Molecular Design of Highly Efficient Thermally Activated Delayed

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A Molecular Design of Highly Efficient Thermally Activated Delayed Fluorescence Hosts for Blue Phosphorescent and Fluorescent Organic Light-Emitting Diodes Chih-Chun Lin, Min-Jie Huang, Ming-Jui Chiu, Man-Ping Huang, Ching-Chih Chang, Chuang-Yi Liao, Kai-Ming Chiang, Yu-Jeng Shiau, Tsu-Yu Chou, Li-Kang Chu, Hao-Wu Lin, and Chien-Hong Cheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03979 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Chemistry of Materials

A Molecular Design of Highly Efficient Thermally Activated Delayed Fluorescence Hosts for Blue Phosphorescent and Fluorescent Organic Light-Emitting Diodes Chih-Chun Lin,†,§ Min-Jie Huang,†,§ Ming-Jui Chiu,† Man-Ping Huang,† Ching-Chih Chang,† ChuangYi Liao,† Kai-Ming Chiang,‡ Yu-Jeng Shiau,‡ Tsu-Yu Chou,‡ Li-Kang Chu,† Hao-Wu Lin,‡ and ChienHong Cheng*,† † ‡

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT: Recently, thermally activated delayed fluorescence (TADF) materials become the most promising hosts for realizing high-performance phosphorescent and fluorescent organic light-emitting diodes (OLEDs) because of their ability of upconverting triplet excitons to singlet excitons. However, despite that a few TADF hosts have been introduced for low energy phosphorescent and fluorescent dopants, developing host materials with TADF properties for blue phosphorescent and TADF OLEDs is still a great challenge to date. In this study, bipolar hosts exhibiting TADF behavior and high triplet energy, consisting of the carbazole group as the donor, diphenylsulphone moiety as the acceptor, and m-bitolyl as π-conjugated bridge, are synthesized and applied for the first time to blue devices. The ∆EST value of TADF host is tuned via the introduction of a cyano group in the carbazole moiety due to the increase of the LE contribution in the CT excited state. Detailed photophysical studies confirm the efficient TADF properties of bipolar hosts. The blue phosphorescent and TADF devices using BT-01 as the host give external quantum efficiencies of 31.8% and 25.5%, respectively. The blue devices based on BT-01 host exhibit superior electroluminescence performance and more reduced efficiency roll-off compared with those hosted by BT-02, ascribed to the faster reverse intersystem crossing process on BT01 host. These excellent results manifest that the use of bipolar host with TADF behavior is a promising approach for the realization of highly efficient blue phosphorescent and TADF devices in the future.

1. Introduction A crucial issue for the realization of organic light-emitting diodes (OLEDs) in the lighting and display application is how to maximize the device performance through materials design and device engineering.1 Among the important components of OLEDs, emitters play a decisive role in the exciton harvesting and external quantum efficiency (EQE) of the devices.2-4 A promising approach to achieve high efficient OLEDs is using phosphorescent chromophores, especially iridium complexes, as the emitter, because the heavy metal complexes can convert singlet into triplet excitons and subsequently emit phosphorescent light via efficient spin-orbital coupling, and thus achieve electroluminescence (EL) performance four times superior to the conventional fluorescent ones.5-7 In recent years, a significant breakthrough in high-performance fluorescent OLEDs was realized by Adachi’s group8-10 and other laboratories4, 11-16 using thermally activated delayed fluorescence (TADF) emitters. In general, the design of TADF materials is to use a large twisted structure to connect the donor and acceptor units, in which the twisted structure minimizes the HOMO–LUMO overlap and thus results in a small singlet–triplet gaps (∆EST).17-20 The small ∆EST is conducive to the up-conversion of the nonradiative triplets to radiative singlet excitons via thermally activated reverse intersystem crossing (RISC).16, 21-23 Thus, TADF-based OLEDs can give a maximal internal quantum efficiency (IQE) of 100% as high as phosphorescent devices. To achieve excellent device performance, both phosphorescent and TADF materials should be doped into a suitable host to reduce the concentration quenching by triplet–

triplet annihilation (TTA) and triplet–polaron quenching (TPQ).24-26 Similar to phosphorescent OLEDs, the host material in a TADF device27-30 should have (1) sufficient overlap between host emission and emitter absorption; (2) high singlet and triplet energies to guarantee efficient energy transfer to the TADF emitters; (3) excellent and balanced charge transport properties to enhance recombination efficiency of holes and electrons in the emitting layer; (4) promising thermal and morphological stability to lengthen the device lifetime. However, of known host materials for phosphorescent OLEDs, only few can be successfully utilized in the TADF OLEDs, particularly, the blue ones, because of the different absorption and HOMO/LUMO levels for these two kinds of emitters.27 It is still problematic to find efficient hosts for both blue TADF and phosphorescent emitters. To date, most host materials in TADF devices exhibit deep HOMO energy level and shallow LUMO energy level, making difficult the injection of hole or electron carriers in the emitting layer31-33 This leads to a high turn-on voltage and low power efficiency (PE) in the TADF devices.34, 35 To solve the issue, intramolecular charge transfer (ICT) materials consisting of electron-donating and electronaccepting moieties are considered as the most favorable molecular approach for the host of TADF devices.36, 37 The bipolar transporting property facilitates both hole and electron injection and the transport, leading to good charge balance, wide exciton-formation zone, and low operating voltage. Recently, bipolar materials exhibiting TADF behavior were used as hosts for high-performance phosphorescent and fluorescent

