Novel blue fluorescent materials for high-performance nondoped blue OLEDs and hybrid pure white OLEDs with ultrahigh color rendering index

Novel blue fluorescent materials for high-performance nondoped blue OLEDs and hybrid pure white OLEDs with ultrahigh color rendering index

Journal Pre-proof Novel Blue Fluorescent Materials for High-Performance Nondoped Blue OLEDs and Hybrid Pure White OLEDs with Ultrahigh Color Rendering...

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Journal Pre-proof Novel Blue Fluorescent Materials for High-Performance Nondoped Blue OLEDs and Hybrid Pure White OLEDs with Ultrahigh Color Rendering Index Futong Liu, Hui Liu, Xiangyang Tang, Shenghong Ren, Xin He, Jinyu Li, Chunya Du, Zijun Feng, Ping Lu PII:

S2211-2855(19)31032-8

DOI:

https://doi.org/10.1016/j.nanoen.2019.104325

Reference:

NANOEN 104325

To appear in:

Nano Energy

Received Date: 9 September 2019 Revised Date:

1 November 2019

Accepted Date: 24 November 2019

Please cite this article as: F. Liu, H. Liu, X. Tang, S. Ren, X. He, J. Li, C. Du, Z. Feng, P. Lu, Novel Blue Fluorescent Materials for High-Performance Nondoped Blue OLEDs and Hybrid Pure White OLEDs with Ultrahigh Color Rendering Index, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104325. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Novel Blue Fluorescent Materials for High-Performance Nondoped Blue OLEDs and Hybrid Pure White OLEDs with Ultrahigh Color Rendering Index

Keywords: blue luminogen, anthracene, energy transfer, ultrahigh color rendering index, pure white OLEDs

Graphical abstract By adopting TPAATPE as the blue-emitting component, thermally activated delayed fluorescent molecule PTZMes2B as the green emitter as well as the host for long-wavelength phosphors, the target devices can achieve external quantum efficiencies exceeding 25% in two-color warm white OLED and three-color pure white OLED with ultrahigh CRI of 92.

Novel Blue Fluorescent Materials for High-Performance Nondoped Blue OLEDs and Hybrid Pure White OLEDs with Ultrahigh Color Rendering Index Futong Liua+, Hui Liua+, Xiangyang Tanga, Shenghong Rena, Xin Hea, Jinyu Lia, Chunya Dua, Zijun Fenga and Ping Lua*

a

State Key Laboratory of Supramolecular Structure and Materials, Department of

Chemistry, Jilin University, Changchun, 130012, P. R. China

*Corresponding authors: E-mail: [email protected]

Keywords: blue luminogen, anthracene, energy transfer, ultrahigh color rendering index, pure white OLEDs

Abstract Pure blue luminogens that can be applied in high-efficiency nondoped blue organic light-emitting diodes (OLEDs) and act as main component to generate white OLEDs with high color rendering index (CRI) simultaneously are rarely reported. Herein, two blue

emitters,

TPAATPE

and

PPIATPE,

consisting

of

triphenylamine/phenanthroimidazole and tetraphenylethene-substituted anthracene with asymmetric structures are designed and synthesized. The nondoped OLED using TPAATPE as the emitter exhibits pure blue emission with the maximum external 1

quantum efficiency (EQE) of 6.97% and Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.16). Employing TPAATPE as the blue-emitting component, combined with a thermally activated delayed fluorescent (TADF) molecule PTZMes2B, which is adopted as the green emitter and the host for phosphors to modulate the long wavelength emission, a series of highly efficient hybrid white OLEDs are successfully achieved. Among them, the two-color white OLED exhibits eye-friendly warm white light with a maximum forward-viewing EQE of 25.2%. In particular, the three-color white OLED achieves pure white emission with CIE coordinates of (0.34, 0.38), a maximum forward-viewing EQE of 25.3% and an ultrahigh CRI of 92. All of nondoped blue OLEDs and hybrid white OLEDs exhibit very small efficiency roll-offs with excellent color stabilities. To the best of our knowledge, these results are among the best outcomes for white OLEDs reported so far.

1. Introduction High-efficiency organic light-emitting diodes (OLEDs) have aroused tremendous attentions and progressed rapidly during the past decades because of their intrinsic advantages of quick response, flexible display potential, energy-saving nature, and free of backlight.[1-3] Although much endeavor has been made to develop high-performance OLEDs, and some have been applied in various flat-panel displays such as televisions and mobile phones in recent years, there still exists much restriction in practical applications.[4-5] In particular, the performance of blue 2

materials is far from satisfication in terms of device efficiency and stability for commercialization. Recently, extensive efficient phosphorescent materials and thermally activated delayed fluorescence (TADF) blue-emitting materials can harvest all triplet excitons and achieve high external quantum efficiency (EQE).[6-13] However, a series of challenges and crucial issues remain unresolved. For example, acquiring efficient pure blue emission with high luminance and stability that fulfill practical application requirements is still difficult for phosphorescent and TADF materials due to their working mechanism.[6, 10] In addition, they have to be doped into appropriate host materials with high triplet energy levels to avoid concentration-caused emission quenching and exciton annihilation, which needs complicated OLEDs fabrication procedures and increase commercial cost. Even though, their device efficiencies still suffer from rapid roll-offs at relatively high brightness.[14-15] Therefore, high-quality pure blue emitters with balanced charge injection and transportation property that can be applied in simple nondoped device and achieve high-efficiency at high luminescence are extremely desired. On the other hand, blue emitters can not only act as excitation sources to generate light of all colors, including green, red, and white by energy transfer processes, but also can increase the color gamut and reduce power consumption in full-color applications.[16-20] To satisfy

