New confined p-phenylenevinylene (PPV)-type polymer analogue of poly(phenylene sulfide)

European Polymer Journal 44 (2008) 2886–2892

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New confined p-phenylenevinylene (PPV)-type polymer analogue of poly(phenylene sulfide) Najmeddine Jaballah a, Mustapha Majdoub a,*, Jean Louis Fave b, Carlos Barthou b, Mohamed Jouini c, Jean Tanguy c a

Laboratoire des Polymères-Biopolymères-Matériaux Organiques (LPBMO), Faculté des Sciences de Monastir, Bd. de l’Environnement, 5019 Monastir, Tunisia Institut des NanoSciences de Paris (INSP), Université Paris 6 et 7 et CNRS, 140 rue de Lourmel, 75015 Paris, France c ITODYS, Université Paris 7-Denis Diderot, 1 rue Guy de La Brosse, 75005 Paris, France b

a r t i c l e

i n f o

Article history: Received 17 September 2007 Received in revised form 23 April 2008 Accepted 4 July 2008 Available online 10 July 2008

Keywords: Conjugated polymers Confined p-phenylenevinylene Poly(p-phenylene sulfide) Photoluminescence Light-emitting diodes

a b s t r a c t A new confined p-phenylenevinylene (PPV)-type polymer (PPVS) has been synthesized using Wittig condensation. The chemical structure of the polymer was well defined by 1 H NMR, 13C NMR, and FTIR spectroscopic analysis. PPVS contains oligomeric PPV units separated by sulfide bridges in the main chain; it is fully soluble in common organic solvents and has a number-average molecular weight of 3500 g mol1. Thermogravimetric analysis and differential scanning calorimetry indicate that PPVS is amorphous, stable up to 360 °C in air and displays a glass transition temperature of 98 °C. The optical properties of the polymer were investigated by UV–visible absorption and photoluminescence spectroscopies. The polymer film absorbs at 375 nm and emits at 517 nm with a narrow emission spectrum. From the cyclic voltammetry analysis, the electrochemical bandgap was estimated to be 2.78 eV. A single-layer diode device of the configuration indium-tin oxide/PPVS/aluminium has been fabricated and has a relatively low turn-on voltage of 3.4 V. An electroluminescent emission similar to photoluminescence is demonstrated in a multilayer device. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The discovery of electroluminescence in poly(p-phenylenevinylene) (PPV), [1], has produced a new area in polymer sciences: the p-conjugated semi-conducting polymers. Since then, a tremendous progress has been made in the molecular engineering of conjugated polymers and in their uses as active components in polymeric lightemitting diodes (PLEDs) [2,3]. These organic materials are currently expanding their applications to others devices, such as light-emitting electrochemical cells [4,5], thin film transistors [6], solar cells [7,8] and chemical sensors [9,10]. PPV and its derivatives remain at the center of focus in conjugated polymers field and many efforts have been devoted * Corresponding author. Tel.: +216 73 500 280; fax: +216 73 500 278. E-mail addresses: [email protected], mustaphamajdoub @gmail.com (M. Majdoub). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.003

to further improving their synthesis and investigating their physical properties. Several PPV derivatives were prepared by varying the synthetic routes and the monomer chemical structures. According to its chemical structure, each one of these derivatives presents its distinctive physico-chemical properties which militate for its use in such or such application. Concerning the optical properties, introducing donor and acceptor substituents in the PPV backbone leads to photoluminescent polymers emitting in various regions of the visible spectrum. Furthermore, disorder reduces the effective length of conjugation of these fully conjugated PPVs and shifts their emission. Hence, the emission occurs usually from the more highly conjugated segments, and the emission spectrum is broad [3,11]. To solve this problem, the confinement of the conjugation into a welldefined length is one of the most successful developed strategies [12]. Some confined PPV-type polymers, containing well-defined oligomeric PPV units separated with

