High-Performance Doping-Free Hybrid White OLEDs Based on Blue

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High-Performance Doping-Free Hybrid White OLEDs Based on Blue Aggregation-Induced Emission Luminogens Baiquan Liu, Han Nie, Gengwei Lin, Shiben Hu, Dongyu Gao, Jianhua Zou, Miao Xu, Lei Wang, Zujin Zhao, Honglong Ning, Junbiao Peng, Yong Cao, and Ben Zhong Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11422 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

High-Performance Doping-Free Hybrid White OLEDs Based on Blue Aggregation-Induced Emission Luminogens Baiquan Liu,† Han Nie,† Gengwei Lin,† Shiben Hu,† Dongyu Gao,‡ Jianhua Zou,*† Miao Xu,† Lei Wang,† Zujin Zhao,*† Honglong Ning,† Junbiao Peng,*† Yong Cao,† and Ben Zhong Tang†§ †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡

§

New Vision Opto-Electronic Technology Co., Ltd, Guangzhou 510530, China

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research

Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

ABSTRACT: Doping-free white organic light-emitting diodes (DF-WOLEDs) have aroused research interest for simple properties. However, to achieve doping-free hybrid WOLEDs (DFH-WOLEDs), avoiding aggregation-caused quenching is challenging. Herein, blue luminogens with aggregation-induced emission (AIE) characteristics, for the first time, have been demonstrated to develop DFH-WOLEDs. Unlike previous DFH-WOLEDs, both thin (10 nm) AIE luminogen (AIEgen) can be used for devices, enhancing the flexibility. Two-color devices show i) pure-white emission, ii) high CRI (85), iii) high efficiency. Particularly, 19.0 lm W‒1 is the highest for pure-white DF-WOLEDs, while 35.0 lm W‒1 is the best for two-color hybrid WOLEDs with CRI ≥80. A three-color DFH-WOLED shows broad color correlated temperature span (2301‒11628 K), i) the first sunlight-like OLED (2500‒8000 K) operating at low voltages, ii) the broadest span among sunlight-like OLED, iii) possessing comparable efficiency with best doping counterpart. Another three1 ACS Paragon Plus Environment


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color DFH-WOLED exhibits CRI >90 at ≥3000 cd m‒2, i) the first DF-WOLED with CRI ≥90 at high luminances, ii) the CRI (92.8) is not only the highest among AIE-based WOLEDs but the highest among DF-WOLEDs. Such findings may unlock an alternative concept to develop DFH-WOLEDs.

Keywords: organic light-emitting diodes, aggregation-induced emission, doping-free, sunlight, color rendering index.

1. Introduction White organic light-emitting diodes (WOLEDs) have been extensively investigated for lighting and displays owing to their splendid characteristics, such as high efficiency, low power consumption and flexibility.1-5 To enhance performance, the doping technique is usually adopted (e.g. doping emitting layers (EMLs) and p-doping/n-doping charge transporting layers). Although high performance can be achieved, several disadvantages of the doping technique remain unsettled. For instance, i) the required precise dopant concentration and codeposition rate are not easy to control (i.e., 0.1% or even lower) in device fabrication,6 causing the problem of reproducibility; ii) the hosts have to be carefully selected for differentcolor guests, since it is difficult to obtain universal hosts (i.e., three hosts may be required for three-color WOLEDs); iii) the influences of adjacent functional layers should be taken into consideration (i.e., the triplet energy levels (T1) of functional layers should be higher than those of hosts in phosphorescent and thermally activated delayed fluorescent (TADF) OLEDs, otherwise triplets will be quenched, decreasing the efficiency); iv) the cost will be increased due to the utilization of hosts. To overcome the problems, the introduction of doping-free technology is believed to be conducive, since this technology may minimize fabrication processes, eliminate hosts, predigest structures and reduce the cost.7-11 In fact, both doping-free monochromatic and 2 ACS Paragon Plus Environment

