Manipulation of Thermally Activated Delayed Fluorescence of Blue

Publication Date (Web): June 5, 2017 ... The application of exciplex energy has become a unique way to achieve organic light-emitting diodes (OLEDs) w...
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Manipulation of Thermally Activated Delayed Fluorescence of Blue Exciplex Emission: Fully Utilizing Exciton Energy for Highly Efficient Organic Light Emitting Diodes with Low Roll-Off Zixing Wang,*,† Hedan Wang,†,‡,§ Jun Zhu,†,§ Peng Wu,†,‡ Bowen Shen,† Dehai Dou,†,‡ and Bin Wei† †

Key Laboratory of Advanced Display and System Applications, Ministry of Education, and ‡Department of Chemistry, Shanghai University, 149 Yanchang Rd, Shanghai 200072, P. R. China S Supporting Information *

ABSTRACT: The application of exciplex energy has become a unique way to achieve organic light-emitting diodes (OLEDs) with high efficiencies, low turn-on voltage, and low roll-off. Novel δ-carboline derivatives with high triplet energy (T1 ≈ 2.92 eV) and high glass transition temperature (Tg ≈ 153 °C) were employed to manipulate exciplex emissions in this paper. Deep blue (peak at 436 nm) and pure blue (peak at 468 nm) thermally activated delayed fluorescence (TADF) of exciplex OLEDs were demonstrated by utilizing them as emitters with the maximum current efficiency (CE) of 4.64 cd A−1, power efficiency (PE) of 2.91 lm W−1, and external quantum efficiency (EQE) of 2.36%. Highly efficient blue phosphorescent OLEDs doped with FIrpic showed a maximum CE of 55.6 cd A−1, PE of 52.9 lm W−1, and EQE of 24.6% respectively with very low turn on voltage at 2.7 V. The devices still remain high CE of 46.5 cd A−1 at 100 cd m−2, 45.4 cd A−1 at 1000 cd m−2 and 42.3 cd A−1 at 5000 cd m−2 with EQE close to 20% indicating low roll-off. Manipulating blue exciplex emissions by chemical structure gives an ideal strategy to fully utilize all exciton energies for lighting of OLEDs. KEYWORDS: exciplex emission, thermally activated delayed fluorescence, organic light emitting diodes, δ-carboline derivatives, blue emission

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INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted much interest in scientific and industrial research because of the promising applications in display and solid state lighting.1,2 However, the nonideal operating lifetime and low efficiency of blue OLEDs limit their commercial applications. In the past decade, lots of efforts have been focused on demonstrating highly efficient and long-life blue OLEDs, such as the developments of new bipolar materials,3−8 the novel hole or electron transport materials for dual emitting layer (EML),9,10 or the mixed host systems.11−15 These methods were utilized to tune the charge balance, confine exciton combination, and reduce the operating voltage resulting in highly efficient blue OLEDs. It has been supposed that combination of the injected holes and electrons in EML to form excitons are controlled by spin statistics, consequently singlet (S1) and triplet (T1) excitons are generated in an approximately ratio of 1:3.16 Hence fully utilizing exciton energy of singlets and triplets is a unique artifice to achieve efficient OLEDs by employing the heavy metal complexes,17,18 p-n heterojunction (exciplex systems),19,20 and thermally activated delayed fluorescence (TADF) materials as emitters.21,22 Adachi et al. reported that TADF behavior could be observed in the dimeric AVBBF2:mCBP system by forming exciplex.23 Moreover, the exciplexes formed between electron-donating molecule of © XXXX American Chemical Society

excited state and electron-accepting molecule of ground state, such as electron transporting layer (ETL)/EML, hole transporting layer (HTL)/EML, or HTL/ETL interfaces, usually exhibit bipolar property for ideal host of phosphorescent OLEDs (PhOLEDs). The application of exciplex energy has been used to tune emission colors24 and white OLEDs.25 In addition, high efficiencies, low turn-on voltage (Von), and low roll-off would be achieved.26−30 The normal HTL materials used in exciplex systems were N,N′-dicarbazolyl-3,5-benzene (mCP), 4,4′-(cyclohexane-1,1diyl)bis(N-phenyl-N-p-tolylaniline) (TAPC), 4,4′,4″-tris(Ncarbazolyl) triphenylamine (TCTA), N,N′-bis(1-naphthyl)N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPB), et al. with either low Tg or low T1 level. Moreover, these materials containing aryl-amine or carbazole units as the major holetransporting components make the highest occupied molecular orbital (HOMO) level close to −5.0 to −5.7 eV. However, the electron injection from cathode needs the lowest unoccupied molecular orbital (LUMO) level lower (−2.73 ∼ −3.2 eV for common ETL, such as TPBi,25 PO-T2T,27 TmPyPB,14 and B3PYMPM,26 et al.), which becomes a challenging issue to Received: April 11, 2017 Accepted: June 5, 2017 Published: June 5, 2017 A