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OLEDs.38, 39 The use of TADF host is expected to result in full singlet and triplet energy transfer to emitter via efficient upconversion from triplet to singlet and thus enhance device performance. Furthermore, the upconversion process from triplet to singlet of the host can reduce the triplet exciton density and thus render low efficiency roll-off in OLEDs.22, 40, 41 Although highly efficient TADF hosts have been successfully used for phosphorescent and fluorescent devices, these developed host materials are only applied to low energy emitters.4246 This is because the intramolecular charge transfer characteristic of TADF host from the donor to acceptor moieties reduces triplet energy gap,47 which makes the design of TADF host for blue emitter difficult. Thus, there is still no report on host materials with TADF properties for blue phosphorescent and TADF OLEDs, which remains challenge to date. Presented herein is the first example of efficient TADFbased host material with both high triplet energy and small ∆EST for blue phosphorescence and TADF devices. For a TADF host that exhibits high triplet energy, we adopt the D– π–A structure, where the carbazole group acts as the donor due to its deep HOMO level, high-energy triplet state, and moderate hole-transporting capability, while the diphenylsulphone moiety is chosen as the weak electron acceptor with the πconjugation-breaking feature to increase the LUMO level. A m-bitolyl as π-conjugated bridges is used to increase twist angle, lower the coupling between donor and acceptor units. The design strategy endows the resulting BT-01 film with small ∆EST and high triplet energy contributed by the carbazole unit. Moreover, a CN group is introduced to the carbazole unit of BT-01 molecule to modulate the ∆EST and understand its influence on the device performance. It is found that the device performance increases and efficiency roll-off decreases with decreasing ∆EST of TADF host. In the developed TADF hosts, BT-01 shows the best performance with maximum quantum efficiencies of 31.8% and 25.5% and gives low operating voltage and efficiency roll-off in the blue phosphorescent and TADF OLEDs due to the bipolar characteristic and efficient up-conversion from triplet to singlet of BT-01 film . These superior performance demonstrate that the use of TADF host is conducive to the realization of highly efficient blue phosphorescent and TADF devices.

Experimental Section Generals. Chemicals and reagents were purchased from commercial providers without further purification. NMR spectra were recorded with a Varian Mercury 400 spectrometer and mass spectra were performed on a Bruker Impact HD, EVOQ instrument. UV-Vis spectra were recorded using a Hitachi U-3300 model, while the r.t. and low temperature PL spectra were taken using a Hitachi F-7000 fluorescence spectrophotometer. Transient PL decay of the materials in film were obtained using an optical measurement system equipped with a 355 nm pulsed Nd-YAG laser (INDI-40-10, SpectraPhysics) as the excitation source. The laser pulse was introduced via fiber optics (Model 77532, Newport Corp) to the sample. The emission signals were collected by a photodiode (DET10A/M, Thorlabs) and the data were acquired with an oscilloscope (WaveSurfer 24MXs-B, LeCroy). A highpass filter (GG-400-25.4, Lamda) at 410 nm in front of the photodiode was used to prevent the scattering of 355-nm laser. The time-resolved emission spectra were measured on an Edinburgh Instruments FLS980 spectrometer with a gated photomultiplier tube (PMT). The excitation power was monitored

