Robust Luminescent Materials with Prominent Aggregation-Induced

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Robust Luminescent Materials with Prominent AggregationInduced Emission and Thermally Activated Delayed Fluorescence for High-Performance Organic Light-Emitting Diodes Jingjing Guo, Xiang-Long Li, Han Nie, Wenwen Luo, Rongrong Hu, Anjun Qin, Zujin Zhao, Shi-Jian Su, and Ben Zhong Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00450 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Chemistry of Materials

Robust Luminescent Materials with Prominent Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence for HighPerformance Organic Light-Emitting Diodes Jingjing Guo,† Xiang-Long Li,† Han Nie,† Wenwen Luo,† Rongrong Hu,† Anjun Qin,† Zujin Zhao,*,† Shi-Jian Su,*,† and Ben Zhong Tang*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, 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: Aggregation-induced emission (AIE) materials have excellent solid-state emission by suppressing concentration quenching and exciton annihiliation, while thermally activated delayed fluorescence (TADF) materials are able to fully utilize electrogenerated singlet and triplet excitons. The collaboration of AIE and TADF should be a rational strategy to design novel robust luminescent materials. Herein, two new materials with both prominent AIE and TADF properties are developed based on a central benzoyl acceptor core and different donor units. Their crystal and electronic structures, thermal stabilities, photophysical properties and energy levels are investigated systematically. The doped organic lightemitting diodes (OLEDs) based on them show green lights and perform outstandingly, providing excellent electroluminescence (EL) efficiencies of up to 19.2%, 60.6 cd A‒1 and 59.2 lm W‒1. Their nondoped OLEDs are turned on at very low torn-on voltages (2.7 V), and afford yellow lights and high EL efficiencies of 9.7%, 26.5 cd A‒1 and 29.1 lm W‒1, with low efficiency roll-off. These results actually demonstrate the feasibility to explore new efficient emitters by the marriage of AIE and TADF.

INTRODUCTION Pure organic thermally activated delayed fluorescence (TADF) materials have attracted intense attentions due to their promising application for the construction of highly efficient organic light-emitting diodes (OLEDs) in recent years.1-4 TADF materials have a small singlet–triplet energy splitting (ΔEST) and thus can harvest electrogenerated singlet and triplet excitons through efficient reverse intersystem crossing (RISC) process. Therefore, ideally, the maximum internal quantum efficiency (IQE) of the device can approach 100%.5-8 Theoretically, the ΔEST value can be easily reduced by separating the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) via molecular engineering. According to this principle, numerous TADF materials have been developed with different kernels, such as triazines,9,10 oxadiazoles,11 cyanobenzenes,12,13 sulfones,14-17 and

spirofluorenes.18,19 The OLEDs based on them have afforded excellent external quantum efficiencies (ηexts) comparable with the results from the best phosphorescent OLEDs.20 Hence the TADF materials have now been regarded as the third generation emitters for OLEDs after the conventional fluorescence and phosphorescence materials. However, there are still some challenges and problems that limit their practical applications to some extent. Just like phosphorescence materials, the TADF materials also suffer from concentration quenching and exciton annihilation processes, such as triplet–triplet annihilation (TTA), singlet–triplet annihilation (STA), and triplet– polaron annihilation (TPA),21 which impede the efficiency enhancement of OLEDs and cause serious efficiency rolloff at a high luminance. Thereby, TADF materials commonly have to be doped in appropriate host matrices to weaken the intermolecular interactions and to mitigate above detrimental processes.

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The materials with aggregation-induced emission (AIE) property show weak emissions in dilute solutions but emit intensely upon aggregation.22-28 Most AIE materials possess twisted molecular conformations that can prevent the intermolecular π-π stacking interactions, and efficiently block the exciton quenching channels as collision induction. So far, many AIE materials have been extensively studied and employed to fabricate stable, efficient, and simplified non-doped fluorescent OLEDs.29-32 Based on robust blue AIEgens, high-performance white OLEDs were also achieved.33 However, generally, these AIE materials can only utilize 25% singlet (S1) excitons for electroluminescence (EL), and their 75% triplet (T1) excitons remain unexploited, implying that there are much space for further improvement in EL efficiency via rational molecular design to make full use of the excitons. In view of the mechanisms for high exciton utility in pure organic materials, TADF process should be highly promising, which is capable of harvesting both S1 and T1 excitons for light emission. Therefore, a wise approach to develop robust organic luminescent materials may be incorporating TADF process into AIE materials, which are anticipated to contribute to high efficiency, low roll-off and low-cost nondoped OLEDs.