N. Jaballah et al. / European Polymer Journal 44 (2008) 2886–2892

non-conjugated aliphatic spacer units in the polymer backbone, were reported [12,13]. Such polymers usually exhibit good solubility, they are homogeneous in terms of conjugation length and can be designed to emit in any portion of the visible spectrum with a narrow emission spectrum. Nevertheless, the aliphatic non-conjugated segments, used as spacers in these polymers, can act as a barrier to injection and mobility of the charge carriers, resulting in higher turn-on voltages [2]. Poly(p-phenylene sulfide) (PPS) is a high-performance thermoplastic with many desirable characteristics, including outstanding thermal, oxidative and chemical resistance [14]. In particular, PPS has a Tg of 85 °C and a Td above 420 °C [15] and shows electrical conductivity greater than 10 S cm1 when doped with AsF5 [16]. In this paper, we report the first PPV analogue of PPS. This new confined PPV-type polymer contains a sulfide bridge as spacer unit. Such a structure maximizes the active conjugated moiety. The synthesis, structural, thermal, optical, electrochemical and electrical characterizations are presented here in detail.

2. Experimental 2.1. Materials and measurements 4,40 -Thiodiphenol (TDP) (Aldrich, 99%), bromoethane (Acros, 98%), potassium carbonate (Acros, 99%), paraformaldehyde (Acros, 96%), triphenylphosphine (Acros, 99%), terephthaldicarboxaldehyde (Aldrich, 99%) and potassium tert-butoxide (Acros, 98%) were used as received. Tetrahydrofuran (THF) was dried over Na/benzophenone and freshly distilled before use. Acetone was stirred over and distilled from potassium carbonate under argon. Other solvents were commercially available and were used without further purification. Melting points (mp) were determined on an Electrothermal Mode 9100 digital analyzer and were not corrected. 1H NMR and 13C NMR spectral data were obtained on a Bruker AV 300 spectrometer. FTIR spectra were acquired on a Perkin–Elmer BX FT-IR system spectrometer by dispersing samples in KBr disks. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on a Setaram TG-DTA 92-10 thermal analyzer under ambient atmospheric conditions at a heating rate of 10 °C min1. DSC was performed on a Setaram instrumentation-regulation DSC 131 system with a heating rate of 10 °C min1. UV–vis absorption spectra were recorded on a Cary 5000 UV–vis-NIR spectrophotometer. Photoluminescence (PL) spectra were obtained on a Jobin–Yvon spectrometer HR460 coupled to a nitrogencooled Si Charged-Coupled Device (CCD) detector. Samples were excited with a 450 W Xenon lamp at 370 nm. For solid state measurements, the films were spin coated onto a silica substrate from a 2  102 M chloroform solution. All measurements were performed at room temperature (25 °C). Cyclic voltammetry (CV) was performed on an EG&G model 273 potentiostat/galvanostat (Princeton Applied Research) in a three-electrode cell using polymer films that were drop-cast onto an indium tin oxide (ITO) as working electrode. The measurements were carried

