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A Method for Reducing the Singlet-Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency Pachaiyappan Rajamalli, Natarajan Senthilkumar, Parthasarathy Gandeepan, Chen-Cheng Ren-Wu, Hao-Wu Lin, and Chien-Hong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10678 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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ACS Applied Materials & Interfaces

A Method for Reducing the Singlet-Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency Pachaiyappan Rajamalli,† Natarajan Senthilkumar,† Parthasarathy Gandeepan,† Chen-Cheng Ren-Wu,‡ Hao-Wu Lin‡ and Chien-Hong Cheng*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.

‡

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

KEYWORDS: (blue OLEDs, TADF, reverse intersystem crossing, benzoylpyridine, upconversion) ABSTRACT: We have successfully synthesized a series of blue thermally activated delayed fluorescence (TADF) emitters BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C bearing a 4benzoylpyridine core as the electron-accepting unit and carbazolyl, t-butyl carbazolyl, dicarbazolyl and tercarbazolyl groups as the electron-donating unit, respectively. The DFT calculation shows that all the compounds have their LUMOs on the benzoylpyridine moiety. However, the HOMO of BPy-p3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-pC and BPy-pTC are mainly on the carbazolyl and extended to the phenyl 1

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ring. As a result, the ∆EST is reduced from 0.29 eV for BPy-pC to 0.05 eV for BPy-p3C and the OLEDs using these materials as dopants emit blue light and maximum EQEs increase from 4.2% to 23.9%, for BPy-pC and BPy-p3C, respectively. The EQE of the BPy-p3C-based device increases 2 times more than that of BPy-pTC-based device without significant change in the color coordinates. INTRODUCTION In the last few decades, the efficiency of organic light-emitting diodes (OLEDs) had grown tremendously. The external quantum efficiency (EQE) improved from less than 1% to over 30% by the improvement of device structures and by using phosphorescent emitters.1-10 The internal quantum efficiency (IQE) of the phosphorescent OLEDs could reach 100% by harvesting both singlet and triplet excitons using noble metal complexes.11-19 Although 100% IQE can be achieved by using phosphorescent iridium or platinum complexes, these transition metal complexes are expensive and still not reliable for blue OLEDs. The blue emitting OLED materials are highly important for both display and lighting application. At present, blue fluorescent TTA materials are used in practical application, although the external quantum efficiency of a TTA material-based device is limited to only 12.5%.20-25 Very recently, thermally activated delayed fluorescence (TADF) materials and devices have attracted great attention as the new generation of OLEDs because the devices can also achieve 100% IQE by harvesting both singlet and triplet excitons using pure organic materials.26-32 The molecular design of TADF emitters is a critical factor for the development of high efficiency TADF devices. It is known that a TADF emitter needs a small singlet-triplet energy gap (∆EST) by separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the molecule to achieve high device efficiency. However, a weakly 2


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overlapping between the HOMO and LUMO is also important to enhance the radiative decay from S1 to S0.33-35 The former condition is desired for up-conversion of triplet excitons into singlet excitons and the latter is required for high photoluminescence (PL) quantum yield. Under these criteria, various red, green, and blue TADF emitters had been developed.36-38 However, the optimum molecular design for efficient blue TADF emitters is not well understood and guidelines for the rational molecular design are highly desired for the practical application of TADF-based OLEDs. Additionally, wide dispersion of the HOMO is proposed as an effective way to reduce the ∆EST and improve the efficiency.39 Although the EQE increased by dispersing the HOMO, the color coordinates red-shift significantly from blue to green, which makes difficult the development of efficient blue TADF OLEDs.40-44 Herein, we report a new molecular design for TADF molecules by expanding the donor units in the molecule, while keeping the acceptor unit unaltered. By this design, the HOMO of the molecule disperses all over the donor units and less overlap between the HOMO and LUMO is resulted. Moreover, the EQE of the blue device increased from 9.4% to 23.9% without significant change in the color coordinates. This concept could be important in the future for the molecular design of efficient TADF materials. EXPERIMENTAL SECTION Synthesis of (4-(9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (BPy-pC): To an oven dried seal tube (4-bromophenyl)(pyridin-4-yl)methanone (BPy-pBr) (2.0 g, 7.63 mmol), carbazole (1.66 g, 9.90 mmol), CuI (0.15 g, 0.79 mmol), 1,10-phenanthroline (0.15 g, 0.79 mmol), K2CO3 (2.11 g, 15.27 mmol) and p-xylene (20 mL) were added. The system was evacuated and purged with nitrogen and the reaction mixture was heated at 150 °C for 24 h. 3



