High-efficiency electroluminescent polymers with stable high work function metal Al and Au as cathode

European Polymer Journal 42 (2006) 2320–2327 www.elsevier.com/locate/europolj

High-eYciency electroluminescent polymers with stable high work function metal Al and Au as cathode Fei Huang, Lintao Hou, Wei Shi, Wei Cao, Qiong Hou, Wei Yang, Yong Cao

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Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials and Advanced Manufacturing Technology, South China University of Technology, Guangzhou 510640, China Received 13 April 2006; received in revised form 19 May 2006; accepted 20 May 2006 Available online 7 July 2006

Abstract Soluble conjugated copolymers (PFN-TPA) derived from 2,1,3-benzothiadiazole (BTDZ), triphenylamine (TPA) and 9,9-bis(3⬘-(N,N-dimethylamino)propyl)Xuorene (DMAPF) were synthesized by palladium(0)-catalyzed Suzuki coupling reactions. Optoelectronic properties of the copolymers were characterized by UV–vis absorption, cyclic voltammetry, photoluminescence and electroluminescence. All these copolymers show excellent EL performances in the devices with Ba/Al, Al and even Au as cathode and are promising candidate for fabrication and patterning of air-stable Xat panel displays. © 2006 Elsevier Ltd. All rights reserved. Keywords: Light emitting diode; Electroluminescence; PolyXuorene; Cationic polyelectrolyte

1. Introduction Since the Wrst polymer light-emitting diode (PLED) based on poly(p-phenylenevinylene) (PPV) was reported in 1990, conjugated polymers have attracted considerable attention due to possible application in large area Xat panel displays [1]. During the past decades, many improvements have been made in PLEDs including good eYciency, high brightness, diVerent emission color and low drive voltage [2]. Now it is well known that in order to obtain highly eYcient PLED devices, good injection and transportation properties are important [3].

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Corresponding author. Fax: +86 20 87110606. E-mail address: [email protected] (Y. Cao).

0014-3057/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.05.026

There are many reports showing that introducing the hole—(such as triphenylamine (TPA)) and electron—(such as oxadiazole (OXD)) transporting moieties to the polymer’s side chain or main chain could improve balance of the hole and electron injection into the light-emitting polymers [4]. However, in most of these reports, the polymers only show high performance when using low work-function metals (such as Ca, Ba or Mg) as cathode. Since the low work-function metals are easy to degradation upon water vapor or oxygen which will decrease the life time of the PLEDs devices, more stable electron injection cathodes are desirable. Recently, we have reported that incorporation of (N,N-dimethylamino)propyl) group into 9-position of Xuorene ring in the polyXuorene improves electron injection from high work-function metals

F. Huang et al. / European Polymer Journal 42 (2006) 2320–2327

2321

Br nBuLi THF

DMF N

O

NBS

N

Br

O B O O

O

N

B

B

O O

1 O 1

+

B

B O

O C8H17

2

O

+

Br

Br

Br

+

Br N

C8 H17 3

4

N

N

S

N

5

Suzuki coupling

N N HC C H 8 17

C8H17 C8 H17

17 8

m

C8H17 C8 H17

N

S

N

N n

p

PFN-TPA 1 m=98.5 n=0.5 p=1 PFN-TPA 5 m=94.5 n=0.5 p=5 PFN-TPA 1 0 m=89.5 n=0.5 P = 10

N N Suzuki coupling 2 +

4 +

5

N

N m PFN-TPA50

m= 99.5

N

S

N

n

n= 0.5

Scheme 1. Synthetic procedures for the PFN-TPA copolymers.

such as Al or Au via interface dipole mechanism [5–7]. In this paper, we introduce triphenylamine moiety into such copolymers as a hole-transport element in order to improve hole-injection and transport in such type of electroluminescent polymer and to get more balanced carrier injection. A novel series of copolymers (PFN-TPA, Scheme 1) containing hole-transporting unit TPA and a novel electron injection unit 9,9-bis(3⬘-(N,N-dimethylamino)propyl)Xuorene (DMAPF) have been synthesized and characterized. A small portion of narrow band gap comonomer, 2,1,3-benzothiadiazole (BTDZ) was also copolymerized in the polymer chains in order to turn emission color and to suppress the excimer formation, which is a common problem for most of polyXuorenes [6]. As a result, the resulted copolymers PFN-TPA show an excellent electroluminescence (EL) performance not only in the device with low work-function metal Ba cathode but also in the device with high work-function metal Al and Au cathode.

