Dual Encapsulation of Electron Transporting Materials To Simplify

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Dual Encapsulation of Electron Transporting Materials to Simplify HighEfficiency Blue Thermally Activated Delayed Fluorescence Devices Wenjing Kan, Chunbo Duan, Mingzhi Sun, Chunmiao Han, and Hui Xu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03518 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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Chemistry of Materials

Dual Encapsulation of Electron Transporting Materials to Simplify High-Efficiency Blue Thermally Activated Delayed Fluorescence Devices Wenjing Kan,‡ Chunbo Duan,‡ Mingzhi Sun, Chunmiao Han and Hui Xu* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, 74 Xuefu Road, Harbin 150080, P. R. China

ABSTRACT: The charge flux balance and interfacial optimization are two core concerns when simplifying blue thermally activated delayed fluorescence (TADF) diodes, which reflects the more stringent demand on carrier transporting materials (CTM) as the embodiment of the contradiction between charge transportation and quenching suppression with the opposite requirement on intermolecular interactions. Herein, phenylbenzimidazole (PBI) was used as core substituted with two diphenylphosphine oxide (DPPO) groups to form six dual-encapsulated charge-exciton separation (CES) type electron transporting materials (ETM) with the collective name of xyPBIDPO. Through tuning the substitution positions of DPPO group, its two functions of resonance and steric effects were integrated and optimized to enhance charged moiety encapsulation without cost of reducing electroactivity. As the result, among xyPBIDPO, mmPBIDPO successfully realizes the balance of favorable electrical performance and interfacial interaction suppressions in virtue of its doubled mesa-substitution structure and roughly

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symmetrical configuration, rendering the good electron affinity of 2.8 eV, the high electron mobility by the level of 10-6 cm2 V-1 s-1 and effective PBI-core encapsulation. Consequently, mmPBIDPO was used to extremely simplify the blue TADF devices with the state-of-the-art performance from trilayer and quadruple-layer configurations, such as the maximum external quantum efficiency (EQE) beyond 20% and improved efficiency stability. This work not only established a solid example of CES-type ETM for high-performance simple structured blue TADF devices, but also provided the direction of developing this kind of materials in the future.

1. Introduction Recently, organic light-emitting diodes (OLED) employing thermally activated delayed fluorescence (TADF) emitters arouse the extensive attention, which can realize 100% exciton harvesting ratio in virtue of reverse intersystem crossing (RISC) for triplet→singlet upconversion and all exciton utilization.1-5 Most of TADF systems are donor-acceptor (D-A) featured organic molecules with merits in low cost and environmental conservation, however, whose high polarities and the strong intermolecular interactions render their optical properties sensitive to environment and worsen quenching effects.5-21 In most cases, TADF devices directly use the materials developed for phosphorescent OLEDs (PHOLED) on account of their similar electroluminescence (EL) processes involving in both singlet and triplet excitons, overlooking the essential differences of phosphorescence and TADF dyes in molecular components and characteristics.6, 11, 12, 22-29 It can be noteworthy that in contrast to phosphorescent counterparts, blue TADF diodes often suffered the remarkable efficiency roll-offs as a combination result of triplet-triplet and singlet-triplet annihilations (TTA and STA), which should be attributed to the high densities of both singlet and triplet excitons.17, 29-40 In this sense, the molecular design of host, dopant and carrier-transporting materials (CTM) for blue TADF diodes should give

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considerations to not only their respective intrinsic optoelectronic properties but also quenching suppression, which can hardly be satisfied simultaneously.41, 42 Actually, the most popular blue TADF host material bis{2-[di(phenyl)phosphino]phenyl}ether oxide (DPEPO) is inferior in electrical properties, but superior in host-dopant energy transfer with high triplet energy (T1) beyond 3.0 eV and weak host-host and host-dopant intermolecular interactions,43 the latter of which endows its blue TADF devices with the state-ofthe-art performances, e.g. the maximum external quantum efficiencies (EQE) more than 20%.24, 25, 36, 44-47

However, the charge flux unbalance and worse interfacial interactions between

emissive layer (EML) and carrier transporting layers (CTL) rendered the serious efficiency rolloffs in simple-structured devices, forcing the utilization of modified layers and the consequent complicated configurations.12 Therefore, blue TADF host materials featured twisted and/or unsymmetrical structures were developed to realize the 20% EQE and reduced roll-offs on the basis of the simplified five-layer configurations, manifesting the significance of intermolecular interaction regulation.48-57 Obviously, this strategy can be reference to the CTM design for blue TADF diodes. Actually, the negative influence of interfacial interactions between EML and electron transporting layer (ETL) on EL efficiencies and efficiency stability of bis[4-(9,9dimethyl-9,10-dihydroacridine)-phenyl]sulfone (DMAC-DPS)-based blue TADF devices was already demonstrated, which can be suppressed through incorporating steric hindrance groups in either host materials or electron transporting materials (ETM).58,

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The recent reports about

phenylbenzimidazole (PBI)-based ETMs revealed that even for blue phosphorescence OLEDs, interfacial exciton-polaron interaction can be dramatically suppressed to improve device performance through separating charged PBI cores of these materials from EML, however, accompanied by the weakened electrical performance, which is the embodiment of the

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contradiction between quenching suppression and charge transportation with the opposite requirement on intermolecular interactions.60 Therefore, although the effectiveness of so-called “charge-exciton separation (CES)” strategy was demonstrated, how to construct CES-type ETM with preserved electron transporting ability is still a big challenge. In recent decades, phosphine oxide (PO)-based ETMs emerge for phosphorescence and TADF devices in virtue of the electron inductive effect and steric hindrance of PO groups, corresponding to their two opposite but also complementary functions as enhancing electron affinity (EA) and reducing the involvement of electron transporting (ET) cores in intermolecular interactions, which doubtlessly make it possible to restore the electron transporting ability of the CES-featured systems.61-72 It was demonstrated that diphenylphosphine oxide (DPPO) substituent on N-phenyl at PBI has a single effect on separating ET core from EML|ETL interface, while DPPO on 2-phenyl in PBI can simultaneously further polarizes the core and thereby enhance electrical performance, which just embodies these two functions of PO groups (Figure 1a).60 In this case, through rational combination of these two substitution positions, the charge flux balance and the exciton quenching suppression can be optimized on the basis of the bi-DPPO functionalized PBI-type ETMs, providing a feasible strategy to realize the "ideal" CES-type ETMs.

