Oxadiazole- and indolocarbazole-based bipolar materials for green and yellow phosphorescent organic light emitting diodes

Journal Pre-proof Oxadiazole- and indolocarbazole-based bipolar materials for green and yellow phosphorescent organic light emitting diodes Qiong Wu, Ramanaskanda Braveenth, Il-Ji Bae, Heng-Qiang Zhang, Hasu Jung, Miyoung Kim, Kyu Yun Chai PII:

S0143-7208(19)32004-2

DOI:

https://doi.org/10.1016/j.dyepig.2019.108052

Reference:

DYPI 108052

To appear in:

Dyes and Pigments

Received Date: 26 August 2019 Revised Date:

12 November 2019

Accepted Date: 15 November 2019

Please cite this article as: Wu Q, Braveenth R, Bae I-J, Zhang H-Q, Jung H, Kim M, Chai KY, Oxadiazole- and indolocarbazole-based bipolar materials for green and yellow phosphorescent organic light emitting diodes, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108052. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical abstract

Oxadiazole- and indolocarbazole-based bipolar materials for green and yellow phosphorescent organic light emitting diodes Qiong Wu a,1 Ramanaskanda Braveenth b,1, Il-Ji Bae c, Heng-Qiang Zhang a, Hasu Jung b, Miyoung Kim c,*, and Kyu Yun Chai b,* a

Department of Chemistry and Chemical Engineering, Hebei Normal University for Nationalities, Chengde, 067-000, PR China; b

Division of Bio-Nanochemistry, College of Natural Sciences, Wonkwang University, Iksan City, Chonbuk, 570-749, Republic of Korea. c

Nano-Convergence Research Center, Korea Electronics Technology Institute, Jeonju, Korea. *

Corresponding Authors: Email: [email protected]; Tel.: +82-632-190-011 (M. Kim), Email: [email protected]; Tel.: +82-63-850-6230; Fax: +82-63-841-4893 (K.Y. Chai). 1

These authors contributed equally to this work

Abstract New bipolar materials, namely 2-phenyl-5-(4-(5-phenylindolo[3,2-a]carbazol12(5H)-yl)phenyl)-1,3,4-oxadiazole (ICz-OXD) and 2,5-bis(4-(5phenylindolo[3,2-a]carbazol-12(5H)-yl)phenyl)-1,3,4-oxadiazole (2ICz-OXD), were designed and synthesized. Tree different devices were fabricated using ICz-OXD and 2ICz-OXD as host and fluorescent materials: a green phosphorescent, yellow phosphorescent, and non-doped fluorescent OLED emitter. The yellow phosphorescent OLED device based on the 2ICz-OXD host presented good maximum current, power, and external quantum efficiencies, whose values were 47.55 cd/A, 49.80 lm/W, and 21.54%, respectively. Its efficiencies were better than those of the devices based on ICz-OXD and on the reference material 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP). The green phosphorescent OLED device with ICz-OXD revealed higher efficiencies than the device based on 2ICz-OXD. The current, power, and external quantum efficiencies based on ICz-OXD were 49.79 cd/A, 52.14 lm/W, and 16.50%, respectively. The non-doped fluorescent devices that used our bipolar materials (ICz-OXD, 2ICz-OXD) exhibited blue emission at 435 and 442 nm.

Keywords: organic indolocarbazole.

light

emitting

diodes;

oxadiazole;

bipolar

host;

1. Introduction Compared to the traditional liquid crystal display (LCD), organic light-emitting diodes (OLEDs) have become a research hotspot in organic optoelectronics due to their ability to actively emit light, lack of back light requirement, wide viewing angle, high contrast, and possibility of fabrication in flexible, stretchable, and bendable substrate [1-3]. OLED has wide applications in the field of flat panel and solid lighting, such as next generation displays [4-6]. The development of OLEDs was motivated by the characteristics of traditional fluorescent emitters, whose efficiencies were limited due to the low possibility of singlet emission. In this context, phosphorescent and thermally activated delayed fluorescent (TADF) materials might achieve 100% internal quantum efficiency (IQE) [7-10]. However, phosphorescent materials comprising noble transition metals have serious issues related to synthetic yield and costs [5, 11]. Recently, an increasing number of studies have focused on metal free dopant materials through the activation of the TADF mechanism [1215]. The TADF mechanism is enabled when a small energy difference between the singlet and triplet excited state (∆EST) is obtained [16-19]. Phosphorescent and TADF devices require suitable host materials to enhance their efficiencies [20-25]. These host materials contribute to the energy supply to the dopant material while preventing energy flow back from the dopant to the host [26-29]. The inevitable demand of organic host materials encourages researchers to design suitable and efficient host molecules. Traditional host materials are mostly of p- and n-types, and their molecules are developed by using hole and electron transport building blocks, respectively [1, 30-33]. These p- and n-based molecules are known as unipolar host materials [34, 35], which are materials affected by the narrow recombination zone. This zone leads to an imbalance in the nature of carries at the emission layer, which suppresses the energy supply and device efficiencies [36, 37]. The most common hole and electron transporting host materials are 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), respectively [38-42]. These molecules exhibit low thermal stability, which suppresses the morphological stability of the device [43]. Therefore, the development of host materials for OLED with proper balanced carriers are essential to improve the performance of the devices. Charge-carrier balance is essential for the OLED device performance. The

