Efficient red phosphorescent OLEDs employing carbazole-based materials as the emitting host

Efficient red phosphorescent OLEDs employing carbazole-based materials as the emitting host

Dyes and Pigments 122 (2015) 257e263 Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig E...

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Dyes and Pigments 122 (2015) 257e263

Contents lists available at ScienceDirect

Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig

Efficient red phosphorescent OLEDs employing carbazole-based materials as the emitting host Chih-Hao Chang a, *, Raimonda Griniene b, Yu-De Su a, Chia-Chi Yeh a, Hao-Che Kao a, Juozas Vidas Grazulevicius b, Dmytro Volyniuk b, Saulius Grigalevicius b, ** a b

Department of Photonics Engineering, Yuan Ze University, Chung-Li, 32003, Taiwan Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu Plentas 19, LT50254, Kaunas, Lithuania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2015 Received in revised form 15 June 2015 Accepted 29 June 2015 Available online 11 July 2015

We report on the synthesis and characterization of a new series of electro-active carbazole-based compounds. The derivatives are thermally stable amorphous materials with glass transition temperatures in the range of 54e93  C. Electron photoemission spectra of thin layers of the materials show ionization potential in the range of 5.4e5.5 eV. The carbazole-based derivatives are fully characterized and their spectroscopic properties are determined by absorption and photoluminescence. All developed materials and commonly-used tris(4-carbazoyl-9-ylphenyl)amine (TCTA) were used as hosts in red phosphorescent organic light-emitting diodes (OLEDs) for comparison. Results indicate that a device with 3-[bis(9-ethylcarbazol-3-yl)methyl]-9-hexylcarbazole exhibited superior performance with peak efficiencies of 8.4%, 5.3 cd/A and 5.5 lm/W. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Electro-active Carbazole Thermally stable TCTA Phosphorescent Organic light-emitting diodes

1. Introduction Applications for organic light-emitting devices (OLED) displays are expanding rapidly due to their superior performance and flexibility as compared with liquid crystal display. Furthermore, phosphorescent OLEDs (PhOLEDs) have attracted research attention because of their intrinsically higher efficiencies as compared to fluorescent OLEDs [1]. To achieve theoretical efficiency levels, the triplet energy should be confined on the phosphorescent dopant without endothermic energy transfer to the host material. Thus, multiple-layer configurations have been widely developed to enhance device efficiency of PhOLEDs, especially in blue and green devices. In contrast, red phosphorescent materials with lower triplet energy gaps allow for simplified device architectures [2]. However, the lower gap of red phosphors usually induces serious carrier trapping, resulting in higher operation voltages. Accordingly, realizing red PhOLEDs with low power consumption and high performance requires the development of new host materials.

* Corresponding author. Tel.: þ886 3 4638800x7517; fax: þ886 3 4514281. ** Corresponding author. Tel.: þ370 37 300192; fax: þ370 37 300152. E-mail addresses: [email protected] (C.-H. Chang), [email protected] ktu.lt (S. Grigalevicius). http://dx.doi.org/10.1016/j.dyepig.2015.06.038 0143-7208/© 2015 Elsevier Ltd. All rights reserved.

To guarantee exothermic energy transfer, the triplet energy of host materials must be higher than that of the dopant [3]. In addition, a host material with a bulky structure is desirable to favor a spatially-dispersed triplet exciton, which could reduce the incidence of tripletetriplet annihilation (TTA) [4]. The carbazole moiety possesses several commendable properties, including chemical stability, easy modification, large triplet energy etc. [5] Consequently, many successful host or hole transport materials adopted carbazole-containing designs. For instance, in 2007 our group synthesized and characterized carbazole-based aromatic amines with oxetanyl functional groups. The adequate ionization potentials (4.9e5.0 eV) and the charge mobility demonstrated the suitability of these carbazole-based compounds for use in organic electronics [6]. At the same time, we also synthesized a carbazole-based material, 3,6-di(9-carbazolyl)-9-(2-ethylhexyl) carbazole (TCz1), which possesses structurally rigid moieties and a nonplanar molecular configuration, resulting in a morphologically-stable molecule with a wide triplet energy gap [7]. Sky blue PhOLEDs with a TCz1 host were demonstrated with efficiencies of up to 15%, 31 cd/A, and 28 lm/W. In 2011, Chang et al. developed a bipolar carbazole-based material 9(4,6-diphenyl-1,3,5-triazin-2-yl)-90 -phenyl-3,3'-bicarbazole (CzT) comprising a dicarbazole donor linked to an electron deficient 1,3,5triazine acceptor [8]. Sufficient triplet energy (ET ¼ 2.67 eV) together