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces realize blue or deep blue exciplex systems,13 as well as the high T1 for blue PhOLEDs. Meanwhile, in terms of device design, because of rare materials, there are few ways to modify exciplex energy levels. Obviously, design new materials with better charge mobility, higher Tg, and suitable T1 to manipulate exciplex energy level becomes an interesting improvement to achieve highly efficient OLEDs with low operating voltage and low roll-off. Carbolines, including the position of α-, β-, γ-, and δ-, have been proposed as electron-transporting moieties of host materials in blue PhOLEDs, with an electron deficient pyridine unit in the backbone.31−37 Our previous report has studied the relationship between molecular structure and photophysical properties, as well as performances of devices based on δ-carboline (DCb) derivatives with the LUMO energy level adjusting.37 Herein we present three new materials DCb-PCz, BTDCbPCz and BTCz-PCz (Figure 1). In this work, we report novel

Photoluminescence (PL) spectra were taken using FLSP920 fluorescence spectrophotometer, in both solution and solid state. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out using a CH Instrument 660C electrochemical analyzer with Hg/Hg2Cl2 electrode as reference electrode, with tetra(n-butyl)ammonium hexa-fluorophosphate (TBAPF6) in DMF as supporting electrolytes. Tg values of compounds were determined under a nitrogen atmosphere by differential scanning calorimetry (DSC) on TA Q500 HiRes thermal analyzer using a scanning rate of 10 °C/min with nitrogen flushing. The decomposition temperature (Td) corresponding to 5% weight loss was conducted on a TA Q500 HiRes TGA thermal analyzer. 4-(2-Nitrophenyl)dibenzo[b,d]thiophene (1). 1 was synthesized as our previous paper by general Suzuki coupling reaction.37 Yield: 78%. 1 H NMR (500 MHz, CDCl3, δ): 8.18−8.21 (m, 2H), 8.09 (d, J = 8.5 Hz, 1H), 7.78 (dd, J = 7.0 Hz, J = 1.5 Hz, 1H), 7.71−7.75 (m, 1H), 7.60−7.65 (m, 2H), 7.54 (dd, J = 7.5 Hz, J = 7.5 Hz, 1H), 7.43−7.50 (m, 2H), 7.33 (dd, J = 7.0 Hz, J = 0.5 Hz, 1H). 13C NMR (125 MHz, CDCl3, δ):148.8, 139.2, 139.0, 136.0, 135.7, 134.9, 133.0, 132.7, 132.0, 129.2, 127.0, 126.3, 124.9, 124.8, 124.6, 122.7, 121.9, 121.3. 2-(Dibenzo[b,d]thiophen-4-yl)-3-nitropyridine (2). 2 was synthesized as our previous paper by general Suzuki coupling reaction.37 Yield: 73%. 1H NMR (500 MHz, CDCl3, δ): 8.94 (dd, J = 4.5 Hz, J = 3.0 Hz, 1H), 8.25−8.29 (m, 2H), 8.18−8.20 (m, 1H), 7.82−7.85 (m, 1H), 7.55 (dd, J = 7.5 Hz, J = 7.5 Hz, 1H), 7.48−7.52 (m, 4H). 13C NMR (125 MHz, CDCl3, δ): 152.2, 151.9, 146.0, 139.8, 138.6, 136.9, 135.1, 132.7, 131.1, 127.1, 126.5, 124.8, 124.5, 123.2, 122.7, 122.6, 121.7. 5H-Benzo[4,5]thieno[3,2-c]carbazole (3). 4-(2-Nitrophenyl)dibenzo[b,d]thiophene (20 g, 65.57 mmol) and 1,2-bis(diphenylphosphino)ethane (35.11 g, 163.93 mmol) were dissolved in 1,2-dichlorobenzene (150 mL) and refluxed 12 h. Solvent was removed by reduced pressure distillation and washed with CH2Cl2. The title compound was obtained as a white powder. Yield: 97%. 1H NMR (500 MHz, DMSO-d6, δ): 10.57(s, 1H), 8.26 (d, J = 7.5 Hz, 1H), 8.16 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.5 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.59−7.64 (m, 2H), 7.33−7.54 (m, 4H). 13C NMR (125 MHz, DMSO-d6, δ): 139.7, 139.0, 138.5, 136.3, 132.1, 130.6, 128.9, 125.4, 125.0, 124.5, 123.0, 122.4, 121.5, 120.7, 119.6, 119.1, 111.1, 108.7. 5H-Benzo[4,5]thieno[2,3-e]pyrido[3,2-b]indole (4). Yield: 96%. 1H NMR (500 MHz, DMSO-d6, δ): 11.94 (s, 1H), 8.64 (dd, J = 9.5 Hz, J = 1.0 Hz, 1H), 8.46 (d, J = 8.5 Hz, 1H), 8.37 (d, J = 7.5 Hz, 1H), 8.11 (d, J = 7.5 Hz, 1H), 8.01 (dd, J = 8.5 Hz, J = 1.0 Hz, 1H),7.74 (d, J = 8.5 Hz, 1H), 7.45−7.54 (m, 3H). 13C NMR (125 MHz, DMSO-d6, δ): 142.6, 140.8, 140.1, 138.8, 135.9, 133.3, 131.7, 128.3, 125.8, 125.2, 123.6, 121.7, 121.5, 120.4, 118.8, 115.6, 110.2. General Procedure of Ullmann-Type Reaction. Compound 3 or 4 (18.3 mmol), 1, 3-dibromobenzene (21.57 g, 91.43 mmol), copper iodide (1.04 g, 5.46 mmol), trans-1,2-diaminocyclohexane (1.33 g, 10.92 mmol), and potassium phosphate tribasic (11.65 g, 54.9 mmol) were dissolved in 1,4-dioxane (100 mL) and toluene (100 mL) under a N2 atmosphere. The mixture was allowed to stir at 110 °C for 12 h. After cooling to room temperature, the mixture was diluted with tetrahydrofuran (THF). The copper catalyst and inorganic were removed by filtration under reduced pressure and the residue washed with CH2Cl2. The solvent was extracted with water and dichloromethane, then solvent was evaporated and the residue was recrystallized by CH2Cl2. Compounds 5 and 6 were obtained as powder. 5-(3-Bromophenyl)-5H-benzo[4,5]thieno[3,2-c]carbazole (5). yield: 52%. 1H NMR (500 MHz, CDCl3, δ): 8.30 (dd, J = 1.0 Hz, J = 1.0 Hz, 1H), 8.20 (dd, J = 8.5 Hz, J = 8.5 Hz, 2H), 7.99 (d, J = 7.5 Hz, 1H), 7.80 (dd, J = 2.0 Hz, J = 2.0 Hz, 1H), 7.66 (m, 1H), 7.58 (m, 1H), 7.49 (m, 7H). 13C NMR (125 MHz, CDCl3, δ): 142.9, 142.4, 141.2, 138.3, 136.8, 135.6, 134.2, 131.4, 130.9, 129.8, 128.2, 126.1, 125.5, 123.4, 122.7, 121.3, 121.1, 116.7, 109.8. 5-(3-Bromophenyl)-5H-benzo[4,5]thieno[2,3-e]pyrido[3,2-b]indole (6). yield: 43%. 1H NMR (500 MHz, CDCl3, δ): 8.81 (d, J = 4.0 Hz, 1H), 8.25 (d, J = 8.5 Hz, 1H), 8.20 (d, J = 7.5 Hz, 1H), 8.01