charge-recombination factor is dominated by the proper balance of holes and electrons at the emissive layer [44, 45]. In this context, bipolar host materials have attracted attention from the commercial and research communities. They contain both donor (hole transporting unit) and accepter (electron transporting unit) moieties of a molecule. Moreover, the use of bipolar host materials can influence the device efficiencies and simplify the OLED device structure [4651]. In addition, bipolar host materials require high triplet energy while maintaining a fast charge transportation from the adjacent layers. Bipolar materials were often reported as fluorescent emitters in non-doped OLED devices due to their balanced charge in one molecule [52-55]. The nondoped fluorescent emitters require a suitable frontier molecular orbital (FMO) energy with the adjacent layer to improve the charge injection and transportation [48, 56-61]. Therefore, bipolar materials with suitable donor and accepter should be developed to meet the above requirements. Bipolar materials based on 1,3,4-oxadiazole are of interest as an electron withdrawing moiety for OLED [62-64]. Oxadiazole derivatives are effective towards improving the injection and transport of electrons. For instance, 2-(4biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [65] and 1,3-bis[4tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7) are usually applied in OLEDs as electron-transport materials [66]. Moreover, indolocarbazole donor moiety has been identified as a rigid building block for bipolar OLED and has a high triplet of 2.96 eV [67]. In the present study, two new bipolar host materials, namely 2-phenyl-5-(4(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)phenyl)-1,3,4-oxadiazole (ICz-OXD) and 2,5-bis(4-(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)phenyl)-1,3,4oxadiazole (2ICz-OXD) were designed and synthesized with indolocarbazole donor and oxadiazole acceptor derivatives. Three different types of OLED devices were fabricated to investigate the performance of our bipolar materials. We expect better device performances by using green and yellow phosphorescent OLED devices.

2. Materials and methods 2.1 Materials The 4-bromobenzoyl chloride and benzohydrazide were purchased from TCI chemicals (Seoul, Korea). Sodium bicarbonate, sodium tert-butoxide, and N'benzoyl-4-bromobenzohydrazide were purchased from Sigma Aldrich (Seoul

Korea). Palladium acetate, tri-tert-butylphosphine, and anhydrous MgSO4 were obtained from Alfa Aesar (Seoul, Korea). POCl3 was purchased from Daejeong Chemicals (Seoul, Korea). Toluene, n-hexane, ethanol, and dichloromethane were purchased from SK Chemicals (Gyeonggi-do, Korea). Anhydrous toluene was prepared through a distillation process using benzophenone and sodium metal. Deionized water was used for all purposes in this work. Aluminumbacked silica thin-layer chromatography plates and silica gel (mesh size 200300) were purchased from Merck (Seoul, Korea). 2.2 Instrumentation A JEON JNM-ECP FT-NMR spectrometer (Peabody, MA, USA) operating at 500 MHz was used to analyze the 1H and 13C NMR data. The mass spectrometry was evaluated using a Xevo TQ-S spectrometer (Waters, Milford, MA, USA). UV absorption spectra were obtained using a Perkin-Elmer Lambda 950 spectrophotometer (Shelton, CT, USA). An FLSP920 spectrophotometer (Wales, England) was used to record the photoluminescence (PL) spectra. The highest occupied molecular orbital (HOMO) level was recorded by a CHI 660D electrochemical workstation (Austin, TX, USA). The lowest unoccupied molecular orbital (LUMO) level was estimated by adding the bandgap energy to the recorded HOMO energy value. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were obtained by using SDT 2960 and DSC 2910 TA4100 (TA Instruments, New Castle, DE, USA) instruments at a heating rate of 10 °C/min in an N2 atmosphere. OLED devices with a FMO distribution were obtained by applying the density functional theory (DFT) in a Gaussian 09 program (Wallingford, CT, USA). Current density-voltage-luminescence (JVL) efficiencies were determined by an OLED IVL test system (Polaronix M6100, Suwon, Republic of Korea). The electroluminescence (EL) spectra analysis was conducted using a spectroradiometer (Konica Minolta CS-2000, Japan). The elemental analysis was recorded by a Thermo Fisher Flash 2000 elemental analyzer (Loughborough, England).