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with promising physical properties and morphological stability (Tg ¼ 134  C) make CzT a potential host material for yellowish-green and red phosphorescent dopants. The yellowish-green OLEDs demonstrated high performance, with maximum efficiencies of 20.0%, 75.7 cd/A and 71.3 lm/W achieved in trilayer device architectures. In 2012, Curiel et al. reported the successful use of a series of carbazolo[4,3-c]carbazoles as hole transporting and emitting host materials in red PhOLEDs [9]. Different N-substituents attached to the carbazolo[4,3-c]carbazole skeleton were used to condition the charge transporting properties. The disparate transport capabilities of these materials provide advantages including flexibility for adjusting the carrier balance in the device architecture design. Recently, our group also synthesized a series of phenyl, naphthyl or biphenyl disubstituted 9-alkylcarbazoles for hole transporting materials [10]. These derivatives show high thermal stability and form amorphous layers with glass transition temperatures of 50e62  C. A green PhOLEDs using 3,6-diphenyl-9-ethylcarbazole as hole transporting layer demonstrated a maximum current efficiency of 22.5%. In this paper, we describe a series of carbazole-based derivatives for use as emitting hosts. The red PhOLEDs with 3-[bis(9-ethylcarbazol3-yl)methyl]-9-hexylcarbazole (3) as host material exhibited superior performance with peak efficiencies of 8.4% and a low turn-on voltage of 3.5 V.

2. Experimental 2.1. Materials and synthesis

IR (KBr, cm1): 3048 (CeH, Ar); 2953, 2926, 2853 (CeH); 1626, 1598 (C]C Ar); 1483, 1479, 1467 (C]C, Ar and CeH); 1348, 1332, 1316 (CeN, Ar); 1230 (CeN); 798, 770, 747, 724, 627 (CeH Ar). N-{[bis(9-hexylcarbazol-3-yl)methyl]phenyl}-N,N-diphenylamine (2). 9-hexylcarbazole (1.2 g, 4.6 mmol) hydrochloric acid (10 ml, 35%) was slowly added to an acetic acid (16 ml) solution of 4-(diphenylamino)benzaldehyde (0.5 g, 1.83 mmol) under stirring and the mixture was refluxed for 48 h. The resulting solution was poured into H2O and filtered. The deposit was dissolved in methylene chloride and washed with H2O three times. The separated organic layer was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:25) as an eluent, yielding 0.4 g (30%) of a white amorphous solid. 1 H NMR spectrum (400 MHz, CDCl3, d, ppm): 8.00 (d, 2H, J ¼ 7.6 Hz, Ar); 7.90 (s, 2H, Ar); 7.42e7.33 (m, 8H, Ar); 7.24e7.25 (m, 5H, Ar); 7.20 (t, 2H, J ¼ 6.8 Hz, Ar); 7.18e7.09 (m, 6H, Ar); 7.02 (d, 2H, J ¼ 8.4 Hz, Ar); 6.97 (t, 2H, J ¼ 7.6 Hz, Ar); 5.91 (s, 1H, CH); 4.26 (t, 4H, J ¼ 7.2 Hz, NCH2); 1.89e1.82 (m, 4H, NCH2CH2); 1.41e1.36 (m, 4H, NCH2CH2CH2); 1.34e1.25 (m, 8H, CH2CH2CH3); 0.86 (t, 6H, J ¼ 7.0 Hz, CH3). 13 C NMR spectrum (400 MHz, CDCl3, d, ppm): 14.0 (CH3(2C)), 22.5 (CH2CH3(2C)), 27.0 (CH2CH2CH3(2C)), 29.0 (CH2CH2 CH2CH3(2C)), 31.5 (CH2CH2N(2C)), 43.2 (CH2N(2C)), 56.5 (CH); 108.4 (2C); 108.6 (2C); 118.5 (2C), 120.4 (2C), 121.0 (2C), 122.5 (2C), 122.7 (2C), 122.8 (2C), 123.85 (C), 123.95 (2C), 124.05 (3C), 124.15 (C), 125.5 (2C), 127.5 (2C), 129.0 (3C), 130.0 (2C), 135.5 (2C), 139.2 (2C), 140.0 (C), 140.7 (2C), 140.7 (C), 148.0 (2C). MS (APCIþ, 20 V, m/z): 759.43 ([M þ H]þ, 100%).

