Highly efficient greenish blue-emitting organic diodes based on pyrene derivatives

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Journal of Luminescence 124 (2007) 93–98 www.elsevier.com/locate/jlumin

Highly efficient greenish blue-emitting organic diodes based on pyrene derivatives Cheng-Hsien Yanga, Tzung-Fang Guob, I.-Wen Suna, a

Department of Chemistry, National Cheng Kung University, No. 1, Ta-Hsueh Road, Tainan, Taiwan 701, Republic of China Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

b

Received 8 November 2005; received in revised form 26 January 2006; accepted 3 February 2006 Available online 23 March 2006

Abstract We report the synthesis of pyrene derivatives as the light emissive layer for highly efficient organic electroluminescence (EL) diodes. Multilayer devices were fabricated with pyrene derivatives (ITO/NPB (50 nm)/blue material (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al). By using 1,10 -dipyrene (DP) and 1,4-dipyrenyl benzene (DPB), the devices produced the blue EL emissions with 1931 Commission International de L’Eclairage coordinates of (x ¼ 0:21, y ¼ 0:35) and (x ¼ 0:19, y ¼ 0:25), respectively. The device with DPB shows a maximum brightness of 42,445 cd/m2 at 400 mA/cm2 and the luminance efficiency of 8.57 cd/A and 5.18 lm/W at 20 mA/cm2. r 2006 Elsevier B.V. All rights reserved. Keywords: Pyrene derivatives; Organic light-emitting diodes

1. Introduction Organic light-emitting diodes (OLED) have drawn intensive interests in recent years since the Kodak’s report in 1987 [1]. Among the studies that have been reported for blue organic emitting diodes [2–5], the Idemitsu’s products—distyrylarylene derivatives appeared to be the best ones. The data showed that distyrylarylene host DPVBi (4,40 -bis(2,2-diphenyl-ethen-1-yl)-diphenyl) and distyrylarylene dopant BCzVBi (4,40 -(bis(9-ethyl-3-carbazovinylene)1,10 -biphenyl) exhibit the highest luminous efficiency and external quantum efficiency among the materials that have been reported in the literature so far. There are great progresses for device fabrication techniques and materials development in the past decade [6–9]. However, further improvements in both the efficiency and brightness for blue OLEDs are still necessary. Organic small molecules containing polyaromatics such as pyrene, carbazole and anthracene have been reported as efficient emitting hole transporters, host materials and Corresponding author. Tel.: +886 6 2757575 65355; fax: +886 6 2740552. E-mail addresses: [email protected] (C.-H. Yang), [email protected] (I.-W. Sun).

0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.02.003

emitters [6,10–13]. Although pyrene is a blue-emitting chromophore and has been proposed as a fluorescent temperature indicator [14], investigations focused on using pyrene derivatives as an emitter for OLEDs are very limited. In this report, we study the photophysical properties of a series of pyrene derivatives and using as an emitter for greenish blue devices. Through molecular structure design, that the close packing/luminescence quenching effect in pyrene type of materials can be reduced or controlled.

2. Experimental 2.1. Instrumentation 1

H-NMR and 13C-NMR spectra were obtained using a Bruker AMX-400 (400 MHz). EI Mass spectra were collected with a Bruker APEX II. The melting point data were measured with a PYRIS Diamond TG/DTA-10 in a heating rate: 20 1C/min. The HOMO was measured with a RIKEN KEIKI AC-2 instrument. Ultraviolet-visible (UV-vis) spectra were taken with an Agilent 8453 spectrophotometer, and Photoluminesence spectra were recorded with a HITACHI F-4500.

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2.2. OLED fabrication and measurement Electroluminescent devices with a configuration of ITO/NPB (50 nm)/blue material (30 nm)/BCP (10 nm)/ Alq3 (30 nm)/LiF (1 nm)/Al were fabricated on a prepatterned ITO glasses with an effective device of 0.16 cm2 [15]. In the devices, N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB), tris-(8-hydroxyquinoline)aluminum(III) (Alq3), and 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP) were used as the hole transport, electron transport, and hole blocking layer, respectively. DP and DPB were used as the light emissive layer. The organic layers as well as the device cathode were deposited on pre-cleaned indium-tin-oxide (ITO)/glass substrates sequentially without breaking the vacuum in a thermal evaporator under 1  106 Torr. The current–brightness–voltage (I–L–V) measurements were carried out by a programmable source meter with PR650 spectrometer (Photo Research). 3. Synthesis 3.1. 1,10 -Dipyrene 1,10 -Dipyrene (DP) was synthesized by self-reaction of Grignard reagent in the presence of palladium catalyst. To Mg turning (1.5 g, 60.0 mmol) was added a solution of 1-bromo-pyrene (1.5 g, 5.3 mmol) in THF (50 ml) in order to initiate the Grignard reaction. An additional amount of 1-bromo-pyrene (12.5 g, 44 mmol) in THF (100 ml) was added drop-wise. When the addition was finished, the solution was heated at 50 1C until the solution became clear. 1,4-dibromobenzene (4.72 g, 20 mmol) and PdCl2(PPh3)2 (88 mg) in THF (100 ml) were added to this clear solution, and refluxed under N2 atmosphere for 3 h. When the reaction was completed, water (30 ml) was added to quench the reaction. The product was extracted with CH2Cl2. The organic layer was

collected, dried over anhydrous MgSO4 and evaporated under vacuum. The crude product was purified with zone sublimation and used for advanced analysis and device fabrication. 1 H-NMR (CDCl3, 400 MHz): d 8.36 (d, J ¼ 7:7 Hz, 2 H), 8.27–8.14 (m, 10 H), 8.04 (t, J ¼ 7:6 Hz, 2 H), 7.88 (d, J ¼ 9:2 Hz, 2 H), 7.66 (d, J ¼ 9:2 Hz, 2 H). 13C-NMR (CDCl3, 100 MHz) d: 131.5, 131.0, 130.9, 130.1, 128.8, 127.6, 127.5, 126.1, 125.8, 125.3, 125.1, 124.8, 124.5. T m ¼ 335:7 1C, EIMS: m/z 402, [M]+.

3.2. 1,4-Dipyrenyl benzene 1,4-Dipyrenyl benzene (DPB) was obtained with the procedure shown in Scheme 1. 1,4-diiodobenzene (6.6 g, 20 mmole) and PdCl2(PPh3)2 (88 mg) were added to the mixture solution of triethylamine (16.7 ml) and dioxane (20 ml). Pinacolborane (8.7 ml, 60 mmole) was added by several portions to the solution within 30 min. After the addition, the reaction solution was stirred at 80 1C for 40 min. After the reaction was completed, the slurry was filtered to obtain boron acid ether. Boron acid ether 1 equiv. was added to the solution of 1-bromo-pyrene 2.4 equiv., sodium tert-butoxide 1 equiv., PdCl2(PPh3)2 1%mol and Toluene. The mixture was stirred overnight at 100 1C. After cooling to room temperature, ethanol was added to the mixture. This slurry was filtered and the solid was dried in vacuum and zone sublimed to give pure product which was used for advanced analysis and device fabrication. 1 H-NMR (CDCl3, 400 MHz): d 8.41 (d, J ¼ 9:2 Hz, 2 H), d 8.30 (d, J ¼ 7:8 Hz, 2 H), 8.23 (m, 4 H), 8.17–8.11 (m, 8 H), 8.05 (t, J ¼ 7:6 Hz, 2 H), 7.86 (s, 4 H). 13C-NMR (CDCl3, 100 MHz) d: 140.2, 137.5, 131.6, 131.1, 130.7, 130.6, 128.6, 127.7, 127.6, 127.5, 127.4, 126.1, 125.4, 125.2, 125.1, 125.0, 124.9, 124.8. T m ¼ 301 1C, EIMS: m/z 478, [M]+.

O H B O Triethylamine PdCl2(PPh3)2 Dioxane

O

O B

O

Br Sodium tert-butoxide PdCl2(PPh3)2 Toluene

Scheme 1. Synthesis of DPB.

B O

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DP DPB Pyrene

PL intensity (a.u.)

In this work, pyrene derivatives, DP and DPB, were synthesized and aimed to adjust the symmetric structure of pyrene. The synthetic routes of DP and DPB were described in experimental section. Both compounds were characterized by 1H and 13C-NMR as well as mass spectrometry. The signals in the 1H-NMR spectra could be exactly assigned to the various hydrogen atoms. In the mass spectra, all these compounds show the corresponding ion peak of pyrene. The molecular structures of DP and DPB that were modeled at B3LYP 6-31G* level using Gaussian 98 software package [16] are presented in the Fig. 1. Based on the molecular modeling, the two pyrene moieties in DP and DPB are twisted, preventing the close packing of p-electron clouds that disrupt intermolecular interactions, and suppressing the problematic recrystallization. This fact would reduce the self-aggregation and improve the morphological stability of the thin film of OLEDs, and thus reduces the self-quenching effect of photoluminescence (PL). Indeed, the films of DP and DPB show much higher PL intensities than that of pyrene (as shown in Fig. 2), suggesting the pyrene derivatives can be used as the efficient light emissive layers for the fabrication of blueemitting organic diodes. Fig. 3 shows the UV-vis absorption and PL spectra of DP and DPB in chloroform. All the relevant parameters are collected in Table 1. The UV-vis absorption of DP shows the characteristic vibration pattern of the pyrene group (lmax ¼ 280, 330, 349 nm). Upon introducing the benzene group in DPB, the vibration pattern of pyrene group becomes less apparent and only two prominent bands are observed (lmax ¼ 281, 352 nm). When DP and DPB are excited in dilute chloroform solution, the PL spectra with lmax ¼ 430, 426 nm are observed and the full-

widths at half-maximum (FWHM) of DP and DPB are 63 and 64 nm, respectively. The fluorescence quantum yield (F) measured in chloroform using DPVBi as a standard (F ¼ 1) are 1.03 and 1.99 for DP and DPB, respectively.

350

400

450

500 550 600 Wavelength (nm)

650

700

Fig. 2. PL emission spectra of the films (100 nm) for pyrene, DP and DPB under the identical exciting power for measurement.

DP-Abs DP-PL DPB-Abs DPB-PL

Intensity (a.u.)

4. Results and discussion

95

250

300

350

400 450 500 Wavelength (nm)

550

600

Fig. 3. The optical absorption and photoluminescence spectra of DP and DPB in a dilute chloroform solution.

Fig. 1. Molecular structure of DP and DPB.

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Table 1 Summaries of physical measurements of the DP and DPB Compound

a labs max (nm)

a lem max (nm)

FWHM (nm)a

Fa,b

Tg (1C)c

Tm (1C)d

Td (1C)d

HOMO (eV)e

LUMO (eV)f

Energy gap (eV)g

DP DPB

280,330,349 281,352

430 426

63 64

1.03 1.99

ND 145.5

335.7 301.0

416.5 465.7

5.40 5.70

2.26 2.50

3.14 3.20

a

Absorption and emission spectra were recorded in chloroform solution. Using DPVBi (F ¼ 1) as a standard. c Determined by differential scanning calorimeter. d Determined by PYRIS Diamond TG/DTA-10. e Determined by AC-2. f LUMO ¼ HOMO—energy gap. g Energy gap was estimated from the absorption edge. b

2.3

5.5

5.4

2.4

2.5

ITO

BCP (10nm)

3.1

LiF/Al

5.8

Blue Emitter (30nm) 6.7

DPB

NPB ITO 5.5

BCP

3.2

ITO Glass

5.7

3.1

LiF/Al

Alq3

NPB (50nm)

5.8 6.7

DPB PL DP PL DP EL DPB EL

Normalized Intensity (a.u.)

DP

AlQ3 (30nm)

BCP

Al LiF (1nm)

NPB

3.2

Alq3

2.4

350

400

450

500 550 600 Wavelength (nm)

650

700

Fig. 5. Normalized PL and EL spectra of the devices using DP and DPB as the light emissive layers.

400

35000

350

30000

300

25000

250

20000

200

15000

150

10000

100

5000

50

Current density (mA/cm2)

40000

0

0 0

2

4

6 8 Voltage (V)

10

12

350 40000

300 250

30000

200 20000

150 100

10000

50 0

Current density (mA/cm2)

400

50000

Brightness (cd/m2)

The results of quantum yield in solution and intensity in film agree with the concept of molecular structure design. Thermal properties of the compounds were investigated by differential scanning calorimetry and thermogravimetry/differential thermal analysis. The glass transition temperatures, melting points and the thermal decomposition temperatures are listed in Table 1. Both DP and DPB display impressive thermal stability and amorphous propensity. The high Tg recorded for these compounds is attributed to the presence of the rigid pyrene segment in the molecular architecture [10,11]. The energy diagram and the configuration of multiplayer devices are shown in Fig. 4. AC-2 measurements showed that DP and DPB have high HOMO value in 5.4 and 5.7 eV, respectively. The LUMO values were evaluated from the long-wavelength absorption edge using the theory reported by Burrows and co-worker [17]. The optical energy band gaps of DP and DPB are 3.14 and 3.20 eV, respectively, calculated from the threshold of the optical absorption. The band gapes, HOMO and LUMO values are summarized in Table 1. To study the EL properties of DP and DPB, multiplayer devices with the configuration ITO/NPB (50 nm)/Blue material (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al were fabricated. Fig. 5 shows the normalized PL and EL

Brightness (cd/m2)

Fig. 4. Configuration and energy diagrams of DP and DPB EL devices.

0 0

2

4

6 8 Voltage (V)

10

12

Fig. 6. I–L–V characteristics of the devices using DP (up) and DPB (down) as the light emissive layers.

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Table 2 Electroluminescence data for DP and DPB devices Brightness (cd/m2) DP

DPB

LE (cd/A)

Power efficiency (lm/W)

Voltage (V)

1459a 3841b 7875c 16,614d 25,947e 35,713f

7.30 7.68 7.88 8.31 8.65 8.93

4.09 3.66 3.21 2.81 2.56 2.40

5.6 6.6 7.7 9.3 10.6 11.7

1714a 4402b 9441c 20,128d 30,613e 42,445f

8.57 8.80 9.22 9.79 10.20 10.61

5.18 4.69 4.24 3.72 3.34 3.12

5.2 5.9 7.0 8.5 9.6 10.7

CIE

EL (nm) 488

x ¼ 0:21 y ¼ 0:35

468 x ¼ 0:19 y ¼ 0:25

spectra for the devices using DP and DPB as the light emissive layers. The shift of emission spectra in films of DP and DPB probably are due to the difference in dielectric constant of the environment [6,18] and the improvement in the coplanarity of pyrenyl units in the solid state [19]. For DP device, the maximum EL intensity is located at 488 nm and the CIE coordinates are (x ¼ 0:21, y ¼ 0:35). The color of the emission is bluish green, covering the visible range of 420–600 nm. It is evident in the energy level diagrams of Fig. 4 that the HOMO of DP is lower than NPB and we suspect that part of the injected charge carriers are recombined at NPB layer, broadening the EL spectrum for DP device. For DPB device, EL emission is centered at 468 nm and the CIE coordinates are (x ¼ 0:19, y ¼ 0:25). EL spectrum of DPB device basically resembles the features of PL spectrum, which infers that the emission of the device mainly comes from the DPB layer. The DPB device gives the higher luminescence efficiency than that of DP device. We regard that the higher luminescence efficiency of DPB device is due to the superior PL efficiency of DPB layer. The I–L–V characteristics of DP and DPB devices are displayed in Fig. 6 and the performance parameters are summarized in Table 2. The best power efficiency obtained for the DPB device was 5.18 lm/W at a voltage, current density, and luminance of 5.2 V, 20 mA/cm2, and 1714 cd/m2, respectively. For the DP device the best power efficiency was 4.09 lm/W at a voltage, current density, and luminance of 5.6 V, 20 mA/cm2, and 1459 cd/m2, respectively. The luminescence efficiency remains stable while biased under the high current density as depicted in Fig. 7. This experimental result was also observed by Kim et al. [8] and it would be useful in high voltage applications. In summary, we developed a new greenish blue emitter based on pyrene. DPB shows better brightness, luminance efficiency and power efficiency. As was expected, pyrenyl group seems to be a good luminescent

Luminescence efficiency (cd/A)

For each parameter, the data in different rows correspond to those measured at different current density: a20 mA/cm2, b50 mA/cm2, c100 mA/cm2, d 200 mA/cm2, e300 mA/cm2, f400 mA/cm2.

12 10 8 6 DPB DP

4 2 0 0

50

100 150 200 250 300 350 400 450 Current density (mA/cm2)

Fig. 7. Dependence of the luminescence efficiency with the driving current density for DP and DPB devices.

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