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Electroluminescent polycarbazole thin films obtained by electrochemical polymerization
Synthetic Metals 126 (2002) 1±6
Electroluminescent polycarbazole thin ®lms obtained by electrochemical polymerization S. Yapi Abea,*, J.C. Bernedea, M.A. Delvalleb, Y. Tregoueta, F. Ragotc, F.R. Diazb, S. Lefrantc a
Laboratoire de Physique des Solides pour l'Electronique, Universite de Nantes, BP 922008, 44322 Nantes Cedex 3, France b Departamento Quimica Organica, Laboratorio de Polimeros, Facultad de Quimica, PUCC, Casilla 306 Santiago, Chile c Laboratoire de Physique Cristalline, Institut des MateÂriaux Jean Rouxel, BP 922008, 44322 Nantes Cedex 3, France Received 27 March 2000; received in revised form 19 February 2001; accepted 26 February 2001
Abstract Polycarbazole (PCZ) thin ®lms have been electrochemically synthesized on SnO2 coated glass substrates. The electrolyte used was the anhydric LiClO4. It is shown that, after optimization of the potential scanning domain, PCZ ®lms are systematically obtained. The physical and morphological properties of ®lms are described. The ®lms obtained with LiClO4 as electrolyte are homogeneous and their coverage ef®ciency on the SnO2 underlayer is very high. Films are exempt of pinholes, cracks and other morphological defaults as shown by scanning electron microscopy. Cyclic voltammograms are reversible, attesting the high quality of the structural properties of the ®lms. Moreover these ®lms are photoluminescent. After deposition of an aluminum upper layer, the structures SnO2/PCZ/Al behave like diodes with a forward polarization when the SnO2 electrode is positively biased. Also these structures exhibit electroluminescent properties. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polycarbazole; Photoluminescent; Organic light-emitting diodes
1. Introduction Polymer based light-emitting devices are nowadays a topic of great interest. While the ®rst electroluminescence (EL) effect in organic compounds has been obtained in poly(phenylene vinylene) [1], green and blue light emission can be achieved by using carbazole derivatives [2±7]. Organic light-emitting diodes (OLED) are composed of, at least, three layers [8]. The ®rst one is a transparent conductive oxide (TCO), such as indium tin oxide (ITO), indium oxide (In2O3) or tin oxide (SnO2). The second layer is an organic active ®lm, which is in fact often a multilayer, in order to improve the EL ef®ciency [8±13]. The third layer is most of the time an Al ®lm because of its relatively low work function and its good stability compared to that of metal with lower work function, such as Ca. The TCO and metallic ®lms are deposited by evaporation under vacuum. Usually the organic ®lm is deposited by spincoating. However, other techniques, such as electrochemistry can be used. It has been rarely used up to now [14,15], because of the higher processability of spin coating, elec*
Corresponding author.
trochemical polymerization presents interesting advantages, such as: direct deposition of the polymer onto the TCO substrate; formation of a film with good adhesion and mechanical properties; possibility to control different parameters during the polymerization by varying parameters, such as the concentration of starting material, the solvent±salt combination, as well as electrochemical ones. All this makes possible to tailor the ®nal polymer ®lm material into a certain structure and morphology. In the present paper, we show that electroluminescent diodes can be achieved by electrochemical deposition of polycarbazole (PCZ) (Scheme 1) on SnO2 substrate, the main purpose being optimization of the polymer by varying the electrochemical parameters. We have shown earlier that PCZ can be synthesized by carbazole oxidation in a solution with acetonitrile and water in the volume ratio 1/2, containing 0.1 M tetraethylammonium perchlorate as electrolyte [16,17]. It was shown that homogeneous ®lms with substrate coverage ef®ciency could be obtained only when a thin ®lm of carbazole was
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 1 ) 0 0 3 5 2 - 6
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S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Scheme 1. The PCZ polymer structure.