Scheme 1: Synthetic route to starting materials 4 and 7. 2.3. Synthetic procedure 2.3.1. Synthesis of N'-benzoyl-4-bromobenzohydrazide (3, Scheme 1)

A solution of 4-bromobenzoyl chloride (1, 4.84 g, 22.03 mmol) in tetrahydrofuran (40 mL) was added dropwise to a mixture of benzohydrazide (2, 3 g, 22.03 mmol) and sodium bicarbonate (1.85 g, 22.03 mmol). After stirring at room temperature overnight, the resulting mixture was extracted with deionized water (3 × 50 mL) and dried in a vacuum oven for 8 h. The intermediate 3 was obtained as a white solid (7.01 g, yield: 99.7%) without further purification, and it was directly used in the next reaction. 1

H NMR (500 MHz, CDCl3) δ 10.65 (s, 2H), 7.85 (d, 2H), 7.75 (d, 2H), 7.64 (d, 4H), 7.49 (t, 1H). 2.3.2. Synthesis of 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (4) N'-benzoyl-4-bromobenzohydrazide (3, 2 g, 6.26 mmol) was placed in a 100 mL two-neck flask equipped with a condenser. The sample was vacuumed for 15 minutes, after which of anhydrous toluene (15 mL) and POCl3 (15 mL) were injected. Under an N2 atmosphere, the mixture was refluxed until the reaction was finished. After the reaction, the solvent was removed on a rotary evaporator under reduced pressure. Then, the mixture was recrystallized with ethanol to produce the target molecule 4 (1.75 g, yield: 93%). 1

H NMR (500 MHz, CDCl3) δ 8.15 (dd, J = 5.30, 3.40 Hz, 2H), 8.04-7.99 (m, 2H), 7.63-7.58 (m, 2H), 7.57-7.52 (m, 1H), 7.52-7.46 (m, 2H), 13C NMR (500 MHz, CDCl3) δ 144.8, 143.5, 133.6, 132.6, 132.0, 131.9, 130.0, 128.7, 128.7, 126.8 2.3.3. Synthesis of 4-bromo-N'-(4-bromobenzoyl)benzohydrazide (6, Scheme 1) 4-Bromobenzoyl chloride (1, 3 g, 13.76 mmol) was dissolved in THF (30 mL) and stirred at 0 °C for 30 minutes. Then, hydrazine monohydrate (0.28 mL, 5.44 mmol) was added dropwise to the above mixture. Subsequently, the reaction mixture was stirred for 3 h at room temperature. The resulting precipitate was filtered and washed with saturated aqueous NaHCO3 solution. Finally, the solid was dried in a vacuum oven to produce the target molecule as a white solid (1.94 g; yield: 84%). 1

H NMR (500 MHz, DMSO-d6) δ 10.65 (s, 2H), 7.88-7.83 (m, 4H), 7.75 (dd, J = 6.42, 4.53 Hz, 4H).

2.3.4. Synthesis 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (7) 4-Bromo-N’-(4-bromobenzoyl)benzohydrazide (6, 5 g, 12.56 mmol) was

placed in a 250 ml two-neck flask equipped with a condenser. The sample was vacuumed for 15 minutes, after which dry toluene (30 mL) and POCl3 (30 mL) were injected. Under an N2 atmosphere, the mixture was refluxed until the reaction was finished. After the reaction, the solvent was removed on a rotary evaporator under reduced pressure. Then, the mixture was recrystallized with ethanol to produce the target molecule 7 (4.75 g, yield: 99.5%). 1

H NMR (500 MHz, CDCl3) δ 8.00 (td, J = 8.00, 1.72, 1.72 Hz, 4H), 7.68 (td, J = 7.99, 1.70, 1.70 Hz, 4H), 13C NMR (500 MHz, CDCl3) δ 164.1,132.5, 128.4, 126.7, 122.7.

Scheme 2: Synthetic route for the target bipolar molecules ICz-OXD and 2ICzOXD.