the

standards

of

the

next-generation

flat-panel

lighting

and

red/green/blue/white-television (RGBW-TV), white OLEDs (WOLEDs) with high color rendering index (CRI) are key to be intensively explored for lighting applications in houses, museums, art galleries, and other commercial places. The 3

realization of high-efficiency, high CRI, superior color stability, and small efficiency roll-off WOLEDs simultaneously in simplifed devices is still a great challenge. Thus far, pure blue luminogens that can be applied as emitter to achieve highly efficient nondoped

blue

OLEDs

and

function

as

main

component

to

construct

high-performance WOLEDs with high CRI simultaneously remain quite rare. Anthracene represents the earliest experimentally explored building block for blue-emitting molecules, and it still attracts considerable attentions up to date owing to its good thermo-stability, outstanding carrier-transporting ability as well as easier modification.[21-23] Anthracene is prone to form intermolecular π-π stacking between adjacent molecules in aggregated state originating from the rigid and planar structure, which often leads to relatively low photoluminescence quantum yield (PLQY) and red-shifted emission zone to undesirable sky blue region. To address this issue, introducing chromorphore with characteristic of aggregation-induced emission (AIE) might be an efficacious solution because the nonradiative decay channels can be largely depressed in aggregated state due to the intrinsic highly twisted conformation and weak intermolecular interactions as reported in prior studies.[24-27] This superior feature also makes it possible for the resulting anthracene derivatives to be utilized in nondoped device. Herein, we propose a novel molecular design strategy by conjugating anthracene moiety with typical AIE unit tetraphenylethene (TPE).[24] In line with this strategy towards blue emitters, TPAATPE and PPIATPE which composed of an anthracene core, TPE at the 9-position of anthracene and triphenylamine (TPA)/phenanthroimidazole (PPI) at the 10-position of anthracene 4

based on an asymmetric structure are successfully synthesized.[28-29] They both show typical AIE property and pure blue fluorescence with high PLQYs (81% and 69%) in neat films. The nondoped OLED device using TPAATPE as the emitting layer exhibits pure blue EL with the Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.16) and the maximum EQE of 6.97%. More importantly, at a practical high luminance of 1000 cd m−2, the EQE can still remain as high as 5.96%. The excellent EL performance of this newly synthesized material is comparable to the best results of pure blue AIE-type emitters among the previously reported devices (summarized in Table S5).[30-34] To further develop the hybrid WOLED, a TADF material PTZMes2B which can also be applied in nondoped device is selected as the co-component.[35] By adopting TPAATPE as the blue component, PTZMes2B as the green emitter as well as the host for long-wavelength phosphors, two types of high-efficiency hybrid WOLEDs are achieved. Two-color WOLED shows eye-friendly warm white light with the maximum forward-viewing EQE of 25.2% taking the merits of blue emission of TPAATPE and yellow emission by the full energy cascade from PTZMes2B to phosphorescent dye PO-01. Furthermore, utilizing incomplete energy transfer between host PTZMes2B and guest Ir(piq)3, three-color WOLED is fabricated and achieves pure white emission with CIE coordinates of (0.34, 0.38), maximum forward-viewing EQE value of 25.3% and ultrahigh CRI of 92. The excellent device performance and high CRI value should be the best results for pure white hybrid WOLEDs including using TADF materials, traditional fluorescent molecules and AIEgens as the blue emitter reported to date (summarized in Table S6 5

– S9).[36-44] These findings will unlock a novel concept that introducing both blue AIEgen and TADF molecule in one device to achieve high-performance hybrid WOLEDs.

2. Experimental section Experimental: N,N-diphenyl-4-(10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)aniline(TPAATP E) A mixture of (4-(diphenylamino)phenyl)boronic acid (0.58 g, 2.0 mmol), TPEABr (1.17 g, 2.0 mmol), Pd(PPh3)4 (70 mg, 0.06 mmol), K2CO3 (5.52 g, 40 mmol), distilled water (20 mL), and toluene (40 mL) was added in 100 mL round flask and refluxed at 90 °C under nitrogen atmosphere for 24 h. After cooling to room temperature, water (30 mL) was added to the mixture and then it was extracted twice with dichloromethane. The organic phase was washed with water and dried over anhydrous Na2SO4. After removal of volatiles, the residue was purified by silica-gel column chromatography using petroleum ether/dichloromethane (1:3, v/v) as eluent to obtain the product as yellow powder. (1.18 g, yield: 79%). 1H NMR (500 MHz, CD2Cl2) δ (ppm): 7.90 - 7.85 (m, 2H), 7.68 (dd, J = 5.6, 2.0 Hz, 2H), 7.40 (m, 6H), 7.36 (d, J = 7.5 Hz, 3H), 7.34 - 7.17 (m, 26H), 7.13 (t, J = 5.8 Hz, 2H).