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simultaneously with a power meter (Model 1916-C, Newport). The temperature dependent experiments were performed with an Oxford Optistat DN cryostat. The glass transition temperatures were determined by DSC under a nitrogen atmosphere using a TA Instrument Q10 instrument. The decomposition temperature corresponding to 5% weight loss was conducted on a Perkin-Elmer Pyris 1 TGA thermal analyzer. Elemental analyses were performed using an Elementar Vario EL III microanalyzer. The HOMO levels were determined by a photoelectron spectrometer (AC2). The PL quantum efficiencies of doped films were measured with a calibrated integrating sphere under a nitrogen atmosphere. The molecular geometry optimizations and electronic states were computed by carrying out the Gaussian 09 program with density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, in which the Becke’s three-parameter functional combined with Lee, Yang, and Parr’s correlation functional (B3LYP) hybrid exchange correlation functional with the 6-31G* basic set were used. The singlet and triplet excited-state properties were analyzed by using natural transition orbital and visualized on the Gaussview 5.0 software. Preparation of BT-01 and BT-02. 9-(4-Bromo-3methylphenyl)-9H-carbazole (0.34 g, 1 mmol) or 9-(4-bromo3-methylphenyl)-9H-carbazole-3-carbonitrile (0.36 g, 1 mmol), 4,4,5,5-tetramethyl2-(2-methyl-4(phenylsulfonyl)phenyl)-1,3,2-dioxaborolane (0.39 g, 1.1 mmol), K2CO3 (0.41 g, 3.0 mmol), and Pd(PPh3)4 (0.060 g, 5 mol%) were placed in a two-neck flask which was vacuumed and filled with nitrogen gas three times, and then toluene (3.0 mL), ethanol (1.0 mL), and water (1 mL) were added. The system was vacuumed and refilled with nitrogen three times again and was stirred at 80 °C for 12 h. After the reaction was completed and cooled to room temperature, the mixture was filtered through a Celite pad and the filtrate was extracted with ethyl acetate. The combined organic layer was dried by magnesium sulfate and concentrated under reduced pressure to give the crude product. The crude material was purified by column chromatography to give the expected white material. Further purification by vacuum sublimation afforded pure product. BT-01. White powder (80%). 1H NMR (400MHz, CD2Cl2, δ): 8.16 (d, J = 8.0 Hz, 2H ), 8.03–8.00 (m, 2H), 7.91 (s, 1H), 7.85 (dd, J = 8.0, 2.0 Hz, 1H ), 7.66–7.56 (m, 3H), 7.49 (d, J = 8.4 Hz, 3H), 7.46–7.41 (m, 3H), 7.39 (d, J = 8.0 Hz, 1H), 7.32–7.26 (m, 3H), 2.24 (s, 3H), 2.21 (s, 3H); 13C NMR (100 MHz, CDCl3, δ): 146.1, 141.6, 140.7, 140.6, 138.8, 137.9, 137.4, 137.3, 133.2, 130.4, 130.1, 129.3, 129.0, 128.4, 127.8, 125.9, 125.1, 124.3, 123.4, 120.3, 120.0, 109.8, 20.0, 19.9; HRMS (ESI) m/z: [M + H]+ calcd for C32H26NO2S, 488.1684; found, 488.1669. Anal. calcd for C32H25NO2S: C 78.82, H 5.17, N 2.87; found: C 78.87, H 5.08, N 2.87. BT-02. White powder (77%). 1 H NMR (400MHz, CD2Cl2, δ): 8.48 (dd, J = 16.0, 4.0 Hz, 1H), 8.20–8.18 (m, 1H), 8.03–8.00 (m, 2H), 7.92 (m, 1H ), 7.87–7.84 (m, 1H), 7.70–7.67 (m, 1H), 7.66–7.55 (m, 3H), 7.53–7.47 (m, 4H), 7.43–7.37 (m, 3H), 7.30 (d, J = 8.0 Hz, 1H), 2.33 (s, 3H), 2.12 (s, 3H); 13C NMR (100 MHz, CDCl3, δ): 145.7, 142.4, 141.5, 141.5, 140.8, 139.8, 137.9, 137.7, 136.0, 133.2, 130.4, 130.3, 129.3, 129.2, 129.0, 128.4, 127.8, 127.4, 125.3, 125.1, 124.4, 123.5, 122.2, 121.3, 120.7, 120.3, 110.6, 110.4, 102.7, 20.0, 19.9; HRMS (ESI) m/z: [M + H]+ calcd for C33H25N2O2S,


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Figure 1. Molecular structures and natural transition orbitals (NTO) in the S1 state of BT-01 and BT-02.

513.1637; found, 513.1635. Anal. calcd for C33H22N2O2S: C 77.32, H 4.72, N 5.46; found: C 77.49, H 4.52, N 5.66. OLED Fabrication and Measurements. The EL devices were fabricated by vacuum deposition of the materials at 10-6 Torr onto a UV-ozone cleaned ITO glass with a sheet resistance of 15 Ω/square. The deposition rate for the organic compounds was 1–2 Å/s. The cathode, consisting of Al/LiF, was deposited by evaporation of LiF with a deposition rate of 0.1 Å/s and then by the evaporation of Al metal with a rate of 4 Å/s. The effective area of the emitting diode was 9.00 mm2. Current, voltage, and light intensity measurements were made simultaneously using a Keithley 2400 source meter and an optical meter (Newport 1835-C) equipped with a calibrated silicon photodiode (Newport 818-ST). Electroluminescence spectra were measured on Konica Minolta CS-2000 spectrophotometer. Angle-Dependent PL Measurement. To analyze the orientation of transition dipole moments in doped films, Angular-dependent PL experiments were carried out on the setup composed of a half-cylindrical lens, a motorized rotation stage to control the emission angle, and a polarizer to select the polarization of the emitting light. The BTs films doped with 10% emitters were thermally evaporated onto 0.7-mm pristine glass substrates and encapsulated with UV epoxy and glass lid under a nitrogen atmosphere. A He-Cd continuous-wave laser (325 nm) was used as an excitation source. The emission was collected by using a fiber-coupled spectrometer (Acton SpectraPro SP-2300, Princeton Instruments) with a photon counting photomultiplier detector.

Results and Discussion To verify the feasibility of this design for the tuning of ∆EST, the electronic structures on the ground states of both BT-01 and BT-02 were first simulated by using density functional theory (DFT) at B3LYP/6-31G* level. In the optimized structures, the carbazole moiety is twisted from the diphenylsulphone unit with a very large dihedral angles of ca. 89° for BT-01 and BT-02 due to the steric hindrance of m-bitolyl. The large twist angles lead to small overlap between their HOMO and LUMO, decrease the electronic coupling of the CT state effectively and induce the separation of the MOs. Moreover, we further use the time-dependent DFT (TDDFT) method to calculate the ∆EST of BT-01 and BT-02 to be 0.50 and 0.86 eV, respectively. The results show that the introduction of a cyano moiety to the carbazole unit increases the ∆EST of the material. To gain more insights of their ∆EST, the natural transition orbitals (NTOs) of the lowest excited state (Figure 1) were evaluated. As expected, the particle and hole NTOs of BT-01 in the lowest S1 state are mainly localized on the diphenylsulphone group and the carbazole moiety, respectively, suggesting a predominant CT transition nature. It is worth noting that only a small overlap in the electron-hole wavefunction appears at the tolyl group near to the carbazole moiety. For the CN-containing BT-02, the NTOs of the S1 state exhibit a combined character of a CT transition from the carbazole to diphenylsulphone and a locally excited state (LE) transition on the carbazole and thus a relatively large wave function overlap was observed for the hole and particle of the NTO, resulting in a larger ∆EST than that for BT-01.



