The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interest. Acknowledgment The authors gratefully thank the National Key Research and Development Plan of China (Grant No. 2016YFB0400701), the National Natural Science Foundation of China (Grant Nos. 91833304, 51527804, 21788102, 51673067, 11661131001) and Guangzhou science & technology plan project (No. 201707020040) for the support of this research. REFERENCE (1) Wang, Q.; Ma, D. Management of Charges and Excitons for High-Performance White Organic Light-Emitting Diodes. Chem. Soc. Rev. 2010, 39, 2387–2398. (2) Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes: Status and Perspective. Rev. Mod. Phys. 2013, 85, 1245–1293. (3) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. Recent Advances in White Organic LightEmitting Materials and Devices (WOLEDs). Adv. Mater. 2010, 22, 572–582. (4) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Management of Singlet and Triplet Excitons for Efficient White Organic Light-Emitting Devices. Nature 2006, 440, 908–912. (5) Zhao, F.; Ma, D. Approaches to High Performance White Organic Light-Emitting Diodes for General Lighting. Mater. Chem. Front. 2017, 1, 1933–1950. (6) Tao, Y.; Yang, C.; Qin, J. Organic Host Materials for Phosphorescent Organic Light-Emitting 27



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Superior Efficiency and Low-Efficiency Roll-Off White Organic Light

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