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out at a scanning rate of 50 mV s1 against a saturated calomel reference electrode (SCE) and using the tetrabutylammonium perchlorate (n-Bu4ClO4) in acetonitrile (0.1 M) as supporting electrolyte. The electrochemical cell was externally calibrated by a ferrocene standard. The measurements were performed at 25 °C and the cell was deoxygenated with argon before each scan. 2.2. Synthesis of 1,10 -thiobis(4-ethoxybenzene) (TDPEt) To a stirred mixture of TDP (2.20 g, 10 mmol) and potassium carbonate (5.57 g, 40 mmol) in 20 mL of dimethylformamide, was added dropwise bromoethane (2.5 mL, 33 mmol). After stirring for 8 h at room temperature (25 °C), the reaction mixture was poured into distilled water and extracted with diethyl ether. The extract was washed with distilled water, dried over anhydrous magnesium sulfate and then evaporated. The resultant crude product was purified by recrystallization from ethanol to afford TDPEt as white crystals. Yield: 90%; mp: 66 °C; 1H NMR (300 MHz, CDCl3, d): 7.29 (d, J = 8.7 Hz, 4H, ArAH), 6.85 (d, J = 8.7 Hz, 4 H, ArAH), 4.02 (q, J = 6.9 Hz, 4 H, OCH2), 1.43 (t, J = 6.9 Hz, 6H, CH3); 13C NMR (75.5 MHz, CDCl3, d): 158.7, 133.1, 127.6, 115.7, 63.9, 15.2; FT-IR (cm1): 3082, 3062, 3030 (w, aromatic CAH stretching), 2982, 2932, 2885 (w, aliphatic CAH stretching), 1592, 1570 (s, C@C stretching), 1254 (s, CAOAC asymmetric stretching), 1043 (m, CAOAC symmetric stretching), 827 (s, aromatic CAH out-of-plane bending). 2.3. Synthesis of 1,10 -thiobis(3-chloromethyl-4ethoxybenzene) (TDPEtCl) A mixture of TDPEt (2.74 g, 10 mmol), paraformaldehyde (2.50 g, 80 mmol of CH2O) and 37% aqueous HCl (8.5 mL, 102 mmol) in acetic acid (30 mL) was stirred at room temperature (25 °C) for 10 h. The resulting mixture was then poured into distilled water and extracted with diethyl ether. The organic layer was washed several times with distilled water and dried over anhydrous magnesium sulfate. After solvent removal and recrystallization from methanol, we obtained TDPEtCl as white powder. Yield: 50%; mp: 91 °C; 1H NMR (300 MHz, CDCl3, d): 7.36 (s, 2H, ArAH), 7.26 (d, J = 8.4 Hz, 2H, ArAH), 6.81 (d, J = 8.4 Hz, 2H, ArAH), 4.58 (s, 4H, CH2Cl), 4.07 (q, J = 6.9 Hz, 4H, OCH2), 1.44 (t, J = 6.9 Hz, 6H, CH3); 13C NMR (75.5 MHz, CDCl3, d): 156.6, 133.9, 133.5, 127.4, 112.9, 64.5, 41.6, 15.1; FT-IR (cm1): 3022 (w, aromatic CAH stretching), 2990, 2934, 2887 (w, aliphatic CAH stretching), 1590, 1574 (s, C@C stretching), 1250 (s, CAOAC asymmetric stretching + CH2Cl out-of-plane bending), 1045 (s, CAOAC symmetric stretching), 806 (s, aromatic CAH out-of-plane bending), 582 (s, CACl stretching). 2.4. Synthesis of 1,10 -thiobis(4-ethoxy-3-triphenylphosphoniomethylbenzene) dichloride (TDPEtP) A solution of TDPEtCl (3.70 g, 10 mmol) and triphenylphosphine (5.82 g, 22 mmol) in anhydrous acetone (50 mL) was stirred and heated at reflux for 5 h in an argon atmosphere. After cooling the reaction mixture, the result-