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After completion, the reaction mixture was filtered through Celite and washed with EtOAc (50 mL). Solvent evaporation under reduced pressure, followed by column chromatography purification using 25% EtOAc/hexane as eluent afforded BPy-pTC in 81% yield. 1H NMR (400 MHz, CDCl3): δ 8.86 (d, J = 4.8 Hz, 2 H), 8.13 (d, J = 7.6 Hz, 2 H), 8.08-8.05 (m, 2 H), 7.767.73 (m, 2 H), 7.67-7.65 (m, 2 H), 7.52 (d, J = 8.4 Hz, 2 H), 7.45-7.41 (m, 2 H), 7.32 (t, J = 7.2 Hz, 2 H).

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C NMR (100 MHz, CDCl3): δ 193.89 (-CO-), 150.42, 144.21, 142.69, 139.97,

133.97, 131.93, 126.35, 126.23, 123.91, 122.76, 120.78, 120.48, 109.67; HRMS (FAB+) cal for C24H16N2O 349.1296, found 349.1340. Synthesis of (4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (BPypTC): A procedure similar to the synthesis of BPy-pC was used to synthesize BPy-pTC in 85% yield using BPy-pBr (2.0 g, 7.63 mmol), 3,6-di-tert-butyl-9H-carbazole (2.76 g, 9.90 mmol), CuI (0.15 g, 0.79 mmol), 1,10-phenanthroline (0.15 g, 0.79 mmol), K2CO3 (2.11 g, 15.27 mmol) and p-xylene (20 mL). 1H NMR (400 MHz, CDCl3): δ 8.86-8.85 (m, 2 H), 8.14 (s, 2 H), 8.06 (d, J = 8.4 Hz, 2 H), 7.75 (d, J = 8.4 Hz, 2 H), 7.67-7.65 (m, 2 H), 7.50-7.45 (m, 4 H), 1.46 (s, 18 H); 13

C NMR (100 MHz, CDCl3): δ 193.95 (-CO-), 150.44, 144.38, 143.89, 143.28, 138.29, 133.41,

131.95, 125.77, 124.02, 123.91, 122.78, 116.45, 109.24, 34.75, 31.91; HRMS (FAB+) cal for C32H32N2O 460.2515, found 460.2524. Synthesis of (4-(9H-[3,9’-bicarbazol]-9-yl)phenyl)(pyridin-4-yl)methanone (BPy-p2C) A procedure similar to the synthesis of BPy-pC was used to synthesize BPy-p2C in 60% yield using BPy-pBr (1.0 g, 3.82 mmol), 9H-3,9'-bicarbazole (1.65 g, 4.95 mmol), CuI (0.08 g, 0.40 mmol), 1,10-phenanthroline (0.08 g, 0.40 mmol), K2CO3 (1.06 g, 7.68 mmol) and p-xylene (20 mL). 1H NMR (400 MHz, CDCl3): δ 8.92 (bs, 2 H), 8.28 (d, J = 2.0 Hz, 1 H), 8.18 (d, J = 8.0 4