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white OLEDs have been actively studied. For example, doping-free red OLEDs with Ptcomplexes have been established by Gnade and Omary’s group, showing the external quantum efficiency (EQE) of >20%.12,13 Doping-free green OLEDs with TADF emitters have been organized by Adachi’s group, exhibiting the EQE of 18%.14 Doping-free yellow OLEDs with TADF exciplexes have been built by Su’s group, yielding the EQE of 12%.15 To satisfy the demand of RGBW-TV and the next-generation solid state lighting, some doping-free WOLEDs (DF-WOLEDs) have also been reported. For instance, Gnade and Omary’s group used a metal-organic Pt(II)-pyridylazolate phosphor to fabricate doping-free phosphorescent WOLEDs, acquiring a peak forward-viewing power efficiency (PE) of 49.5 lm W‒1.16 Hence, it can be summarized that the doping-free technology is of great potential in developing highperformance OLEDs. However, despite some DF-WOLEDs have been made, there are a large number of issues needing to be solved. On one hand, the efficiency of doping-free fluorescent WOLEDs is usually low. On the other hand, although the efficiency of doping-free phosphorescent WOLEDs can be high,16 the neat phosphorescent layers are as thick as ~100 nm, indicating that the cost is expensive due to the great use of noble metals, which is even more expensive than the doping WOLEDs. Besides, since no stable blue phosphor is available until now, the lifetime of doping-free phosphorescent WOLEDs is poor. Hence, this strategy may be not feasible for the real commercialization. Alternatively, doping-free hybrid WOLEDs (DFHWOLEDs) are more promising owing to the harvest of triplets and utilization of blue fluorophors.17,18 For hybrid WOLEDs, the key issue is the management of blue fluorophors.17,18 In fact, several state-of-the-art concepts have been proposed to manage blue fluorophors in doping hybrid WOLEDs, such as separating blue fluorophors from phosphors via spacers (22.1 lm W‒1),19 harvesting the triplets of blue fluorophor via the diffusion mechanism (33.9 lm W‒1 at 100 cd m‒2),20,21 using blue fluorophors with high T1 to develop single-EML structure (39.5 lm W‒1),22,23 introducing mixed-hosts with bipolar transport to 3 ACS Paragon Plus Environment


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manipulate the blue fluorophor (41.7 lm W‒1),24 utilizing blue TADF fluorophor to prohibit the external heavy-atom effect (50.2 lm W‒1)25 and exploiting double blue fluorophors to possess multifunctional roles (52.5 lm W‒1 at 100 cd m‒2).26 However, only negligible attention has been paid to the DFH-WOLEDs, leading to the fact that their performance is urgently required to be enhanced. To develop high-performance DFH-WOLEDs, how to avoid the aggregation-caused quenching (ACQ) problem is challenging, particularly for the blue fluorophors. This is because the host, which is used to avoid the ACQ problem in doping devices, is averted in doping-free devices. As a result, when conventional blue fluorophors are used in DFHWOLEDs, the reported concept to avoid the ACQ problem is the use of ultrathin blue EMLs (90 at ≥3000 cd m‒2, i) the first DF-WOLED with CRI ≥90 at high luminances, ii) the CRI (92.8) is not only the highest among AIE-based WOLEDs but the highest among DFWOLEDs. Such findings may unlock an alternative concept to develop DFH-WOLEDs. 2. Results and discussions 2.1 Architectures of DFH-WOLEDs based on blue AIEgens As shown in Figure 1, the architecture of the p-type interlayer based two-color devices is indium tin oxide (ITO)/ 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN, 100 nm)/ N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine (NPB, 15 nm)/ 1-bis[4-[N,N-di(4tolyl)amino]phenyl]-cyclohexane (TAPC, 5 nm)/ is bis(2-phenyl-4,5-dimethylpyridinato)[2(biphenyl-3-yl)pyridinato] iridium(III) (Ir(dmppy)2(dpp), 0.9 nm, yellow emitter)/TAPC (3.5 nm)/ 9,10-bis[4-(1,2,2-triphenylvinyl)phenyl]anthracene (BTPEAn, A nm)/ 1,3,5-tri(m-pyrid3-yl-phenyl)benzene (TmPyPB, 35 nm)/Cs2CO3 (1 nm)/Al (160 nm), where BTPEAn is a blue AIEgen (blue emitter), and A = 0.4, 1, 5, 10 and 15 for device W11, W12, W13, W14 and W15, respectively. For the n-type interlayer based two-color WOLEDs and three-color WOLEDs, their EMLs are different while other layers are the same as those of two-color devices (the detailed structures will be shown later).

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Figure 1. The schematic configurations of two-color and three-color DFH-WOLEDs, and the molecular structure of emitters. 2.2 Device strategies For the high performance, some device design strategies have been adopted. Firstly, since BTPEAn is a very efficient deep-blue AIEgens (~449 nm), it has been selected as the blue emitter to guarantee the high efficiency.39 Due to the AIE characteristic of BTPEAn, i) no host is required for this emitter, ii) the blue EML can be varied from thin (i.e., 10 nm). Then, different from the representative DF-WOLED requiring ~100 nm neat phosphorescent layers,16 we have used doping-free thin phosphorescent EMLs, which can vastly reduce the cost owing to the reduction of noble metals. Hence, by combining BTPEAn with the complementary phorsphors Ir(dmppy)2(dpp) (~550 nm), two-color white emissions can be expected.40 Besides, by inserting interlayers (i.e., TAPC or TmPyPB) between fluorophors and phosphors, the mutual exciton quenching is prohibited.19