13

C NMR

(125 MHz, CDCl3) δ (ppm): 147.84, 147.14, 144.73, 143.93, 143.46, 143.11, 141.59, 141.02, 136.98, 132.74, 132.67, 132.14, 131.57, 131.34, 130.70, 130.08, 129.80, 129.40, 127.79, 127.62, 127.05, 126.58, 124.81, 123.09. MS (MALDI-TOF)

6

m/z: [M+] Calcd for C58H41N, 751.97; Found, 752.13. Elem. Anal. Calcd (%) for C58H41N: C, 92.64; H, 5.50; N, 1.86. Found: C, 92.42; H, 5.55; N, 2.01. 1-phenyl-2-(4-(10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)phenyl)-1H-phenant hro[9,10-d]imidazole (PPIATPE) The synthetic steps are the similar as compound TPAATPE

except

using

PPIB

as

the

reactant

instead

of

(4-(diphenylamino)phenyl)boronic acid. The target compound is obtained as white powder. (1.26 g, yield: 72%). 1H NMR (500 MHz, CD2Cl2) δ (ppm): 8.87 (d, J = 8.3 Hz, 1H), 8.82 (d, J = 8.4 Hz, 1H), 7.92 (d, J = 7.4 Hz, 2H), 7.85 (t, J = 7.3 Hz, 1H), 7.80 - 7.71 (m, 6H), 7.66 (m, 5H), 7.46 (d, J = 8.0 Hz, 2H), 7.43 - 7.35 (m, 5H), 7.43 - 7.35 (m, 21H).

13

C NMR (125 MHz, CDCl3) δ (ppm): 150.77, 144.06, 143.45,

143.20, 141.63, 140.98, 139.81, 138.88, 137.60, 137.33, 136.91, 136.15, 131.42, 130.67, 130.28, 130.00, 129.73, 129.31, 128.37, 127.77, 127.63, 127.37, 127.07, 126.69, 126.36, 125.72, 125.04, 124.19, 123.03, 120.95, MS (MALDI-TOF) m/z: [M+] Calcd for C67H44N2, 877.10; Found, 876.84. Elem. Anal. Calcd (%) for C67H44N2: C, 91.75; H, 5.06; N, 3.19. Found: C, 91.53; H, 5.18; N, 3.24. 3. Results and discussion 3.1. Synthesis of Materials Scheme 1 showed the molecular structures and the synthetic route to TPAATPE and PPIATPE. The starting materials TPEB and PPIB were prepared according to the reported methods.[45-46] Then the intermediate TPEABr was prepared between the TPEB and 9, 10-dibromoanthracene under mild conditions. The Suzuki coupling reactions were performed between the intermediate TPEABr with respective 7

(4-(diphenylamino)phenyl)boronic acid and PPIB to obtain TPAATPE and PPIATPE in good yields. The new compounds were characterized by 1H and spectroscopies,

high-resolution

mass

spectrometry,

elemental

13

C NMR

analyses

and

corresponded well with their expected structure.

Scheme 1. Molecular structures and synthetic routes of TPAATPE and PPIATPE. (i) Suzuki coupling reactions: Pd(PPh3)4, K2CO3, toluene, H2O, 90 °C, 24 h.

3.2. Thermal Properties The thermal properties of these new compounds were examined by thermal gravimetric

analysis

(TGA)

and

differential

scanning

calorimetry

(DSC)

measurements under a nitrogen atmosphere (Fig. S5, Supporting Information). TPAATPE and PPIATPE exhibited decomposition temperatures (Td: corresponding to 5% weight loss) as high as 427 and 496 °C, respectively. In the DSC measurement, TPAATPE and PPIATPE exhibited apparent melting point temperatures (Tm) of 354 and 362 °C, respectively. PPIATPE showed a high crystallization (Tc) peak at 251 °C, no glass transition (Tg) temperature can be observed revealing that they are morphologically stable. The good thermal properties are beneficial for the process of vacuum deposition and subsequent operating stability of the devices. 8

3.3. Photophysical Properties The absorption spectra and PL spectra of the two compounds in the tetrahydrofuran (THF) solutions (10-5 M) as well as neat thin films were shown in Fig. 1. The key optical data were summarized in Table 1. Absorption spectra of TPAATPE and PPIATPE in solutions and films showed subtle difference which were all dominated by a series of peaks resulting from the combination of anthracene, TPE groups, imidazole and TPA groups.[47-48] The optical energy gaps for TPAATPE and PPIATPE were calculated to be 2.91 and 2.95 eV from the absorption onsets in THF. The PL spectrum of PPIATPE in THF showed deep blue emission peak centered at 441 nm, while that of the film was 454 nm, corresponding to a 13 nm red-shift from the solution to the solid state. However, the emission maximum of TPAATPE was at 465 nm in THF solution, compared to the 462 nm in the film state, indicating the absence of intermolecular interactions in the film state. The twisted molecular conformation of TPAATPE prevented close molecular packing in the solid state, resulting in reduced intermolecular interactions while also effectively inhibiting excimer formation and fluorescence quenching.