2.3.5. Synthesis of 2-phenyl-5-(4-(5-phenylindolo[3,2-a]carbazol-12(5H)yl)phenyl)-1,3,4-oxadiazole (ICz-OXD) 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (4, 3 g, 11.62 mmol), 5-phenyl5,12-dihydroindolo[3,2-a]carbazole ICz (4.2 g, 12.78 mmol), sodium tertbutoxide (2.23 g, 23.2 mmol), and palladium acetate (0.09 g, 0.35 mmol) were added to a 500 mL two-neck flask equipped with a condenser, and vacuumed for 15 minutes. Then, dry toluene (200 mL ) was injected, and the reaction

mixture was reflexed under constant stirring. Next, tri-tert-butylphosphine (3.2 mL ,10% in toluene) was injected to the mixture. After the reaction was complete, the mixture was extracted with dichloromethane (3 × 100 mL) and deionized water (350 mL). Subsequently, the organic layer was dried over anhydrous MgSO4, and concentration on a rotary evaporator was performed under reduced pressure. Finally, the crude residue was purified by silica column chromatography to produce the ICz-OXD (5.75 g). Yield of 85.5%; White solid; 1H NMR (500 MHz, CDCl3) δ 8.49-8.41 (m, 2H), 8.26-8.12 (m, 4H), 7.87-7.79 (m, 2H), 7.69-7.53 (m, 8H), 7.47 (d, J = 7.49 Hz, 1H), 7.42-7.33 (m, 4H), 7.28-7.21 (m, 1H), 6.89-6.81 (m, 1H), 6.27 (d, J = 8.15 Hz, 1H); 13C NMR (500 MHz, CDCl3) δ 165.0, 164.1, 143.5, 141.8, 141.6, 140.7, 137.7, 136.3, 132.0, 130.0, 129.3, 129.2, 128.5, 128.1, 127.150, 125.102, 124.705, 124.6, 123.9, 123.4, 123.3, 121.1, 121.1, 119.6, 119.2, 118.5, 118.0, 110.1, 109.5, 108.0, 104.4. MS (APCI): 552.64 for C38H24N4O [M+H+]; Anal. Calcd for C38H24N4O (%): C, 82.59; H, 4.38; N, 10.14. Found: C, 82.71; H, 4.40; N, 10.25. 2.3.6. Synthesis of 2,5-bis(4-(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)phenyl)1,3,4-oxadiazole (2ICz-OXD) 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (7, 2.5 g, 6.58 mmol), 5-phenyl5,12-dihydroindolo[3,2-a]carbazole ICz (4.8 g, 14.47 mmol), sodium tertbutoxide (2.5 g, 26.3 mmol), and palladium acetate (0.10 g, 0.44 mmol) were added to a 500 mL two-neck flask equipped with a condenser. Then, the sample was vacuumed for 15 minutes, and dry toluene (200 mL) was added under stirring. Next, tri-tert-butylphosphine (3.8 mL ,10% in toluene) was injected to the mixture, which was stirred and refluxed for 12 hours. After the reaction was complete, the mixture was cooled to room temperature and extracted three times with dichloromethane (100 mL) and deionized water (250 mL). Subsequently, the organic layer was dried over anhydrous MgSO4, then filtered and concentrated on a rotary evaporator under reduced pressure. Finally, the residue was purified by a silica column chromatography to produce 4.68 g of 2ICzOXD. Yield of 80.7%; Yellow solid; 1H NMR (500 MHz, CDCl3) δ 8.51-8.47 (m, 4H), 8.20-8.15 (m, 4H), 7.88-7.83 (m, 4H), 7.68-7.63 (m, 4H), 7.63-7.59 (m, 4H), 7.57-7.51 (m, 2H), 7.47 (d, J = 7.48 Hz, 2H), 7.43-7.33 (m, 8H), 7.30-7.20 (m, 2H), 6.90-6.82 (m, 2H), 6.28 (d, J = 8.03 Hz, 2H); 13C NMR (500 MHz, CDCl3) δ 164.5, 143.7, 141.9, 141.6, 140.7, 137.7, 136.3, 130.0, 129.3, 128.6, 128.1, 125.1, 124.7, 124.6, 123.3, 123.2, 121.1, 119.6, 119.2, 118.5, 118.1, 116.9,

110.1, 109.6, 108.0, 104.4, 100.0, 96.69. MS (APCI): 883.70 for C62H38N6O [M+H+]; Anal. Calcd for C62H38N4O (%): C, 84.33; H, 4.34; N, 9.52. Found: C, 83.51; H, 4.49; N, 9.16.