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Hz, 2 H), 8.12 (t, J = 8.0 Hz, 3 H), 7.85 (d, J = 8.4 Hz, 2 H), 7.70-7.69 (m, 3 H), 7.59-7.56 ( m, 2 H), 7.52-7.48 (m, 1 H), 7.43-7.34 (m, 5 H), 7.31-7.27 (m, 2 H); 13C NMR (100 MHz, CDCl3): δ 193.92 (-CO-), 150.47, 144.14 142.38, 141.70, 140.76 139.14, 134.46, 132.12, 130.72, 127.02, 126.55, 125.88, 125.78, 125.05, 123.48, 123.15, 123.14, 121.24, 120.81, 120.32, 119.72, 119.64, 110.85, 110.06, 109.68; HRMS (FAB+) cal for C36H23N3O 513.1841, found 513.1844. Synthesis of (4-(9'H-[9,3':6',9''-tercarbazol]-9'-yl)phenyl)(pyridin-4-yl)methanone (BPyp3C): A procedure similar to the synthesis of BPy-pC was used to synthesize BPy-p3C in 87% yield using BPy-pBr (2.0 g, 7.63 mmol), 9'H-9,3':6',9''-tercarbazole (4.93 g, 9.90 mmol) (4.93 g, 9.90 mmol), CuI (0.15 g, 0.79 mmol), 1,10-phenanthroline (0.15 g, 0.79 mmol), K2CO3 (2.11 g, 15.27 mmol) and p-xylene (20 mL). 1H NMR (400 MHz, CDCl3): δ 8.90 (d, J = 6.0 Hz, 2 H), 8.30 (d, J = 1.2 Hz, 2 H), 8.17 (t, J = 8.8 Hz, 6 H), 7.93 (d, J = 8.4 Hz, 2 H), 7.77 (d, J = 8.4 Hz, 2 H), 7.70 (d, J = 5.6 Hz, 2 H), 7.67-7.64 (m, 2 H), 7.40-7.39 (m, 8 H), 7.30-7.26 (m, 4 H); 13C NMR (100 MHz, CDCl3): δ 193.86 (-CO-), 150.56, 143.97, 141.96, 141.56, 139.81, 134.86, 132.23, 131.14, 126.66, 126.51, 125.92, 124.57, 123.18, 120.33, 119.88, 119.80, 111.24, 109.55; HRMS (FAB+) cal for C48H30N4O 678.2420, found 678.2416. RESULTS AND DISCUSSION Synthesis and DFT Calculation Designed compounds were readily synthesized with high yields via the Ullmann coupling reaction of 4-bromobenzoylpyridine with carbazole derivatives (Scheme 1). The synthetic route is displayed in Scheme 1, while the detailed synthesis procedures and characterization data are given in the Experimental Section and Supporting Information. All the compounds were further purified by temperature gradient vacuum sublimation, and characterized by 1H, and 5


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and high resolution mass spectrometry. The crystal structure of BPy-p3C confirms the proposed molecular structure and the molecular packing in the crystalline state are shown in Figure S1. Scheme 1. Synthesis of BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C

To understand the molecular orbital distribution of these emitters, the HOMOs and LUMOs were calculated using the B3LYP 6-31G basis set implemented in the Gaussian 09 program and were shown in Figure 1. Based on the DFT calculation, the HOMO of BPy-pC and BPy-pTC is mainly distributed over the carbazolyl group and extended to the BPy phenyl ring. The LUMO is mainly on the BPy core and slightly dispersed to the nitrogen of the carbazolyl group as depicted in Figure 1. However, the HOMO and LUMO are well separated for BPy-p2C and BPy-p3C. Its HOMOs are distributed over the dicarbazole or tercarbazole group and slightly over the phenyl ring of BPy group, while the LUMO is over the BPy core and is slightly dispersed to the nitrogen 6