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Figure 1. (a) Molecular design, chemical structures and DFT-optimized spatial configurations of xyPBIDPO (RU: resonance unit; ETU: electron transfer unit; SU: separation unit) and synthetic procedure: i. Ph2PH, Pd(Ac)2, NaAc, DMF, reflux, 24 h; ii. Na2S2O5, DMF, 80 oC, 24 h; (b) Single crystal structures of omPBIDPO, opPBIDPO and pmPBIDPO.

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In this contribution, six constitutionally isomeric PBI-cored ETMs with two DPPOs as peripheral groups, collectively named xyPBIDPO (x = o, m, p; y = m, p), were designed and prepared (Figure 1a), in which x and y refer to the substitution positions of DPPO groups on 2phenyl and N-phenyl, respectively, namely ortho- (o), meso- (m) and para- (p). xyPBIDPO showed the identical optical properties and T1 energy of ~3.0 eV, owing to the insulating effect of P=O linkage. As expected, their EA is completely consistent with the resonance effects of their x-DPPOs, without any additional assistances from y-DPPO groups. Both single-crystal packing diagrams and density functional theory (DFT) simulations reveal the correlation between encapsulation effect of DPPO groups and their substitution positions (o > m > p), which is roughly opposite to the electron transporting abilities of xyPBIDPO. In consequence, mmPBIDPO successfully harmonizes the functions of its two DPPO groups to achieve the favorable electron mobility (e) by the level of 10-6 cm2 V-1 s-1 and the effective encapsulation of its PBI core, supporting the state-of-the-art performances to its simplified DMAC-DPS-based TADF devices, e.g. the maximum EQE beyond 19% and 20% on the basis of trilayer and quadruple-layer configurations, respectively, as well as the reduced efficiency roll-offs. Importantly, the interfacial exciton trap (IET) on hole-transporting layer (HTL)|EML interfaces can halve the device efficiencies in the same way, which further indicates the significance of host and CTM optimization and compatibility for the improvement of blue TADF device performance. 2. Experimental Section General procedure of imidazole cyclization reaction: In Ar2, aromatic aldehyde (1 mmol) and arylamine (1 mmol) were dissolved in DMF (5 mL). Then, saturated Na2S2O5 aqueous solution was added in the mixture and stirred for 20 h at 80 oC. After cooled to room temperature, the

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system was poured into water. The precipitate was filtered as crude product, which was further recrystalized with methanol to afford the phenylbenzimidazole derivatives. 1-(3-Bromophenyl)-2-(2-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole: prepared with 2-(diphenylphosphoryl)benzaldehyde and N1-(3-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 70%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.884 (d, J = 8.0 Hz, 2H); 7.778 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 2H); 7.442-7.484 (m, 9H); 7.420 (t, J = 2.2 Hz, 1H); 7.412 (d, J = 1.6 Hz, 1H); 7.334-7.382 (m, 4H), 7.278 (dd, J1 = 8.2 Hz, J2 = 1.0 Hz, 1H); 7.008 ppm (d, J = 8.0 Hz, 2H); LDI-TOF: m/z (%): 548 (100) [M+]; elemental analysis (%) for C31H22BrN2OP: C 67.77, H 4.04, N 5.10; found: C 67.76, H 4.06, N 5.15. 1-(4-Bromophenyl)-2-(2-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole: prepared with 2-(diphenylphosphoryl)benzaldehyde and N1-(4-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 72%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.930 (d, J = 8.0 Hz, 2H); 7.822 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 2H); 7.431-7.539 (m, 11H); 7.334-7.382 (m, 4H), 7.322 (dd, J1 = 8.0 Hz, J2 = 1.2 Hz, 1H); 7.050 ppm (d, J = 7.6 Hz, 2H); LDI-TOF: m/z (%): 548 (100) [M+]; elemental analysis (%) for C31H22BrN2OP: C 67.77, H 4.04, N 5.10; found: C 67.78, H 4.07, N 5.17. 1,2-Bis(3-bromophenyl)-1H-benzo[d]imidazole: prepared with 3-bromobenzaldehyde and N1(3-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 74%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.887 (d, J = 8.0 Hz, 1H); 7.645 (d, J = 8.0 Hz, 1H); 7.547 (s, 1H); 7.411-7.521 (m, 4H); 7.324-7.410 (m, 2H); 7.296 (t, J = 8.0 Hz, 1H); 7.146-7.267 ppm (m, 2H); LDI-TOF: m/z (%):428 (100) [M+]; elemental analysis (%) for C19H12Br2N2: C 53.30, H 2.83, N 6.54; found: C 53.28, H 2.84, N 6.57.

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2-(3-Bromophenyl)-1-(4-bromophenyl)-1H-benzo[d]imidazole:

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prepared

with

3-

bromobenzaldehyde and N1-(4-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 70%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.916 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 2H); 7.680 (d, J = 7.6 Hz, 1H); 7.591-7.630 (m, 1H); 7.516-7.558 (m, 2H); 7.392-7.478 (m, 3H); 7.314(d, J = 8.0 Hz, 1H), 7.234 ppm (t, J = 3.8 Hz, 2H); LDI-TOF: m/z (%):428 (100) [M+]; elemental analysis (%) for C19H12Br2N2: C 53.30, H 2.83, N 6.54; found: C 53.31, H 2.85, N 6.58. 1-(3-Bromophenyl)-2-(4-bromophenyl)-1H-benzo[d]imidazole:

prepared

with

4-

bromobenzaldehyde and N1-(3-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 72%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.879 (d, J = 8.0 Hz, 1H); 7.624 (d, J = 8.0 Hz, 1H); 7.54 (s, 1H); 7.428 (dd, J1 = 16.0 Hz, J2 = 8.0 Hz, 4H); 7.349 (dd, J1 = 8.4 Hz, J2 = 1.6 Hz, 2H); 7.281 (t, J = 7.4 Hz, 1H); 7.184 ppm (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 2H); LDI-TOF: m/z (%):428 (100) [M+]; elemental analysis (%) for C19H12Br2N2: C 53.30, H 2.83, N 6.54; found: C 53.33, H 2.80, N 6.59. 1,2-Bis(4-bromophenyl)-1H-benzo[d]imidazole: prepared with 4-bromobenzaldehyde and N1(4-bromophenyl)benzene-1,2-diamine to afford white powder with a yield of 75%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.878 (d, J = 8.0 Hz, 1H); 7.646 (d, J = 8.4 Hz, 2H); 7.429 (dd, J1 = 8.4 Hz, J2 = 1.6 Hz, 4H); 7.337 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H); 7.211 (t, J = 7.6 Hz, 1H); 7.202(d, J = 8.4 Hz, 1H), 7.179 ppm (d, J = 8.8 Hz, 2H); LDI-TOF: m/z (%):428 (100) [M+]; elemental analysis (%) for C19H12Br2N2: C 53.30, H 2.83, N 6.54; found: C 53.31, H 2.84, N 6.60. General phosphorylation procedure of bromides: In Ar2, bromide (1 mmol), NaAc (1.1 mmol), Pd(Ac)2 (0.05 mmol) and diphenylphosphine (1.1 mmol) were dissolved in DMF (10 mL). The

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mixture was heated to reflux under stirring for 24h. After cooled to room temperature, the reaction was quenched with water (10 mL). The system was then extracted with CH2Cl2 (3×3 mL). The organic layer was combined and dried with anhydrous sodium sulfate. The solvent was removed in vacuo to afford the crude phosphine intermediate product. Then, the phosphine was dissolved in CH2Cl2 (10 mL). H2O2 (30%, 4 mL) was added to the solution in dropwise at 0 oC and stirred for 2 h. Then, the mixture was extracted with CH2Cl2 (3×3 mL). The organic layer was combined and dried with anhydrous sodium sulfate. The solvent was removed in vacuo, and then the residue was purified by flash column chromatography to afford phosphine oxide derivatives. 2-(2-(Diphenylphosphoryl)phenyl)-1-(3-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (omPBIDPO):

prepared

from

1-(3-bromophenyl)-2-(2-(diphenylphosphoryl)phenyl)-1H-

benzo[d]imidazole to afford white powder with a yield of 75%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.814 (d, J = 8.0 Hz, 2H); 7.756 (dd, J1 = 11.6 Hz, J2 = 3.8 Hz, 1H); 7.638 (dd, J1 = 12.6 Hz, J2 = 7.4 Hz, 1H); 7.582 (d, J = 8.0 Hz, 1H); 7.492-7.556 (m, 9H); 7.402-7.460 (m, 10H); 7.155-7.244 (m, 7H); 7.057 ppm (d, J = 7.6 Hz, 1H); LDI-TOF: m/z (%): 670 (100) [M+]; elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.03, H 4.80, N 4.21. 2-(2-(Diphenylphosphoryl)phenyl)-1-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (opPBIDPO):

prepared

from

1-(4-bromophenyl)-2-(2-(diphenylphosphoryl)phenyl)-1H-

benzo[d]imidazole to afford white powder with a yield of 73%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.675 (d, J = 7.2 Hz, 1H); 7.647 (dd, J1 = 9.6 Hz, J2 = 2.8 Hz, 3H); 7.612 (d, J = 8.4 Hz, 1H); 7.548 (dd, J1 = 7.6 Hz, J2 = 5.6 Hz, 6H); 7.483-7.529 (m, 9H); 7.414-7.458 (m, 4H); 7.302-7.350 (m, 4H), 7.244 ppm (d, J = 3.2 Hz, 4H); LDI-TOF: m/z (%): 670 (100) [M+];

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elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.02, H 4.83, N 4.22. 1,2-Bis(3-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (mmPBIDPO): prepared from 1,2-bis(3-bromophenyl)-1H-benzo[d]imidazole to afford white powder with a yield of 52%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.813 (d, J = 8.0 Hz, 2H); 7.758 (d, J = 7.6 Hz, 2H); 7.614 (d, J = 12 Hz, 1H); 7.482-7.564 (m, 14H); 7.403 (t, J = 7.4 Hz, 9H); 7.306 (dd, J1 = 13.6 Hz, J2 = 6.4 Hz, 2H); 7.225 (t, J = 7.6 Hz, 1H); 7.114 ppm (d, J = 8.0 Hz, 1H); LDI-TOF: m/z (%): 670 (100) [M+]; elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.00, H 4.84, N 4.22. 2-(3-(diphenylphosphoryl)phenyl)-1-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (mpPBIDPO): prepared from 2-(3-bromophenyl)-1-(4-bromophenyl)-1H-benzo[d]imidazole to afford white powder with a yield of 55%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.852 (dd, J1 = 4.0 Hz, J2 = 10.0 Hz, 2H); 7.732-7.794 (m, 3H); 7.642-7.722 (m, 6H); 7.592 (d, J = 6.4 Hz, 1H); 7.557 (dd, J1 = 7.4 Hz, J2 = 1.6 Hz, 2H); 7.474-7.526 (m, 10H); 7.352-7.408 (m, 4H), 7.281 (dd, J1 = 2.0 Hz, J2 = 8.4 Hz, 3H), 7.212 ppm (d, J = 7.6 Hz, 1H); LDI-TOF: m/z (%): 670 (100) [M+]; elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.03, H 4.81, N 4.23. 1-(3-(Diphenylphosphoryl)phenyl)-2-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (pmPBIDPO): prepared from 1-(3-bromophenyl)-2-(4-bromophenyl)-1H-benzo[d]imidazole to afford white powder with a yield of 61%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.892 (d, J = 8.0 Hz, 1H); 7.805 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H); 7.612-7.714 (m, 9H); 7.454-7.572 (m, 14H); 7.366-7.442 (m, 5H); 7.2943 (t, J = 7.6 Hz, 1H); 7.19-7.21 ppm (d, J = 8.0 Hz, 1H); LDI-

