Novel fluorine containing polyfluorenes with efficient blue electroluminescence

Polymer 45 (2004) 7071–7081 www.elsevier.com/locate/polymer

Novel fluorine containing polyfluorenes with efficient blue electroluminescence Andressa M. Assakaa,b, Paula C. Rodriguesa, Alfredo R.M. de Oliveiraa, Liming Dingc, Bin Hud, Frank E. Karaszc, Leni Akcelruda,* a

Departamento de Quı´mica, Universidade Federal do Parana´ UFPR, Centro Polite´cnico da UFPR, Caixa Postal 19081, CEP 81531-990, Curitiba/PR, Brazil b Instituto Tecnolo´gico para o Desenvolvimento LACTEC, Centro Polite´cnico UFPR, CEP 81531-990, Curitiba/PR, Brazil c Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA d Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN-37996-2200, USA Received 12 March 2004; received in revised form 25 July 2004; accepted 29 July 2004 Available online 3 September 2004

Abstract The synthesis, structural characterization, photo and electroluminescence, thermal and electrochemical properties of a new fluorinated fluorene-containing copolymer are described. The copolymer is formed by alternating mers of [2,3,5,6 tetrafluoro-1,4 phenylene] and [9,9 0 dihexyl-2,7 fluorene] and emits blue light with low turn on voltages. The EL performance of the fluorinated copolymer was superior to those of the non-fluorinated analog copolymer and of the corresponding poly(9,9 0 dihexyl-2,7 polyfluorene) homopolymer. q 2004 Elsevier Ltd. All rights reserved. Keywords: Polyfluorenes; Electroluminescence; Fluorinated polyfluorenes

1. Introduction Since the discovery of electroluminescence in polymers [1], the fabrication of blue emitting devices has been the object of a great deal of investment. Polymers emitting light with wavelengths shorter than 495 nm and their characteristics have been recently reviewed [2]. Recently, polyfluorenes were introduced as prospective emitting layer for polymer light emitting devices (LEDs) with blue emission. These materials are thermally stable and display high photoluminescent efficiencies. Their photostability and thermal stability are also found to be better than those of the poly(phenylene vinylene)s. Polyfluorenes contain a rigidly planarized biphenyl structure in the fluorene repeating unit, while the remote substitution at C-

* Corresponding author. Address: Departamento de Quı´mica, Universidade Federal do Parana´ UFPR, Centro Polite´cnico da UFPR, Caixa Postal 19081, CEP 81531-990, Curitiba/PR, Brazil. Tel.: C55-413-367-507; fax: C55-413-613-396. E-mail address: [email protected] (L. Akcelrud). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.07.072

9 produces less steric interaction in the backbone itself, in comparison with polyphenylenes [3–18]. A critical issue in the design of blue polymer LEDs is how to achieve high brightness at low operating voltage. The particular problem is that the large optical gap required for blue emission means that the barriers to carrier injection from common electrodes materials are generally large [19]. Turn on voltages vary from 7 to 20 V [5,20,21] for pure polymers, and 5 V for polymer blends [22]. To circumvent this problem the copolymerization with electron withdrawing moieties such as oxadiazole [20] or the insertion of electronegative groups as cyano [23–27] in the polymer chain have been used as strategies to impart high electron affinity and transport properties to polyfluorenes. In this line of pursuit, we combined the properties of polyfluorene with a fluorinated phenylene monomer and obtained a blue emitting polymer with low turn on voltages. This structure is shown in Scheme 1(a) and its electroluminescent performance and electro-optical properties were compared with those of a similar polymer without the fluorine atoms (b) and of polyfluorene homopolymer (c). The Suzuki crosscoupling polymerization [28,29] was used as chemical

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Scheme 1. Structures of synthesized polymers. (a) poly(2,7-9,9 0 -dihexylfluorene-dyil-alt-1,4-tetrafluorobenzene) (PDHF-TB), (b) poly(2,7-9,9 0 dihexylfluorene-dyil-alt-p-phenylene) (PDHF-PP) and (c) poly(2,7-9,9 0 -dihexylfluorene-dyil) (PDHF).

route to synthesize the materials. To our knowledge, the enhanced emission properties due to the incorporation of fluorine to polyfluorene homopolymers has not been addressed before.

