An alkylsilylphenoxy PPV chromophore

Synthetic Metals 130 (2002) 203±212

An alkylsilylphenoxy PPV chromophore Sung-Ho Jina, Hyung-Jong Leeb, Yeong-Soon Galc, Taehyoung Zyungd, Hyun-Nam Choe, E. Elif GuÈrelf, F.E. Karaszg,* a

Department of Chemistry Education, Pusan National University, Pusan 609-735, South Korea b Zen Photonics, Daeduk-gu, Daejon 306-230, South Korea c Polymer Chemistry Laboratory, College of General Education, Kyungil University, Hayang 712-701, South Korea d Korean Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea e Electronics and Telecommunications Research Institute, Kajong-Dong, Taejon 305-350, South Korea f Quantira Technologies Inc., 250 14th Street NW, Suite 4004, Atlanta, GA 30318, USA g Department of Polymer Science and Engineering, University of Massachusetts, 120 Governor's Drive Amherst, Amherst, MA 01003, USA Received 27 December 2001; accepted 16 May 2002

Abstract The synthesis and characterization of a poly(p-phenylene vinylene) (PPV) based chromophore containing a long chain alkylsilylphenoxy side group have been reported. Physical and electroluminescent (EL) properties of the green light emitting polymer, poly[2-40 -dimethyldodecylsilylphenyloxy-1,4-phenylene vinylene] (SiPhOPPV), in single and multilayer con®gurations have been investigated. The maximum brightness and ef®ciency were found to be 2600 cd/m2 at 23 V and 0.24 lm/W at 12 V for double layer device con®guration in which PEDOT (25 nm) was used a hole transport layer. The use the SiPhOPPVas one of the components in a chromophore blend system was also investigated and the results compared to those for other related blends. When SiPhOPPV is used in a double layer blend con®guration, EL intensity as well as device ef®ciency increased by a factor of 5. The composition of blend systems, the ratio of hole and electron transport components and the degree of phase separation between different chromophores are found to be important parameters for the device ef®ciency and balanced charge injection. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electroluminescence; Silicon containing PPV; Polymer blend chromophore

1. Introduction Extensive studies have been made on organic and polymeric light emitting diodes (LEDs) because of their practical application as well as inherent scienti®c interest [1±4]. Polymer LEDs using poly(p-phenylene vinylene) (PPV) were ®rst reported in 1990 by Burroughs et al. [5] and since then various other kinds of conjugated polymers, such as PPV containing substitutients, poly(9,90 -dialkyl¯uorene)s [6] and polyalkylthiophenes [7] have been developed which exhibit electroluminescence. Among the conjugated polymers, PPV derivatives and poly(9,90 -dialkyl¯uorene)s have been the most used chromophores. Conjugated polymers have many advantages in terms of versatile multicolor emission controlled readily by the molecular structure, and facile fabrication of small and large area active layers by spin coating or printing technology. Some problems, such as polymer purity, * Corresponding author. Tel.: ‡1-413-545-4783; fax: ‡1-413-253-5295. E-mail address: [email protected] (F.E. Karasz).

and puri®cation, control of polymerization and low luminance ef®ciency remain. To achieve an ef®cient polymer LED, we have synthesized and reported several types of novel electroluminescent polymers which are PPV derivatives containing bulky phenyl substitutents [8] and single hetero-atom linkages [9] in the polymer main chain to control the effective conjugation length. Carbazole containing polymers and multicomponent systems incorporating an electron transporting polymer with an oxadiazole repeating unit to control the color tuning of the emitting polymer have also been reported [10,11]. Asymmetric PPV derivatives have been designed to decrease crystallinity and to reduce morphological defects [12]. We have also reported synthesis of a soluble green light emitting PPV derivative containing alkyl silylphenyl side groups. In this contribution, we discuss the properties of a PPV chromophore containing a long chain alkylsilylphenoxy side group designed to increase ef®ciency. LEDs using this material in a double layer architecture with a PEDOT HTL were studied as well as blue and green emitting chromophore blend systems using PPV as an HTL.