9

Fig. 1. UV-vis absorption and PL spectra of TPAATPE and PPIATPE measured in dilute THF solutions (a) and neat thin films (b). The solvatochromic effect was negligible for PPIATPE, as the PL spectra did not change much upon increasing solvent polarity, rendering the localized excited (LE) property of the lowest singlet (S1) excited state. However, the PL spectra for TPAATPE exhibited large spectral shift in solvents, from 443 nm in nonpolar solvent n-hexane to 516 nm in high polar solvent acetonitrile with a 73 nm variation (Fig. S6, Supporting Information). This shift indicated that owing to the enhanced electron donating ability of the TPA group, the intramolecular charge transfer (ICT) transition is dominant in the emission mechanism.[49] The transient PL decay characteristics of neat films were further measured in an oxygen-free environment (Fig. S7, Supporting Information), and TPAATPE and PPIATPE showed a single-exponential fluorescence decay process with short lifetimes of 2.04 and 1.26 ns, respectively. No delayed component was observed for these compounds since the low triplet energy of the anthracene moiety (1.7-1.8 eV) prohibited reverse intersystem crossing from the lowest triplet (T1) to S1 state.[50-53] These results excluded the possibility of TADF 10

mechanism for these compounds. The twisted and rotatable conformation of TPE unite gave rise to vigorous intramolecular motion in the solution state, which led to PLQY values in THF of 4.6% and 3.9%, respectively. In thin film, the PLQY values were found to be as high as 81% for TPAATPE and 69% for PPIATPE, the radiative decay constant (Kr) of S1 state can thus be calculated to be 3.97 × 108 and 5.48 × 108 s−1 for TPAATPE and PPIATPE according to Equation S2 (Supporting Information). The significant emission enhancement of the film PLQY illustrated that these compounds possessed AIE characteristics and were promising high-efficiency solid-state emitters (Fig. S8, Supporting Information). 3.4. Electrochemical Behaviors The electrochemical properties of TPAATPE and PPIATPE were measured by cyclic voltammetry (CV) measurements (Fig. S9, Supporting Information). The onset oxidation potentials against ferrocenium/ferrocene (Fc+/Fc) redox couple were 0.74 and 1.00 V, respectively. Thus, the highest occupied molecular orbital (HOMO) energy levels of TPAATPE and PPIATPE were calculated to be -5.34 and -5.60 eV, respectively, according to the Equation S5 (Supporting Information). The stronger electron-donating ability of TPA unit can heighten the HOMO energy level and contribute to the device performance, owing to the relatively easy transfer of holes. These compounds have almost identical cyclic voltammograms with similar reversible reduction peaks. The reduction onset potentials of TPAATPE and PPIATPE were -1.93 and -1.95 V, respectively, being very close to each other. Their corresponding lowest unoccupied molecular orbital (LUMO) energy levels were 11

around -2.78 eV. The LUMO energy levels matched well with that of the widely used electron-transporting

material

1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)-benzene

(TPBi) (LUMO: -2.70 eV), suggesting the feasible electron injection from TPBi to emitters. Table 1. Key thermal and photophysical properties of TPAATPE and PPIATPE. Compound

λmax, abs (nm)d)

Tda)/Tcb)/Tmc) (°C)

sol

427/-/354

292,355,376,396

PPIATPE

496/251/362

315,360,376,396

Td: decomposition temperature.

point.

d)

λmax,

abs:

Egf)

HOMO/LUMOg)

PLQY (%)h)

sol/film

(eV)

(eV)

sol/film

310,362,382,401

465/462

2.91

-5.34/-2.80

4.6/81

316,361,381,402

441/454

2.95

-5.60/-2.78

3.9/69

film

TPAATPE

a)

λmax, PL (nm)e)

b)

Tc: crystallization temperature.

c)

Tm: melting

absorption maximum in 10-5 M THF and thin films.

emission peak in 10-5 M THF and thin films. absorption onset in THF.

g)

f)

e)

λmax,

PL:

Eg: Optical gap calculated from the

HOMO/LUMO energy levels estimated by cyclic

voltammetry measurement. h) PLQY: Photoluminescence quantum yields. 3.5. Theoretical Calculations To gain insight into the molecular properties, Density Functional Theory (DFT) calculations were performed on TPAATPE and PPIATPE using B3LYP/6-31G(d,p) method.[54] As depicted in Fig. 2, these compounds exhibited a highly twisted molecular conformation, with large dihedral angle between anthracene and the adjacent moiety (TPA and PPI) of 83.2° for TPAATPE and 78.8° for PPIATPE. The LUMOs of the two compounds had similar distributions, being predominantly located on the anthracene core. However, the HOMO distributions of these compounds were significantly different. For PPIATPE, the HOMO was mainly concentrated on the anthracene unit with little contributions from PPI and TPE groups, while that of TPAATPE was mainly located on the anthracene and TPA moieties. The calculated

12

HOMO and LUMO density maps suggested that TADF behavior should be eliminated. TPAATPE and PPIATPE had the close LUMO energy levels of -1.60 eV and -1.64 eV, respectively, and owing to the good electron-donating ability of TPA unit, the calculated HOMO level of TPAATPE was higher than PPIATPE. This result is consistent well with the phenomenon observed in the measurement of CV.

Fig. 2. Molecular conformations, spatial distributions and energy levels of frontier molecular orbitals of TPAATPE and PPIATPE. 3.6. Single Carrier Devices To better investigate the hole and electron transporting abilities of TPAATPE and PPIATPE, we fabricated the hole- and electron- only devices with configurations of indium tin oxide (ITO)/1,4,5,8,9,11- hexaazatriphenylenehexacarbonitrile (HATCN) (5

nm)/N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine

(NPB)

(10

nm)/

TPAATPE or PPIATPE (80 nm)/NPB (20 nm)/Al (120 nm) and ITO/TPBi (20 nm)/TPAATPE or PPIATPE (80 nm)/TPBi (10 nm)/LiF (1 nm)/Al (120 nm), respectively. Herein, NPB and TPBi were employed aiming at preventing electron and hole injection. As shown in Fig. S13 (Supporting Information), TPAATPE and 13

PPIATPE both possessed bipolar transporting capacities. With increasing voltage, TPAATPE-based single carrier devices showed better transporting ability for holes and electrons than PPIATPE, demonstrating that TPAATPE is more likely to possess balanced charge-transporting ability. 3.7. Blue OLEDs Inspired by the excellent thermal stability and high solid fluorescence quantum yield, we utilized the new TPE derivatives as active layers to fabricate nondoped OLEDs. As seen in Fig. S14 (Supporting Information), the device configuration was ITO/HATCN (5 nm)/(di-(4-(N,N-ditolyl-amino)-phenyl) cyclohexane) (TAPC) (25 nm)/(tris(4-carbazoyl-9-ylphenyl)amine) (TCTA) (15 nm)/emitting layer (EML, TPAATPE for B1 and PPIATPE for B2) (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (120 nm), in which ITO was the anode, HATCN, TAPC and TCTA served as the hole injecting layer, hole transporting layer and buffer layer, respectively. TPBi, LiF and Al functioned as the electron transporting layer, electron injecting layer and cathode, respectively. As shown in Fig. 3, the emission peak wavelengths of the devices were located at 460 nm, which was close to the nondoped PL spectrum in solid film, indicating that the exciton recombination occured in emitting layer. The EL spectra kept stable at a wide range of driving voltages, confirming the devices had good color stability (Fig. S15, Supporting Information). The device B1 and B2 displayed extremely low turn-on voltages of 2.9 V and 3.0 V, indicating the smaller injection barriers between transporting layers and emitters. Among them, the device B1 exhibited the best EL performance, with a maximum luminance of 27500 cd m-2, a 14

maximum current efficiency (CE) of 9.4 cd A-1, a maximum power efficiency (PE) of 8.8 lm W-1 and EQE of 6.97%, probably owing to the higher PLQY of TPAATPE. In addition, the EL efficiencies reduced slowly, even at a luminescence of 1000 cd m-2, the EQE can still remain 5.96%, demonstrating a relatively low efficiency roll-off and the good efficiency stability of the device. The excellent EL performance of TPAATPE was comparable to the best results of pure blue AIE-type emitters among the previously reported devices (summarized in Table S5). Therefore, the new AIE-type blue emitter TPAATPE has great potential in flat-panel lighting and full-color

display.

The

doped

blue

device

employed

4,4’-Bis(Ncarbazolyl)-1,1’-biphenyl (CBP) as the host material with a configuration of ITO/HATCN (5 nm)/TAPC (25 nm)/TCTA (15 nm)/emitting layer (CBP-10 wt%, TPAATPE for D1 and PPIATPE for D2) (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (120 nm). As shown in Fig. S16 and Table S3 (Supporting Information), both the doped OLED exhibited slightly improved blue color purities, with the CIE coordinates of (0.15, 0.12) for TPAATPE and (0.15, 0.14) for PPIATPE at 1000 cd m−2, which was stable at different luminance. The doped devices D1 and D2 showed the maximum EQEs of 5.70% and 5.33%, respectively, which were slightly lower than those of nondoped device B1 and B2. But these devices also displayed small efficiency roll-off, and the EL spectra were also very stable at different voltages (Fig. S17, Supporting Information).

15

Fig. 3 (a) Energy level diagrams and molecular structures of the materials used in the devices B1 and B2. (b) EQEs and normalized EL spectra of devices B1 and B2. (c) Current density-voltage-luminance characteristics. (d) Current efficiency and power efficiency versus luminance curves.

The nondoped device B1 exhibited low efficiency roll-off and the maximum EQE was up to 6.97%, which greatly exceeded the theoretical limitation of 5% for fluorescent OLEDs (singlet excitons ratio ≈ 25%). This indicated that some triplet excitons may participate in a radiative process and were converted to singlet states via reverse intersystem crossing (RISC). At present, RISC could occur through the TADF or 16

triplet-triplet annihilation (TTA) process. The fluorescence lifetimes of films, the low triplet energy of the anthracene moiety (1.7-1.8 eV) and theoretical calculations were not in favor of the TADF process. To further explore the origin of high-efficiency, we measured the transient EL decay of nondoped devices B1 and B2 by an electrical excitation pulse generator (Fig. S19, Supporting Information). The transient EL decay of device B1 and B2 showed a prompt EL decay at the timescale of submicroseconds and a microsecond-scale delayed fluorescence. We presumed that these molecules may harvest triplet excitons through the TTA mechanism in OLEDs, increasing the singlet excitons ratio above 25%.[55-57] Moreover, the ratio of delayed component of device B1 was slightly larger than device B2, illustrating that more triplet excitons have been converted to singlet state. The combined effects of the PLQY, balanced carrier transport ability and the contribution of TTA process resulted in different EQEs for the two devices. 3.8. Hybrid WOLEDs Because TPAATPE exhibits marvelous performance of nondoped blue OLEDs, it is anticipated that using the TPAATPE as the blue-emitting component can realize high-performance hybrid WOLEDs. To accomplish this goal, several device design strategies have been adopted. First, an efficient yellow phosphor acetylacetonato bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl] iridium(III) (PO-01) has been introduced into the optimal device to design two-color hybrid WOLEDs. Then, to reduce the quenching effect of TTA and triplet-polaron quenching (TPQ), the phosphorescent EML should be doped into bipolar host materials. Double host systems were widely 17

