Triphenylsilane-substituted arenes as host materials for use in green phosphorescent organic light emitting diodes

Author’s Accepted Manuscript Triphenylsilane-substituted Arenes as Host Materials for use in Green Phosphorescent Organic Light Emitting Diodes Jwajin Kim, Kum Hee Lee, Young Seok Kim, Hyun Woo Lee, Ho Won Lee, Young Kwan Kim, Seung Soo Yoon www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(15)30050-8 http://dx.doi.org/10.1016/j.jlumin.2015.10.056 LUMIN13679

To appear in: Journal of Luminescence Received date: 2 June 2015 Revised date: 20 October 2015 Accepted date: 24 October 2015 Cite this article as: Jwajin Kim, Kum Hee Lee, Young Seok Kim, Hyun Woo Lee, Ho Won Lee, Young Kwan Kim and Seung Soo Yoon, Triphenylsilanesubstituted Arenes as Host Materials for use in Green Phosphorescent Organic Light Emitting Diodes, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.10.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Triphenylsilane-substituted Arenes as Host Materials for use in Green Phosphorescent Organic Light Emitting Diodes.

Jwajin Kim1, Kum Hee Lee1, Young Seok Kim1, Hyun Woo Lee1, Ho Won Lee2, Young Kwan Kim2* and Seung Soo Yoon1*

1

Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea

2

Department of Information Display, Hongik University, Seoul, 121-791, Korea

*Corresponding author. 1

Tel.: +82 31 290 7071; Fax: +82 31 290 7075.

2

Tel.: +82 2 320 1646; Fax: +82 2 3141 8928.

E-mail address: [email protected]; [email protected].

1

Abstract We demonstrated triphenylsilane-substituted arenes (1-4) as host materials for green phosphorescent organic light-emitting diodes. Particularly, a device using 9,9-dimethyl2-(triphenylsilyl)-7-[4-(triphenylsilyl)phenyl]-9H-fluorene (compound 4) as the host material

with

the

green

phosphorescence

dopant

bis[2-(1,1',2',1''-terphen-3-

yl)pyridinato-C,N]iridium(III)(acetylacetonate) showed the efficient green emission with an external quantum efficiency of 4.64 %, a power efficiency of 7.2 lm/W and luminous efficiency of 16.6 cd/A at 20 mA/cm2, respectively, with the Commission International de L’Eclairage chromaticity coordinates of (0.33, 0.59) at 8.0 V.

Keywords: Phosphorescent Organic Light-Emitting Diodes; Triphenylsilane; Suzuki cross-coupling reaction

2

1. Introduction Phosphorescent organic light-emitting diodes (PHOLEDs) containing transition metal complexes as emitters have attracted much attentions, because it can achieve increased internal quantum efficiency of the devices up to 100 % by harvesting both single and triplet excitons.1 However, there are problems caused by the harvested triplet excitons, such as triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) that reduce the efficiency of diodes. Therefore, in order to prevent these efficiency decrement problems in neat film state, PHOLEDs usually use a host-dopant system.2,3 To achieve high efficiency from host-dopant system PHOLEDs, the host material should meet the following requirements: (i) the triplet energy (ET) of the host material must be higher than that of the dopant to ensure efficient confinement of exciton;4,5 (ii) the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of host material need to be well-matched with the HOMO energy level of hole transporting layer (HTL) and the LUMO energy level of electron transporting layer (ETL) to obtain charge carriers from neighboring layer; and (iii) there must be balanced and efficient carrier transporting properties to get a well-define recombination zone within the emitting layer (EML) and to decrease the efficiency rolloff.

3

Several classes of host materials for PHOLEDs have been reported, and the most popular core structures which can meet the requirements of host material are arylsilane derivatives.6-14 Arylsilane derivatives are known as host materials to have high energygap

with

triplet

energies

larger

than

those

of

carbazole

derivatives.

Bis(triphenylsilyl)arene derivatives such as 4,4’-bis(triphenylsilyl)-biphenyl (BSB, compound 1)15 and 1,4-bis(triphenylsilyl)benzene (UGH2)16 were developed as host materials for blue phosphorescence emitters. However, their electroluminescence (EL) performances still need to be improved for the practical applications. In this paper, we report three bis(triphenylsilanyl)arene derivatives as host materials for PHOLEDs to produce the high efficient phosphorescent devices. Triphenylsilane units were introduced to enhance thermal stability and to increase energy bandgap of host materials17-19. Compared to the known 4,4’-bis(triphenylsilyl)-biphenyl (1), triphenylsilane groups in compounds 2 and 3 are introduced in different positions on biphenyl-backbone. In compound 4, one phenyl group of biphenyl-backbone are replaced to 9,9’-dimethylfluorenyl unit to investigate the systematic information on the effect of the structural changes in the emitters on their EL performances.

2. Experimental details

4

2.1 General information

All solvents were dried using standard procedures and all reagents were used as received from commercial sources. All reactions were performed under a N2 atmosphere. 4,4'-Dibromobiphenyl and chlorotriphenylsilane were used as received from Aldrich or TCI.

(3-Bromophenyl)triphenylsilane,20

triphenyl[3-(4,4,5,5-tetramethyl-1,3,2-

dioxaborolan-2-yl)phenyl]silane,21 triphenyl[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)phenyl]silane,22 (2-bromo-9,9-dimethyl-9H-fluoren-7-yl)triphenylsilane,23 and 4,4'bis(triphenylsilyl)biphenyl (1)15 were prepared by using a reported method. The 1H- and 13

C- NMR spectra were recorded on a Varian Unity Inova 300Nb spectrometer. FT-IR

spectra were recorded using a Bruker VERTEX70 FT-IR spectrometer. Elemental analysis (EA) was measured using an EA-1108 spectrometer. The low resolution mass spectra were measured using a Jeol JMS-AX505WA spectrometer in FAB mode and a JMS-T100TD (AccuTOF-TLC) in positive ion mode. The UV-Vis absorption and photoluminescence spectra of the newly designed host materials were measured in a CH2Cl2 solution (10-5 M) using a Shimadzu UV-1650PC and AMINCO-Bowman Series 2 luminescence spectrometer. The ionization potentials (or HOMO energy levels) of the compounds were measured using a low-energy photo-electron spectrometer (Riken-

5

Keiki, AC-2). The energy bandgaps were determined from the intersection of the absorption and photoluminescence spectra. The thermal properties were measured by thermogravimetric analysis (TGA) (DTA-TGA, TA-4000) and differential scanning calorimetry (DSC) (Mettler Toledo; DSC 822) under N2 at a heating rate of 10 oC/min. A heat-cold-heat method was used with an initial heating rate of 10 oC/min, rapidly quenched-cooled in liquid nitrogen, and finally heated at rate of 10 oC/min.

General procedure for the Suzuki cross-coupling reaction Triphenyl[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]silane (1.2mol) and the corresponding aryltriphenylsilyl bromide derivatives (1.0 mol), Pd(PPh3)4 (0.04 mol), aqueous 2.0 M K2CO3 (10.0 mol), Aliquat 336 (0.1 mol), and toluene were mixed in a flask, and heated under reflux for 4 h. After the reaction was complete, the reaction mixture was extracted with ethyl acetate and washed with water. The organic layer was dried with anhydrous MgSO4 and filtered with silica gel. The solution was then evaporated and the crude product was recrystallized from CH2Cl2/EtOH.

3,3’-Bis(triphenylsilyl)biphenyl (2). Yield = 53%. 1H NMR (300 MHz, CDCl3): δ ppm 7.75 (s, 2H), 7.57-7.50 (m, 14H), 7.43-7.31 (m, 22H);

6

13

C NMR (75 MHz, CDCl3): δ

ppm 140.8, 136.6, 135.6, 135.2, 134.3, 129.9, 128.8, 128.5, 128.3, 128.1; FT-IR (KBr, cm-1): 3015, 2970, 2946, 1435, 1366, 898, 786, 698; MS (FAB+, m/z): 670 [M+]; Anal. Calcd: C, 85.92; H, 5.71. Found: C, 83.96; H, 5.64.

3,4’-Bis(triphenylsilyl)biphenyl (3). Yield = 56%. 1H NMR (300 MHz, CDCl3): δ ppm 7.85 (s, 1H), 7.69-7.54 (m, 18H), 7.47-7.35 (m, 19H);

13

C NMR (75 MHz, CDCl3): δ

ppm 142.4, 137.1, 136.6, 135.9, 135.2, 134.4, 134.3, 129.9, 129.8, 128.7, 128.6, 128.2, 128.1, 126.8; FT-IR (KBr, cm-1): 3015, 2970, 2946, 1435, 1366, 899, 787, 699; MS (FAB+, m/z): 670 [M+]; Anal. Calcd: C, 85.92; H, 5.71. Found: C, 84.81; H, 5.65.

9,9-dimethyl-2-(triphenylsilyl)-7-[4-(triphenylsilyl)phenyl]-9H-fluorene (4). Yield = 54%. 1H NMR (300 MHz, CDCl3): δ ppm 7.79 (d, J = 7.9 Hz, 2H), 7.74 (d, J = 7.6 Hz, 4H), 7.67 (s, 6H), 7.63-7.60 (m, 12H), 7.47-7.39 (m, 16H), 1.47 (s, 6H). 13C NMR (75 MHz, CDCl3): δ ppm 153.4, 137.2, 136.7, 135.7, 134.8, 134.5, 133.1, 130.6, 128.9, 129.8, 128.2, 128.1, 126.9, 126.5, 121.7, 120.9, 119.8, 47.2, 27.4. FT-IR (KBr, cm-1): 3015, 2970, 2946, 1435, 1366, 898, 785; MS (FAB+, m/z): 786 [M+]; Anal. Calcd: C, 85.97; H, 5.89. Found: C, 86.73; H, 5.90.

7

2.2 Device fabrication and characterization

OLEDs using green-light-emitting molecules were fabricated by vacuum (5×10-7 torr) thermal evaporation onto pre-cleaned ITO coated glass substrates. The structure was as follows: ITO/ N,N'-diphenyl-N,N'-(1-napthyl)-(1,1'-phenyl)-4,4'-diamine (NPB)(50nm)/ 4,4',4''(-tris(N-carbazole)triphenylamine

(TcTa)(10nm)/

(tphpy)2Ir(acac)(8%):host(30nm)/ 4,7-diphenyl-1,10-phenanthroline (Bphen)(30nm)/ lithium quinolate (Liq)(2nm)/ Al(100nm). The current density (J), luminance(L), LE, PE and CIE chromaticity coordinates of the PHOLEDs were measured with a Keithly 2400 Chroma meter CS-1000A. The EL was measured using a Roper Scientific Pro 300i instrument.

3. Results and discussion

3.1. Synthesis of triphenylsilane derivatives

Triphenylsilane derivatives were synthesized with a yield of 53-56%. As described in Scheme 1, product 1 were synthesized from 4,4’-bis(triphenylsilyl)-biphenyl and

8

chlorotriphenylsilane by organolithium reagent in THF. Products 2–4 were synthesized by Suzuki coupling from the corresponding boronic esters and corresponding bromides in the presence of a palladium catalyst Pd(PPh3)4 and a K2CO3 base. The products were characterized by 1H- and 13C-NMR, mass spectrometry, and element analysis.

3.2. Physical and photophysical properties of compounds

The thermal stability and morphological property of compounds 1–4 were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively (Table 1). The decomposition temperature (Td), corresponding to 5% weight loss, were ranged 246-399 oC, and the glass transition temperatures (Tg) were varied from 65 oC to 103 oC. It is clear that the thermal and morphological stability closely correlate to the molecular weights. Particularly, in compound 4, the introduction of the fluorene moiety greatly improves the Td and Tg as compared to those of aryl silanes (1–3) without fluorene unit, and thus these high thermal stability of compound 4 gives morphological stability in device. Photophysical properties of compounds 1–4 were analyzed using UV-visible and photoluminescence (PL) spectrometers. Fig. 1 shows UV-visible spectra and PL spectra

9

of compounds 1–4. Comparing with biphenyl derivatives (1–3), compound 4 exhibited red-shifted UV-visible absorption and PL emission spectra due to the increased πconjugation length by replaced fluorene unit. Triplet energy of compounds 1–4, which were determined from the first phosphorescent emission peaks at 77 K, were 2.56, 2.71, 2.67 and 2.73 eV, respectively. These values are higher than the 2.43 eV triplet energy of the (typhpy)2Ir(acac) dopant.24 Furthermore, triplet energy of compounds 1–4 were higher than CBP (ET = 2.56 eV), which is commonly used as a host material for green PHOLEDs. The high ET values of compounds 1–4 are sufficient for green PHOLEDs host materials, and it fulfills the condition required for good host materials in PHOLEDs.

3.3. Performance of electroluminescence devices

A series of green EL devices A–D were fabricated using (tphpy)2Ir(acac) as the dopant in the EML and compounds 1–4 as the host material. The energy diagram of materials selected in each layers of the device is described in Fig 2. Devices A–D are consisted of the following layer structure: Indium tin oxide (ITO)/ N,N'-diphenyl-N,N'-(1-napthyl)(1,1'-phenyl)-4,4'-diamine

(NPB)(50nm)/

10

4,4',4''-tris(N-carbazole)triphenylamine

(TcTa)(10nm)/

(tphpy)2Ir(acac)(8%):host(30nm)/

4,7-diphenyl-1,10-phenanthroline

(Bphen)(30nm)/ lithium quinolate (Liq)(2nm)/ Al(100nm). NPB and TcTa were chosen as the hole transporting layers (HTLs) and compounds 1–4 were used as the host materials. Bphen was selected as the electron transporting layer (ETL), and Al as the cathode. The EL spectra of (tphpy)2Ir(acac) doped devices A–D are plotted in Fig. 3 and their EL performances data are listed in Table 2. In all devices, the major emission peaks were observed at 524-527 nm, in green emission range, respectively. This implies that the emission of devices A–D originated from triplet excitons of dopant. However, in EL spectra of all devices, unexpected peaks arise in range about 450 nm, which considered emission came from the exciplex formed at the interface between the EML and the ETL due to over-populated hole carriers in the EML.25 Particularly, compared to the other devices A–C, device D showed the most significant peaks in range about 450 nm and thus EL of device D showed the color impurity. Presumably, the hole carriers in the EML were populated highly in device D in comparison with the other devices A–C, due to the smaller HOMO energy difference between HTL and compound 4 than those between HTL and compounds 1–3, and thus the more effective hole injection from HTL to EML in device D than those in devices A–C. The highly populated hole carriers in EML of device D would contribute to the significant peaks in range about 450 nm of EL

11

spectrum of device D. Turn-on voltages of devices A–D, which were determined at 1.0 cd/m2, were 3.23, 5.26, 4.23 and 3.24 V, respectively. Fig. 4 showed (a) current density-voltage-luminance (J-V-L) curve, (b) luminance efficiency (LE) and power efficiency (PE) versus current density and (c) the external quantum efficiency (EQE) versus current density of devices A–D. While devices A–C using compounds 1–3 as the host show moderate performance, device D shows higher efficiencies than those of devices A–C. A device D had an EQE of 4.64 %, a PE of 7.2 lm/W and LE of 16.6 cd/A at 20 mA/cm2, respectively, with the Commission International de L’Eclairage (CIE) chromaticity coordinates of (0.33, 0.59) at 8.0 V. The improvement of efficiencies of device D in compared with devices A–C could be explained based on (i) the relatively well-matched HOMO energy level with HTL and (ii) the well-overlapped emission spectrum of host with the absorption spectrum of dopant. Firstly, compared to the energy gap between the HOMO energy level of compound 4 and that of TcTa, compounds 1–3 have too large energy gap and thus holeinjections from TcTa to host materials 1–3 in devices A–C are very difficult. On the other hand, in device D, holes move easily from HTL to host materials in EML, and thus it would lead the increment of EL efficiencies in device D. Secondly, a singlet metal-to-