2.4. OLED fabrication and characterization To fabricate the OLED devices, indium-tin-oxide (ITO) coated glass substrates were cleaned in an ultrasonic bath with isopropyl alcohol and deionized for 20 minutes. The cleaned substrates were then dried using inert gas, and they were subjected to a UV-ozone treatment for 6 min. All organic layers and a metal cathode were deposited on the pre-cleaned ITO glass. A deposition pressure of ∼5×10−7 Torr was applied using a Sunic organic evaporator (Suwon, Korea). All deposition processes were conducted inside a nitrogen-filled glove box. The constructed device area was set as 2 mm2. 3. Results and discussion The thermal properties of the bipolar materials were analyzed by using DSC and TGA (Figure 1). For 10% weight reduction, the thermal degradation temperature of ICz-OXD and 2ICz-OXD were 468.72 and 474.61 °C, respectively. The glass transition temperature of ICz-OXD and 2ICz-OXD were 131.30 and 173.17 °C, and the melting points were 240.46 and 333.46 °C, respectively. 2ICz-OXD bipolar molecule with two substituted indolocarbazole donor exhibited better thermal stability compared to a single ICz-OXD substituted. 0.0

ICz-OXD 2ICz-OXD

100

131.30 C

+ -1.0

+

o

173.17 C

Heat Flow (W/g)

80

Weight (%)

ICz-OXD 2ICz-OXD

o

-0.5

60

40

+

o

333.46 C

-1.5 -2.0 -2.5 -3.0 -3.5

20

o

+ 240.46 C

-4.0

100

200

300

400

500 o

Temperature ( C)

600

700

800

100

200

300

400

o

Temperature ( C)

Figure 1. Differential scanning calorimetry and thermal gravimetric analysis of ICz-OXD and 2ICz-OXD.

Figure 2 shows the UV-vis absorption and photoluminescent (PL) spectra of

ICz-OXD and 2ICz-OXD in the THF solution and film state. The absorption and PL emission data are summarized in Table 1. In the film state, ICz-OXD and 2ICz-OXD showed absorption at 280, 346, 360, and at 292, 349, and 365 nm, respectively. The identical UV-vis absorption of both molecules around 346-365 nm can be attributed to the intramolecular charge transfer between indolocarbazole donor and oxadiazole acceptor moieties. The UV-vis absorption in the solution and film states did not show any significant difference of absorption pattern, and they were identical to each other. The band gap energy of ICz-OXD and 2ICz-OXD were recorded at 3.25 and 3.10 eV, respectively, and they were calculated from the onset UV-vis absorption at 381 and 400 nm, respectively. PL emission was observed at 447 and 452 nm for ICz-OXD and 2ICz-OXD, respectively. The indolocarbazole donor with one site attached to the molecule of ICz-OXD showed a blue shifted emission when compare to two-sites attached 2ICz-OXD. The PL spectra of ICz-OXD and 2ICz-OXD in solution was more red-shifted than that of the film state (456 and 459 nm, respectively). However, the peak patterns were similar to each other. The triplet energy was 2.77 and 2.74 eV for ICz-OXD and 2ICz-OXD, respectively. Therefore, ICz-OXD and 2ICz-OXD are suitable as host materials for green and yellow phosphorescent OLEDs. The photoluminescent quantum yield in solution were 15.6 and 20% for ICz-OXD and 2ICz-OXD, respectively. The higher triplet energy of host materials will support an effective energy transfer to the dopant. ICz-OXD neat film ICz-OXD in THF 2ICz-OXD neat film 2ICz-OXD in THF

1.0

Normalized Intensity

Absobance (a.u.)

0.8

ICz-OXD neat film 2ICZ-OXD neat film ICz-OXD in THF 2ICz-OXD in THF

1.0

0.6

0.4

0.2

0.8

0.6

0.4

0.2

0.0 0.0

300

400

Wavelength (nm)

500

300

400

500

600

700

800

Wavelength (nm)

Figure 2. UV-Vis absorption and photoluminescent spectra of ICz-OXD and 2ICz-OXD. The electrochemical analysis was performed by using a cyclic voltammetry measurement (Figure 3). Identical HOMO energy levels were recorded for ICzOXD and 2ICz-OXD (-5.60 and -5.59 eV, respectively). LUMO energy levels were calculated by adding the bandgap energy to the HOMO energy level. The LUMO energy level of ICz-OXD and 2ICz-OXD were -2.25 and -2.49 eV, respectively. Higher HOMO and LUMO energy levels of both materials could

help enable the charge hopping pathway from both electrodes, thus contributing to a proper charge recombination at the emission layer.