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of the carbazolyl group attached to the BPy. In the BPy-p2C and BPy-p3C contains two and three electron-donating carbazole moieties, respectively. The outer carbazole moieties enhance the separation of the HOMO and LUMO by withdrawing the HOMO in the opposite direction to the electron acceptor (Figure 1) in BPy-p2C and BPy-p3C. The resulting greater separation between the HOMO and LUMO leads to a smaller ∆EST and is expected to have an efficient upconversion of triplet to singlet excitons compared with BPy-pTC. On the other hand, the small overlap between the HOMO and LUMO appears sufficient to keep a very efficient S1 to S0 radiative transition (see below). The TDDFT calculation shows that the widely distributed compounds possess small singlet and triplet energy gap (∆EST) of 0.1 eV and 0.09 for BPy-p2C and BPy-p3C, respectively compared to the less HOMO distributed BPy-pC (0.34 eV) and BPypTC (0.32 eV). The main transitions of these materials along with oscillator strengths and contour plots of the occupied and unoccupied molecular orbitals are listed in Table S1.

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Figure 1. The structures and molecular orbitals of BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C. Photophysical Properties The absorption and emission spectra of these compounds in various solvents are shown in Figure 2 and Figures S2 and S3. These materials exhibit a strong π-π* absorption around 290 nm, and a broad band around 350~373 nm assigned to the intramolecular charge transfer (ICT) absorption from the carbazole groups to the benzoylpyridine moiety. All the compounds show similar absorption spectra in various solvents. However, the fluorescent emission of BPy-pC and BPypTC displays significant solvatochromic effect from violet (406 nm) in n-hexane to green (521 nm) in dichloromethane (DCM) for BPy-pC and deep blue (418 nm) in n-hexane to yellow (562 nm) in dichloromethane (DCM) for BPy-pTC (Figure 2, S3). The emission of BPy-p2C and 8


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BPy-p3C also shows solvatochromic effect. However, BPy-p2C exhibit dual emissions in all solvents, one is from ICT state (longer wavelength) and the other emission is from the LE emission of the donor unit (shorter wavelength) and the donor emission (LE) does not change with solvents. The BPy-p3C exhibits dual emissions only in DCM at 405 and 581 nm, likely from LE and CT emission, respectively. The LE emission at 405 nm is attributed to the tercarbazole-centered π-π* transition as supported by the observation that tercarbazole molecule also exhibits a similar emission at the same wavelength. The emission at 581 nm is assigned as an intramolecular charge transfer from the tercarbazole to benzoyl pyridine group (Table S1).

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Figure 2. Absorption (Abs.) and fluorescence (Fl.) spectra at room temperature and phosphorescence (Phos.) spectra at 77 K of the BPy-pTC (a) and BPy-p3C (b) in toluene (10-5 M) (excitation wavelength : 360 nm); emission spectra of BPy-pTC (c) and BPy-p3C (d) in various solvents.

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Table 1. Physical properties of BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C

Compound

λabs (nm)a

λem (nm)a

λem (nm)b

Tg (°C)c

Td (°C)d

HOMO (eV)e

LUMO (eV)f

Eg (eV)g

ET (eV)h

∆EST (eV)i

ΦPL (%) Solj/filmk

BPy-pC

290, 338, 359

440

453

N.D.

370

-5.68

-2.54

3.14

2.85

0.29

1.2/38

BPy-pTC

295, 327, 373

467

462

86

382

-5.60

-2.63

2.97

2.84

0.13

18.6/70

BPy-p2C

292, 342, 362

480

454

108

399

-5.58

-2.63

2.95

2.88

0.07

17.8/72

BPy-p3C

291, 341, 363

482

455

106

530

-5.53

-2.58

2.93

2.88

0.05

24.3/96

a

Measured in toluene (1×10-5 M) at room temperature. bPhosphorescence measured in toluene (1×10-5 M) at 77 K. cObtained from DSC measurements. dObtained from TGA measurement. eMeasured from the oxidation potential in 10-3 M DCM solution by cyclic voltammetry. fCalculated from HOMO - Eg. gEstimated from the onset of fluorescence spectrum. hEstimated from the onset of phosphorescence spectrum. i∆EST = ES - ET. jMeasured in degassed toluene solution using diphenyl anthracene as a standard. kAbsolute PL quantum yield evaluated in co-doped mCBP: dopant (7 wt%) thin films using an integrating sphere under N2 atmosphere.