used in AIE-based hybrid WOLEDs in prior studies,[24, 40] which can broaden the charge and exciton distribution and reduce annihilation, but it is difficult to precisely control the doping concentration which makes the fabrication process more complex. Herein, we choose a new TADF type molecule PTZMes2B reported by our group previously as bipolar host.[35] Employing PTZMes2B as host material, the fabricated orange and red phosphorescent OLEDs exhibited excellent EL performances with the maximum EQEs of 27.3% for orange, and 24.6% for red; even at the practical luminance of 1000 cd m−2, their efficiencies remained at 25.7%, and 23.0%, respectively (Fig. S20 and Table S4, Supporting Information). Detailed experiments that supporting PTZMes2B as a good host material for phosphorescent dyes was illustrated in Supporting Information. The bipolar character is beneficial for balanced charge injection and recombination in the emitting layer, leading to the low efficiency roll-off. Meanwhile, the T1 of PTZMes2B (2.41 eV) is higher than that of PO-01 (T1 = 2.2 eV), which can prevent the reverse energy transfer from PO-01 to the host. Therefore, triplet excitons can be well confined in the EML, producing the yellow emission effectively by the full energy cascade from PTZMes2B to phosphorescent dye PO-01(Fig. S23, Supporting Information). In addition, an appropriate

mixed

interlayer

consisting

of

hole-transporting

TAPC

and

electron-transporting bis[2-(2- hydroxyphenyl)-pyridine] beryllium (Bepp2) is prerequisite to ensure the high-performance. This interlayer (4 nm thickness) can block energy transfer between the fluorophors and phosphors, and the high T1s of TAPC (2.87 eV) and Bepp2 (2.6 eV) can avoid quenching of the triplet excitons in the 18

phosphorescent EML.[58] Besides, TAPC and Bepp2 possess moderate HOMO and LUMO energy levels as well as high hole mobility and electron mobility, which ensures that both holes and electrons can easily pass through interlayer to acquire the low turn-on voltages. Taking the above factors into account, we fabricated the optimized

two-color

hybrid

WOLED

with

multilayer

configurations

of

ITO/HATCN(5 nm)/TAPC (35 nm)/TCTA (5 nm)/PTZMes2B: PO-01 (12 nm, 10%)/TAPC: Bepp2 (4 nm, x: y)/TPAATPE (8 nm)/BmPyPB (40 nm)/LiF (1 nm)/Al (120 nm), x: y is the codoping ratio of TAPC: Bepp2 (Fig. 4a). First, when the codoping ratio of interlayer TAPC: Bepp2 was 5: 5, a high-performance hybrid WOLED (W3) had been developed. As shown in Fig. 4 and Table 2, the device W3 exhibited the maximum forward-viewing CE, PE, and EQE values of 69.0 cd A-1, 69.5 lm W-1, and 25.2%, respectively, which represented the highest values for AIE-based hybrid WOLEDs without employing outcoupling enhancement. Even at a high luminance of 1000 cd m-2, its CE and EQE values remained 60.0 cd A-1 and 21.9%, respectively, demonstrating the low efficiency roll-off. Moreover, the device W3 exhibited desirable warm color white light, with a CIE coordinates of (0.44, 0.44) at 1000 cd m−2, which was very eye-friendly lighting source owing to the low blue intensity. The color of device W3 was very stable, the CIE coordinates variation was only (0.01, 0.01) when the luminance went from 384 to 5211 cd m-2 (Fig. S25c, Supporting Information). In addition, the device W3 showed a very low turn-on voltage of 2.8 V, indicating the balanced carrier transport and injection. For comparison, we further fabricated devices W2 and W4 with the same device 19

structures as W3, except for the interlayer with the codoping ratio of TAPC: Bepp2 = 7: 3 (W2) and 3: 7 (W4). The device W2 also showed good forward-viewing CE, PE, and EQE values of 66.1 cd A-1, 67.5 lm W-1, and 24.1%, respectively, with CIE coordinates of (0.43, 0.43). Compared with device W3, the EL efficiencies of device W2 were slightly subordinate, which might be assigned to the higher doping ratio of TAPC. With an increased ratio of hole-transporting TAPC, exciton recombination region slightly moved from yellow layer to blue region and triplet excitons might be quenched through nonradiative relaxation. On the contrary, owing to the absence of TAPC, electrons readily arrived at the yellow region, achieving the full utilization of excitons. As expected, W4 exhibited a yellowish white colors with the CIE coordinates of (0.48, 0.47) and the maximum forward-viewing CE, PE, and EQE values of 74.0 cd A-1, 68.1 lm W-1, and 25.8%. To further comprehend the effect of interlayer for high-performance WOLEDs, the device with an interlayer of TAPC (W1) and Bepp2 (W5) were fabricated and investigated, where other layers were the same as those of W3. The EL efficiencies of device W1 (with maxima values of 44.7 cd A-1, 35.0 lm W-1, and 17.1%) were obviously lower than W3. Similar to the phenomena of W4, W5 showed a strong yellow emission and poor blue intensity with the CIE coordinates of (0.49, 0.49). As illustrated above, by adjusting different codoping ratio of TAPC: Bepp2, these white devices displayed excellent efficiency and color stability, demonstrating TPAATPE is a promising candidate for highly efficient hybrid WOLEDs.