0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 1 7 - 0

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2. Experimental section 2.1. Materials 1,4-Dibromobenzene, chlorododecyldimethylsilane, 1,4dimethylphenol, n-butyllithium (1.6 M hexane solution), copper (I) bromide, N-bromosuccinimide (NBS), and potassium tert-butoxide (1.0 M solution in THF) (Aldrich) were all used as received without further puri®cation unless otherwise noted. Solvents were puri®ed by normal procedures and handled in a moisture free atmosphere. Moisture contents of THF were <10 ppm. Column chromatography was performed using silica-gel (Merck, 250±430 mesh). 2.2. Synthesis of 1-bromo-4-(dimethyldodecylsilyl)benzene (1) 1,3-dibromobenzene (4.62 g, 19 mmol) and dry THF (150 ml) were added into a 250 ml ¯ask under a N2 atmosphere. The ¯ask was cooled to 78 8C. With stirring, a n-BuLi (11.9 ml, 1.6 M n-hexane solution, 19 mmol) was added dropwise causing a lightening of the yellow color. After 1 h stirring at 78 8C, the mixture was warmed to room temperature and stirred additionally for 1 h. After cooling to 78 8C, chlorododecyldimethylsilane (5.0 g, 19 mmol) was added dropwise, and the system slowly warmed to room temperature and stirred for an additional 3 h. Most of the solvent was evaporated under reduced pressure to give a yellow oil. The crude product was further puri®ed by vacuum distillation (bp 155 8C under 0.8 mmHg, yield 74%). 1 H-NMR (CDCl3, d) 0.27 (s, 6H, Si(CH3)2, 0.77 (t, 2H, SiCH2), 0.92 (t, 3H, ±CH3), 1.27 (m, 20H, (CH2)10), 7.21 and 7.49 (d, 4H, aromatic protons). 2.3. Synthesis of 1,4-dimethyl-2(40 -dimethyldodecylsilylphenyl)oxy benzene (2) A mixture of 1 (20 g, 59 mmol), 1,4-dimethylphenol (21.6 g, 178 mmol) and potassium carbonate (12.2 g, 89 mmol) in pyridine (200 ml) was heated under nitrogen atmosphere to 100 8C. Copper (I) bromide (0.6 g, 4.2 mmol) was added and the reaction mixture was heated to 115 8C overnight. The reaction mixture was then cooled to room temperature and poured into ice water. After acidi®cation with 1N HCl, the crude product was extracted with diethylether. After evaporation of the solvent, the product was chromatographed on silica with hexane as an eluent to yield 65% of 2. 1 H-NMR (CDCl3, d): 0.25 (s, 6H, Si(CH3)2), 0.74 (t, 2H, SiCH2±), 0.90 (t, 3H, ±CH3), 1.27±1.56 (b, 20 H, (CH2)10), 2.28, 2.56 (s, 6H, 2CH3). 2.4. Synthesis of 1,4-bis(bromomethyl)-2(40 -dimethyldodecylsilylphenyl)oxybenzene (3) A mixture of 2 (15 g, 39.6 mmol), NBS (16.2 g, 91.1 mmol) and catalytic amount of benzoyl peroxide in

CCl4 (150 ml) was heated to re¯ux for 6 h under a N2 atmosphere. As the reaction proceeded, succinimide was precipitated and the reaction progress was monitored by TLC on silica plate. The warm reaction mixture was ®ltered under suction and washed with hot CCl4 twice. After the solvent was evaporated, the crude product was puri®ed by column chromatography by using hexane as an eluent to yield 25% (4.14 g). 1 H-NMR (CDCl3, d): 0.1 (s, 6H, Si(CH3)2), 0.48 (t, 2H, SiCH2), 0.62 (t, 3H, ±CH3), 0.99± 1.03 (b, 20H, (CH2)10), 4.1, 4.3 (s, 4H, ±CH2Br), 6.62±7.24 (m, 7H, aromatic protons). 2.5. Preparation of poly[2-(40 -dimethyldodecylsilylphenyl)oxy-1,4-phenylene vinylene] A solution of 3 (0.2 g, 374 mmol) in dry, degassed THF (30 ml) was heated to 60 8C under a nitrogen atmosphere. With stirring, potassium tert-butoxide (3.74 ml, 1 M solution in THF) was slowly added dropwise for 30 min using a syringe pump. The reaction mixture became viscous and changed to a greenish color. After 3 h, a small amount of 4-tert-butylbenzyl bromide was added to the polymerization solution to endcap. After further stirring for 1 h, the mixture was precipitated into methanol (200 ml). The resulting yellow polymer, poly[2-(40 -dimethyldodecylsilylphenyl)oxy-1,4phenylene vinylene] (hereafter, SiPhOPPV) was ®ltered and Soxhlet extracted with methanol to remove the low molecular weight oligomers and other impurities. The SiPhOPPV was redissolved in chloroform and reprecipitated into methanol. After ®ltration and drying under vacuum, a bright yellow ®brous polymer was obtained (0.13 g). 2.6. Characterization 1 H-NMR spectra were obtained using a Bruker AM-300 spectrometer, and chemical shifts were recorded in ppm units against the residual proton solvent resonance (chloroform: 7.26 ppm). UV±VIS spectra were carried out with a Shimadzu UV-3100 spectrophotometer with baseline correction and normalization obtained using Microsoft Excel software. FTIR spectra were measured using KBr pellets and a Nicolet DX-5B spectrometer. The molecular weight and polydispersity of the polymer in THF solution were determined by gel permeation chromatography (GPC) analysis relative to polystyrene calibration (Waters high-pressure GPC assembly Model M590 pump, m-Styragel columns of 105, 104, 103, 500, and 100, refractive index detectors). Emission spectra were made with dilute solutions (10 6 M) on a Perkin-Elmer LS-50 ¯uorometer utilizing a lock-in ampli®er system with a chopping frequency of 150 Hz. Solid-state emission measurements were performed using ®lms supported on glass substrates and mounted with front-face excitation at an angle of <458. For the measurement of EL, a light emitting diode was constructed as follows. A glass substrate coated with transparent ITO electrode was thoroughly cleaned by successive ultrasonic

S.-H. Jin et al. / Synthetic Metals 130 (2002) 203±212

treatments in acetone and isopropyl alcohol and dried with nitrogen gas. The polymer ®lm was prepared by spin-casting a polymer solution containing 1% by weight in chlorobenzene. Uniform and pinhole free ®lms with a thickness around 80 nm were easily obtained from the resulting polymer solution. For the double layer devices using PEDOT [poly(3,4-ethylenedioxythiophene)] as a hole injection/ transport layer, a modi®ed water dispersion of this polymer doped with poly(styrene sulfonate) (PSS) (Bayer AG, Germany) was used. Metal contact was achieved by depositing an Al:Li alloy through a mask by vacuum evaporation at pressure below 4  10 6 Torr, yielding active areas of 4 mm2. For the measurements of the device characteristics, current±voltage (I±V) curves were measured using a current±voltage source (Keithley 238) and an optical power meter (Newport 818-SL). All processes and measurements mentioned above were carried out in air at room temperature. Devices using PPV as a hole injection layer were also constructed using methods previously described [13±16]. In these devices electron (PBD) and hole transport (PVK) materials were added to a mixed chromophore layer in which alternating block copolymers containing phenylene vinylene blocks [13,16] were added to the SiPhOPPV to enhance ef®ciency.

205

3. Results and discussion 3.1. Synthesis Scheme 1 shows the relevant synthetic scheme for the monomer and corresponding polymer. The asymmetric monomer 3 was prepared in a three-step reaction. The coupling of chlorodimethylsilane and 1,4-dibromobenzene was almost quantitative to give 4-dimethyldodecyl-1-bromobenzene. The long chain alkylsilyiphenyloxy group was introduced into the xylene backbone by means of a copper catalyzed Ullmann coupling reaction. During an Ullmann reaction, most alkyloxy side chains on the phenyl ring tend to be cleaved by copper catalysts. However, in our system the alkylsilylphenyl bond was stable, and we did not detect any bond cleavage. After puri®cation of 2 using column chromatography, the bromomethylation reaction was carried out with N-bromosuccinimide in the presence of a catalytic amount of benzoyl peroxide to give the asymmetric monomer 3. The important concept of this monomer design is the introduction of silicon compounds into the side chain phenyl ring and the synthesis of an asymmetric monomer which was to differentiate the acidity of the a-proton to control the stereoregularity of the resulting polymer. The polymerization of the bis(bromomethyl) benzene derivative containing

Scheme 1.

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the alkylsilylphenyloxy side chain was carried out with an excess of potassium tert-butoxide in anhydrous THF, (Gilch method). The present monomer was allowed to polymerize to a moderate extent with an excess of potassium tertbutoxide in THF. An intense green photoluminescence was observed in the reaction mixture but no precipitate or gelation was observed as has often been found with phenyl substituted PPV polymerizations. The polymerization mixture was quenched with a few drops of 4-(tert-butyl)benzyl bromide and further stirred for 1 h. The resulting polymer was completely soluble in common organic solvents, e.g. chloroform, THF, xylene, toluene, etc. The number (Mn) and weight (Mw) average molecular weights found are 154,100 and 353,000 with a polydispersity index of 2.29. The structural analysis of the polymers was con®rmed by 1 H-NMR and IR spectroscopy. As the polymerization proceeded, the bromomethyl peaks of the monomer at 4.10 and 4.30 ppm disappeared and new vinylic proton peaks together with aromatic proton peaks appeared at 6.8±7.2 ppm. The assignment of other proton peaks in the spectrum were identical in both the monomer and polymer. Because of the good solubility of the polymer, an optical thin ®lm could be readily cast onto the ITO substrate with ®lm thickness controlled by changing the solution concentration. The UV± VIS, photoluminescence and electroluminescence spectra of the polymer are shown in Fig. 1. The absorption spectrum of the thin ®lm has a relatively sharp maximum at 450 nm with a band edge of 545 nm. The emission spectrum was obtained by excitation at the maximum absorption wavelength of the polymer. The maximum emission of the polymer was at 540 nm (yellow±green) with a shoulder at 575 nm. The EL spectrum of a SiPhOPPV device (ITO/PEDOT/SiPhOPPV/ Li:Al) is almost identical to that of the PL spectrum and

result indicates that EL of the device occurs at the same emission center as does PL in the SiPhOPPV ®lm. 3.2. LEDs with PEDOT layer The work function and the surface roughness of the ITO substrate have important effects on the device performances, such as turn-on voltage and luminance ef®ciency. Most p-conjugated semiconducting polymers have a HOMO level more than 5 eV relative to vacuum and there is therefore, a signi®cant energy barrier at the HOMO/ITO interface responsible for an increase in the turn-on voltage of the device. To facilitate hole injection, and to improve the luminescence ef®ciency, polyaniline (PANI) doped with camphor sulphonic acid (CSA) or polythiophene derivatives (PEDOT) doped with PSS have been used [17] as a holetransporting layer and which also has the effect of smoothing the interface. In the present device PEDOT was used on the surface treated substrate. Fig. 2 shows the voltage±currentluminance characteristics of the ITO/(PEDOT)/Polymer/ Al:Li device. The turn-on voltages of devices with and without PEDOT are 6 and 10 V, respectively. The introduction of a PEDOT buffer layer thus signi®cantly decreases the turn-on voltage. Fig. 3 shows the current density and luminescence intensity of the polymer as a function of applied voltage. The linear dependence of the intensity on current density is attributed to a linear response of the recombination of the charge carriers to form singlet excitons. The maximum ef®ciency of the SiPHOPPV single layer device (80 nm on ITO glass) is 0.313 lm/W at 13.8 V with brightness of 44.3 cd/m2. The maximum brightness is 787 cd/m2 at around 20 V for this device. The brightness of the device increased to 2600 cd/m2 at 23 V when

Fig. 1. UV±VIS absorption, PL spectra in solid state and EL spectrum of the ITO/PEDOT/SiPhOPPV/Al:Li device (PEDOT: 25 nm; SiOPhPPV: 80 nm; Al: 100 nm; Li: 1 nm). The active pixel area is 4 mm2.

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207

Fig. 2. Voltage-luminescence characteristics of an ITO/(PEDOT)/SiPhOPPV/Al:Li devices. Current luminescence characteristics of the device is shown in the inset. Thicknesses of each layer are the same as in the device shown in Fig. 1.

SiPhOPPV was used in a double layer con®guration with PEDOT (25 nm). The maximum ef®ciency of the double layer device was 0.24 lm/W at 12 V. 3.3. LEDs with multicomponent architectures Multicomponent single and double layer devices were also prepared using the SiPhOPPV chromophore. Blends

containing hole and electron transport materials and blends of blue and green chromophores were used in double layer device con®gurations in which PPVwas interspersed between ITO and the chromophore blend. In these systems, poly(N-vinylcarbazole) (PVK) and butyl-PBD(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole) were used as the hole and electron transport materials, respectively. The structures of the blue light

Fig. 3. Dependence of luminescence intensity on injection current of an ITO/PEDOT/SiPhOPPV/Al:Li device. Thicknesses of each layer are the same as in the device shown in Fig. 1.

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Fig. 4. Molecular structures of blue I and green II light emitting copolymers used in multicomponent polymer blend systems.

emitting (I) and green light-emitting (II) alternating block copolymers are shown in Fig. 4. The syntheses and characterization of these copolymers were reported earlier [13±15]. As a reference SiPhOPPV was also used in a single layer con®guration without charge transport layers in which the polymer was dissolved in chloroform (2 wt.%) and coated (90 nm) on ITO glass by spincasting (3800 rpm for 30 s). Ca and Al (as a protecting layer) alloy were used as cathode (vacuum deposition, 10 7 Torr, 400 nm). In the double layer con®guration, a PPV layer was used as a HTL to improve the longevity of the devices. The PPV layer thicknesses were 15 and 60 nm for the blue and green emitting blends [17,18,19], respectively. In this process, a PPV precursor [20] was dissolved in methanol and spin-cast on ITO glass, eliminated at 250 8C under argon atmosphere for 2.5 h. In the double layer devices, multicomponent blends of selected compositions were spun-cast from chloroform solutions as a second layer onto the insoluble PPV. The compositions of blue and green emitting chromophores blends are given in Table 1.

Table 1 Double layer, multicomponent polymer blend compositions ratios Blend

PPV layer (nm)

PVK

ButylPBD

I

II

SiPhOPPV

Blue 1a 1b

15 15

12 12

4 4

4 4

± 0.4

0.4 ±

Green 2a 2b

60 60

12 12

2 2

4 4

± 2

2 ±

4. Results and discussions Fig. 5 shows electroluminescence (EL) spectra of the SiPhOPPV single layer device at constant bias (7 V). The lmax of 531 nm is 9 nm blue shifted relative to the PEDOT containing device (Fig. 1). In the blue emitting multicomponent LEDs we are able to compare the effectiveness of SiPhOPPV as an additive to that of II. EL emission spectra (7 V pulse bias) from blue blends (1a and b) are shown in Fig. 6. The spectrum of the blend 1a (SiPhOPPV is the green chromophore component) shows two maxima at 455 and 511 nm, indicating a possible phase separation between the blue I and green (SiPhOPPV) chromophore components in the blend structure. However, the blend 1b (lmax ˆ 464 nm) shows only a shoulder around 490 nm, which is related to diluted green chromophore II in the blend. The comparison of EL intensity with applied voltage for blends 1a and b are given in Fig. 7a. Turn-on voltages were around 7.5 and 5.5 V for 1a and b, respectively. Fig. 7b shows the EL intensity versus current density. It is evident that the green copolymer II is more compatible in the blue light emitting blend system. This copolymer (Fig. 4) contains 80 mol% of structure I and 20 mol% of additional PV units randomly inserted in the structure. Although SiPhOPPV containing bulky alkylsilylphenyl pendant group is a highly ef®cient green light emitter, its contribution is less effective in the blue emitting blend system (1a) in which it was inserted with copolymer I in a weight ratio of 0.4:4. The copolymer II enhances the ef®ciency of blue emission because of phase separation and quantum dot effects [20±22]. We believe that the chromophore (copolymer II) forms domains in PVK and butyl-PBD matrix because of the immiscible character of the components [16]. This microphase separation will serve to con®ne injected charge carriers while increasing the device ef®-

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209

Fig. 5. EL spectrum of SiPhOPPV, under pulse voltage (7 V).

ciency. The domains of chromophore II thereby function analogously to quantum wells in inorganic semiconductors [21,22]. SiPhOPPV also phase separates but because of a greater incompatibility of this material compared to II, the latter effects are absent. The difference in morphology

clearly accounts for the less ef®cient performance of device 1a. The SiPhOPPV is a strong green light emitting chromophore and it is of interest to further improve the EL ef®ciency of this chromophore by blending with hole and

Fig. 6. Comparison of EL spectra of blue emitting blend devices prepared by SiPhOPPV and II. Compositions of 1a and 1b are given in Table 1.

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Fig. 7. (a) EL intensity vs. dc voltage for blue emitting blend devices 1a and b; (b) comparison of EL intensity±current density characteristics of 1a and b blue emitting blend devices (PPV: 15 nm; polymer chromophore: 90 nm; Ca: 400 nm, active pixel area: 6 mm2).

electron transporting materials as well as in multicomponent chromophore architectures. EL spectra of green emitting blends (2a) and the single layer device of SiPhOPPV under 7 V bias are compared in Fig. 8. For the blend used (2a) no blue shift or shoulder is observed in lmax of the EL spectra because of higher concentration of the green emitter and EL intensity as well as device ef®ciency

increases by a factor of 5 when SiPhOPPV is used in double layer blend con®guration (blend 2a) as shown in Fig. 9. The EL intensity and current density dependence on the applied dc voltage are more balanced (as compared to single layer devices of SiPhOPPV) as indicated in Fig. 10. The turn-on voltage remains almost the same (5.5 V) for both architectures.

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Fig. 8. Comparison of EL spectra for SiPhOPPV single layer and green blend (composition 2a, Table 1 devices, SiPhOPPV layer: 90 nm; PPV: 60 nm; Ca: 400 nm, active pixel area 6 mm2).

Fig. 9. EL intensity vs. current density characteristics of green emitting blend (2a) and single layer device using SiPhOPPV. Thickness of each component is given in the caption of Fig. 8.

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Fig. 10. EL characteristics of green blend (2a) device with balanced charge injection. Thickness of each component is given in the caption of Fig. 8.

5. Conclusion An alkylsilylphenoxy derivative of PPV has been synthesized and shown to be an ef®cient green light emitter in single and double layer LED con®gurations. In multicomponent chromophore blends this derivative is more completely phase separated from other PPV derivatives whose properties had been previously shown to be enhanced in blends. This more extensive phase separation and the resulting changed morphology prevents ef®cient energy transfer and consequently, in these combinations enhancement is absent. Acknowledgements This work was supported by the Institute of Information Technology Assessment Grant (IITA-C1-0019-0001). EG and FEK gratefully acknowledge support from AFOSR.

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