2. Experimental 2.1. Materials Fluorene, n-butyllithium, n-bromohexane, triisopropylborate and tetrakis(triphenylphosphine)palladium(0) were purchased from commercial sources (Acros Organics) and used without further purification. 2-Bromofluorene, 1,4dibromobenzene and 1,4-dibromotetrafluorobenzene were purchased from Aldrich Chemical Co. and were used without further treatment as well. The solvents dimethylsulfoxide (DMSO), ethanol, hexane, chloroform, diethyl ether and toluene were previously treated as described in the literature [30]. 2.2. Monomer synthesis 2.2.1. 9,9 0 -Dihexylfluorene [31] To a mechanically stirred mixture of fluorene (Acros,

33.62 g, 0.205 mol) were added powdered potassium hydroxide (56 g, 1.0 mol), potassium iodide (3.4 g), and dimethyl sulfoxide (DMSO, 150 ml) cooled to 10 8C. Hexyl bromide (88.44 g, 0.536 mol) was added dropwise over 45 min, and the mixture turned from red to light purple. The temperature was increased to 20 8C, and the mixture was left overnight, with constant stirring. It was then poured into water and extracted with toluene. The organic extracts were washed with water and dried over magnesium sulfate. The solvent was removed under reduced pressure. The excess of hexyl bromide was removed by distillation to give a bright yellow oil with mp 30–32 8C. (95% yield, 63.5 g). 2.2.2. 2,7-Dibromo-9,9-dihexylfluorene [32] To a solution of 9,9-dihexylfluorene (34.32 mmol, 12.83 g) in CHCl3 (58 ml) at 0 8C were added 96 mg (0.59 mmol) of ferric chloride and 4.14 ml (80.52 mmol) of bromine. It is important that the reaction proceeds in the dark to avoid any bromination of the aliphatic part of the molecule. The solution was warmed to room temperature and was stirred during 3 h. The resulting slurry was poured into water and washed with sodium thiosulfate until the red color disappeared. The aqueous layer was extracted twice with CHCl3 and the combined organic layers were dried over magnesium sulfate. The solvent was removed and the

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residue was purified by recrystallization from ethanol three times to yield a white crystalline product. Mp 70–72 8C, 18.40 g (97% yield). 2.2.3. 9,9-Dihexylfluorene-2,7-diboronic acid [23,33] A 2.5 M solution of n-butyllithium (42 ml, 105 mmol) was added to an argon purged solution of 2,7-dibromo-9,9dihexylfluorene (24.60 g, 50 mmol) in anhydrous diethyl ether (300 ml) at K78 8C using a syringe. The solution was then allowed to slowly warm to room temperature, and stirred for a further 1 h before it was cooled again to K78 8C. Triisopropyl boronate (39.5 g, 210 mmol) was added with a syringe. The resulting mixture was once again allowed to warm to room temperature and was stirred for an additional 20 h. Then 2 N HCl (200 ml) was added to the stirred solution while maintained at room temperature during 1 h. The organic layer was extracted with 200 ml of diethyl ether. The combined ether layers were washed twice with 200 ml of water. The solvent was then removed under reduced pressure. The crude product was purified by Soxhlet extraction with hexane to give a white powder in a yield of 23% (4.85 g). Mp O400 8C. 2.2.4. Polymerization General procedure of polymerization through the Suzuki cross coupling. To a mixture of 9,9 0 -dihexylfluorene-2,7diboronic acid (1.05 equiv.), dibromo compound (1 equiv.) and tetrakis(triphenylphosphine)palladium(0) (1.0 mol%) was added a degassed mixture of toluene and aqueous 2 M potassium carbonate (1:1 in volume), in such a way that the total monomer concentration was 0.5 M. The mixture was vigorously stirred at 80 8C during 48 h. Degassed 2bromofluorene (1.0 mol%) in toluene solution (5% of the total volume), was then added and vigorously stirred at 80 8C during 12 h. After that the mixture was cooled to room temperature, and poured into 500 ml of methanol. A fibrous solid was obtained by filtration. The solid was washed with acetone. After extracting during 12 h in a Soxlet apparatus with acetone, the resulting polymers were collected and dried under reduced pressure. 2.2.5. Poly(2,7-9,9 0 -dihexylfluorene-dyil) (PDHF) Yellow powder in a yield of 60%. 1H NMR (CDCl3, 400 MHz, ppm) d 7.84–7.32 (m, 6H), 2.12 (br, 4H), 1.15– 0.80 (m, 22H). 13C NMR (CDCl3, 100 MHz, ppm) d 151.79, 140.52, 139.19, 126.14, 121.51, 119.95, 55.32, 40.33, 31.42, 29.63, 23.82, 22.51, 13.96. FTIR (cmK1): 2961, 2926, 2859, 1630, 1454, 1386, 1260, 1103, 1016, 806, 810. 2.2.6. Poly(2,7-9,9 0 -dihexylfluorene-dyil-alt-p-phenylene) (PDHF-PP) Pale yellow powder in a yield 58%. 1H NMR (CDCl3, 400 MHz, ppm) d 7.80–7.55 (m, 8H), 7.30(br, 2H), 2.07 (br, 4H), 1.10–0.82 (m, 22H). 13C NMR (CDCl3, 100 MHz, ppm) d 151.87, 140.25, 134.69, 131.15, 128.75, 127.94,