9H-carbazole, 9-ethylcarbazole, 1-bromohexane, 4fluorobenzaldehyde, phosphorus oxychloride (POCl3) and triphenylamine were purchased from Aldrich and used as received. 9Hexylcarbazole [11], 4-(9-carbazolyl)benzaldehyde (1a), 4-(diphenylamino)benzaldehyde (2a) [12] and 9-hexyl-3-formylcarbazole (3a) were synthesized according to the procedures outlined in the literature [13]. 9-{4-[bis(9-hexylcarbazol-3-yl)methyl]phenyl}carbazole (1). 9hexylcarbazole (1.2 g, 4.6 mmol) hydrochloric acid (10 ml, 35%) was slowly added to an acetic acid (16 ml) solution of 4-(9carbazolyl)benzaldehyde (0.5 g, 1.85 mmol) under stirring and the mixture was refluxed for 3 h. The resulting solution was poured into H2O and filtered. The deposit was dissolved in methylene chloride and washed with H2O three times. The separated organic layer was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using a mixture of ethyl acetate and hexane (vol. ratio 1:25) as an eluent, yielding 0.23 g. (30%) of a white amorphous solid. 1 H NMR spectrum (400 MHz, CDCl3, d, ppm): 8.13 (d, 2H, J ¼ 8.0 Hz, Ar); 8.02 (d, 2H, J ¼ 7.6 Hz, Ar); 7.96 (s, 2H, Ar); 7.51 -7.36 (m, 16H, Ar); 7.27 (t, 2H, J ¼ 7.4 Hz, Ar); 7.17 (t, 2H, J ¼ 7.4 Hz, Ar); 6.09 (s, 1H, CH); 4.29 (t, 4H, J ¼ 7.4 Hz, NCH2); 1.91e1.84 (m, 4H, NCH2CH2); 1.44e1.37 (m, 4H, NCH2CH2CH2); 1.36e1.24 (m, 8H, CH2CH2CH3); 0.86 (t, 6H, J ¼ 7.0 Hz, CH3). 13 C NMR spectrum (400 MHz, CDCl3, d, ppm): 14.1 (CH3(2C)), 22.5 (CH2CH3(2C)), 27.1 (CH2CH2CH3(2C)), 29.1 (CH2CH2 CH2CH3(2C)), 31.6 (CH2CH2N(2C)), 43.3 (CH2N(2C)), 56.7 (CH); 108.7 (2C); 108.8 (2C); 110.0 (2C), 118,7 (2C), 119.9 (2C), 120.3 (2C), 120.5 (2C), 121.2 (2C), 122.7 (2C), 122.8 (2C), 123.4 (2C), 125.7 (2C), 125.9 (2C), 126.8 (2C), 127.6 (2C), 131.0 (2C), 134.0 (2C), 135.6 (C), 139.7 (2C), 140.2 (2C), 141.0 (2C), 145.0 (C).

Elemental analysis for C55H55N3 % Calc.: C 87.14, H 7.31, N 5.54; % Found: C 87.16, H 7.36, N 5.51. IR (KBr, cm1): 3049 (CeH, Ar); 2953, 2926, 2853 (CeH); 1626, 1627, 1590 (C]C Ar); 1505, 1490, 1467 (C]C, Ar and CeH); 1328, 1276 (CeN, Ar); 1242 (CeN); 770, 746, 695 (CeH Ar). 3-[bis(9-ethylcarbazol-3-yl)methyl]-9-hexylcarbazole (3). 9ethylcarbazole (5.6 g, 28.6 mmol) hydrochloric acid (60 ml, 35%) was slowly added to an acetic acid solution (90 ml) of 9-hexyl-3formylcarbazole (3.2 g, 11.5 mmol) under stirring and the mixture was refluxed for 2 h. The resulting mixture was poured into H2O and filtered. The deposit was dissolved in methylene chloride and washed with H2O three times. The separated organic layer was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using the mixture of chloroform and hexane (vol. ratio 1:6) as an eluent. yielding 1.8 g (25%) of white crystals. M.p.: 135  C (DSC). 1 H NMR spectrum (400 MHz, CDCl3, d, ppm): 7.97e7.92 (m, 6H, Ar); 7.43 -7.30 (m, 12H, Ar); 7.12 (t, 3H, J ¼ 7.2 Hz, Ar); 6.20 (s, 1H, CH); 4.33 (q, 4H, J ¼ 7.0 Hz, NCH2CH3); 4.26 (t, 2H, J ¼ 7.2 Hz, NCH2); 1.90e1.80 (m, 2H, NCH2CH2); 1.42 (t, 6H, J ¼ 7.2 Hz, NCH2CH3); 1.39e1.35 (m, 2H, NCH2CH2CH2); 1.32e1.23 (m, 4H, CH2CH2CH3); 0.85 (t, 3H, J ¼ 7.2 Hz, CH3). 13 C NMR spectrum (400 MHz, CDCl3, d, ppm): 13.95 (CH3CH2N(2C)), 14.1 (CH3), 22.5 (CH2CH3), 27.0 (CH2CH2CH3), 29.0 (CH2CH2 CH2CH3), 31.5 (CH2CH2N), 37.5 (CH3CH2N(2C)), 43.3 (CH2N), 56.8 (CH); 108.2 (2C); 108.35 (2C); 108.4 (C),108.45 (C),118,5 (C), 118.6 (2C), 120.5 (C), 120.6 (2C), 121.2 (C), 121.3 (2C), 122.7 (C), 122.8 (2C), 122.9 (C), 123.0 (2C), 125.3 (C), 125.4 (2C), 127.7 (C), 127.8 (2C), 136.2 (C), 136.3 (2C), 138.6 (2C), 139.1 (C), 140.2 (2C), 140.7 (C).

MS (APCIþ, 20 V, m/z): 757.43 ([M þ H]þ, 100%).

MS (APCIþ, 20 V, m/z): 652.37 ([M þ H]þ, 100%).

Elemental analysis for C55H53N3 % Calc.: C 87.38, H 7.07, N 5.56; % Found: C 87.36, H 7.11, N 5.54.

Elemental analysis for C47H45N3 % Calc.: C 86.60, H 6.96, N 6.45; % Found: C 86.57, H 6.99, N 6.41.

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IR (KBr, cm1): 3048 (CeH, Ar); 2952, 2927, 2852 (CeH); 1625, 1597 (C]C Ar); 1484, 1478, 1466 (C]C, Ar and CeH); 1346, 1331, 1315 (CeN, Ar); 1231 (CeN); 798, 771, 747, 725, 627 (CeH Ar). 2.2. Thermal, photophysical and electrical characterization Differential scanning calorimetry (DSC) measurements were carried out using a Bruker Reflex II thermosystem and PerkineElmer DSC-7 series thermal analyzer. Thermogravimetric analysis (TGA) was performed on a TGAQ50 apparatus. The TGA and DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10  C/min. The ionization potentials (Ip) of the layers of the compounds synthesized were measured by electron photoemission using a method similar to that demonstrated by Miyamoto et al. [14] The absorption spectra were measured using a UVeVIS Spectrophotometer (Shimadzu, UV-1650PC). The fluorescent and phosphorescent spectra of materials in CH2Cl2 were measured using a Fluorolog III photoluminescence spectrometer (Horiba Jovin Yvon). The charge carrier mobility measurements were carried by the time of flight method [15]. Preparation of samples from the materials and the measurement were organized as we described earlier [16]. 2.2.1. OLED fabrication The organic materials used for the small molecules were purchased from Nichem. All organic compounds were subject to temperature-gradient sublimation under high vacuum before use. ITO glass with a sheet resistance of ~15 U/square was washed with detergent solution and treated with UV-ozone to increase the work function. The organic and metal layers were deposited by vacuum evaporation in a vacuum chamber with a base pressure of <106 torr. The deposition system enabled the fabrication of the complete device structure without breaking the vacuum. The deposition rates for organics and aluminum were respectively kept at around 0.1 nm/s and 0.5 nm/s. The active area of the device was 2  2 mm2, as defined by the shadow mask for cathode deposition. Currentevoltageeluminance (IeVeL) characterization of the devices was performed using an Agilent 4156C semiconductor parameter analyzer and a Si photodiode calibrated with a Photo Research PR650. Electroluminescence spectra of the devices were recorded using an Ocean Optics spectrometer. The external quantum efficiencies of the devices were determined from the measured (0 ) EL spectra while the Lambertian distribution was assumed. 3. Results and discussion 3.1. Synthesis and thermal properties of carbazole-based materials Synthesis of the host materials 1e3 containing electronically isolated carbazolyl fragments is outlined in Scheme 1. The key starting materials, 4-(9-carbazolyl)benzaldehyde (1a), 4-(diphenylamino)benzaldehyde (2a) and 3-[bis(9-ethylcarbazol-3-yl) methyl]-9-hexylcarbazole (3a) were prepared procedures welldescribed in the literature [11e13]. The aldehydes 1a-3a were then reacted with an excess of 9-hexylcarbazole or 9ethylcarbazole to respectively produce 9-{4-[bis(9-hexylcarbazol3-yl)methyl]phenyl}carbazole (1), N-{[bis(9-hexylcarbazol-3-yl) methyl]phenyl}-N,N-diphenylamine (2) and 3-[bis(9ethylcarbazol-3-yl)methyl]-9-hexylcarbazole. The behavior under heating of derivatives 1e3 was studied by TGA and DSC under a nitrogen atmosphere. It was observed during the TGA analyses that the synthesized materials demonstrate very high thermal stability. A 5% weight loss (Td) occurred at the temperature of 417  C for 1, at 413  C for 2 and at 411  C for 3. It was

Scheme 1. Compounds 1e3 investigated in this study.

observed that the values of Td did not particularly depend on the chemical structure of the materials. Compounds 1 and 2 were obtained as amorphous materials as confirmed by DSC. The thermograms of derivative 1 are shown in Fig. 1(a) as an example. When the amorphous sample was heated during the DSC test, no endothermic peak due to melting was observed. When the sample was cooled down and reheated, the glass-transition was clearly seen at 73  C and, upon further heating no peaks due to crystallization and melting appeared. The amorphous sample of derivative 2 demonstrated analogous behavior during the DSC experiment. When the sample was heated, only the glass-transition was observed at 54  C and no peaks due to crystallization or melting appeared. Derivative 3 was obtained after synthesis as crystalline material demonstrated different behavior in the DSC measurements. DSC thermograms of material 3 are depicted in Fig. 1(b). The crystalline sample of 3 melted at 135  C on first heating and formed glass upon cooling. When the amorphous sample was heated again, the glasstransition was observed at 93  C and on further heating no peaks due to crystallization and melting appeared. Our investigation confirmed that the crystalline derivatives 3 can be converted into an amorphous material and used for the preparation of thin amorphous layers on substrates.

3.2. Photoemission, absorption and photoluminescence spectra The ionization potentials (Ip) of thin amorphous films of the compounds 1e3 were determined from the electron photoemission spectra of the layers. The spectra as well as the values of Ip are presented in Fig. 2. It was established that the Ip value of the materials was only slightly dependent on the chemical structure of the derivative and were in the range of 5.4e5.5 eV. The layers of

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Fig. 1. DSC curves of the synthesized materials (a) 1 and (b) 3.

Fig. 2. Electron photoemission spectra of the layers using prepared materials 1e3.

derivative 2 had the lowest Ip of 5.4 eV probably due to the conjugated system of triphenylamine. The Ip of the layers of compound 3 with only carbazolyl fragments reached 5.5 eV. It is evident that the values of Ip of the newly synthesized derivatives were lower than those of polymers with pendant carbazole rings [17]. This suggests that the synthesized derivatives 1e3 are suitable for the preparation of hole transporting layers or as hole transporting host materials in OLED devices. Fig. 3(a) shows room temperature absorption spectra of compounds 1, 2, and 3 in dichloromethane at room temperature. The concentrations of the various materials were kept at about 104 M in CH2Cl2 solution. The absorption spectra of each material exhibited absorption peaks around 271, 303, and 353 nm, similar to

those of the carbazole based compounds [18]. Thus, the lowest absorption band and fluorescence of the compounds 1e3 can be unambiguously attributed to the lowest p-p* transition of the carbazole moieties [19]. The energy bandgaps could be evaluated by the onset of absorption spectra and the respective values of 1, 2 and 3 are 3.43, 3.35, and 3.42 eV. On the other hand, the normalized photoluminescence (PL) spectra of 1, 2 and 3 in chlorobenzene was measured under excitation by a xenon lamp (l ¼ 355 nm), as shown in Fig. 3(b). Because the experiments were performed in liquid nitrogen without gate delays, their PL spectra exhibited distinguishable fluorescence and phosphorescence. The fluorescent spectra of each material showed two main peaks and one shoulder, which are similar to their triplet vibronic features. Their PL spectra respectively exhibited main peaks at 358, 359, and 361 nm. The slightly redshifted fluorescence and absorption of 3 may be attributed to its higher polarity and thus stronger molecular interaction. In addition, because the phosphorescence of the three materials exhibited similar peak positions, the corresponding triplet energy level can be assumed to be 2.97 eV. These wide triplet energy gaps make these materials suitable for use in phosphorescent OLEDs (PhOLEDs). The corresponding photophysical as well as thermal properties are summarized in Table 1. 3.3. Hole drift mobility Time of flight measurements (TOF) were used to characterize the magnitudes of charge drift mobility in thin layers of the synthesized materials 1e3. It was observed from the measurements that positive charges (holes) are transported in the thin films. Electric field dependencies of the hole drift mobility (mh) for the

Fig. 3. (a) Absorption spectra of 1, 2, and 3 in dichloromethane at room temperature. (b) PL spectra of 1, 2 and 3 in dichloromethane at 77 K.

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Table 1 Photophysical and thermal properties of compounds 1, 2 and 3.

1 2 3 a b c d e f g

labsa (nm)

lPLb (nm)

ETc (eV)

Egd (eV)

Ipe (eV)

Tgf ( C)

Tdg ( C)

271, 294, 301, (329), 341, 352 273, (292), 303, (336), 353 270, (291), 302, (323), 337, 353

358, 375, 396 359, 375, 397 361, 377, 397

2.97 2.97 2.97

3.43 3.35 3.42

5.45 5.4 5.5

73 54 93

417 413 411

labs: absorption maxima in CH2Cl2. lPL: emission maxima in chlorobenzene. Estimated from phosphorescence spectrum in chlorobenzene at 77 K. Calculated from the absorption onset. Estimated from electron photoemission spectra. Determined by DSC. Analyzed using TGA (5% weight loss).

layers are shown in Fig. 4. The mh of the materials 1e3 range from 3  103 to 2  104 cm2/(V$s) at high electric field at 25  C. It is obvious that charge transporting properties depend on chemical structures of the electroactive materials. The highest mh, exceeding 3  103 cm2/(V$s) at high electric field, was observed for the layers of the derivative 3 containing three carbazolyl fragments connected to central carbon atom. Charge mobility observed in the layers of materials 1 or 2 is noticeably lower and reaches only 104 cm2/(V$s) at high electric fields. It is evident that these results correlate with characteristics of OLED devices using these materials. The research results are described below.

3.4. Red phosphorescent OLEDs with different host materials To further examine the electroluminescent properties, these carbazole-based materials were used as hosts in red PhOLEDs. For hole-transport capabilities and energy level matching, the commonly used N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) was chosen as the HTL in the red PhOLEDs [20]. In addition, the red iridium emitter Ir(piq)3 was used as the dopant for the fabrication of red OLEDs due to its adequate triplet energy gap [21]. The photoluminescent quantum yield (PLQY) value of Ir(piq)3 in the N2-bubbled dichloromethane is found to be 41%. The procedure involved dispersing dopants into host materials with opposite transport properties, so that the carrier recombination zone can be defined and shifted away from the cathode to avoid exciton quenching [22]. Moreover, the barrier located at the interface of the double emitting layers has advantages for achieving carrier balance, which is crucial if the host is capable of single carrier transport. Therefore, considering the relationship between the energy levels, the newly-developed materials were used as emitting host materials adjacent to the HTL and the corresponding emitting layer (EML) was named EML1. In addition, to ensure

Fig. 4. Electric field dependencies of mh in charge transport layers of the materials 1e3.

efficient exothermic energy transfer and exiton confinement, the device was completed using the wide triplet-gap material 3,5,30 ,50 tetra(m-pyrid-3-yl)phenyl[1,10 ]biphenyl (BP4mPy) as the host and electron-transport layer (ETL) [23]. The second EML, consisting of BP4mPy and Ir(piq)3 as a host-guest system, was named EML2. On the other hand, to fairly evaluate the performance of these carbazole-based materials, a control device was fabricated with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the host material in EML1 due to its common application as a host material in green or red emitting layers [24]. The hole mobility of TCTA is about 3  104 cm2/V [25]. Furthermore, this carbazole-based TCTA exhibits a wide triplet energy gap of 2.85 eV which is similar to those of the tested materials, making it a suitable reference point for host comparisons. The device architecture consists of ITO/NPB (40 nm)/ Host doped with 8 wt.% of Ir(piq)3 (20 nm)/BP4mPy doped with 8 wt.% of Ir(piq)3 (10 nm)/BP4mPy (40 nm)/LiF (0.8 nm)/Al (150 nm). Fig. 5(a) shows a structural drawing of the materials employed in OLEDs while Fig. 5(b) shows the schematic structures of the tested red PhOLEDs with different host materials. The electroluminescence (EL) characteristics of all tested devices and numeric data are depicted in Fig. 6 and Table 2. Fig. 6(a) shows the normalized EL spectra of all tested devices at a luminance of 103 cd/m2. All tested devices exhibited nearly identical spectral profiles, implying that the carrier recombination zone was located within the EML and that exciton diffusion to the adjacent layers could be avoided. Moreover, the location of the exciton formation zone was similar for all devices. An exothermic energy transfer was also achieved between the host and guest in both emitting layers. The EL emission of each device is stable within a wide luminance range from 102 to 104 cd/m2. According to the JeVeL curves (Fig. 6(b)), the order of the current density of the Devices is C > B > D > A, while a similar tendency can be observed for luminance. Furthermore, the respective turn-on voltages of Devices A, B, C and D are 5.3 V, 3.5 V, 3.5 V and 4.9 V. As expect, Device C with 3 possessing higher hole mobility showed a lower turn on voltage. In addition, the similar hole mobilities of both TCTA and compound 1 led alike turn on voltages. On the other hand, the low turn on voltage of Device B might be due to the lower Ip of compound 2, which mitigating the energy barrier between HTL/EML interface. The LeV curves clearly show that Devices A and D form one group, while Devices B and C form another. The steric hindrance imposed by the bis(9-hexylcarbazol-3-yl)methyl decreases carrier transport capabilities [9]. Lower current densities are obtained in Device A, even though compounds 1 and 2 possess the same structure of the bis(9-hexylcarbazol-3-yl)methyl moiety. On closer inspection of molecules 1 and 2, the only molecular difference is that the phenyl of 1 links with the carbazole moiety, while 2 has an phenyl linked N,N-diphenylamine. Thus, we deduced that the superior current density shown in Device B should result from the amine group [26]. In contrast to the compounds 1 and 2 with two 9hexylcarbazol moieties, compound 3 succinctly adopts two shorter

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Fig. 5. (a) Structural drawing of the materials and (b) schematic structures of the tested red PhOLEDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

9-ethylcarbazol moieties, leading to an even higher current density. The detected differences in terms of current density could likely be related to the different molecular structures which in turn would influence the film morphology. Moreover, the maximum luminance of Device C is recorded at 8843 cd/m2 which is double that of the other devices, indicating the superior electrical stability of 3. Overall, these structural designs endow compounds 1e3 with a wide range of transport capabilities, allowing for the adjustment of the OLED carrier balance. As shown by the efficiency curves, Device A exhibits efficiency comparable to that of the control device (i.e., Device D) in a very wide luminance range. Devices A and D showed similar maximum efficiencies of 6.9% and 7.0%. Accompanied with the JeVeL curves, compound 1 possesses intrinsic properties similar to that of TCTA,

