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Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly efficient nondoped OLEDs with low efficiency roll-off
Accepted Manuscript Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly efficient nondoped OLEDs with low efficiency roll-off Jinshan Wang, Chao Liu, Cuifeng Jiang, Chuang Yao, Min Gu, Wei Wang PII:
S1566-1199(18)30588-3
DOI:
https://doi.org/10.1016/j.orgel.2018.11.018
Reference:
ORGELE 4981
To appear in:
Organic Electronics
Received Date: 27 September 2018 Revised Date:
12 November 2018
Accepted Date: 12 November 2018
Please cite this article as: J. Wang, C. Liu, C. Jiang, C. Yao, M. Gu, W. Wang, Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly efficient nondoped OLEDs with low efficiency roll-off, Organic Electronics (2018), doi: https:// doi.org/10.1016/j.orgel.2018.11.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstracts Solution-processed aggregation-induced delayed fluorescence (AIDF)
nondoped OLEDs with low efficiency roll-off
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emitters based on strong π-accepting triazine cores for highly effcient
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, ***
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Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly effcient nondoped OLEDs with low efficiency roll-off
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, *** a
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School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, P. R. China. b Chongqing Key Laboratory of Extraordinary Bond Engineering and Advance Materials Technology (EBEAM), Yangtze Normal University, Chongqing 408100, China. c School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, China. ABSTRACT
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Currently, aggregation-induced delayed fluorescence (AIDF) is the most potential method that overcomes the limitations of thermally activated delayed fluorescence (TADF) emitters. Herein, to develop high efficient AIDF emitters, a novel strategy of introducing carbazole dendrites to a strengthen electron acceptor to construct D−π−A structure has been presented. Two emitters, namely diphenyl(4-(4-phenyl-6-(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''- tercarbazol]-9'-yl)phen phosphine
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yl)-1,3,5-triazin-2-yl)phenyl)
oxide
(CzTAZPO)
and
diphenyl(4-(4-phenyl-6-(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)phenyl)-1,3,5triazin-2-yl)phenyl)phosphine oxide (sCzTAZPO) with a twisted carbazole dendrites structure are
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synthesized and investigated theoretically and experimentally. Both compounds show aggregation-induced emission, a prominent TADF and bipolar properties. The reasonable molecular
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design strategy allows CzTAZPO to exhibit high oscillator strengths (f) and small singlet–triplet energy gap (∆EST) at the same time, which signifcantly increase the rate of reverse intersystem crossing process and fluorescence quantum efficiency. High-performance nondoped OLEDs are fabricated with CzTAZPO neat films as the emission layers, providing excellent maximum current efficiency (CEmax) and maximum external quantum efficiency (EQEmax) of 29.1 cd A−1 and 12.8%, respectively. More importantly, nondoped OLEDs provided negligible EQE roll-off of 1.6% from the maximum values to those at 1000 cd m−2. The AIDF emitters with small ∆EST, high photoluminescence quantum yields (ΦPL), bipolar charge transport and high rate constant of reverse
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Molecular design; High effciency; Delayed fluorescence; Aggregation-induced
emission; Low effciency roll-off. ________________________________________
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* Corresponding author. ** Corresponding author. ***Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (C. Yao), Email: [email protected] (W. Wang).
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ACCEPTED MANUSCRIPT 1. Introduction Pure organic molecules with TADF have attracted significant interest because they can utilizes the excitons from the excited triplet to the singlet (T1→S1) states by reverse intersystem crossing (RISC)
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process to achieve nearly 100% internal quantum efficiency[1−5]. Without exception, all these TADF emitters are characterized by a small ∆EST. A small ∆EST can be easily obtained by minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest
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unoccupied molecular orbital (LUMO) by molecular engineering. Based on this design concept, numerous TADF emitters have been synthesized with a variety building blocks, such as
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benzophenones [6], sulfones [7,8] and triazines [9,10]. External quantum efficiencies (EQEs) of OLEDs based on TADF emitters have reached an impressive 25% [9,11], which is comparable to the best phosphorescent devices and even exceeds their performances in some cases. However,
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there are still many significant challenges that limit the development and application of TADF materials. One of the pressing problems involves aggregation-caused quenching (ACQ) and serious efficiency roll-off caused by exciton annihilations [12,13]. Since most TADF materials need to be
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doped in the host materials to suppress concentration quenching and excitons annihilation, adopting
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a host-guest system leads to increased difficulty in the precise control of film composition. Another problem involves the deposition process of TADF materials, which are generally suitable for vacuum deposition. Only a few reports have demonstrated solution-processed OLEDs comprising of highly soluble small or polymer TADF molecules [14−17]. However, solution based deposition process is simple to realize cost effective and precise controllable in the large-area fabrication of OLEDs. Except as mentioned above, the unbalanced transfer of hole and electron in the emitting layer and the slow dynamics process relates to the triplet exciton mainly responsible for the 3
ACCEPTED MANUSCRIPT efficiency roll-off by arousing severe triplet-involved annihilation processes under high current densities [18]. Currently, aggregation-induced emission (AIE) mechanism provides the most effective method
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to restrain the ACQ problem [19,20]. AIE Materials typically possess highly distorted molecular structures that can weaken the intermolecular π–π interactions and efficiently block the non-radiative decay of exciton in the aggregated state [21−23]. As a result, they show weak
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emissions molecularly dispersed state in solutions but excellent solid-state emission and high ΦPL in
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aggregated state. Hence, AIE emitters are considered as ideal emitters for non-doped OLEDs and have provided high EL effciencies approaching the theoretical limit of pure organic fluorescent emitters along with low efficiency roll-off. However, only 25% singlet (S1) excitons can be utilize for electroluminescence for most AIE emitters, implying that exciton utilization offers a scope for
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improvement. To this end, molecules incorporating prominent TADF and AIE properties have been reported by several groups [21−26]. The slow RISC process and then caused slow radiative deactivation result in significant triplet excitions annihilate at high current density [24]. According
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to the current molecular design theory, small ∆EST are the key in facilitating the T1 → S1 RISC
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process in TADF emitters [27]. Early studies have revealed that small ∆EST can be obtained by effectively separating the wave function distributions of HOMO and LUMO. Therefore, an effective approach to obtain aggregation-enhanced delayed fluorescence is to design molecules with a D-A-D′, D-A and D-π-A structure [28,29]. This configuration can easily form a highly twisted molecular configuration, which can weaken intermolecular π–π interactions in the aggregated state and give rise to weak frontier orbital overlap at the same time [21]. Therefore, the distorted molecular design rule for AIE emitters is consistent with that of TADF 4
[21], which can not only
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yielding a low ΦPL, since the ΦPL is direct proportional to such an overlap [30,31]. Therefore, a trade-off exists between ∆EST and ΦPL to maintain excellent fluorescence efficiency in the aggregation-enhanced delayed fluorescence molecular design.
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Although a small number of molecules have been synthesized based on the AIDF design, there
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is scope for potential applicability of these emitters in OLEDs [22]. Herein, we present two new emitters CzTAZPO and sCzTAZPO with AIE and TADF properties using traditional design principles of molecular charge transfer. Both the molecules have an asymmetric D−π−A configuration, where triazine serves an electron acceptor (A), carbazole dendrites as electron donors
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(D) and phenyl spacer as the π-bridge. Furthermore, phosphorus oxygen groups were added to improve the electron transport of the molecules. As expected, a highly twisted conformation was formed between the carbazole dendrites and the phenyl spacer, resulting in the desired AIE and
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TADF properties. In addition, theoretical calculations and experimental results showed that the
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increase of the number of carbazole dendrites led to the fluorescence oscillator strength increased, while for ∆EST the value decrease. CzTAZPO and sCzTAZPO exhibit remarkably AIE and TADF properties, and prominent high EL efficiencies of up to 29.1 cd A−1 and 12.8% in nondoped OLEDs. Furthermore, their nondoped OLEDs also show outstanding EL efficiencies with extremely low efficiency roll-off. 2. Experimental section The
compounds
3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole
2,4-bis(4-bromophenyl)-6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine 5
(TAZPDBr)
(Cz), and
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(TAZPBr)
were
prepared according to literature procedures [30,32]. 2.1 Synthesis of 9',9''''-((6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(4,1phenylene))bis(3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole) (CzTAZP)
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Compound 3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole (Cz) (0.40 g, 0.62 mmol), 2,4-bis(4-bromophenyl)-6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine (TAZPDBr) (1.06 g, 1.5
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mmol), CuI (0.001 g, 0.06mmol), K3PO4 (0.5 g, 2.5 mmol) and (±)-trans-1,2-diaminocyclohexane (0.20 mL, 1.6 mmol) were added into toluene (50 ml) and the solution was heated under Ar
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atmosphere. After 72 h under reflux, the mixture was extracted with CH2Cl2, and the combined organic solution was dried over anhydrous Na2SO4. After solvent removal, the crude product was purified by column chromatography to yield a bright yellow powder (0.45g, 36% yield). The intermediate was used in the following steps without further purification.