deposited by evaporation under vacuum before electropolymerization. Here, we show also that when an anhydride solution is used, after optimization of the experimental conditions, PCZ ®lms with good coverage ef®ciency and better conjugated properties, as demonstrated by their oxidation±reduction reversibility, can be grown. These ®lms, after dedoping, are photoluminescent allowing then the fabrication of an OLED prototype. 2. Experimental 2.1. Apparatus As previously reported [16,17], the electrochemical experiments were monitored with a potentiostat/galvanostat PGP201. A classical one compartment cell with three electrodes was used. The reference electrode was Ag/AgCl (TMACl aÁ 0.1 M). The working electrode was a SnO2 coated glass substrate and the third one was a platinum sheet counter electrode. The SnO2 active area was about 1 cm2. The SnO2 substrates used were provided by Solem. The morphology of the ®lm was visualized by using a scanning electron microscope (SEMÐJEOL 6400 F ®eld effect microscope).1 The thickness of the ®lms was measured by using cross-section visualization. The X-ray photoelectron spectroscopy (XPS)2 measurements have been performed using a magnesium X-ray source (1253.6 eV) operating at 10 kV and 10 mA. The energy resolution was 1 eV at a pass energy of 50 eV. High resolution scans with a good signal/noise ratio were obtained in the C 1s, N 1s, O 1s, Cl 2p and Li 1s regions of the spectrum. The quantitative analysis was based on the determination of the C 1s, N 1s, O 1s, Cl 2p and Li 1s peak areas with 0.2, 0.36, 0.61, 0.58 and 0.014, respectively, as the sensitivity factors. The sensitivity factors of the spectrometer are given by the manufacturer. The vacuum in the analysis chamber was about 10 6 Pa. All the spectra were recorded under identical conditions. The decomposition of the XPS peaks into different components and the quantitativeinterpretation were made after the subtraction of the background using the Shirley method [18]. 1 SEM measurements were performed at the SEM service of the University of Nantes, Nantes. 2 XPS measurements were performed at the University of Nantes, CNRS, Nantes.
The developed curve ®tting programs permit the variation of parameters, such as the Gaussian/Lorentzian ratio, the full width at half maximum (FWHM), the position and the intensity of the contribution. These parameters were optimized by the curve-®tting program to obtain the best ®t. The optical measurements were carried out at room temperature using a CARY 2300 spectrometer. The optical density (OD) was measured at wavelengths from 300 to 800 nm. The reference spectrum of SnO2 coated glass substrate was subtracted from the measured signal. The photoluminescence (PL) spectra were recorded using the Jobin±Yvon TG HG2S spectrophotometer with holographic gratings. The signal was detected with a Peltiercooled photomultiplier. The sample was excited with the 337 nm line of a ®ltered xenon lamp (150 W). The laser power at the sample was kept below 2 mW. Experiments were performed at room temperature. For current±voltage (I±V) and EL measurements, sandwich structures SnO2/PCZ/Al were grown by vacuum evaporation of the aluminum upper electrode. The active area of the OLED was 4 mm2. An EL was detected through the transparent ITO electrode and glass. The light output was detected using a silicon photodiode and a Keithley 617 electrometer. The I±V curves of the LEDs were measured using a Lambda power supply as source voltage and a Keithley 617 electrometer. 2.2. Chemicals The carbazole monomer was provided by Fluka with a purity of 99%. Acetonitrile (anhydrous, 99.98%) was provided by Aldrich. The electrolyte salt used was the lithium perchlorate (LiClO4) purchased at Fluka. 2.3. Procedure The electrochemical polymerization was made by cycling the SnO2 coated glass. The potential scanning domain was varied to achieve the optimum conditions, the largest scans were run between 200 and 2000 mV. As described above, the solution used was acetonitrile with LiClO4 as electrolyte. The total amount of 0.1 M of carbazole was dissolved in the solution. The solution was purged with argon before polymerization and during the electrosynthesis, argon was passed over the solution. The scan rate used was 500 mV/mn. The electrical contact to SnO2 was made with a metal caiman clip, which was not inserted in the solution. 3. Experimental results When LiClO4 is used as salt, in the observed cyclic voltammogram (0±1800 mV) anodic peaks at Va1 920 mV and Va2 1230 mV and cathodic complements at Vc1
S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
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Fig. 3. Scanning electron micrographs (SEMs) of PCZ films electrochemically synthesized using a LiClO4 as electrolyte. Fig. 1. Cyclic voltammogram (0±1800 mV) of PCZ using LiClO4 as electrolyte, first and second cycle.