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Next, HAT-CN has been used as the hole injection layer owing to the superior hole injection ability,41 while NPB/TAPC can function as the hole transport layer (HTL)/electron blocking layer due to i) the high hole mobility of these two materials (10‒4 and 10‒2 cm2/V s for NPB and TAPC, respectively), ii) high lowest unoccupied molecular orbital (LUMO) of TAPC (1.8 eV).17 Besides, since TAPC is the interlayer, the number of evaporation source in the fabrication process can be reduced. On the other hand, TmPyPB can function as the electron transport layer (ETL) and hole blocking layer owing to i) the high electron mobility of 10‒3 cm2/V s and suitable LUMO of 2.7 eV, ii) deep highest occupied molecular orbital (HOMO) of 6.7 eV.42 Hence, an efficient charge transport/exciton blocking architecture has been realized, which can improve device performance.5 Finally, since the effect of thick film of the blue emitter in DFH-WOLEDs is not explored, we have comprehensively studied their influence, which is expected to gain deep insight into DFH-WOLEDs. Besides, to demonstrate the flexibility of this kind of DFH-WOLEDs, both thin and thick films of the blue AIEgen (BTPEAn) are used for DFH-WOLEDs. 2.3 Two-color DFH-WOLEDs based on blue AIEgens 2.3.1 Performance of two-color DFH-WOLEDs Based on above strategies, high-performance DFH-WOLEDs based on the blue AIE BTPEAn can be expected. Figure 2 inset shows that the devices (W11, W12, W13, W14 and W15) exhibit white light, indicating the possibility of DFH-WOLEDs based on BTPEAn.Besides, because of balanced intensities (yellow and blue), the Commission International de I’Eclairage (CIE) coordinates of W11 and W12 are (0.34, 0.35) and (0.32, 0.32), respectively, near white equivalent point (0.33, 0.33). As hybrid WOLEDs usually exhibit yellowish/warm white colors and the pure-white color is rarely documented [i.e., (0.40, 0.41) for Forrest’s device,19 (0.44, 0.47) and (0.45, 0.43) for Leo’s device,20,21 (0.46, 0.44) and (0.38, 0.45) for Zhang and Lee’s device,22,23 (0.43, 0.43) for Ma’s device,24 (0.42, 0.48) for 7 ACS Paragon Plus Environment


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Duan’s device,25 (0.43, 0.48) for Liu’s device26]. Therefore, the present finding may open a new avenue to develop pure-white hybrid WOLEDs, which is extremely desired for display applications. Remarkably, the maximum CRIs of W11, W12, W13, W14 and W15 are as high as 85, 84, 83, 82 and 82, respectively (Supporting Information, Figure S1), which is even higher than that of many representative doping three-color hybrid WOLEDs.23,24,43-45 Despite it is challenging to achieve two-color WOLEDs with high CRI (≥80) currently, here, we have demonstrated that WOLEDs with high CRIs can be realized via such simple manner. Besides, the phenomenon that the color of devices are blue-shifted can be attributed to the fact that more excitons are recombined in the blue region with the increasing thickness of BTPEAn. As shown in Figure 2 and Table 1, the maximum forward-viewing current efficiency (CE) of W11, W12, W13, W14 and W15 are 9.3, 10.0, 15.0, 18.1 and 18.4 cd A‒1, respectively, and maximum forward-viewing PE of W11, W12, W13, W14 and W15 are 10.5, 11.2, 16.8, 20.3 and 20.6 lm W‒1, respectively (at >1 cd m-2). As illumination sources are generally characterized by their total emitted power,19-23 the maximum total PE of W11, W12, W13, W14 and W15 are 17.9, 19.0, 28.6, 34.5 and 35.0 lm W‒1, respectively. Particularly, the PE of W12 becomes the best for DF-WOLEDs having so pure-white colors while that of W15 becomes best for two-color hybrid WOLEDs possessing CRI ≥80. Besides, unlike the DFHWOLEDs with conventional fluorophors in which blue fluorophors should be fabricated as very thin films to avoid the ACQ problem,27 the film of BTPEAn in DFH-WOLEDs can be modulated from thin (i.e., 10 nm), rendering good flexibility of this novel kind of DFH-WOLEDs. Therefore, regardless of thin or thick films of BTPEAn, all of them can be used to develop DFH-WOLEDs, which is unlike previous DFH-WOLEDs.27


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Figure 2. Forward-viewing efficiencies of W11, W12, W13, W14 and W15. Inset: electroluminescent (EL) spectra at 1000 cd m-2. Table 1. Performances of optimized devices. Vona

PEmax b

CEmax c

(V)

(lm W-1)

(cd A-1)

W11

2.65

17.9

15.8

85

(0.34, 0.35)

W12

2.65

19.0

17.0

84

(0.32, 0.32)

W13

2.65

28.6

25.5

83

(0.28, 0.29)

W14

2.70

34.5

30.8

82

(0.26, 0.28)

W15

2.70

35.0

31.3

82

(0.25, 0.27)

W52

2.80

10.5

10.1

58

(0.35, 0.26)f

W7

2.70

14.5

11.8

92.8

(0.48, 0.37)

WOLEDs

a

CRId

CIEe

The turn-on voltage (>1 cd m-2). b Maximum total PE. c Maximum total CE. d Maximum CRI. e

CIE coordinates at 1000 cd m-2. f Measured at 100 cd m-2.