terials by introducing various functional groups and/or making modifications on them. In this work, the planar dibenzothiophene (DBT) is used as the invariable donor to increase charge-transporting ability of the materials. Phenoxazine (PXZ) and phenothiazine (PTZ) have good electron-donating and hole-transporting ability and can promote the separation of HOMO and LUMO with BZ acceptor. According to the reported results, the critical point of rational molecular design is the combination of a small ΔEST with a reasonable radiative decay rate. Hence, it is very important to control the overlap degree of HOMO and LUMO.13,39 Therefore, a phenyl linker is introduced between PXZ (or PTZ) and carbonyl to strengthen the radiative internal conversion rate constant to some degree, and meanwhile to maintain a small ΔEST. Successfully, these new materials exhibit prominent AIE and TADF properties, and remarkably high-performance EL efficiencies of up to 60.6 cd A-1, 59.2 lm W-1 and 19.2% are achieved in doped OLEDs based on them. In addition, their nondoped OLEDs also show excellent EL efficiencies with small efficiency roll-off.

To date, several groups have reported the successful cases of the combination of prominent TADF and AIE properties.34-38 However, the potential applicability of these materials as emitters for OLED devices had not been fully exploited. To further validate the concept, herein, we design and synthesize two new emitters with an asymmetric D-A-D’ electronic configuration comprised of a central benzoyl (BZ) acceptor (A) core and various donor (D) units (Figure 1). It is envisioned that the asymmetric D-A-D’ structure can enhance the variety of the materials and allow higher possibility to modulate the optical property, energy level, carrier transport as well as intermolecular distance and interaction (in the solid state) of the ma-

The syntheses of DBT-BZ-PXZ and DBT-BZ-PTZ can be easily accomplished in two steps via Friedel-Crafts acylation, and then coupling with PXZ or PTZ in good yields. The detailed synthetic procedures are described in the

RESULTS AND DISCUSSION Synthesis and Characterization

Figure 2. Crystal structure (CCDC 1455106) and packing pattern of DBT-BZ-PXZ in crystals.

Figure 1. Molecular structures and frontier orbital amplitude plot of DBT-BZ-PXZ and DBT-BZ-PTZ, calculated by M06-2X hybrid functional at the basis set level of 631G (d, p).

Supporting Information (SI). Palladium or other rareearth-metal catalysts are not required for the syntheses, making the materials cost low and suitable for large-scale productions. Both target products are further purified by temperature gradient vacuum sublimation after column chromatography before optical property measurements and OLED device fabrication by vacuum deposition. Both

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are mainly centered on the donor units of PXZ and PTZ, and slightly extended to the phenyl linkers, while the LUMOs are predominantly delocalized over the central BZ acceptor core and about half of DBT moiety (Figure 1). The small spatial overlap between HOMO and LUMO is conducive to the occurrence of TADF. The calculated ΔEST values of DBT-BZ-PXZ and DBT-BZ-PTZ are 0.31 and 0.48 eV, respectively. The experimental energy levels of the two materials are investigated by cyclic voltammetry. As shown in Figure 3, they show reversible oxidation processes, indicating their good electrochemical stability. The oxidation peak potentials of DBT-BZ-PXZ and DBTBZ-PTZ are 0.87 and 0.88 V, respectively, and their half-