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TOF: m/z (%): 670 (100) [M+]; elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.01, H 4.83, N 4.22. 1,2-Bis(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazole (ppPBIDPO): prepared from 1,2-bis(4-bromophenyl)-1H-benzo[d]imidazole to afford white powder with a yield of 63%. 1H NMR (TMS, CDCl3, 400 MHz):  = 7.878 (d, J = 8.0 Hz, 1H); 7.784 (dd, J1 = 11.2 Hz, J2 = 8.4 Hz, 2H); 7.634-7.726 (m, 12H); 7.548-7.616 (m, 4H); 7.452-7.514 (m, 8H); 7.414 (d, J = 6.8 Hz, 2H); 7.33-7.37(m, 1H), 7.27-7.30 ppm (t, J = 6.8 Hz, 2H); LDI-TOF: m/z (%): 670 (100) [M+]; elemental analysis (%) for C43H32N2O2P2: C 77.01, H 4.81, N 4.18; found: C 77.01, H 4.83, N 4.22. 3. Results and Discussions 3.1. Design, Synthesis and Structures Two main functions of DPPO, namely electron-withdrawing effect and steric hindrance, were considered when designing CES-type ETMs with PBI as carrier transporting channel for appropriate electron mobility, in which the dominant function of DPPO depends on its substitution positions.73 Firstly, the inductive effect of DPPO on 2-phenyl is much stronger than that of DPPO on N-phenyl, ascribed to the insulating effect of saturated N atom on intramolecular electronic coupling, which can remarkably enhance the electron affinity. Furthermore, the resonance effect of P=O is dependent on its substitution positions, namely m > p ≈ o. Secondly, these two kinds of DPPO groups have dual encapsulation effect on PBI core, which should be in direct proportion to the distance between them. The steric hindrance of orthoDPPO on 2-phenyl would significantly distort the quasi-planar PBI core, additionally influencing its electroactivity. Thirdly, the involvement of two DPPO groups in one PBI derivative would be beneficial to enhance molecular symmetry and thereby inter-core

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interactions, making it possible to realize the optimization and perfect harmonization of paradoxical core encapsulation and inter-core charge hopping. In consequence, DPPO bissubstitution can not only double the core encapsulation, but also improve electron transportation (ET). The final performance of the ETM is dependent on the balance of two functions for DPPO groups and molecular configuration optimization, which can be actually determined by the combination of DPPO substitution positions. In addition, our previous works showed that DPPO hardly contributes to the T1 location.74-79 Therefore, it serves as an electroactive peripheral groups for encapsulation and configuration adjustment with negligible influences on optical properties, which establishes the basis for selectively enhancing electrical performance of CES-type ETMs. Consequently, six bisubstituted PBI derivatives named xyPBIDPO containing one DPPO on 2-phenyl with three different substitution positions (o, m and p, corresponding to x) and one DPPO on N-phenyl with two different substitution positions (m and p, corresponding to y) were designed, in which besides of the steric hindrance similar to y-DPPOs as separation units (SU), x-DPPOs as resonance units (RU) have various inductive effects in accord with their resonance effects. The different orientation directions for 2- and N-phenyls further render the two-dimensional (2D) encapsulation of xyPBIDPO (Figure 1a). Nevertheless, their configurations optimized by DFT simulation indicate the diverse exposure degrees of their PBI cores directly proportional to DPPO-PBI distances increased from o, m to p, rendering the encapsulation order of xyPBIDPO as omPBIDPO > opPBIDPO > mmPBIDPO > mpPBIDPO ≈ pmPBIDPO > ppPBIDPO. Furthermore, for x-DPPO, m- and p-DPPOs hardly twist the molecular configurations with the normal dihedral angles of ~30o for quasi-planar PBI; while, the strong steric hindrance of oDPPO remarkably distorts the PBI cores of omPBIDPO and opPBIDPO with the doubled

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dihedral angles of 63 and 78o, respectively, which would impede the intermolecular interactions and weaken inductive effect of DPPOs. xyPBIDPO can be conveniently prepared through the typical cyclization and Pd-catalyzed phosphorylation with moderate yields (Figure 1a). The chemical structure characteristics were established on the basis of NMR spectra, mass spectra and elementary analysis. The exact structures of omPBIDPO, opPBIDPO and pmPBIDPO were further demonstrated with single crystal x-ray diffraction, which directly verify the highly twisted PBI cores of omPBIDPO and opPBIDPO and the degressive encapsulation capabilities of o, m and p-DPPO groups (Figure 1b). xyPBIDPO showed the outstanding thermal stability with the temperature of decomposition (Td) at weight loss of 5% beyond 400 oC, beneficial to the device fabrication through vacuum evaporation (Figure S1 and Table 1). Suffering from the high intramolecular intension, Td of omPBIDPO and opPBIDPO are 30-40 oC lower than those of their isomers. While, their compact and rigid structures contrarily render their highest melting points (Tm) of 251 and 220 o

C, respectively, which are more than 20 oC higher than those of pmPBIDPO and ppPBIDPO. It

is noteworthy that no Tm was observed for mmPBIDPO and mpPBIDPO, in accord with their suppressed intermolecular and inter-core interactions by the direct linkage of m-DPPOs on their PBI cores. Nevertheless, mmPBIDPO reveals the temperature of glass transition (Tg) as high as 120 oC, supporting the favorable morphological stability and amorphous film formability. 3.2. DFT Simulation The ground states (S0) configurations of xyPBIDPO were optimized with DFT simulation at the level of B3LYP/6-31g(d) to figure out the influence of DPPO substitution positions on molecular configuration and electronic characteristics (Figure 2).

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Figure 2. Energy levels and contours of frontier molecular orbitals for ground-state xyPBIDPO optimized with DFT simulation. Consistent with the single crystal results, the molecular configurations of omPBIDPO and opPBIDPO are highly twisted, in contrast to the preserved quasi-planar PBI configurations of the other isomers, resulting in their various frontier molecular orbital (FMO) locations. In this case, PBI cores of mmPBIDPO, mpPBIDPO, pmPBIDPO and ppPBIDPO are their main carrier transporting channels with the major contributions to the HOMO and the LUMO, accompanied with the peripheral DPPO groups served as intermolecular electron hopping bridges due to their major contributions to the LUMO+1 or the LUMO+2. On contrary, the HOMO and the LUMO of omPBIDPO and opPBIDPO are separated with respective locations on benzimidazole and phenyl of their PBI cores. Nevertheless, their LUMO+1 and the LUMO+2 are also dispersed to the peripheral DPPO groups. The anion reorganization energy (ER) of