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Figure 2. (a) and (b) The UV-Vis absorption spectra, fluorescence emission spectra measured in difference solvents and film at 300 K, and phosphorescence spectra in neat film at 77 K for BT-01 and BT-02, respectively; (c) and (d) their transient PL characteristics in the neat films. Displayed in the insets are their prompt and delayed emission spectra.

Scheme 1. Synthetic procedure for BT-01 and BT-02.

The two m-bitolyls, BT-01 and BT-02, with one end connecting to a N-carbazolyl unit and the other end connecting to a phenylsulfonyl group were synthesized by the Pd-catalyzed Suzuki coupling reactions of the 4,4,5,5-tetramethyl-2-(2methyl-4-(phenylsulfonyl)phenyl)-1,3,2-dioxaborolane with corresponding 9-p-bromo-3-methylphenylcarbazole derivatives in ca. 80% yield after purification by column chromatography and further by sublimation. The chemical structures and the synthetic routes are depicted in Scheme 1, while the details of the synthetic procedures are described in Experimental Section and Supporting Information. The chemical structures were fully confirmed by the respective 1H and 13C spectra, mass data, and elemental analysis. Their thermal properties were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (see Figure S1 and Table 1). The decomposition temperature (Td, corresponding to 5% weight loss) of BT-02 is ca. 30 °C higher than that of BT-01 due to the introduction of the polar CN substituent on

the carbazole.48 In the DSC measurements, only BT-01 exhibits a distinct glass-transition temperature (Tg) of 96 °C. Figure 2 displays the UV-Vis absorption and photoluminescence (PL) spectra of BT-01 and BT-02 at room temperature and 77 K. These two materials exhibit similar UV-vis spectra with a strong π-π* absorption around 290 nm and two weak absorptions centered at 327 and 341 nm, characteristics of carbazole-centered n-π* absorptions. No CT absorption appears at longer wavelength region of their UV-vis spectra probably because the strong steric hindrance of m-bitolyl linkage significantly reduces the conjugation and the interaction between donor and acceptor units. However, an obvious disparity between BT-01 and BT-02 is found for their PL spectra in solution. As the solvent polarity increases, the PL spectrum of BT-01 broadens and shifts to a longer wavelength, accompanied by the gradual loss of the vibronic fine structure, assigned as a CT state emission. In contrast, the emission of BT-02 shows a much less degree of red shift with the solvent polarity, because of the presence of the nitrile group that reduces the electron donating ability of the carbazole unit. In most solvents, BT-02 exhibits a LE-state emission which is known to be less sensitive to the solvent polarity. In acetonitrile, a new shoulder, in addition to the CT state, appears in the red region, indicating a combination of LE and CT character for its emission. The observed spectral behavior in the solvatochromic experiments is in good agreement with the aforementioned DFT results. All emission spectra in the neat film show broader bands at longer wavelength compared with those


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Table 1. Physical data of BT-01 and BT-02

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Compound

λabsa [nm]

λema [nm]

λemb [nm]

λphosb [nm]

Es/ETc [eV]

HOMO/LUMOd [eV]

Tg/Tde [°C]

BT-01

294, 326, 340

349, 364

396

413, 441, 462

3.45/3.00

-5.88/-2.33

96/366

BT-02

286, 327, 341

352, 369

375

410, 438, 458

3.55/3.03

-6.01/-2.47

N.D.f/395

a

Measured in toluene solution of 10-5 M at r.t.. bMeasured in the neat film. cThe singlet energy (Es) was obtained from the emission onset in the film. The triplet energy (ET) was estimated from the high-energy emission maxima in the phosphorescence spectra. dHOMO values were determined using photoelectron spectrometry (AC-II). LUMO levels were calculated based on the equation of LUMO = HOMO – Eg. The energy gap (Eg) was estimated from the absorption threshold. eThermal data were obtained via TGA and DSC measurements. fN.D.: not detected.

in toluene. The degree of red-shift correlates well with the polarity of the molecule. In particular, the emission band of BT-02 film is red shifted in the spectra in comparison with those in different polar solvents. These results suggest that the bathochromic shift in the film is likely originated from the intermolecular charge transfer from carbazole to adjacent diphenylsulphone between BTs molecules as well as solid state polarization effect. Moreover, the phosphorescent spectra of both hosts show nearly the same well-defined patterns as that of N-phenyl carbazole, indicative of the 3LE contribution of carbazole core for their T1. This result is in good agreement with the theoretical predication, where the holes and particles of BTs in the first triplet excited state are located only at the electron-donating carbazole moiety (Figure S2). Their singlet (Es) and triplet energy (ET) were obtained from the onset of the fluorescence band and the highest energy peak of the phosphorescence spectra in the neat film, respectively. The calculated Es/ET values are 3.45/3.00 and 3.55/3.03 eV for BT-01 and BT-02, respectively. The respective ∆EST of BT-01 and BT02 are estimated to be 0.45 and 0.52 eV. It is found that the introduction of a CN group into carbazole core in BT-02 increases the Es value by 0.10 eV relative to BT-01 with its triplet energy remaining nearly the same as that of BT-01, leading to a larger ∆EST for BT-02. It is interesting that a DF characteristic is found in transient PL of BTs films even through BT-01 and BT-02 have a relative large ∆EST value. As displayed in Figure 2c and 2d, both the transient PL curves of BT-01 and BT-02 films exhibit bi-exponential decay, indication of two emission pathways. One is a normal fluorescence decay with the lifetime on the scale of nanosecond and the other is a delayed PL emission with the decay time falling in the microsecond range. A second-order exponential decay fitting gave the transient decay times of 11 ns and 1.3 µs for the prompt and delayed decay components of BT-01. For BT-02, the lifetimes of the prompt and delayed fluorescence were 18 ns and 1.8 µs, respectively. Furthermore, the prompt decays of BT-01 and BT-02 films are insensitive to oxygen and the delayed components disappear in the presence of oxygen environment. The PL spectrum of the delayed component show broad featureless band and is similar to that found in the prompt component, as shown in the insets of Figure 2c and 2d. To further verify the origin of the delayed emission of BTs, we conducted delayed fluorescence spectra of neat films as a function of excitation power (Figure S3). Generally speaking, the TADF-based delayed emission intensity exhibits a strictly linear proportionality on incident light intensity, while a transition from quadratic to linear de-