830 mV and Vc2 960 mV can be seen (Fig. 1), where Va1 and Va2 are oxidation peaks, while Vc1 and Vc2 are reduction peaks. Since Va2 1230 mV allows to obtain electrochemical deposition of PCZ on the SnO2, scanning between 0 and 1450 mV has been used (Fig. 2). The visualization of the ®lms is shown in Fig. 3. It can be seen that in the case of LiClO4 as salt (Fig. 3), the coverage
ef®ciency of the ®lm is very high, without pinholes, cracks or inhomogeneities. The features visible on this picture duplicate the pyramidal shape of the crystallites of the SnO2 under layer. This shows the good adherence of the ®lm to the substrate. The optical density measurement is reported in Fig. 4. One can observe that the threshold absorption edge is located at about 400 nm as expected in such polymer [19]. No other feature is visible on the spectrum.
Fig. 2. Cyclic voltammograms (0±1400 mV) of PCZ using LiClO4 as electrolyte, first and fifth cycle.
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S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Fig. 4. The UV±VIS spectrum of PCZ films electrochemically synthesized using a LiClO4 as electrolyte.
Films have been characterized by X-ray photoelectron spectroscopy. The presence of chlorine and lithium in the ®lm cannot be excluded, but, even if present, their relative atomic percentages are too small (<1 at.%) to be quanti®ed. The decomposition of the C 1s is reported in Fig. 5. The C 1s peak can be decomposed in three contributions. The ®rst one situated at 285 eV should be attributed at the C±C bonds. The second, peaked at about 286.8 eV, could be assigned to the C±N bonds [20] while the third one corresponds to some oxygen surface contamination (C±OH. . .). The N 1s peak corresponds to only one peak, which can be attributed to the C±N covalent bond. Photoluminescent measurements are reported in Fig. 6. A signal has been obtained. It can be seen that the emission is
Fig. 6. Absorbance and photoluminescence spectra of PCZ electrochemically synthesized with LiClO4 as electrolyte (excitation: 337 nm).
recorded with a maximum at about 430 nm, which corresponds to blue emission light. Such photoluminescent samples were also tested in EL. After aluminum deposition on such ®lms, the I±V
Fig. 5. The XPS spectrum of the C 1s peak: (^-^-^) experimental result; (- - -) theoretical result; (Ð) different components.
S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Fig. 7. The I±V curve and corresponding (EL±V) signal (photodiode response Vdp±V) characteristics of a structure SnO2/PCZ/Al, the PCZ being electrochemically synthesized with LiClO4 as electrolyte.
Fig. 8. Luminescence spectrum of PCZ electrochemically synthesized.
characteristics of the structures are represented in Fig. 7. Diode characteristics are obtained. The forward direction corresponds, as expected, to a positive polarization of the SnO2 electrode. On the same ®gure the EL characteristic is reported, while the spectrum is reported in Fig. 8. These results demonstrate that PCZ based OLED can be obtained by electrochemical synthesis. 4. Discussion In the voltammogram of Figs. 1 and 2, the Va1 and Va2 peaks can be attributed to the oxidation of the carbazole monomers and of the polymer chains, respectively, and to the doping process. In Fig. 1, a nucleation loop is clearly visible on the ®rst cycle. Moreover an irreversible oxidation of carbazole appears at about 1.2 V. In Fig. 2, it can be seen that the redox process is reversible, since the two reduction peaks situated at Vc1 and Vc2 are systematically observed.