2.3.2 Insight of the high performance 9 ACS Paragon Plus Environment


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In order to further comprehend DFH-WOLEDs based on AIEgens, the origin of the high performance has been unveiled. Previously, the effect of thick blue EML is difficult to be explored in DFH-WOLED with conventional fluorophors due to the ACQ. However, this effect can be studied in DFH-WOLED with the AIEgen. Hence, since the thickness of AIEgens can be flexible together with the fact that W14 shows almost similar efficiency to W15, we select 0.4 and 10 nm as the thin and thick AIEgens for the following devices, respectively. I. The effect of charge transport layers For DF-WOLEDs, charges and excitons are more easily escaped from the EMLs than doping WOLEDs since no host is used, particularly for devices with thin EMLs, which can deteriorate the device performance. Since the effect of charge transport layers, which is significant to the performance, on DF-WOLEDs has been usually overlooked, we have studied their influence before further enhancing the performance of this kind of DFHWOLEDs. In the case of charge transport layers, charge transport ability or charge mobility, charge confining ability, triplets confining ability coupled with energy gap have critical influences on phorsphor based devices.5,46 First, to investigate the effect of the HTL, we have replaced the 5 nm TAPC HTL with 5 nm NPB to fabricate two devices with thin (0.4 nm, device W21) or thick (10 nm, device W22) EMLs of BTPEAn, while the thickness of other layers are same as that of W11. Similar to above phenomena, the efficiency of W22 with thick BTPEAn is higher than that of W22 with thin BTPEAn (Figure 3a). However, due to the single NPB HTL, the maximum forward-viewing PE of W21 and W22 are only 4.7 and 8.7 lm W‒1, respectively. Regardless of the device with thin or thick BTPEAn films, the efficiencies are dramatically decreased (>2 times) upon using only NPB as the HTL, indicating that the 5 nm TAPC HTL plays a vital role on the performance because only this HTL has been changed. As shown in Figure 3b, the LUMO of TAPC (1.8 eV) is much higher than that of NPB (2.4 eV), indicating that TAPC 10 ACS Paragon Plus Environment

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has a stronger electron confining ability than NPB. Besides, TAPC has a much higher T1 (2.87 eV) than the adjacent yellow phorsphor (~2.2 eV) while the T1 of NPB (2.3 eV) is almost similar to that of Ir(dmppy)2(dpp), indicating that TAPC has a stronger confining ability for triplets.5 In other words, both electrons and triplets are more difficult to escape from the EML in TAPC based WOLEDs than the NPB based WOLEDs. Therefore, DFHWOLEDs with the TAPC HTL show much higher efficiencies than DFH-WOLEDs with single NPB HTL, although DFH-WOLEDs with single NPB HTL have a better hole transport ability since they have higher current densities which may be attributed to the increased heterojunctions in TAPC based devices and the large energy gap of TAPC (Supporting Information, Figure S2). Then, to explore the effect of the ETL, we have replaced 35 nm TmPyPB with 35 nm Bepp2 to fabricate two devices with thin (0.4 nm, device W23) or thick (10 nm, device W24) EMLs of BTPEAn, while the thickness of other layers are same as that of W11. As shown in Figure 3a, the maximum forward-viewing PE of W23and W24 are 9.2 and 16.6 lm W‒1, respectively. Hence, the efficiencies are decreased upon using Bepp2, indicating that the ETL has a great influence on DFH-WOLEDs. As shown in Figure 3b, the HOMO of TmPyPB (6.7 eV) is higher than that of Bepp2 (5.7 eV), indicating that TmPyPB has a stronger electron confining ability than Bepp2. In fact, due to the almost same HOMOs between Bepp2 and BTPEAn, it is difficult for Bepp2 to confine holes.42 Besides, TmPyPB has a higher exciton confining ability than Bepp2 since the T1 of TmPyPB (2.75 eV) is higher than that of Bepp2 (2.6 eV).6 Therefore, DFH-WOLEDs with the TmPyPB ETL show higher efficiency than DFHWOLEDs with Bepp2 ETL, although DFH-WOLEDs with Bepp2 ETL have a better electron transport ability which may be attributed to no LUMO barrier for electron injection between Bepp2 and BTPEAn as well as the large energy gap of TmPyPB (Supporting Information, Figure S2). In a word, the above facts demonstrate that the charge and exciton confining abilities of charge transport layers play a vital role on the performance of DF-WOLEDs. 11 ACS Paragon Plus Environment