materials are highly soluble in common organic solvents, such as tetrahydrofuran (THF), chloroform, dichloromethane, and so on, but insoluble in water and methanol. The thermal properties of DBT-BZ-PXZ and DBT-BZ-PTZ are examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). DBT-BZ-PXZ and DBT-BZ-PTZ show good thermal stability with high decomposition temperatures (Td) of 377 and 349 °C, respectively. Adequate glass-transition temperatures (Tg) of 71 and 75 °C are determined for DBT-BZ-PXZ and DBT-BZPTZ, respectively, revealing the good morphological stability (Figure S1). Crystal Structure Single crystals of DBT-BZ-PXZ are grown from a CH2Cl2/n-hexane mixture and analyzed by X-ray crystallography. As depicted in Figure 2, highly distorted geometry with a dihedral angle of 66° between PXZ unit and the nearby phenyl linker is observed from the crystal structure. Such a twisted molecular conformation, arising from the steric repulsion by the hydrogen atoms at the peripositions in the PXZ units, contributes to the effective separation of HOMO and LUMO. Figure 2 also illustrates the molecular packing pattern of DBT-BZ-PXZ in crystals. Due to the twisted conformation, the molecules pile up loosely in the crystal lattice without close π‒π stacking among DBT-BZ fragments. Multiple C‒H···π and C=O···H hydrogen bonds with distances of 2.998‒3.136 Å are observed. All these multiple interactions can lock intramolecular motions and rigidify molecular structures, which are conducive to largely depressing the nonradiative decay and enhancing the emission efficiency in the condensed phase. In addition, pairs of PXZ units align parallel with an interplane distance of ~3.603 Å, allowing partial intermolecular overlap of π-orbitals. The collective effect of the aromatic stacking could promote the charge transport.

DBT-BZ-PXZ DBT-BZ-PTZ

Current

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Figure 3. Cyclic voltammograms of DBT-BZ-PXZ and DBT-BZ-PTZ measured in dichloromethane containing 0.1 M tetra-n-butylammonium hexafluorophosphate. Scan rate: 100 mV s‒1. wave potentials are 0.81 and 0.82 V, respectively. The energy levels of HOMOs and LUMOs are estimated to be −5.13 and −2.51 eV for DBT-BZ-PXZ, and −5.13 and −2.18 eV for DBT-BZ-PTZ, respectively, based on onset potential of oxidation and optical band gap.40

Electronic Structures and Energy Levels To gain deep insights into the geometric and electronic structures of these asymmetric BZ-based materials, quantum-chemical calculations are performed using density functional theory (DFT) calculations with M06-2X hybrid functional at the basis set level of 6-31G (d, p).13 As expected, the optimized structures of DBT-BZ-PXZ and DBT-BZ-PTZ are apparently twisted, consistent with the single-crystal structure. The HOMOs of both materials

Physical Properties The photophysical properties of DBT-BZ-PXZ and DBTBZ-PTZ are analyzed based on UV-vis absorption, steady photoluminescence (PL) and transient PL spectra. As shown in Figure 4A, DBT-BZ-PXZ exhibits an obvious broad and relatively weak absorption band at 394 nm in THF solution, which can be ascribed to the intramolecu-

Table 1. Photophysical properties of DBT-BZ-PXZ and DBT-BZ-PTZ. solna λabs (nm)

doped filmf

neat film b

λem (nm)

ΦF (%)

λem (nm)

ΦF (%)

τprompt (ns)

c

τTADFd

e

(μs)

ΔEST (eV)

λem (nm)

ΦF (%)

τprompt (ns)

τTADF (μs)

ΔEST (eV)

DBT-BZ-PXZ

394, 312

594

0.5

549

38.1

38.3

1.8

0.09

524

57.5

39.4

12.4

0.11

DBT-BZ-PTZ

330

604

0.5

547

40.3

27.6

1.3

0.05

524

76.8

37.4

2.2

0.08

In THF solution (10−5 M). b Fluorescence quantum yield determined by a calibrated integrating sphere. c Lifetime of the prompt component in transient PL. d Lifetime of the delayed component in transient PL. e Estimated from the fluorescence spectra at room temperature and the phosphorescence spectra at 77 K. f 6 wt% DBT-BZ-PXZ:CBP film and 10 wt%3 DBT-BZ-PTZ:CBP film. a



lar charge transfer (ICT) from PXZ donor to BZ acceptor. However, no obvious ICT band is apparently discernable in the absorption spectrum of DBT-BZ-PTZ, probably due to the relatively weaker electron-donating ability of PTZ than PXZ. DBT-BZ-PXZ and DBT-BZ-PTZ exhibit weak PL peaks at 594 and 604 nm, respectively, in dilute THF solutions, with low PL quantum yields (ΦPL) of ~0.5%. In the vacuum-deposited neat films, DBT-BZ-PXZ and DBT-BZPTZ emit strong yellow PL at 549 and 547 nm, and their ΦPL values are enhanced to 38.1% and 40.3%, respectively, indicative of their good AIE nature. In comparison with the PL spectra in THF solutions, the PL spectra in neat films are blue-shifted apparently by ~50 nm. This should be attributed to the decreased polarity of neat film relative to THF solvent, namely, the DBT-BZ-PXZ and DBTBZ-PTZ molecules feel less polarity in neat film than in THF. To confirm this, their PL spectra in binary THF/toluene solvents are measured (Figure S2). As the increase of THF fraction, the polarity of the mixture is enhanced. The PL peaks of both materials are red-shifted progressively, and the intensity is decreased, which manifest the ICT effect. The PL spectra of both materials in a basically non-polar host matrix of 4,4'-di(9H-carbazol-9yl)-1,1'-biphenyl (CBP) are further investigated. As appended in Figure 4B, the PL peaks of 6 wt% DBT-BZPXZ:CBP film and 10 wt% DBT-BZ-PTZ:CBP film are all located at 524 nm, which are significantly blue-shifted by ~25 nm than those of their neat films. And the ΦPL values of DBT-BZ-PXZ and DBT-BZ-PTZ in the doped films inDBT-BZ-PTZ:CBP DBT-BZ-PTZ DBT-BZ-PXZ:CBP DBT-BZ-PXZ

B


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Figure 4. (A) Absorption spectra of DBT-BZ-PXZ and DBT-BZ-PTZ in THF solutions. (B) PL spectra of DBTBZ-PXZ neat film, 6 wt% DBT-BZ-PXZ:CBP film, DBTBZ-PTZ neat film and 10 wt% DBT-BZ-PTZ:CBP film. (C) PL spectra of DBT-BZ-PXZ in THF/water mixtures with different water fractions (fw). (D) Plots of the PL peak intensity versus the water fraction (fw). Inset: photos of DBT-BZ-PXZ and DBT-BZ-PTZ in THF/water mixtures (fw = 0 and 99%), taken under 365 nm excitation.

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crease to 58% and 77%, respectively. Since CBP is much less polar than DBT-BZ-PXZ and DBT-BZ-PTZ, the polarity of the CBP matrix is weaker than that of the films of DBT-BZ-PXZ and DBT-BZ-PTZ. Therefore, DBT-BZ-PXZ and DBT-BZ-PTZ undergo less ICT process by dispersed into the CBP matrix, and exhibit blue-shifted PL from local excited state. Meanwhile, by dispersed in CBP matrix, the intermolecular interactions between fluorescent molecules are also weakened, which also accounts for the blue-shifted PL. To further validate the AIE activity, the PL behaviors of DBT-BZ-PXZ and DBT-BZ-PTZ in nanoaggregates formed in poor solvents are investigated (Figures 4C and S3). Taking DBT-BZ-PXZ as an example, its PL intensity remains low in the mixtures with water fraction (fw, vol%) less than 80%, above which it increases swiftly. The higher the water fraction is, the stronger is the PL intensity. Similar PL enhancements are also observed for DBT-BZ-PTZ. These results demonstrate the AIE property of both materials. The aggregate formation can restrict intermolecular motions and thus block nonradiative decay channel, which surely contributes to the AIE effect.41-43 Another factor needs to be included for consideration is that the aggregate formation excludes the majority of high-polar solvent molecules, and thus fluorescent molecules feel less polarity in aggregate and their ICT process is weakened. The dipole-dipole interaction among molecules becomes weaker than that between polar solvent molecules and fluorescent molecules.44,45 Therefore, the materials show blue-shifted PL peaks and increased PL intensity in aggregate. The twisted molecular conformation of both materials can obstruct the intermolecular π–π stacking interaction in the aggregated state in some degree. This can availably avoid the concentration quenching and/or exciton annihilation caused by the interactions between fluorescent molecules, which also plays a constructive role for the OLEDs performance. In order to deepen the understanding of PL mechanism of both materials, the ΔEST values of their neat and doped films are determined from the onsets of PL spectra at room temperature and phosphorescence spectra at 77 K (Figure S4). The measured ΔEST values for DBT-BZ-PXZ and DBT-BZ-PTZ (0.09 and 0.05 eV, respectively) are small enough to allow efficient thermally promoted upconversion from T1 to S1, revealing the high potential as TADF emitters. The transient PL spectra of the neat and doped films are further measured at room temperature (Figures 5A and 5B). The transient decay curves of the neat and doped films can be roughly divided into a nanosecond-scale prompt decay component and a microsecond-scale delayed component as fitted by doubleexponential functions. The TADF property is further confirmed by temperature-dependent transient PL measurements of DBT-BZ-PXZ and DBT-BZ-PTZ in neat and doped films (Figure 5). As the temperature increases from


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100 K to 300 K, the ratio of the delayed components gradually enhances, further demonstrate the presence of typical TADF characteristic for these two materials. 100

DBT-BZ-PXZ 6 wt% in CBP neat film

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DBT-BZ-PXZ:CBP doped film are 15.9% and 41.6%, respectively. Similarly, the Φprompt and ΦTADF of DBT-BZ-PTZ neat film are 31.3% and 9.0% and those of 10 wt% DBTBZ-PTZ:CBP doped film are increased to 44.0% and 32.8% (Table 1). Furthermore, the intersystem crossing rate constant (KISC), the TADF rate constant (KTADF) and the fluorescence decay rate (KF), etc., are also calculated on the basis of the PL efficiency and lifetime data (Table S1). The KTADF for the neat and doped films of DBT-BZ-PXZ are estimated to be 2.14 × 105 s‒1 and 4.65 × 104 s‒1, respectively; meanwhile, the KISC and KF of neat and doped films are comparable. As for DBT-BZ-PTZ, the results are similar to those of DBT-BZ-PXZ. Electroluminescence

DBT-BZ-PXZ doped film

D

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DBT-BZ-PTZ 10 wt% in CBP neat film

B

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50

Time (µs)

Figure 5. Transient decay spectra of (A) DBT-BZ-PXZ neat film and 6 wt% DBT-BZ-PXZ:CBP film and (B) DBTBZ-PTZ neat film and 10 wt% DBT-BZ-PTZ:CBP film. Temperature dependent transient decay spectra of (C) DBT-BZ-PXZ neat film, (D) 6 wt% DBT-BZ-PXZ:CBP film, (E) DBT-BZ-PTZ neat film and (F) 10 wt% DBT-BZPTZ:CBP film. The lifetime of delayed component of DBT-BZ-PTZ neat film (τTADF = 1.3 μs) is found to be shorter than that of DBT-BZ-PXZ neat film (τTADF = 1.8 μs), indicating a more efficient up-conversion from T1 to S1, probably due to the smaller ΔEST.46-49 Compared with the neat films, the doped films show delayed components with longer lifetimes. But the doped films have larger ΦPL values than the neat films (Table 1) because of the reduced ICT effect and dipoledipole interactions in doped films. According to the proportion of the integrated area of the prompt and delayed components in the transient spectra to the total integrated area, the PL quantum yields for prompt fluorescence (Φprompt) and TADF (ΦTADF) of DBT-BZ-PXZ neat film are calculated to be 8.2% and 29.9%, while those for 6 wt%

OLEDs using DBT-BZ-PXZ or DBT-BZ-PTZ either as the emitting dopant in a host or as the non-doped (neat) emitting layer are fabricated. The doped devices A-D or FI have configurations of ITO/TAPC (25 nm)/DBT-BZ-PXZ (x wt%):CBP (35 nm) or DBT-BZ-PTZ (x wt%):CBP (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al. The non-doped OLEDs (device E or J) are fabricated as ITO/TAPC (25 nm)/DBT-BZ-PXZ (35 nm) or DBT-BZ-PTZ (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al. In these devices, 1,1'bis(di-4-tolylaminophenyl) cyclohexane (TAPC) functions as a hole-transporting layer, while 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPB) serves as an electrontransporting layer. The CBP doped with different concentrations (x wt%) of DBT-BZ-PXZ or DBT-BZ-PTZ, and the neat DBT-BZ-PXZ and DBT-BZ-PTZ films are adopted as the emitting layers. Since CBP has a high triplet energy of 2.6 eV, it is selected as a host to suppress backward energy transfer from the guest to the host materials and to confine the triplet excitons within the guest materials. The key data and characteristic curves of these OLEDs are given in Table 2 and Figure 6, respectively. The doped OLEDs of DBT-BZ-PXZ or DBT-BZ-PTZ in CBP host are turned on at low voltages of 3.1‒3.4 V, and radiate stable green EL emissions with the peaks at about 528 and 538 nm, respectively, which are close to their PL peaks of doped films. Through doping concentration optimization (1 wt%, 6 wt%, 10 wt% and 10 wt%), it is found that all the doped devices exhibit excellent EL efficiencies. The device H with 10 wt% DBT-BZ-PTZ doped in CBP as an emitting layer gives the high external quantum (ηext), current (ηC) and power efficiencies (ηP) of up to 15.1%, 46.0 cd A-1 and 43.3 lm W-1, respectively. The doped OLED B of 6 wt% DBT-BZ-PXZ in CBP shows even better peak ηext, ηC and ηP of 19.2%, 60.6 cd A-1 and 59.2 lm W‒1, respectively, which are very close to those of previously reported most efficient TADF OLEDs,13 such as 4CzIPN:CBP-based OLED (ηext = 19.3%).50 It is also noteworthy that the doped device B allows relatively smaller efficiency roll-off, and at the luminance of 1000 cd m-2, the ηext still remains as high as 15.4%, being higher than the data in the literatures.13

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−2


Figure 6. (A) and (D) EL spectra at 10 mA cm−2, (B) and (E) luminance–voltage–current density, (C) and (F) current efficiency–luminance–power efficiency characteristics of device A-E and device F-J. Device configurations: for device A-D, ITO/TAPC (25 nm)/DBT-BZ-PXZ (x wt%):CBP (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; x = 1, 6, 10 and 20, respectively; for device F-I, ITO/TAPC (25 nm)/DBT-BZ-PTZ (x wt%):CBP (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; x = 1, 6, 10 and 20, respectively; for device E, ITO/TAPC (25 nm)/DBT-BZ-PXZ (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; for device J, ITO/TAPC (25 nm)/DBT-BZ-PTZ (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al. In addition, non-doped devices E and J, incorporating DBT-BZ-PXZ and DBT-BZ-PTZ as emitting layers, display yellow EL emissions with peaks at 557 and 563 nm, respectively, which are red-shifted to some degree relative to the doped devices. And they are red-shifted by 8 and 16 nm than the corresponding PL peaks of their neat films. The turn-on voltages of the non-doped devices E and J (2.9 V and 2.7 V, respectively) are lower than those of the corresponding doped devices, because it is easier for carrier injection and transfer in the neat emitting layers without host matrix with a large energy barrier. Besides, the non-doped device J gives maximum ηext, ηC and ηP of 9.7%, 26.5 cd A-1 and 29.1 lm W-1, which are higher than those of the DBT-BZ-PXZ-based device E (9.2%, 26.6 cd A1 and 27.9 lm W-1, respectively). More importantly, the non-doped OLED of DBT-BZ-PTZ retains a relatively high ηext of 8.5% at 1000 cd m-2. The efficiency roll-off is as small as 12.4%, indicating the greatly advanced efficiency

stability of the device. This effect can be ascribed to, in some degree, the suppression of exciton and/or concentration quenching of the materials. Although the peak values of this non-doped OLED are slightly lower than those of the most efficient non-doped OLEDs based on TADF materials, but the efficiency roll-off has advanced apparently (Table S2).14,38,51-53 The EL efficiencies of these new materials with AIE and TADF property in non-doped devices are considerably more excellent than those achieved by conventional AIE emitters. The key factor for attaining high ηext values and low efficiency roll-off in devices should be its efficient T1 to S1 up-conversion, because the ηext values are beyond the theoretical limit for the conventional fluorescent OLEDs. In general, the theoretical value of ηext can be expressed by Equation (1): ηext = ηint × ηout = (γ × ηST × ΦPL) × ηout

(1)

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Table 2. EL performances of OLEDs based on DBT-BZ-PXZ and DBT-BZ-PTZ.a values at 1000 cd m‒2

maximum values Device

DBT-BZ-PXZ

DBT-BZ-PTZ

A B C D E F G H I J

Von (V) 3.4 3.2 3.4 3.4 2.9 3.4 3.3 3.2 3.1 2.7

ηC (cd A‒1) 58.8 60.6 57.5 41.4 26.6 45.0 45.0 46.0 43.1 26.5

ηP (lm W‒1) 51.7 59.2 51.3 40.6 27.9 41.5 41.0 43.3 44.1 29.1

ηext (%) 18.7 19.2 18.5 13.4 9.2 14.1 14.6 15.1 14.3 9.7

V (V) 6.6 7.0 7.9 5.5 7.6 5.7 6.8 6.6 4.8

ηC (cd A‒1) 47.7 48.6 30.0 19.6 19.1 32.3 29.5 29.2 23.5

ηP (lm W‒1) 22.5 21.8 12.0 11.3 7.9 17.8 13.6 13.9 15.4

ηext (%) 15.2 15.4 9.7 6.8 5.8 10.5 9.7 9.7 8.5

RO (%) 18.7 19.8 47.6 26.1 58.9 28.1 35.8 32.2 12.4

CIE (x,y) (0.310, 0.566) (0.340, 0.576) (0.338, 0.567) (0.434, 0.542) (0.352, 0.557) (0.362, 0.565) (0.370, 0.563) (0.385, 0.559) (0.448, 0.531)

a

Abbreviations: Von = turn-on voltage at 1 cd m‒2; ηc = current efficiency; ηp = power efficiency; ηext = external quantum efficiency; RO = external quantum efficiency roll-off from maximum value to that at 1000 cd m‒2; CIE = Commission Internationale de I’Eclairage coordinates. Device configurations: for device A-D, ITO/TAPC (25 nm)/DBT-BZ-PXZ (x wt%):CBP (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; x = 1, 6, 10 and 20, respectively; for device F-I, ITO/TAPC (25 nm)/DBT-BZ-PTZ (x wt%):CBP (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; x = 1, 6, 10 and 20, respectively; for device E, ITO/TAPC (25 nm)/DBT-BZ-PXZ (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; for device J, ITO/TAPC (25 nm)/DBT-BZPTZ (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al. where ηint is the internal quantum efficiency, ηout is the light out-coupling factor (0.2~0.3), γ is the charge balance factor (ideally 1.0), ηST is the fraction of excitons involving radiative decay (0.25 for conventional fluorescent emitters), and ΦPL is the PL quantum yield of the emitting layer. Therefore, if DBT-BZ-PXZ and DBT-BZ-PTZ are assumed to be the conventional fluorescent emitters, the theoretical ηext values of the nondoped devices are calculated to be 1.9~2.9% and 2.0~3.0%, respectively. The actual maximum ηext values are about 3.2-fold higher than the theoretical limit. Hence, these results explicitly demonstrate the strong potential of these new materials to be candidates for fabricating highly efficient OLEDs with low efficiency roll-off.

CONCLUSION In summary, two simple and robust materials, DBT-BZPXZ and DBT-BZ-PTZ, with an unsymmetrical D-A-D’ structure are synthesized and characterized. They simultaneously have AIE and TADF characteristics, allowing not only high solid-state PL efficiencies but also low concentration quenching and exciton annihilation. The OLED using DBT-BZ-PXZ as the emitting dopant achieves excellent peak EL efficiencies of 19.2%, 60.6 cd A-1 and 59.2 lm W-1. The efficiency roll-off is lower than most reported doped OLEDs based on TADF materials. The nondoped DBT-BZ-PTZ device shows very small turn-on voltage of 2.7 V, and affords EL efficiencies as high as 9.7%, 26.5 cd A-1 and 29.1 lm W-1, with much smaller efficiency roll-off. In view of the facts that most reported TADF materials suffer from severe efficiency low-off in the doped OLEDs, and recent studies demonstrate the prominent role of host-dopant interactions in the RISC processes,54 our non-doped OLEDs based on the new materials are

promising alternatives to make these complications effectively eliminated. The design strategy proposed in this work of the combination of AIE and TADF should have important guidance to develop robust light-emitting materials to improve EL efficiency and lower efficiency rolloff of OLEDs.

ASSOCIATED CONTENT Supporting Information Experimental details, device fabrication, X-ray structure refinement data, photophysical data in neat and doped films, TGA and DSC curves, PL spectra in THF/toluene mixtures and THF/water mixtures, fluorescence and phosphorescence 1 spectra in neat and doped films, and H NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected]. *Email: [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306035), the National Natural Science Foundation of China (21673082), the Nation Key Basic Research and Development Program of China (973 program, 2015CB655004 and 2013CB834702) Founded by MOST, the Guangdong In-

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novative Research Team Program of China (201101C0105067115), the Natural Science Foundation of Guangdong Province (2016A030312002), the Science and Technology Project of Guangdong Province (2016B090907001), the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01) and the Fundamental Research Funds for the Central Universities (2015ZY013).

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