Aryl substituted 9-(2,2-diphenylvinyl)carbazoles as efficient materials for hole transporting layers of OLEDs

Synthetic Metals 161 (2011) 2466–2470

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Aryl substituted 9-(2,2-diphenylvinyl)carbazoles as efficient materials for hole transporting layers of OLEDs R. Griniene a , J.V. Grazulevicius a , K.Y. Tseng b , W.B. Wang b , J.H. Jou b , S. Grigalevicius a,∗ a b

Department of Organic Technology, Kaunas University of Technology, Radvilenu Plentas 19, LT50254 Kaunas, Lithuania Department of Materials Science and Engineering, National Tsing-Hua University, No. 101, Kaungfu Rd., Hsin-Chu 30013, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 25 June 2011 Received in revised form 28 July 2011 Accepted 19 September 2011 Available online 13 October 2011 Keywords: Carbazole derivatives Amorphous material Ionization potential Light emitting diode

a b s t r a c t Phenyl or naphthyl substituted 9-(2,2-diphenylvinyl)carbazoles were synthesized by the multi-step synthetic route. The materials were examined by various techniques including thermogravimetry, differential scanning calorimetry, and electron photoemission technique. These derivatives were also tested as hole-transporting materials in bilayer OLEDs with Alq3 as the emitter. The devices exhibited promising overall performance with a turn-on voltage of 3.2 V, a maximal photometric efficiency of about 4.8 cd/A and maximum brightness of 12,400–13,100 cd/m2 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) based on organic small molecules and polymers have attracted much attention because of their potential use in flat panel displays and lighting [1–4]. Efficient OLEDs can be obtained only by building multilayer structures [5,6]. One approach that has been employed to improve efficiency of the devices is the appliance of effective hole-transporting layers in the multilayer devices. Carbazole-containing polymers and low-molecular-weight derivatives are among the most studied materials for optoelectronic and electronic applications due to their high hole mobility and excellent thermal stability [7]. Some carbazole based materials have been commercialized in a number of devices and processes (photocopy machines, laser printers, etc.) [8]. We have earlier synthesized series of hole-transporting carbazole-, [3,3]bicarbazoleand indolo[3,2-b]carbazole-based derivatives [9–11]. It was observed that phenyl substituted indolo[3,2-b]carbazole derivatives demonstrate better charge transporting properties than the derivatives containing unsubstituted indolo[3,2-b]carbazole fragments [12]. We have also established that 2,2-diphenylvinylsubstituted carbazoles, i.e. enamines, show better hole-injecting and transporting properties than the corresponding compounds with electronically isolated carbazole rings [13]. In this work, we have designed and synthesized phenyl or naphthyl substituted

∗ Corresponding author. E-mail address: [email protected] (S. Grigalevicius). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.09.032

9-(2,2-diphenylvinyl)carbazoles, which were expected to show enhanced hole injection and transport properties and could be suitable as hole-transporting materials for multilayer OLEDs.

2. Experimental 2.1. Instrumentation 1H

NMR spectra were recorded using a Varian Unity Inova (300 MHz) apparatus. Mass spectra were obtained on a Waters ZQ 2000 spectrometer. Differential scanning calorimetry (DSC) measurements were carried out using a Bruker Reflex II thermosystem. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409. TGA and DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10 ◦ C/min. The ionization potentials of the layers of the compounds synthesized were measured by the electron photoemission method in air, which was described earlier [14]. The measurement method was, in principle, similar to that described by Miyamoto et al. [15]. The samples for the ionization potential measurements were prepared as follows [16]. The materials were dissolved in THF and were coated on Al plates pre-coated with ∼0.5 ␮m thick methylmethacrylate and methacrylic acid copolymer (MKM) adhesive layer. The function of this layer is not only to improve adhesion, but also to eliminate the electron photoemission from Al layer. In addition, the MKM layer is conductive enough to avoid charge accumulation on it during the measurements. The thickness of the layers was 0.5–1 ␮m.