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ing white precipitate was filtered off, washed with diethyl ether several times and dried under vacuum. Yield: 90%; mp: 189 °C; 1H NMR (300 MHz, CDCl3, d): 7.80–7.45 (m, 30H, P(Ph)3), 7.33 (s, 2H, ArAH), 7.15 (d, J = 8.4 Hz, 2H, ArAH), 6.54 (d, J = 8.4 Hz, 2H, ArAH), 5.34 (d, 2JHA P = 13.8 Hz, 4H, CH2P), 3.48 (q, J = 6.9 Hz, 4H, OCH2), 1.02 (t, J = 6.9 Hz, 6H, CH3); 13C NMR (75.5 MHz, CDCl3, d): 156.6, 135.6, 135.3, 134.4 (d, J = 9.7 Hz), 133.1, 130.4 (d, J = 12.5 Hz), 118.4 (d, J = 85.7 Hz), 114.4 (d, J = 8.9 Hz), 112.2, 64.1, 24.6 (d, 1JCAP = 48.5 Hz, CH2P), 14.9; FT-IR (cm1): 3054 (w, aromatic CAH stretching), 2977, 2855, 2770 (w, aliphatic CAH stretching), 1589, 1488 (s, C@C stretching), 1255 (m, CAOAC asymmetric stretching), 1112 (s, PAC stretching), 1038 (s, CAOAC symmetric stretching), 750, 724, 690 (s, aromatic CAH out-of-plane bending), 509 (s, PACl stretching). 2.5. Synthesis of the polymer (PPVS) To a stirred mixture of an equimolar amount of a TDPEtP (0.894 g, 1 mmol) and terephthaldicarboxaldehyde (0.135 g, 1 mmol) in 10 mL of anhydrous THF, 10 mL of a 0.5 M t-BuOK solution in THF (5 mmol) was added dropwise at room temperature under a nitrogen atmosphere. The reaction mixture was stirred for 24 h after the addition and then acidified with 3% aqueous hydrochloric acid, poured into water and extracted with chloroform. The organic phase was washed with water, concentrated and then precipitated into methanol. A yellow powder was obtained, filtered and dried under vacuum for 24 h. Yield: 60%; 1H NMR (300 MHz, CDCl3, d): 8.10–6.20 (br m, aromatic and vinylic H), 4.20–3.60 (br m, OCH2), 2.10 (s, ArACH3 end-group), 1.50–1.00 (br m, CH3); 13C NMR (75.5 MHz, CDCl3, d): 156.1, 137.5, 136.8, 134.1, 132.0, 130.8, 130.4, 129.7, 129.5, 128.8, 128.3, 128.0, 127.7, 127.3, 126.7, 123.1, 113.3, 112.0, 64.6, 16.6, 15.3; FT-IR (cm1): 3051, 3020 (w, aromatic and vinylic CAH stretching), 2976, 2931, 2879 (w, aliphatic CAH stretching), 1719, 1696, 1585 (w–m, C@C stretching), 1245 (s, CAOAC asymmetric stretching), 1042 (m, CAOAC symmetric stretching), 803 (s, aromatic CAH out-of-plane bending), 966 (m, trans-HC@CH out-of-plane bending), 865 (w, cisHC@CH out-of-plane bending). 2.6. Fabrication and characterization of diodes The single-layer device was fabricated as a sandwich structure between aluminium (Al) cathode and indiumtin oxide (ITO) anode. Polymer solution (8 mg mL1 in chloroform) was spin-cast (2000 rpm) onto ITO glass to obtain a film with a thickness of about 50 nm after annealing at 40 °C for 1 h. The thin aluminium layer (150 nm) was deposited as the cathode by thermal evaporation under 3  106 torr. For multilayer device, the poly(ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) layer (25 nm) were spin-cast onto ITO glass and then dried at 60 °C for 12 h. The bathocuproine (BCP) layer (20 nm) and the aluminium electrode (150 nm) were thermally deposited through shadow masks under a vacuum of 2  106 and 105 torr, respectively. The current–voltage (I–V) characteristics of the devices were recorded with a

digital oscilloscope Tektronix 5034. EL and PL spectra were obtained using a cooled CCD camera attached to a Jobin– Yvon Triax 190 spectrometer. For PL measurement, UV excitation at 337 nm was provided by a pulsed nitrogen laser. All the device fabrication and characterization steps were done under ambient laboratory conditions.

3. Results and discussion 3.1. Synthesis and structural characterization The polymer was obtained according to a four steps synthetic route as showed in Scheme 1. A new bischloromethyl aromatic derivative (TDPEtCl) was synthesised from TDP by direct chloromethylation of the benzene ring of the corresponding ethylated ether (TDPEt) in an HCl/paraformadehyde/acetic acid system following a conventional described procedure [17]. Bis(triphenylphosphonium) salt (TDPEtP) could be easily obtained by acetone reflux of TDPEtCl in presence of triphenylphosphine. The synthesis of the polymer was carried out via a conventional Wittig reaction [18]. Thus, the addition of t-BuOK/THF solution to a suspension of TDPEtP and terephthaldicarboxaldehyde produced PPVS which was easily isolated and purified by precipitation from methanol. The synthetic pathway of PPVS is quite simple, with a good yield in every step and using relatively cheap materials, standard synthesis methods and common purification techniques. The polymer was found to have good solubility in common organic solvents such as THF, chloroform, methylene chloride and so forth. The polymer structure was well established by 1H NMR, 13 C NMR and FT-IR spectroscopic analysis. The 1H NMR spectrum (Fig. 1) showed a broad peak between 8.10 and 6.20 ppm assigned to phenylene and vinylene protons. The CH2O group and the methyl group appear in the ranges 4.20-3.60 ppm and 1.50-1.00 ppm, respectively. The absence of the aldehyde terminal groups was supported by the absence of the corresponding peak from the 1H NMR spectrum (10 ppm). On the other hand, the appearance of a weak signal at 2.10 ppm suggests a toluene methyl end-group (ArACH3). In fact, regarding the quantum efficiency, this feature is very significant knowing that carbonyl is a fluorescence-quencher group [19]. The IR spectrum (Fig. 2) showed absorption bands due to alkyl CAH stretching at 3051 and 3020 cm1. The valence bands of the aliphatic CAH groups are seen between 3000 and 2820 cm1. The aromatic ring and vinylic C@C stretching vibrations appear between 1720 and 1580 cm1. The strong band at 1245 cm1 is attributed to the asymmetric CAOAC vibration. The band at 1042 cm1 is attributed to the symmetric CAOAC vibration. The out-of-plane vibration of the aromatic hydrogen shows a strong absorption at 803 cm-1. The spectrum showed the presence of both cis (865 cm1) and trans (966 cm1) vinylic absorptions [20]. Indeed, according to its chemical structure, the yield used in this work can be classified as semi-stabilized yield, and its normal Wittig reaction with aldehydes produces mixtures of Z- and E-configurations non-stereospecifically [21]. By comparing the 1H NMR signal integration for toluene methyl terminal groups and OCH2

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S

S HO

K2CO3, EtBr DMF, rt

OH

O

O TDPEt

TDP

Cl

-

Cl , (Ph)3P+

Cl

P+(Ph)3, Cl

S (H2CO)n, HCl AcOH, rt

P(Ph)3 acetone, reflux

O

O

O TDPEtCl

TDPEtP

O

O TDPEtP

O +

-

S

S

O

t-BuOK/THF THF, rt

CH HC

O

CH HC

O

S PPVS

O n

Scheme 1. Synthetic route to PPVS.

Fig. 1. 1H NMR spectrum of PPVS.

units, the degree of polymerization was estimated to be 8, which gives a number-average molecular weight (Mn) of 3500 g mol1.

Transmittance (%)

100

3.2. Thermal characterization

865 966

80 3500

3000

2500

2000

1500

Wavenumber (cm-1) Fig. 2. FT-IR spectrum of PPVS.

1000

Thermal properties of luminescent polymer are among the most important properties for both processing and PLED application [22]. The use of material with high glass transition temperature (Tg) and high onset thermal decomposition temperature (Td) is of interest in PLED since it has been reported that they improve the thermal stability of the device [23]. The thermal stability of PPVS was investigated by TGA, TDA (Fig. 3) and DSC (Fig. 4) under ambient atmospheric conditions. The results show Tg at 98 °C and Td at 360 °C. Relatively lower Tg and Td values have been reported for polymers containing almost the same chromo-

Heat flow Exo

80

100

200

300

360 400

60

Temperature (°C) Fig. 3. TGA and TDA thermograms of PPVS.

Absorption

375

Photoluminescence

517

459

Solution Film

300

400

500

600

700

Normalized PL intensity (a. u.)

100

Normalized UV-Vis absorbance (a. u.)

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Weight loss (%)

2890

800

Wavelength (nm) Fig. 5. UV–vis absorption and PL spectra of PPVS in chloroform solution and as thin film.

and well defined structure of the emitting chromophore. Moreover, self-absorption of the emission is negligible as indicated by the small overlap of its absorption and emission spectra in the solid state (Stokes shift about 153 nm). From the onset of film absorption (449 nm), the optical band-gap is calculated to be 2.76 eV. All these features make PPVS a good candidate for application in PLEDs. 3.4. Electrochemical characterizations

phore of PPVS but with an aliphatic flexible spacer segment [12]. No melting or other thermal events were seen before thermal decomposition, which suggested that the polymer was amorphous. So, PPVS possesses good thermal properties which may be favourable for long-life operation when used in light-emitting devices. 3.3. Optical characterization The UV–vis absorption and PL spectra of a dilute chloroform solution (5  105 M) and of a thin film (45 nm) of PPVS spin coated on a silica plate are depicted in Fig. 5. In solution or as film, the polymer exhibits nearly the same absorption spectrum with a maximum at 375 nm (3.3 eV). The absorption coefficient was 3.19  104 M1 cm1 in solution and 7.18  104 cm1 in solid state. From the chromophore structure point of view, PPVS is similar to di(alkoxystyryl)benzenes investigated elsewhere [21,24,25]. In comparison with di(ethyloxystyryl)benzene [21], PPVS show a 7 nm red-shifted absorption maximum due to the donor effect of spacer sulfur atom. In PL spectroscopic analysis, the polymer solution shows an emission maximum at 484 nm (2.6 eV). In the solid state, the emission spectrum is slightly red-shifted (33 nm i.e., 0.2 eV) with an emission maximum in the blue-green region at 517 nm (2.4 eV). Noteworthy, the film fluorescence band is especially narrow with a full width at half-maximum of 96 nm (0.6 eV), which is in agreement with the confined

To investigate the redox behavior of PPVS and to estimate its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, CV was applied to the polymer film. In fact, the knowledge of the HOMO and LUMO energy levels is of crucial importance to the selection of cathode and anode materials for PLED devices [26]. The use of CV analysis is of good reliability as the electrochemical processes probed thereby are similar to those involved in charge injection and transport processes in PLED devices [27]. PPVS was drop-coated onto ITO glass substrate and scanned both positively and negatively in (n-Bu)4NClO4)/acetonitrile solution. As shown in the cyclic voltammogram depicted in Fig. 6, the onset of

0.6 0.4

Current (mA)

Fig. 4. DSC thermogram of PPVS.

0.86

0.2 0.0 -0.2

-1.92

-0.4 -0.6 -0.8 -1.0 -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V) vs. SCE Fig. 6. Cyclic voltammogram of PPVS film coated on an ITO electrode. In M 0.1 M (n-Bu)4ClO4/acetonitrile at a scanning rate of 50 mV s1.

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3.0 2.5

Current (mA)

oxidation (Vonset-ox) was found to occur at 0.86 V and the onset of reduction (Vonset-red) was at 1.92 V (vs SCE). According to an empirical method [28,29] and by assuming that the energy level of the ferrocene/ferrocenium is 4.8 V below the vacuum level, the HOMO and LUMO energy levels can be calculated as follows: EHOMO (IP, ionisation potential) = (Vonset-ox  VFOC + 4.8) eV ELUMO (EA, electron affinity) = (Vonset-red  VFOC + 4.8) eV (Where VFOC is 0.55 V, the ferrocene half-wave potential measured versus SCE). Thus, EHOMO, ELUMO and electrochemical bandgap values were estimated to be 5.11 eV, 2.33 eV and 2.78 eV, respectively. This result is indeed very close to the optical gap value.

2.0 1.5 1.0 0.5 0.0 0

5

10

15

20

25

30

Voltage (V) Fig. 8. Current–voltage curve of ITO/PEDOT:PPS/PPVS/BCP/Al diode.

We first fabricated and characterized a single-layer device with the configuration ITO/PPVS/Al. As shown in Fig. 7, the I–V curve indicates typical diode behavior with a relatively low turn-on voltage of 3.4 V. However, no electroluminescence could be recorded for this simple device. The reason is probably the confinement that increases the exciton binding energy and the electrical bandgap, and thus the energy barriers at electrodes. Furthermore, for a single-layer diode, it is difficult to balance the injections of holes and electrons. Thus, the turn-on voltage indicates rather the threshold of unipolar injection. Therefore, new device was built with a layer (20 nm) of spin coated PEDOT:PPS to improve the surface quality of the ITO anode and reduce the probability of electrical shorts as well as to facilitate the hole injection [30]. Besides, before the deposition of the 150 nm aluminium cathode, a thin layer (20 nm) of bathocuproine (BCP) was evaporated on the spin coated emitting layer to prevent the holes from escaping from the desired recombination zone in the polymer layer [31]. Thus, a multilayer device with an ITO/PEDOT:PPS/PPVS/BCP/Al configuration was constructed. This diode turned on at approximately 25 V (Fig. 8), emitting a blue-green light. Fig. 9 shows the EL spectrum of the multilayer device. The high value of its EL threshold can be probably explained by the excessive

0.004

Normalized intensity (a. u.)

3.5. Diodes characterizations 506

1.0

EL PL

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) Fig. 9. EL spectrum for the multilayer device ITO/PEDOT:PPS/PPVS/BCP/Al and PL spectrum of PPVS thin film.

thickness (60 nm) of the active emitting layer; in the conditions of the experiment, the current was moderate (20 mA cm2). For convenience of comparison, the PL spectrum of the polymer film is also given in the same Fig. 9. The emission maximum of EL spectrum appears at a wavelength of about 506 nm. The EL spectrum is quite similar to the film PL spectrum, indicating that both EL and PL originate from the same radiative decay process of the singlet exciton [32].

Current (mA)

4. Conclusion

0.002

0.000 0

2

3.4

4

Voltage (V) Fig. 7. Current–voltage curve of ITO/PPVS/Al diode.

A new confined PPV-type polymer (PPVS) has been synthesized and characterized. The well-defined degree of conjugation in the oligomeric PPV segment results in a chromophore that exhibits blue-green PL in thin film. The polymer is intrinsically soluble and has a number-average molecular weight of 3500 g mol1. PPVS is thermally stable up to 360 °C under ambient atmospheric conditions and displays a Tg of 98 °C. The electrochemical bandgap was about 2.78 eV. The I–V characteristic of the device with an ITO/PPVS/Al configuration demonstrates typical diode behavior and a relatively low turn-on voltage of 3.4 V.

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Acknowledgements This work was partially supported by the contact CMCU code 05S1304. The authors thank the members of the ‘‘Laboratoire de Physique et Chimie des Interfaces” (LPCI) for the electrical measurements. Special thanks go to Dr. John Lomas for his help to improve the English quality of the manuscript and for his scientific remarks. References [1] Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, et al. Nature (London) 1990;347:539–41. [2] Kraft A, Grimsdale AC, Holmes AB. Angew Chem Int Ed Eng 1998;37:402–28. [3] Akcelrud L. Prog Polym Sci 2003;28:875–962. [4] Xiang D, Shen Q, Zhang S, Jiang X. J Appl Polym Sci 2003;88:1350–6. [5] Sun Q, Yang C, Wang H, He G, Li Y. Polym Adv Technol 2002;13:663–9. [6] Murphy AR, Fréchet JMJ. Chem Rev 2007;107:1066–96. [7] Segura JL, Martin N, Guldi DM. Chem Soc Rev 2005;34:31–47. [8] Gunes S, Neugebauer H, Sariciftci NS. Chem Rev 2007;107:1324–38. [9] McQuade DT, Pullen AE, Swager TM. Chem Rev 2000;100:2537–74. [10] Thomas III SW, Joly GD, Swager TM. Chem Rev 2007;107:1339–86. [11] Menon A, Dong H, Niazimbetova ZI, Rothberg LJ, Galvin ME. Chem Mater 2002;14:3668–75. [12] Yang Z, Sokolik I, Karasz FE. Macromolecules 1993;26:1188–90. [13] Liau CY, Gan YY, Zhou Y, Lam YL, Gan LH. Polymer 2000;41:7339–46.

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