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121.27, 55.31, 40.41, 31.43, 29.62, 13.95. FTIR (cmK1): 2959, 2926, 2854, 1625, 1464, 1378, 1289, 1100, 1010, 806. 2.2.7. Poly(2,7-9,9 0 -dihexylfluorene-dyil-alt-1,4tetrafluorobenzene) (PDHF-TB) Greenish powder in a yield of 80%. 1H NMR (CDCl3, 400 MHz, ppm) d 7.87–7.26 (m, 6H), 2.05 (br, 4H), 1.08– 0.75 (m, 22H). 13C NMR (CDCl3, 100 MHz, ppm) d 151.45, 141.32, 134.51, 130.28, 128.08, 125.02, 120.24, 55.52, 40.06, 31.33, 29.55, 23.74, 22.44, 13.87. FTIR (cmK1): 3041, 2928, 2855, 1450, 1266, 980, 867, 815. 2.3. Equipment Gel permeation chromatography (GPC) was used for molecular weight measurements with a GPC HPLC Agilent 1100 equipment. The PL gel (mixed C) columns were connected to a refraction index detector, using THF as solvent, and the calibration was based on PS standards. FTIR spectra were obtained in a BIORAD FTS 3500 GX Spectrometer in the range of 400–4000 cmK1, in transmittance mode, using KBr pellets. UV–vis spectra were taken in a HP 8452 A spectrophotometer, single beam, in the range 200–400 nm. Emission spectra were taken in a Shimadzu 5301PC spectrofluorimeter, in the visible range (390–780 nm). The concentration of the solutions was 10K5 M for the diluted and 10K2 M for the concentrated ones and films were cast from polymer solution and dried at 10 mmHg, 60 8C, during 48 h. Thermogravimetric analysis (TGA) was performed in a Netzsch Thermisch Analyse TG 209 with a heating rate of 10 8C/min under a nitrogen flow of 15 ml/min from room temperature to 450 8C. NMR Bru¨cker 400 MHz Avance series with 13C at 100 MHz and 1H at 400 MHz or AC 200 MHz Bru¨cker with 13 C at 50 MHz and 1H at 200 MHz. All NMR experiments used CDCl3 as a solvent and TMS as internal standard. 2.4. Device fabrication and measurements Polymer solutions (20 mg/ml in chloroform) were filtered through 0.2 mm Millex-FGS filters (Millipore) and spin cast onto ITO glass, PEDOT/ITO substrates under a nitrogen atmosphere in a glove box. PEDOT-PSS were spin cast onto ITO glass and then dried in vacuo at 60 8C for 2 days. The polymer films were typically 75 nm thick as measured by ellipsometry using Si wafer. Calcium electrodes of 400 nm thickness were evaporated onto the polymer films at about 0.1 mTorr pressure through a mask, followed by deposition of a protective coating of aluminum, at room temperature. The devices were characterized using a spectral measurement system constructed in UMASS laboratory that is described elsewhere [34]. The devices were operated using pulsed voltage for spectral measurements and dc voltage for current density determinations.