Figure 1. Molecular structures of CzBPDCb, DCb-PCz, BTCz-PCz, and BTDCb-PCz.

δ-carboline derivatives with the high T1 (∼2.92 eV) and high Tg (at 105−153 °C) to manipulate exciplex energy level. Deep blue (peak at 436 nm) or pure blue (peak at 468 nm) exciplex OLEDs were demonstrated by utilizing them as emitters with the maximum current efficiency (CE) of 4.64 cd A−1, power efficiency (PE) of 2.91 lm W−1, and external quantum efficiency (EQE) of 2.36%. Highly efficient blue PhOLEDs doped with Iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) showed a maximum CE of 55.6 cd A−1, PE of 52.9 lm W−1, and EQE of 24.6% respectively with very low Von at 2.7 V. The devices still remain high CE of 46.5 cd A−1 at 100 cd m−2, 45.4 cd A−1 at 1000 cd m−2 and 42.3 cd A−1 at 5000 cd m−2 with EQE close to 20% indicating the low efficiency rolloff. Manipulating exciplex emissions by chemical structure gave an ideal strategy to avoid exciton energy back transfer from exciplex to host or ETL. Then all excitons energies were utilized for lighting, which promises a bright future for OLEDs.

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EXPERIMENTAL SECTION

General Information. All chemicals and reagents were used as received from commercial sources without further purification unless stated otherwise. The auxiliary materials for OLEDs fabrication such as 1,4,5,8,9,11-hexaaza triphenylene-hexacarbonitrile (HAT-CN), TCTA, NPB, 3,3′-(5′-(3-(pyridin-3-yl)phenyl)-[1,1′:3′,1″-terphenyl]-3,3″diyl)dipyridine (TmPyPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), FIrpic, were purchased from Yurui (Shanghai) Chemical Co. Ltd. 1H NMR and 13C NMR spectra were recorded on a Bruker AV-500 spectrometer at room temperature. High resolution mass spectra (HRMS) were determined on Thermo Fisher Scientific LTQ FT Ultra mass spectrometer. Ultraviolet−visible (UV−vis) absorption spectra were recorded on UV-2501PC instrument. B

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Route for BTCz-PCz, BTDCb-PCz, and DCb-PCz