The absorption and emission spectra of BPy-p3C could not be measured in n-hexane due to its poor solubility. BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C in toluene at room temperature show fluorescence peaks centred at 440, 467, 480 and 482 nm, respectively. Their singlet, triplet energies were calculated from the onset of fluorescence and phosphorescent emission spectra, respectively and are summarized in Table 1. Phosphorescent spectra were collected at 77 K with 6.25 ms delay time. Based on these data, the ∆EST was estimated to be 0.29 eV for BPy-pC, 0.13 eV for BPy-pTC, 0.07 eV for BPy-p2C and 0.05 eV for BPy-p3C (Figure 2 and S3). The small values of ∆EST of BPy-pTC, BPy-p2C and BPy-p3C indicate that they are TADF materials with efficient thermal up-conversion from T1 to S1 than the BPy-pC. The electrochemical properties of these compounds were investigated by cyclic voltammetry (Figure S4). From the oxidation waves of these compounds (Figure S4), the HOMO levels were calculated to be ˗5.68, ˗5.60, ˗5.58 and ˗5.53 eV for BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C, respectively, from the onset of oxidation waves. The LUMO energy levels were estimated from 10


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HOMO ˗ Eg to be ˗ 2.54, ˗2.63, 2.63 and ˗2.58 eV, respectively, for BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C. The photoluminescence quantum yields (PLQY) in oxygen-free toluene are 1.2%, 18.6%, 17.8% and 24.3%, for BPy-pTC and BPy-p3C, respectively. In the presence of oxygen the PLQY values decrease to 1.1%, 12.5%, 8.4% and 9.8%. The substantial decrease of PL quantum efficiencies in the presence of oxygen also supports that these three (BPy-pTC, BPyp2C and BPy-p3C) molecules possess TADF property and suggests that the T1 states of these molecules were readily quenched by the triplet ground state oxygen molecules.45 The quench of T1 by oxygen leads to the quench of delayed fluorescence and the decrease of the fluorescence intensity. Conversely, PLQY is almost remains the same for BPy-pC in the presence of oxygen suggesting that there is no triplet excitons involved in the emission due to large ∆EST. In addition, 7 wt% of dopant-doped mCBP films were fabricated by vacuum deposition, where mCBP = 3,3′-bis(N-carbazolyl)-1,1’-biphenyl and have been used previously as the host materials for blue TADF emitters. The quantum yields (PLQY) of these films (mCBP:BPy-pC (7 wt%), mCBP:BPy-pTC (7 wt%), mCBP:BPy-p2C (7 wt%) and mCBP:BPy-p3C (7 wt%)) were 38%, 70%, 72% and 96%, respectively, measured by using an integrating sphere under N2 atmosphere. The quantum yield enhancement in the thin film state relative to that in the toluene solution is likely due to the suppression of the collisional quenching in the thin film state.46,47 The PLQY of BPy-pTC having a para t-butyl on the carbazole group is nearly doubled compared to that BPy-pC. This is likely because BPy-pTC has small ∆EST and is a TADF material showing up-conversion from T1 to S1. On the other hand, BPy-pC has large ∆EST and does not show up-conversion from T1 to S1 (see Table 1). The electron-donating ability of t-butyl group is expected to reduce the Eg value, but without much changing the ET value of BPy-pTC