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Figure 3. a) Efficiencies for devices. Inset: Device spectra (1000 cd m-2). b) Energy levels in devices as well as the molecular structures of charge transport materials. II. The influence of interlayers


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For DFH-WOLEDs with conventional fluorophors, the interlayer has been demonstrated to be significant to the performance.27 However, whether the interlayer will exert great impact on DFH-WOLEDs based on AIEgens is still not clear, particularly for DFH-WOLEDs with thick blue AIEgens because the charge transport can be easily affected when the thick EMLs are employed.5 First, to understand the influence of interlayer, we have develop two interlayer-free devices with thin or thick BTPEAn. The EMLs are Ir(dmppy)2(dpp) (0.9 nm)/BTPEAn (B nm), where other layers are the same as those of W11, B = 0.4 and 10 for device W31 and W32, respectively. Except for the interlayer, structures of W31 and W32 are similar to those of W11 and W14, respectively. As shown in Figure 4a inset, W31 possesses a very weak blue emission while W32 exhibits a white emission. Therefore, the interlayer is essential for DFHWOLEDs with thin blue EMLs to achieve balanced white emissions, similar to the previous study.27 However, despite no interlayer is adopted, DFH-WOLEDs with thick blue EMLs can achieve white emissions, unlike DFH-WOLEDs with thin blue EMLs.27 This surprising phenomenon can be explained as follows. Without TAPC interlayer, electrons are barrier-free to transport from the blue EML to yellow EML, since Ir(dmppy)2(dpp) has a lower LUMO than BTPEAn (Figure 3b). On the other hand, there is big HOMO barrier (0.57 eV) at the BTPEAn/ Ir(dmppy)2(dpp) interface, indicating that holes are relatively difficult to arrive at the blue region.5 Therefore, main exciton generation zone (MEGZ) of W32 is located at the Ir(dmppy)2(dpp)/BTPEAn interface, forming singlets and triplets with a ratio of 1: 3.19 On the other hand, holes still arrive at blue EMLs upon obtaining enough energy (i.e., at high voltages).5 To demonstrate the above analyses, we have measured the spectra of W32 at different luminances (Figure 4b). The blue intensity is increased as luminance increasing, suggesting that more holes reach blue region by overcoming the HOMO barrier at high voltages. In fact, the color of W31 is also blue-shifted with the increasing luminance, further demonstrating the analyses (Supporting Information, Figure S3). 13 ACS Paragon Plus Environment


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It is noteworthy that the above analyses about the the MEGZ are based on the assumption that BTPEAn can effectively transport electrons rather than being a p-type material, otherwise the MEGZ is located at the BTPEAn/TmPyPB interface. To demonstrate this assumption, we have fabricated electron-only devices by comparing the electron transport ability of BTPEAn with those of 4,4-N,N-dicarbazolebiphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq3). As shown in Figure 4c, the BTPEAn based device shows much higher current density than CBP and Alq3 based devices, suggesting that BTPEAn possesses a strong electrontransporting ability since the electron mobility of CBP and Alq3 are 10‒4 and 10‒7 cm2 V‒1 s‒1, respectively.17,46 Hence, electrons can easily reach the yellow EML from the blue EML due to the barrier-free LUMO and high electron mobility of BTPEAn. W31 has a peak efficiency of 6.4 lm W‒1, 4.3 times better than W32 (1.5 lm W‒1). This is different from the above phenomenon that the device with thick BTPEAn shows higher efficiency than that with thin BTPEAn, indicating that the interlayer have great influence on device performance, which can be explained as follows. Due to absence of interlayer, the efficiency of W32 is 14 times lower than that of W14, indicating that excitons are greatly quenched in W32, although white emissions can be obtained. According to Leo’s report,47 such dramatically decreased efficiency (14 times) is attributed to the low-lying triplet state of BTPEAn acting as a phosphorescence quencher. However, despite no interlayer is used in W31, the efficiency of W31 is only 1.6 times lower than that of W11, which is not so dramatically decreased as W32.This is because the thickness of the blue EML is too thin and yellow EML has 2 times thicker than blue one in W31,22 leading to inefficient quenching of triplets by BTPEAn. Combining these factors and the fact that BTPEAn possesses strong electron-transporting ability, it is reasonable that the yellow intensity is stronger than the blue intensity in W31 (Figure 4a inset), although BTPEAn is a phosphorescence quencher. However, the blue EML is 11 times thicker than yellow one in W32, indicating that most of triplets in the MEGZ can be quenched by BTPEAn, leading to the extremely low efficiency of 14 ACS Paragon Plus Environment

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W32.47 In a word, the interlayer have great influence on DFH-WOLEDs based on AIEgens, regardless of the thickness of AIEgens.