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Scheme 2. Monomer synthesis.

2.5. Electrochemical characterization The redox behavior of the polymers was investigated using a potenciostat/galvanostat EG&G-PARC, model 273A. Pt-wire, Pt-coil and saturated calomel electrode were used as working, counter and reference electrodes, respectively. The polymer films were deposited onto the working electrode by solution casting. Measurements were carried out at a rate of 50 mV/s in acetonitrile/LiClO4 0.5 mol/l. The electrolyte was deoxygenated before each measurement, at room temperature.

ane when catalyzed by concentrated KOH in DMSO solution with potassium iodine in a process adapted from the literature [31]. This reaction affords 9,9 0 -dihexylfluorene (1) in a high yield (95%). The main disubstituted derivative can be isolated from the monosubstituted byproduct through distillation at low pressure to afford a product with higher purity. The bromination of 1 was catalysed by FeCl3. After bromination, the crude product was easily purified by recrystallization with hexane to produce white crystals with overall yield of 97% with higher purity for this one step reaction [32]. The bromine (2) was converted to the boronic acid (3) by lithiating the compound in diethyl ether at low Table 1 Molecular weights and weight distribution of homopolymer and copolymers

3. Results and discussion 3.1. Synthesis and structure characterization The synthetic approach used in this study started with fluorene (Scheme 2). The hydrogen atoms on the 9 position of fluorene are sufficiently acidic to react with 1-bromohex-

Polymer

Mn

Mw

Mw/Mn

PDHF PDHF-PP PDHF-TB

8500 9500 9000

12,000 14,500 13,500

1.4 1.5 1.5

Scheme 3. Polymer synthesis.

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Fig. 1a. 1H NMR spectra for PDHF, PDHF-PP and PDHF-TB.

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Fig. 1b. 13C NMR spectra for PDHF, PDHF-PP and PDHF-TB.

temperatures, followed by reaction with trimethylborate [23,33]. The resulting boronate ester was then hydrolysed with HCl 2 N to give the boronic acid 9,9 0 -dihexylfluorene2,7-diboronic acid with 23% yield. The monomers were used to afford the polyfluorene homopolymer or, in combination with other di-bromoarenes to give the products depicted in Scheme 3. The polymerization reaction was run with excess of monomer 2, during 48 h under reflux and

further end capping with the addition of 2-bromofluorene. The molecular weights obtained by GPC are shown in Table 1 and the assigned NMR spectra of the polymers are shown in Fig. 1. 3.2. Optical characterization Figs. 2–4 depict the absorption and emission profiles of

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Fig. 2. UV–vis absorption and photoluminescence spectra of PDHF.

Fig. 4. UV–vis absorption and photoluminescence spectra of PDHF-TB.

poly(2,7-9,9 0 -dihexylfluorene-dyil-alt-1,4-tetrafluorobenzene) (PDHF-TB), poly(2,7-9,9 0 -dihexylfluorene-dyil-alt-pphenylene) (PDHF-PP) and poly(2,7-9,9 0 -dihexylfluorenedyil) (PDHF), respectively. The absorption in the solid state was coincident with that in solution and is not shown. In all cases, a redshift in emission was observed in going from dilute solution (10K5 mol lK1, chloroform) to film: 2, 8 and 16 nm, for each polymer, respectively. This effect, very common in photoluminescent polymers [35–38] is usually attributed to the stronger interchain interactions that could significantly reduce the energy difference between LUMO and HOMO in solid states. Another feature that can be observed in Figs. 2–4 is that the copolymer’s emission is shifted to deeper UV value in relation to the homopolymer. In liquid state, emission of the phenylene copolymer shifts about 13 nm, and the fluorine copolymer shifts around 29 nm, whereas the emission is less shifted in relation to the homopolymer: around 7–15 nm for both copolymers in solid state. The larger blue shift in liquid state can be explained as follows: in liquid state, the interchain