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2.2 Synthesis of CzTAZPO
Compound CzTAZP (200 mg, 0.103 mmol) and H2O2 (5 mL, 30wt%) were added to 50 mL of
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CH2Cl2. The mixture was then vigorously stirred overnight at room temperature. After solvent removal, the reaction mixture was extracted with CH2Cl2, and the combined organic solution was
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dried over anhydrous MgSO4. The crude product was purified by column chromatography to obtain CzTAZPO in the form of a bright yellow solid. Yield: 190 mg (94.6%). 1HNMR (400 MHz, CDCl3) δ = 9.18 (d, J=7.0, 4H), 8.99 (s, 2H), 8.29 (s, 4H), 8.17 (s, 8H), 8.03 (d, J=7.4, 4H), 7.98 (d, J=9.7, 2H), 7.81 (d, J=8.4, 4H), 7.76 (d, J=10.5, 4H), 7.67 (d, J=7.4, 4H), 7.61 (s, 2H), 7.53 (s, 4H), 7.47 (d, J=7.7, 8H), 7.37 (d, J=7.8, 8H), 1.47 (s, 72H). 13CNMR (101 MHz, CDCl3) δ = 171.29, 164.41, 142.67, 141.56, 140.08, 139.87, 139.21, 135.08, 132.66, 132.20, 132.10, 131.62, 131.45, 131.07, 128.76, 128.64, 126.95, 126.18, 124.48, 123.61, 123.18, 119.44, 116.26, 111.25, 109.05, 34.85, 6
ACCEPTED MANUSCRIPT 32.04. MS (MALDI-TOF) m/z calcd for C137H130N9OP, 1949.5; found, 1947.5[M-2]+ 2.3 Synthesis of 3,3'',6,6''-tetra-tert-butyl-9'-(4-(4-(4-(diphenylphosphanyl)phenyl)-6-phenyl -1,3,5-triazin-2-yl)phenyl)-9'H-9,3':6',9''-tercarbazole (sCzTAZP)
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To a mixture of TAZPBr (200 mg, 0.35 mmol), Cz (303 mg, 0.42 mmol), CuI (13.3 mg, 0.07 mmol), K3PO4 (297.1 mg, 1.40 mmol) in degassed toluene (50 mL), (±)-trans-1,2-diaminocyclohexane (0.10 mL, 0.80 mmol) was added in a dropwise manner under an Ar atmosphere. The resulting
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mixture was refluxed and stirred for 72 h. The mixture was then filtered, and the filtrate was poured
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into water and extracted with CH2Cl2. Further purification was carried out by chromatography on silica gel with CH2Cl2: petroleum ether: methanol (100:200:1, v/v) as eluent. Yield: 220 mg (51.7%). Without further purification the intermediate was utilized directly in the next step. 2.4 Synthesis of sCzTAZPO
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Compound sCzTAZP (200 mg, 0.165 mmol) and H2O2 (5 mL, 30wt%) were added to 50 mL of CH2Cl2. The mixture was then vigorously stirred overnight at room temperature. After solvent removal, the reaction mixture was extracted with CH2Cl2, and the combined organic solution was
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dried over anhydrous MgSO4. The crude product was purified by column chromatography to give
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CzTAZPO to yield a bright yellow solid. Yield: 195 mg (96.4%). 1HNMR (400 MHz, CDCl3) δ = 9.11 (d, J=8.3, 2H), 8.92 (d, J=6.3, 2H), 8.83 (d, J=7.1, 2H), 8.27 (s, 2H), 8.17 (s, 4H), 7.99 (d, J=8.6, 2H), 7.96 – 7.90 (m, 2H), 7.80 (s, 1H), 7.77 (s, 2H), 7.75 (d, J=4.4, 4H), 7.75 (d, J=4.4, 2H), 7.72 (s, 1H), 7.67 (s, 2H), 7.64 (s, 2H), 7.62 (s, 1H), 7.60 (s, 1H), 7.58 (s, 1H), 7.52 (d, J=5.3, 2H), 7.48 (s, 2H), 7.46 (s, 2H), 7.36 (d, J=8.6, 4H), 1.47 (s, 36H).
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CNMR (101 MHz, CDCl3) δ =
171.95, 171.10, 170.47, 142.64, 141.32, 140.09, 139.90, 135.76, 135.28, 133.01, 132.71, 132.20, 132.10, 131.67, 131.38, 130.99, 129.09, 128.83, 128.73, 128.61, 126.80, 126.15, 124.48, 123.61, 7
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The proposed luminogens were synthesized in a straightforward procedure according to routes demonstrated in Scheme 1. The triazine derivative coupled with carbazole dendrites to provide
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intermediate CzTAZP and sCzTAZP, followed by oxidation with hydrogen peroxide to furnish CzTAZPO and sCzTAZPO in high yields. Both compounds were soluble in common organic
HNMR, 13CNMR spectroscopy and mass spectrometry as shown in Fig. S1-S6.
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solvents such as tetrahydrofuran (THF) and CH2Cl2. The target compounds were characterized by
Scheme 1. The synthesis route to CzTAZPO and sCzTAZPO. 3.2. Thermal properties Determined through thermogravimetric analysis (TGA) and differential scanning calorimetry 8
ACCEPTED MANUSCRIPT (DSC) were used to investigate the thermal properties of CzTAZPO and sCzTAZPO, As illustrated in Fig. S7. The decomposition temperatures (Td, corresponding to 5% weight loss) are as high as 455 and 478 °C for CzTAZPO and sCzTAZPO, respectively. In addition, these two emitters have
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high glass transition temperatures (Tg) of 153 and 157 °C. High Td and Tg are attributed to the bulky substituents of the carbazole dendrites and the non-planar structures, which can effectively
fabrication and long-term stability of the OLEDs.
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3.3. Electrochemical properties and energy level
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overcome unfavorable morphological changes. This ensures uniform amorphous films upon
The energy levels and electrochemical behavior of both emitters were investigated by cyclic voltammetry as shown in Fig. S8. Both compounds display two reversible oxidation processes suggesting excellent electrochemical stability. These oxidation peaks are attributed to two types
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electron donating carbazole moiety of the carbazole dendritic, similar to previously reported dendritic [33]. From the experimental onset potentials, the HOMO energy levels of CzTAZPO and sCzTAZPO are estimated to be −5.41 and −5.40 eV, respectively. Based on the values of HOMO
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levels and optical band gaps [21], LUMO energy levels of CzTAZPO and sCzTAZPO are obtained
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as −2.72 eV and −2.66 eV, respectively.
3.4. DFT calculations
The optimized geometry and the electronic structures of the frontier molecule orbitals (FMOs) were investigated using time-dependent density functional theory (TD-DFT) calculations based on the PBE0/def2-SVPD level. As depicted in Fig. 1, both compounds exhibited twisted configurations owing to the bulky steric effect of the carbazole dendrites. In their optimized structures, the two molecules adopted moderate dihedral angles of 49.9° and 50.6° between the carbazole planes of the 9
ACCEPTED MANUSCRIPT dendrite and the phenyl spacer for sCzTAZPO and CzTAZPO, respectively. As shown in Fig. 1, their optimized geometries reveal nearly separated FMOs distributions. According to the FMOs, HOMO levels are predominantly localized on the electron-donor carbazole dendrite, while the
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LUMO levels are mainly distributed over the electron-acceptor triazine rings and adjacent phenyl bridges. FMOs of both compounds that are almost completely divided result in extremely small ∆EST, which facilitate the T1→S1 RISC process for both TADF emitters [34]. The small ∆EST
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confirmed by the calculations were 0.084 eV and 0.104 eV for CzTAZPO and sCzTAZPO,
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respectively. In addition, the oscillator strengths (f) which are significant indicators of S1→S0 transition dipole moments and ΦPL, were estimated to be 0.159 and 0.236 for sCzTAZPO and CzTAZPO, respectively. The high f values of sCzTAZPO and CzTAZPO are beneficial for realizing large radiative decay rate (krS) and thus promoting (ΦPL) [35]. This indicates that the increase of the
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carbazole dendrites number results in CzTAZPO being a superior emitter that combines a small ∆EST and a large fluorescence oscillator strength. The calculations clear indication that an increase in the number of carbazole dendrites leads to a decrease in the S0 → S1 and an increase S0 → T1
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excitation energies, which leads to a net reduction of the ∆EST. With the increase in the number of
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donor moieties, the hole and particle tend to delocalize in order to stabilize the S1 and T1 energies, especially the particle of S1, as shown in the natural transition orbitals (NTO) (Fig. 2). Meanwhile, the introduction of carbazole dendrites onto the triphenyl-triazine is beneficial for the formation of large relevant orbital overlaps due to multiple electronic configurations, which results in an large oscillator strength. The similar observation between the NTO and molecular architectures also have been theoretical studied in other TADF molecules [35].
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Fig. 1. The optimized geometries, the density distributions of the FMOs and calculated energy
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levels for sCzTAZPO and CzTAZPO based on TD-DFT at the PBE0/def2-SVPD basis set.
Fig. 2. The natural transition orbitals (hole and particle) of the S1 and T1 states for the sCzTAZPO and CzTAZPO. The weight of the contribution to the excitation is also included.