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Figure 4. a) Forward-viewing efficiencies of W31 and W32. Inset: EL spectra at 1000 cd m-2. b) EL spectra of W31 at various luminances. c) The current density of electron-only devices, where the structure is ITO/Cs2CO3 (1 nm)/Bphen (60 nm)/BTPEAn, CBP or Alq3 (20 nm)/TmPyPB (35 nm)/Cs2CO3 (1 nm)/Al (160 nm). III. The location of emitters To further understand DFH-WOLEDs based on AIEgens, we have fabricated another devices by exchanging the location of blue and yellow EMLs. The EMLs are BTPEAn (C nm)/TmPyPB (3.5 nm)/Ir(dmppy)2(dpp) (0.9 nm), where other layers are the same as those of W11, C = 0.4 and 10 for device W41 and W42, respectively. The 3.5 nm TmPyPB is used to be interlayer due to the n-type property and high T1 (2.75 eV),42 which can avoid the exciton quenching and guarantee the blue emissions. As shown in Figure 5 inset, W41 exhibits a white emission while W42 only shows a strong blue emission, indicating that the location of emitters in DFH-WOLEDs based on AIEgens should be well manipulated to achieve the white emission. This phenomenon can be explained as follows. As demonstrated above, BTPEAn is a strong electron-transporting materials, which may be unfavorable to the hole transport. Besides, there is a HOMO barrier between TAPC and BTPEAn (0.32 eV).33 Hence, 16 ACS Paragon Plus Environment

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holes become difficult to reach yellow EML when a thick BTPEAn is used in W42, leading to almost no yellow emission. However, holes can easily be transported to yellow EML in W41 due to the very thin BTPEAn, generating excitons for yellow emissions. One may expect that the n-type interlayer TmPyPB is the main factor to confine the hole transport in W42. However, a strong yellow emission can be obtained in W41, indicating that holes can pass through TmPyPB to generate excitons for yellow emissions via the tunneling effect.48 To further demonstrate the analyses, we have fabricated a device without TmPyPB interlayer (W43), where other layers are the same as those in W42. As shown in Figure S4 (Supporting Information), W43 shows a strong sole blue emission, further indicating that the hole transport in W42 is mainly confined by BTPEAn instead of TmPyPB. Hence, the MEGZ of W42 and W43 is located at the TAPC/BTPEAn interface. Triplets in the MEGZ are quenched by BTPEAn due to its low T1, otherwise they are harnessed by Ir(dmppy)2(dpp) via the triplets diffusion mechanism because the triplets diffusion distance can be very long (i.e., 100 nm).19 These analyses can also be verified by the fact that the efficiency of W42 is similar to that of W43, although an interlayer has been used (Supporting Information, Figure S5). Therefore, the charge transport of emitters should be considered when develop DFHWOLEDs with thick EMLs, which is not clarifed previously.



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Figure 5. Forward-viewing efficiencies of W41 and W42. Inset: EL spectra at 1000 cd m-2. IV. Emission mechanisms of DFH-WOLEDs based on blue AIEgens In general, when the organic layer is thin (i.e., 0.4 nm), the formed film can partially penetrate the adjacent layers.49 Hence, the BTPEAn layer can partially penetrate TAPC and TmPyPB at their interface in W11 and W41. Besides, the MEGZ is usually narrow (~3 nm).50 For TAPC interlayer based DFH-WOLEDs (W11, W14), holes can easily go across TAPC to blue region, while electrons difficultly arrive at the yellow region by means of tunneling effect since TAPC can block the electron transport.48 Therefore, considering the thin blue EML and strong hole blocking ability of TmPyPB, the MEGZ of W11 includes BTPEAn EML, partially TAPC/BTPEAn, BTPEAn/TmPyPB interface (Figure 6a).49 Since BTPEAn has a lower singlet energy level than TmPyPB as well as TAPC, singlet excitons generated at TmPyPB, BTPEAn and TAPC in the MEGZ are directly recombined within BTPEAn for the blue emission.33 On the other hand, since a portion of electrons can arrive at yellow region together with the fact that there are sufficient holes on Ir(dmppy)2(dpp), singlet/triplet