Table 1. Physical properties of ICz-OXD and 2ICz-OXD. Materials

UV absorption (nm)

PL emission (nm)

Neat film

THF

Neat film

THF

PLQY (%) (Toluene)

ICz-OXD

280, 346, 360

283, 357, 360

447

456

15.6

2ICz-OXD

292, 349, 365

280, 346, 360

452

459

20.0

1.2

ICz-OXD 2ICz-OXD

1.0

Current(a.u.)

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Potential(V vs Ag/AgCl)

Figure 3. Cyclic voltammetry measurements of ICz-OXD and 2ICz-OXD. To understand the electronic properties of the bipolar molecules, we performed a DFT calculation for both ICz-OXD and 2ICz-OXD molecules by using Gaussian 09 (Figure 4). The calculated physical properties are summarized in Table 2. A lower dihedral angle of 46o was noticed between the donor and acceptor of both molecules. The HOMO distribution of both molecules was observed in the indolocarbazole donor moiety, whereas LUMO was distributed over the oxadiazole acceptor moiety. Although both molecules revealed clear separation between donor and acceptor units, the energy difference between singlet and triplet was 0.65 and 0.62 eV for ICz-OXD and 2ICz-OXD, respectively. Higher energy difference between singlet and triplet states does not support any effective reverse intersystem crossing (RICS) mechanism of TADF. The higher calculated triplet energy around 3.2 eV will provide an effective energy transfer to the phosphorescent dopant of the OLED device. The

calculated HOMO values for ICz-OXD and 2ICz-OXD were -5.52 and -5.54 eV, respectively.

Table 2. Physical properties and calculated values of ICz-OXD and 2ICz-OXD. Materials

S1 (eV)

T1 (eV)

∆EST (S1-T1) eV

ICz-OXD 2ICz-OXD

3.93 3.82

3.27 3.20

0.65 0.62

D-A rotation angle (o) 46.37 46.14

HOMO LUMO (eV) (eV) -5.52 -5.54

-2.13 -2.24

Band gap (eV) 3.39 3.29

Figure 4. Frontier molecular orbital distribution of ICz-OXD and 2ICz-OXD. To evaluate the electroluminescent characteristics of our bipolar host materials, various multilayer OLED devices were constructed with different phosphorescent dopant materials. The yellow device structure was as follows: indium tin oxide (ITO) (150 nm); 1,4,5,8,9,11Hexaazatriphenylenehexacarbonitrile (HATCN) (7 nm); 4,4′Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) (43 nm); host: 10 wt% Iridium(III) bis(4-(4-tert-butylphenyl) thieno[3,2-c]pyridinatoN,C2') acetylacetonate (PO-O1) (20 nm); 1,3,5-Tri(m-pyridin-3ylphenyl)benzene (TmPyPB) (35 nm); 8-Quinolinolato lithium (Liq) (1.5 nm);

and Al (100 nm). The green device structure included: ITO (150 nm); HATCN (7 nm); TAPC (43 nm); host: 5 wt% Tris[2-phenylpyridinato-C2,N] iridium(III) (Ir(ppy)3) (20 nm); TmPyPB (35 nm); Liq (1.5 nm); and Al (100 nm). ITO and Al were used for the anode and cathode, HATCN as the hole injecting layer (HIL), TAPC as the hole transporting layer (HTL), TmPyPB for the electron transporting layer (ETL), and Liq for the electron injecting layer (EIL). PO-O1 and Ir(ppy)3 were applied as yellow and green phosphorescent emitters, respectively. To evaluate the yellow phosphorescent OLED performance, we used the CBP host material for the reference device, which presented a similar structure. The device structures are summarized in Figure 5.

Figure 5. Device structure of yellow, green, and non-doped OLEDs. The current density-voltage and luminescence voltage performances are summarized in Table 3 and depicted in Figure 6. The turn-on voltage of the yellow phosphorescent device based on ICz-OXD and on 2ICz-OXD was 3 V, which is lower than that of the reference CBP-based device. The power efficiency of the yellow devices based on ICz-OXD and 2ICz-OXD was 48.71 and 49.80 lm/W, respectively. The current efficiency of the device based on 2ICz-OXD was higher (47.55 cd/A) than the 46.91 cd/A of the reference device. The bipolar materials ICz-OXD and 2ICz-OXD exhibited excellent external quantum efficiencies, 19.59 and 21.54%, respectively. These values are significantly better than that of the reference device (16.58%), which