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ACCEPTED MANUSCRIPT 3.5. Aggregation-induced emission properties Water/THF hybrid solvent with different water volume fractions (fw(vol%)) were used to investigate the AIE behavior of CzTAZPO and sCzTAZPO in nanoaggregates, because both
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molecules are insoluble in water. In dilute THF solution (10−5 M), CzTAZPO and sCzTAZPO exhibited weak PL emission with the peaks located at 551 and 539 nm, and low ΦPLs of 3.2 and 4.1%, respectively (Fig. 3). The addition of water to the THF solution (fw(vol%) < 10%), resulted in
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the lowering of the emission as compared to the THF solution. This observation can be attributed to the twisted intramolecular charge transfer (TICT) state as a resulted of the increased polarity of the
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solvent mixture upon addition of water [36,37]. For fw(vol%) above 70%, the PL was reinvigorated and greatly enhanced demonstrating the AIE nature of both materials. A near ten-fold enhancement of 61.3% and 45.9% was observed in the ΦPL for solutions of fw(vol%) = 99% as compared to dilute
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THF solution. This can be explained by the formation of aggregation that restricted the intramolecular torsional/vibrational motions that are present in solution, thereby reducing the non-radiative transition of the excited state [22,24]. In addition, the aggregate formation of
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fluorescent molecules increased the weak dipole-dipole interaction among polar solvent and
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fluorescent molecules, which partially accounts for the blue-shifted PL peaks relative to that in THF solution [22].
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Fig. 3. PL spectra of (a) CzTAZPO and (b) sCzTAZPO in THF/water mixtures with different water fractions (fw). (b) The plots of I/I0 values versus fw(vol%). Where I0 is the PL intensity in dilute THF solution. The excitation wavelength is 350 nm. Inset: photographs of CzTAZPO and sCzTAZPO in fw(vol%) = 0 and 99% at 365 nm UV irradiation. 3.6. Photophysical properties The photophysical properties of CzTAZPO and sCzTAZPO are analyzed by UV-vis absorption, PL and phosphorescence (Phos) spectra. As shown in Fig. 4, Both the compounds exhibit strong 13
ACCEPTED MANUSCRIPT absorption bands in dilute CH2Cl2 solution (10−5 M) with peaks (λabs) below 300 nm, which can be assigned to the π-π* transitions of the aromatic moieties. In addition, the weak and broad absorption bands around 380 nm can be attributed to the intramolecular charge transfer (ICT) mainly derived
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from the carbazole dendritic to the triazine. From the edge of absorbance spectrum in film (Fig. S9), the band gaps were calculated to be 2.69 and 2.74 eV for CzTAZPO and sCzTAZPO, respectively. Both materials reveal broad and featureless emission profile with peaks at 512 and 502 nm,
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suggesting that the singlets are identified as CT excited state. In films, CzTAZPO and sCzTAZPO
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exhibit a remarkable ΦPLs of 71% and 57% (Table 1), respectively. With the polar increase of the solvent, the emission spectra of both compounds exhibited red-shifted emission (Fig. S10), further validating that the emissions originate from the CT states [35]. The triplet states are mainly composed of triplets of the donor and acceptor moieties, due to the considerable overlap between
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the hole and particle of the triplet states. The triplet energies derived from the onset edges of the neat films of Phos spectra at 77K were 2.62 eV and 2.65 eV for CzTAZPO and sCzTAZPO, respectively (Fig. S11). The S1 energies estimated from the onset edges of FL spectra at 77K were
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2.70 and 2.75 eV, respectively. Therefore, the ∆EST values of 0.08 and 0.10 eV, are consistent with
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their theoretical values. These values are among the smallest reported for TADF materials [21,35,36], which help facilitate the RISC process in the excited state.
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Fig. 4. The absorption spectrum of CzTAZPO and sCzTAZPO in CH2Cl2 solution and PL spectra in
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neat film.
Table 1. Photophysical properties of the emitters. calc. λabs[a] λPL[b] ΦPL τp[d] τd[d] S1[e] T1[e] HOMO[f] LUMO[g] ∆EST [c] [nm] [nm] [ns] [µs] [eV] [eV] [eV] [eV] [eV] ∆EST[h] [%] [eV] CzTAZPO 267, 512 71 40.6 1.1 2.70 2.62 −5.41 −2.72 0.08 0.084 351 sCzTAZPO 269, 502 57 46.9 0.81 2.75 2.65 −5.40 −2.66 0.10 0.104 350 [a] In CH2Cl2 solution (10−5 M). [b] Detected in neat films. [c] Absolute PL quantum yield of neat films determined by a calibrated integrating sphere. [d] PL lifetimes of prompt (τp) and delayed (τd) decay components evaluated at 300 K under vacuum. [e] Singlet (S1) and triplet (T1) energies estimated from onset wavelengths of neat film at 77 K. [f] Obtained from cyclic voltammograms in dichloromethane solution. [g] Calculated from the HOMO levels and optical energy gaps (Eg). [h] Deduced by TD-DFT at PBE0/def2-SVPD.
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3.7. Delayed fluorescence and theoretical calculation In order to study the photophysical processes, PL transient decay of neat films of CzTAZPO and sCzTAZPO were acquired. As shown in Fig. 5a and 5b, the PL transient decay revealed a distinct double exponential feature, which can be attributed to the prompt and delayed fluorescence 15
ACCEPTED MANUSCRIPT of CzTAZPO and sCzTAZPO neat films. Time-resolved PL spectra of CzTAZPO and sCzTAZPO also exhibit the identical prompt and delayed PL emission, suggesting the TADF property of both compounds, as shown in Fig. 5c and 5d. At 300K, the neat films of CzTAZPO and sCzTAZPO have
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a clear prompt fluorescence decay component (τp) of 40.6 and 46.9 ns, a delayed fluorescence decay component (τd) of 1.1 and 0.81 µs, corresponding to a ratio of delayed component (Rd) of 40.2% and 37.8% respectively. The kinetic constants of CzTAZPO and sCzTAZPO were calculated from
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the PL quantum yield and lifetime data, and are listed in Table S1. The radiative decay rate
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constants (krS) of both the compounds are in the range of 107 s−1 due to favored FMO distributions and are among the highest values reported in similar emitters [21,24,35]. Interestingly, despite the high krS value, CzTAZPO showed a higher rate constant of RISC (kRISC) than sCzTAZPO, indicating that a trade-off exists between krS and ∆EST, which needs to be tuned for efficient TADF emission.
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Fig. 5. The PL transient decay curves of the neat film of (a) CzTAZPO and (b) sCzTAZPO, and
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time-dependent emission spectra of (c) CzTAZPO and (d) sCzTAZPO. Excitation wavelength: 360 nm. Detection wavelengths are 520 nm for CzTAZPO and 510 nm for sCzTAZPO.
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3.8. Electroluminescent properties
The AIE and TADF nature endows CzTAZPO and sCzTAZPO with outstanding solid state
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emission effciency and high utilization of exctions. In order to evaluate the electroluminescence (EL) performance of CzTAZPO and sCzTAZPO, a device consisting of a structure of ITO/PEDOT:PSS (40 nm)/CzTAZPO and sCzTAZPO (50 nm)/TmPyPB (20 nm)/Ca (10 nm)/Al (100 nm) was fabricated. Here, PEDOT:PSS and TmPyPB are poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, respectively, serving as hole- and electron-transporting materials, and the neat film of CzTAZPO and sCzTAZPO functions as light-emitting layer. The energy level diagram of these devices is shown in Fig. 6. The EL emission 17
ACCEPTED MANUSCRIPT spectra are shown in Fig. 6a , with the emission peaks at 537 and 531 nm with the CIE color coordinates of (0.37, 0.56) and (0.36, 0.56), for devices with CzTAZPO and sCzTAZPO as the emitters, respectively.
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Fig. 6b and 6c shows the current density–voltage–luminance (J–V–L) and current efficiency– luminance–external quantum efficiency (CE–L–EQE) characteristic curves of the nondoped devices, and the key data of the OLEDs are listed in Table 2. The nondoped OLEDs exhibit low turn-on
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voltages (Von) of 4.5 and 4.1 V for CzTAZPO and sCzTAZPO, respectively, which can be attributed
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to the optimum energy levels with low electron and hole injection barriers. The CzTAZPO and sCzTAZPO based devices demonstrated a maximum luminance (Lmax) of 9776 and 8283 cd m−2, respectively. Remarkably high maximum current efficiency (CEmax) and maximum external quantum efficiency (EQEmax) of 29.1 cd A−1 and 12.8%, and 20.6 cd A−1 and 9.2% were obtained for
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OLEDs with CzTAZPO and sCzTAZPO as the emitters, respectively. Notably, the nondoped OLEDs maintained high EL efficiencies with increase in the current density. For instance at 1000 cd m−2, the CE and EQE are 28.6 cd A−1 and 12.6% for CzTAZPO device, whereas they are are 20.4 cd
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A−1 and 9.1% for the sCzTAZPO device, respectively. These values are similar to CEmax and
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EQEmax in both cases. The roll-off values can be calculated from peak values to those at 1000 cd m−2. Roll-off of CE is 1.8%, and EQE is 1.6% for CzTAZPO based device, while for sCzTAZPO based device it is 0.97% and 1.1%, respectively, demonstrating enhanced efficiency and stability of nondoped OLEDs.
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Fig. 6. a) The EL spectra of CzTAZPO- and sCzTAZPO-based devices. (Inset) Device photographs. b) The current density–voltage–luminance and c) external quantum efficiency–luminance–current
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effciency characteristics of CzTAZPO- and sCzTAZPO-based devices. d) Energy level diagrams for nondoped devices.
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Table 2. Electroluminescence Characteristics of the Devices @1000cd/m2
Maximum
Von[a] (V)
Lmax (cd/m2)
CE (cd/A)
EQE (%)
CE (cd/A)
EQE (%)
RO[b] [%]
CIE (x,y)
CzTAZPO
4.5
9776
29.1
12.8
28.6
12.6
1.8
0.37,0.56
sCzTAZPO
4.1
8283
20.6
9.6
20.4
9.1
0.97
0.36,0.56
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Emission layer
[a] Von = turn-on voltage at the luminance of 1 cd m‒2. [b] RO = current effciency roll-off from peak value to that at 1000 cd m−2. Such low turn-on voltage, high efficiency and negligible efficiency roll-off could be attributed to the excellent injection and balance of excitons in neat films. To evaluate the transport properties, 19
ACCEPTED MANUSCRIPT CzTAZPO- and sCzTAZPO-based hole- and electron-only devices were fabricated and investigated. The single-carrier devices had the configuration of ITO/TmPyPB (30 nm)/ CzTAZPO or sCzTAZPO (40 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) for electron-only and ITO/
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1,1′-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (30 nm)/CzTAZPO or sCzTAZPO (40 nm)/ TAPC (50 nm)/Au(100 nm) for hole-only cases. As can be seen from Fig. 7, CzTAZPO and sCzTAZPO exhibit almost the identical electron- and hole-current density under the same voltage.
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This indicative that well-balanced electron and hole transporting abilities within neat films of TADF
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dendrimers, which benefit from their small ∆EST [38]. Compared with CzTAZPO-based device, the current densities of sCzTAZPO-based devices are more balanced under the same voltage. The superior bipolar characteristic of sCzTAZPO promote the balance of charges in the emitting layers, which assist in suppressing efficiency roll-offs [35]. This is consistent with the observation that the
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device based on sCzTAZPO as the emission layer has a smaller roll-off. These devices are among the most efficient nondoped AIDF OLEDs along with negligible effciency roll-off reported up till
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now, which can be attributed to rapid RISC processes and well-balanced carrier transport property.
Fig. 7. The current density–voltage curves of the hole- and electron-only devices based on CzTAZPO and sCzTAZPO. To evaluate the utilization efficiency of excitons and validate the EL efficiencies of nondoped 20
ACCEPTED MANUSCRIPT OLEDs, quantitative estimation of the theoretical values of external quantum efficiencies was performed. The maximum external quantum efficiency ηext can be expressed by Equation (1) and (2) [24]. (1)
ηint = γ × [ηS × ΦP + (ηS × ΦISC + ηT) × ΦRISC]
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ηext = ηint × ηout
(2)
Where, ηint refers to the internal quantum efficiency, ηout is the optical out-coupling factor
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(usually 0.2~ 0.3), γ is the charge balance factor (ideally γ = 1.0), and ηS and ηT are the fractions of
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singlet (25%) and triplet excitons (75%), ΦP = fluorescent components quantum yield, ΦISC = the intersystem crossing quantum yield. ΦRISC = the reverse intersystem crossing quantum yield, respectively. According to the kinetic parameters in Table S1, the theoretical ηext are estimated to be 14.5%–21.6% for the devices based on CzTAZPO and 12.6%–18.9% for sCzTAZPO. Theoretical
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calculations are in reasonable consistent with the experimentally obtained parameters, implying high exciton utilization have been realized in the emission layers. The utilization of excitons of nondoped OLED is expressed by Equation (3) [21].
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ηext = (γ × ηST × ΦPL ) × ηout
(3)
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Where ηST is the fraction of excitons involving radiative decay, and ΦPL is the PL quantum yield of the nondoped films. Given a general ηout of 0.2–0.3 and experimental values of ηext (12.8 and 9.6% for CzTAZPO and sCzTAZPO) and ΦPL (71 and 57% for CzTAZPO and sCzTAZPO), the ηST are calculated to be 60.1%–90.1% for CzTAZPO based OLEDs and 56.1–84.2% for sCzTAZPO based OLEDs, which is about 2.2–3.6-fold higher than the limit of spin statistical ratio (25%) for conventional fluorescent emitters. There are only a few reported studies involving nondoped OLEDs by solution process based on 21
ACCEPTED MANUSCRIPT TADF materials and the relevant key performance of these devices are shown in Table 3. As is evident from the literature, only a few nondoped/doped devices with simple configuration gave a high EQEmax that is comparable to the results in this work. However, these devices suffered from
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significant efficiency decline at high current density. For instance, the solution processed nondoped OLED reported by Lee et al. providing a EQEmax of 18.3% declined to 12.0% at the luminance of 1000 cd m−2, leading to sharply effciency roll-off of 34.4%. The EQEmax values of other nondoped
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OLEDs were below 12%, which cannot be compared with the efficiency in this work. In addition,
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some devices adopted TADF as a dopant for solution processed devices. Even though the efficiency of CzTAZPO-based nondoped device is somewhat lower than the top values, the effciency roll-off is superior to those of reported nondoped and even some doped OLEDs. Therefore, the combination of AIE and TADF properties in CzTAZPO makes it one of the most promising materials for
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nondoped OLEDs.
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ACCEPTED MANUSCRIPT Table 3. EL performances of nondoped OLEDs in this work and the reported TADF emitters. Emitter
EML
Von a)
EQEmax
EQE b)
Roll-off c)
[%]
[%]
[%]
Reference
nondoped
−
18.3
12.0
34.4
[39]
CzDMAC-DPS
nondoped
3.6
12.2
4.5
63.1
[40]
tBuG2TAZ
nondoped
3.5
9.5
−
−
[41]
G3TAZ
nondoped
3.5
3.4
−
−
[16]
G2B
nondoped
3.4
5.7
−
−
[42]
TZ-3Cz
nondoped
3.6
10.1
−
−
[43]
B-oCz
nondoped
4.4
8.0
−
−
[44]
Cz-CzCN
nondoped
3.1
15.5
−
−
[45]
CDE1
nondoped
−
12.0
11.9
0.83
[46]
TPA–AQ
doped
3.8
7.5
−
−
[47]
3a
doped
4.5
5.6
−
−
[48]
ACRDSO2
doped
3.7
17.5
14.0
20
[13]
G3Ph
doped
2.5
16.1
10.2
36.6
[49]
MA–TA
doped
−
22.1
−
−
[50]
doped
−
11.2
−
−
[50]
doped
6.2
11.3
10.6
6.2
[51]
4.5
12.8
12.6
1.6
This work
CzTAZPO
nondoped
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4CzCNPy
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t4CzIPN
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[a] Turn-on voltage at 1 cd m‒2. [b] External quantum efficiency at the luminance of 1000 cd m−2. [c] RO = EQE roll-off from peak value to that at 1000 cd m−2.
4. Conclusions
Two OLED emitters were developed in this work, namely CzTAZPO and sCzTAZPO that possessed highly twisted D-π-A carbazole dendritic structures. The properties of AIE, TADF, and bipolar charge transport were combined to establish a molecular design strategy that allowed
23
ACCEPTED MANUSCRIPT CzTAZPO to exhibit a high ΦPL and small ∆EST at the same time. Solution-processed nondoped OLEDs were fabricated by adopting CzTAZPO and sCzTAZPO as the emission layer. The OLEDs exhibited excellent EL efficiencies of 29.1 cd A−1 and 12.8% for CzTAZPO, and 20.4 cd A−1 and 9.1%
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for sCzTAZPO, respectively. Extremely flat CE roll-off of 1.8% and 0.97% were obtained for CzTAZPO and sCzTAZPO based OLEDs, while the EQE roll-off were 1.6% and 1.1% from maximum values to those at 1000 cd m−2. Detailed analyses showed that the flat efficiency decline
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was derived from the well-balanced charges transfer and fast reverse intersystem crossing and
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fluorescence decay rate, which not only shortened exciton lifetimes but also reduced exciton concentration. The promotion of this molecular design strategy is a promising method for developing ideal materials for efficient OLEDs with simplifed structures. The stability of these devices will be study to confirm the superiority of CzTAZPO and sCzTAZPO in our future work.
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Acknowledgments
The authors would like to thank the Natural Science Foundation of Jiangsu Province (BK20160440,
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Highlights
Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly effcient
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nondoped OLEDs with low efficiency roll-off
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, ***
Highlights
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• A strategy to construct solution-processed AIDF was proposed
• Two solution-processed AIDF emitters were designed and synthesized. • CzTAZPO exhibit high oscillator strengths and small singlet–triplet energy gap. • CzTAZPO based nondoped OLED shown high efficiencies (EQE>12%) and low roll-off (< 1.6%).
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• This strategy would be a promising method for efficient and stable OLEDs.
S1566-1199(18)30588-3
DOI:
https://doi.org/10.1016/j.orgel.2018.11.018
Reference:
ORGELE 4981
To appear in:
Organic Electronics
Received Date: 27 September 2018 Revised Date:
12 November 2018
Accepted Date: 12 November 2018
Please cite this article as: J. Wang, C. Liu, C. Jiang, C. Yao, M. Gu, W. Wang, Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly efficient nondoped OLEDs with low efficiency roll-off, Organic Electronics (2018), doi: https:// doi.org/10.1016/j.orgel.2018.11.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstracts Solution-processed aggregation-induced delayed fluorescence (AIDF)
nondoped OLEDs with low efficiency roll-off
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emitters based on strong π-accepting triazine cores for highly effcient
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, ***
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Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly effcient nondoped OLEDs with low efficiency roll-off
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, *** a
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School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, P. R. China. b Chongqing Key Laboratory of Extraordinary Bond Engineering and Advance Materials Technology (EBEAM), Yangtze Normal University, Chongqing 408100, China. c School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, China. ABSTRACT
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Currently, aggregation-induced delayed fluorescence (AIDF) is the most potential method that overcomes the limitations of thermally activated delayed fluorescence (TADF) emitters. Herein, to develop high efficient AIDF emitters, a novel strategy of introducing carbazole dendrites to a strengthen electron acceptor to construct D−π−A structure has been presented. Two emitters, namely diphenyl(4-(4-phenyl-6-(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''- tercarbazol]-9'-yl)phen phosphine
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yl)-1,3,5-triazin-2-yl)phenyl)
oxide
(CzTAZPO)
and
diphenyl(4-(4-phenyl-6-(4-(3,3'',6,6''-tetra-tert-butyl-9'H-[9,3':6',9''-tercarbazol]-9'-yl)phenyl)-1,3,5triazin-2-yl)phenyl)phosphine oxide (sCzTAZPO) with a twisted carbazole dendrites structure are
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synthesized and investigated theoretically and experimentally. Both compounds show aggregation-induced emission, a prominent TADF and bipolar properties. The reasonable molecular
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design strategy allows CzTAZPO to exhibit high oscillator strengths (f) and small singlet–triplet energy gap (∆EST) at the same time, which signifcantly increase the rate of reverse intersystem crossing process and fluorescence quantum efficiency. High-performance nondoped OLEDs are fabricated with CzTAZPO neat films as the emission layers, providing excellent maximum current efficiency (CEmax) and maximum external quantum efficiency (EQEmax) of 29.1 cd A−1 and 12.8%, respectively. More importantly, nondoped OLEDs provided negligible EQE roll-off of 1.6% from the maximum values to those at 1000 cd m−2. The AIDF emitters with small ∆EST, high photoluminescence quantum yields (ΦPL), bipolar charge transport and high rate constant of reverse
1
ACCEPTED MANUSCRIPT intersystem crossing process are promising candidates for OLEDs that are roll-off-free and possess high efficiency. Keywords:
Molecular design; High effciency; Delayed fluorescence; Aggregation-induced
emission; Low effciency roll-off. ________________________________________
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* Corresponding author. ** Corresponding author. ***Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (C. Yao), Email: [email protected] (W. Wang).
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ACCEPTED MANUSCRIPT 1. Introduction Pure organic molecules with TADF have attracted significant interest because they can utilizes the excitons from the excited triplet to the singlet (T1→S1) states by reverse intersystem crossing (RISC)
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process to achieve nearly 100% internal quantum efficiency[1−5]. Without exception, all these TADF emitters are characterized by a small ∆EST. A small ∆EST can be easily obtained by minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest
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unoccupied molecular orbital (LUMO) by molecular engineering. Based on this design concept, numerous TADF emitters have been synthesized with a variety building blocks, such as
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benzophenones [6], sulfones [7,8] and triazines [9,10]. External quantum efficiencies (EQEs) of OLEDs based on TADF emitters have reached an impressive 25% [9,11], which is comparable to the best phosphorescent devices and even exceeds their performances in some cases. However,
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there are still many significant challenges that limit the development and application of TADF materials. One of the pressing problems involves aggregation-caused quenching (ACQ) and serious efficiency roll-off caused by exciton annihilations [12,13]. Since most TADF materials need to be
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doped in the host materials to suppress concentration quenching and excitons annihilation, adopting
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a host-guest system leads to increased difficulty in the precise control of film composition. Another problem involves the deposition process of TADF materials, which are generally suitable for vacuum deposition. Only a few reports have demonstrated solution-processed OLEDs comprising of highly soluble small or polymer TADF molecules [14−17]. However, solution based deposition process is simple to realize cost effective and precise controllable in the large-area fabrication of OLEDs. Except as mentioned above, the unbalanced transfer of hole and electron in the emitting layer and the slow dynamics process relates to the triplet exciton mainly responsible for the 3
ACCEPTED MANUSCRIPT efficiency roll-off by arousing severe triplet-involved annihilation processes under high current densities [18]. Currently, aggregation-induced emission (AIE) mechanism provides the most effective method
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to restrain the ACQ problem [19,20]. AIE Materials typically possess highly distorted molecular structures that can weaken the intermolecular π–π interactions and efficiently block the non-radiative decay of exciton in the aggregated state [21−23]. As a result, they show weak
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emissions molecularly dispersed state in solutions but excellent solid-state emission and high ΦPL in
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aggregated state. Hence, AIE emitters are considered as ideal emitters for non-doped OLEDs and have provided high EL effciencies approaching the theoretical limit of pure organic fluorescent emitters along with low efficiency roll-off. However, only 25% singlet (S1) excitons can be utilize for electroluminescence for most AIE emitters, implying that exciton utilization offers a scope for
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improvement. To this end, molecules incorporating prominent TADF and AIE properties have been reported by several groups [21−26]. The slow RISC process and then caused slow radiative deactivation result in significant triplet excitions annihilate at high current density [24]. According
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to the current molecular design theory, small ∆EST are the key in facilitating the T1 → S1 RISC
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process in TADF emitters [27]. Early studies have revealed that small ∆EST can be obtained by effectively separating the wave function distributions of HOMO and LUMO. Therefore, an effective approach to obtain aggregation-enhanced delayed fluorescence is to design molecules with a D-A-D′, D-A and D-π-A structure [28,29]. This configuration can easily form a highly twisted molecular configuration, which can weaken intermolecular π–π interactions in the aggregated state and give rise to weak frontier orbital overlap at the same time [21]. Therefore, the distorted molecular design rule for AIE emitters is consistent with that of TADF 4
[21], which can not only
ACCEPTED MANUSCRIPT avoid intermolecular π–π interactions, but also improve RISC process and radiative deactivation, eventually resulting in improved device efficiency and suppressed efficiency roll-off. Nevertheless, such low overlap would results in a weak intramolecular charge transfer (ICT),
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yielding a low ΦPL, since the ΦPL is direct proportional to such an overlap [30,31]. Therefore, a trade-off exists between ∆EST and ΦPL to maintain excellent fluorescence efficiency in the aggregation-enhanced delayed fluorescence molecular design.
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Although a small number of molecules have been synthesized based on the AIDF design, there
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is scope for potential applicability of these emitters in OLEDs [22]. Herein, we present two new emitters CzTAZPO and sCzTAZPO with AIE and TADF properties using traditional design principles of molecular charge transfer. Both the molecules have an asymmetric D−π−A configuration, where triazine serves an electron acceptor (A), carbazole dendrites as electron donors
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(D) and phenyl spacer as the π-bridge. Furthermore, phosphorus oxygen groups were added to improve the electron transport of the molecules. As expected, a highly twisted conformation was formed between the carbazole dendrites and the phenyl spacer, resulting in the desired AIE and
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TADF properties. In addition, theoretical calculations and experimental results showed that the
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increase of the number of carbazole dendrites led to the fluorescence oscillator strength increased, while for ∆EST the value decrease. CzTAZPO and sCzTAZPO exhibit remarkably AIE and TADF properties, and prominent high EL efficiencies of up to 29.1 cd A−1 and 12.8% in nondoped OLEDs. Furthermore, their nondoped OLEDs also show outstanding EL efficiencies with extremely low efficiency roll-off. 2. Experimental section The
compounds
3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole
2,4-bis(4-bromophenyl)-6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine 5
(TAZPDBr)
(Cz), and
ACCEPTED MANUSCRIPT 2-(4-bromophenyl)-4-(4-(diphenylphosphanyl)phenyl)-6-phenyl-1,3,5-triazine
(TAZPBr)
were
prepared according to literature procedures [30,32]. 2.1 Synthesis of 9',9''''-((6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(4,1phenylene))bis(3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole) (CzTAZP)
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Compound 3,3'',6,6''-tetra-tert-butyl-9'H-9,3':6',9''-tercarbazole (Cz) (0.40 g, 0.62 mmol), 2,4-bis(4-bromophenyl)-6-(4-(diphenylphosphanyl)phenyl)-1,3,5-triazine (TAZPDBr) (1.06 g, 1.5
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mmol), CuI (0.001 g, 0.06mmol), K3PO4 (0.5 g, 2.5 mmol) and (±)-trans-1,2-diaminocyclohexane (0.20 mL, 1.6 mmol) were added into toluene (50 ml) and the solution was heated under Ar
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atmosphere. After 72 h under reflux, the mixture was extracted with CH2Cl2, and the combined organic solution was dried over anhydrous Na2SO4. After solvent removal, the crude product was purified by column chromatography to yield a bright yellow powder (0.45g, 36% yield). The intermediate was used in the following steps without further purification.
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2.2 Synthesis of CzTAZPO
Compound CzTAZP (200 mg, 0.103 mmol) and H2O2 (5 mL, 30wt%) were added to 50 mL of
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CH2Cl2. The mixture was then vigorously stirred overnight at room temperature. After solvent removal, the reaction mixture was extracted with CH2Cl2, and the combined organic solution was
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dried over anhydrous MgSO4. The crude product was purified by column chromatography to obtain CzTAZPO in the form of a bright yellow solid. Yield: 190 mg (94.6%). 1HNMR (400 MHz, CDCl3) δ = 9.18 (d, J=7.0, 4H), 8.99 (s, 2H), 8.29 (s, 4H), 8.17 (s, 8H), 8.03 (d, J=7.4, 4H), 7.98 (d, J=9.7, 2H), 7.81 (d, J=8.4, 4H), 7.76 (d, J=10.5, 4H), 7.67 (d, J=7.4, 4H), 7.61 (s, 2H), 7.53 (s, 4H), 7.47 (d, J=7.7, 8H), 7.37 (d, J=7.8, 8H), 1.47 (s, 72H). 13CNMR (101 MHz, CDCl3) δ = 171.29, 164.41, 142.67, 141.56, 140.08, 139.87, 139.21, 135.08, 132.66, 132.20, 132.10, 131.62, 131.45, 131.07, 128.76, 128.64, 126.95, 126.18, 124.48, 123.61, 123.18, 119.44, 116.26, 111.25, 109.05, 34.85, 6
ACCEPTED MANUSCRIPT 32.04. MS (MALDI-TOF) m/z calcd for C137H130N9OP, 1949.5; found, 1947.5[M-2]+ 2.3 Synthesis of 3,3'',6,6''-tetra-tert-butyl-9'-(4-(4-(4-(diphenylphosphanyl)phenyl)-6-phenyl -1,3,5-triazin-2-yl)phenyl)-9'H-9,3':6',9''-tercarbazole (sCzTAZP)
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To a mixture of TAZPBr (200 mg, 0.35 mmol), Cz (303 mg, 0.42 mmol), CuI (13.3 mg, 0.07 mmol), K3PO4 (297.1 mg, 1.40 mmol) in degassed toluene (50 mL), (±)-trans-1,2-diaminocyclohexane (0.10 mL, 0.80 mmol) was added in a dropwise manner under an Ar atmosphere. The resulting
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mixture was refluxed and stirred for 72 h. The mixture was then filtered, and the filtrate was poured
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into water and extracted with CH2Cl2. Further purification was carried out by chromatography on silica gel with CH2Cl2: petroleum ether: methanol (100:200:1, v/v) as eluent. Yield: 220 mg (51.7%). Without further purification the intermediate was utilized directly in the next step. 2.4 Synthesis of sCzTAZPO
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Compound sCzTAZP (200 mg, 0.165 mmol) and H2O2 (5 mL, 30wt%) were added to 50 mL of CH2Cl2. The mixture was then vigorously stirred overnight at room temperature. After solvent removal, the reaction mixture was extracted with CH2Cl2, and the combined organic solution was
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dried over anhydrous MgSO4. The crude product was purified by column chromatography to give
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CzTAZPO to yield a bright yellow solid. Yield: 195 mg (96.4%). 1HNMR (400 MHz, CDCl3) δ = 9.11 (d, J=8.3, 2H), 8.92 (d, J=6.3, 2H), 8.83 (d, J=7.1, 2H), 8.27 (s, 2H), 8.17 (s, 4H), 7.99 (d, J=8.6, 2H), 7.96 – 7.90 (m, 2H), 7.80 (s, 1H), 7.77 (s, 2H), 7.75 (d, J=4.4, 4H), 7.75 (d, J=4.4, 2H), 7.72 (s, 1H), 7.67 (s, 2H), 7.64 (s, 2H), 7.62 (s, 1H), 7.60 (s, 1H), 7.58 (s, 1H), 7.52 (d, J=5.3, 2H), 7.48 (s, 2H), 7.46 (s, 2H), 7.36 (d, J=8.6, 4H), 1.47 (s, 36H).
13
CNMR (101 MHz, CDCl3) δ =
171.95, 171.10, 170.47, 142.64, 141.32, 140.09, 139.90, 135.76, 135.28, 133.01, 132.71, 132.20, 132.10, 131.67, 131.38, 130.99, 129.09, 128.83, 128.73, 128.61, 126.80, 126.15, 124.48, 123.61, 7
ACCEPTED MANUSCRIPT 123.09, 119.28, 116.25, 111.03, 109.07, 34.74, 32.05. MS (MALDI-TOF) m/z calcd for C85H77N6OP, 1229.5; found, 1229.2[M]+. 3. Results and discussions 3.1. Synthesis and characterization
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The proposed luminogens were synthesized in a straightforward procedure according to routes demonstrated in Scheme 1. The triazine derivative coupled with carbazole dendrites to provide
SC
intermediate CzTAZP and sCzTAZP, followed by oxidation with hydrogen peroxide to furnish CzTAZPO and sCzTAZPO in high yields. Both compounds were soluble in common organic
HNMR, 13CNMR spectroscopy and mass spectrometry as shown in Fig. S1-S6.
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1
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solvents such as tetrahydrofuran (THF) and CH2Cl2. The target compounds were characterized by
Scheme 1. The synthesis route to CzTAZPO and sCzTAZPO. 3.2. Thermal properties Determined through thermogravimetric analysis (TGA) and differential scanning calorimetry 8
ACCEPTED MANUSCRIPT (DSC) were used to investigate the thermal properties of CzTAZPO and sCzTAZPO, As illustrated in Fig. S7. The decomposition temperatures (Td, corresponding to 5% weight loss) are as high as 455 and 478 °C for CzTAZPO and sCzTAZPO, respectively. In addition, these two emitters have
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high glass transition temperatures (Tg) of 153 and 157 °C. High Td and Tg are attributed to the bulky substituents of the carbazole dendrites and the non-planar structures, which can effectively
fabrication and long-term stability of the OLEDs.
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3.3. Electrochemical properties and energy level
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overcome unfavorable morphological changes. This ensures uniform amorphous films upon
The energy levels and electrochemical behavior of both emitters were investigated by cyclic voltammetry as shown in Fig. S8. Both compounds display two reversible oxidation processes suggesting excellent electrochemical stability. These oxidation peaks are attributed to two types
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electron donating carbazole moiety of the carbazole dendritic, similar to previously reported dendritic [33]. From the experimental onset potentials, the HOMO energy levels of CzTAZPO and sCzTAZPO are estimated to be −5.41 and −5.40 eV, respectively. Based on the values of HOMO
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levels and optical band gaps [21], LUMO energy levels of CzTAZPO and sCzTAZPO are obtained
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as −2.72 eV and −2.66 eV, respectively.
3.4. DFT calculations
The optimized geometry and the electronic structures of the frontier molecule orbitals (FMOs) were investigated using time-dependent density functional theory (TD-DFT) calculations based on the PBE0/def2-SVPD level. As depicted in Fig. 1, both compounds exhibited twisted configurations owing to the bulky steric effect of the carbazole dendrites. In their optimized structures, the two molecules adopted moderate dihedral angles of 49.9° and 50.6° between the carbazole planes of the 9
ACCEPTED MANUSCRIPT dendrite and the phenyl spacer for sCzTAZPO and CzTAZPO, respectively. As shown in Fig. 1, their optimized geometries reveal nearly separated FMOs distributions. According to the FMOs, HOMO levels are predominantly localized on the electron-donor carbazole dendrite, while the
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LUMO levels are mainly distributed over the electron-acceptor triazine rings and adjacent phenyl bridges. FMOs of both compounds that are almost completely divided result in extremely small ∆EST, which facilitate the T1→S1 RISC process for both TADF emitters [34]. The small ∆EST
SC
confirmed by the calculations were 0.084 eV and 0.104 eV for CzTAZPO and sCzTAZPO,
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respectively. In addition, the oscillator strengths (f) which are significant indicators of S1→S0 transition dipole moments and ΦPL, were estimated to be 0.159 and 0.236 for sCzTAZPO and CzTAZPO, respectively. The high f values of sCzTAZPO and CzTAZPO are beneficial for realizing large radiative decay rate (krS) and thus promoting (ΦPL) [35]. This indicates that the increase of the
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carbazole dendrites number results in CzTAZPO being a superior emitter that combines a small ∆EST and a large fluorescence oscillator strength. The calculations clear indication that an increase in the number of carbazole dendrites leads to a decrease in the S0 → S1 and an increase S0 → T1
EP
excitation energies, which leads to a net reduction of the ∆EST. With the increase in the number of
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donor moieties, the hole and particle tend to delocalize in order to stabilize the S1 and T1 energies, especially the particle of S1, as shown in the natural transition orbitals (NTO) (Fig. 2). Meanwhile, the introduction of carbazole dendrites onto the triphenyl-triazine is beneficial for the formation of large relevant orbital overlaps due to multiple electronic configurations, which results in an large oscillator strength. The similar observation between the NTO and molecular architectures also have been theoretical studied in other TADF molecules [35].
10
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ACCEPTED MANUSCRIPT
Fig. 1. The optimized geometries, the density distributions of the FMOs and calculated energy
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levels for sCzTAZPO and CzTAZPO based on TD-DFT at the PBE0/def2-SVPD basis set.
Fig. 2. The natural transition orbitals (hole and particle) of the S1 and T1 states for the sCzTAZPO and CzTAZPO. The weight of the contribution to the excitation is also included.
11
ACCEPTED MANUSCRIPT 3.5. Aggregation-induced emission properties Water/THF hybrid solvent with different water volume fractions (fw(vol%)) were used to investigate the AIE behavior of CzTAZPO and sCzTAZPO in nanoaggregates, because both
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molecules are insoluble in water. In dilute THF solution (10−5 M), CzTAZPO and sCzTAZPO exhibited weak PL emission with the peaks located at 551 and 539 nm, and low ΦPLs of 3.2 and 4.1%, respectively (Fig. 3). The addition of water to the THF solution (fw(vol%) < 10%), resulted in
SC
the lowering of the emission as compared to the THF solution. This observation can be attributed to the twisted intramolecular charge transfer (TICT) state as a resulted of the increased polarity of the
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solvent mixture upon addition of water [36,37]. For fw(vol%) above 70%, the PL was reinvigorated and greatly enhanced demonstrating the AIE nature of both materials. A near ten-fold enhancement of 61.3% and 45.9% was observed in the ΦPL for solutions of fw(vol%) = 99% as compared to dilute
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THF solution. This can be explained by the formation of aggregation that restricted the intramolecular torsional/vibrational motions that are present in solution, thereby reducing the non-radiative transition of the excited state [22,24]. In addition, the aggregate formation of
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fluorescent molecules increased the weak dipole-dipole interaction among polar solvent and
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fluorescent molecules, which partially accounts for the blue-shifted PL peaks relative to that in THF solution [22].
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ACCEPTED MANUSCRIPT
Fig. 3. PL spectra of (a) CzTAZPO and (b) sCzTAZPO in THF/water mixtures with different water fractions (fw). (b) The plots of I/I0 values versus fw(vol%). Where I0 is the PL intensity in dilute THF solution. The excitation wavelength is 350 nm. Inset: photographs of CzTAZPO and sCzTAZPO in fw(vol%) = 0 and 99% at 365 nm UV irradiation. 3.6. Photophysical properties The photophysical properties of CzTAZPO and sCzTAZPO are analyzed by UV-vis absorption, PL and phosphorescence (Phos) spectra. As shown in Fig. 4, Both the compounds exhibit strong 13
ACCEPTED MANUSCRIPT absorption bands in dilute CH2Cl2 solution (10−5 M) with peaks (λabs) below 300 nm, which can be assigned to the π-π* transitions of the aromatic moieties. In addition, the weak and broad absorption bands around 380 nm can be attributed to the intramolecular charge transfer (ICT) mainly derived
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from the carbazole dendritic to the triazine. From the edge of absorbance spectrum in film (Fig. S9), the band gaps were calculated to be 2.69 and 2.74 eV for CzTAZPO and sCzTAZPO, respectively. Both materials reveal broad and featureless emission profile with peaks at 512 and 502 nm,
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suggesting that the singlets are identified as CT excited state. In films, CzTAZPO and sCzTAZPO
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exhibit a remarkable ΦPLs of 71% and 57% (Table 1), respectively. With the polar increase of the solvent, the emission spectra of both compounds exhibited red-shifted emission (Fig. S10), further validating that the emissions originate from the CT states [35]. The triplet states are mainly composed of triplets of the donor and acceptor moieties, due to the considerable overlap between
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the hole and particle of the triplet states. The triplet energies derived from the onset edges of the neat films of Phos spectra at 77K were 2.62 eV and 2.65 eV for CzTAZPO and sCzTAZPO, respectively (Fig. S11). The S1 energies estimated from the onset edges of FL spectra at 77K were
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2.70 and 2.75 eV, respectively. Therefore, the ∆EST values of 0.08 and 0.10 eV, are consistent with
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their theoretical values. These values are among the smallest reported for TADF materials [21,35,36], which help facilitate the RISC process in the excited state.
14
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Fig. 4. The absorption spectrum of CzTAZPO and sCzTAZPO in CH2Cl2 solution and PL spectra in
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neat film.
Table 1. Photophysical properties of the emitters. calc. λabs[a] λPL[b] ΦPL τp[d] τd[d] S1[e] T1[e] HOMO[f] LUMO[g] ∆EST [c] [nm] [nm] [ns] [µs] [eV] [eV] [eV] [eV] [eV] ∆EST[h] [%] [eV] CzTAZPO 267, 512 71 40.6 1.1 2.70 2.62 −5.41 −2.72 0.08 0.084 351 sCzTAZPO 269, 502 57 46.9 0.81 2.75 2.65 −5.40 −2.66 0.10 0.104 350 [a] In CH2Cl2 solution (10−5 M). [b] Detected in neat films. [c] Absolute PL quantum yield of neat films determined by a calibrated integrating sphere. [d] PL lifetimes of prompt (τp) and delayed (τd) decay components evaluated at 300 K under vacuum. [e] Singlet (S1) and triplet (T1) energies estimated from onset wavelengths of neat film at 77 K. [f] Obtained from cyclic voltammograms in dichloromethane solution. [g] Calculated from the HOMO levels and optical energy gaps (Eg). [h] Deduced by TD-DFT at PBE0/def2-SVPD.
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Emitter
3.7. Delayed fluorescence and theoretical calculation In order to study the photophysical processes, PL transient decay of neat films of CzTAZPO and sCzTAZPO were acquired. As shown in Fig. 5a and 5b, the PL transient decay revealed a distinct double exponential feature, which can be attributed to the prompt and delayed fluorescence 15
ACCEPTED MANUSCRIPT of CzTAZPO and sCzTAZPO neat films. Time-resolved PL spectra of CzTAZPO and sCzTAZPO also exhibit the identical prompt and delayed PL emission, suggesting the TADF property of both compounds, as shown in Fig. 5c and 5d. At 300K, the neat films of CzTAZPO and sCzTAZPO have
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a clear prompt fluorescence decay component (τp) of 40.6 and 46.9 ns, a delayed fluorescence decay component (τd) of 1.1 and 0.81 µs, corresponding to a ratio of delayed component (Rd) of 40.2% and 37.8% respectively. The kinetic constants of CzTAZPO and sCzTAZPO were calculated from
SC
the PL quantum yield and lifetime data, and are listed in Table S1. The radiative decay rate
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constants (krS) of both the compounds are in the range of 107 s−1 due to favored FMO distributions and are among the highest values reported in similar emitters [21,24,35]. Interestingly, despite the high krS value, CzTAZPO showed a higher rate constant of RISC (kRISC) than sCzTAZPO, indicating that a trade-off exists between krS and ∆EST, which needs to be tuned for efficient TADF emission.
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The high kRISC and krS can ensure the triplet excitions undergo rapid RISC and efficient radiative
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decay, which resulting in excellent EL efficiency and suppressed efficiency roll-off [24].
16
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Fig. 5. The PL transient decay curves of the neat film of (a) CzTAZPO and (b) sCzTAZPO, and
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time-dependent emission spectra of (c) CzTAZPO and (d) sCzTAZPO. Excitation wavelength: 360 nm. Detection wavelengths are 520 nm for CzTAZPO and 510 nm for sCzTAZPO.
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3.8. Electroluminescent properties
The AIE and TADF nature endows CzTAZPO and sCzTAZPO with outstanding solid state
AC C
emission effciency and high utilization of exctions. In order to evaluate the electroluminescence (EL) performance of CzTAZPO and sCzTAZPO, a device consisting of a structure of ITO/PEDOT:PSS (40 nm)/CzTAZPO and sCzTAZPO (50 nm)/TmPyPB (20 nm)/Ca (10 nm)/Al (100 nm) was fabricated. Here, PEDOT:PSS and TmPyPB are poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, respectively, serving as hole- and electron-transporting materials, and the neat film of CzTAZPO and sCzTAZPO functions as light-emitting layer. The energy level diagram of these devices is shown in Fig. 6. The EL emission 17
ACCEPTED MANUSCRIPT spectra are shown in Fig. 6a , with the emission peaks at 537 and 531 nm with the CIE color coordinates of (0.37, 0.56) and (0.36, 0.56), for devices with CzTAZPO and sCzTAZPO as the emitters, respectively.
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Fig. 6b and 6c shows the current density–voltage–luminance (J–V–L) and current efficiency– luminance–external quantum efficiency (CE–L–EQE) characteristic curves of the nondoped devices, and the key data of the OLEDs are listed in Table 2. The nondoped OLEDs exhibit low turn-on
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voltages (Von) of 4.5 and 4.1 V for CzTAZPO and sCzTAZPO, respectively, which can be attributed
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to the optimum energy levels with low electron and hole injection barriers. The CzTAZPO and sCzTAZPO based devices demonstrated a maximum luminance (Lmax) of 9776 and 8283 cd m−2, respectively. Remarkably high maximum current efficiency (CEmax) and maximum external quantum efficiency (EQEmax) of 29.1 cd A−1 and 12.8%, and 20.6 cd A−1 and 9.2% were obtained for
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OLEDs with CzTAZPO and sCzTAZPO as the emitters, respectively. Notably, the nondoped OLEDs maintained high EL efficiencies with increase in the current density. For instance at 1000 cd m−2, the CE and EQE are 28.6 cd A−1 and 12.6% for CzTAZPO device, whereas they are are 20.4 cd
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A−1 and 9.1% for the sCzTAZPO device, respectively. These values are similar to CEmax and
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EQEmax in both cases. The roll-off values can be calculated from peak values to those at 1000 cd m−2. Roll-off of CE is 1.8%, and EQE is 1.6% for CzTAZPO based device, while for sCzTAZPO based device it is 0.97% and 1.1%, respectively, demonstrating enhanced efficiency and stability of nondoped OLEDs.
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Fig. 6. a) The EL spectra of CzTAZPO- and sCzTAZPO-based devices. (Inset) Device photographs. b) The current density–voltage–luminance and c) external quantum efficiency–luminance–current
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effciency characteristics of CzTAZPO- and sCzTAZPO-based devices. d) Energy level diagrams for nondoped devices.
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Table 2. Electroluminescence Characteristics of the Devices @1000cd/m2
Maximum
Von[a] (V)
Lmax (cd/m2)
CE (cd/A)
EQE (%)
CE (cd/A)
EQE (%)
RO[b] [%]
CIE (x,y)
CzTAZPO
4.5
9776
29.1
12.8
28.6
12.6
1.8
0.37,0.56
sCzTAZPO
4.1
8283
20.6
9.6
20.4
9.1
0.97
0.36,0.56
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Emission layer
[a] Von = turn-on voltage at the luminance of 1 cd m‒2. [b] RO = current effciency roll-off from peak value to that at 1000 cd m−2. Such low turn-on voltage, high efficiency and negligible efficiency roll-off could be attributed to the excellent injection and balance of excitons in neat films. To evaluate the transport properties, 19
ACCEPTED MANUSCRIPT CzTAZPO- and sCzTAZPO-based hole- and electron-only devices were fabricated and investigated. The single-carrier devices had the configuration of ITO/TmPyPB (30 nm)/ CzTAZPO or sCzTAZPO (40 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) for electron-only and ITO/
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1,1′-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (30 nm)/CzTAZPO or sCzTAZPO (40 nm)/ TAPC (50 nm)/Au(100 nm) for hole-only cases. As can be seen from Fig. 7, CzTAZPO and sCzTAZPO exhibit almost the identical electron- and hole-current density under the same voltage.
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This indicative that well-balanced electron and hole transporting abilities within neat films of TADF
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dendrimers, which benefit from their small ∆EST [38]. Compared with CzTAZPO-based device, the current densities of sCzTAZPO-based devices are more balanced under the same voltage. The superior bipolar characteristic of sCzTAZPO promote the balance of charges in the emitting layers, which assist in suppressing efficiency roll-offs [35]. This is consistent with the observation that the
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device based on sCzTAZPO as the emission layer has a smaller roll-off. These devices are among the most efficient nondoped AIDF OLEDs along with negligible effciency roll-off reported up till
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now, which can be attributed to rapid RISC processes and well-balanced carrier transport property.
Fig. 7. The current density–voltage curves of the hole- and electron-only devices based on CzTAZPO and sCzTAZPO. To evaluate the utilization efficiency of excitons and validate the EL efficiencies of nondoped 20
ACCEPTED MANUSCRIPT OLEDs, quantitative estimation of the theoretical values of external quantum efficiencies was performed. The maximum external quantum efficiency ηext can be expressed by Equation (1) and (2) [24]. (1)
ηint = γ × [ηS × ΦP + (ηS × ΦISC + ηT) × ΦRISC]
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ηext = ηint × ηout
(2)
Where, ηint refers to the internal quantum efficiency, ηout is the optical out-coupling factor
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(usually 0.2~ 0.3), γ is the charge balance factor (ideally γ = 1.0), and ηS and ηT are the fractions of
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singlet (25%) and triplet excitons (75%), ΦP = fluorescent components quantum yield, ΦISC = the intersystem crossing quantum yield. ΦRISC = the reverse intersystem crossing quantum yield, respectively. According to the kinetic parameters in Table S1, the theoretical ηext are estimated to be 14.5%–21.6% for the devices based on CzTAZPO and 12.6%–18.9% for sCzTAZPO. Theoretical
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calculations are in reasonable consistent with the experimentally obtained parameters, implying high exciton utilization have been realized in the emission layers. The utilization of excitons of nondoped OLED is expressed by Equation (3) [21].
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ηext = (γ × ηST × ΦPL ) × ηout
(3)
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Where ηST is the fraction of excitons involving radiative decay, and ΦPL is the PL quantum yield of the nondoped films. Given a general ηout of 0.2–0.3 and experimental values of ηext (12.8 and 9.6% for CzTAZPO and sCzTAZPO) and ΦPL (71 and 57% for CzTAZPO and sCzTAZPO), the ηST are calculated to be 60.1%–90.1% for CzTAZPO based OLEDs and 56.1–84.2% for sCzTAZPO based OLEDs, which is about 2.2–3.6-fold higher than the limit of spin statistical ratio (25%) for conventional fluorescent emitters. There are only a few reported studies involving nondoped OLEDs by solution process based on 21
ACCEPTED MANUSCRIPT TADF materials and the relevant key performance of these devices are shown in Table 3. As is evident from the literature, only a few nondoped/doped devices with simple configuration gave a high EQEmax that is comparable to the results in this work. However, these devices suffered from
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significant efficiency decline at high current density. For instance, the solution processed nondoped OLED reported by Lee et al. providing a EQEmax of 18.3% declined to 12.0% at the luminance of 1000 cd m−2, leading to sharply effciency roll-off of 34.4%. The EQEmax values of other nondoped
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OLEDs were below 12%, which cannot be compared with the efficiency in this work. In addition,
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some devices adopted TADF as a dopant for solution processed devices. Even though the efficiency of CzTAZPO-based nondoped device is somewhat lower than the top values, the effciency roll-off is superior to those of reported nondoped and even some doped OLEDs. Therefore, the combination of AIE and TADF properties in CzTAZPO makes it one of the most promising materials for
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nondoped OLEDs.
22
ACCEPTED MANUSCRIPT Table 3. EL performances of nondoped OLEDs in this work and the reported TADF emitters. Emitter
EML
Von a)
EQEmax
EQE b)
Roll-off c)
[%]
[%]
[%]
Reference
nondoped
−
18.3
12.0
34.4
[39]
CzDMAC-DPS
nondoped
3.6
12.2
4.5
63.1
[40]
tBuG2TAZ
nondoped
3.5
9.5
−
−
[41]
G3TAZ
nondoped
3.5
3.4
−
−
[16]
G2B
nondoped
3.4
5.7
−
−
[42]
TZ-3Cz
nondoped
3.6
10.1
−
−
[43]
B-oCz
nondoped
4.4
8.0
−
−
[44]
Cz-CzCN
nondoped
3.1
15.5
−
−
[45]
CDE1
nondoped
−
12.0
11.9
0.83
[46]
TPA–AQ
doped
3.8
7.5
−
−
[47]
3a
doped
4.5
5.6
−
−
[48]
ACRDSO2
doped
3.7
17.5
14.0
20
[13]
G3Ph
doped
2.5
16.1
10.2
36.6
[49]
MA–TA
doped
−
22.1
−
−
[50]
doped
−
11.2
−
−
[50]
doped
6.2
11.3
10.6
6.2
[51]
4.5
12.8
12.6
1.6
This work
CzTAZPO
nondoped
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4CzCNPy
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t4CzIPN
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[a] Turn-on voltage at 1 cd m‒2. [b] External quantum efficiency at the luminance of 1000 cd m−2. [c] RO = EQE roll-off from peak value to that at 1000 cd m−2.
4. Conclusions
Two OLED emitters were developed in this work, namely CzTAZPO and sCzTAZPO that possessed highly twisted D-π-A carbazole dendritic structures. The properties of AIE, TADF, and bipolar charge transport were combined to establish a molecular design strategy that allowed
23
ACCEPTED MANUSCRIPT CzTAZPO to exhibit a high ΦPL and small ∆EST at the same time. Solution-processed nondoped OLEDs were fabricated by adopting CzTAZPO and sCzTAZPO as the emission layer. The OLEDs exhibited excellent EL efficiencies of 29.1 cd A−1 and 12.8% for CzTAZPO, and 20.4 cd A−1 and 9.1%
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for sCzTAZPO, respectively. Extremely flat CE roll-off of 1.8% and 0.97% were obtained for CzTAZPO and sCzTAZPO based OLEDs, while the EQE roll-off were 1.6% and 1.1% from maximum values to those at 1000 cd m−2. Detailed analyses showed that the flat efficiency decline
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was derived from the well-balanced charges transfer and fast reverse intersystem crossing and
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fluorescence decay rate, which not only shortened exciton lifetimes but also reduced exciton concentration. The promotion of this molecular design strategy is a promising method for developing ideal materials for efficient OLEDs with simplifed structures. The stability of these devices will be study to confirm the superiority of CzTAZPO and sCzTAZPO in our future work.
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Acknowledgments
The authors would like to thank the Natural Science Foundation of Jiangsu Province (BK20160440,
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Highlights
Solution-processed aggregation-induced delayed fluorescence (AIDF) emitters based on strong π-accepting triazine cores for highly effcient
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nondoped OLEDs with low efficiency roll-off
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Jinshan Wang a ,*, Chao Liu a, Cuifeng Jiang a, Chuang Yao b, **, Min Gu a, Wei Wang c, ***
Highlights
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• A strategy to construct solution-processed AIDF was proposed
• Two solution-processed AIDF emitters were designed and synthesized. • CzTAZPO exhibit high oscillator strengths and small singlet–triplet energy gap. • CzTAZPO based nondoped OLED shown high efficiencies (EQE>12%) and low roll-off (< 1.6%).
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• This strategy would be a promising method for efficient and stable OLEDs.