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The ®rst peak, Vc2 830 mV, corresponds to a partial chain reduction. The area of the peaks is proportional to the injected charges. Therefore, the reduction peaks concern only the dedoping process and a small part of the oxidized chains, since the oxidation peaks area is far higher than that of reduction peaks. This means that only weakly bonded carbazoles, defects. . . are reduced, leading to the highly homogeneous ®lm visualized in Fig. 4a. As usual with carbazole units, an absorption peak is visible at 310 nm and a shoulder at 335 nm (Fig. 4). Respectively, they correspond to the formation of the carbazole dimer excimer [2] and to the formation of a polymer where the carbazole chain have a statistical distribution length, as justi®ed by the band tail visible in the spectra. As described above, no feature is visible in the OD spectrum before the absorption edge. However, peaks at 385 and 735 nm have been described by Verghese et al. [19] in the case of irreversible oxidation. These peaks correspond to the doping of the ®lms. Here it can be seen (Fig. 2) that after extended cycling, the redox process stays reversible, moreover not only is no feature corresponding to doping visible in the OD spectrum, but Li and Cl are nearly absent from the ®lms. All these results are in good agreement with the dedoping process of the PCZ during the cyclic voltammogram and the attribution of Vc2 to this dedoping process. The redox process irreversibility of the electrochemical synthesis is usually attributed to some degradation reaction (reticulation. . .) of the electrochemically synthesized polymer [21]. Therefore, the present reversibility puts in evidence the high quality of the synthesized PCZ. Moreover, it is well known that doped polymer are not luminescent because doping states quench the luminescence. Therefore, the presence of photoluminescence in these polymerized ®lms (Fig. 6) corroborates the dedoping of the ®lms. 5. Conclusion It has been shown that highly homogeneous PCZ ®lms can be electrochemically synthesized in anhydrous solution by using LiClO4 salt and acetonitrile solvent. It is shown that in this case, the redox process is reversible which allows a dedoping of the polymer, which is suf®cient to obtain a photoluminescence signal from the PCZ ®lms. The SnO2/PCZ/Al structure exhibits diode characteristics and EL, when the SnO2 electrode is positively biased. The device performance will be systematically studied and improved by the deposition on the PCZ of an electron transporting layer. Acknowledgements This work has been supported by the Project ECOSCONICYT No. C99E05 and the FONDECYT Project No. 1990544.
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References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] D.B. Romero, M. Schaer, M. Leclerc, D. Ades, A. Siove, L. Zuppiroli, Synth. Met. 80 (1996) 271. [3] D.D. Gelber, Y.Z. Wang, D.K. Fu, J.M. Swager, A.J. Eptain, J. Chem. Phys. 108 (1998) 7842. [4] K.S. Marayan, G.L. Murthy, Chem. Phys. Lett. 276 (1997) 441. [5] C.I. Chao, S.-A. Chen, Appl. Phys. Lett. 73 (1998) 426. [6] H.N. Cho, J.K. Kim, D.Y. Kim, C.Y. Kim, Macromol. Symp. 125 (1997) 133. [7] H. Meng, Z.-K. Chen, W.-L. Yu, J. Pei, X.-L. Liu, Y.-H. Lai, W. Huang, Synth. Met. 100 (1999) 297. [8] J.J.M. Halls, D.R. Baigent, F. Cacialli, N.C. Greenham, R.H. Friend, S.C. Moratti, A.B. Holmes, Thin Solid Films 276 (1996) 13. [9] C. Hosokana, H. Higashi, H. Nahamura, T. Kusumoto, Appl. Phys. Lett. 67 (1995) 3853. [10] H. Vestweber, W. Rie, Synth. Met. 91 (1997) 181.