R. Griniene et al. / Synthetic Metals 161 (2011) 2466–2470

cooling

< endo - exo >

The multilayer electroluminescent devices were fabricated on glass substrates and had the typical structure with the organic layers sandwiched between a bottom ITO (125 nm) anode and a top metal cathode. Before use in device fabrication, the ITO-coated glass substrates were carefully cleaned and treated with UV/ozone right before deposition of the organic layers. PEDOT:PSS layers (35 nm) were deposited by spin-coating and heated at 120 ◦ C for 30 min. It was described earlier that these conditions are suitable for preparation of PEDOT layers with optimum electrical properties [17]. The hole-transporting layers (HTL) were prepared by spin-coating a 40 nm layer of the derivatives 3 or 4. Bis(naphthalen-1-yl)-N,N bis(phenyl)benzidine (NPB)-based device was also prepared for the comparison. NPB is one of the best hole transporting materials, which is widely used as a reference for new hole transporting OLED materials [18]. Tris(quinolin-8-olato)aluminium (Alq3 ) was used as green light emitter. Evaporation of Alq3 as well as of LiF/Al cathode was done at a pressure of 10−5 Torr in vacuum evaporation equipment. The current–voltage and luminance–voltage characteristics were recorded as we described earlier [19].

2467

nd

o

2 heating

Tg = 90 C

st

1 heating

o

Tm = 178 C 0

40

80

120

160

200

o

Temperature ( C) Fig. 1. DSC curves of compound 3. Heating rate: 10 ◦ C/min.

2.2. Materials 9H-carbazole, 2,2-diphenylacetaldehyde, (±)-camphor-10sulfonic acid, 1-naphtalene boronic acid, phenyl boronic acid, bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3 )2 Cl2 ), Alq3 , and potassium hydroxide were purchased from Aldrich and used as received. 3-Iodo-9H-carbazole (1) was obtained by a procedure of Tucker [20]. 3-Iodo-9-(2,2-diphenylvinyl)carbazole (2). 3-Iodo-9Hcarbazole (1) (1.5 g, 5 mmol) was dissolved in toluene (15 ml) at 80 ◦ C. 2,2-Diphenylacetaldehyde (1.54 ml, 5.1 mmol) was added dropwise to the stirred mixture and a catalytic amount of (±)camphor-10-sulfonic acid was added. The solution was stirred at 110 ◦ C for 36 h. When the reaction was finished (TLC control), the solvent was evaporated under vacuum. The crude product was purified by silica gel column chromatography using hexane/diethyl ether (vol. ratio 25:1) as an eluent. The yield of compound 2 was 1.3 g (54%). MS (APCI+ , 20 V): 472.5 ([M+1], 100%). 1 H NMR spectrum (CDCl3 , ı, ppm): 8.29 (s, 1H, Ar); 8.24 (d, 1H, J = 7.8 Hz, Ar); 7.48 (d, 2H, J = 8.7 Hz, Ar); 7.43–7.18 (m, 7H, Ar); 7.15 (s, 1H, NCH); 7.08–7.02 (m, 4H, Ar); 6.95 (d, 2H, J = 8.4 Hz, Ar); Elemental analysis for C26 H18 IN % Calc.: C 66.26, H 3.85, N 2.97; % Found: C 66.31, H 3.78, N 2.93. 3-(1-Naphthyl)-9-(2,2-diphenylvinyl)carbazole (3). 0.45 g (0.95 mmol) of 3-iodo-9-(2,2-diphenylvinyl)carbazole (2), 0.3 g (1.75 mmol) of 1-naphtalene boronic acid, 0.024 g (0.035 mmol) of PdCl2 (PPh3 )2 and 0.24 g (4.2 mmol) of powdered potassium hydroxide were stirred in 8 ml of THF containing degassed water (0.5 ml) at 80 ◦ C under nitrogen for 24 h. After TLC control the reaction mixture was cooled and quenched by the addition of ice water. The product was extracted by chloroform. The combined extract was dried over anhydrous Na2 SO4 . The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:30) as an eluent. Yield: 0.4 g (89%) of white crystals. M.p.: 178 ◦ C (DSC). MS (APCI+ , 20 V): 472.7 ([M+1], 100%). 1 H NMR spectrum (CDCl3 , ı, ppm): 8.12 (s, 1H, Ar); 8.01 (d, 1H, J = 7.8 Hz, Ar); 7.91 (t, 2H, J = 7.8 Hz, Ar); 7.84 (d, 1H, J = 7.8 Hz, Ar); 7.53 (d, 1H, J = 6.9 Hz, Ar); 7.49–7.11 (m, 18H, Ar); 7.10 (s, 1H, NCH). Elemental analysis for C36 H25 N % Calc.: C 91.69, H 5.34, N 2.97; % Found: C 91.63, H 5.36, N 2.93. 3-Phenyl-9-(2,2-diphenylvinyl)carbazole (4). 0.45 g (0.64 mmol) of 3-iodo-9-(2,2-diphenylvinyl)carbazole (2), 0.16 g (0.95 mmol) of phenyl boronic acid, 0.02 g (0.03 mmol) of PdCl2 (PPh3 )2 and 0.18 g (3.2 mmol) of powdered potassium

