Efficient Thermally Activated Delayed Fluorescence Conjugated

Jun 12, 2018 - All the polymers were characterized by 1H NMR spectroscopies, elemental ..... into conjugated polymeric main chains to modulate the upc...
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Efficient Thermally Activated Delayed Fluorescence Conjugated Polymeric Emitters with Tunable Nature of Excited States Regulated via Carbazole Derivatives for Solution-Processed OLEDs Yuchao Liu,† Yukun Wang,‡,⊥ Chensen Li,† Zhongjie Ren,*,† Dongge Ma,§ and Shouke Yan*,† †

State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry Chinese Academy of Sciences, Beijing 130022, China ⊥ University of Chinese Academy of Sciences, Beijing 10039, China § Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: For main-chain-type thermally activated delayed fluorescence (TADF) polymeric emitters, conjugated or nonconjugated monomers are generally copolymerized as the spacers to suppress the inter- and intramolecular exciton concentration quenching. In this work, carbazole derivatives are introduced into the main chains of the conjugated polymers based on the TADF unit, 2-(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide (DBTO2-PTZ). It is found that the content of carbazole derivatives units could not only effectively suppress exciton quenching and nonradiative transition but also manipulate the distribution of molecular orbits and ΔEST values and even regulate the nature of excited states. Therefore, the upconversion of triplet exciton from triplet to singlet excited state could be regulated by introducing the different contents of carbazole derivatives. Among three synthesized polymers, COP-10 displays a relatively higher kRISC and PLQY in the film state. The optimized OLED device without any TADF assistant dopant could reach to an EQEmax of 15.7% with a lower turn-on voltage of 3.2 V.

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INTRODUCTION Since polymeric light-emitting diode (PLED) based on poly(pphenylenevinylene) (PPV) as the single semiconductor layer was reported,1 PLED is getting more extensive attention. Up to date, various π-conjugated polymers have been studied as the potential emitting materials, even though the internal quantum efficiencies (IQEs) suffer from a limit of 25% because of the spin statistics.2−5 In 2011, Adachi et al.6 reported the first purely aromatic compounds with the highly efficient thermally activated delayed fluorescence (TADF) characteristics, and the device displayed a maximum external quantum efficiency (EQEmax) of 5.3% at low current density. Following the design tactics of the twisted electron donor−acceptor (D−A) structures together with a high fluorescent radiative decay rate, highly efficient TADF emitters with a wide range of emission colors have been reported.7−11 In this context, polymeric TADF emitting materials are attractive candidates because of their low-cost solution processes, such as spin coating, die casting, or inkjet printing.12,13 For instance, Nikolaenko et al.14 first reported a main-chain polymeric TADF emitter by introducing nonconjugated monomers with high triplet energy to separate TADF units and prevent © XXXX American Chemical Society

quenching of triplet excitons. Yang’s group and our group reported a series of side-chain polymeric TADF emitters, where the TADF features of monomers can be easily inherited due to the relatively independent TADF units grafted on the polymeric main-chain backbone.15−20 Generally, a definitely valid approach to get TADF conjugated polymers is to fix donors on the main chain and graft the acceptors on the side chain. In this way, the obtained conjugated polymers generally remain the twisted D−A structures, and small enough excited singlet−triplet energy splitting (ΔEST) will be achieved.21−24 For the early investigated TADF system, a small ΔEST is ever considered as the foremost factor in facilitating reverse intersystem crossing (RISC).25,26 Thus, the significant chargetransfer (CT) contribution to both the singlet (S1) and triplet (T1) excited states and highly twisted D−A structures appropriately separating the wave function distributions of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are indeed required. Received: March 15, 2018 Revised: May 12, 2018

A

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high content of aliphatic chains due to high ratio of carbazole blocks. The optimized geometrical structures and frontier molecular orbital distributions and energy-level diagrams of the repeat unit of Homo, Cop-50, and Cop-10 were characterized via time-dependent density functional theory (TD-DFT) calculations (Figure 1). The LUMOs are primarily localized on