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xyPBIDPO was further estimated by DFT calculation, which can quantitatively evaluate the single molecular electron exchangability.80 ER of mmPBIDPO is the lowest among xyPBIDPO, while that of mpPBIDPO is the second, which should be attributed to the enhanced electron stability by overlapped resonance effects of electron-withdrawing benzimidazole and P=O groups at mutual mesa positions. In the same way, omPBIDPO, opPBIDPO, pmPBIDPO and ppPBIDPO reveal 1 eV larger ER, due to the staggered resonance effects of their two DPPO groups. Therefore, mmPBIDPO is superior in the molecular electron exchange process, even though its LUMO energy level is not the deepest among xyPBIDPO. It can be noticed that for mmPBIDPO and ppPBIDPO, their LUMO+1s are mainly localized on N-phenyl and the relative y-DPPO groups with the minor contributions from PBI cores to facilitate the electron migration between peripheral DPPO bridges and PBI cores, thereby enhancing the DPPOintermediate intermolecular ET, which should be ascribed to the roughly symmetrical molecular configurations of mmPBIDPO and ppPBIDPO. Furthermore, it is known that the regular intermolecular stacking is beneficial to the carrier transportation on the basis of molecular symmetry. Nevertheless, on account of the encapsulation effects decreasing from ortho-, mesoto para-substitution and the synergistic steric hindrances of both DPPO groups, the electron transporting abilities of xyPBIDPO would be comprehensively resulted by molecular electron exchangability, packing mode and intermolecular interactions with a rough order of pyPBIPO ≥ myPBIPO >> oyPBIPO; meanwhile, mmPBIDPO and ppPBIDPO would be superior to their asymmetrical congeners in molecular symmetry. The FMO energy levels of xyPBIDPO are roughly determined by the electron-withdrawing effects of P=O groups, which is in reverse proportion to their resonance effects, viz. p ≈ o > m (Figure 2 and Table 1). The strong inductive effect of para-DPPO renders simultaneous

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decrease of the LUMO and HOMO. Therefore, the LUMOs and the HOMOs of pyPBIDPO are the deepest by the levels of -1.5 and -5.9 eV, respectively, revealing their strong electroninjecting and hole-blocking ability. In contrast, the twisted configurations and the insulated conjugation of PBI cores remarkably weaken the electron affinities of oyPBIDPO, rendering their anomalous FMO energy levels around -1.0 and -5.7 eV, which are even slightly shallower than those of myPBIDPO (-1.2 and -5.8 eV). Due to their insulated conjugations, the HOMOLUMO gaps of oyPBIDPO, corresponding to the first singletexcited energy level (S1), are around 4.9 eV, which are significantly larger than those of myPBIDPO (4.5 eV) and pyPBIDPO (4.4 eV). In the same way, the calculated T1 energy of xyPBIDPO is gradually decreased from oyPBIDPO (2.8 eV), myPBIDPO (2.7 eV) to pyPBIDPO (2.6 eV), which should be attributed to the overestimated conjugation extension by P=O groups (Table 1). Nevertheless, the T1 locations of xyPBIDPO are identical with the major contributions from their PBI cores and the slight dispersions on the N-phenyls (Figure S2). The effective encapsulation of the tripletlocalized moieties with more relatively inertia DPPO groups can further suppress triplet exciton diffusion and interfacial-interaction induced quenching. In consequence, compared to its isomers, besides of its more effectively encapsulated PBI core, mmPBIDPO can remedy its moderate electron affinity with its strongest molecular electron exchangability and improved molecular symmetry, making the balance of charged moiety separation and electron transportability can be expected. 3.3. Optical Properties All of xyPBIDPO reveal the similar absorption spectra with three main peaks around 230, 280 and 300 nm, corresponding to n→* and →* transitions of PBI and →* transition of phenyls, in dilute solutions (10-6 M in CH2Cl2) (Figure 3). Nevertheless, in accord with DFT

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results, their absorption edges gradually shift red from oyPBIDPO, myPBIDPO to pyPBIDPO, corresponding to the S1 energy of 3.85, 3.75 and 3.55 eV, which should be attributed to the different conjugations of their PBI chromophores increased from nonplanar, planar to para-P=O extended configurations (Table 1). In contrast, the fluorescence (FL) spectra in dilute solutions of xyPBIDPO are almost identical with the same profiles and peak wavelengths, indicating their analogical S1 characteristics. The largest stokes shifts of oyPBIDPO reflect its worst excitedstate structural relaxation. It is noteworthy that the emissions of myPBIDPO are hyperchromic in comparison to their isomers, which should be ascribed to their weakest solvatochromic effects due to their smallest molecular polarities as demonstrated by DFT simulation. Obviously, the rationally controlled molecular polarities of myPBIDPO are beneficial to reduce the interfacial charge-exciton interactions in their devices. The time-resolved phosphorescence (PH) spectra of xyPBIDPO are measured at 77 K after a delay of 300 s (inset of Figure 3). The spectra are almost identical with the same profiles and peak wavelengths. Estimated with 0→0 transitions in these spectra, the T1 value of xyPBIDPO is exactly the same as 2.98 eV, consistent with their identical T1 locations on PBI cores and the negligible contributions from DPPO groups (Figure S2), which can effectively block the triplet exciton diffusion from EMLs of DMAC-DPS (T1 = 2.91 eV) based blue TADF diodes.

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Figure 3. Electronic absorption and fluorescence (FL) spectra of xyPBIDPO in CH2Cl2 (10-6 mol L-1) and phosphorescence (PH) spectra of xyPBIDPO in CH2Cl2 glass measured with timeresolved technology at 77 K after a delay of 100 s. Therefore, although DPPO groups at different substitution positions render the various S1 energy to xyPBIDPO, their T1 energy is identical, excluding the interferences of their optical properties to the comparison on their EL performances as CES-type ETMs. 3.4. Electrical Properties The correlation between DPPO substitution position and electrical performance was further experimentally investigated with cyclic voltammetry (CV) analysis and single-carrier transporting experiment (Figure 5).

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Figure 4. (a) Cyclic voltammograms of xyPBIDPO measured in tetrahydrofuran for reduction and dichloromethane for oxidation, respectively, at room temperature with tetrabutylammonium hexafluorophosphate as electrolyte under the scanning rate of 100 mV s−1; (b) IV characteristics of electron-only-transporting devices with the configuration of ITO|LiF (1 nm)|xyPBIDPO (100 nm)|LiF (1 nm)|Al, fitted with field-dependent SCLC model; (c) single-crystal packing diagrams of omPBIDPO (left), opPBIDPO (center) and pmPBIDPO (right). xyPBIDPO reveal two irreversible anodic peaks characteristic of PBI and DPPO-originated oxidations, respectively (Figure 5a). Nevertheless, with the distorted PBI and the imidazolelocalized HOMOs, the inductive effect of P=O groups on imidaole of oyPBIDPO is restrained, rendering their first oxidation peaks 0.1 V lower than those of myPBIDPO and pyPBIDPO.