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leading to reduced ∆EST when compared to BPy-pC. As revealed below, these observed PLQYs are parallel to the corresponding quantum efficiency of their electroluminescence devices. To further confirm the TADF property of these compounds, their transient PL decay characteristics in 10-5 M toluene solution under vacuum were measured. As shown in Figure S5, each transient decay curve appears to contain two components. The fast one is the prompt emission decay curve from S1 to the ground state (S0) and the second one is the delayed emission component. The delayed emissions may be rationalized as the thermal up-conversion from T1 to S1, followed by fluorescence to the ground state. However, BPy-pC shows only the fast component (prompt emission), suggesting that BPy-pC is not a TADF emitter consistent with the relative low quantum yield and large ∆EST (Table 1). In addition, the thermal stability of these materials were measured by thermo gravimetric analysis. These materials possess high thermal stability with decomposition temperatures of 370, 382, 399 and 530 °C for BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C, respectively, (Figure S6). Notably, the tercarbazole containing BPy-p3C shows high quantum yield in the thin films, but also gives high thermal stability. The fluorescence and phosphorescence spectra of the co-doped (mCBP) films were also measured and presented in Figure 3 and S7. The ∆EST measured from the onset of the PL spectra at room temperature and phosphorescence spectra at 77 K are 0.30 and 0.06 eV, respectively for BPy-pC and BPy-p3C. The observed ∆EST value for BPy-p3C is low enough to have efficient RISC and comparable with reported values.26-32 The prompt and delayed PL spectra of the codoped film are shown in Figure 3b. The prompt and delayed emission spectra coincide with each other well suggesting that the delayed PL component can be ascribed to the TADF emission.36

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Wavelength, nm

Figure 3. a) Fluorescence (Fl.) and phosphorescence (Phos.) spectra of the mCBP: BPy-p3C codoped film at 300 K and 77 K, respectively; b) prompt and delayed PL spectra of the co-doped film measured using a 355 nm pulsed laser at 300 K. The delayed PL spectra was collected at 10 µs delay time. The transient decay curve of the co-doped BPy-p3C film were also measured at room temperature under vacuum. As shown in Figure 4, the transient decay curve, similar to that in solution, can be divided into two components. The first one is the prompt emission decay curve from S1 to ground state (S0) with a lifetime (τ) calculated from intensity vs time to be 10.6 ns and the second one is a delayed emission component with τ = 0.65 µs. The delayed emission may be rationalized as the thermal up-conversion of T1 to S1, followed by fluorescence to the ground state. The short excited state lifetime of these molecules is comparable, or even lower than the highly emissive phosphorescent iridium complexes.48 The short delayed emission lifetimes with high PL quantum yields in the thin-film state of BPy-p3C is important features of TADF emitters to realize high device efficiencies. To further confirm the TADF property, the temperature dependent transient PL measurements of the doped film, mCBP: BPy-p3C (7 wt%), at a temperature between 200 to 300 K under vacuum were carried out. As shown in Figure 4, the intense emission and long tail are from prompt and delayed fluorescence, respectively. The delayed emission component gradually increased with the increase of temperature from 200 K to 13



300 K. The results reveal that the T1 to S1 up-conversion increased with increasing temperature, further supports that TADF occurred in the BPy-p3C (7 wt%) doped mCBP thin film. Unlike reported materials, BPy-p3C showed high up-conversion even at 300 K.50 1 200 K 250 K 300 K

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

0.1

0.01

1E-3 0

2

4

6

8

10

12

Time, µs

Figure 4. Temperature-dependent transient PL characteristics of a 7 wt% BPy-p3C:mCBP codoped film measured using a 355 nm pulsed laser. Electroluminescence Performance To evaluate the potential of these emitters for OLEDs, we fabricated electroluminescence devices using the developed dopants. The molecular structures used in the devices and schematic representation of the device structure are shown in Figure 5. Devices A-D2 were fabricated using the following layers: ITO/NPB (30 nm)/TAPC (20 nm)/mCBP: Dopant (7 wt% (A-D) or 20 wt% (D1) or 30 wt% (D2)) (30 nm)/PPT (10 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm), respectively. In these devices, N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) acts as the hole injection material, 1,1-bis[4-[N,N′-di(ptolyl)amino]phenyl] cyclohexane (TAPC) is the hole transporting material, and mCBP is the host material, while 1,3,5-tri(m-pyrid3-yl-phenyl)benzene