demonstrates that the devices based on our bipolar materials present an effective energy transfer mechanism. The lower turn-on voltage and higher device performance are supported by the effective charge transportation of our new bipolar materials. The performance of the green PhOLED device based on ICz-OXD host material was better than the one based on the two-site molecule 2ICz-OXD (Figure 7). The current and power efficiencies of ICz-OXD and 2ICz-OXD were 49.79 and 44.44 cd/A, and 52.14 and 46.53 lm/W, respectively. Nevertheless, both materials presented the same turn-on voltage of 3 V. The external quantum efficiencies of the green devices based on ICz-OXD and 2ICz-OXD were 16.50 and 16.36%, respectively. Thus, there was no significant difference in external quantum efficiencies. The higher triplet energy of ICz-OXD (2.77 eV) provided an effective energy flow to the green dopant when compared to that of the 2ICzOXD (2.74 eV).

60 CBP ICz-OXD 2ICz-OXD

Current efficiency (cd/A)

50

50

40

40

30

30

20

20

10

10

0 0

20

40

60

80

Power efficiency (lm/W)

60

0 100

2

Current density (mA/cm )

25

CBP ICz-OXD 2ICz-OXD

20

EQE (%)

15

10

5

0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Voltage (V)

Figure 6. Current density-voltage and luminescence-voltage, and external quantum efficiencies of yellow phosphorescent OLEDs.

60

60 CBP ICz-OXD

50

2 ICz-OXD

40

40

30

30

20

20

10

10

0

Power efficiency (lm/W)

Current efficiency (cd/A)

50

0 0

20

40

60

80

100

2

Current density (mA/cm )

20

ICz-OXD 2ICz-OXD

18 16 14

EQE (%)

12 10 8 6 4 2 0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Row Numbers

Figure 7. Current density-voltage and luminescence-voltage, and external quantum efficiencies of green phosphorescent OLEDs. The electroluminescent spectra of yellow and green phosphorescent OLEDs are depicted in Figure 8. The yellow phosphorescent OLED device presented a maximum peak at 563 nm, and its peak pattern was the same than the one from the reference device. The CIE coordinate values were recorded at (x, y) (0.50, 0.49), (0.49,0.48), and (0.49,0.48) for devices based on CBP, ICz-OXD, and 2ICz-OXD, respectively. The electroluminescent spectra of the green phosphorescent OLEDs was observed at 521 nm, and both devices presented a similar peak.

Figure 8. Electroluminescent spectra of green and yellow phosphorescent OLEDs.

The bipolar materials ICz-OXD and 2ICz-OXD showed excellent device properties with green and yellow phosphorescent OLEDs. ICz-OXD materials revealed better performances with green PhOLED, whereas 2ICz-OXD showed better properties with yellow PhOLEDs. The triplet energy of ICz-OXD and 2ICz-OXD were higher than that of green (2.42 eV) and yellow (2.2 eV) dopants. These higher triplet energy levels suppress the energy flow back from the dopant and improve the device efficiency. The low turn-on voltage of devices can be attributed to the proper FMO energy alignment with adjacent layers. As a result, the charge transportation from the electrodes and recombination of charges at the emission layer was facilitated. Moreover, we fabricated non-doped fluorescent devices to understand the emission properties of ICz-OXD and 2ICz-OXD (Figure 9). The device structure presented: ITO (150 nm); HATCN (7 nm); TAPC (43 nm); emission layer (20 nm); TmPyPB (35 nm); Liq (1.5 nm); and Al (100 nm). The nondoped device based on ICz-OXD exhibited lower current and power efficiencies (1.38 cd/A and 1.24 lm/W) when compared to those of 2ICz-OXD (1.89 cd/A, 1.49 lm/W). Nevertheless, the external quantum efficiency of devices based on ICz-OXD (1.77%) was higher than those based on 2ICz-OXD (1.34%). Moreover, ICz-OXD presented a lower turn-on voltage of 3.5 V, compared to 4.0 V of 2ICz-OXD. 2.0

2.0 ICz-OXD

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0 0

50

100

150 2

Current density (mA/cm )

200

Power efficiency (lm/W)

Current efficiency (cd/A)

2ICz-OXD

3.0

ICz-OXD 2ICz-OXD

2.5

EQE (%)

2.0

1.5

1.0

0.5

0.0 3

4

5

6

7

8

Voltage (V)

Figure 9. Current density-voltage and luminescence-voltage, and external quantum efficiencies (EQE) of non-doped fluorescent OLEDs. Overall, the bipolar materials employed with non-doped devices did not produce significant performances. Non-doped devices based on ICz-OXD presented a blue shifted emission (CIE (x, y) = 0.16, 0.12) compared to those based on 2ICz-OXD (0.19, 0.20) (Figure 10). The peak maxima of the electroluminescent spectra were observed at 435 and 442 nm for ICz-OXD and 2ICz-OXD, respectively. Both bipolar materials exhibited emissions in the blue wavelength region. This pure emission from the materials did not have any influence from the host material. Table 3. Characteristics of phosphorescent and fluorescent devices based on ICz-

OXD and 2ICz-OXD.