(ESI) m/z: [M + H]+ calcd for C41H25N3S: 592.1769, found: 592.1843. 5-(3-(9-Phenyl-9H-carbazol-3-yl)phenyl)-5H-pyrido[3,2-b]indole (DCb-PCz). yield:81%. 1H NMR (500 MHz, CDCl3, δ): 8.65 (dd, J = 1.0 Hz, J = 4.5 Hz, 1H), 8.51 (d, J = 7.5 Hz, 1H), 8.41 (d, J = 1.5 Hz, 1H), 8.18 (d, J = 7.5 Hz, 1H), 7.91 (m, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.82 (dd, J = 1.0 Hz, J = 8.5 Hz, 1H), 7.71 (m, 2H), 7.59 (m, 6H), 7.51 (m, 3H), 7.42 (m, 3H), 7.36 (dd, J = 4.5 Hz, J = 8.5 Hz, 1H), 7.31 (m, 1H). 13C NMR (125 MHz, CDCl3, δ): 144.2, 142.6, 142.3, 141.6, 141.4, 140.7, 137.4, 137.3, 134.4, 132.0, 130.4, 130.0, 128.0, 127.7, 127.0,126.7, 126.4, 125.5, 125.3, 124.7, 124.0, 123.3, 122.5, 121.0, 120.9, 120.4, 120.3, 118.9, 117.0, 110.3, 110.2, 110.0. HRMS (ESI) m/ z: [M + H]+ calcd for C35H23N3: 486.1892, found: 486.1968. Device Fabrication. The devices were fabricated by conventional vacuum deposition under a base pressure lower than 1.0 × 10−5 Pa. All of the organic layers and aluminum cathode were subsequently deposited on the prepatterned glass substrates (26 × 31 mm) containing three indium tin oxide (ITO) pixels (2 × 2 mm) which had been cleaned by scouring powder, acetone, and isopropyl alcohol followed by UV ozone treatment for 20 min. The entire organic layers and the Al cathode were deposited without exposure to the atmosphere. The deposition rates for the organic materials and Al were typically 1.0 and 5.0 Å s−1, respectively. Device Measurement. The current density−voltage−luminescence (J−V−L) characteristics were measured by a Keithley 2400 source meter and a PR-650 Spectra Colorimeter in the direction perpendicular to the substrate at room temperature under ambient conditions.

(d, J = 7.5 Hz, 1H), 7.79 (s, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.67 (d, J = 7.5 Hz, 1H), 7.41−7.57 (m, 5H), 7.35−7.39 (m, 1H). 13C NMR (125 MHz, CDCl3, δ): 143.9, 141.8, 140.2, 139.9, 138.2, 135.4, 134.2, 134.0, 132.9, 131.4, 131.2, 130.2, 130.0, 129.9, 125.6, 124.6, 123.5, 123.2, 121.3, 121.0, 119.9, 116.7, 107.4. General Procedure of Suzuki-Coupling Reaction. Compounds 5 or 6 (2.33 mmol), 9-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (1.03 g, 2.8 mmol), and potassium carbonate (0.97 g, 6.99 mmol) were dissolved in THF (60 mL) and water (20 mL) . After 15 min of N2 bubbling, Pd(OAc)2 (0.1 g, 0.44 mmol) and PPh3 (0.4 g, 1.52 mmol) were added and then refluxed for 12 h under a N2 atmosphere. After being cooled to room temperature, the mixture was poured into water and then extracted with dichloromethane. The organic phase was dried with anhydrous MgSO4 before evaporated. The residue was purified by column chromatography on silica gel with CH2Cl2/ether acetate as eluent to yield BTCz-PCz, BTDCb-PCz and DCb-PCz. 5-(3-(9-Phenyl-9H-carbazol-3-yl)phenyl)-5H-benzo[4,5]thieno[3,2-c]carbazole (BTCz-PCz). Yield:78%. 1H NMR (500 MHz, CDCl3, δ): 8.47 (s, 1H), 8.40 (d, J = 7.5 Hz, 1H), 8.18−8.23 (m, 3H), 8.01 (s, 1H), 7.90 (d, J = 7.5 Hz, 1H), 7.65−7.76 (m, 2H), 7.61−7.65 (m, 7H), 7.46−7.52 (m, 8H), 7.33−7.35 (m, 1H). 13C NMR (125 MHz, CDCl3, δ): 144.0, 141.4, 140.8, 140.7, 140.1, 138.7, 138.0, 137.5, 136.0, 132.9, 132.1, 130.9, 130.3, 129.9, 129.0, 127.7, 127.1, 126.7, 126.3, 126.0, 125.8, 125.3, 125.2, 124.6, 124.0, 123.3, 123.0, 122.5, 121.7, 120.9, 120.6, 120.4, 120.2, 119.3, 118.9, 117.1, 110.2, 110.1, 110.0, 107.8. HRMS (ESI) m/z: [M]+ calcd for C42H26N2S 590.1817, found: 590.1814. 5-(3-(9-Phenyl-9H-carbazol-3-yl)phenyl)-5H-benzo[4,5]thieno[2,3-e]pyrido[3,2-b]indole (BTDCb-PCz). Yield: 69%. 1H NMR (500 MHz, CDCl3, δ): 8.84 (dd, J = 1.0 Hz, J = 4.5 Hz, 1H), 8.43 (d, J = 1.5 Hz, 1H), 8.28 (d, J = 8.0 Hz, 1H), 8.22 (d, J = 7.0 Hz, 1H), 8.19 (d, J = 7.5 Hz, 1H), 8.03 (d, J = 7.5 Hz, 1H), 7.97 (m, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.87 (dd, J = 1.0 Hz, J = 8.5 Hz, 1H), 7.76 (t, J = 7.0 Hz, 1H), 7.72 (dd, J = 1.0 Hz, J = 8.5 Hz, 1H), 7.67 (d, J = 8.5 Hz, 1H), 7.61 (m, 5H), 7.49 (m, 4H), 7.43 (m, 2H), 7.39 (dd, J = 4.0 Hz, J = 8.5 Hz, 1H), 7.31 (m,1H). 13C NMR (125 MHz, CDCl3, δ): 144.2, 143.5, 141.7, 141.4, 140.7, 140.6, 139.8, 137.5, 137.4, 137.3, 135.5, 134.4, 132.8, 131.9, 130.5, 130.0, 129.6, 127.7, 127.1, 126.9, 126.4, 125.6, 125.4, 125.3, 124.8, 124.5, 124.0, 123.3, 123.2, 121.1, 120.9, 120.4, 120.3, 119.8, 118.9, 117.0, 116.5, 110.3, 110.0, 107.8. HRMS