2. Experimental part 2.1. Materials All manipulations involving air-sensitive reagents were performed under an atmosphere of dry argon. All reagents, unless otherwise speciWed, were obtained from Aldrich, Acros and TCI and used as received. All the solvents used were further puriWed before use. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)-9,9-dioctyl-Xuorene (3) and 2,7-dibromo-9,9bis(3⬘-(N,N-dimethylamino)propyl)Xuorene (4) were prepared according to published procedures [6,7]. 2.2. Instrumentations The 1H and 13C NMR spectra were collected on a Bruker DRX 400 spectrometer operating, respectively, at 400 MHz and 100 MHz for 1H and 13C, respectively, in deuterated chloroform solution with

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tetramethylsilane as reference. Number-average (Mn) and weight-average (Mw) molecular weights were determined by a Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve of polystyrene standards. Elemental analyses were performed on Vario EL Elemental Analysis Instrument (Elementar Co.). UV–vis absorption spectra were recorded on a HP 8453 UV–vis spectrophotometer. The PL quantum yields were determined in an Integrating sphere IS080 (Labsphere) with 325 nm excitation of HeCd laser (Mells Griot). PL an EL spectra were recorded on Instaspec IV CCD spectrophotometer (Oriel Co.). Cyclic voltammetry was carried out on a CHI660A electrochemical workstation in a solution of tetrabutylammonium hexaXuorophosphate (Bu4NPF6) (0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature under argon. A platinum electrode was coated with a thin polymer Wlm and was used as the working electrode. A Pt wire was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. 2.3. Synthesis of 4,4⬘-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-triphenylamine (2) In a 250 ml three-necked Xask in an argon Xow, N,N-bis(4-bromophenyl)aniline (1) (7.357 g, 18.2 mmol) was dissolved in 120 ml of anhydrous tetrahydrofuran (THF). The solution was cooled down to ¡78 °C, and then n-butyllithium solution (n-BuLi) (1.6 M in hexane, 24 ml, 38.4 mmol) was slowly added. The reaction mixture was stirred at this temperature for 2 h. Then 2,2-isopropoxy-4,4,5,5-tetra-methyl-1,3,2-dioxaborolane (25 ml, 120 mmol) was quickly added by syringe. The resulted mixture was stirred at ¡78 °C for 2 h, and then was slowly warmed to a room temperature and kept on stirring for 48 h. 50 ml of distilled water was added to the reaction mixture, which was extracted by dichloromethane (3 £ 100 ml). The organic phase was separated and dried by MgSO4 overnight. The solvent was distilled and the crude product was puriWed by column chromatography (silica gel, 5% ethyl acetate in hexane) to give white solid (4.3 g, 47%). 1 H NMR (400 MHz, CDCl3).  (ppm): 7.72–7.69 (d, 4H), 7.29–7.26 (m, 2H), 7.15–7.07 (m, 7H), 1.36 (s, 24H, –CH3). 13C NMR (100 MHz, CDCl3).  (ppm): 150.14, 147.07, 135.89, 129.37, 125.61, 123.89, 122.75, 83.62, 24.88. Anal. calcd for C30H37B2NO4: C, 72.46; H, 7.50; N, 2.82. Found: C, 72.54; H, 7.73; N, 2.64.

3. 4,4⬘-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)-triphenylamine (2) 3.1. Polymerization The preparation of polymer PFN-TPA1 is given as a typical example. (1) 0.0081 g (0.02 mmol), (3) 0.642 g (1.00 mmol), (4) 0.479 g (0.97 mmol), (5) 0.0034 g (0.01 mmol), tetrakis(triphenylphosphine) palladium [(PPh3)4Pd(0)] (8 mg), and several drops Aliquat 336 were dissolved in a mixture of 6 ml toluene and 4 ml of 2 M Na2CO3 aqueous solution. The mixture was reXuxed with vigorous stirring for 2 days under argon atmosphere. After the mixture was cooled to room temperature, the solution was Wltered and the Wltrate was poured into methanol. The precipitated material was recovered by Wltration through a funnel. The resulting solid material was washed for 24 h using acetone to remove oligomers and catalyst residues (0.59 g, 81%). 1H NMR (400 MHz, CDCl3).  (ppm): 7.86–7.84, 7.73–7.60, 2.11, 1.26–1.14, 0.98, 0.84–0.76. Anal. Found: C, 85.97%; H, 9.10%; N, 4.51%. Mn D 48300, Mw D 76,900 g/mol. 3.1.1. PFN-TPA5 (1) (0.05 equiv), (3) (0.50 equiv), (4) (0.40 equiv) and (5) (0.05 equiv) were used in this polymerization. 1 H NMR (400 MHz, CDCl3).  (ppm): 7.86–7.84, 7.73–7.61, 2.11, 1.26–1.15, 0.98, 0.84–0.81. Anal. Found: C, 86.79%; H, 8.68%; N, 4.40%. Mn D 80500, Mw D 194,600 g/mol. 3.1.2. PFN-TPA10 (1) (0.10 equiv), (3) (0.50 equiv), (4) (0.35 equiv) and (5) (0.05 equiv) were used in this polymerization. 1 H NMR (400 MHz, CDCl3).  (ppm): 7.86–7.84, 7.72–7.61, 2.11, 1.27–1.14, 1.01, 0.83–0.80. Anal. Found: C, 86.52%; H, 8.58%; N, 4.31%. Mn D 75600, Mw D 172,900 g/mol. 3.1.3. PFN-TPA50 (2) (0.50 equiv), (4) (0.45 equiv) and (5) (0.05 equiv) were used in this polymerization.1H NMR (400 MHz, CDCl3).  (ppm): 7.78–7.76 (d, 2H), 7.61–7.59(m, 8H), 7.36–7.32(t, 2H), 7.29–7.24(m, 6H), 7.12–7.08 (t, 1H), 2.19–2.07 (m, 4H), 2.05–2.03 (m, 16H), 0.92 (m, 4H). 13C NMR (100 MHz, CDCl3).  (ppm): 151.06, 147.56, 146.90, 139.82, 139.61, 135.84, 129.38, 127.88, 125.84, 124.58, 124.26, 123.15, 120.95, 120.06, 59.83, 54.89, 45.32, 37.92, 22.13. Anal. Found: C, 85.12%; H, 7.62%; N, 7.21%. Mn D 14600, Mw D 25,300 g/mol.

F. Huang et al. / European Polymer Journal 42 (2006) 2320–2327

Polymers were dissolved in toluene (for neutral polymers) or ethanol (for polyelectrolytes) and Wltered through a 0.45 m Wlter. Patterned indium tin oxide (ITO) coated glass substrates were cleaned with acetone, detergent, distilled water, isopropanol and subsequently in an ultrasonic bath. After treatment with oxygen plasma, 150 nm of poly-(3,4ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (Batron-P 4083, Bayer AG) or polyvinylcarbazole (PVK, Aldrich) from 1,1,2,2,-tetrachloroethane solution was spin-coated onto the substrate followed by drying in a vacuum oven at 80 °C for 8 h and evacuation at room temperature for PEDOT and PVK coating, respectively. Active layer copolymers for this study were spincoated onto top of PEDOT layer from toluene solution followed by deposition of 200 nm Al or Au (or 4 nm Ba and 200 nm Al) as a cathode under a vacuum of 1 £ 10¡4 Pa. The Wlm thickness of the active layers was around 70 nm, determined by an Alfa step 500 Surface ProWler (Tencor). Polymer processing (except PEDOT coating) and measurement device of performances was conducted in a dry box. In order to avoid contamination of volatile low work-function metal (Ba, Ca) during Al or Au cathode deposition on top of PFN-TPA layer, an evaporation chamber was thoroughly cleaned by baking at 120–130 °C and at a high vacuum (<1 £ 10¡4 Pa) before Al or Au deposition. A control device [ITO/ PEDOT (PVK)/MEH-PPV/Al(or Au)] was fabricated and PFN-TPA/Al (or Au) device to insure no low work-function metal residue left in the evaporation chamber from previous deposition. Current– voltage (I–V) characteristics were recorded with a Keithley 236 source meter. EL spectra were recorded with an Oriel Instaspec IV CCD Spectrograph. Luminance and external quantum eYciencies were determined by a calibrated photodiode referenced to a PR-705 SpectraScan Spectrophotometer (Photo Research) and integrating sphere (model IS 080, Labspere), respectively. 4. Results and discussion 4.1. Synthesis and characterization The synthetic procedures of the PFN-TPA copolymers are shown in Scheme 1. 2,7-bis(4,4,5, 5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylXuorene (3), 2,7-dibromo-9,9-bis(3⬘-(N,N-dimethylamino)-

propyl)Xuorene (4) were prepared according published procedures [6,7]. The copolymers derived from monomer 2, 3 and 4 were obtained by using Pd-catalyzed Suzuki coupling methods with Pd(PPh3)4 as the catalyst, in a mixture of toluene and aqueous Na2CO3 (2.0 M) in the presence of Aliquat 336 as a phase transfer reagent. The TPAs feed ratios are 1%, 5% and 10%, and the corresponding polymers are named PFN-TPA1, PFN-TPA5 and PFN-TPA10, respectively. A 0.5% of monomer 5 was introduced in all polymerization. In order to get a copolymer with high TPA content, PFN-TPA50, a TPAs diboronic ester 2 was used in the copolymerization with monomer 4 and 5 and the resulted copolymer was named PFN-TPA50. All the copolymers have a good solubility in common solvents such as chloroform, toluene, and tetrahydrofuran, etc. The amino-functionalized side chains give these polymers some new solubility properties, and all these polymers are readily soluble in polar solvents, such as methanol by adding a trace of weak organic acid (such as acetic acid), due to a weak interaction formed between the nitrogen atoms and the acetic acid [8]. The molecular weights of the copolymers were measured by gel permeation chromatography (GPC) against the polystyrene standard with THF as an eluting solvent. The number-average molecular weights (Mn) of PFN-TPA1, PFNTPA5, PFNTPA10 and PFN-TPA50 are 48,300, 80,500, 75,600 and 14,600 g/mol with a polydispersity index (Mw/ Mn) of 1.6, 2.4, 2.3 and 1.7, respectively. 4.2. UV–vis absorption and electrochemical properties The UV–vis absorption spectra of the copolymers in thin solid Wlms are shown in Fig. 1. For the copolymers with small amount of TPA units, PFNTPA1, PFN-TPA5 and PFN-TPA10 have similar

1 Normalized Intensity

3.2. LED fabrication and characterization

2323

PFN-TPA1 PFN-TPA5 PFN-TPA10 PFN-TPA50

0.8 0.6 0.4 0.2 0

350

400

450 500 Wavelength (nm)

Fig. 1. Polymers’ UV–vis absorption spectra.

550

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F. Huang et al. / European Polymer Journal 42 (2006) 2320–2327

Table 1 UV–vis absorption, electrochemical and photoluminescence properties of the polymers (in solid Wlms) Polymers

PFN-TPA1 PFN-TPA5 PFN-TPA10 PFN-TPA50 a b c

abs/nm 382 382 383 390

Optical band gapa

2.93 2.93 2.93 2.86

Eox/V 1.21 1.21 1.12 0.71

HOMOb/eV

¡5.61 ¡5.61 ¡5.52 ¡5.11

LUMOc/eV

¡2.68 ¡2.68 ¡2.59 ¡2.25

Photoluminescence PLsmax/nm

QPLsmax (%)

535 538 541 598

85 83 77 73

Estimated from the onset wavelength of optical absorption of thin solid Wlm. Calculated according to HOMO D ¡e(Eox + 4.4). Calculated from HOMO level and the optical band gap.

absorption spectra and show an absorption peak at around 382 nm, while PFN-TPA50 shows a redshifted absorption spectrum with a peak at 390 nm due to more regular alternating PFN-TPA mainchain structure compared with the other copolymers. The absorption edge of PFN-TPA50 is also red shifted to 434 nm from 423 nm for the other copolymers, demonstrating that the optical band gap of PFN-TPA50 (2.86 eV) is narrower than those of the other copolymers (2.93 eV) (Table 1). Because the content of BTDZ units in these copolymers is very low (0.5%), it is hard to detect 450 nm absorption feature corresponding to BTDZ unit [6]. The electrochemical behavior of the polymers was investigated by cyclic voltammetry (CV). A platinum electrode was coated with a thin polymer Wlm and was used as the working electrode. A Pt wire was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The measurement was performed in a 0.1 M n-Bu4NPF6 solution in acetonitrile at room temperature. Fig. 2 shows the CV curves of the copolymers. PFN-TPA1 and PFN-TPA5 show a quasi-reversible oxidation wave with onset at around 1.21 V. With the increase of TPA content in the polymers, the onset of oxidation potential of PFN-TPA10

PFN-TPA50 PFN-TPA10 PFN-TPA5 PFN-TPA1

0

0.5

1 Voltage (V)

1.5

Fig. 2. Cyclic voltammograms of the polymers Wlms.

2

decrease to 1.12 V (Table 1). When the TPA content in the polymer chains increase to 50% percent, PFNTPA50’s oxidation wave becomes irreversible and its onset of oxidation potential decrease to 0.71 V. According to an empirical relation [9], the ionization potential (HOMO) of a conjugated polymer are approximately equal to the onset oxidation potential (vs. SCE) and the onset reduction potential (vs. SCE) plus 4.4 eV (the SCE energy level below the vacuum level), respectively. Table 1 lists the calculated HOMO levels of the copolymers. It can be found that HOMO levels of the copolymers are decreasing with increasing TPA content. When the TPAs content is increased to 50%, PFN-TPA50’ HOMO energy level is 5.11 eV, which is similar to the data reported previously for copolymers incorporating triarylamine units into the polyXuorene main chains [4d]. We were unable to record n-doping processes after many attempts. In order to get some idea about LUMO levels in these copolymers, LUMO were estimated from the optical band gap and HOMO energies (Table 1). The LUMO levels of the copolymers increase with the increasing of TPA content. 4.3. Photoluminescence properties Photoluminescence spectra of the thin solid Wlms of the copolymers are shown in Fig. 3. Though there are only 0.5% BTDZ units in the copolymers, PL emission of all the copolymers consists exclusively of BTDZ units’ emission [6] except for PFN-TPA50, indicating the eYcient energy transfer from PFNTPA segments to BTDZ units under photoexcitation. PFN-TPA50 displays a red-shifted emission with a maximum at 598 nm, consistent with its redshifted absorption spectrum compared to the other copolymers (Fig. 1). There is a very small peak observed at around 460 nm from PFN-TPA units in PFN-TPA50’ PL spectrum, demonstrating the

F. Huang et al. / European Polymer Journal 42 (2006) 2320–2327 1 PFN-TPA1 PFN-TPA5 PFN-TPA10 PFN-TPA50

0.8 0.6

Normalized PL

Normalized PL

1

0.4 0.2 0 400

2325

500

600 700 Wavelength (nm)

0.6 0.4 0.2 0 400

800

PFN-TPA1 PFN-TPA5 PFN-TPA10 PFN-TPA50

0.8

500

600 700 Wavelength (nm)

800

Fig. 3. PL emission spectra for the polymers in thin Wlm.

Fig. 4. EL emission spectra of the polymers with a device conWguration of ITO/PVK/copolymer/Al.

slightly incomplete energy transfer among PFNTPA50. The absolute PL quantum eYciencies (QEs) are also listed in Table 1. All the copolymers possess very high PL eYciency (QEPL) due to the eVective intra- and inter-molecular energy transfers in these copolymers. PFN-TPA1 and PFN-TPA5 possess QEPL values of 85% and 83%, respectively. With the increasing TPA content, the QE values of copolymers are slightly decreased to 77% for PFN-TPA10 and 73% for PFN-TPA50.

indicates that intra- and inter-molecular energy transfer among the copolymers is an eYcient process. Similar to PL emission in the solid state, EL emission was from BTDZ unit with an emission peak at around 541–587 nm. Table 2 lists the device performances of these copolymers in diVerent conWgurations at currents of around 5 mA. It is found that all the copolymers show excellent performance with Al as the cathode, similarly as reported previously for polyXuorenes containing the electron injection units DMAPF in the copolymers [5–7]. For the copolymer containing 1% TPA, PFN-TPA1 shows a very poor device performance with external quantum eYciency around 0.2%, due to the large injection barrier between the emission layer and PEDOT layer. With the increasing TPA content in the copolymers, device eYciency from PFN-TPA10, PFN-TPA50 are greatly enhanced indicating more balanced carrier injection due to the improved hole-injection. The devices with PFN-TPA10 and PFN-TPA50 exhibits great improved QE (more than 1%) both in Al cathode device and in Ba cathode device, with a luminescence more that 500 cd/m2 at the currents density around 30–40 mA/cm2 (Table 2). Because of using

4.4. Electroluminescence properties Electroluminescence (EL) devices from PFNTPA copolymers were fabricated with the conWguration: ITO/anode buVer/polymer/cathode, where anode buVer was PEDOT and cathode was Al or Ba/Al. Fig. 4 shows the EL spectra of the devices (conWguration: ITO/PEDOT/copolymer/Al) based on these polymers. It can be found that all the copolymers’EL emissions were exclusively characteristic from the BTDZ unit. Especially for PFNTPA50, the small peak at around 460 nm in its PL emission caused by the incomplete energy transfer is completely disappeared in its EL emission. This

Table 2 Electroluminance properties and device performances of the copolymers with a device conWguration ITO/PEDOT/copolymer/Al (or Ba/Al) Polymers

PFN-TPA1 PFN-TPA5 PFN-TPA10 PFN-TPA50

max/nm 541 541 550 552 552 549 585 587

Cathode

Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al

Device performances Voltage (V)

Current intensity (mA/cm2)

Lumin. ( cd/m2)

QE%

19.2 19.5 19.5 20.3 17.6 18.3 20.0 20.1

36.6 32.7 35.3 35.2 32.7 32.7 38.0 30.7

94 78 305 298 512 539 579 704

0.22 0.20 0.72 0.72 1.3 1.4 1.1 1.9

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Table 3 Device performances of the copolymers (device conWguration: ITO/PVK/PFN-TPA/Au) max/nm

Polymers

PFN-TPA10 PFN-TPA50

553 594

Device performances Voltage (V)

Current intensity (mA/cm2)

Lumin. ( cd/m2)

QE%

23.4 26.2

34.7 36.0

446 418

1.1 0.98

novel electron injection units DMAPF, all the polymers show good performance with Al as the cathode, which is favorable for stability of the LEDs. It has been reported by us that the electron injection ability of DMAPF almost works for all the high work function metal cathode, including Au [5b], which is very diVerent from the other established ways [10–12]. For example, by inserting insulating polar or ionic species such as LiF [10]. Al2O3 [11] or organic surfactant [12] between Al metal electrode and light emitting layer all can improve the electron injection from Al cathode. But all these ways have a cathode metal dependence and can not improve the electron injection from other high work function metals (such as Au). In order to get some idea of these copolymers’ EL performance in Au cathode device, PFN-TPA10 and PFN-TPA50 were used as the emission layer in a device conWguration (ITO/ PVK/PFN-TPA/Au). Table 3 lists the results of these devices. The results are very promising. The external EL QE reached 1.10% for PFN-TPA10 with a luminescence of 446 cd/m2 at the current density of 34.7 mA in the device conWguration of ITO/ PVK/copolymer/Au. PFN-TPA50 also exhibits good performance in Au cathode device with an EL QE of 0.98% and a luminescence of 418 cd/m2 at the current density of 36.0 mA/cm2. Fig. 5 shows their EL spectra in device conWguration of ITO/PVK/

copolymer/Au, which is almost identical to their EL spectra in Al cathode device (Fig. 4). Au is a most noble metal with a high work-function (u D 5.2 eV). The excellent performance of PFN-TPA copolymers in Au cathode device make them a promising candidate for fabrication and patterning of air-stable Xat panel displays. 5. Conclusions We have synthesized a novel series of copolymers (PFN-TPA) containing a small amount (0.5%) of narrow band gap BTDZ units, hole-transporting unit TPA and a novel electron injection unit 9,9-bis(3⬘(N,N-dimethylamino)propyl)Xuorene (DMAPF) by Suzuiki coupling reactions. The eYcient energy transfer due to exciton trapping on the narrow band gap BTDZ sites has been observed. The added TPA and DMAPF units make the copolymers have a balanced exciton transporting ability and all the copolymers show good EL performances in the devices with Ba, Al and even Au as cathode. Acknowledgements This work was supported by research grants from Ministry of Science and Technology (Project No. 2002CB613402) and the National Natural Science Foundation of China (Project No. 50433030).

1 PFN-TPA10 PFN-TPA50

References

Normalized EL

0.8 0.6 0.4 0.2 0 300

400

500

600 700 Wavelength (nm)

800

900

Fig. 5. EL emission spectra of the polymers with a device conWguration of ITO/PVK/copolymer/Au.

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