12

ligand charge transfer (1MLCT) absorption band of dopant was observed at 350-450 nm, broadly.24 A singlet emission band of compound 4 was located in range 350-400 nm, while those of compounds 1–3 were observed in range 300-350 nm. This implies that the Förster energy transfer from compound 4 to the dopant occurs more effectively than that from other host materials (1–3). Transferred singlet excitons of dopants are converted to triplet excitons by spin-orbit interaction originated from heavy atom effect of Ir. This is an important pathway of achieving the better EL efficiencies of device D compared with devices A–C.26 Furthermore, compound 4 has the higher Tg than those of compounds 1–3 and thus device D would have the improved morphological stability of EML in comparison with devices A–C. Presumably, this morphological stability of EML in device D would contribute the improved EL efficiencies of device D. In devices A–C, the external quantum efficiencies at 20 mA/cm2 increase in the order of B < C < A. Furthermore, the turn-on voltages of devices A–C decrease in the order of B > C > A. Interestingly, Tg values of host materials 1–3 in the corresponding devices A–C increase in the order of 2 < 3 < 1. As a result, compound 2 showed the lowest Tg value in compounds 1–4 and thus device B showed the highest turn-on voltage and largest efficiency roll-off in devices A–D.27 These observations imply that the morphological stability of EML in devices would play a significant role in control of

13

EL efficiencies of electrophosphorescent devices.28

4. Conclusion

We have synthesized triphenylsilane-containing host materials based on biaryl backbone. Green phosphorescent devices using these compounds as host materials were fabricated and their EL properties were investigated. Among those, a device using 9,9dimethyl-2-(triphenylsilyl)-7-[4-(triphenylsilyl)phenyl]-9H-fluorene (4) as the host and bis[2-(1,1',2',1''-terphen-3-yl)pyridinato-C,N]iridium(III)(acetylacetonate), (tphpy)2Ir(acac), as the dopant exhibited improved EL efficiencies (EQE: 4.64 %, PE: 7.2 lm/W and LE: 16.6 cd/A at 20 mA/cm2) compared with a device using 4,4’bis(triphenylsilyl)-biphenyl (BSB, 1) as the host (EQE: 2.56 %, PE: 3.02 lm/W and LE: 9.33 cd/A at 20 mA/cm2). The effective charge injection and effective energy transfer of compound 4 act significant roles in enhancing the EL efficiencies of PHOLED devices.

Acknowledgment: This research was supported by Basic Science Research Program through the NRF funded by the Ministry of Education, Science and Technology (NRF2013R1A1A2A10008105).

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Table captions Table 1. Physical properties of host materials 1–4. Table 2. EL performance characteristics of devices A–D.

Table 1. Physical properties of host materials 1–4. Compounds 1 2 3 4

Tda

Tga

λabsmax, b λemmax, b

Egc

HOMOd/LUMOe

ETf

(°C) (°C)

(nm)

(nm)

(eV)

(eV)

(eV)

338 246 305 399

272 256 262 321

323 323 324 351

4.16 4.16 4.14 3.52

-7.09/-2.93 -7.02/-2.86 -6.99/-2.85 -6.27/-2.75

2.56 2.71 2.67 2.73

94 65 81 103

a

The thermal properties were measured by thermogravimetric and differential scanning calorimetry. bλabsmax and λemmax measured in CH2Cl2 solution (ca.1×10-5M). cThe energy bandgaps were determined from the intersection of the absorption and photoluminescence spectra. dThe HOMO energy level was determined by a low-energy photo-electron spectrometer (Riken-Keiki, AC-2). eThe LUMO energy levels were estimated by subtracting the Eg from the HOMO energy levels. fThe ET was determined by the phosphorescent emission energy from the phosphorescent emission spectra of the materials. Table 2. EL properties of devices A–D. Devices A B C D a

Vona (V)

Lb

PEb/c

(cd/m2) (cd/A) (lm/W)

4729 (13.0V) 109.1 5.26 (22.5V) 443.2 4.23 (15.5V) 10080 3.24 (11.0V) 3.23

LEb/c 30.2/ 9.33 43.3/ 25.2/ 1.25 25.4/ 16.6

31.6/ 3.02 27.2/ 20.0/ 0.29 25.0/ 7.2

EQEb/c λELd (%) 9.32/ 2.56 12.0/ 7.20/ 0.37 11.7/ 4.64

CIEd

(nm) (x, y) 525 527 524 526

(0.34, 0.62) (0.34, 0.62) (0.33, 0.61) (0.33, 0.59)

Turn-on voltage measured at 1.0 cd/m2. bMaximum values. cAt 20 mA/cm2. dAt 8.0V.

20

Scheme 1. Synthetic routes to host materials. a) n-BuLi, THF, -78 oC; b) Pd(PPh3)4, 2M K2CO3, aliquat 336, toluene, ethanol, 90 oC reflux for 2 hours.

Figure Captions

Figure 1. (a) UV-visible absorption spectra for host materials 1–4 and (b) Photoluminescence emission spectra of host materials 1–4 and UV-visible absorption spectrum of the dopant material (tphph)2Ir(acac). (The patterned area is 1MLCT absorpion region of dopant.)

Figure 2. Energy level diagram of device A–D.

Figure 3. Normalized EL emission spectra of devices A–D.

Figure 4. (a) Current density and luminance versus voltage, (b) luminance efficiency and power efficiency versus current density and (c) external quantum efficiency versus current density plots of devices A–D.

21

22

Br

Br

+

Cl

(a)

SiPh3

Ph3Si

SiPh3 1

Ph3Si

O B O

Ph3Si +

Br SiPh3

Ph3Si

O B O

+

(b)

Br

O B O

+ Br

SiPh3

SiPh3

3

SiPh3

Ph3Si

SiPh3 Ph3Si

2

Ph3Si

SiPh3 4

Scheme 1.

23

UV-vis Abs. Intensity (a.u.)

(a)1.0

1 2 3 4

0.8

0.6

0.4

0.2

0.0 250

300

350

400

450

Wavelength (nm) 1.0 1 2 3 0.8 4 (tphpy)2Ir(acac)

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

320

340

360

380

400

Wavelength (nm)

Figure 1.

24

420

440

460

UV-vis Abs. Intensity (a.u.)

PL Em. Intensity (a.u.)

(b) 1.0

O

N Ir

N O

2

N

(tphpy)2Ir(acac)

N

N

TcTa N

N Li O

NPB

N

Liq Bphen N

N

Figure 2.

25

Normalized EL Em. Intensity (a.u.)

1.0

A B C D

0.8

0.6

0.4

0.2

0.0 450

500

550

600

Wavelength (nm)

Figure 3.

26

650

A B C D

2

140 120

10000 1000 100

100 80

10

60

1

40

2

Luminance (cd/m )

Current density (mA/cm )

(a) 160

0.1

20 0

0.01 0

2

4

6

8

10

Voltage (V) (b)

Luminance efficiency (cd/A)

30

20

20

10

0

0 0

20

40

60

80

100

120

140

160

2

Current density (mA/cm )

External quantum efficiency (%)

(c) 12

A B C D

10 8 6 4 2 0 0

20

40

60 2

Current density (mA/cm )

27

80

Power efficiency (lm/W)

PE A PE B 40 PE C PE D

LE A LE B LE C LE D

40

Figure 4.

28