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Self-host blue dendrimer comprised of thermally activated delayed fluorescence core and bipolar dendrons for efficient solution-processible nondoped electroluminescence Xinxin Ban, Wei Jiang, Kaiyong Sun, Baoping Lin, and Yueming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14922 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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ACS Applied Materials & Interfaces

Self-Host Blue Dendrimer Comprised of Thermally Activated Delayed Fluorescence Core and Bipolar Dendrons for Efficient Solution-Processible Nondoped Electroluminescence Xinxin Ban†‡ Wei Jiang*† Kaiyong Sun† Baoping Lin† and Yueming Sun*† †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, P. R. China 211189

‡

School of Chemical Engineering, Huaihai Institute of Technology, Lianyungang, 222005, China

Abstract: A self-host thermally activated delayed fluorescence (TADF) dendrimer POCz-DPS for solution-processed nondoped blue organic light-emitting diodes (OLEDs) was designed and synthesized, in which the bipolar phosphine oxide carbazole moiety was introduced by alkyl chain to ensure the balanced charge transfer. The investigation of physical properties showed that the bipolar dendrons not only improve the morphological stability, but also restrain the concentration quenching effect of the TADF emissive core. The spin-coated OLEDs featuring POCz-DPS as the host-free blue emitter achieved the highest external quantum efficiency (7.3 %) and color purity comparing with the doped or nondoped devices based on the parent molecule DMOC-DPS, which indicates that incorporating the merits of encapsulation and bipolar dendron is an effective way to improve the electroluminescent performance of the TADF emitter used for solution-processed nondoped device. Keywords: self-host, bipolar, TADF, dendrimer, solution process, organic light-emitting diodes

Introduction Thermally activated delayed fluorescence (TADF) with the feature of harvesting both singlet and triplet excitons for 100% internal quantum efficiency have been widely used in organic light-emitting diodes (TADF) in recent years.1-7 Up to now, the TADF researches have focused on the improvement of the device efficiency and color diversity. By using small molecule evaporation TADF materials, the great progresses have been achieved even at deep blue range.8-12 Therefore, after the conventional fluorescence and phosphorescence emitters, the TADF material have been accepted as the third generation of OLEDs material.13-18 Nevertheless, some challenges still impede them from practical application in low-cost large-area flexible devices. Firstly, although numerous small molecular TADF emitters have been developed, only a few of them can be used for solution processing. Secondly, the reported TADF emitters generally suffer from the concentration quenching effect, which impels them to use doping technique to achieve high performance.14, 19-24 In order to solve these problems, molecular modification will be an effective way due to the direct relationship between property and structure.25-28 Similar to TADF counterparts, phosphorescent emitters also should be dispersed in suitable matrixes to suppress triplet-triplet annihilation.29-32 In order to avoid the host-guest blend system induced phase separation, Wang et al. have developed a series of nondoped phosphorescent materials with carbazole dendrons attached to the Ir complex core, in which the dendrons act as the hosts and the core plays the same role as the dopant .33-35 This strategy not only reduces the intermolecular interactions between the molecules, but also maintains the excellent electroluminescent efficiencies of the luminescence cores For the purpose of transferring this technology to the TADF platform, some polymers10-11, 14 and dedrimers36-38 composed self-host nature have been developed for application in solution-processed OLEDs. A suitable self-host

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TADF material for nondoped solution-processed OLEDs should meet the requirement of (i) the favorable thermal and morphological stability for solution processing; (ii) the effective encapsulation of the emissive core for concentration quenching restraining; (iii) the excellent carrier injection and transporting ability for balanced charge transfer. So far, carbazole dendrons have been mainly studied to attain the objectives.36, 38 However, the reported host-free TADF materials are all confronted with the charge unbalance problem result from the strong hole transporting ability of carbazole. To achieve high performance along with simple device structure, functional dendrons with bipolar characteristics have to been developed to facilitate the charge-transporting and charge-balance properties. In this work, a blue TADF dendrimer POCz-DPS was designed and synthesized as solution-processible self-host materials with bipolar feature. The well-known blue emitter bis[4-(3,6-dimethoxycarbazole)phenyl] sulfone (DMOC-DPS) was used as the TADF emissive core, and the high triplet energy phosphine oxide substituted carbazole acted as bifunctional dendrons. Direct substitution of electron donor and acceptor at the core by π-conjugation would change the frontier orbital distribution and further impair the color purity of the blue emission. The strategy to overcome the drawback is introducing the alkyl chain as a bridge between the TADF core and bipolar dendrons. The non-conjugated alkyl chains favorably make the fluorescence of the TADF core independent of the peripheral dendrons, while the encapsulation of the emitting core effectively restrained the concentration quenching effect of the TADF materials. Moreover, the self-host TADF dendrimer POCz-DPS exhibits good solubility and thermal stability, which is beneficial to the spin-coating process. Using POCz-DPS as the nondoped TADF emitter, solution-processed blue OLEDs achieved a high EQE of 7.3 %, which is comparable with the doping device and much higher than the device with TADF core as nondoped emitter. This is the first work reporting solution-processible self-host TADF dendrimer with bipolar dendrons, which not only successfully restrain the intermolecular interaction between the emissive cores, but also simplify the device fabrication processes.

Experimental section The synthesis of POCz-DPS is outlined in Scheme 1. All manipulations involving air-sensitive reagents were performed under a dry nitrogen atmosphere. 9,9'-[4,4'-sulfonylbis(4,1-phenylene)]bis(9H-carbazole-3,6-diol) (4OH-DPS) DMOC-DPS (0.67 g, 1 mmol) was dissolved in 50 mL dry CH2Cl2. The solution was stirred at 0 °C and BBr3 (0.4 mL 1 M solution in CH2Cl2, 4.2 mmol) was added dropwise. After stirring for 6 h, the reaction was returned to room temperature and carefully quenched with methanol. After that, a saturated solution of NaHCO3 was added, and the mixture was washed with water for three times. The organic layers were combined and dried with MgSO4. The product was then obtained by column chromatography on silica gel with CH2Cl2/ethyl acetate (2 :1) as the eluent to yield a brown solid (0.45 g, 72%). 1H NMR (500 MHz, DMSO-d6, δ): 9.19 (s, 4H), 8.25 (d, J = 8.7 Hz, 4H), 7.90 (d, J = 8.5 Hz), 7.40 (dd, J = 15.4, 5.6 Hz, 8H), 6.89 (dt, J = 11.0, 5.5 Hz, 4H). 13C NMR (126 MHz, DMSO-d6, δ): 151.98, 142.76, 137.67, 133.62, 129.54, 126.06, 124.35, 115.45, 110.61, 105.33. MS (MALDI-TOF) [m/z]: calcd for C36H24N2O6S, 612.1; found, 612.1. Anal. Calcd. for C36H24N2O6S: C, 70.58; H, 3.95; N, 4.57. Found: C, 70.58; H, 3.96; N, 4.56.


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Scheme 1 Synthetic route and chemical structures of the compounds.

9-(6-bromohexyl)-3,6-bis(diphenylphosphoryl)-9H-carbazole (Br-POCz) A mixture of POCz (5.67 g, 10.0 mmol), 1,6-dibromohexane (9.76 g, 40.0 mmol) and KOH (2.8 g, 20.0mmol) was dissolved in to the 100 mL mixed solvent of toluene and water (100 : 1). The reaction was refluxed for 8h under nitrogen. After cooling, the solvent was removed and the residue was purified by column chromatography on silica gel with ethyl acetate as the eluent to yield a white solid (5.8 g, 80%). 1H NMR (300 MHz, CDCl3, δ): 8.38 (d, J = 12.2 Hz, 2H), 7.87 7.62 (m, 10H), 7.60 - 7.40 (m, 14H), 4.35 (t, J = 5.4 Hz, 2H), 3.35 (t, J = 6.5 Hz, 2H), 1.95-1.76 (m, 4H), 1.57-1.34 (m, 2H). 13C NMR (75 MHz, CDCl3, δ) 142.74, 132.37, 132.15, 132.02, 131.89, 129.97, 129.81, 128.60, 128.44, 125.61, 125.47, 109.33, 109.15, 108.99, 43.37, 33.52, 32.38, 28.75, 27.77, 26.37. MS (MALDI-TOF) [m/z]: calcd for C42H38BrNO2P2, 729.1; found, 729.2. Anal. Calcd. for C42H38BrNO2P2: C, 69.05; H, 5.24; N, 1.92. Found: C, 69.04; H, 5.24; N, 1.93. 9,9',9'',9'''-[6,6',6'',6'''-[9,9'-[4,4'-sulfonylbis(4,1-phenylene)]bis(9H-carbazole-9,6,3-triyl)]tet rakis(oxy)tetrakis(hexane-6,1-diyl)]tetrakis[3,6-bis(diphenylphosphoryl)-9H-carbazole] (POCz-DPS) 4OH-DPS (0.6 g, 1 mmol), Br-POCz (3.6 g, 5 mmol) and K2CO3 (1.4 g, 10 mmol) were dissolved in to 100 mL acetone. The reaction was refluxed for 24h under nitrogen. After cooling, the solvent was removed and the residue was purified by column chromatography on silica gel with ethyl acetate/ethanol (30:1) as the eluent to yield a light yellow powder (1.9 g, 60%). 1H NMR (500 MHz, DMSO-d6, δ): 8.53 (dd, J = 11.8, 7.2 Hz, 8H), 8.27 (s, 4H), 7.87 - 7.70 (m, 21), 7.64 (dd, J = 18.3, 9.8 Hz, 44H), 7.54 (d, J = 6.2 Hz, 15H), 7.51 - 7.41 (m, 30), 6.98 - 6.79 (m, 2),



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4.36 (t, J = 8.0 Hz, 8H), 3.99 (t, J = 7.8 Hz, 8H), 1.78-1.64 (m, 16 H), 1.37-1.22 (m, 16H). 13C NMR (125 MHz, DMSO-d6, δ) 154.15, 142.76, 138.52, 134.83, 134.73, 133.62, 132.19, 132.04, 131.91, 130.11, 129.80, 129.64, 129.11, 128.96, 126.71, 125.52, 125.37, 124.65, 123.68, 122.29, 122.10, 116.17, 111.14, 110.48, 110.40, 104.97, 70.21, 70.13, 68.52, 43.12, 29.82, 29.22, 26.62, 26.04.. MS (MALDI-TOF) [m/z]: calcd for C204H172N6O14P8S, 3209.0; found, 3209.1. Anal. Calcd. for C204H172N6O14P8S: C, 76.30; H, 5.40; N, 2.62. Found: C, 76.28; H, 5.40; N, 2.63.

Results and discussion The initial compounds DMOC-DPS39 and POCz40 were prepared according to the previous reports. OH-DPS was formed by demethylation of DMOC-DPS with BBr3. POCz was simply coupled with 1,6-dibromohexane by nucleophilic substitution to form the bromide terminated phosphine oxide carbazole dendron. Then the final dendrimer was conveniently prepared by alkylation of the OH-DPS with the alkyl bromides under alkaline conditions. 1H/13C NMR, mass spectrometry and elemental analysis were employed to confirm the chemical structure of the dendrimer (Figure S8, S9 and S10). Before device fabrication, the material was purified by sedimentation in the mixed solvent (ethyl acetate and petroleum ether) for several times to give light yellow solid. The thermal gravimetric analyses and differential scanning calorimetry were performed to investigate the thermal properties of this TADF dendrimer. As shown in Figure 1, the thermal decomposition temperature (Td) and glass transition temperature (Tg) of POCz-DPS are 450 °C and 145 °C, respectively, which are higher than those of its parent molecule DMOC-DPS due to the increased molecular size. Moreover, the alkyl chains make POCz-DPS more soluble in common solvents, such as toluene, chlorobenzene and dichloroethane. The enhanced thermal stability and solubility make POCz-DPS more favorable to the solution process. The morphologies of DMOC-DPS and POCz-DPS were characterized by atomic force microscopy (AFM). As shown in Figure S1, the film of POCz-DPS was quite smooth with a root-mean-square (RMS) value of 0.30 nm, while DMOC-DPS exhibited some kind of pinholes on the surface with the RMS value of 1.82 nm. The results demonstrate that the introduced flexible chains and bipolar dendrons effectively enhance the film forming ability of POCz-DPS through solution-processing, which is highly important in improving the device efficiency.

Figure 1. TGA curve of DMOC-DPS and POCz-DPS recorded at a heating rate of 10 °C min-1; Inset: DSC trace recorded at a heating rate of 10 °C min-1.

The absorption and photoluminescence spectra of the POCz-DPS in CH2Cl2 are presented in Figure 2. Comparing with DMOC-DPS, the intramolecular charge transition (ICT) induced weak


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absorption bands around 300-400 nm and the lowest excited singlet state corresponding emission peaks at 534 nm keep unchanged, which means the non-conjugated aliphatic chains favorably make the fluorescence of the core independent of the peripheral dendrons. The intense absorptions of DMOC-DPS and POCz-DPS below 320 nm can be attributed to the π-π* transitions of the carbazole units, which enhanced in intensity due to the increased carbazole units. The optical band gap (Eg) of POCz-DPS was estimated from the onset of the absorption spectrum and is 3.13 eV, which is the same as that of DMOC-DPS. However, the emission peak of POCz-DPS in the solid state was 10 nm blue shift compared to the emissive core DMOC-DPS, which indicates that the intermolecular interaction have been reduced due to the effective encapsulation by peripheral dendrons. To further confirm the encapsulation effect, photoluminescence spectra of DMOC-DPS and POCz-DPS in various solvents with different polarities were measured. As shown in Figure 3, POCz-DPS exhibits a relatively weaker solvent-dependent shift than the emissive core DMOC-DPS, which can be attributed to the reduced interactions between the solvent molecules and the emissive core by the wrapping groups. It has been reported that the singlet (S1) and triplet (T1) energy levels of DMOC-DPS are 3.12 and 2.88 eV, respectively.39 Furthermore, the singlet-triplet energy gap (∆EST) of DMOC-DPS is 0.24 eV. Figure S2 shows the PL spectra of POCz-DPS in toluene at room temperature (RT) and 77K. Based on the previous studies, in aromatic media, the S1 state can be determined from the onset of the PL spectrum, while the T1 state can be calculated by the highest energy peak of the phosphorescence spectrum. Thus, the S1 level of POCz-DPS is at 3.03 eV and its T1 level is at 2.80 eV. As a result, the ∆EST of POCz-DPS is 0.23 eV, which is similar to the emissive core DMOC-DPS and fully confirm that the introduction of peripheral dendrons by alkyl chain will keep the TADF property unchanged. Figure 4 shows the transient photoluminescence curves of DMOC-DPS and POCz-DPS in neat films at room temperature. The proportion of the delayed component was increased after introducing the flexible dendrons, which indicates that the reduced quenching effect by molecular encapsulation facilitates the reverse intersystem crossing (RISC) and radiative deactivation process of triplet exitons. The time-resolved photoluminescence spectra of POCz-DPS were measured. As shown in Figure S3, the similar photoluminescence spectra were obtained before and after applying delay time, which confirms that the delayed emission comes from the singlet states by reversed process. The film PL quantum efficiencies (∅F) of POCz-DPS and DMOC-DPS were measured by integrating sphere with flowing nitrogen under an excitation wavelength of 350 nm. As a result, the ∅F of POCz-DPS (0.61) was higher than that of DMOC-DPS (0.42), which indicates that the molecular encapsulation can efficiently suppress the concentration quenching and facilitate the radiative decay.