20

Fig. 4. (a) Device configuration and corresponding thickness of Hybrid WOLEDs, and the molecular structures of emitters. (b) EQEs and normalized EL spectra of devices W1-W5. (c) Current density-voltage-luminance characteristics. (d) Current efficiency and power efficiency versus luminance curves. It is difficult to achieve high CRI over 85 for two-color hybrid WOLEDs (blue-orange) on account of the absence of the green region in EL spectra. Here, by introducing the red phosphor guest tris(1-phenylisoquinolinolato-C2,N) iridium(III) (Ir(piq)3) and utilizing incomplete energy transfer between TADF type host PTZMes2B and guest Ir(piq)3, three-color hybrid WOLEDs (blue-green-red) were fabricated. Fig. 5a showed the energy transfer diagram for the emitting layers. The 21

optimized device W6 had the structure of ITO/HATCN (5 nm)/TAPC (35 nm)/TCTA (5 nm)/PTZMes2B: Ir(piq)3 (12 nm, 0.5%)/TAPC (4 nm)/TPAATPE (8 nm)/BmPyPB (40 nm)/LiF (1 nm)/Al (120 nm). As displayed in Fig. 5, W6 showed decent forward-viewing CE, PE, and EQE values of 49.7 cd A-1, 47.3 lm W-1, and 25.3%, respectively. More importantly, W6 exhibited pure white emission, the CIE coordinates of this device were very close to the theoretical white point (0.33, 0.33). For full-color displays and solid-state lightings, pure white colors were extremely desired. However, hybrid WOLEDs usually exhibited warm white or yellowish white light, while the pure white color is rarely reported. Remarkably, W6 exhibited not only ultrahigh CRI value of 92 but also high color stability with a slight EL spectra change when the luminance changed from 513 to 5276 cd m-2 (Fig. S25f, Supporting Information), which should be the best results of pure white hybrid WOLEDs with a high CRI including using TADF materials, traditional fluorescent molecules and AIEgens as the blue emitter to date (summarized in Table S6 – S9).[12]

22

Fig. 5. (a) Schematic diagram of energy transfer mechanism and the molecular structure of emitters in the three-color WOLED. (b) EQE and normalized EL spectra of

device

W6.

Inset:

the

photograph

of

white

emission.

(c)

Current

density-voltage-luminance characteristics. (d) Current efficiencies and power efficiencies versus luminance curves.

Here, our hybrid WOLEDs exhibit excellent EL performance, the highlights are as following: (i) this is the first hybrid WOLED that uses AIEgen as the blue component and TADF molecule acts as the green emitter as well as the host for the long-wavelength phosphors. (ii) This is the best results of pure white OLEDs with high-efficiency, ultrahigh CRI and good color stability. (iii) The devices not only simultaneously simplify the fabrication process and improve the reproducibility of the devices, but also achieve both high-efficiency and low efficiency roll-off, combining the advantages of TADF and AIE. Therefore, we believe the novel concept that introducing both blue AIEgen and TADF molecule in one device is an effective way to achieve high-performance hybrid WOLEDs. 23

Table 2. Key Performance Parameters of the EL performances of the resulting OLEDs. device B1 B2 W1 W2 W3 W4 W5 W6 a) c)

Vona) (V) 2.9 3.0 2.8 2.7 2.8 2.7 2.7 2.7

Lmaxb) (cd m-2) 27500 23609 27588 59927 40530 82298 89821 13915

CEmax/1000c) (cd A-1) 9.4/7.7 7.9/6.3 44.7/44.1 66.1/60.4 69.0/60.0 74.0/73.5 79.1/76.2 49.7/39.0

PEmax/1000d) (lm W-1) 8.8/4.8 7.3/3.2 35.0/32.5 67.5/46.4 69.5/41.7 68.1/56.9 79.2/59.6 47.3/28.9

EQE max/1000e) (%) 6.97/5.96 6.10/5.00 17.1/16.8 24.1/22.5 25.2/21.9 25.8/25.7 27.1/26.2 25.3/19.7

Von: turn-on voltage at the luminescence of 1 cd m-2.

EL λmaxf) (nm) 460 460 b)

CRIg)

CCTh)

56 53 52 43 41 92

3550 3295 3228 2906 2781 5074

Lmax: maximum luminance.

Maximum forward-viewing CE and CE at 1000 cd m-2.

forward-viewing PE and PE at 1000 cd m-2. EQE at 1000 cd m-2.

f)

e)

CIEi) (x, y) (0.15, 0.16) (0.16, 0.16) (0.41, 0.41) (0.43, 0.43) (0.44, 0.44) (0.48, 0.47) (0.49, 0.49) (0.34, 0.38)

d)

Maximum

Maximum forward-viewing EQE and

EL λmax: EL emission peak of EL spectrum at 1000 cd m -2.

CRI: color rendering index measured at 1000 cd m-2.

h)

g)

CCT: color correlated

temperature measured at 1000 cd m-2. i) CIE: Commission International de l’Éclairage coordinates at 1000 cd m-2.

4. Conclusions In summary, we have designed and synthesized two novel AIE-type blue emitters composed of triphenylamine/phenanthroimidazole and TPE-substituted anthracene. These compounds exhibit good thermal stabilities, excellent charge-transporting capabilities and high PLQYs in neat films. Nondoped device based on TPAATPE demonstrates stable pure blue light with CIE coordinates of (0.15, 0.16) and a maximum EQE of 6.97%. In addition, two types of high-efficiency hybrid WOLEDs are achieved by adopting TPAATPE as a blue-emitting component and TADF molecule PTZMes2B acts as the green emitter and the host for the long-wavelength phosphors. Two-color device shows eye-friendly warm white light with the maximum 24

forward-viewing CE, PE and EQE of 69.0 cd A-1, 69.5 lm W-1, and 25.2%, respectively. Furthermore, utilizing incomplete energy transfer between host PTZMes2B and guest Ir(piq)3, three-color WOLED is fabricated and achieves pure white emission with CIE coordinates of (0.34, 0.38), the maximum forward-viewing EQE value of 25.3% and ultrahigh color rendering index of 92. Furthermore, all of blue and white OLEDs exhibit small efficiency roll-offs and excellent color stabilities. Such results systematically demonstrate the efficient pure blue luminogens are promising candidates to develop high-performance hybrid WOLEDs, which is valuable for the future practical applications, such as display and lightings. Appendix A. Supplementary data The following is the Supplementary data to this article: Acknowledgements This research is supported by the National Key R&D Program of China (Grant No. 2016YFB0401001), National Natural Science Foundation of China (91833304, 21774047),

the

Jilin

Provincial

Science

and

Technology

Department

(20180201084GX) and the Fundamental Research Funds for the Central Universities. +

Futong Liu and Hui Liu contributed equally to this paper.

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Futong Liu received his Bachelor degree in Chemistry from Jilin University in 2017. He is now a Ph.D. candidate at State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University. His current scientific interests focus on the white organic light-emitting materials and devices.

Hui Liu got his Bachelor degree in Chemistry from Langfang Normal University in 2015. He is now a Ph.D. candidate at State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University. His research interests focus on the device physics of organic light-emitting diodes.

Xiangyang Tang received his Ph.D. from college of chemistry, Jilin University in 2018. His research interests focus on the synthesis, spectroscopic study and device physics of new-generation organic light-emitting materials.

Shenghong Ren is an undergraduate student at Jilin University expected to graduate in June 2020 and has received the offer as a Ph.D. candidate at the Institute of Metal Research, Chinese Academy of Sciences. 35

Xin He got his Bachelor degree in Chemistry from Jilin University, China, in 2014. He is now a Ph.D. candidate at State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University. His research interests focus on the red organic light-emitting materials and theoretical calculations.

Jinyu Li received her Bachelor degree in chemistry and Ph.D. degree in polymer chemistry and physics from Jilin University in 2013 and 2018, respectively. Her research interests focus on the D-A type luminous materials and their applications in organic light-emitting diodes.

Chunya Du received her Bachelor degree in Chemical Engineering and Technology from Jilin University in 2018. Now she is a second-year Master’s candidate working in State Key Laboratory of Supramolecular Structure and Materials, Jilin University. Her research interests focus on the design and synthesis of red thermally activated delayed fluorescence materials and theoretical calculations.

Zijun Feng received his Bachelor degree in 2019 in Chemistry from Jilin University. Now, he is a working in State Key Laboratory of Supramolecular Structure and Materials of Jilin University as a Ph.D. candidate. His research interests focus on the design and synthesis of thermally activated delayed fluorescence materials.

Ping Lu received her Ph.D. degree in polymer chemistry and physics at Jilin University in 2005. And then, she joined the State Key Lab of Supramolecular Structure and Materials, Jilin University. She worked as a postdoctoral research fellow at Hong Kong University of Science and Technology from 2009 to 2010. She is promoted to be a full professor in 2014 in Jilin University. Her current research interests concern organic/polymer optoelectronic materials and devices.

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Highlights A nondoped OLED using TPAATPE as the emitting layer exhibits stable pure blue emission with a maximum EQE of 6.97% and CIE coordinates of (0.15, 0.16). At the practical luminance of 1000 cd m−2, the EQE can still remain as high as 5.96% demonstrating a small efficiency roll-off. A two-color white OLED is obtained by adopting TPAATPE as the blue component and phosphorescent dye PO-01 as yellow component, and yields an eye-friendly warm white light with a maximum forward-viewing EQE of 25.2%. By employing TPAATPE as blue emitter and utilizing incomplete energy transfer between host PTZMes2B and guest Ir(piq)3, a three-color white OLED is successfully fabricated and achieves pure white light emission with CIE coordinates of (0.34, 0.38), a maximum forward-viewing EQE value of 25.3% and an ultrahigh CRI of 92.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: