Thermally stable efficient hole transporting materials based on carbazole and triphenylamine core for red phosphorescent OLEDs

Thermally stable efficient hole transporting materials based on carbazole and triphenylamine core for red phosphorescent OLEDs

Organic Electronics 51 (2017) 463–470 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel ...

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Organic Electronics 51 (2017) 463–470

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Thermally stable efficient hole transporting materials based on carbazole and triphenylamine core for red phosphorescent OLEDs

MARK

Ramanaskanda Braveentha, Hyeong Woo Baeb, Ik Jang Kob, Wu Qionga, Quynh Pham Bao Nguyena, Pothupitiya Gamage Sudesh Jayashanthaa, Jang Hyuk Kwonb,∗∗, Kyu Yun Chaia,∗ a b

Division of Bio-Nanochemistry, College of Natural Sciences, Wonkwang University, Iksan City, Chonbuk, 570-749, Republic of Korea Department of Information Display, Kyung Hee University, Dongdaemoon-gu, Seoul, 130-701, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Organic light emitting diodes Hole transporting materials Red phosphorescent Carbazole Triphenylamine

In this work, a series of hole transporting materials with carbazole and triphenylamine cores have been synthesized and characterized. In the carbazole's 3rd and 6th positions, two site tryphenylamine para positions are end capped with the same types of branching derivatives to compare the overall performances of constructed devices. All of our hole transporting materials showed good thermal stabilities without any crystallized features which expressed in higher decomposition temperature (Over 500 °C at 5% weight reduction). All synthesized materials revealed HOMO energy levels between −5.62 and −5.48 eV, which values are lying between HOMO energy values of anode and emission layer; as a result, it made an effective path for hole transportation. Higher lying LUMO values between −2.51 and −2.31 can block the electrons from adjacent layer to ensure the perfect recombination in the middle layer. Triphenylamine based HTMs indicated better performances than carbazole based HTMs. Further comparisons were done by using NPB as hole transporting material with the same red phosphorescent based OLED device. HTM2A based device IV was exhibited higher maximum current efficiency of 30.6 cd/A and higher maximum external quantum efficiency of 26.7% than reference NPB based device. Measured Hole mobility value of HTM2A with hole dominant device was 5.3 × 10−4 cm2 V−1 s−1, which was better than NPB. Synthesized HTM2A would be a promising hole transporting material for various phosphorescent based OLEDs.

1. Introduction Organic light-emitting diodes (OLEDs) firstly reported by Tang in 1987 have received remarkable attention from both research and industrial communities for their prospective application in flat panel displays and light emitting sources. OLEDs are influencing the next generation technologies because of their practical advantages, such as wide view angle, high contrast, flexible, potable and low power consumption [1–4]. Multilayer OLEDs have been given much attention due to the enhancement of device efficiencies. Standard OLED structural construct use several layers located between the cathode and transparent anode, which consists of the electron transporting layer (ETL), hole transporting layer (HTL) and emission layer (EML) [5–10]. Development of quality hole transporting material (HTM) is one of the key parameters to providing significant improvement in device performances by lowering the hurdle for hole injection from the anode to EML



and efficient recombination of holes and electron at the middle layer [9,11]. Many HTMs have been synthesized based on their lower molecular weight, amorphous organic materials [12,13]. The most extensively used amorphous HTMs are N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and N,N′-Bis(3methylphenyl)-N,N′-diphenylbenzidine (TPD). Those materials have excellent hole transporting/mobility properties and highly transparency to visible region. However, their thermal instability manifested at low transition glass temperature (Tg). For instance, Tg of N,N′-Di(1naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and N,N′Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) widely used as HTMs are 95 °C and 65 °C, respectively [6–10,14]. The OLED device during operation period caused by Joule heating leads to elevate the temperature and such low Tg diminished the lifetime of the device [5,15,16]. The HTMs with lower thermal stability hampered their commercial application. Over the past few decades, many HTMs have

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J.H. Kwon), [email protected] (K.Y. Chai).

∗∗

http://dx.doi.org/10.1016/j.orgel.2017.09.050 Received 9 March 2017; Received in revised form 27 September 2017; Accepted 29 September 2017 Available online 30 September 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.

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Scheme 1. Synthesis of HTM1A to 2B. (a) Pd(Ph3P)4, K2CO3, tBu3P, toluene, 110 °C.

N

N

B(OH)2

N

N

A 1A

(a) N (a) Br

Br

N

N

1

N N

N

N N

1B

B(OH)2

B N

N

B(OH)2

N

N

A 2A

(a) N (a) Br

2

Br

N

N N N

N

N

N 2B B(OH)2

B

Table 1 Thermal, photophysical and electrical properties of HTM1A, 1B, 2A and 2B. HTMs

Tga (°C)

Tdb (°C)

UV- Vis (nm)

PL max (nm)

HOMO (eV)

LUMO (eV)

Egc (eV)

ETd (eV)

Hole mobilityg (cm2·V−1·s−1)

HTM1A

165

500

333

444

575

313

402

2.59

5.6 × 10−5

HTM2A

160

500

365

442

2.37

5.3 × 10−4

HTM2B

172

550

312, 358

420

3.04e 3.06f 3.31e 3.54f 2.98e 2.85f 3.05e 3.21f

2.4 × 10−4

260

2.51e 2.33f 2.31e 2.03f 2.50e 2.43f 2.50e 2.18f

2.35

HTM1B

5.55e 5.39f 5.62e 5.57f 5.48e 5.28f 5.55e 5.39f

2.45

1.0 × 10−4

a

Transition glass temperature. Decomposition temperature. c Band gap energy. d Triplet energy. e Experimental data. f Data from DFT calculation. g Calculated at 0.3 MV/cm. b

464

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carbazole and TPA core. At the same time we use well known NPB material as a common reference for our current study. 2. Materials and methods 2.1. General procedures All reagents and solvents were obtained from commercial suppliers and were used without further purification otherwise stated. 1H and 13C NMR spectra were recorded by using JEON JNM-ECP FT-NMR spectrometer operating at 500 MHz. Absorbance spectra were obtained from UV–visable spectrophotometer (SINCO S-4100). Band gap (Eg) was estimated from the on-set of the Uv–visible absorbance spectra. Photoluminescence (PL) spectra were measured by using JASCO FP8500 spectrofluorometer and tetrahydrofuran (THF) was used as solvent. Triplet energy level (ET) was found out by comparing between the PL spectra of room temperature and low temperature (∼77 K). To measure the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) level, cyclic voltammetry (CV) was carried out by using BioLogic SP-50 and HOMO level was calculated by subtracting the oxidation potential shift between ferrocene and the HTMs. LUMO level was estimated from the obtained HOMO level and band gap by adding them. Current density-voltage-luminance (J-VL) characteristics of each device were measured with a luminance color meter (Konica Minolta CS-100A) and a source meter unit (Keithley 2635A). Electroluminescence (EL) spectra and CIE (Commission Internationale l’Eclairage) 1931 color coordinators were obtained using a spectroradiometer (Konica Minolta CS-2000). Molecular simulations were carried by using density function theory (DFT) calculation with B3LYP (beck three parameter hybrid functional and Lee-Yang-Parr correlation functional) and 6-31G (d) basic set implemented in Gaussian 09. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted by SDT Q600 V20.9 Build 20 and DSC Q200 V24.9 Build 121 instruments with the heating rate of 10 °C/min.

Fig. 1. Thermal stabilities of devices.

been developed to solve the critical issues such as thermal stability, low hole mobility, high hole injection barrier and higher ionization potential [17]. There are two major structural groups that have been reported for their considerable enhancement towards better properties, which are Triphenylamine (TPA) and Carbazole based compounds. All these compounds have extraordinary electron donating properties, which helps to improve the hole carrier mobility [18,19]. Among these, carbazole based HTMs have shown astonishing achievement for their following characteristics; (1) long conjugation length. (2) Excellent carrier mobility. (3) Thermal and morphological durability. (4) Photo chemical steadiness. (5) Possibility of modification at 3, 6 and 2, 7, N positions [16–18,20–23]. Meanwhile TPA core based molecules, in which several linear arms are joined together can cause non-planner structures and longer conjugation length. The above characters have shown to possess good amorphous film forming properties and lower ionization potential, where a low barrier for hole injection from the anode are noticeable [23–25]. When compared to the carbazole core, TPA based HTMs show lower thermal stabilities. However we have recently synthesized TPA core based HTMs with better thermal stabilities. Which have demonstrated the presence of bulky and rigid structures at the end-capped location indicated that can suppress the crystallization [25–28]. The presence of non-planar geometries around the core region can help to prevent the close π-π stacking in order to increase its physical strength [29]. Herein, we report new hole transporting materials with carbazole and TPA functional moieties, 1A, 1B and 2A, 2B. In order to enhance the overall device properties, we have introduced similar bulky structures at the 3, 6 orientation of carbazole and para position of TPA as branching unit. We compare the overall performance between

2.2. Synthesis procedure 2.2.1. Typical procedure for synthesis of compounds 1A and 1B A mixture of 3,6-dibromo-9-phenyl-9H-carbazole 1 (1 g, 1 equiv), 4(N-(naphthalen-2-yl)-N-(naphthalen-4-yl)amino)phenylboronic acid A (3.5 g, 3 equiv) or 9-(4-diphenylamino)phenyl)-9H-carbazol-2-yl-2boronic acid B (4 g, 3.5 equiv), Pd(Ph3P)4 (0.8 g, 0.3 equiv), t-Bu3P 50% in toluene (0.15 ml, 0.6 equiv), K2CO3 (2 M, 50 ml) and 100 ml of toluene were added then stirred under 110° C for 48 h. After completion of the reaction, the reaction mixtures were extracted with dichloromethane and water. The organic layer was dried over anhydrous magnesium sulphate then filtered and removed the solvent through

Fig. 2. Spatial distribution of HOMO and LUMO from DFT calculation.

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Fig. 3. (a) UV–visible absorption and (b) PL spectra of synthesized HTMs.

Fig. 4. Cyclic voltammetry measurements of HTM1A to 2B.

Fig. 5. Configuration and energy band gap diagram of fabricated red phosphorescent based OLED device I-V with HTM NPB, 1A, 1B, 2A and 2B, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

146.22, 143.63, 140.56, 135.59, 135.42, 134.53, 133.93, 131.26, 130.02, 129.61, 128.92, 128.53, 127.98, 127.64, 127.39, 127.02, 126.96, 126.68, 126.62, 126.51, 126.36, 126.31, 125.32, 125.31, 124.43, 124.16, 124.12, 123.00, 122.60, 118.40, 117.69, 110.21; GCMS: 929.98 for C70H47N3 [M+H+]. Anal.calcd for C70H47N3 (%): C 90.39, H 5.09, N 4.52; found C 89.76, H 5.05, N 4.56. 1B Yield: 59%; white solid; 1H NMR (500 MHz, CDCl3) δ 8.46 (s, 2H),

rotary evaporation method. Crude residues were purified by silica gel column chromatography by using n-hexane: dichloromethane solvent system to achieve the requisite products 1A and 1B. 1A Yield: 52%; yellow solid; 1H NMR (500 MHz, CDCl3) δ 8.38 (s, 2H), 8.02–8.03 (d, J = 5 Hz, 2H), 7.90–7.91 (d, J = 5 Hz, 2H), 7.80–7.81 (d, J = 5 Hz, 2H), 7.68–7.73 (m, 4H), 7.43–7.60 (m, 22H), 7.33–7.36 (m, 9H), 7.16–7.17 (d, J = 5 Hz, 4H); 13C NMR (500 MHz, CDCl3) δ 147.28, 466

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Fig. 6. (a) J-V-L (b) luminescence-current efficiency characteristics of HTM1A to 2B based OLED devices.

Table 2 Device characteristics. Characteristics

Device I (NPB)

Device II (HTM1A)

Device III (HTM1B)

Device IV (HTM2A)

Device V (HTM2B)

Turn-on voltage (V) Driving voltagea (V) Maximum Current efficiency (cd/A) Current efficiency at 1000 cd/m2 (cd/A) Max EQE CIE 1931 (x, y)

2.2 4.6 27.9 27.9 24.2% 0.65,0.35

2.5 4.8 24.6 24.4 23.2% 0.65,0.35

2.2 5.4 17.6 15.2 16.3% 0.65,0.34

2.5 4.4 30.6 28.6 26.7% 0.65,0.34

2.3 5.0 22.1 21.6 19.7% 0.65,0.35

2A Yield: 69%; yellow solid; 1H NMR (500 MHz, CDCl3) δ 7.97–7.99 (d, J = 10 Hz, 2H), 7.88–7.90 (d, J = 10 Hz, 2H), 7.78–7.80 (d, J = 10 Hz, 2H), 7.71–7.73 (d, J = 10 Hz, 2H), 7.68–7.70 (d, J = 10 Hz, 2H), 7.24–7.50 (m, 29H), 7.09–7.15 (m, 8H), 7.00–7.03 (t, J = 6 Hz, 2H); 13 C NMR (500 MHz, CDCl3) δ 147.47, 143.52, 135.41, 134.15, 133.82, 131.12, 129.56, 129.21, 128.36, 127.64, 127.37, 126.95, 126.60, 126.54, 126.49, 126.31, 124.37, 124.13, 123.02, 122.32, 117.97; GCMS: 932.03 for C70H49N3 [M+H+]. Anal.calcd for C70H49N3 (%): C 90.19, H 5.30, N 4.51; found C 89.57, H 5.23, N 4.57. 2B Yield: 78%; white solid; 1H NMR (500 MHz, CDCl3) δ 8.09–8.17 (dd, J = 10 Hz, 4H), 7.58–7.60 (d, J = 10 Hz, 6H), 7.57 (s, 2H), 7.50–7.52 (dd, J = 10 Hz, 2H), 7.24–7.32 (m, 9H), 7.19–7.22 (m, 30H), 7.06–7.08 (t, J = 6 Hz, 2H); 13C NMR (500 MHz, CDCl3) δ 147.57, 147.23, 141.31, 140.97, 138.41, 135.86, 131.14, 129.59, 129.32, 128.43, 128.03, 125.98, 125.01, 124.63, 124.04, 123.84, 123.52, 122.56, 122.41, 122.06, 121.04, 120.74, 120.09, 118.89, 110.03, 108.12; GCMS: 1062.05 for C78H55N5 [M+H+]. Anal.calcd for C78H55N5 (%): C 89.19, H 5.22, N 6.59; found C 87.60, H 4.77, N 6.12.

Fig. 7. Normalized EL spectra of Device I-V.

8.21–8.23 (d, J = 10 Hz, 2H), 8.15–8.17 (d, J = 10 Hz, 2H), 7.63–7.74 (m, 11H), 7.42–7.50 (m, 12H), 7.24–7.31 (m, 22H), 6.99–7.03 (t, J = 10 Hz, 2H); 13C NMR (500 MHz, CDCl3) δ 147.55, 147.27, 141.89, 141.74, 140.82, 140.41, 137.75, 134.72, 131.31, 130.09, 129.55, 128.07, 127.69, 127.09, 126.36, 125.80, 124.96, 124.12, 123.89, 123.52, 123.22, 122.06, 120.64, 120.34, 120.03, 119.96, 119.55, 110.21, 109.96, 108.62; GC-MS: 1060.01 for C78H53N5 [M+H+]. Anal.calcd for C78H53N5 (%): C 88.36, H 5.04, N 6.61; found C 87.51, H 4.97, N 6.63.

2.3. OLED fabrication and characterization In order to estimate the performances of synthesized HTMs, red phosphorescence OLED devices were fabricated. The ITO substrate of 150 nm thickness with emission area of 2 × 2 mm2 was used as an anode electrode. Substrates are ultrasonically cleaned with acetone, isopropyl alcohol, deionized water and finally treated with ultravioletozone. 4,4′-Bis[N-[4-{N,N-bis(3-methylphenyl)amino}phenyl]-N- phenylamino] biphenyl (DNTPD) was used as hole injection layer. To do the comparison with our synthesized HTMs, NPB based device was used as a reference for our current study. 5 wt% of bis[2,4-dimethyl-6-(4methyl-2-quinolinyl-κN)phenyl-κC](2,2,6,6-tetramethyl-3,5-heptanedionato-κO3 (Ir(mphmq)2 (tmd)) doped bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2) was introduced as the host of EML. Bebq2 also used as electron transporting layer and Aluminum with LiF was used as a cathode. Organic and cathode materials were deposited in thermal evaporator system under ∼1.0 × 10−7 torr pressure. After deposition, devices were encapsulated by the glass cap.

2.2.2. Typical procedure for synthesis of compounds 2A and 2B A mixture of N,N-bis(4-bromophenyl)benzenamine 2 (1 g, 1 equiv), 4-(N-(naphthalen-2-yl)-N-(naphthalen-4-yl)amino)phenylboronic acid A (3.5 g, 3 equiv) or 9-(4-diphenylamino)phenyl)-9H-carbazol-2-yl-2boronic acid B (4 g, 3.5 equiv), Pd(Ph3P)4 (1 g, 0.35 equiv), t-Bu3P 50% in toluene (0.15 ml, 0.6 equiv), 2 M K2CO3 in 50 ml distilled water and 100 ml of toluene were added then stirred under 110° C for 48 h. After completion of the reaction, the reaction mixtures were extracted with dichloromethane and water. The organic layer was dried over anhydrous magnesium sulphate then filtered and removed the solvent through rotary evaporation method. Crude residues were purified by silica gel column chromatography by using n-hexane: dichloromethane solvent system to achieve the requisite products 2A and 2B. 467

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3. Results and discussion

Scheme 1 displays the synthetic path of designed hole transporting materials (HTMs). All materials were synthesized by using well known Suzuki cross coupling reaction between derivatives 3,6-dibromo-9phenyl-9H-carbazole 1, N,N-bis(4-bromophenyl)benzenamine 2 and 4(N-(naphthalen-2-yl)-N-(naphthalen-4-yl)amino)phenylboronic acid A, 9-(4-diphenylamino)phenyl)-9H-carbazol-2-yl-2-boronic acid B by using tetrakis catalyst, tributylphosphine ligand, potassium carbonate base and toluene as solvent were stirred under reflux condition. Designed HTMs were obtained with good yields of 52%, 59%, 69% and 78%, respectively and revealed good solubility in common organic solvents like dichloromethane and chloroform. The obtained compounds were confirmed by NMR (1H,13C), mass spectrometry and elemental analysis.

data are epitomized in Table 1. Absorption spectra maximum peak values of these HTMs were vary between 312 nm and 365 nm. There was not any evidence of absorption in the visible region, which substantiates to boost the device efficiencies. At the same time, band gap values of HTM1A to 2B were able to obtain from the absorption spectra, which were 3.04, 3.31, 2.98 and 3.05 eV, respectively. HTM2A exhibited little lower gap value (2.9 eV) than NPB material (3.0 eV), which helps to transport carriers effectively. PL spectra of HTMs 1A to 2B were 444, 402, 442, 420 nm and consequently the PL spectra in solid states were identified as 438, 399, 437 and 415 nm, sequentially. Above values did not show any bathochromic shift in the solid state, which is a strong evidence of weak intermolecular interactions. Triplet energies of the synthesized HTMs were between the limit of 2.35 eV–2.59 eV and these data are little higher than that of NPB (2.3 eV) reference material. It implies that the synthesized HTMs are appropriate for hole transporting materials of red phosphorescent based OLED because of sufficiently high triplet energy levels to confine triplet excitons in EML.

3.2. Thermal properties

3.5. Electrochemical properties

Thermal properties were demonstrated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The above studies were carried out under a nitrogen atmosphere. All HTMs exhibited very high decomposition temperatures (Td) above 500 °C as summarized in Table 1. The glass transition temperatures (Tg) of 1A, 1B, 2A and 2B were found to be 165, 260, 160 and 172 °C, respectively. Our synthesized HTMs significantly expressed superior thermal stability than reference NPB which had lower Tg of 95 °C [29]. From the summarized data in Table 1, carbazole based materials render higher thermal steadiness than TPA derivatives attributed to its rigid core structure. These thermal solidity of our synthesized materials lead to support several good properties such as good film formation, enhanced the device properties and durability. Further, we have investigated operational thermal stabilities of fabricated devices are shown in Fig. 1. We performed the stability study while increasing the temperature and record operational luminescence change of different HTM devices [30–32]. NPB based device I showed the poor thermal stability at 90 °C as reported. The luminescence was decreased significantly at near NPB Tg value. While the other devices (HTM1A to 2B) II to V were exhibited good device thermal property up to 110 °C due to the stable glass transition temperature. All II to V devices show luminescence decrease after 110 °C because we used DNTPD as a hole injecting material (HIL) has Tg value about 106 °C [33]. This is a solid evidence that our synthesized hole transporting materials have good thermal properties in the devices.

HOMO energy levels were determined by using ionization potential measurements, which were obtained from CV values (Fig. 4). HOMO levels of HTM1A, 1B, 2A and 2B were −5.55, −5.62, −5.48 and −5.55 eV, respectively. The above measurements done by using the following equation, EHOMO = −4.8 - (EOX - EFc). We noticed that HOMO of TPA based 2A and 2B were exhibited considerably higher level than carbazole based 1A and 1B, which are beneficial for hole injection. Calculated LUMO levels of our HTMs were −2.51, −2.31, −2.50 and −2.50 eV (Table 1) and these are higher than that of EML (−2.80 eV), which can effectively block electrons from adjacent layer and make sure the recombination only at the ideal layer.

3.1. Synthesis

3.6. Device performance In order to investigate the performance of synthesized HTMs, we have fabricated OLED device consist of red phosphorescence based emitter. The device I-V configuration is ITO (150 nm)/DNTPD (40 nm)/ HTM (65 nm)/Bebq2: 5% Ir(mphmq)2 (tmd) (20 nm)/Bebq2 (60 nm)/ LiF (1.5 nm)/Al (100 nm) and shown in Fig. 5. The current density-voltage-luminescence (J-V-L) characteristics and current efficiency spectra are placed in Fig. 6 and summarized in Table 2. The maximum current efficiencies of our HTM1A to 2B based devices were 24.6 cd/A, 17.6 cd/A, 30.6 cd/A and 22.1 cd/A, respectively. Those noticed values are better than NPB based device (27.9 cd/ A) and generally current efficiency of the device depends on driving voltage and quantum efficiency, which explained that HTM2A was given lower driving voltage (4.4 V) and possessed quantum efficiencies of 26.7%. Which is higher than that of reference NPB based device I (24.2%). When Comparing the J-V curves, HTM2A showed the fastest JV curve. Therefore, it implies that the charge balance was tempered well due to the fast hole transport ability. The overall optimized device data was observed better in device IV, which substantiate the HTM2A has good impact on device performances compared to NPB material. It suggests that hole transport and injection property of TPA core based HTM is better than that of carbazole based HTMs by fast hole mobility with small hole injection barrier. Meanwhile HTMs with moiety A (HTM1A and HTM2A) showed considerably faster J-V characteristics than HTMs with moiety B. which explain that moiety A has higher hole mobility and low hole injection barrier. To understand the hole mobility property further, hole only devices (HOD) of each HTMs were fabricated. Hole dominant devices were fabricated with the structure of ITO (50 nm)/MoO3 (5 nm)/HTL (100 nm)/MoO3 (5 nm)/Al (100 nm). Hole mobility values were calculated from the space charge limited current (SCLC) measurement. From the HOD result, hole mobility values of NPB, HTM1A to HTM2B were calculated as 3.0 × 10−4 cm2 V−1 s−1, 2.4 × 10−4 cm2 V−1 s−1,

3.3. DFT simulation Density functional theory calculations were conducted to optimize the molecular structure and predict HOMO/LUMO distributions and energy levels. HOMO and LUMO distribution of each HTMs are depicted in Fig. 2. Calculated HOMO, LUMO energy levels and band gap were summarized on Table 1. From the simulation results, TPA core based HTMs (HTM2A and HTM2B) showed shallower HOMO level and deeper LUMO level than carbazole based HTMs (HTM1A and HTM1B). Consequently, HTMs contain moiety A (4-(N-(naphthalen-2-yl)-N(naphthalen-4-yl)amino)phenyl) showed the shallower HOMO level, deeper LUMO level and smaller band gap when compare to moiety B (9(4-diphenylamino)phenyl)-9H-carbazol-2-yl) based HTMs. The calculated values were almost corresponding with the measured results. 3.4. Photophysical properties The optical analysis of HTMs were accomplished on the basis of UV–visible spectra and PL spectra under room temperature and low temperature (77 K). Fig. 3 shows UV–vis absorption and PL spectra, all 468

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5.6 × 10−5 cm2 V−1 s−1, 5.3 × 10−4 cm2 V−1 s−1 and 1.0 × 10−4 cm2 V−1 s−1 at 0.3 MV/cm of electric field. Hole mobility of each HTMs are summarized in Table 1. Both HTM1A and HTM2A showed considerably higher hole mobility compared with other HTMs and HTM2A based HOD device was exhibited little higher hole mobility than NPB. Hole mobility values were perfectly correlated with device efficiencies and J-V curves in the order of HTM2A > NPB > HTM1A > HTM2B > HTM1B. Additionally, we have extended our study towards geometrical analysis of NPB, HTM1B and HTM2A to figure out the planarity of the molecules. From the DFT simulation, we calculated the ratio of the length of each direction of x-, y- and z-axis when the molecules are put on the x-y plane. Each ratio of Δx: Δy: Δz of NPB, HTM1A, HTM1B, HTM2A and HTM2B were 2.63: 1.48: 1.00, 3.53: 2.32: 1.00, 2.96: 1.47: 1.00, 3.14: 1.76: 1.00 and 5.75: 2.74: 1.00. From the above study, Δx & Δy values of HTM2A were higher than NPB and provides more planar structure. This molecular shape can facilitate the holes hopping path further easily with increasing the hole mobility compared with NPB. The HTM2A molecule with planar structure could improve the hole mobility and device efficiencies. In case of other HTMs, aspect ratio roughly matched because carrier mobility can be predicted by using two main factors which are molecular shape and reorganization energy. So we believe that HTM1A exhibited lower hole mobility when compare with NPB which may cause due to reorganization energy. Derivative A based HTM1A and HTM 2A were exhibited higher hole mobility when compare with derivative B based HTM 1B and 2B. At the same time we can notice that, TPA based central core has good approach towards hole mobility enhancement. Electroluminescent spectra (EL) of NPB, HTM1A to 2B are displayed in Fig. 7. All of our device performed the typical EL spectra of Bebq2: 5% Ir(mphmq)2 (tmd) with red emission. Mostly, we can observe the shoulder intensity of the EL spectra is increased when the recombination zone formed near HTL interface, device III based on HTM 1B was shown strong shoulder intensity and slower hole mobility. Which result was attributed to poor device performances compared to other HTMs based devices.

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4. Conclusion In summary, we have designed and synthesized a series of HTMs based on carbazole (HTM1A and 1B) and TPA central core (HTM2A and 2B) incorporating with same types of branching units. All HTMs were exhibited higher thermal and morphological stabilities due to their rigid and bulky structure. All synthesized compounds showed better HOMO energy levels to match with the adjacent layers and make sure the effective charge hopping pathway, at the same time LUMO energy levels of the synthesized compounds were much enough to block the electrons and act as electron blocking layer as well. HTM2A based device IV was revealed higher maximum current efficiency, better external quantum efficiency (26.7%) and higher hole mobility (5.3 × 10−4 cm2V−1·s−1) than NPB based device. TPA based HTM2A was found to be the best molecule out of our synthesized HTMs. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the education (NRF-2016R1D1A3B01015531). References [1] V. Promarak, M. Ichikawa, D. Meunmart, T. Sudyoadsuk, S. Saengsuwan, T. Keawin, Synthesis and properties of stable amorphous hole-transporting molecules for electroluminescent devices, Tetrahedron Lett. 47 (2006) 8949–8952, http://dx.doi.org/10.1016/j.tetlet.2006.10.031. [2] V. Promarak, M. Ichikawa, T. Sudyoadsuk, S. Saengsuwan, S. Jungsuttiwong, T. Keawin, Thermally and electrochemically stable amorphous hole-transporting materials based on carbazole dendrimers for electroluminescent devices, Thin Solid

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