However, according to the recent researches, the energetic proximity between triplet CT states and locally excited (LE) states and the high spin−orbit coupling (SOC) are really favorable to enhance the RISC process simultaneously.27−29 For polymeric TADF systems with definitely hybrid excited states, a mediated SOC process might predominate the process that triplet excitons upconvert to the emissive singlet states. When polymeric TADF emitters possess two excited states with the clear CT nature, the direct intersystem crossing between 1 CT and 3CT might be suppressed due to the weak SOC. In some cases, the efficient RISC can also be achieved between 1 CT and 3CT, which is ascribed to hyperfine coupling (HFC) of the spins.29,30 Therefore, the rational regulation of the nature of excited states is possibly useful for guiding the synthesis of high efficiency TADF materials. Herein, we synthesized three π-conjugated TADF polymers based on an efficient D−A structure TADF unit, 2-(10Hphenothiazin-10-yl)dibenzothiophene-S,S-dioxide (DBTO2PTZ), in which carbazole derivatives units were introduced into main chains of the conjugated polymers to tune TADF characteristics. Actually, carbazole and its related derivatives are widely used as emitting and host materials and even other functional layers in OLED devices due to their higher triplet energy level and excellent hole transporting properties.31−33 We found that the content of carbazole derivatives units could manipulate the distribution of molecular orbits (MOs) and ΔEST values, suppress nonradiative transition, and even regulate the characteristics of excited states while the orthogonal dihedral angle between donors and acceptors still remains unchanged. With the increased ratio of carbazole units, ΔEST decreases and photoluminescence quantum yield (PLQY) in the film state increases obviously. By utilizing these TADF polymers as the emitters in the solution-processed multilayer PLEDs, the best one shows a relatively high EQEmax of 15.7% with a lower turn-on voltage of 3.2 V.

Figure 1. Molecular structures of Homo, Cop-50, and Cop-10, Frontier molecular orbital distributions and energy-level diagrams for the three repetitive units of Homo, two repetitive units of Cop-50, and a fragment of Cop-10 containing four bilateral symmetric carbazole derivatives distributed around TADF moiety. The structural optimization and calculation of the ground states are obtained by density functional theory (DFT) [rb3lyp/6-31G(d,p)].

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RESULTS AND DISCUSSION All these potential TADF polymeric emitters were synthesized by Suzuki−Miyaura coupling condensation of the TADF monomers, DBTO2-PTZ,34 and carbazole derivatives spacer with the feed molar ratios of 100:0 for homopolymer (Homo), 50:50 for copolymer-50 (Cop-50), and 10:90 for copolymer-10 (Cop-10), as depicted in Scheme S1 (Supporting Information). In order to improve the solubility of these polymers, 2ethylhexyl and n-heptyl side chains were introduced to dibenzothiophene-S,S-dioxide fragments and carbazole derivatives units, respectively. Hence, these three polymers exhibit good solubility in the common solvents, such as dichloromethane, toluene, and chlorobenzene. All the polymers were characterized by 1H NMR spectroscopies, elemental analysis, and gel permeation chromatography as shown in Figure S1 and Table S1. From the 1H NMR spectroscopies, the ratio of TADF units in the polymers are 51% for Cop-50 and 11% for Cop-10. In thermal analysis, these polymers show the reduced glass transition temperatures (Tg) in the range of 176−255 °C with increasing the carbazole units into the backbone, indicating that introduction of carbazole derivatives into the main chain can increase the mobility of polymer chains due to high flexible n-heptyl groups (Figure S2); the 5% weight loss temperature (Td) was determined to be 344 °C for Homo, 422 °C for Cop-50, and 370 °C for Cop-10 (Figure S3). The lower thermal stability of Cop-10 than Cop-50 may be caused by the

electron-acceptor dibenzothiophene-S,S-dioxide moiety, and the HOMOs are mainly localized on the main chain, which is generally regarded as the charge transport channel in conjugated polymeric emitters. With the increased carbazole units in backbone, the MOs energy levels are elevated and the energy gaps tend to be decreasing. It can be partly attributed to the distribution of HOMOs extend to the extra carbazole moieties. However, this extension is limited to the scope of two carbazole moieties, as shown in Cop-10 of Figure 1.35,36 Actually, the experimental HOMOs measured from cyclic voltammograms are also increased gradually from −5.32 eV for Homo and −4.95 eV for Cop-50 to −4.83 eV for Cop-10 (shown in Figure S4). The LUMOs values also decrease with the increased content of carbazole units (Table 1). For TADF emitters, the smaller value of ΔEST is regarded as an essential prerequisite to facilitate the RISC process from T1 to S1.37−39 The ΔEST values are decreased gradually with the increased proportion of carbazole moieties in the backbone as well, that is, 38.1 meV for Homo, 25.8 meV for Cop-50, and 21.7 meV for Cop-10 determined by TD-DFT. Therefore, small ΔEST values are deemed to be favorable for efficient RISC and thus harvesting triplet exciton by upconversion, especially for Cop10. B

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Macromolecules Table 1. Photophysical Properties of Polymers polymers

ΔEST [meV]a

HOMO [eV]b

LUMO [eV]c

λabs [nm] sold

λPL [nm] sold/ filme

τp/ratio [ns]/ [%]f

τd/ratio [μs]/ [%]g

PLQY [%] solh/filmi

kISC (107 s−1)j

kRISC (106 s−1)k

Homo Cop-50 Cop-10

72.3 52.0 42.8

−5.32 −4.95 −4.83

−3.08 −2.91 −2.85

285, 380 286 303

545/537 549/538 437, 552/540

15.2/21 33.4/38 82.0/53

50.6/79 120/62 2650/47

60 ± 0.3/10 ± 0.5 59 ± 0.2/18 ± 0.3 44 ± 0.4/21 ± 0.2

1.49 0.99 0.73

0.027 2.63 1.44

Excited singlet−triplet energy gap determined by fluorescent and phosphorescent spectra in neat film. bDetermined from the cyclic voltammograms. cDeduced from the HOMO and the optical energy gap (Eg) values. dMeasured in toluene. eMeasured in neat film. fThe lifetime and the ratio of the prompt fluorescence in neat film without O2. gThe lifetime and ratio of the delayed fluorescence in neat film without O2. hMeasured in toluene without O2. iMeasured in blend film (65 wt % TCTA and 25 wt % TAPC) in the atmosphere. jThe intersystem crossing (ISC) rate constant. kThe RISC rate constant. a

between singlet and triplet excited states determined from fluorescent and phosphorescent spectra tend to decrease with increasing the proportion of carbazole derivatives, which are favorable to boost the RISC process. In order to figure out more details about the excited states and the corresponding natures, TD-DFT analyses by Gaussian 09 were obtained to understand the excitation energies and oscillator strength (f). The natural transition orbital (NTO) analyses were performed to examine the natures of the excited states by Multiwfn. The TD-DFT calculations indicate that the increase in the numbers of carbazole moieties leads to decrease in both the S0 → S1 and T0 → T1 excitation energies, but the bigger drop occurs in the S0 → S1 process; in addition, the f values of S0 → S1 decrease when increasing the content of carbazole moieties in the backbone, and thus the fluorescence radiative decay will be weakened, which are consistent with transient PL decay characteristics for these polymers (Table S2).29,40 According to the NTOs analyses, the nature of excited states is shown in Figure 3. For the S1 and T1 states of these polymers, the wave function distributions are exactly similar. It is noted that the NTO electron and hole wave functions are mainly distributed on the backbone and electron acceptor moieties, respectively. There are also partially overlapped wave functions found in the joint of donor and acceptor moieties especially for Homo and Cop-50, so the S1 and T1 states of these two polymers have relatively distinct hybrid characteristics combining LE and CT states. Furthermore, to understand the overlap of NTO electron−hole wave functions quantitatively, the centroid distance of the two orbitals and the overlap integral of norm of the two orbitals were gained, as shown in Table S2. For Cop-50, the centroid distance and overlap integral value are 12.11 Å and 0.143, respectively, indicating a higher degree of overlap compared to Homo of 12.73 Å and 0.141. However, for Cop-10, the centroid distance and overlap integral value of 13.67 Å and 0.126 are achieved, which can be ascribed to the more carbazole derivatives in the backbone, and thus the electron density distribution is changed. Generally, a larger value of centroid distance indicates a smaller degree of overlap and the more CT characteristic for the excited states. Thus, Homo and Cop-50 feature relatively hybrid excited states with CT and LE natures, whereas Cop-10 displays the characteristics of CT nature and comparatively lower energy splitting between 1CT and 3CT, demonstrating that the increased content of carbazole units would elevate the CT ratio in the excited state and enhance the HFC that interconverts from 3CT to 1CT. This result is consistent with the solvatochromic effect of polymers. Solvatochromic studies revealed that the emission peak maxima of three polymers are sensitive to solvent polarity. In Figure 4a, the Homo displays an obvious red-shift of PL peaks with the increased solvent polarity

The UV−vis absorption and steady-state photoluminescence (PL) spectra of Homo, Cop-50, and Cop-10 in toluene solutions and in neat films are shown in Figure 2, and their

Figure 2. UV−vis absorption and PL spectra of the polymers: (a) Homo, (b) Cop-50, and (c) Cop-10 in toluene and (d) Homo, (e) Cop-50, and (f) Cop-10 in neat film.

photophysical properties are summarized in Table 1. The UV− vis absorption spectra in toluene show the CT bands at approximately 380 nm, and the absorption peak at 285−302 nm can be ascribed to the π−π* transition (Figure 2). It is noted that the absorption coefficients of CT band decrease when introducing the extra carbazole moiety into backbone. The PL spectra of Homo and Cop-50 in toluene show the explicit bright green emission band at 550 nm at 300 K. No emission band of carbazole derivatives can be observed for Cop-50, indicating that adjacent single carbazole derivatives can achieve charge transfer with TADF units efficiently. In contrast, the PL spectrum of Cop-10 in toluene exhibits an extra emission band of carbazole derivatives in addition to 550 nm emission because of the poor charge transfer efficiency from the backbone of carbazole to the TADF moiety. However, the extra emission band of carbazole derivatives disappears in the neat film because of the efficient inter- and intrachain interactions in solid state. Phosphorescent spectra of Homo, Cop-50, and Cop-10 show the broad band without the characteristic structure. In line with the calculated result, the energy gaps C

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Figure 3. Natural transition orbital (NTO) analyses of the representative excited states of Homo, Cop-50, and Cop-10. The values of v represent the weight of the hole−electron wave function’s contribution to the excitation.

Figure 4. Emission spectra in different solvents at room temperature and the corresponding Lippert−Mataga plot (νabs − νem against orientation polarizability of solvent) of (a, b) Homo, (c, d) Cop-50, and (e, f) Cop-10. D

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Figure 5. Transient PL decay characteristics of (a) Homo, (b) Cop-50, and (c) Cop-10 in blend film at room temperature. (Black curves and corresponding fitting line are measured with O2; the blue curves and corresponding fitting line are measured without O2. Inset exhibits the decay curves in the prompt section. kd and kp are the delayed and prompt decay rate of the blend film, respectively.) For clearly calculating kp and kd only, the decay lifetime of Homo, Cop-50, and Cop-10 is cut to 5, 50, and 50 μs, respectively. The temperature-dependent transient PL decay spectra of the (d) Homo, (e) Cop-50, and (f) Cop-10 in the doped film from 77 to 300 K under nitrogen conditions.

s−1, kd = 7.64 × 105 s−1 for Cop-10. Because of the severe intramolecular quenching effect of triplet exciton in the blend film of Homo, the delayed fluorescence decay rate is quite low despite the fact that Homo is at very low concentration, so it is absolutely essential to isolate the TADF moieties by introducing a spacer. However, the prompt emission rates decrease with increasing the proportion of carbazole derivatives, suggesting that the attachment of carbazole can effectively lower the f values of S1 → S0 transition, and this result is consistent with molecular simulation and calculation results (Table S2). However, the decay rates for Cop-50 and Cop-10 are the same order, and only a subtle difference can be observed.42,43 The intersystem crossing (ISC) and RISC rate (kISC and kRISC) in blend film, as shown in Table 1, can be calculated by eqs 1 and 2:25

(λmax 534 nm in n-hexane, 542 nm in toluene, 566 nm in chlorobenzene, and 586 nm in dichloromethane). The Lippert−Mataga plot (νabs − νem against polarizability of solvent) exhibits a slope of ∼6822 cm−1 for Homo (Figure 4b). The relatively inconspicuous solvatochromic effect observed for Cop-50 (λmax 551 nm in toluene, 560 nm in chlorobenzene, 566 nm in anisole, and 572 nm in tetrahydrofuran as shown in Figure 4c) and the corresponding lower slope of ∼3336 cm−1 (Figure 4d) indicate obvious decrease in dipole moment of the emissive excited state. Additionally, PL of Cop-10 in the different solvents is shown in Figure 4e. The emission peak at 550 nm of Cop-10 is suppressed in most of solvents, and thus, the different ratios of hexane and toluene are used. The Lippert−Mataga plot of Cop-10 exhibits a more positive slope of ∼11 000 cm−1 (Figure 4f), suggesting a larger increase in dipole moment of the emissive excited state and more CT characteristics in the excited state.41 To suppress the concentration quenching, the emitting molecules are usually blended into host materials. Therefore, the mixed hosts of 65 wt % tris(4-carbazol-9-yl-phenyl)amine (TCTA) and 25 wt % 4,4-cyclohexylidenebis[N,N-bis(4methylphenyl)benzenamine] (TAPC) are adopted to improve the charge transfer and combination properties and reduce the efficiency roll-off.36 The RISC processes from the lower energy level of T1 to S1 state of these polymers were explored by transient PL decay of the blend film (Figure 5). As shown in Figure 5, the longer lifetime of delayed component is detected in the transient decay spectra without O2, and the prompt decay section keeps invariant, signifying that the delayed component is only composed of triplet exciton which is easily quenched in the atmospheric environment. When introducing carbazole moieties into the polymeric backbone, the portion of delayed emission increases remarkably; longer PL lifetimes are observed with increasing content of carbazole units (Table 1). According to the slope of prompt and delayed decay section of the transient PL curves measured in a vacuum, the prompt and decayed emission rates constants (kp and kd) are determined to be kp = 1.88 × 107 s−1, kd = 5.77 × 103 s−1 for Homo, kp = 1.60 × 107 s−1, kd = 9.98 × 105 s−1 for Cop-50, and kp = 1.55 × 107

kISC = (1 − ϕPF)k p

kRISC =

(1)

k pkdϕDF kISCϕPF

(2)

where the ϕPF and ϕDF are the quantum yield of the prompt and delayed fluorescence emission, respectively. The values of kISC gradually decrease with the increased proportion of carbazole units. Compared with the value of kRISC for Homo, the kRISC for Cop-50 is enhanced to a quite larger degree, which is favorable to improve the efficiency. kRISC of Cop-10 is slightly lower than that of Cop-50. That may be attributed to the weak SOC caused by the absence of LE states in the excited states, indicating that hybrid excited states with definite CT and LE natures are favorable to boost the rate of RISC process. In addition, the ϕDF values gradually decrease when introducing more carbazole derivatives into polymeric backbones (Figure S5), but the photoluminance intensity of these polymers with O2 exhibits completely opposite tendency, indicating that higher proportion of carbazole derivatives can boost the stability of emitters in the air environment, and that may improve the device performance. E

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Figure 6. (a) Structures of the devices and the corresponding energy level diagram with the HOMO/LUMO levels. (b) Current efficiency− luminance−power efficiency curves of Homo, Cop-50, and Cop-10. (c) Current density−voltage−luminance curves. (d) External quantum efficiency versus luminance curves of the devices. The inset is normalized EL spectra.