[11] Z.-L. Zhang, X.-Y. Jiang, S.-H. Xu, J. Nagatomo, O. Omoto, Synth. Met. 91 (1997) 131. [12] P. Jolnat, R. Clergereaux, J. Farenc, P. Destruel, J. Phys.: Appl. Phys. 31 (1998) 1. [13] R. Clergeaux, P. Jolinat, T.P. Nguyen, P. Destruel, J. Farenc, J. Chim. Phys. 95 (1998) 1347. [14] T. Ostergrad, C. Kvarnstrom, H. Stubb, A. Ivaska, Thin Solid Films 311 (1997) 58. [15] P. Damlin, T. Ostergard, A. Ivaska, H. Stubb, Synth. Met. 102 (1999) 947. [16] H. Taoudi, J.C. Bernede, A. Bonnet, M. Morsli, A. Godoy, Thin Solid Films 304 (1997) 48. [17] H. Taoudi, J.C. Bernede, M.A. Del valle, A. Bonnet, P. Molinie, M. Morsli, F. Diaz, Y. Tregouet, A. Barreau, Appl. Polym. J. [18] D.A. Shirley, Phys. Rev. B5 (1972) 4709. [19] M.M. Verghese, N.S. Sundaresan, T. Basu, B.D. Malhotra, J. Mater. Sci. Lett. 14 (1995) 401. [20] G. Beamon, Briggs, High resolution XPS of organic polymers, The Scienta ESCA 300 Data Base. Wiley, New York, 1992. [21] J. Simmonet, J. Rault-Berthelot, Progress Solid State Chem. 21 (1991) 1.
Electroluminescent polycarbazole thin ®lms obtained by electrochemical polymerization S. Yapi Abea,*, J.C. Bernedea, M.A. Delvalleb, Y. Tregoueta, F. Ragotc, F.R. Diazb, S. Lefrantc a
Laboratoire de Physique des Solides pour l'Electronique, Universite de Nantes, BP 922008, 44322 Nantes Cedex 3, France b Departamento Quimica Organica, Laboratorio de Polimeros, Facultad de Quimica, PUCC, Casilla 306 Santiago, Chile c Laboratoire de Physique Cristalline, Institut des MateÂriaux Jean Rouxel, BP 922008, 44322 Nantes Cedex 3, France Received 27 March 2000; received in revised form 19 February 2001; accepted 26 February 2001
Abstract Polycarbazole (PCZ) thin ®lms have been electrochemically synthesized on SnO2 coated glass substrates. The electrolyte used was the anhydric LiClO4. It is shown that, after optimization of the potential scanning domain, PCZ ®lms are systematically obtained. The physical and morphological properties of ®lms are described. The ®lms obtained with LiClO4 as electrolyte are homogeneous and their coverage ef®ciency on the SnO2 underlayer is very high. Films are exempt of pinholes, cracks and other morphological defaults as shown by scanning electron microscopy. Cyclic voltammograms are reversible, attesting the high quality of the structural properties of the ®lms. Moreover these ®lms are photoluminescent. After deposition of an aluminum upper layer, the structures SnO2/PCZ/Al behave like diodes with a forward polarization when the SnO2 electrode is positively biased. Also these structures exhibit electroluminescent properties. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polycarbazole; Photoluminescent; Organic light-emitting diodes
1. Introduction Polymer based light-emitting devices are nowadays a topic of great interest. While the ®rst electroluminescence (EL) effect in organic compounds has been obtained in poly(phenylene vinylene) [1], green and blue light emission can be achieved by using carbazole derivatives [2±7]. Organic light-emitting diodes (OLED) are composed of, at least, three layers [8]. The ®rst one is a transparent conductive oxide (TCO), such as indium tin oxide (ITO), indium oxide (In2O3) or tin oxide (SnO2). The second layer is an organic active ®lm, which is in fact often a multilayer, in order to improve the EL ef®ciency [8±13]. The third layer is most of the time an Al ®lm because of its relatively low work function and its good stability compared to that of metal with lower work function, such as Ca. The TCO and metallic ®lms are deposited by evaporation under vacuum. Usually the organic ®lm is deposited by spincoating. However, other techniques, such as electrochemistry can be used. It has been rarely used up to now [14,15], because of the higher processability of spin coating, elec*
Corresponding author.