pendence is found for TTA-based delayed fluorescence. Figure 3a presents the double-logarithmic plots of delayed fluorescence intensities from BTs versus the power of the incident light. As anticipated, the delayed fluorescence intensity of BT films is found to raise with the increase of incident power and shows a first-order dependence over more than two orders of magnitude in power density. Such a relationship indicates a single photon process that a delayed output photon is generated from one input photon, in good agreement with the photochemical nature of the TADF process for BTs films. Considering a relative large ∆EST value of BT-01 and BT-02, the TADF property of BT-01 and BT-02 films do not arise from molecule itself and thus is likely attributed to the upconversion involved in intermolecular charge transfer process. Such charge transfer induced from intermolecular interaction can

Figure 3. (a) Dependence of DF intensity on the incident light power density; (b) Arrhenius plot of the DF intensity integrated from 320 to 550 nm for BT-01 and BT-02. The straight lines represent the best linear fits to the experimental data.



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Table 2. Electroluminescence performance of blue phosphorescent and TADF devices

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Devicea

Host/Dopant

Vonb (V)

Lmax (cd/m2, V)

EQEc (%)

CEc (cd/A)

PEc (lm/W)

CIEd (x, y)

A

BT-01/FIrpic

3.2

53300, 11.5

31.8, 31.6, 31.2

73.5, 73.3, 72.3

64.4, 53.4, 42.6

(0.16, 0.36)

B

BT-02/FIrpic

3.4

36029, 12.5

30.7, 30.5, 29.9

78.8, 78.2, 76.7

64.0, 56.0, 43.3

(0.17, 0.40)

C

BT-01/2CzPN

3.0

16923, 14.0

25.5, 19.5, 10.0

53.0, 40.2, 20.2

47.5, 26.6, 9.0

(0.16, 0.31)

D

BT-02/2CzPM

3.4

11797, 14.0

22.3, 13.9, 6.2

47.5, 30.3, 13.3

42.6, 19.2, 5.9

(0.17, 0.31)

a

ITO/NPB (30 nm)/mCP (20 nm)/hosts: 10% dopant (30 nm)/TmPyPb (60 nm)/LiF(0.8 nm)/Al (100 nm). bthe operating voltage at 1 cd/m2. the efficiencies at max, 100, and 1000 cd/m2, respectively. dmeasured at 8 V.

c

offer adequate overlap between the donor and adjacent acceptor units and result in a new intermolecular CT excited state, leading to a small ∆EST and thus fast RISC.49 To gain detailed insight into the TADF mechanism resulting from intermolecular CT for BTs films, the experiments of temperature-dependent delayed fluorescence spectra were examined from room temperature to 80 K (Figure S3). Similar to the previously reported TADF materials, BTs film exhibits enhanced delayed fluorescence emission band upon increasing the temperature, demonstrating the presence of the thermally activated RISC in the BTs films. Between 100 and 250 K, an Arrhenius like temperature dependence is observed in the plot of the delayed fluorescence intensity with respect to the inverse T (Figure 3b). The thermal activation energy (Ea) corresponding to the difference between S1 and T1 energy levels of intermolecular CT excited state can further derived from the slope of the least-squares line in Arrhenius plot and is estimated to be 0.067 and 0.109 eV for BT-01 and BT-02, respectively. The trend in Ea value is consistent with the experimental results of ∆EST obtained from PL spectra. However, an interesting phenomenon of calculated ∆EST > measured ∆EST >> Ea is observed. In the solid state, the 1CT state of BTs can strongly interact with the dipole fields of BTs,50 resulting in a lower S1 state and thus smaller ∆EST than those obtained from DFT calculation. Moreover, the obtained Ea values of BTs are significantly smaller than singlet–triplet energy gap determined from PL spectra. This is because intermolecular excited states of BT films can offer a smaller exchange energy owing to the electron and hole being located on two different molecules relative to that of intramolecular excited states,51-53 leading to the triplet levels being much closer to the singlet levels. Such disparity between measured ∆EST and Ea could also be resulted from the presence of a 3nπ* state13 or the molecular configuration changes in triplet excited states.54 These aforementioned results demonstrate that BT-01 and BT-02 films undergo reversible intersystem crossing, leading to a substantial amount of TADF. It is noteworthy that the delayed lifetime of BT-01 film is about 0.5 µs shorter than that of BT-02 because the small ∆EST of BT-01 film results in a faster RISC rate and thus a shorter delayed lifetime. The delayed lifetime of BT-01 is shorter compared with most TADF materials.8, 9, 14, 55, 56 This is favorable for inducing full singlet and triplet energy transfer from BT-01 to phosphorescent or TADF emitters and prevent the accumulation of triplet exciton at high operational voltage, achieving