Yellow device

Device characteristics

Green device

Non-Doped device

Reference CBP

ICzOXD

2ICzOXD

ICzOXD

2ICzOXD

ICzOXD

2ICzOXD

Turn-on voltage (V)

3.5

3.0

3.0

3.0

3.0

3.5

4.0

Current efficiency (Cd/A)

46.91

46.52

47.55

49.79

44.44

1.38

1.89

Power efficiency (lm/W)

42.11

48.71

49.80

52.14

46.53

1.24

1.49

EQE (%)

16.58

19.59

21.54

16.50

16.36

1.77

1.34

CIE (x, y)

0.50, 0.49

0.49, 0.48

0.49, 0.48

0.34, 0.61

0.34, 0.59

0.16, 0.12

0.19, 0.20

1.2 ICz-OXD 2ICz-OXD

Normalized EL intensity

1.0

0.8

0.6

0.4

0.2

0.0 400

450

500

550

600

650

Wavelength (nm)

Figure 10. Electroluminescent spectra of non-doped fluorescent OLEDs.

In our previous work, we incorporated an oxadiazole acceptor into the

phenothiazine donor moiety for better external quantum efficiencies in a nondoped device (2PTZ-OXD = 3.99%). As a result, we obtained a higher dihedral angle (78o) between donor and acceptor. However, the device electroluminescent spectra revealed a bathochromic emission of 550 nm due to the phenoxazine donor [68]. In the present work, we employed an indolocarbazole donor and oxadiazole acceptor. They presented a control in the emission color towards blue. However, they could not enhance the device efficiency. Therefore, it can be concluded that a low dihedral angle between the donor and acceptor does not produce an effective intramolecular charge transfer to improve the device efficiency.

4. Conclusion Two bipolar materials were constructed with an oxadiazole acceptor and indolocarbazole donor and applied in phosphorescent and non-doped fluorescent OLEDs. The green and yellow phosphorescent OLEDs with ICzOXD and 2ICz-OXD presented a turn-on voltage of 3.0 V, which is lower than that of the reference CBP-based device (3.5 V). The yellow device with 2ICzOXD revealed an excellent external quantum efficiency of 21.54%, which is significantly higher than that of the CBP-based reference device (16.58%). The current and power efficiencies of the green device based on ICz-OXD were 49.79 cd/A and 52.14 lm/W, respectively. Both bipolar materials exhibited excellent device performances as host materials on green and yellow phosphorescent OLEDs. However, these materials did not enhance the device performance as non-doped fluorescent emitter, despite the presence of blue emission. Therefore, oxadiazole and indolocarbazole bipolar materials are more suitable as host materials for phosphorescent OLEDs. Acknowledgement The authors are thankful for the financial support from the Hebei Education Department Science Foundation (No. QN2018313) and the Science Foundation of Hebei Normal University for Nationalities (No. QN2018005). This research also was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the education (NRF-2016R1D1A3B01015531).

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Highlights •

Maximum current efficiency achieved 49.79 cd/A for ICz-OXD based green PhOLED.

•

Excellent external quantum efficiency of 21.54 % obtained with 2ICz-OXD based yellow PhOLEDs.

•

The bipolar materials had good thermal stabilities.

•

Low turn on voltage of 3 V recorded.

•

Oxadiazole and indolocarbazole bipolar materials are suitable as host moieties for phosphorescent OLEDs.

11th November 2019 Dear Editor,

I would like to resubmit the manuscript entitled “Oxadiazole- indolocarbazolebased bipolar materials for green and yellow phosphorescent organic light emitting diodes” by our group for publication as article in the Dyes and Pigments journal.

We declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We know of no conflict of interest towards this publication.

Sincerely, Professor. Kyu Yun Chai Division of Bio-Nanochemistry, Wonkwang University Iksan, 570-749, Republic of Korea Tel: +82-63-850-6230/ Fax: 82-63-841-4893 Email: [email protected]