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excitons are formed and then radiatively decay for the yellow emission. Hence, the white emission is produced. For W14, due to the thick BTPEAn, the MEGZ is located at the TAPC/BTPEAn interface, different from W11 (Figure 6b). Hence, although the generation of yellow emission is similar to W11, the formation of blue emission in W14 is originated from singlets generated on TAPC and BTPEAn. For TmPyPB interlayer based devices (W41, W42), electrons are easy while holes are difficult to transit TmPyPB since TmPyPB can block holes.48 Therefore, considering the thin blue EML and electron blocking ability of TAPC, the MEGZ of W41 includes BTPEAn EML, partially TAPC/BTPEAn, BTPEAn/TmPyPB interfaces (Figure 6c).18 Similar to W11, singlets generated in the MEGZ are used for blue color by BTPEAn,33 while holes transported from the n-type interlayer can meet electrons to form excitons for yellow color by Ir(dmppy)2(dpp), leading to a white emission. However, due to thick BTPEAn in W42 and the hole blocking property of BTPEAn, the MEGZ is located at the TAPC/BTPEAn interface and holes are very difficult to reach yellow EML (Figure 6d). Hence, although blue emission can be generated from the MEGZ, almost no hole are transported to the yellow EML, resulting in no white emission. In a word, the emission mechanism of DFH-WOLEDs with thick AIEgen is different from DFH-WOLEDs with thin AIEgen. This is because the charge transport property of thick EMLs, which is usually neglected previously, plays a vital role, while thin EMLs have little influence on the performance. Therefore, the present results may provide a deep insight on DFH-WOLEDs.



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Figure 6. Emission mechanisms for DFH-WOLEDs based on AIEgens. IL is the interlayer, B-EML and Y-EML are blue and yellow EML, respectively. Gray filled rectangles represent the MEGZ. Charges transport and excitons decay in W11 (a), W14 (b), W41 (c) and W42 (d). 2.4 Three-color DFH-WOLEDs based on blue AIEgens 2.4.1 The purpose of three-color DFH-WOLEDs To demonstrate the universality of DFH-WOLEDs based on AIEgens, we have developed three-color devices. By adding a red emitter, the CCT span is possible to be broadened since red emitters can ensure low CCTs, while the CRI is potential to be enhanced because red emitters can widen spectra. Here, tris(1-phenylisoquinolinolato-C2,N) iridium(III) (Ir(piq)3) is selected as the deep-red emitter (~624 nm) to produce RYB instead of RGB white color, since the emission of Ir(dmppy)2(dpp) well locates between those of the deep-blue and deep-red emitters.40 2.4.2 DFH-WOLEDs with broad CCT span We have first to develop the p-type interlayer based three-color DFH-WOLEDs. The EMLs are Ir(piq)3 (0.3 nm)/TAPC (1.5 nm)/Ir(dmppy)2(dpp) (0.9 nm)/TAPC (3.5 20 ACS Paragon Plus Environment

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nm)/BTPEAn (D nm), where other layers are the same as those of W11, D= 0.4 and 10 for device W51 and W52, respectively. The TAPC between Ir(piq)3 and Ir(dmppy)2(dpp) is an emission-regulating layer, which has been optimized to 1.5 nm to broaden the CCT span. W51 exhibits a CCT span of 2314‒6910 K (Supporting Information, Figure S6). The lowest CCT (2314 K) is much lower than that of W11 (3954 K, Supporting Information, Figure S7), indicating that Ir(piq)3 is required to low CCTs. However, the CCT span is not broad enough, which may be attributed to the fact that the blue EML is too thin, leading to insufficient blue emission for high CCTs. Remarkably, because CCT of W52 ranges from 2301 to 11628 K, which shows the sunlight-like emission (Figure 7 inset), totally covering those of the entire daylight at different times and regions (i.e., 2500, 3500, 5000 and 8000 K at sunset, sunrise, noon and noon in high-latitude countries, respectively).33 Despite lots of lighting sources (i.e., candles, carbon arc lamps, incandescent bulbs, mercury lamps, fluorescent tubes and inorganic LEDs) have been created, only limited CCTs can be obtained for these artificial lightings (i.e., 2500‒3000 K for warm-white fluorescent tubes and 2700 K for incandescent bulbs), which cannot satisfy the demand for natural light.51,52 Therefore, to better meet the demands of life, lighting should possess emissions with CCT covering sunlight (2500‒8000 K). Besides, the voltage of previous WOLEDs with sunlight-like emissions is high (≥8 V),33 which is not beneficial to the power consumption and device lifetime. Here, sunlight-like emissions (2500‒8000 K) can be realized at very low voltages (≤4.3 V), which may be attributed to the high electron transport ability of BTPEAn and low HOMO barrier between TAPC and BTPEAn, providing a new avenue to develop sunlight-like emissions. In fact, i) this is the first OLED exhibiting sunlight-like color that can operate at low voltages; ii) the CCT span (2301‒11628 K) is the broadest among OLEDs with sunlight-like emissions. Moreover, although W31 is doping-free, the maximum total efficiency (10.5 lm W‒1) can be comparable



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to that of the best doping sunlight-like WOLEDs (11.6 lm W‒1),33 further indicating the advantage of this kind of DFH-WOLEDs.