interactions are usually very weak so that the energy difference between LUMO and HOMO is mainly determined by intrachain interactions (effective conjugation length). In solid state, the interchain interactions become, however, so strong that the energy difference between the LUMO and HOMO depends not only on intrachain interactions but also on interchain interactions. It is known that the interchain interactions can dramatically reduce the energy difference (red shift) in conjugated polymers [39]. The reduced effective conjugation length are due to the electron withdrawing effect from fluorine and to the chain twisting introduced by the phenylene ring, increasing the band gap and consequently leading to a blue shift of the emission spectrum of the two copolymers relative to the homopolymer. On the other hand, the decrease of the energy difference (red shift) due to the involved interchain interactions compensates the increase of energy difference (blue shift) due to the reduced effective conjugation length, leading to a larger blue shift of the emission spectrum in liquid state compared to solid state for fluorine and phenylene copolymers relative to polyfluorene homopolymer. 3.3. Electroluminescence characterization

Fig. 3. UV–vis absorption and photoluminescence spectra of PDHF-PP.

The EL spectra of the three polymers are compared in Fig. 5. The main feature is noteworthy here: all spectra are red-shifted in comparison to PL emission. This is in accordance with previous published data for PDHF and PDHF-PP [23]. For PDHF, we believe that the new emission band at 488 nm observed in the EL spectrum is attributed to the excimers formed between the molecular chains in the thin film under electrical excitation. For PDHF-TB or PDHF-PP, we suggest that the red shift of the EL spectrum compared to the PL spectrum is related to the different emission zone in the thin films. The injected charge carriers may recombine and give rise to radiative emission in the area close to the polymer/Ca interface due to the unbalanced

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Fig. 5. Electroluminescence spectra of homopolymer and copolymers.

double injection possibly occurring in the devices. The surface area could show a red shift compared to the bulk area due to varied electron–phonon coupling at the interface [34] and self-absorption when the emitted photons exit from ITO side. It should be noted that the PL usually measures a bulk area in a thin film. Therefore, the red shift of the EL spectrum compared to the PL can be explained by different emission zone involved in EL and PL processes, respectively, for PDHF-TB and PDHF-PP copolymers. The tailing in the long wavelength region is largely due to defects in the emissive polymer layer which act as new recombination centers in which excitons radiatively decay giving emissions different from those from the pristine polymer backbone [40–42]. More importantly, the fact that the EL spectra in PDHFTB and PDHF-PP do not show clear emission from excimers (or exciplexes) suggests a mechanism of hindering excimer or exciplex formation and thus improving the EL efficiency by incorporating the fluorobenzene and phenylene units in polyfluorene copolymers. The LED performance of the polymers is compared in Fig. 6. It can be seen in the I–V curves that the turn-on voltage of injection current is significantly reduced for the fluorine copolymer, in comparison with the phenylene copolymer and the polyfluorene homopolymer. This result clearly indicates that the fluorinated copolymer (PDHF-TB) forms lower potential barriers at the electrode interfaces, leading to a facile charge injection. Interfacial dipoles have been observed at the polymer/metal interface and can either increase or decrease the overall injection current depending on the directions of dipoles [43]. We propose here that the reduced threshold voltages in both copolymers compared to the homopolymer are due to the contribution of the interfacial dipoles. In another words, the interfacial dipoles facilitate the charge injection in double-layer PDHF-TB and PDHF-PP devices. This suggestion is currently under investigations in collaboration with the University of Tennessee.

Fig. 6. J–V–L characteristics of double layer LEDs.