(TmPyPb)51

is

the

electron-transporting

material

and

dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) (PPT) is an exciton blocker. The electroluminescent properties of these devices are displayed in Figures 6, S8 and S9 and are summarized in Table 2. 14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Me

Me

N N

N

N

Me

Me

NPB

TAPC N

S O

O P

P

N

N

N

N PPT

mCBP

TmPyPb

Figure 5. Chemical structures of the materials used in the devices (left), and schematic representation of the devices A-D2 (right). Devices A-D show maximum EQEs of 4.2, 9.4, 11.0, and 23.9%, respectively. The current efficiencies and power efficiencies for devices A-D are 5.0, 16.3, 20.8, 56.5 cd/A and 3.9, 14.6, 16.2, 50.6 lm/W, respectively. A dramatic improvement in the electroluminescent performance was realized by increasing the number of carbazole units. The EQE of 23.9% of blue device D is one of the best reported in the literature using lower concentration of the dopants.39,52 More than 2 times improvement of the EQE was made by tuning the molecular structure with three carbazole units instead of two carbazole and one t-butyl carbazole. The high EQE of the BPyp3C devices can be correlated to the higher PL quantum yield of BPy-p3C as well as the efficient up-conversion from triplet to singlet because of the small ∆EST.

15



a)

b)

A B C D

1.0 Device A Device B Device C Device D

0.8

E.Q.E.

Intensity, a.u.

10

0.6 0.4 0.2

1

0.0 10

100

1000

400

10000

Luminance, cd/m2

500

600

700

Wavelength, nm

d)

c) Device A Device B Device C Device D

Device A Device B Device C Device D

10000

100 200 10

10 cd/A

1000

400

Luminance, cd/m2

600 Current density, mA/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

1

0 2

4

6

8

10

12

14

10

16

100

1000

Luminance, cd/m

V

10000

2

Figure 6. Electroluminescent performance of devices A-D: a) EQE vs luminance, b) EL spectra measured at 8 V, c) current density and luminance vs driving voltage and d) current efficiency vs luminance. The EL spectra of devices A-D in Figure 6b exhibited no emission from other layers. The observation indicates that the excitons are confined in the emission layer without leakage to the adjacent layers. Devices B-D gave sky blue electroluminescence with color coordinates of (0.17, 0.27), (0.18; 0.28) and (0.19; 0.32), respectively. The efficiency of device D is comparable to those of blue phosphorescent OLEDs and much higher than the conventional fluorescent devices.53,54 As revealed in Table 2, device D showed a maximum EQE of 23.9, current efficiency of 56.5 cd/A and power efficiency of 50.6 lm/W with luminance up to 16700 cd/m2

16


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without light out-coupling enhancement. Moreover, the electroluminescence spectra remains stable with increasing driving voltages, as shown in Figure S9. Table 2. The EL performances of the BPy-pC, BPy-pTC, BPy-p2C and BPy-p3C

Devicea

Dopant

Vd (V)b

L (cd/m2, V)

EQE (%, V)

CE (cd/A, V)

PE (lm/W, V)

λmax (nm)

CIE (x,y), 8V

A

BPy-pC

3.8

2183 (13.0)

4.2 (4.0)

5.0 (4.0)

3.9 (4.0)

452

0.16;0.13

B

BPy-pTC

3.4

8610 (14.5)

9.4 (3.5)

16.3 (3.5)

14.6 (3.5)

476

0.17;0.27

C

BPy-p2C

3.6

10800 (12.5)

11.0 (4.0)

20.8 (4.0)

16.2 (4.0)

477

0.18;0.28

D

BPy-p3C

2.9

16700 (14.0)

23.9 (3.5)

56.5 (3.5)

50.6 (3.5)

485

0.19;0.32

D1

BPy-p3C

3.1

25850 (12.0)

14.5 (3.5)

35.2 (3.0)

31.5 (3.0)

492

0.20;0.36

D2

BPy-p3C

3.1

28660 (13.0)

9.9 (5.0)

25.3 (5.0)

18.1 (4.0)

492

0.20;0.38

a Device configuration for A-D2: ITO/NPB (30 nm)/TAPC (20 nm)/mCBP: BPy-p3C (7 (A-D) or 20 (D1) or 30 wt% (D2)) (30 nm)/PPT (10 nm)/ TmPyPb (60 nm)/LiF (1 nm)/Al (100 nm); bVd, The operating voltage at a brightness of 1 cd/m2; L, maximum luminance; EQE, maximum external quantum efficiency; CE, maximum current efficiency; PE, maximum power efficiency; and λmax, the wavelength where the EL spectrum has the highest intensity

To see the effect of emitter concentration on the device performance, devices D1 and D2 were fabricated using 20 and 30 wt% of BPy-p3C dopant. The EL spectra of these devices show slight red shift as the doping concentration of BPy-p3C increases (Figure S10). The color coordinates of devices D-D2 shifted from (0.19, 0.32) at 7 wt% to bluegreen (0.20, 0.38) at 30 wt% doping. The relative small change of color coordinates with doping concentration is likely a result of the weak intermolecular interaction of BPy-p3C due to the bulkiness of the tercarbazole group. The EQE of the device decreased gradually from D to D2, but the luminance increased, as the emitter concentration increased (Table 2) and reached up to 28660 cd/m2 in device D2. Notably, the device roll-off at higher brightness is reduced with increasing emitter concentration. The device performance at 100 cd/m2 and 1000 cd/m2 were summarized in Table S2. Further, to confirm the TADF property of the devices under electrical excitation, the transient 17



electroluminescence decay was measured for device D at room temperature (Figure 7). The electroluminescence delayed component lasts for several tens of microseconds and this result strongly supports the existence of TADF property in the device.

1 Device D

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

0

20

40 Time, µs

60

Figure 7. Transient electroluminescence characteristics of device D measured at 4V. CONCLUSION We have designed four benzoyl pyridine derived compounds bearing the 4-benzoylpyridine core as the electron-accepting unit and a different type of carbazole-based electron-donating unit. A comparison of these materials shows that the wide dispersion of HOMO in BPy-p3C increases the PL quantum yield, reduces the ∆EST and thus enhances the up-conversion from T1 to S1, but does not much change the color coordinates of the device. The photoluminescence quantum efficiency reaches 96% in the mCBP film and the EQE of a BPy-p3C-based device reaches 23.9% with light blue emission. The transient PL characteristics and electroluminescence characteristics of BPy-p3C confirm the TADF properties. Therefore, a take-home message from this study for the molecular design of efficient TADF material is to include donor units with widely dispersed HOMO. This method opens up a new possibility to enhance the efficiency and is highly desirable for the development of blue OLEDs based on TADF emitters.

18 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. General information, computational details, UV-Vis, Pl, single crystal packing, CV, TGA, transient Pl, EL spectra, device performance, Electroluminescence performance table. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT We thank the Ministry of Science and Technology of Republic of China (MOST 104-2633-M007-001) for support of this research and the National Center for High-Performance Computing (Account number: u32chc04) of Taiwan for providing the computing time. REFERENCES (1) Highly Efficient OLEDs with Phosphorescent Materials (Ed.: Yersin, H.), Wiley-VCH, Weinheim, 2008. (2) Organic Light Emitting Materials and Devices (Eds.: Li, Z.; Meng, H.) Taylor & Francis, New York, 2007. (3) Organic Light-Emitting Devices: Synthesis Properties and Applications (Eds.: Müllen, K.; Scherf, U.), Wiley-VCH, Weinheim, 2006.



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