Figure 2. (a) Absorption and fluorescence spectra of DMOC-DPS and POCz-DPS in CH2Cl2.



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(b) Photoluminescence spectra of DMOC-DPS and POCz-DPS in neat films.

Figure 3. Photoluminescence spectra of DMOC-DPS (a) and POCz-DPS (b) in different solvents.

Figure 4. Transient fluorescence decays of DMOC-DPS and POCz-DPS in films at 300 K.

Cyclic voltammetry was performed to investigate the electrochemical behavior of the dendrimer. As shown in Figure 5, POCz-DPS displays two oxidation waves. The first one located at the lower potential was attributed to the electron-donating carbazole moiety of the emissive core, which was similar to DMOC-DPS. The other one located at the higher potential was ascribed to the peripheral carbazole based dendrons. Due to the introduction of phosphine oxide groups at the 3,6-position of carbazole, no peaks belong to the couple of carbazoles were observed. On the basis of the onset potentials, the highest occupied molecular orbital (HOMO) energy levels of DMOC-DPS and POCz-DPS were estimated to be -5.43 eV and -5.40 eV, respectively. By subtracting of the optical energy band gaps, the lowest unoccupied molecular orbital (LUMO) energy levels of DMOC-DPS and POCz-DPS were calculated to be -2.30 eV and -2.27 eV, respectively. The redox behavior of POCz-DPS was measured in anhydrous tetrahydrofuran. As shown in Figure S4, POCz-DPS exhibited obvious reduction peak, which indicates the electron-transporting property of POCz-DPS. Gaussian simulation showed that the HOMO and LUMO of the dendrimer were located on the carbazole unit and the diphenyl sulfone of the emissive core, respectively (Figure 6). The frontier orbitals do not distribute to the peripheral carbazole or diphenylphosphine oxide units, which indicates that the alkyl chains make the electronic property of emissive core independent of the dendrons. According to the time dependent-density functional theory (TD-DFT), the fully separated HOMO and LUMO lead to a small ∆EST of 0.26 eV, which is equal to that of DMOC-DPS. Furthermore the calculated


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HOMO and LUMO energy levels of POCz-DPS are also similar to those of DMOC-DPS (Table 1). Therefore, the flexible linked dendrons actually form the peripheral shell to isolate the emissive core, while keeping the energy levels of the TADF molecule unchanged.

Figure 5. Cyclic voltammogram of DMOC-DPS and POCz-DPS with a concentration of 10-3 M in CH2Cl2 solution.

Figure 6. Optimized geometries and calculated HOMO and LUMO density maps for DMOC-DPS and POCz-DPS.



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Table 1. Physical properties of DMOC-DPS and POCz-DPS. Td/Tg [°C] DMOC-DPS

394/85

POCz-DPS

450/145

a

λabs [nm]

λems a

244,277,

∆EST

Eg

c

[nm]

[eV]

a

c

0.26

d

g

490

3.51

b

[eV]

HOMO

LUMO

[eV]

[eV]

-5.05

c

-1.54c

e

-2.30f

294,367

470

3.13

0.24

-5.43

236,267,

490a

3.47c

0.26

-5.03c

-1.56c

309,370

460b

3.13d

0.23h

-5.40e

-2.27f

Measured in CH2Cl2 solution at 300 K. b Measured in deposited films at 300 K. c Obtained from Gauss simulation. d Estimated from

the absorption edges in CH2Cl2. e Determined by the CV measurement.

f

Calculated from the energy gap and HOMO. hThe calculated

∆EST according to the experimental values. g According to reference[39].

In order to investigate the charge injection and transporting properties of this newly designed dendrimer compared with its parent emissive core, single carrier devices with the structures of ITO/Al/POCz-DPS or DMOC-DPS/TPBI/Cs2CO3/Al for electron-only and ITO/PEDOT:PSS/ POCz-DPS or DMOC-DPS/MoO3/Al for hole-only were fabricated. As shown in Figure 7, the electron current density of DMOC-DPS was negligible comparing to the hole one, which indicates the dominant hole injection and transporting capability of DMOC-DPS. In contrast, POCz-DPS exhibits an equal size of hole and electron current, which suggests the balanced charge transfer in this bipolar dendron modified TADF dendrimer. Inspired by the favorable thermal stability and charge balance property, solution-processsed nondoped OLEDs with the configuration of ITO/PEDOT:PSS(25nm)/POCz-DPS(35nm)/TPBI(35nm)/Cs2CO3(2nm)/Al(100nm) was fabricated. Here, POCz-DPS was alone used as the emission layer (EML) without any host material. For comparison, the control device with DMOC-DPS as EML and the doping device with PPO2:10% DMOC-DPS as EML were also prepared under the same condition. PPO2 was reported as bipolar host material with high triplet energy of 3.02 eV41. The device energy level diagram was present in Figure 8. Figure 9 shows the electroluminescence (EL) spectra of these three devices. Obviously, the host-free device based on POCz-DPS exhibits a blue emission with a maximum peak at 480 nm, which suggest the emitting occurs from the TADF core. However, the EL spectrum of the control device based on DMOC-DPS observed an additional emission at the long wavelength, which was not observed in the PL spectrum of DMOC-DPS neat film. The additional emission in the EL spectrum indicates that the electro-excitation facilitates the intermolecular interaction of DMOC-DPS comparing with the photo-excitation. Although the doping device can suppress the electromer emission,42 the red-shift emission peak and broad EL spectrum also indicate the existence of excitons interactions in the doping layer. As shown in Figure S5, the EL spectra of POCz-DPS are independent of the applied voltages from 9 V to 15 V. Therefore, the alkyl chain based self-host dendrimer not only restrains the intermolecular interactions between TADF cores, but also ensures the high color purity of the nondoped electroluminescent device. According to the current density-voltage-luminescence curves, the maximum brightness of POCz-DPS based device are much higher than that of DMOC-DPS, which can be attributed to the reduction of the exciton quenching effect by the wrapping of the emissive cores. As shown in Figure 10, the peak current efficiency of POCz-DPS based nondoped device is 12.6 cd A-1 and the corresponding external quantum efficiency (EQE) is 7.3% (Figure S6), which are among the highest performance of solution-processed blue fluorescent OLEDs with


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the Commission Internationale de L’Eclairage (CIE) coordinateds of (0.18, 0.30). However, the current efficiency of DMOC-DPS based nondoped device is only 1.56 cd A-1, which should be assigned to the seriously concentration quenching effect of the pure TADF emitter. Moreover, the reduction of the film PL quantum yield, the imbalanced charge transfer and the pinhole film morphology of DMOC-DPS are also responsible for the low device efficiency. The performance of doped device using bipolar compound PPO2 as host and parent emissive core DMOC-DPS as guest was optimized by changing the doping concentration (Figure S7). Considering the mass fraction of TADF core in POCz-DPS is about 18.9%, the control doping devices with the concentration from 5% to 20% were prepared, As a result, the device with PPO2:10% DMOC-DPS as EML achieves the highest device efficient of 11.2 cd A-1, which is very similar to the nondoped device using POCz-DPS as EML (Table 2). This further demonstrates that the enhanced performance of POCz-DPS based device should be attributed to the comprehensive effect of the balanced charge transfer by bipolar dendrons and reduced exciton quenching by the effective separation of TADF cores through multi-position encapsulation. All these results indicate the self-host dendrimer is an effective way to improve the electroluminescent efficiency of the nondoped TADF device. However, the turn-on voltages of POCz-DPS and PPO2:DMOC-DPS based devices are higher than that of DMOC-DPS based nondoped device. Comparing to the reported vacuum-deposited device based on DMOC-DPS39, which has a turn-on voltage of 4.3 V and a maximum brightness of 2500 cd A-1, the solution-processed device based on PPO2:DMOC-DPS has a comparable brightness of 2600 cd A-1 and a higher turn-on voltage of 6.0 V. According to the energy level diagram, the low-lying HOMO of the host PPO2 may be the main reason for the high turn-on voltage of this doping device. As for POCz-DPS, although the alkyl chains reduce the interaction between the emissive cores, the carrier injection and transporting ability of the material was remarkably weaken due to the electric inertia of the alkyls, which can be proved by the reduced current density of POCz-DPS in the carrier-only devices. Therefore, the relatively higher operating voltage of POCz-DPS based nondoped device was assigned to the reduced charge injection and transporting abilities. It has been reported that there is an appropriate dendron structure and size to balance the dilemma between luminescence quenching and charge transport of self-host phosphorescent emitters. Thus, the exploration of novel bipolar dendrons with good conductivity and high triplet energy for low operating voltage and high performance solution-processed nondoped blue fluorescence devices still have great potential.43-50

Figure 7. Current density-voltage (J-V) characteristics of carrier-only devices: (a) DMOC-DPS and (b) POCz-DPS.



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Figure 8. Schematic diagram of the device with the energy levels and molecular structures of the organic compounds.

Figure 9. EL spectra for the DMOC-DPS, POCz-DPS and PPO2:DMOC-DPS based devices.

Figure 10. (a) Current density-voltage-luminance (J-V-L) characteristics; (b) Calibrated current efficiencies of the OLEDs as a function of current density.

Table 2

Device Performances of the solution-processed OLEDs. Von

ηc,max

[V]

-1

ηext,max

L max

[cd A ]

[%]

[cd m-2]

DMOC-DPS

3.6±0.2

1.56±0.02

0.6±0.1

1500

(0.26, 0.33)

POCz-DPS

5.4±0.1

12.6±0.2

7.3±0.1

2700

(0.18, 0.30)

PPO2:DMOC-DPS

6.0±0.1

11.2±0.1

4.8±0.1

2600

(0.19, 0.32)

Emitter


CIE [x, y]

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Conclusions In summary, a self-host TADF dendrimer POCz-DPS with four-arm encapsulation was designed and synthesized. By using the bipolar groups as the functional dendrons, the unwanted concentration quenching and unbalance charge transfer of TADF core can be significantly reduced, while the small ∆EST remains unchanged. Function test shows that the phosphine oxide carbazole end-capped aliphatic groups not only facilitate the improvement of the thermal stabilities, but also effectively restrain the intermolecular interactions between the emissive cores. The superiority of such self-host TADF dendrimer in electroluminescence was demonstrated by comparing the EL performance of the host-free and TADF-doping devices. As a result, the solution-processed OLEDs using POCz-DPS as the non-doped TADF emitter achieves the highest current efficiency and color purity, which indicate that the introduction of bipolar dendrons to the edge of emissive core through flexible alkyl chains is a feasible approach to enhance the solution-processed nondoped TADF devices.

Supporting Information Experimental details for physical measurements; DFT calculations; Device fabrication and measurement; This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors * E-mail: [email protected] (W. Jiang). * E-mail: [email protected] (Y.M. Sun) Author Contributions X. X. Ban and W. Jiang contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declares no competing financial interest.

Acknowledgements We are grateful for the Grants from the National Basic Research Program of China (2013CB932902), National Natural Science Foundation of China (51103023, 21173042).

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