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According to the onset voltages of anodic curves, the HOMO energy levels of oyPBIDPO are estimated as -6.32 eV in contrast to -6.48 eV of their isomers (Table 1). Significantly, the reduction voltammograms of xyPBIDPO are diverse that oyPBIDPO and myPBIDPO show one irreversible and one reversible cathodic peaks; while, the para-substituted x-DPPO in pyPBIDPO with the strongest inductive effect remarkably enhances electron-withdrawing ability, giving rise to its two reversible cathodic peaks. Accordingly, the LUMO energy of oyPBIDPO and myPBIDPO is similar as -2.80 eV, which is equivalent to that of the conventional PBIbased ETM 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI, -2.81 eV). Meanwhile, pyPBIDPO have the deeper LUMO by the level of -2.95 eV, comparable to that of the most popular high-energy-gap ETM 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB, -3.0 eV). Therefore, in accord with DFT simulation, oyPBIDPO and pyPBIDPO are superior to myPBIDPO in electron-withdrawing inductive effect, making pyPBIDPO achieve the highest electron affinity. On contrary, the distorted PBI cores of oyPBIDPO reduce their electron affinity to the level of myPBIDPO. It can be noticed that the substitution position of y-DPPO groups hardly influences the FMO energy levels, conversely reflecting their single steric effect as designed. The nominal single-layer electron-only transporting devices with the configuration of ITO|LiF (1 nm)|xyPBIDPO (100 nm)|LiF (1 nm)|Al were fabricated to evaluate the electron mobility of these ETMs, in which LiF was used as electron-injecting layers (Figure 5b). At the same voltages, current densities (J) of these devices were sharply increased from oyPBIDPO, myPBIDPO to pyPBIDPO, which was roughly consistent with their electron affinity (inset of Figure 5b). Nevertheless, it was noticed that in spite of their similar electron affinity, J of oyPBIDPO-based devices was remarkably lower than that of myPBIDPO-based analogues,

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indicating the significant influence of intermolecular interactions on electron transportation. The packing diagrams of oyPBIDPO show the negligible intermolecular interactions with PBI-PBI distances beyond 10 Å, which should be attributed to their highly twisted configurations (Figure 5c). In contrast, the strong - stacking between PBI cores of adjacent pmPBIDPO molecules are recognized with a centroid-centroid distance of 3.479 Å, which establishes the efficient intermolecular electron hopping channels. Although we did not obtain the single crystal of mmPBIDPO, the slightly increased steric hindrance of mesa-substituted x-DPPO in the potential reverse dimer packing mode as the situation of pmPBIDPO would support the effective intermolecular interactions in mmPBIDPO-based films, assistant with its roughly symmetrical configuration. According to field-dependent space charge limited current (SCLC) model, the zero-field electron mobility (e) of mmPBIDPO reached to 1.21×10-6 cm2 V-1 s-1, which is close to those of pyPBIDPO (~2×10-6 cm2 V-1 s-1) and 5-10 folds of those of oyPBIDPO (~2×10-7 cm2 V-1 s-1) (Table 1). Obviously, the similar intermolecular interplays for mmPBIDPO and pyPBIDPO bridge their gap in electron affinity, rendering their comparable e. As estimated by DFT simulation, e of mmPBIDPO and ppPBIDPO is higher than those of mpPBIDPO and pmPBIDPO, which should be ascribed to the enhanced intermolecular interactions with more symmetrical configurations of the former. Nevertheless, the differences of e between oyPBIDPO, myPBIDPO and pyPBIDPO are remarkably larger than their internal discrepancy, manifesting the secondary role of y-DPPO in steric hindrance. Taken together, mmPBIDPO successfully realizes the balance between electrical performance and PBI encapsulation with the favorable electron affinity of 2.8 eV and the high e comparable to that of TPBI (3.7×10-6 cm2 V-1 s-1),60 in virtue of its rationally dual-substituted and roughly symmetrical configuration, making it close to the "ideal" CES-type ETM.

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Table 1. Physical properties of xyPBIDPO. HOMO/LUMO (eV)

ERi (eV) j (10-6 2 cm /V/s)

3.85e/2.97f 251/422

-6.32/-2.78h

0.6451

0.22

387c

4.93/2.86g

-5.81/-1.12g

228, 275, 300b

384b

3.86e/2.97f 220/434

-6.32/-2.78h

0.6504

0.14

231, 283, 335c

387c

4.88/2.80g

-5.69/-0.94g

mmPBIDPO 228, 286, 304b

376b

3.76e/2.98f -/466

-6.48/-2.81h

0.4025

1.21

232, 283, 340c

387c

4.58/2.70g

-5.73/-1.15g

mpPBIDPO 228, 285, 304b

376b

3.75e/2.98f -/457

-6.49/-2.81h

0.5013

0.81

233, 283, 341c

387c

4.54/2.72g

-5.90/-1.36g

pmPBIDPO 228, 286, 314b

384b

3.55e/2.98f 204/460

-6.48/-2.93h

0.6529

2.18

235, 313, 363c

387c

4.39/2.64g

-5.93/-1.54g

228, 285, 314b

384b

3.55e/2.98f 209/468

-6.48/-2.96h

0.5551

2.44

235, 313, 363c

387c

4.43/2.65g

-5.91/-1.48g

Compound omPBIDPO

opPBIDPO

ppPBIDPO

Absa (nm)

Emd

S1/T1 (eV)

227, 275, 300b

384b

230, 282, 332c

(nm)

