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Dec 25, 2018 - ABSTRACT: Through the A4 + B2 type Suzuki−Miyaura coupling reaction, hyperbranched polymers CP1, CP2, and. CP3 were easily prepared, ...
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Tetraphenylcyclopentadiene-Based Hyperbranched Polymers: Convenient Syntheses from One Pot “A4 + B2” Polymerization and High External Quantum Yields up to 9.74% in OLED Devices Yujun Xie,†,‡ Yanbin Gong,† Mengmeng Han,† Fengyuan Zhang,† Qian Peng,§ Guohua Xie,*,† and Zhen Li*,†,‡ Macromolecules Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/15/19. For personal use only.

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Department of Chemistry, Wuhan University, Wuhan 430072, China Institute of Molecular Aggregation Science, Tianjin University, Tianjin 300072, China § Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Through the A4 + B2 type Suzuki−Miyaura coupling reaction, hyperbranched polymers CP1, CP2, and CP3 were easily prepared, which exhibited high luminous efficiency in solution, neat film, and solid state as well as good thermal stability. When fabricated into organic light-emitting diodes (OLEDs) with mCP as host, the device based on CP1 demonstrated the good EL performance with external quantum efficiency (EQE) up to 6.36%. While the bipolar TADF host of CzAcSF was utilized instead of mCP, the driving voltage (at luminescence of 10 cd m−2) decreased from 5.7 to 4.4 V, and the EQE further increased to 9.74%, which was the highest EQE reported for hyperbranched polymers.

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INTRODUCTION

From the viewpoint of device fabrication, a suitable host material is generally required to improve the EL efficiency, as the conductive host can suppress the aggregation of the emitter (guest), alleviate the triplet-polaron quenching (TPQ) effects, enhance carrier mobility, and so on.21−27 On the other hand, to prevent energy back-transfer from the emitter to host, the T1 energy level of the host should be higher than that of the emitter while the S1 energy level of the host should be relatively low so as to balance the hole/electron injection. Thus, a bipolar TADF compound with a small energy barrier between singlet and triplet states (ΔEST) and a relatively high T1 energy exhibited potential value as host to achieve balanced charge injection and transporting as well as the smaller efficiency roll-off than conventional hosts.28,29 Also, benefiting from the characteristic of reverse intersystem crossing (RISC) from the T1 to S1 state triggered by small ΔEST, the triplet exciton can be converted to the singlet one through excitation of ambient thermal energy. Thus, as host, the TADF compound was able to harvest both singlet and triplet excitons, further improving the OLEDs efficiency through more efficient energy transferring to the emitter.30,31 The effective RISC is also beneficial to reducing the triplet exciton density in the devices and refraining triplet−triplet annihilation, as validated in both phosphorescent and fluorescent OLEDs.21,30,32−36

Since the great breakthrough made by Tang and VanSlyke in the 1980s,1 organic light-emitting diodes (OLEDs) went through rapid development and partially been commercialized as the next generation of flat-panel display and solid state lighting source because of their excellent electroluminescence (EL) performance.2−8 However, the efficiency and lifetime still need to be further improved with low cost. Theoretically, there were typically 25% singlet and 75% triplet excitons produced under the electrical excitation due to the random nature of electrical exciton spin production.9−11 The fluorescent OLEDs always encounter intrinsic low internal quantum efficiency (IQE) with an upper limit of 25%, under the assumption of the electron and hole in the recombination ratio of 100%. Therefore, the utilization of triplet excitons is the fundamental solution to improve EL efficiencies. Thanks to the rapid development of theory and experiment technology, the triplet excitons can be either harvested for EL by the direct transition from the triplet state to ground state as phosphorescence (T1 → S0)12,13 or turned into singlet exciton through the upconversion process (T1 → S1) and followed by decay as delayed fluorescence (S1 → S0). Accordingly, various OLED devices based on mechanism of phosphorescent, triplet−triplet annihilation (TTA),14 hybridized local and charge transfer (HLCT),15,16 or thermally activated delayed fluorescence (TADF)17−20 were developed sequentially, and the internal quantum efficiency (IQE) could reach to 100% theoretically. © XXXX American Chemical Society

Received: September 26, 2018 Revised: December 25, 2018

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further increased to 9.74% and still remained as high as 6.38% at the luminescence of 500 cd m−2. To the best of our knowledge, this was the first OLED device with TADF materials as host and emissive polymers as guest, providing an alternative approach to highly efficient OLED devices with simple syntheses and convenient fabrication.

Actually, Zhang et al. had pointed out that when TADF materials acted as sensitizing host and the fluorescent emitter as guest, the total internal EL efficiency of the device was relevant to the ΦRISC of the host and the ΦPL of the guest.31 Therefore, the utilization of TADF materials with small ΔEST as host and fluorescent emitter with high photoluminescence (PL) efficiency was a convenient way to get high performance OLED devices. However, to the best of our knowledge, in all the reported OLEDs to date with the TADF materials as hosts, the doped phosphorescent/fluorescent emitters were based on small molecules, which required the expensive vacuum deposition for the device fabrication. Alternatively, the solution-processed OLEDs possessed some advantages for its simplicity and lower cost in the rapid fabrication of large area devices at room temperature, especially on flexible substrates.5,37−40 Thus, accordingly, we attempted to fabricate OLEDs devices with TADF host and fluorescent polymers as emitter. Previously, our group has prepared a series of tetraphenylcyclopentadiene (CP) derivatives with the characteristic of aggregation-induced emission (AIE) upon the introduction of a tetraphenylethene (TPE) moiety,41 which exhibited relatively high PL efficiency and good EL performance in fluorescent OLEDs. CP-based polymers also exhibit high PL efficiency because of the high degree of conjugation originating from multiple linkage sites. For example, Patra et al. synthesized A4 + B2 type hyperbranched polymers with CP as core and diethynylbenzene or benzothiadiazole as linkage, which demonstrated good luminous properties had been applied to conjugated porous organic polymer and nitroaromatic sensing.42−44 Here, in this work, we further developed CP-based A4 + B 2 type hyperbranched polymers of CP1, CP2, and CP3 (Scheme 1)

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RESULTS AND DISCUSSION The monomers were synthesized according to the procedure presented in Scheme S1, and the corresponding 1H and 13C NMR spectra are listed in the Supporting Information. The preparation of the hyperbranched polymers was performed through the coupling reaction of Br4−CP and aromatic borates at 65 °C with the catalyst of Pd(PPh3)4 (Scheme 1). After stirring for about 1.5 h, the reactant solution became a little opaque, and then the polymerization was terminated by the addition of end-capped reagents (phenylboronic acid and bromobenzene) to react with end groups, such as bromine and borate groups, to avoid possible harm for the fluorescent properties of the yielded polymers. Finally, the polymers were collected by precipitating the tetrahydrofuran (THF) solution of polymers to the excess amount of n-hexane and then washed thoroughly with acetone by Soxhlet extractions. The yielded polymers of CP1, CP2, and CP3 were soluble in common organic solvents, such as THF, dichloromethane, chloroform, toluene, and so on. The 1H NMR spectra of polymers are presented in Figures S25−S27; no sharp peak was observed in both aromatic and alkyl regions, indicating the completion of polymerization and high molecular weight of the obtained polymers. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) with THF as eluent and polystyrene as calibration standard, the results listed in Table S1. The weight-average molecular weight (Mw) of CP1 was the highest one among three polymers (up to 5.65 × 104 g mol−1), revealing high efficiency of the polymerization reaction. The Mw’s of CP2 and CP3 were also as high as 1.29 and 4.03 × 104 g mol−1. The thermogravimetric property of the polymers was analyzed by thermal gravimetric analysis (TGA) under an atmosphere of nitrogen. As shown in Figure S1A, CP1, CP2, and CP3 exhibited good thermal stability with the 5% weight loss temperatures (Td) as high as 386, 382, and 339 °C, respectively. As measured by differential scanning calorimetry (DSC), the glass transition temperatures (Tg) of the polymers were 100, 124, and 133 °C, respectively (Figure S1B). The high thermal stability and Tg would contribute to the enhancement of the stability of the emissive layer in OLED devices. The photophysical properties of polymers were studied by the ultraviolet−visible (UV−vis) and photoluminescence (PL) spectrometers, with the results listed in Table 1. As demonstrated in Figure S2, the maximum absorption wavelengths of CP1, CP2, and CP3 were 325, 305, and 344 nm, respectively. It was interesting to find that the polymers possessed identical onset of the absorption edge at 415 nm, indicating their optical band gaps were the same, possibly due to their similar highly conjugated electronic geometry. As expected, regardless of the different incorporated copolymer units of TPE, PCz, and Flu, the three polymers exhibited almost similar PL spectra in THF solution with the maximum wavelength at 468 nm (Figure 1A). However, different from CP2 and CP3 exhibiting intense emission under 365 UV light source, the emission intensity of

Scheme 1. Synthetic Route of Polymers CP1, CP2, and CP3

through the Suzuki−Miyaura coupling reaction between four bromine substituted CP (Br4−CP) and aromatic borate of TPE, 9-phenylcarbazole (PCz), and fluorene (Flu). As expected, the obtained polymers exhibited high PL efficiency exceeding 50% in neat films. When fabricated into OLEDs with mCP as host, the device based on CP1 achieved the best EL performance with the maximum external quantum efficiency (EQE) up to 6.36%. More excitedly, once the TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF)45 was used, the EQE B

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Macromolecules Table 1. Photophysical and Electronic Properties of CP1, CP2, and CP3

CP1 CP2 CP3

λAbs(sol) (nm)a

λPL(sol) (nm)/ Φb

λPL(film) (nm)c/Φd/τ (ns)

λPL(solid) (nm)/Φd/ τ (ns)

ES/ET (eV)e

EHOMO/ELUMO (eV)f

EHOMO/ELUMO (eV)g

325 306 344

469/0.028 469/0.431 469/0.489

479/0.501/1.81 482/0.265/1.92 469/0.438/1.90

508/0.237/2.50 496/0.204/2.28 484/0.325/1.68

3.05/3.08 3.05/3.13 3.05/3.10

−5.63/−2.65 −5.58/−2.67 −5.60/−2.66

−5.11/−1.31 −4.89/−1.03 −5.30/−1.16

Dissolved in THF, c = 2 × 10−3 mg mL−1. bThe PLQY in THF was estimated with standard of quinine sulfate (ΦPL = 53% in dilute H2SO4 solution). cMeasured in neat film. dAbsolute PLQY evaluated using an integrating sphere. eSinglet (ES) and triplet (ET) energy levels measured from onset wavelengths of the emission spectra in the neat film at 300 and 77 K, respectively. fCalculated from the empirical formula: HOMO = −[Eox − E(Fc/Fc+) + 4.8] eV, LUMO = −[Ered − E(Fc/Fc+) + 4.8] eV, E(Fc/Fc+) = 0.46 eV, measured in acetonitrile solution with the support of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6). gCalculated from DFT simulation. a

Figure 1. (A) PL spectra of CP1, CP2, and CP3 in THF solution, c = 2 × 10−3 mg/mL, the corresponding λEx = 330, 310, and 348 nm. The inset shows PL images in THF solution under 365 nm UV light illumination. (B) PL spectra of polymers in solid state and neat film with λEx = 365, 370, and 370 nm. (C) AIE curves of CP1 in the mixed solvent of MeOH/THF with the volume fraction increase from 0 to 90%. (D) PL intensity of CP1 at 471 nm in MeOH/THF solvent.

Figure 2. Transient decay spectra of CP1, CP2, and CP3 in the solid state (A−C) and neat film (D−F) at 300 K.

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Figure 3. Top: molecular structure of oligomer unit of polymers CP1, CP2, and CP3. Bottom: frontier molecular orbital distributions and energylevel diagrams for the HOMO and LUMO wave functions as characterized by DFT calculations. The alkyl chains in fluorene are simplified to methyl groups to simplify the calculations.

those of CP1 (3.08 eV) and CP3 (3.10 eV), probably due to the introduction of electron donor PCz groups with higher ET. The electrochemical behavior of polymers was investigated by cyclic voltammetry measurements (CV), with the results shown in Figure S3 and Table 1. The oxidation process of polymers should all arise from the central core of CP because of their similar oxidation potentials. According to the half-wave potential of ferrocene (Fc), the highest occupied molecular orbital (HOMO) levels of CP1, CP2, and CP3 were −5.63, −5.58, and −5.60 eV, while their lowest unoccupied molecular orbital (LUMO) levels were −2.65, −2.67, and −2.66 eV, respectively, as calculated according to optical band gap. As shown in Figure 3, the computational geometrical models of polymers were simplified by the oligomer of symmetric tetra-aromatic substituted CP. The oligomers were optimized via density functional theory (DFT) at the B3LYP/6-31G(d) level, and the vertical excitation energies of singlet and triplet energy were calculated by time-dependent DFT (TD-DFT) theory. The electron contours of HOMO and LUMO are presented in Figure 3, while the rest of important frontier molecular orbitals (FMOs) are listed in Figures S5−S7 (Table S2). The oligomers of three polymers took the highly symmetrical geometry; accordingly, the electrical contour distribution of FMOs was highly symmetrical as well, for the absence of a polar group. CP1, composed of two chromophores of the CP and TPE moiety, exhibited more complicated electrical structure than those of CP2 and CP3, which resulted in orbital degeneracy and very close orbital energy levels. Consequently, the singlet and triplet transition configurations, such as S0 → S1−3 and S0 → T1 (Table S2) of CP1, composed by lots of FMOs, can be mainly attributed to the transition from the CP and TPE moiety. Relatively, the singlet and triplet excited state transition configurations of CP2 and CP3, which could be assigned to HOMO → LUMO that

CP1 was relatively weaker because of the TPE group, a typical AIEgen unit, which could largely quench the fluorescence in solution as a result of its free intramolecular rotation. Thus, it was reasonable that CP1 was AIE active. As shown in Figures 1C and 1D, when the methanol fraction of MeOH/THF mixture solvent increased from 0 to 80%, the emission intensity dramatically increased. However, upon the fraction further increase to 90%, the intensity decreased due to the poor solubility. To evaluate the emission quantitatively, the PL quantum efficiency (PLQY) in THF solution was estimated by using quinine sulfate (ΦPL = 53% in dilute H2SO4 solution) as standard (Table 1). CP1 showed a relatively low PLQY of 2.8%, while CP2 and CP3 exhibited high PLQY up to 43.1 and 48.9%, respectively. While in solid state and neat film, the PL spectra of polymers (Figure 1B) were a little different from those in solution due to the different ambient environment and aggregation effect. Their absolute PLQYs were good as evaluated by using an integrating sphere (Table 1). The neat film of CP1 demonstrated the highest PLQY of 50.1%, much higher than those of CP2 and CP3 (26.5% and 43.8%). Whereas in solid state, PLQY of three polymers (23.7%, 20.4%, and 32.5%) was inferior to that in neat film, indicating the aggregation caused quenching in solid state for the hyperbranched polymer resulted from high conjugation degree, regardless of the introduction of AIE unit of TPE. The room-temperature transient decay spectra of polymers in solid state and neat film were measured and are presented in Figure 2. The prompt fluorescence of polymers displayed second-order exponential decay with lifetimes in the range 1.68−2.50 ns, indicating their fluorescent properties. The PL and phosphorescence of polymers in THF solution at the low temperature of 77 K in liquid nitrogen are shown in Figure S4, and the corresponding triplet state energy levels (ET) were estimated according to onset wavelengths of the phosphorescence (Table 1). The ET of CP2 (3.13 eV) was higher than D

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Table 2. Device Performance of CP1, CP2, and CP3 Based PLEDs with Different Host Materials of MCP (Devices A1, A2, and A3) and CzAcSF (Devices B1, B2, and B3) EQEh (%) device

λEL (nm)

A1 A2 A3 B1 B2 B3

474 478 472 470 474 474

a

V10b

(V)

5.7 7.9 6.5 4.4 4.6 4.7

Lmaxc

−2

(cd m ) 574 121 1041 2441 3224 2673

d

−1

PEmax (lm W )

CEmaxe

6.68 0.81 2.51 10.75 7.75 5.89

−1

(cd A )

10.64 1.81 5.59 16.96 12.32 9.38

f

EQEmaxg

0.25) 0.26) 0.23) 0.24) 0.28) 0.28)

6.36 1.12 3.49 9.74 6.51 4.87

CIE (x, y) (0.18, (0.20, (0.16, (0.17, (0.19, (0.19,

(%)

−2

10 cd m 5.11 0.52 2.46 8.66 5.36 3.61

100 cd m−2

500 cd m−2

2.82 0.47 3.30 8.80 5.77 4.50

1.54 2.06 6.38 4.21 3.45

EL emission maximum. bThe driving voltage at 10 cd m−2, cMaximum luminance, dMaximum power efficiency, eMaximum current efficiency, The Commission Internationale de L’Eclairage coordinates, gMaximum external quantum efficiency (EQE), hEQE value at the luminescence of 10, 100, and 500 cd m−2. a f

Figure 4. Chemical structure of materials employed and energy level diagram of the OLEDs device.

located around the central core of CP, were much simpler than that of CP1.

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should be caused by the exciplex emission due to either the interactions of the adjacent polymer chains or the interactions between the host and polymers. Also, because of the much stronger shoulder emission of CP2 than CP1 and CP3, device A2 showed the biggest Commission Internationale de L’Eclairage (CIE) value of (0.20, 0.26), whereas that of device A3 (0.16, 0.23) was the smallest, much closer to the blue region on the CIE chromaticity coordinates (Figure S10A). The driving voltages (V10, recorded at a luminance of 10 cd m−2) of the devices were in the range 5.7−7.9 V. Device A3 presented the highest luminescence of 1041 cd m−2; however, device A1 achieved the better EL performance for the higher maximum power efficiency (PE, 6.68 lm W−1) and current efficiency (CE, 10.64 cd A−1). It was surprising to find that the CP1-based device A1 exhibited excellent maximum EQE of 6.36% at current density of 0.02 mA cm−2 without employing any light out-coupling technique, which exceeded the theoretical limit of conventional fluorescent OLEDs (5%), assuming the light out-coupling efficiency is 25%. Because the exciton bonding energy was relatively low in polymer for the large conjugation degree, the singlet exciton utilization would increase. On the other hand, multi singlet/triplet energy levels and orbital degeneracy were formed (Table S2) because of the efficient energy transfer between chromophore of the TPE and CP moiety in CP1, which may be in favor of upconversion from the triplet to singlet state through triplet−triplet annihilation (TTA). Actually, the abnormally high singlet exciton utilization and EQE have been reported in derivatives of PPV (poly(p-phenylenevinylene)).46−47 However, the relevant theoretical research was still scarce. Device A1 encountered a severe efficiency roll-off: the EQE was 5.11% at a luminescence of 10 cd m−2, then decreased to 2.82% at a luminescence of 100 cd m−2, and further decreased to 1.54% at luminescence of 500 cd m−2 (Table 2). Comparatively, device A3 showed relatively lower efficiency

ELECTROLUMINESCENCE PROPERTIES

Encouraged by the high PL efficiency and good thermal stability of polymers, we further evaluated the electroluminescence (EL) performances of the polymers. The solution processed multilayer PLED devices were fabricated using 5 wt % emitter:host doped films as emitting layer (EML). The structures of the devices were as follows: indium− tin oxide (ITO)/PEDOT:PSS (60 nm)/mCP:polymers (5 wt %, 50 nm)/DPEPO (10 nm)/TmPyPB (60 nm)/Liq (1 nm)/ Al. Among them, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) and 8-hydroxyquinolinato lithium (Liq) were hole and electron injection layers, while bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) acted as the hole-blocking (HBL) and electrontransport layers (ETL) to prevent the triplet exciton quenching at the interface between emitting layer and metal cathode. The devices using CP1, CP2, and CP3 as emitting layer were assigned as devices A (corresponding to A1, A2, and A3). The current density−voltage−luminescence (J−V−L) characteristics, external quantum efficiency (EQE), and EL spectra of devices are shown in Figure 5A,B. The related current and power efficiency versus current density spectra are presented in Figure S9A,B, and the key data of the device performances are summarized in Table 2. As expected, the devices based on CP1, CP2, and CP3 (A1, A2, and A3) exhibited almost identical EL spectra (Figure 5B) similar to their PL spectra in THF solution, confirming that EL emissions were generated solely from the emitters via the same radiative decay process. However, EL spectra red-shifted a little (from 469 nm in solution to around 474 nm) accompanied by a weak shoulder emission at around 600−700 nm, which E

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Figure 5. Current density−voltage−luminance (J−V−L) characteristics and external EL quantum efficiency (EQE) versus current density of the PLED devices A (A, B) and B (C, D). The inset was normalized EL spectra of the devices.

roll-off, and the EQE was 2.46% at a luminescence of 10 cd m−2 and slightly decreased to 2.06% at a luminescence of 500 cd m−2. The EL performance of device A2 was the worst for its much lower maximum EQE (1.12%) and other critical parameters. We noticed that the V10 of device A2 (7.9 V) was higher than those of A1 (5.7 V) and A3 (6.5 V), possibly due to the difficulty in carrier injection and transport. As mentioned in the Introduction, in recent years, TADF compounds have been attempted widely to serve as host in OLEDs with the doped fluorescent or phosphorescent luminogens as emitter, and extremely excellent EL performance could be achieved. Thus, we attempted to fabricate PLED devices using TADF compounds as host and the hyperbranched polymers of CP1, CP2, and CP3 as emitters. Considering the sky-blue emission and high triplet energy level of the polymers, the deep blue TADF emitter of CzAcSF (the chemical structure is presented in Figure 4) was chosen as host. To demonstrate TADF properties of doped polymers, the temperature-dependent transient PL decay of doped film of 5 wt % polymers in CzAcSF was investigated. As shown in Figure S8, the spectra could be resolved into two components: the prompt decay and the delayed decay. For the doped film at 300 K, the transient decay lifetimes of prompt and the delayed component for CP1, CP2, and CP3 were estimated to be 27.7, 28.4, and 24.0 ns and 3.0, 3.0, and 4.0 μs from second-order exponential decay fitting. From Table S3, when the temperature increased from 100 to 300 K, the delayed component increased as well, and the prompt component decreased, which agreed well with the typical characteristic of TADF compounds that the reverse intersystem crossing (RISC) would increase at higher temperature. The transient decay of pure CzAcSF is presented in Figure S8D, and the lifetime for TADF emission was 5.6 μs according to the numerical fitting. Thus, the largely prolonged prompt lifetime of doped films versus PL lifetime of

polymers (Table 1) and slightly decreased delayed lifetime compared to the case of CzAcSF indicated the Förster energy transferring from CzAcSF to the polymers in the doped films. The solution processed multilayer PLED devices were fabricated with similar structure to previous ones: ITO/ PEDOT:PSS (60 nm)/CzAcSF:polymers (5 wt %, 50 nm)/ DPEPO (10 nm)/TmPyPB (60 nm)/Liq (1 nm)/Al. The devices using CP1, CP2, and CP3 as emitting layer were assigned as devices B (corresponding to B1, B2, and B3), and the corresponding J−V−L characteristics, EQE and EL spectra, current, and power efficiency versus current density spectra are shown in Figure 5C,D and Figure S9C,D, respectively. The EL spectra of devices B were also similar and exhibited relatively weaker shoulder emission than those of devices A in the range 600−700 nm, indicating that CzAcSF could suppress the formation of exciplex in the emissive layer. EL spectra of devices B2 and B3 were almost identical with the maximum emission at 474 nm, but device B1 was a minor blue-shift to 470 nm. As a result, the CIE value of device B1 (0.17, 0.24) was smaller than that of devices B2 and B3 (0.19, 0.28) (Figure S10B). It can be understood from Figure 5C that the curves of luminescence and current density versus voltage showed similar variation tendency. From Table 2, the V10 of devices B1, B2, and B3 was greatly reduced to 4.4, 4.6, and 4.7 V, while the maximum PE greatly improved to 10.75, 7.75, and 5.89 lm W−1; the maximum CE boosted to 16.96, 12.32, and 9.38 cd A−1, respectively, revealing that CzAcSF can also decrease driving voltages and improve carrier mobility efficiently, probably due to its bipolar property. Benefiting from that, the maximum EQEs of devices B1, B2, and B3 were further increased to 9.74, 6.51, and 4.87%, respectively. Furthermore, the device exhibited much smaller efficiency roll-off upon increasing current density. Accordingly, the EQE of device B1 was 8.66% at the luminescence of 10 cd m−2 and F

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Macromolecules still as high as 6.38% at the luminescence of 500 cd m−2. Devices B2 and B3 showed similar character, but the EQE was inferior to that of device B1. The excellent device performance fully indicated that the bipolar TADF host can greatly improve the EL efficiency, and the high EQE can be ascribed to the TADF property that harvested both singlet and triplet excitons. Actually, in the field of EL, TADF materials always encountered the intrinsically small radiative decay rates caused by small ΔEST; the device instability and efficiency roll-off were severe at high current density. On the other hand, the disadvantages can be handled by delivering the singlet excitons of TADF sensitizer to fluorescent emitters through an efficient FET process (Figure 6). The conventional fluorescent

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curves, frontier molecular orbitals, and OLED performances; Figure S1−S27 and Tables S1−S3 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected] (L.Z.). *E-mail: [email protected] (X.G.H.). ORCID

Qian Peng: 0000-0001-8975-8413 Guohua Xie: 0000-0003-0764-7889 Zhen Li: 0000-0002-1512-1345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 51573140, 21734007), Hubei Province (2017CFA002), and Special funds for basic scientific research services in central colleges and Universities (2042017kf0247).

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Figure 6. Schematic diagram of energy transfer process from TADF host to polymer guest. NR: nonradiative transition; PF: prompt fluorescence; and DF: delayed fluorescence.

emitters, however, were abundant in colors, high color purity, convenient to be synthesized, and with high ΦPL. Therefore, the combination of TADF host and a suitable fluorescent dopant was an elegant solution to high performance OLED devices. Also, the conventional high ΦPL emissive polymers also exhibited a great application prospect in EL for the novel OLED device structure.

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CONCLUSION In summary, we designed three hyperbranched polymers of CP1, CP2, and CP3, which exhibited high PLQY in solution, neat film, and solid state as well as good thermal stability. When fabricated into OLEDs with mCP as host material, the CP1-based device achieved best EL performance with EQE up to 6.36%. While replacing the mCP by bipolar TADF host material CzAcSF, the driving voltage of the devices decreased and the corresponding PE and CE increased. As the result, the EQE further increased to 9.74% and was still as high as 6.38% at the luminescence of 500 cd m−2. Coupled with the convenient preparation and the easy solution process in the device fabrication derived from hyperbranched polymers, the marriage of TADF host and fluorescent polymers as emitter provides an alternative approach to high-performance OLEDs devices.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02051. The general method, OLED devices fabrication, the synthesis and characterization of polymers, absorption and phosphorescence spectra, TGA, DSC, and CV G

DOI: 10.1021/acs.macromol.8b02051 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b02051 Macromolecules XXXX, XXX, XXX−XXX