The brightness–voltage shows that (1) the PDHF-TB device shows a lower EL turn-on voltage compared with the PDHF-PP and PDHF devices, (2) the brightness of the two copolymers is comparable at a given voltage, but much higher than the homopolymer. The observed effects confirm that the insertion of the electronegative fluorine atoms in the polyfluorene chain was able to produce a blue emitting polyfluorene with reduced turn-on voltage for current injection. In principle, an efficient polymer electroluminescence requires lower potential barriers at the electrode interfaces, balanced double charge injection for exciton formation, and high quantum yield of emission from the formed excitons. As discussed above, the reduced turn-on voltage both in I–V and EL–V curves and the absence of excimer (or exciplex) emission in the EL spectra implies lower potential barriers for charge injection and higher efficiency of exciton radiative decay in the fluorinated copolymer (PDHF-TB) compared to fluorene copolymer (PDHF-PP) and homopolymer

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2.60 2.31 1.63 5.97 6.05 6.14

HOMO

Energy levels

LUMO

Egelec. (eV)

(PDHF) in the devices. In the brightness–voltage plot, the fact that the EL from the PDHF-TB is not much enhanced with respect to the PDHF-PP may be due to unbalanced double injection, suggesting that the potential-barrier heights are much different at ITO and Ca interfaces in the PDHF-TB device. However, this unbalanced double injection can be adjusted by a better choice of cathode material. As a result, the fluorinated copolymer PDHF-TB demonstrates superior EL properties in charge injection and emission efficiency and therefore should be a promising material for the applications of polymer electroluminescence.

3.37 3.74 4.51

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1.71 1.82 1.88 1.23 1.31 1.40 K2.38 K2.34 K2.70 K2.96 K2.90 K3.85 K2.14 K2.43 K3.11 417 396 375 PDHF PDHF-PP PDHF-TB

2.97 3.13 3.31

Epa Eonset

p/V Dopping

Epa Epc Eonset

n/V Dopping Egopt (eV) l (nm) Polymer

Table 2 Electrochemical parameters of PDHF, PDHF-PP and PDHF-TB

Fig. 7. Cyclic voltamograms of (a) PDHF, (b) PDHF-PP and (c) PDHF-TB.

Epc

The cyclic voltamogram of the three polymers is shown in Fig. 7. The oxidation peak easily permitted the determination of the HOMO level of each one. The reduction peak of the homopolymer and the phenylene copolymer also afforded the LUMO level values for these materials. The fluorinated copolymer, however, was exceptionally resistant to reduction, and the reduction peak seen in Fig. 7 was actually due to the solvent (acetonitrile). Therefore, it was not possible to determine electrochemically the LUMO level of the fluorinated copolymer, and the value shown in Table 2 was taken from the optical gap of the electronic absorption spectrum. The electron affinities and ionization potentials shown in Table 2 predict larger barriers for charge injection in fluorine and phenylene copolymers compared to polyfluorene homopolymer with ITO and Ca electrodes. The disagreement between the predicted larger barriers and observed reduced turn-on voltages suggests that the interfacial dipoles are crucially involved in the charge injection process.

1.19 1.32 –

3.4. Electrochemical characterization

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acknowledged. AMA thanks LACTEC for a fellowship. FEK thanks AFOSR for support.

References

Fig. 8. TGA traces of PDHF, PDHF-PP and PDHF-TB.

3.5. Thermal properties—TGA Fig. 8 compares the thermal resistance of the three polymers through TGA measurements. The onset of mass loss is 163, 225 and 321 8C, for the homopolymer, the phenylene copolymer and fluorinated copolymer, respectively, clearly demonstrating the enhanced thermal properties of the fluorine containing polyfluorene.

4. Conclusions Blue-emitting fluorinated fluorene-containing copolymers were successfully synthesized with excellent thermal stability, reduced turn-on voltage, and enhanced EL performance (injection and brightness). The EL spectra clearly show the formation of excimers in polyfluorene homopolymer but not in fluorine and phenylene copolymers. The elimination of excimer (or exciplex) formation accounts for the enhanced EL brightness in the copolymers. It can therefore be concluded that the fluorine and phenylene the fluorine atoms and phenylene unit significantly hinder excimer or exciplex formation and consequently enhance the EL actions in polyfluorene type polymers. Furthermore, the current–voltage characteristics together with the cyclic voltammetric data suggest that interfacial dipoles were formed at the copolymer/Ca interface and dramatically facilitated the charge injection. In summary, the synthesis of fluorine and phenylene copolymers confirms the direction of producing enhanced blue EL by hindering the formation of excimers. The interfacial dipoles can be used to improve the charge injection in fluorine polymer LEDs.

Acknowledgements Financial support by Brazilian Agency CNPq is fully

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