high efficiency and reduced efficiency roll-off for phosphorescent and TADF OLEDs. The HOMO levels of BT-01 and BT-02 neat films were determined to be -5.88 and -6.01 eV by ultraviolet photoemission spectroscopy (Figure S4), respectively. The addition of a CN group to the carbazolyl moiety makes the HOMO level of BT-02 0.13 eV lower than that for BT-01. From the optical band gap and the HOMO level, the LUMO values were estimated to be -2.33 and -2.47 eV. To gain insight into the carrier-transport character of BTs that contain both an electrondonating carbazolyl moiety and an electron-withdrawing phenylsulfonyl substituent, a hole-only and an electron-only devices were fabricated for these two host materials with the device structures consisting of ITO/NPB (15 nm)/host (40 nm)/NPB (15 nm)/Al (100 nm) and ITO/BCP (15 nm)/host (40 nm)/BCP (15 nm)/LiF (1 nm)/Al (100 nm), respectively. In these devices, NPB (N,N’-bis-(1-naphthyl)-N,N’-diphenyl1,10-biphenyl-4,40-diamine) and BCP (2,9-dimethyl-4,7diphenyl-1,10-phenanthroline) were used to avoid electron and hole injection from the cathode and anode, respectively. Figure S5 displays the current density-voltage curves of these devices. The BT-01 based hole-only and electron-only devices exhibit very close current density-voltage curves, suggesting that BT-01 is an excellent bipolar material capable of transporting both electrons and holes. However, for BT-02, the current density of the electron-only device is several order of magnitude higher than that of the hole-only device at the same applied voltage, indicating BT-02 is a good electron transporter rather than a hole transporter. The result is in agreement with its very low HOMO of 6.01 eV compared with other known hole transporters. The high singlet and triplet energy gaps and bipolar property of BTs prompt us to explore the application of these two materials as the hosts for blue phosphorescent and TADF OLEDs. For the fabrication of phosphorescent and TADF devices, we choose FIrpic (iridium(III) bis(4,6difluorophenylpyridinato-N,C2′)picolinate) and 2CzPN (4,5di(9H-carbazol-9-yl)phthalonitrile) as the emitters, respectively, because their UV-Vis absorptions show substantial spectrum overlap with the PL emissions of BTs films, supporting efficient Förster energy transfer from BTs to the emitters (Figure S6). The FIrpic-based blue OLEDs were first prepared with the device configuration of ITO/NPB (30 nm)/mCP (20 nm)/host: 10% FIrpic (30 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm), in which the hosts were BT-01 and BT-02


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Figure 4. Device performance of phosphorescent and TADF OLEDs hosted by BT-01 and BT-02. (a) and (b) EQE-luminance-current efficiency characteristics; (c) and (d) EL spectra at 8 V for FIrpic- (left) and 2CzPN-based (right) devices.

for devices A and B, respectively. In these devices, NPB acts as the hole-transporting material, mCP (1,3-bis(Ncarbazolyl)benzene) as a hole-transporting material and also as an exciton blocker (ET = 2.9 eV) to prevent the diffusion of excitons to the NPB layer, and TmPyPb (1,3,5-tri[(3-pyridyl)phen-3-yl]benzene) as an electron transporting layer. The representative electroluminescent performance of these two devices are displayed in Figure 4 and Figure S7, and summarized in Table 2. Both devices emit sky-blue light from FIrpic, manifesting complete energy transfer from host to FIrpic and effective exciton blocking by mCP. Devices A and B offer maximum EQEs of 31.8 and 30.7% with the turn-on voltages of 3.2 and 3.4 V, corresponding to maximum current efficiencies (CEs) of 73.5 and 78.8 cd/A and power efficiencies (PEs) of 64.4 and 64.0 lm/W, respectively. Device A using BT-01 as the host exhibits higher efficiency and lower turn on voltage compared with device B hosted by BT-02. To see whether the device efficiency is overestimated, we measured the light distribution patterns of these two BTs-based blue OLEDs A and B. Both devices show ideal emission patterns close to the Lambertian emission profile (Figure S8), indicating that the EQEs of these devices are not overestimated. To the best of our knowledge, the superior EL performance of device A is the best result among the FIrpic-based blue PhOLEDs.57-59 This result can be ascribed to the following factors. First, the emission of BT-01 shows extensive spectral overlap with the MLCT absorption of FIrpic, providing a good Förster energy transfer from the excited state of BT-01 to that of FIrpic. This