hydroxide were stirred in 8 ml of THF containing degassed water (0.5 ml) at 80 ◦ C under nitrogen for 24 h. After TLC control the reaction mixture was cooled and quenched by the addition of ice water. The product was extracted by chloroform. The combined extract was dried over anhydrous Na2 SO4 . The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:50) as an eluent. Yield: 0.31 (78%) g of white crystals. M.p.: 165 ◦ C (DSC). MS (APCI+ , 20 V): 422.5 ([M+1], 100%). 1 H NMR spectrum (CDCl3 , ı, ppm): 8.22 (s, 1H, Ar); 8.04 (d, 1H, J = 7.2 Hz, Ar); 7.65 (d, 2H, J = 8.4 Hz, Ar); 7.52–7.36 (m, 8H, Ar); 7.22–7.18 (m, 6H, Ar); 7.12–7.06 (m, 4H, Ar); 7.05 (s, 1H, NCH). Elemental analysis for C32 H23 N % Calc.: C 91.18, H 5.50, N 3.32; % Found: C 91.23, H 5.46, N 3.26. 3. Results and discussion The synthesis of phenyl- or naphthyl-substituted 9-(2,2diphenylvinyl)carbazoles (3 and 4) was carried out by a multi-step synthetic route shown in Scheme 1. 3-Iodo-9H-carbazole (1) as a key material was synthesized from commercially available 9H-carbazole by Tucker iodination with KI/KIO3 in acetic acid [20]. 3-Iodo-9-(2,2-diphenylvinyl)carbazole (2) was prepared by the reaction of the 3-iododerivative (1) with an excess of 2,2diphenylacetaldehyde under acidic conditions in the presence of (±)-camphor-10-sulfonic acid. Phenyl- and naphthyl-substituted derivatives 3 and 4 were prepared by Suzuki reaction of compound 2 with an excess of 1-naphtalene boronic acid or phenyl boronic acid, correspondingly. The synthesized derivatives 3 and 4 were identified by mass spectrometry, elemental analysis and 1 H NMR spectroscopy. The data were found to be in good agreement with the proposed structures. The materials are soluble in common organic solvents. Transparent thin films of these materials could be prepared by spin coating from solutions or by vacuum evaporation. The behaviour under heating of compounds 3 and 4 was studied by DSC and TGA under a nitrogen atmosphere. The compounds demonstrated high thermal stability. The onsets of mass loss were at 328 ◦ C for 3 and at 324 ◦ C for 4, as confirmed by TGA with a heating rate of 10 ◦ C/min. Both the compounds 3 and 4 were obtained as crystalline materials. The DSC thermo-grams of 3 are shown in Fig. 1 as an example. When the crystalline sample was heated, the endothermic peak due to melting was observed at 178 ◦ C. When the melt sample was

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R. Griniene et al. / Synthetic Metals 161 (2011) 2466–2470

H

1

N

I

HO

(a)

B

3

OH

H N

O

(b)

N

(b)

4

I

HO

2

B

OH

N

Scheme 1. (a) (±)-Camphor-10-sulfonic acid, toluene; (b) Pd(PPh3 )2 Cl2 , KOH, THF.

Photocurrent (a. u.)

cooled down and heated again, the glass-transition was observed at 90 ◦ C and on further heating no peaks due to crystallization and melting appeared. The crystalline sample of compound 4 demonstrated the similar behaviour. It melted on first heating at 165 ◦ C and formed glass (Tg = 74 ◦ C) upon cooling. The ionization potentials (Ip ) of layers of the compounds synthesized were measured by the electron photoemission method. The photoemission spectra of thin amorphous films of compounds 3–4 are presented in Fig. 2. The values of Ip in eV are very close to 5.8 for both the compounds. It could be observed that the val-

4 3

5,6

5,7

5,8

5,9

6,0

6,1

6,2

6,3

hν (eV) Fig. 2. Electron photoemission spectra of the layers prepared using 3 and 4.

ues of Ip of the newly synthesized compounds are lower than that of 9-(2,2-diphenylvinyl)carbazole (Ip = 5.94 eV) [21] as well as of derivatives having un-substituted carbazole rings (Ip > 5.9 eV) [22,23]. The characteristics demonstrate that thin layers of 3 or 4 could be suitable for application in optoelectronic devices. The layers should demonstrate better hole-injecting and transporting properties in multilayer electroluminescent devices than that of widely used PVK [22]. Compounds 3 and 4 were tested in OLEDs as hole-transporting materials. N,N -bis(naphthalen-1-yl)-N,N -bis(phenyl)benzidine (NPB)-based device was also prepared for the comparison. The two-layer OLED devices were prepared using Alq3 for the electroluminescent (EL)/electron transporting layers. The cathode used was aluminium with a thin LiF electron injection layer. When a positive voltage was applied the bright green electroluminescence of Alq3 was observed with an emission maximum at around 520 nm. This observation implies that the charge mobility in the HT layers of 3 and 4 is fully sufficient for an effective charge carrier recombination occurring within the Alq3 layer. Fig. 3 shows current density–voltage (a), luminance–voltage (b) and efficiency–current density (c) characteristics for the OLEDs containing the HTL of 3, 4 and NPB. The current density of the two devices with 3 and 4 is higher than that of the device with NPB when voltage exceeds 7 V. These devices in general exhibit turn-on voltages of 3.2 V (defined as the voltage where electroluminescence becomes 10 cd/m2 ) and a maximum brightness of 12,400–13,100 cd/m2 (at 8 V). Maximal current efficiency of the OLEDs is 4.8 cd/A and 4.85 cd/A for the devices containing materials 3 and 4. All the electroluminescence characteristics are summarized in Table 1. It is evident that incorporation of HTL of compounds 3 or 4 into the devices leads to better performance with respect of current

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Table 1 The electroluminescence data of the devices containing HTL of NPB, 3 and 4. Driving voltage at 10 cd/m2 (V)

HTL

Current efficiency (cd/A)

Power efficiency (lm/W)

CIE 1931 chromatic coordinates (x, y)

2.4/1.7 2.9/2.7 2.8/2.5

(0.32, 0.52)/(0.32, 0.52) (0.30, 0.55)/(0.30, 0.55) (0.31, 0.55)/(0.31, 0.55)

@100/1000 cd/m2 NPB 4 3

2.9 3.2 3.2

2.7/3.0 3.7/4.5 3.6/4.3

Current Density (mA/cm2)

(a) 2

10

1

10

0

10

-1

10

HTL NPB

-2

10

4

3

-3

10

-4

10

-5

10

2

4

6 Voltage (V)

8

10

(b) 4

Luminance (cd/m2)

10

3

10

HTL NPB

2

4 3

10

Acknowledgements This research was funded by a Grant No. MIP-063/2011 from the Research Council of Lithuania. Habil. Dr. V. Gaidelis is thanked for the help in ionization potential measurements.

1

10

2

4

6 Voltage (V)

8

References

Current Efficiency (cd/A)

(c) HTL NPB

4

3 4

2

0 -3 10

-2

10

-1

10

0

10

1

10

efficiency at the same current density in comparison with that of NPB-based device. Higher current efficiency of the devices implies that more photons, in general, were generated at and emitted from the emitting layer. This may arise from the ability of 3 or 4 to make the injection of both holes and electrons more balanced and thus further to improve generation of excitons in emitting layer compared with the device utilizing the commonly used NPB in OLED fabrication. It was established that Ip of the layers of 3 and 4 are close to 5.8 eV. These values are very close to Ip of Alq3 [24] and injection of holes from the hole transporting layers of the synthesized materials into the emitting layer is easier as compared with NPDbased device. Characteristics of OLEDs containing hole transporting layers of 3 or 4 are better due to this reason. It should be pointed out that these characteristics were observed for a non-optimized test device under ordinary laboratory conditions. The device performance may be further improved by an optimization of the layer thicknesses and processing conditions [25]. In conclusion, phenyl- or naphthyl-substituted 9-(2,2diphenylvinyl)carbazoles were synthesized and characterized as materials for hole-transporting layers. The derivatives showed high thermal stability and formed amorphous layers with glass transition temperatures of 74–90 ◦ C. The electron photoemission spectra of the layers showed ionization potentials of about 5.8 eV. The compounds were tested as hole-transporting materials in bilayer OLEDs with Alq3 as the emitter. The devices exhibited good overall performance (turn-on voltage: 3.2 V; maximum photometric efficiency: ∼4.8 cd/A; maximum brightness: 12,400–13,100 cd/m2 ). These OLEDs properties are rather promising among Alq3 -based two-layer devices.

2

10

3

10

4

10

Current Density (mA/cm2) Fig. 3. OLED characteristics of the devices with the configuration: ITO/PEDOT:PSS/3, 4 or NPB/Alq3 /LiF/Al.

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