Figure 7. Forward-viewing efficiencies of W52. Inset: EL spectra at various voltages. 2.4.3 DFH-WOLEDs with ultrahigh CRIs As mentioned above, the TAPC between Ir(piq)3 and Ir(dmppy)2(dpp) can regulate the emissions. Hence, we have further optimized the thickness of TAPC to 3.5 nm to achieve ultrahigh CRIs (≥90). The structures of device W61 and W62 are similar to those of W51 and W52, respectively, except for the 3.5 nm emission-regulating layer. As shown in Figure S8 (Supporting Information), the maximum CRI of W61 and W62 are 91.3 and 87.3, respectively. Although ultrahigh CRIs are obtained in W61, the CRIs are not high enough at high luminances (i.e., ≥3000 cd m-2 for general lighting),2 which is not beneficial to the practical applications. In fact, no DF-WOLED with ultrahigh CRIs (≥90) at high luminances (≥ 3000 cd m-2) has been documented. To loosen the bottleneck, we have developed the TmPyPB interlayer based three-color device (W7), since the above finding (W41) suggests that the TmPyPB interlayer can also be 22 ACS Paragon Plus Environment

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used to manipulate the device spectra. The EML is BTPEAn (0.4 nm)/TmPyPB (3.5 nm)/Ir(dmppy)2(dpp) (0.9 nm)/TmPyPB (3.5 nm)/Ir(piq)3 (0.3 nm), where other layers are the same as those of W11. The 3.5 nm TmPyPB film has been used as an emission-regulating layer between Ir(piq)3 and Ir(dmppy)2(dpp) to enhance CRIs. As shown in Figure 7 inset, W7 can exhibit ultrahigh CRIs (90.3‒92.8) at high luminances (≥ 3000 cd m‒2), indicating that W7 can well satisfy the requirement of practical applications. As far as we known, i) this is the first DF-WOLED with ultrahigh CRIs (≥90) at high luminances (≥ 3000 cd m‒2), ii) the CRI (92.8) is not only the highest among AIE-based WOLEDs but the highest among DFWOLEDs. Besides, the maximum total efficiency reaches 14.2 lm W‒1, even better compared with the doping devices with ultrahigh CRIs (≥90).53-57

Figure 8. Efficiencies for W7 (forward-viewing). Inset: Spectra at high luminances. Finally, we have to point out that there is much room for further improvement in the stability of the DF-WOLEDs (e.g., 3.5 h at 1000 cd m-2 for W15), which may be attributed to the instability of the interlayers (e.g., TAPC, TmPyPB) and BTPEAn.5,33 More stable interlayers and AIEgens are essential for the practical use. Besides, the devices cannot 23 ACS Paragon Plus Environment


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simultaneously achieve very high efficiency and ultrahigh CRI, which may be attributed to the nonradiative decay of triplets in AIEgens. However, it is possible to achieve both high efficiency and high CRI in one device if more efficient blue AIEgens can be adopted (e.g., blue AIEgens with TADF characteristic).58-60 Furthermore, we believe that other AIEgens materials are also suitable for the presented concept. 3. Conclusion In summary, DFH-WOLEDs based on blue AIEgens have been successfully demonstrated. Unlike previous DFH-WOLEDs, both thin and thick blue AIEgens have been demonstrated to achieve high-performance devices, enhancing the flexibility of DFH-WOLEDs. Two-color devices show i) pure-white color, ii) high CRI (85), iii) high efficiency. Particularly, 19.0 lm W‒1 is the best for pure-white DF-WOLEDs, and 35.0 lm W‒1 is best for two-color hybrid WOLEDs with CRI ≥80. Then, a three-color DFH-WOLED shows shows broad CCT span (2301‒11628 K), i) the first sunlight-like OLED (2500‒8000 K) operating at low voltages, ii) the broadest span among sunlight-like OLED, iii) possessing comparable efficiency with best doping counterpart. Another three-color DFH-WOLED exhibits CRI >90 at ≥3000 cd m‒2, i) the first DF-WOLED with CRI ≥90 at high luminances, ii) the CRI (92.8) is not only the highest among AIE-based WOLEDs but the highest among DF-WOLEDs. The findings may unlock an alternative concept to develop DFH-WOLEDs, which is beneficial to the solid-state lighting and displays.

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author


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E-mail: [email protected] (J. Z.); [email protected] (Z. Z); [email protected] (J. P.) Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Key Basic Research and Development Program of China (973 program, Grant No.2015CB655004), the National Natural Science Foundation of China (Grant No. 61401156, 21673082, U1601651and U1301243), the Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306035), the Pearl River S&T Nova Program of Guangzhou (No. 201710010066, 201610010052), the Fundamental Research Funds for the Central Universities (2017MS008 and 2017ZD001), China Postdoctoral Science Foundation(No.2017T100627) and the Tiptop Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (No. 2015TQ01C777, 2016TQ03C331).

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High-Performance Doping-Free Hybrid White OLEDs Based on Blue

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