can further be confirmed by the absolute fluorescent quantum yields (ΦPL) of FIrpic in the BTs host. The ΦPL of BT-01 doped with 10% FIrpic is measured to be 0.99, higher than that of 0.93 for BT-02. Secondly, the suitable HOMO and LUMO levels of BT-01 prevents charge trapping, but provide nice charge balance in the emitting layer for this FIrpic-based OLED. In contrast, the device hosted by BT-02 easily leads to charge trapping due to the larger difference in energy levels between the host and FIrpic. Third, the bipolar behavior of BT-01 provides a higher carrier drift mobility and more balanced carrier injection and transport, and thus increase the electron and hole recombination within the emissive layer. Moreover, the smaller ∆EST further enhances the possibility of full singlet and triplet energy transfer from BT-01 to FIrpic via efficient and fast reverse intersystem crossing from triplet to singlet. On the other hand, Dexter energy transfer from BTs to FIrpic significantly influences the device efficiency at high dopant concentration. Generally, the rate of Dexter energy transfer is proportion to the orbital and spectral overlaps between guest and host.60 Relative to BT-02 with CN substituent, BT-01 can give better emission spectral overlap with FIrpic, facilitating the Dexter process from BT-01 to FIrpic. Intriguingly, both FIrpic-based OLEDs hosted by BTs exhibit the device performance exceeding the EQE limit of 2530% for phosphorescent OLEDs. The excellent efficiency plausibly originates from the predominantly horizontal dipole orientation of the emitting layer as well as their high quantum efficiency. To further evaluate emitting dipole orientation, we



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Figure 5. Angle-dependent PL intensity of BTs films doped with 10% FIrpic. The solid lines represent the simulation results with a different ratio of horizontal dipoles.

measured the angle-dependent and polarization-resolved emission profiles of FIrpic in the BTs films. By analyzing measured angular dependent PL spectra using optical simulation, the horizontal transition dipole (Θ) ratio of the doped film is extracted, in which Θ represents the ratio of the horizontal portion of the transition dipole moments to the all transition dipole moments. Figure 5 displays the measured and theoretical results of the PL intensity as a function of emission angle. The simulated fitting curves are found to accord well with the experimental data and render Θ value of 0.71 for the BTs films doped with FIrpic. The obtained Θ value suggests that the transition dipole moments of FIrpic molecules doped in BT films are preferentially oriented toward the horizontal direction. Such result was also found in some heteroleptic Ir-based phosphorescent dyes with β-diketonate or picolate as the ancillary ligand doped in the host film.61-64 Two mechanisms have been proposed to explain the origin of the preferred emitting dipole orientation of Ir complex in a host matrix.65, 66 For a heteroleptic Ir complex, its distinct asymmetrical structure gives rise to two nearly parallel triplet transition dipoles along the Ir–N direction.67 The first mechanism indicates that the strong electrostatic interaction between Ir complex and host leads to the linear binding geometries of the dopant−host molecules and thus induces the triplet transition dipole moment of Ir complex to orient preferentially toward the horizontal direction.68 The second one mainly focuses on the inherent asymmetry at the surface of the deposited film.66 The vacuum/organic boundary created during the vapor deposition can promote the alignment of the transition dipoles of Ir complex in the amorphous film. In our hosts, the linear BT molecule plausibly facilitates the formation of linear binding geometries between the FIrpic and BT host via electrostatic interaction, which results in negative binding energy and thus induce the emitting dipole orientation of FIrpic to preferentially align along the horizontal direction in BTs films. Such predominantly horizontal dipole orientation of FIrpic can effectively enhance the optical out-coupling of emitting layer and therefore accounts for highly efficient Firpic-based blue OLEDs using BTs as hosts. To further demonstrate the potential of BTs as the hosts for blue TADF device, we fabricated devices C and D using TADF material 2CzPN as the blue dopant and BT-01 and BT02 as the host, respectively. The other layers in the devices are the same as devices A and B. The key device characteris-

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tics are also presented in Figure 4, Figure S7, and Table 2. Similar to the EL behaviors of FIrpic-based devices, device C hosted by BT-01 achieves the best performance with low driving voltage. Device C gives pure 2CzPN emission with a maximum EQE as high as 25.5%, CE of 53.0 cd/A, and PE of 47.5 lm/W, respectively, which are superior to those of device D using BT-02 as the host (EQE = 22.3%, CE = 47.5 cd/A, and PE = 42.6 lm/W). As expected, the performance of blue TADF devices show good correlation with the measured ΦPL of 2CzPN in BTs hosted films, where the ΦPL values reach 99 and 91% for BT-01 and BT-02 films doped with 10% 2CzPN, respectively. It is noteworthy that the EQE of device C hosted by BT-01 is significantly higher than that of 2CzPNbased TADF device containing a mCP host reported by Adachi.8, 69 This is because BT-01 not only possesses the bipolar property and balanced charge transporting characteristic, but also gives higher ΦPL for 2CzPN-doped film compared with mCP.61 To explore the host effect on the TADF behavior of 2CzPN, the transient PL decay was carried out for 2CzPN doped in the two hosts. As displayed in Figure S9 and Table S1, the two doped films exhibit TADF lifetimes of 131 and 169 µs for BT-01 and BT-02, respectively, shorter than 273 µs reported for 2CzPN-doped mCP film.69 The shorter DF lifetime combined with high quantum yield suggests that BTs hosts are beneficial for increasing the RISC rate of 2CzPN and thus enhance the TADF characteristics of 2CzPN. We further analyzed the PL efficiency and lifetime data to gain insight into TADF dynamics of 2CzPN-doped films (see Supporting Information for details) and the corresponding kinetic parameters are listed in Table S1. Interestingly, 2CzPN doped in BT01 film gives higher TADF quantum efficiency (ΦTADF = 53%) and larger RISC rate constant (kRISC = 1.6 × 104 s-1) than those in mCP host reported previously70 (ΦTADF = 38 % and kRISC = 6.2 × 103 s-1). This manifests that BT-01 host can facilitate the RISC rate of 2CzPN and thus increase quantum yield of TADF component, resulting in excellent device performance. These device efficiencies are outstanding in comparison to those of the best blue phosphorescent counterparts71, 72 and appear among the best performance of reported blue TADF devices.34, 35, 47, 73 In particular, the FIrpic- and 2CzPN-based OLEDs hosted by BT-01 (devices A and C) also present remarkably reduced roll-off relative to most reported phosphorescent and TADF devices. For example, the EQE of FIrpicbased device A exhibits an efficiency roll-off of 1.9% at the brightness of 1000 cd/m2, while for 2CzPN-based device D, at a brightness level of 100 cd/m2 and 1000 cd/m2, the external quantum efficiencies retain as high as 19.5% and 10.0%, respectively. Generally, the efficiency roll-off in most phosphorescent and TADF devices is mainly arisen from the TTA. To gain further insight into the influence of TTA mechanism on the efficiency roll-off for these OLEDs, their EQE-current density curves are analyzed by using the TTA model24, 74 as follows EQE J 0 J = ( 1 + 8 − 1) EQE0 4 J J0

(1) where EQE and EQE0 stand for the quantum efficiencies in the presence and absence of TTA mechanism, and J0 represents the current density at EQE = EQE0/2. Because the J0 is found to be inversely proportional to the TTA rate, a larger J0 value indicates a smaller TTA rate. The fitting results and the corresponding parameters of these devices are shown in Figure S10. The best fitting curves of EQE-current density characteristics


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is found to accord well with the experimental values, supporting that the efficiency roll-off of these devices can be attributed to the TTA mechanism. The J0 values deduced from the fitting of the blue phosphorescent OLEDs based on BT-01 and BT-02 are 91 and 60 mA/cm2, respectively. Of these two hosts, BT-01 bears a larger J0 value than that of BT-02 in the FIrpic-based devices, indicative of a lower efficiency roll-off for the BT-01 based device. Similar trend is also observed in blue TADF devices hosted by BT-01 and BT-02. The BT-01 based TADF OLED gives a J0 value of 2.2 mA/cm2, superior to the BT-02 based device of 1.4 mA/cm2. Due to the bipolar behavior of BT-01, the small roll-off in BT-01 based device can be attributed to the wide recombination zone originating from good charge balance in the emitting layer. Moreover, the obtained J0 values of these blue phosphorescent devices are found to be significantly higher than those of previously reported PhOLEDs,24 revealing that a host with TADF ability can distinctly reduce TTA rate and maintain high efficiency over a wide range of current density. The TTA rate and efficiency roll-off is found to increase with increasing ∆EST in the BTs-based devices. This is because the TADF host can thermally upconvert the triplet excitons into its singlet via fast RISC that originates from small ∆EST, followed by a Förster resonance energy transfer (FRET) from the host to the dopant, providing an additional pathway to decrease the concentration of triplet excitons and the occurrence of TTA in the OLEDs using TADF materials as the hosts.75 In this process, the FRET rate from TADF host to guest determines the ratio of triplet to singlet excitons on TADF host.41 For BTs hosts, a better overlap between its emission and absorption of emitters is found for BT-01 than for BT-02, indicative of a faster FRET rate for BT-01. A faster FRET can greatly reduce the singlet exciton density on the BT-01 host, and thus suppress the ISC and increase the RISC on the BT-01 host, resulting in the decrease of the triplet population on BT-01 and thus reduced the efficiency roll-off for the BT-01-based devices.

Conclusions In conclusion, two bipolar host materials BT-01 and BT-02 with TADF behavior, composed of diphenylsulphone as an electron acceptor, a m-bitolyl group as a π bridge, and carbazole with or without cyano moiety as an electron donor, were successfully synthesized and used for both blue TADF and phosphorescent OLEDs. The photophysical and theoretical studies suggest that the incorporation of a cyano group in the carbazole moiety increases the LE contribution in the CT excited state and thus greatly alters the magnitude of ∆EST. In comparison with BT-02 bearing a CN group, BT-01 exhibits TADF behavior with short delayed lifetime due to efficient reverse ISC from triplet to singlet arising from smaller ∆EST, which is favorable for efficient singlet and triplet energy transfer to emitters and the reduction of the triplet density of the host. In particular, blue TADF and phosphorescent OLEDs hosted by BT-01 achieve excellent EQEs of 25.5 and 31.8% with low turn-on voltage, respectively, significantly higher than those found for BT-02. Moreover, all the devices using BT-01 renders low efficiency roll-off at high luminance. This result can be ascribed to the efficient reverse ISC from triplet to singlet and thus the reduced TTA on host as well as the bipolar characteristic of the host. These findings demonstrate a promising design concept for the development of the host

material for TADF and phosphorescent OLEDs with extremely high performance.

ASSOCIATED CONTENT Supporting Information. Experimental details, NMR spectra, photophysical and thermal properties, power- and temperaturedependent DF data, photoelectron spectra, EL characteristics, and triplet-triplet annihilation analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of the Republic of China through Grants MOST 105-2633M-007-003. We are grateful to the National Center for HighPerformance Computing of Taiwan (Account number: u32chc04) for generous amounts of computing time.

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Molecular Design of Highly Efficient Thermally Activated Delayed

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