including hole transporting ability, triplet energy gap and practical application in OLEDs. On the other hand, the higher efficiencies obtained in both Devices B and C indicate that devices using 2 or 3 as hosts achieved superior carrier balance. Devices B and C exhibited respective peak EL efficiencies of up to 7.1% (4.2 cd/A and 4.9 lm/W) and 8.4% (5.3 cd/A and 5.5 lm/W). At a practical brightness of 102 cd/m2, the efficiencies of Device B remained high at around 6.3% (2.0 cd/A and 4.3 lm/W) while the forward efficiency of Device C was also maintained at 7.0% (2.2 cd/A and 4.5 lm/W). By comparing devices with identical architectures but different host materials, the higher efficiencies obtained in Device C suggest that the carrier balance harvested through employing 3 with superior electrical property. In common PhOLEDs, tripletetriplet annihilation (TTA) is seen as being responsible for significant decreases in

Fig. 6. (a) Normalized EL spectra at a luminance of 103 cd/m2, (b) current densityeluminanceevoltage (JeVeL) characteristics, (c) external quantum efficiency versus luminance, and (d) luminance efficiency versus luminance for Devices A, B, C, and D.

C.-H. Chang et al. / Dyes and Pigments 122 (2015) 257e263

263

Table 2 EL characteristics of tested devices with different hosts. Device

A

B

C

D

EML1

1

2

3

TCTA

6.9 5.2 3.2 1.1 4.5 3.4 5.3 54.2 3898 [17.6 V] (0.681, 0.317) (0.681, 0.317)

7.1 6.3 4.2 2.0 4.9 4.3 3.5 67.9 3610 [14.0 V] (0.681, 0.318) (0.679, 0.318)

8.4 7.0 5.3 2.2 5.5 4.5 3.5 118.7 8843 [13.2 V] (0.681, 0.315) (0.681, 0.317)

7.0 5.0 3.4 1.1 4.4 3.1 4.9 46.0 4259 [16.0 V] (0.682, 0.316) (0.682, 0.316)

External quantum efficiency (%) Luminescence efficiency (cd/A) Power efficiency (lm/W) Von (V) J1/2 (mA/cm2) Max. luminance (cd/m2) [Voltage] CIE1931 coordinates

a b a b a b c

b d

a. Maximum efficiency; b. Recorded at 102 cd/m2; c. Turn-on voltage measured at 1 cd/m2; d. Measured at 103 cd/m2.

efficiency in higher luminance regimes [27]. The external quantum efficiency of the PhOLEDs reported here declined by half at a current density (J1/2) of 54.2, 67.9, 118.7 and 46.0 mA/cm2, respectively, for Devices A, B, C and D [28]. The tendency of J1/2 agrees with the current density observed in the J-V curves. Obviously, the mitigated efficiency roll-off behavior of Device C indicates lower exciton concentrations at the EML1/EML2 interface, which implies that an enlarged carrier recombination zone formed in EML1. Consequently, compound 3 might allow for some electron injection into EML1 and thus enlarge the carrier recombination zone. A similar phenomenon of the bipolar transporting property has been observed in other carbazole-based compounds [7]. Overall, using these materials as hosts in EML provides sufficient carrier transport capability along with triplet energy gaps, thus demonstrating the validity of the molecular designs. In summary, the synthesis and characterization of three newly developed carbazole-based compounds were reported. Furthermore, all developed compounds and TCTA were used as hosts in Ir(piq)3-based OLEDs. The EL results indicate that devices with the newly-developed materials exhibited superior performances to devices using TCTA as host. For comparison, devices with TCTA and 3 as the host showed respective peak efficiencies of 7.0% (3.4 cd/A) and 8.4% (5.3 cd/A). The disparate photophysical and electrical properties of these compounds merit adjusting the carrier balance in the device architecture design. Acknowledgments The development of the OLED materials was supported by grant No. MIP-024/2013 from the Research Council of Lithuania. The authors gratefully acknowledge the financial support from National Science Council of Taiwan (NSC 102-2221-E-155-080-MY3 and NSC 103-2623-E-155-008-ET). References [1] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Nature 1998;395:151.

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