Table 2. Electroluminescent Properties of the OLEDs Based on Polymeric Emitters emitters

λEL [nm]a

Von [V]b

Lmax [cd m−2]c

CEmax [cd A−1]d

PEmax [lm W−1]e

EQEmax [%]f

EQE100 [%]g

CIEh

Homo Cop-50 Cop-10

577 581 578

4.4 3.0 3.2

310 1400 3000

8.3 15.1 42.9

6.7 14.9 45.0

2.4 5.7 15.7

0.2 1.8 3.7

(0.47, 0.49) (0.46, 0.47) (0.46, 0.49)

a λEL = electroluminescence emission maximum. bVon = turn-on voltage at 1 cd m−2. cLmax = maximum luminance. dCEmax = maximum current efficiency. ePEmax = maximum power efficiency. fEQEmax = maximum external quantum efficiency. gEQE100 = external quantum efficiency at 100 cd m−2. hCIE = coordinates of Commission Internationale de L’Eclairage.

The morphology of the emitting layer is critical for solutionprocessed OLEDs. Therefore, the morphologies of the spincoated neat films and blend films (65 wt % TCTA and 25 wt % TAPC) of these three polymers were studied using atomic force microscopy. As shown in Figure S6, the neat and blend films have relatively smooth surface topographies with a rootmean-square (RMS) surface roughness of 0.2−0.5 nm, especially in the blend film of Cop-10 with the really low RMS of 0.24 nm. In the AFM height image of Cop-10 blend film, there are no voids and aggregates observed, indicating excellent miscibility between the Cop-10 and host materials. To evaluate the EL characteristics of the polymers, we fabricated and optimized the multilayer OLED devices, in which the polymers are deposited on hole-transporting layer by spin-coating and act as the emitters (Figure 6a). The devices with the structures of indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene:poly(styrenesulfonic acid) (PEDOT:PSS) (50 nm)/Homo or Cop-50 or Cop-10:TCTA:TAPC (10:65:25 wt %) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were fabricated, where PEDOT:PSS and lithium fluoride (LiF) serve as the hole- and electron-injection layer, respectively; 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) acts as the electron transporting layer. In the emitting layer (EML), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and 4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) act as the hosts. The current efficiency−luminance− power efficiency curves, current density−voltage−luminance (J−V−L) characteristics, and external quantum efficiency (EQE)−luminance curves are exhibited in Figure 6, and their

To further confirm the TADF feature of these polymers, their temperature-dependent transient PL decays from 77 to 300 K were investigated (Figure 5). The proportions of delayed fluorescence are enhanced with the elevated temperature, while the prompt fluorescence remains unchanged, showing that the thermal activation can accelerate the endothermic process from triplet excited state upconvert to singlet excited state and boost the quantum efficiency of the polymer.44−46 The photoluminescence quantum yields (PLQYs) measured in the doped film under air condition are 10 ± 0.5% for Homo, 18 ± 0.3% for Cop-50, and 21 ± 0.2% for Cop-10; the values measured in toluene bubbled by pure nitrogen are 60 ± 0.3% for Homo, 59 ± 0.2% for Cop-50, and 44 ± 0.4% for Cop-10. It is noteworthy that PLQYs in the blend film state are enhanced with the increased proportion of carbazole moieties due to suppressing the aggregation effect of TADF units and improving the solidstate stability in air. However, in solution state where the interchain interaction can be neglected, the PLQY decreases with increasing the proportion of carbazole derivatives. The higher PLQY of nearly 60% for Homo in toluene can be ascribed to the relatively stronger SOC induced by hybrid excited state, high fluorescence decay rate from S1 to S0, and negligible interchain interaction in solution. Hence, in the dilute solution of Homo, the PLQY is relatively higher in spite of the high concentration of TADF moieties in the main chain. Consequently, the higher kISC and kRISC values combining with the photoluminance performance, Cop-50 and Cop-10 are expected to obtain the efficient electroluminescence (EL) devices. F

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CT and LE natures; therefore, the SOC is favorable to the RISC. In contrast, Cop-10 has the characteristics of the predominant CT nature, in which HFC makes the efficient RISC because of the small exchange energy between 1CT and 3 CT states.54 We demonstrate that introducing carbazole derivatives into conjugated polymeric main chains to modulate the upconversion of triplet exciton from triplet to singlet excited state by changing the nature of excited states, and this provides a novel strategy to gain the efficient solutionprocessed conjugated polymeric TADF OLEDs.

device performances are summarized in Table 2. Homo shows the lower current density than that of Cop-50 and Cop-10. The differences observed can be attributed to the unbalanced hole and electron fluxes for Homo. For Homo-based devices, poor device performances are exhibited with the EQEmax value of 2.4% and low brightness. For Cop-50, the device performances are getting better as expected with the EQEmax of 5.7%. This improvement in device performance is consistent with photophysical results. Furthermore, Cop-10-based device achieves a higher EQEmax of 15.7% with maximum luminance of 3000 cd m−2, maximum CE of 42.9 cd A−1, maximum PE of 45 lm W−1, and Commission Internationale de l’Eclairage (CIE) coordinates of (0.46, 0.49) (Figure S7). The relatively higher device performance based on Cop-10 can be partly ascribed to the smooth surface topographies of the emitter films, excellent stability of emitters in the air environment driven from the higher proportion of CT nature, and the higher PLQY of blend film.47−49 However, the EQE value at the brightness of 100 cd m−2 (EQE100) are much lower than EQEmax for Cop-10-based device. It can be partly due to the inferior efficiency of exciton transportion and recombination and relatively low decay rate constant.50,51 To evaluate the charge transporting properties of EMLs and find out the recombination zone in the devices, we investigated hole-only devices (HODs) and electron-only devices (EODs) (HODs:ITO/PSS:PEDOT (40 nm)/emitter (65% TCTA:25% TAPC:10% Polymer)/MoO3 (10 nm)/Al (100 nm); EODs:ITO/ZnO:CsCO 3 (40 nm)/emitter (65% TCTA:25% TAPC:10% Polymer)/TmPyPB (20 nm)/LiF (1 nm)/Al (100 nm)). In Figure S8a, we found the neat films of TADF polymers exhibited relatively high hole mobility compared to their electron mobility. And thus, the unbalanced charge fluxes will aggravate the formation of hole trap52 and inevitably reduce the device’s stability.53 In addition, the difference between the hole and electron mobility of Cop-10 neat film is comparatively lower than that of Cop-50 and Homo, and it might be favorable for getting high device efficiency and suppressing the efficiency roll. When blending the TADF polymers into TCTA:TAPC host materials, the hole mobility of EMLs exhibited the obvious decrease, but the electron mobility almost kept consistent as shown in Figure S8b. Therefore, the relatively better device performance was obtained from the doped EMLs, which can be partly attributed to the balanced the charge fluxes and thus no charge and exciton accumulation. Furthermore, for the hostdoped devices, the electron mobilities of EMLs are much lower than their hole mobilities, which will cause unbalanced hole and electron fluxes and account for the efficiency roll-off. As shown in the inset of Figure 6d, the devices based on these polymers produce the yellow EL, which exhibit ca. 20 nm red-shift compared with the PL spectra, indicating that the luminescence process under electroexcitation is different than that in the photophysical process.

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EXPERIMENTAL SECTION

Synthesis of Homo. A mixture of DBSOR-PTZ-BPD (467 mg, 0.6 mmol), DBSOR-PTZ-Br (408.6 mg, 0.6 mmol), Pd(PPh3)4 (42 mg, 0.036 mmol), aqueous potassium carbonate (2 M, 5 mL), Aliquat 336 (15 μL), and dry toluene (20 mL) was stirred for 72 h at 85 °C, and bromobenzene (0.5 mL) was added to cap the ester end groups. After stirring for another 24 h, the reaction mixture was poured into water and was then extracted with dichloromethane. The organic layer was washed with water and dried over MgSO4. After filtration and evaporation, the product was reprecipitated from ethyl alcohol, further purified by Soxhlet extraction using n-hexane as extraction solvent, and then dried under vacuum to give Homo. Mw = 11 514; PDI = 1.44. Synthesis of Cop-50. A mixture of Cz-C7-BPD (550 mg, 1 mmol), DBSOR-PTZ-Br (681 mg, 1 mmol), Pd(PPh3)4 (69 mg, 0.06 mmol), aqueous potassium carbonate (2 M, 5 mL), Aliquat 336 (15 μL), and dry toluene (20 mL) was stirred for 72 h at 85 °C, and bromobenzene (0.5 mL) was added to cap the ester end groups. After stirring for another 24 h, the reaction mixture was poured into water and was then extracted with dichloromethane. The organic layer was washed with water and dried over MgSO4. After filtration and evaporation, the product was reprecipitated from ethyl alcohol, further purified by Soxhlet extraction using n-hexane as extraction solvent, and then dried under vacuum to give Cop-50. Mw = 8241; PDI = 2.04. Synthesis of Cop-10. A mixture of Cz-C7-Br (338 mg, 0.8 mmol), Cz-C7-BPD (517 mg, 1 mmol), DBSOR-PTZ-Br (137 mg, 0.2 mmol), Pd(PPh3)4 (69 mg, 0.06 mmol), aqueous potassium carbonate (2 M, 5 mL), Aliquat 336 (15 μL), and dry toluene (15 mL) was stirred for 72 h at 85 °C, and bromobenzene (0.5 mL) was added to cap the ester end groups. After stirring for another 24 h, the reaction mixture was poured into water and was then extracted with dichloromethane. The organic layer was washed with water and dried over MgSO4. After filtration and evaporation, the product was reprecipitated from ethyl alcohol, further purified by Soxhlet extraction using n-hexane as extraction solvent, and then dried under vacuum to give Cop-10. Mw = 10 441; PDI = 2.17. Devices Fabrication. The hole-injection material poly(3,4ethylenedioxythiophene)−poly(styrenesulfonate) dry redispersible pellets] (PEDOT:PSS) (Al 4083), and electron-transporting and hole-blocking material 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB) were obtained from commercial sources. ITO-coated glass with a sheet resistance of 10 Ω per square was used as the substrate. Before device fabrication, the ITO-coated glass substrate was carefully cleaned with the lotion and deionized water. After flushing with nitrogen and drying at 120 °C for 10 min, the ITO-coated glasses were exposed to UV-ozone for 15 min. PEDOT:PSS was then spincoated onto the clean ITO substrate as a hole-injection layer with the thickness of 40 nm. After drying for another 10 min, ITO substrates were transferred into a glovebox, and the mixture of 10% polymer, 25% 4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC), and 65% 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) in chlorobenzene was spin-coated (10 mg/mL, 3000 rpm) to form a ca. 40 nm thick emissive layer and annealed at 80 °C for 30 min to remove the residual solvent. Finally, a 50 nm thickness electron-transporting layer of TmPyPB was vacuum deposited, and a cathode composed of a 1 nm thick layer of LiF and aluminum was sequentially deposited through shadow masking with a pressure of 10−6 Torr. The current density− voltage−luminance (J−V−L) characteristics of the devices were

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CONCLUSION In summary, we successfully synthesized π-conjugated TADF polymers, Homo, Cop-50, and Cop-10 by the Suzuki coupling reaction. With increasing the proportion of carbazole derivatives, these TADF polymers exhibit the descending ΔEST, the increased MOs values, and the tunable nature of excited states. The Cop-10-based OLED device without any TADF assistant dopant can reach the EQEmax of 15.7% with the CIE coordinates of (0.46, 0.49). The RISC values can be mediated by excited-state mixing. Cop-50 features the hybrid G

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Macromolecules measured using a Keithley 2400 source meter and a Keithley 2000 source multimeter. The EL spectra were recorded using a JYSPEX CCD3000 spectrometer. The EQE values were calculated from the luminance, current density, and electroluminescence spectrum. All measurements were performed at room temperature under ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00565. Experimental procedures including materials, characterization as well as device fabrication and molecular simulation and calculation, materials synthesis, and results and discussion; figures of TGA, DSC, CV, 1H NMR, temperature-dependent transient PL decay spectra, steady-state fluorescence spectra, AFM, and current efficiency−luminance−power efficiency curves of devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.R.). *E-mail: [email protected] (S.Y.). ORCID

Zhongjie Ren: 0000-0002-7981-4431 Shouke Yan: 0000-0003-1627-341X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundations of China under Grant No. 51221002 is gratefully acknowledged.

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REFERENCES

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