trochemical polymerization presents interesting advantages, such as: direct deposition of the polymer onto the TCO substrate; formation of a film with good adhesion and mechanical properties; possibility to control different parameters during the polymerization by varying parameters, such as the concentration of starting material, the solvent±salt combination, as well as electrochemical ones. All this makes possible to tailor the ®nal polymer ®lm material into a certain structure and morphology. In the present paper, we show that electroluminescent diodes can be achieved by electrochemical deposition of polycarbazole (PCZ) (Scheme 1) on SnO2 substrate, the main purpose being optimization of the polymer by varying the electrochemical parameters. We have shown earlier that PCZ can be synthesized by carbazole oxidation in a solution with acetonitrile and water in the volume ratio 1/2, containing 0.1 M tetraethylammonium perchlorate as electrolyte [16,17]. It was shown that homogeneous ®lms with substrate coverage ef®ciency could be obtained only when a thin ®lm of carbazole was
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 1 ) 0 0 3 5 2 - 6
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S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Scheme 1. The PCZ polymer structure.
deposited by evaporation under vacuum before electropolymerization. Here, we show also that when an anhydride solution is used, after optimization of the experimental conditions, PCZ ®lms with good coverage ef®ciency and better conjugated properties, as demonstrated by their oxidation±reduction reversibility, can be grown. These ®lms, after dedoping, are photoluminescent allowing then the fabrication of an OLED prototype. 2. Experimental 2.1. Apparatus As previously reported [16,17], the electrochemical experiments were monitored with a potentiostat/galvanostat PGP201. A classical one compartment cell with three electrodes was used. The reference electrode was Ag/AgCl (TMACl aÁ 0.1 M). The working electrode was a SnO2 coated glass substrate and the third one was a platinum sheet counter electrode. The SnO2 active area was about 1 cm2. The SnO2 substrates used were provided by Solem. The morphology of the ®lm was visualized by using a scanning electron microscope (SEMÐJEOL 6400 F ®eld effect microscope).1 The thickness of the ®lms was measured by using cross-section visualization. The X-ray photoelectron spectroscopy (XPS)2 measurements have been performed using a magnesium X-ray source (1253.6 eV) operating at 10 kV and 10 mA. The energy resolution was 1 eV at a pass energy of 50 eV. High resolution scans with a good signal/noise ratio were obtained in the C 1s, N 1s, O 1s, Cl 2p and Li 1s regions of the spectrum. The quantitative analysis was based on the determination of the C 1s, N 1s, O 1s, Cl 2p and Li 1s peak areas with 0.2, 0.36, 0.61, 0.58 and 0.014, respectively, as the sensitivity factors. The sensitivity factors of the spectrometer are given by the manufacturer. The vacuum in the analysis chamber was about 10 6 Pa. All the spectra were recorded under identical conditions. The decomposition of the XPS peaks into different components and the quantitativeinterpretation were made after the subtraction of the background using the Shirley method [18]. 1 SEM measurements were performed at the SEM service of the University of Nantes, Nantes. 2 XPS measurements were performed at the University of Nantes, CNRS, Nantes.
The developed curve ®tting programs permit the variation of parameters, such as the Gaussian/Lorentzian ratio, the full width at half maximum (FWHM), the position and the intensity of the contribution. These parameters were optimized by the curve-®tting program to obtain the best ®t. The optical measurements were carried out at room temperature using a CARY 2300 spectrometer. The optical density (OD) was measured at wavelengths from 300 to 800 nm. The reference spectrum of SnO2 coated glass substrate was subtracted from the measured signal. The photoluminescence (PL) spectra were recorded using the Jobin±Yvon TG HG2S spectrophotometer with holographic gratings. The signal was detected with a Peltiercooled photomultiplier. The sample was excited with the 337 nm line of a ®ltered xenon lamp (150 W). The laser power at the sample was kept below 2 mW. Experiments were performed at room temperature. For current±voltage (I±V) and EL measurements, sandwich structures SnO2/PCZ/Al were grown by vacuum evaporation of the aluminum upper electrode. The active area of the OLED was 4 mm2. An EL was detected through the transparent ITO electrode and glass. The light output was detected using a silicon photodiode and a Keithley 617 electrometer. The I±V curves of the LEDs were measured using a Lambda power supply as source voltage and a Keithley 617 electrometer. 2.2. Chemicals The carbazole monomer was provided by Fluka with a purity of 99%. Acetonitrile (anhydrous, 99.98%) was provided by Aldrich. The electrolyte salt used was the lithium perchlorate (LiClO4) purchased at Fluka. 2.3. Procedure The electrochemical polymerization was made by cycling the SnO2 coated glass. The potential scanning domain was varied to achieve the optimum conditions, the largest scans were run between 200 and 2000 mV. As described above, the solution used was acetonitrile with LiClO4 as electrolyte. The total amount of 0.1 M of carbazole was dissolved in the solution. The solution was purged with argon before polymerization and during the electrosynthesis, argon was passed over the solution. The scan rate used was 500 mV/mn. The electrical contact to SnO2 was made with a metal caiman clip, which was not inserted in the solution. 3. Experimental results When LiClO4 is used as salt, in the observed cyclic voltammogram (0±1800 mV) anodic peaks at Va1 920 mV and Va2 1230 mV and cathodic complements at Vc1
S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
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Fig. 3. Scanning electron micrographs (SEMs) of PCZ films electrochemically synthesized using a LiClO4 as electrolyte. Fig. 1. Cyclic voltammogram (0±1800 mV) of PCZ using LiClO4 as electrolyte, first and second cycle.
830 mV and Vc2 960 mV can be seen (Fig. 1), where Va1 and Va2 are oxidation peaks, while Vc1 and Vc2 are reduction peaks. Since Va2 1230 mV allows to obtain electrochemical deposition of PCZ on the SnO2, scanning between 0 and 1450 mV has been used (Fig. 2). The visualization of the ®lms is shown in Fig. 3. It can be seen that in the case of LiClO4 as salt (Fig. 3), the coverage
ef®ciency of the ®lm is very high, without pinholes, cracks or inhomogeneities. The features visible on this picture duplicate the pyramidal shape of the crystallites of the SnO2 under layer. This shows the good adherence of the ®lm to the substrate. The optical density measurement is reported in Fig. 4. One can observe that the threshold absorption edge is located at about 400 nm as expected in such polymer [19]. No other feature is visible on the spectrum.
Fig. 2. Cyclic voltammograms (0±1400 mV) of PCZ using LiClO4 as electrolyte, first and fifth cycle.
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S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Fig. 4. The UV±VIS spectrum of PCZ films electrochemically synthesized using a LiClO4 as electrolyte.
Films have been characterized by X-ray photoelectron spectroscopy. The presence of chlorine and lithium in the ®lm cannot be excluded, but, even if present, their relative atomic percentages are too small (<1 at.%) to be quanti®ed. The decomposition of the C 1s is reported in Fig. 5. The C 1s peak can be decomposed in three contributions. The ®rst one situated at 285 eV should be attributed at the C±C bonds. The second, peaked at about 286.8 eV, could be assigned to the C±N bonds [20] while the third one corresponds to some oxygen surface contamination (C±OH. . .). The N 1s peak corresponds to only one peak, which can be attributed to the C±N covalent bond. Photoluminescent measurements are reported in Fig. 6. A signal has been obtained. It can be seen that the emission is
Fig. 6. Absorbance and photoluminescence spectra of PCZ electrochemically synthesized with LiClO4 as electrolyte (excitation: 337 nm).
recorded with a maximum at about 430 nm, which corresponds to blue emission light. Such photoluminescent samples were also tested in EL. After aluminum deposition on such ®lms, the I±V
Fig. 5. The XPS spectrum of the C 1s peak: (^-^-^) experimental result; (- - -) theoretical result; (Ð) different components.
S. Yapi Abe et al. / Synthetic Metals 126 (2002) 1±6
Fig. 7. The I±V curve and corresponding (EL±V) signal (photodiode response Vdp±V) characteristics of a structure SnO2/PCZ/Al, the PCZ being electrochemically synthesized with LiClO4 as electrolyte.
Fig. 8. Luminescence spectrum of PCZ electrochemically synthesized.
characteristics of the structures are represented in Fig. 7. Diode characteristics are obtained. The forward direction corresponds, as expected, to a positive polarization of the SnO2 electrode. On the same ®gure the EL characteristic is reported, while the spectrum is reported in Fig. 8. These results demonstrate that PCZ based OLED can be obtained by electrochemical synthesis. 4. Discussion In the voltammogram of Figs. 1 and 2, the Va1 and Va2 peaks can be attributed to the oxidation of the carbazole monomers and of the polymer chains, respectively, and to the doping process. In Fig. 1, a nucleation loop is clearly visible on the ®rst cycle. Moreover an irreversible oxidation of carbazole appears at about 1.2 V. In Fig. 2, it can be seen that the redox process is reversible, since the two reduction peaks situated at Vc1 and Vc2 are systematically observed.
5
The ®rst peak, Vc2 830 mV, corresponds to a partial chain reduction. The area of the peaks is proportional to the injected charges. Therefore, the reduction peaks concern only the dedoping process and a small part of the oxidized chains, since the oxidation peaks area is far higher than that of reduction peaks. This means that only weakly bonded carbazoles, defects. . . are reduced, leading to the highly homogeneous ®lm visualized in Fig. 4a. As usual with carbazole units, an absorption peak is visible at 310 nm and a shoulder at 335 nm (Fig. 4). Respectively, they correspond to the formation of the carbazole dimer excimer [2] and to the formation of a polymer where the carbazole chain have a statistical distribution length, as justi®ed by the band tail visible in the spectra. As described above, no feature is visible in the OD spectrum before the absorption edge. However, peaks at 385 and 735 nm have been described by Verghese et al. [19] in the case of irreversible oxidation. These peaks correspond to the doping of the ®lms. Here it can be seen (Fig. 2) that after extended cycling, the redox process stays reversible, moreover not only is no feature corresponding to doping visible in the OD spectrum, but Li and Cl are nearly absent from the ®lms. All these results are in good agreement with the dedoping process of the PCZ during the cyclic voltammogram and the attribution of Vc2 to this dedoping process. The redox process irreversibility of the electrochemical synthesis is usually attributed to some degradation reaction (reticulation. . .) of the electrochemically synthesized polymer [21]. Therefore, the present reversibility puts in evidence the high quality of the synthesized PCZ. Moreover, it is well known that doped polymer are not luminescent because doping states quench the luminescence. Therefore, the presence of photoluminescence in these polymerized ®lms (Fig. 6) corroborates the dedoping of the ®lms. 5. Conclusion It has been shown that highly homogeneous PCZ ®lms can be electrochemically synthesized in anhydrous solution by using LiClO4 salt and acetonitrile solvent. It is shown that in this case, the redox process is reversible which allows a dedoping of the polymer, which is suf®cient to obtain a photoluminescence signal from the PCZ ®lms. The SnO2/PCZ/Al structure exhibits diode characteristics and EL, when the SnO2 electrode is positively biased. The device performance will be systematically studied and improved by the deposition on the PCZ of an electron transporting layer. Acknowledgements This work has been supported by the Project ECOSCONICYT No. C99E05 and the FONDECYT Project No. 1990544.
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