Tm/Td (oC)

a

Absorption peaks; b in CH2Cl2 (10-6 mol L-1); c in thin film; d fluorescence peaks at room temperature; e estimated according to the absorption edges; f calculated according to the 0-0 transitions of the phosphorescence spectra; g TDDFT calculated results; h calculated according to the equation HOMO/LUMO = -(4.78 + onset voltage) eV81; i reorganization energy of electron; j electron mobility estimated by I-V characteristics of electron-only devices according fileddependent SCLC model82. 3.5. Device Performance of Blue TADF Diodes with Simplified Structures The high T1 energy, the favorable electron injecting and transporting ability and the effective PBI-core encapsulation of xyPBIDPO make the configuration simplification of blue TADF diodes feasible and encouraged us to fabricate the T-I type trilayer devices with a configuration of ITO|MoO3 (6 nm)|TAPC (70 nm)|DPEPO:DMAC-DPS (20 nm, 10%wt.)|xyPBIDPO (50 nm)|LiF (1 nm)|Al, in which MoO3 and LiF served as hole and electron-injecting layer, TAPC is 1,1-bis(di-4-tolylaminophenyl)cyclohexane as hole transporting layer, and DPEPO and DMAC-

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DPS were used as host and blue TADF dopant, respectively (Figure 5a). The mono-substituted diphenyl(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl)phosphine oxide (PBIPO)60 was used to fabricate the control devices for comparison. The hole injection barrier between TAPC and DPEPO is as large as 1.3 eV, while the electron injection barriers between xyPBIDPO and DPEPO are 0.2-0.4 eV. Furthermore, the energy barriers for direct hole and electron capture on DMAC-DPS are 0.7 and -0.2-0 eV, respectively. Therefore, hole would be accumulated at the TAPC|EML interface. Nevertheless, the single-carrier transporting devices were fabricated with the configurations of ITO|MoO3 (6 nm)|DPEPO:DMAC-DPS (100 nm, 10%wt.)|MoO3 (6 nm)|Al for hole only and ITO|LiF (1 nm)|DPEPO:DMAC-DPS (100 nm, 10%wt.)|LiF (1 nm)|Al for electron only, respectively. Their J-V curves showed the slightly hole-predominant charge carrier flux in EMLs, owing to the significant contributions of DMAC-DPS to carrier transportation in EMLs (Figure S3). In this case, the charge carrier recombination zones of these devices should be roughly uniformly dispersed in EML. Thus, both EML-involved interfaces would influence the device performance with xyPBIDPO|EML interface predominant. The T1 energy of TAPC, DPEPO and xyPBIDPO is higher than that of DMAC-DPS, effectively confining the excitons on the dopant. Therefore, all of the devices showed the DMAC-DPSattributed blue emissions (inset in Figure 5b). mmPBIDPO endowed its devices with the pure blue emissions peaked at 464 nm and the highest color purity at 1000 cd m-2, corresponding to the Commission Internationale Ed I'eclairage (CIE) coordinates of (0.16, 0.20) (Table 2). oyPBIDPO-based devices showed the sky-blue emissions with peaks at 476 nm and CIE coordinates of (0.17, 0.25), which should be attributed to their recombination zones close to oyPBIDPO|EML interfaces. On contrary, the red shifts of emissions from pyPBIDPO-based

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devices were ascribed to the interfacial charge-exciton interactions, due to their weakest PBIcore encapsulation.

Figure 5. (a) Device configuration, energy level diagram, material chemical structures and proposed charge carrier recombination and exciton confinement mechanism of xyPBIDPObased trilayer TADF diodes T-I using DPEPO and DMAC-DPS as host and blue dopant, respectively. PBIPO was used to fabricate the control device; (b) Luminance-J-Voltage curves and EL spectra (inset) and (c) Efficiency-Luminance curves of the devices. On the basis of the same device configuration, it was rational that at the same driving voltages, J of these T-I type devices was in direct proportion to e of their ETMs (Figure 5b). Nevertheless, in contrast to pyPBIDPO, mmPBIDPO-based devices realized the same onset

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voltage of 3.5 V, while along with luminance increasing to 100 and 1000 cd m-2, the operation voltages further decreased from 6.5 and 9.0 V to 5.5 and 8.0 V, respectively (Table 2). The almost equivalent onset voltages of these T-I devices indicated the predominant contribution of DMAC-DPS in carrier injection at the low driving voltages. However, although mmPBIDPO is not advantageous to pyPBIDPO in electrical performance, the superiority in quenching suppression rendered the higher exciton radiative ratio for the former, giving rise to the lower driving voltages of its devices required for specific luminance, which was conversely evidenced by the highest luminance of its devices at the same voltages.83 oyPBIDPO-based devices showed the lowest luminance among these T-I devices, in accord with their lowest J, which resulted in the unbalanced charge flux and the deficient exciton recombination. The luminance and J of PBIPO-based devices were secondary. Although J of pyPBIDPO-based devices was the highest among these T-I devices, their luminance was lower than those of myPBIDPO-based devices, especially at voltages below 10 V. At this stage, the contribution of DMAC-DPS to electron injection and transportation can effectively remedy the inferiority of myPBIDPO in electron affinity and mobility, therefore amplify the advantage of myPBIDPO in interfacial quenching suppression. The efficiency-luminance curves of these T-I devices directly indicated the influences of two core functions for CES-type xyPBIDPO, viz. charge flux balance and quenching suppression, on their device performance (Figure 5c and Table 2). It is noteworthy that with the monoencapsulated structure, PBIPO provided its devices with the lowest efficiencies due to the worst interfacial quenching. oyPBIDPO and pyPBIDPO supported the almost identical efficiency characteristics to their devices with the maximum values of about 15.5 cd A-1 for current efficiency (CE), 13.5 lm W-1 for power efficiency (PE) and 8.5% for EQE, which were rather

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mediocre among reported DMAC-DPS-based devices. According to the optoelectronic analysis mentioned above, oyPBIDPO are superior in PBI-core encapsulation, but inferior in electron injection and transportation; while, the situation of pyPBIDPO is just the opposite. Therefore, the low but identical efficiencies of oyPBIDPO and pyPBIDPO-based devices should be attributed to the out-of-balance charge flux in the former and the interfacial quenching induced by the exposure of charged PBI core in the latter, which actually manifest the equal importance of electrical performance and charged moiety encapsulation when employing CES-type ETMs to simplify blue TADF diodes. As expected, the effective PBI-core encapsulation and the preserved electron injecting and transporting ability of myPBIDPO dramatically improved their device efficiencies. mpPBIDPO-based devices achieved the maximum efficiencies of 26.9 cd A-1, 28.2 lm W-1 and 15.8%, which were 50% larger than those of pyPBIDPO-based analogues; while, mmPBIDPO can further improve the efficiencies to 32.7 cd A-1, 28.4 lm W-1 and 19.2%, accompanied by the EQE roll-offs as low as 5 and 26% at 100 and 1000 cd m-2, respectively, which were almost equivalent with those of DMAC-DPS-based multi-layer devices25 and among the best results of trilayer blue TADF diodes to date.58, 59 In virtue of its more symmetrical configuration and more effective PBI-core encapsulation by two mesa-substituted DPPO groups, the higher e and the stronger CES effect of mmPBIDPO gave rise to its higher device performance than that of mpPBIDPO. It is rational that although mmPBIDPO is inferior to oyPBIDPO and pyPBIDPO in CES effect and electrical performance, respectively, mmPBIDPO is superior in harmonizing and balancing these two functions, rendering the stateof-the-art performance to its extremely simplified trilayer devices. To further enhance the charge flux balance in EML to facilitate exciton formation, 2,2',4tris(di(phenyl)phosphoryl)-diphenylether (DPETPO) with improved charge mobility was used

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instead of DPEPO as the host to form the T-II type trilayer devices (Figure S4).49 However, The employment of DPETPO not only gave rise to the formation of interfacial exciton traps (2.8 eV) with TAPC, but also made the recombination zones of the T-II devices shifted to the holeaccumulated TAPC|EML interfaces, significantly worsening the efficiency reduction by interfacial exciton-polaron quenching (TPQ) effects, which was directly evidenced by their remarkably broadened EL spectra with the full width at half maximum (FWHM) of 88 nm in contrast to 72 nm of the T-I type analogues (Figure S5).

Figure 6. (a) Device configuration, energy level diagram and proposed charge carrier recombination and exciton confinement mechanism of DPETPO-based quadruple-layer blue TADF diodes Q-I with mCP as exciton blocking and interfacial optimizing layer and chemical

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structure of NPB; (b) Luminance-J-Voltage curves and EL spectra (inset) and (c) Efficiency vs. luminance curves of the devices. To suppress the interfacial quenching effects, a conventional high-energy-gap holetransporting host N,N'-dicarbazole-3,5-benzene (mCP) was incorporated as exciton blocking and interfacial modifying layer to form the Q-I type quadruple-layer devices with the configuration of

ITO|MoO3

(6

nm)|NPB

(70

nm)|mCP

(5

nm)|DPETPO:DMAC-DPS

(20

nm,

10%wt.)|xyPBIDPO (50 nm)|LiF (1 nm)|Al, in which NPB is 4,4'-bis[N-(1-naphthyl)-Nphenylamino]biphenyl as HTL (Figure 6a). NPB with higher hole mobility was used instead of TAPC to remedy the negative influence of involving mCP with lower hole mobility on charge flux balance and make the charge carrier recombination zones away from mCP|EML interfaces. Besides of blocking triplet exciton diffusion to NPB layer, the employment of mCP with the HOMO energy of -5.9 eV enlarged the energy gap between its HOMO and the LUMO of DPETPO to ~3.1 eV and also halved the hole injection barrier, thereby restraining the IET formation and alleviating the hole accumulation at EML interface as indicated by the narrowed EL spectra of the Q-I devices with the FWHM of 66 nm (Figure S5). Furthermore, the HOMO energy of mCP and DMAC-DPS is equivalent to facilitate the direct hole capture by the latter. As expected, in comparison to the T-II devices, these Q-I devices achieved the comparable or slightly lower driving voltages, accompanied by the further increased luminance with the maximum value beyond 9000 cd m-2 from mmPBIDPO-based devices (Figure 6b and Table 2). Significantly, the efficiencies of the Q-I devices were dramatically improved to about two folds of those of the T-II devices. mmPBIDPO endowed its Q-I type devices with the state-of-the-art efficiencies up to 38.5 cd A-1, 40.3 lm W-1 and 20.1%, respectively, accompanied by the reduced EQE roll-offs as 6 and 24% at 100 and 1000 cd m-2, which were among the best results reported

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so far for blue TADF diodes24, 36, 44 and comparable to those of DPETPO and DMAC-DPS-based five-layer devices49 (Figure 6c and Table 2). Furthermore, it is noticed that on the basis of Q-I configuration, the efficiencies of pyPBIDPO-based devices were increased for more than 70% in contrast to their T-I devices. The maximum efficiencies of mmPBIDPO-based Q-I devices were 1.2-1.7 folds of those of other isomers based analogues; while, when using T-I configuration, this ratio increased to 1.2-2.37. Since the contributions of xyPBIDPO to electron injection and transportation were basically constant in these devices, it is convincible to attribute the higher efficiencies and reduced discrepancies of the Q-I devices to their recombination zones farther away from ETL|EML interfaces, which relieved xyPBIDPO-involved interfacial quenching and thereby partially remedy the weakness of pyPBIDPO in charged moiety encapsulation. The comparison on the EL performance of xyPBIDPO and the state-of-the-art performances of mmPBIDPO-based trilayer and quadruple-layer devices indicate that to develop high-efficiency ETMs for blue TADF diodes with simple configurations, the original function of ETMs for improving electron injection and transportation should be the principle concern, which determines not only the J-V characteristics and the maximum brightness, but also the charge flux balance for unity exciton formation and TPQ suppression. Nevertheless, the interfacial excitoninvolved interaction induced quenching still significantly decreases the device performance, which can be effectively suppressed by employing CES-type ETMs. Therefore, the almost perfect harmonization of electrical performance and intermolecular interaction regulation is the key point for the success of mmPBIDPO in both high-performance blue TADF and phosphorescent diodes with extremely simple configurations (Figure S6 and Table S1). Table 2. EL performance of xyPBIDPO-based blue TADF OLEDs. Host

Voltagea (V)

Maximum

Efficiency Roll-Offc (%)

Emission

CIE (x, y)

29



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Efficiencyb

CE

PE

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EQE

Peak (nm)

T- I: ITO|MoO3 (6 nm)|TAPC (70 nm)|DPEPO:DMAC-DPS (20 nm, 10%wt.)|xyPBIDPO (50 nm)|LiF (1 nm)|Al omPBIDPO

4.0,

Dual Encapsulation of Electron Transporting Materials To Simplify

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