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RESULTS AND DISCUSSION

Synthesis and Characterization. Synthetic routes of three compounds are showed in Scheme 1. Suzuki-coupling reaction, cyclization reaction, and Ullmann reaction are used to prepare intermediates and target products in good yields. Synthetic method of compound 1 and 2 was under general Suzuki coupling reaction. The synthetic method of compound 5-(3bromophenyl)-5H-pyrido[3,2-b]indole (7) was same as our previous paper.37 1H and 13C NMR spectroscopy, high

C

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Electron densities distribution and the energy levels of CzBPDCb, DCb-PCz, BTDCb-PCz, and BTCz-PCz from DFT calculation.

Table 1. Photophysical, Electrochemical, and Thermal Properties of DCb-PCz, BTDCb-PCz, and BTCz-PCz λabs(nm) a

compd

Sol.

DCb-PCz BTDCb-PCz BTCz-PCz

292,331,345 289,349,365 285,340,355

λPL (nm) Sol.

a

360,376 384 361,376

λPh(nm) c

Film

Sol.b

Eox/Eredd (V)

HOMO/LUMOe (eV)

(Egf/ETg (eV)

Tg/Tdh (°C)

385 410 385,412

445 432 428

0.95/−2.79 0.89/−2.66 0.91/na

−5.75/−2.39 −5.69/−2.49 −5.71/−2.46

3.36/2.96 3.20/2.91 3.25/2.94

105/355 153/450 140/450

Measured in CH2Cl2. bMeasured in 2-methyl-THF solutions at 77 K. cMeasured in vacuum-deposited films on quartz substrates. dEox=oxidation potential and Ered= reduction potential were determined by DPV. eThe HOMO and LUMO values were determined using the following equations: EHOMO(eV) = −(Eox+4.8), ELUMO(eV) = EHOMO + Eg. fThe value of Eg was calculated from the absorption edges of films. gThe value of ET was estimated from onset values of phosphorescence spectra (λPh). hObtained from DSC and TGA measurements; na = none appear. a

Thermal and Electrochemical Properties. The morphological properties and thermal stabilities of three compounds were estimated with the thermogravimetric analysis (TGA) and DSC. The Td values corresponding to 5% weight loss DCbPCz, BTDCb-PCz and BTCz-PCz were 355, 450, and 450 °C, respectively. The Tgs of DCb-PCz, BTDCb-PCz and BTCzPCz were 105, 153, and 140 °C, respectively, indicating good thermal and morphological stability under device fabrication. (Table 1 and Figure S1). Tg of BTDCb-PCz is about 13 °C higher than that of BTCz-PCz ascribing to the influence of the nitrogen atom of DCb component, which could give stronger CH···N hydrogen bond interaction.37 The electrochemical properties of three materials were examined by CV and DPV. Electrochemical reversibility was determined using CV (Figure S2), whereas all redox potentials were found using DPV and reported relative to a ferrocenium/ferocene (Fc+/Fc) redox couple as an internal standard. The oxidation progresses of DCb-PCz, BTDCb-PCz, and BTCz-PCz were taken at 0.95, 0.89, and 0.91 V attributed to PCz, BTDCb, and BTCz moieties, respectively, which was consistent with the results of DFT calculation. The reduction potentials of DCb-PCz and BTDCb-PCz were found to be −2.78 and −2.68 V, respectively, attributed to the δ-carboline unit. There was no obvious reduction progress in BTCz-PCz because of the lack of electron-deficient moiety. Photophysical Properties. As shown in Figure S3, UV− vis absorption of three materials at a short wavelength below 300 nm were attributed to π−π* transition of backbone,

resolution mass spectrometry were measured to characterize these compounds. Theoretical Calculations. On the basis of density functional theory (DFT) calculations (Figure 2), the HOMOs of DCb-PCz was localized on 3-phenyl-9-phenyl9H-carbazole (PCz) unit (HOMO) and on DCb unit (HOMO−1) with close energy level (ΔE (HOMO, HOMO−1) ∼ 0.1 eV), which is similar to that of CzDPDCb.37 The LUMO of CzBPDCb was localized on biphenyl group, while DCb-PCz was localized on δ-carboline moiety due to strong electron-donating of PCz unit. These interesting results gave us a different point to adjust the HOMO and LUMO energy level. Benzothiophene (BT) was usually utilized as the backbone structure of host materials with high triplet energy (∼3 eV),38−41 enhancement of thermal stability, and good mobility for charge transfer.41,42 Meanwhile, BT group can also extend molecular conjugation resulting in stabilizing the LUMO, which will be in favor of the charge injection and facilitating charge transporting.39 Two compounds integrated BT and δ-carboline or carbazole, BTDCb-PCz and BTCz-PCz, were then prepared as comparison. The HOMOs of BTDCbPCz and BTCz-PCz mainly distributed onto BTDCb and BTCz (HOMO) units as well as PCz (HOMO−1). The LUMO of BTDCb-PCz is about ∼0.13 eV lower than that of DCb-PCz. The LUMOs of BTDCb-PCz mainly dispersed on DCb (LUMO) and carbazole (LUMO−1) (ΔE (LUMO, LUMO+1) ∼ 0.1 eV), whereas the LUMO of BTCz-PCz was localized on PCz unit. D

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces whereas the band between 300 and 370 nm were attributed to n−π* transition. The introduction of BTDCb instead of DCb extended the molecular conjugation resulting in red-shift absorption from 345 to 365 nm. The optical bandgap energies (Eg) obtained with the absorption edges tested in film were 3.25, 3.20, and 3.36 eV for BTCz-PCz, BTDCb-PCz and DCbPCz, respectively. The HOMO energy levels of BTCz-PCz, BTDCb-PCz, and DCb-PCz were estimated at −5.71, −5.69, and −5.75 eV, accordingly, the LUMO were defined at −2.46, −2.49, and −2.39 eV, respectively. (HOMO = −(4.8 + EOX) and the LUMO = HOMO + Eg)37 The peaks of PL emissions of BTCz-PCz, BTDCb-PCz, and DCb-PCz were observed at 361/376, 384, and 360/376 nm, a bathochromic shift of emission for BTDCb-PCz compared to BTCz-PCz and DCbPCz was observed. Their T1 energy levels were estimated to be 2.96, 2.91, and 2.94 eV, respectively by phosphorescent PL measured in 2-methyl-THF at 77 K. The triplet energy of these materials was higher than blue phosphor (FIrpic, T1 = 2.63 eV), implying sufficiently application as the host materials for blue PhOLEDs. Device Performances and Electroluminescent Properties. Hole only (HODs: ITO/MoO3 (5 nm)/NPB (5 nm)/ host (100 nm)/Al (100 nm)) and electron only devices (EODs: ITO/BCP (5 nm)/host (100 nm)/Liq (1 nm)/Al (100 nm)) of three materials were fabricated to evaluate the charge transporting properties. BTDCb-PCz showed the best current density behavior both in HOD and EOD because the planar structure of BTDCb enhanced intermolecular packing.43 The current density of HOD and EOD for BTDCb-PCz showed good balance compared to BTCz-PCz and DCb-PCz. (Figure S4) Suitable HOMO/LUMO energy level, high T1, and good carriers transporting properties make these materials acting as host or HTL in OLEDs. Exciplex OLEDs of three hosts were investigated by employing host:TmPyPB (99:1 wt %), dual layer of host/ TmPyPB, and host:TmPyPB (50:50 wt %) as EML respectively. The structure of OLEDs were optimized with the configuration of ITO/HATCN (10 nm)/NPB (10 nm)/ TCTA (5 nm)/TCTA:Host (1:1) (5 nm)/EML (20 nm)/ TmPyPB (50 nm)/Liq (1 nm)/Al. The relative energy levels of the materials employed in the devices, electroluminescence spectra, and the J−V−L characteristics were presented in Figure 2 and in Figures S5−S8. Table 2 summarizes the EL performances of the exciplex OLEDs. Figure 3a shows the transient PL decay curves of the BTDCb-PCz, TmPyPB, and BTDCb-PCz:TmPyPB (99:1 wt %) codeposited films at the wavelength of 468 nm. The codeposited film shows a prompt decay with the lifetime of 3.6 ns and the long-delayed component with the lifetime of 52.5 ns, comparing with neat BTDCb-PCz of 14.4 ns and TmPyPB of 6.2 μs. The long-delayed component of codeposited film indicates that the reverse intersystem crossing exists in its exciplex states. As shown in Figure 3b, deep blue or pure blue exciplex EL emissions were observed at 436, 436 (corresponding to 2.84 eV), and 468 nm (corresponding to 2.65 eV) for BTCz-PCz, BTDCb-PCz, and DCb-PCz, respectively, when host:TmPyPB (99:1 wt %) were used as EML (devices 1, 2, and 3). BTDCb-PCz and BTCz-PCz based devices show lower turn on voltages at 3.4 V compared to DCb-PCz at 3.8 V because of their higher HOMOs and lower LUMOs than those of DCb-PCz. Especially, BTDCb-PCz-based exciplex OLED shows the pure blue emission (CIE (x, y): 0.16, 0.21) with the maximum CE of 4.64 cd A−1, PE of 2.91 lm W−1, EQE of

Table 2. Performances of Exciplex OLEDs device 1 2 3 4 5 6 7

host BTCzPCza BTDCbPCza DCbPCza BTCzPCzb BTDCbPCzb DCbPCzb BTDCbPCzc

V (V)

Lmax (cd m−2)

PE/CE/EQE (lm W1−/cd A−1/%)

2.7

261.4

1.38/1.49/0.96

2.9

3340

2.91/4.64/2.36

2.8

353.4

0.70/0.84/0.56

3.5

624

2.63/4.19/1.89

3.2

1230

1.40/2.11/1.05

4

1534.1

3.84/4.28/2.43

4.2

1080

2.22/1.46/1.0

CIE (x, y) (0.19, 0.19) (0.16, 0.21) (0.19, 0.18) (0.24, 0.32) (0.20, 0.31) (0.19, 0.22) (0.21, 0.32)

a c

Host:TmPyPB (99:1 wt %). bDual layer of host/TmPyPB. Host:TmPyPB (50:50 wt %) as EML.

2.36%, and the full width at half-maximum (fwhm) is about 72 nm. BTCz-PCz and DCb-PCz based OLEDs show much lower efficiencies (1.38 cd A−1, 1.49 lm W−1, 0.96% for BTCz-PCz) and wider fwhm (Table 2, Figure S5). Dual-layer exciplex OLEDs (devices 4, 5, and 6) were fabricated by using neat host/TmPyPB to form p−n heterojunction as EML. All of three OLEDs exhibit two band emissions with peaks around 435 and 500 nm (Figure 3c). Comparison with the literature reports and our test results carefully, the emission around 500 nm might be attributed to excimer emission of TmPyPB formed via triplet−triplet annihilation. Comparing with exciplex OLEDs doped 1% TmPyPB, dual layer OLEDs based on BTCz-PCz and DCbPCz showed better performance, whereas BTDCb-PCz-based OLED showed lower efficiency (Figure S7). Because the electron mobility is much lower than the hole mobility in common OLEDs, the hole and electron recombination zone was very close to ETL. The triplet energy of TmPyPB was at 2.80 eV, which was lower than those T1 states of hosts and exciplex energy level formed between BTCz-PCz or DCb-PCz and TmPyPB (2.84 eV). When we go back to check the EL spectra of OLEDs based on BTCz-PCz and DCb-PCz doped with 1% TmPyPB (Device 1 and 3), there are tiny shoulders at 500 nm, indicating an energy transfer between exciplex and TmPyPB. Overall, both exciplex and original triplet excitons were harvested by T1 of TmPyPB easily, then the excimer emission of TmPyPB is formed resulting in higher efficiencies. On the contrary, exciplex energy level of BTDCb-PCz/ TmPyPB couple was at 2.65 eV, which is lower than T1 of TmPyPB (2.8 eV) resulting in no contribution to excimer emission of TmPyPB. Then dual layer exciplex OLED based on BTDCb-PCz/TmPyPB showed the lowest efficiency because of useless exciplex energy for excimer emission of ETL (Figure 3d). Exciplex OLEDs using codeposited BTDCbPCz:TmPyPB (50:50 wt %) as EML (device 7) were fabricated too. The performances of devices are similar to those of dual layer devices, which is the side evidence of Dexter energy transfer between exciplex and T1 of TmPyPB (Figure S8). Considering the high triplet energy of exciplex, we are interested in the application of these materials as hosts for blue PhOLEDs. FIrpic was selected as phosphorescent dopant due to its common and suitable triplet energy (2.63 eV), which could easily harvest energy from the triplet energy of exciton or exciplex through Dexter energy transfer. Meanwhile, the S1 E

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Transient PL decay curves, (b) EL spectra of device 1, 2, and 3 (host:TmPyPB (99:1 wt %)). (c) EL spectra of device 4, 5, 6, (dual layer of host/TmPyPB) and PL spectrum of neat TmPyPB film. (d) Energy diagrams showing the formation of exciplex and related energy transfer (ET).

molecule, which made its derivatives easier to form C−H···N hydrogen bonds with adjacent molecules.37,46 The intermolecular force induced DCb-PCz well horizontally oriented in the films and the molecular stacking generates channels for hole and electron transfer. The corporation of BT made the BTDCb-PCz more rigid and relatively planar structure, then stronger π−π interactions made it possible to implement charge transport consisting with HOD and EOD results. Moreover, BTDCb-PCz/TmPyPB interface forming the lowest exciplex energy level lead fully exciton energy utilization without energy back transfer to host or ETL. Device 8 showed the maximum brightness of 23400 cd m−2 (30% higher than that of device 10 at 17900 cd m−2 and 150% higher than that of device 9 at 9445 cd m−2) as well as the lowest roll-off of efficiencies. The device still remains high CE of 46.5 cd A−1 at 100 cd m−2, 45.4 cd A−1 at 1000 cd m−2, and 42.3 cd A−1 at 5000 cd m−2 with EQE close to 20% (Figure 4d, Figure S9).

energy of hosts and exciplex would transfer to FIrpic through Förster resonant energy transfer (Figure 3d) as our previous report.44 Multilayer OLEDs were fabricated employing host:FIrpic (8 wt %) as EML with a structure of ITO/HATCN (10 nm)/NPB (10 nm)/TCTA (5 nm)/ TCTA:Host (1:1) (5 nm)/ Host:FIrpic (20 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (Figure 4a). The EL spectra of devices, J−V−L characteristics of these PhOLEDs were exhibited in Figure 4 and Figure S9. Devices performances are summarized in Table 3. As shown in Figure 4b, these devices exhibited the typical emission spectra of FIrpic at 475 and 500 nm without other peak at 380−460 nm range indicating fully energy transfer. However, the shoulder peak of 500 nm was strengthened in device 8 (BTDCb-PCz as host) due to the microcavity effect,45 increasing the CIE y-coordinate to 0.457 compared to that of 0.347 for device 9 or 10 (BTCz-PCz or DCb-PCz as host). Von (defined at 1 cd m−2) at 2.7, 2.8, and 2.9 V for devices 8, 9, and 10 respectively, are achieved, illustrating good carrier injection into the EML, which could be attributed to the smaller HOMO barrier between TCTA and three hosts (∼0.15 eV)46 and full utilization of exciplex energy. All these devices showed high performances; regarding the best one, device 8 exhibits a maximum CE of 55.6 cd A−1 and PE of 52.9 lm W−1 compared to the device 9, having a maximum CE of 44.8 cd A−1 and PE of 43.7 lm W−1, as well as the device 10 holding a maximum CE of 46.4 cd A−1 and PE of 44.6 lm W−1, which are better than our previous PhOLEDs based on CzBPDCb (device 11).37 The nitrogen atom of δ-carboline locates at the outer side of the

■

CONCLUSION In summary, we have successfully developed new materials based on δ-carboline derivatives with good thermal stabilities (Tg over 153 °C). Deep blue (peak at 436 nm) and pure blue (peak at 468 nm) TADF of exciplex OLEDs manipulated by chemical structure were demonstrated by utilizing them as emitters with the maximum CE of 4.64 cd A−1, PE of 2.91 lm W−1, and EQE of 2.36%. We also demonstrated and proved the mechanism of energy transfer between exciplex, ETL, and dopants, gave an ideal strategy to avoid energy back transfer from exciplex to host or ETL, then fully utilized all excitons F

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Device structure, (b) EL spectra, (c) current density−voltage−luminance characteristics, and (d) current efficiency−luminance−power efficiency curves of FIrpic-based PhOLEDs.

Table 3. Performances of FIrpic-Based PhOLEDs with BTDCb-PCz, BTCz-PCz, and DCb-PCz as Host PE/CE/EQE (lm W−1/cd A−1/%) device

host

Von (V)

8 9 10 11

BTDCb-PCz BTCz-PCz DCb-PCz CzBPDCb37

2.7 2.9 2.8 3.2

at 1 cd m

−2

52.9/55.6/23.6 43.7/44.8/23.0 44.6/46.4/24.6 38.9/44.2/21.9

■

energies for lighting of OLEDs. Highly efficient blue PhOLEDs doped with FIrpic showed a maximum CE of 55.6 cd A−1, PE of 52.9 lm W−1, and EQE of 24.6% respectively with very low turn on voltage at 2.7 V. The devices still remain high CE of 46.5 cd A−1 at 100 cd m−2, 45.4 cd A−1 at 1000 cd m−2, and 42.3 cd A−1 at 5000 cd m−2, with EQE close to 20% indicating the low efficiency roll-off. Our results show the importance of exciplex energy utilization in producing efficient blue OLEDs as well as the strategy to develop deep blue exciplex emitters.

■

at 100 cd m−2

at 1000 cd m−2

41.8/46.5/18.9 33.4/39.5/19.7 38.0/45.4/23.9 35.4/43.9/21.3

33.9/45.4/18.6 23.6/33.0/16.6 31.6/42.9/22.8 29.1/41.7/19.7

CIE(x,y) (0.19, (0.15, (0.15, (0.15,

0.46) 0.35) 0.34) 0.35)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-21-56333362. Phone: +86-21-56333362. ORCID

Zixing Wang: 0000-0002-0814-8822 Author Contributions §

H.W. and J.Z. contributed equally to this work.

Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.

S Supporting Information *

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04987.

ACKNOWLEDGMENTS This work was financially supported by the National Key Basic Research and Development Program of China (973 program, 2015CB655005), the National Natural Scientific Foundation of China (21302122)

DSC and TGA curves, CV curves, UV−vis absorption and PL spectra, devices structure and performances, 1H NMR and 13C NMR spectra (PDF) G

DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b04987 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX