Enhancement of the Electrical Conductivity in PEDOT:PSS Films by

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Enhancement of the Electrical Conductivity in PEDOT:PSS Films by the Addition of Dimethyl Sulfate Marisol Reyes-Reyes,† Isidro Cruz-Cruz,† and Roma´n Lo´pez-Sandoval*,‡ ´ ptica, UniVersidad Auto´noma de San Luis Potosı´, A´lVaro Instituto de InVestigacio´n en Comunicacio´n O Obrego´n 64, San Luis Potosı´ 78000, Me´xico, and AdVanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, Col. Lomas 4a seccio´n, San Luis Potosı´ 78216, Me´xico ReceiVed: August 5, 2010; ReVised Manuscript ReceiVed: October 18, 2010

A significant increase of the electrical conductivity of PEDOT:PSS films, brought about by the addition of dimethyl sulfate (DMS, (CH3)2SO4), while preserving the films’ excellent flexibility and visible-light transparency, is reported. The electrical and morphological properties of the films were studied as a function of DMS concentration. At an optimal concentration of around 1:25 (DMS to PEDOT:PSS), the conductivity of the films is enhanced by a factor on the order of 1880 times that of pristine PEDOT:PSS films. Extensive spectroscopic measurements using absorbance, Raman, and FTIR techniques, as well as structural characterization by AFM microscopy, were performed. These measurements support the idea that the mechanism responsible for the conductivity enhancement is the partial replacement of the PSS- segments by SO4-2 anionic sulfates when a small amount of DMS is added to a PEDOT:PSS solution. This mechanism is associated with an increase of doping, and this doping can be understood in the following manner: due to that the SO3- ions of the PSS segment only carry one negative charge, it is more probable for them to create polaronic states, whereas the SO4-2 ions are double charged, increasing the possibility of creating bipolaron carriers in the PEDOT backbone. In this way, the partial replacement of the PSS- segments by SO4-2 ions increases the bipolaron population by an ion exchange process, and, as a consequence, the doping level is increased. Introduction In the past few years, the interest in conductive polymers for the fabrication of organic electro-optical devices has progressively increased. Of particular interest is poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS). This polymer has excellent qualities as compared to other conductive polymers, such as a high conductivity and transparency, as well as good processability and flexibility.1,2 Because of these reasons, PEDOT:PSS has been proposed as an alternative to indium tin oxide (ITO) in different organic electrooptical devices like organic solar cells,3 organic light emitting diodes,4,5 etc. The electrical conductivity of PEDOT:PSS, however, is still low in comparison to ITO, although it has been reported by several groups6-9 that the conductivity can be enhanced by the addition of organic compounds called secondary dopants.10-15 Among these dopants, we find solvents with high boiling points,10 ionic liquids,11,12 or solvents with a determined number of polar groups.13,14 In this work, we study how the doping of PEDOT:PSS (Baytron P) films with dimethyl sulfate (DMS, (CH3)2SO4), which is a strong methylating agent, modifies the electrical and morphological properties of the films. The films were characterized as a function of DMS concentration, and, from these results, an optimal doping value of DMS was determined, thus leading to a significant increase of the electrical conductivity of the films of around 1880 times the conductivity of pristine PEDOT:PSS films. From spectroscopic measurements, using Raman and FTIR techniques, we found * Corresponding author. Tel.: +52 444 834 2000. Fax: +52 444 834 2010. E-mail: [email protected]. † Universidad Auto´noma de San Luis Potosı´. ‡ IPICYT.

that the increase of the conductivity is due to a replacement of some PSS- segments by SO4-2 anionic sulfates. Experimental Section Thin films were prepared by dissolving DMS (Sigma-Aldrich, 99.8% purity) in an aqueous dispersion of PEDOT:PSS (Baytron P, Clevios). The weight ratios were set to values of 1:100, 1:50, 1:35, 1:25, 1:20, and 1:15. The solutions were stirred for 6 h, at a temperature between 16 and 18 °C, with the purpose of slowing the decomposition of DMS by hydrolysis.16,17 This temperature range was also chosen to improve the solubility of DMS in the PEDOT:PSS solution; we note that the polymer increases its viscosity at lower temperatures and thus hinders the processability of the sample. Before the deposition of the films, square substrates of Dow corning glass (2.5 cm in length) were prepared. The substrates were ultrasonically cleaned successively in acetone, methanol, and isopropyl alcohol for 20 min each time. After the evaporation of the solvent, the substrates were maintained in a UVozone ambient for 45 min. The DMS/PEDOT:PSS solutions were subsequently deposited on the substrates by spin-casting and then dried at a reduced pressure (21 in. Hg vacuum), at 100 °C, for 3 h, with the continuous extraction of vapor during 15 min of each hour. Once the samples were cooled, the conductivity was measured by using the four-point probe technique. The pristine PEDOT: PSS films exhibited conductivities on the order of 0.07 S/cm, which are in good agreement with the values specified in the data sheet provided by the supplier. Regarding the film thickness, we performed six measurements on each sample using an Alpha Step 500 surface profiler along two parallel scratches going from the center to an edge of the film. The reported thicknesses

10.1021/jp107386x  2010 American Chemical Society Published on Web 11/08/2010

Electrical Conductivity in PEDOT:PSS Films

Figure 1. Electrical conductivity of the thin films as a function of the amount of DMS added to the PEDOT:PSS aqueous dispersion. The DMS/PEDOT:PSS ratios of 0.01, 0.02, 0.03, 0.04, 0.05, and 0.07 correspond to the mixture (DMS into PEDOT:PSS) in a ratio by weight of 1:100, 1:50, 1:35, 1:25, 1:20, and 1:15, respectively.

correspond to an average of these measurements. The absorbance and transmittance spectra of the films were measured using a UV-vis-NIR Varian Cary 5E spectrophotometer. The Raman and FTIR measurements were performed using a JobinYvon T64000 spectrometer in backscattering configuration with a 514.5 nm Ar-ion laser and a Perkin-Elmer 1600 FTIR spectrophotometer. To collect the FTIR spectra, the DMS/ PEDOT:PSS solution was spin coated on a KBr window and then baked at 120 °C, in reduced pressure, for 3 h, with the continuous extraction of vapor during 15 min of each hour, and followed by a bake at the same temperature and pressure for 12 h. Results and Discussion It is known that DMS is slightly soluble in water and hydrolyzes slowly to sulfuric acid and methanol at 18 °C or above.16,17 For this reason, the solutions were processed at temperatures between 16 and 18 °C to avoid low quality films and to promote reproducibility of the process. When the solutions were prepared above 18 °C, for example, at 50 °C, the viscosity of the solutions increased quickly. A viscous liquid forms; this formation can be attributed to the presence of sulfuric acid and methanol in the solutions due to the rapid hydrolysis16 and their interaction with the PEDOT:PSS chains. The good dispersion of the DMS/PEDOT:PSS solutions allows for the attainment of high-quality, thin polymer films. Figure 1 shows the electrical conductivities and thicknesses obtained for different concentrations of DMS. It was found that a correlation between conductivity and secondary doping concentration exists, which is similar to that reported by other groups using different secondary dopants.11,12,18,19 The conductivity of the films rapidly increases as a function of DMS concentration, from 0.07 S cm-1 for the pristine films, up to a maximum of ∼132 S cm-1, and then gradually decreases. Note that, when the concentration corresponds to a weight ratio of 1:25 (DMS/PEDOT:PSS), the maximum conductivity is around 1880 times larger than that measured for the pristine films. In addition, the films’ thicknesses, as a function of DMS concentration, show a trend similar to that of the conductivity; that is, both quantities first increase up to a maximum value and then decrease. To explain the conductivity enhancement, due to the addition of DMS into the PEDOT:PSS solutions, the films were characterized using Raman, FTIR, and UV-vis-NIR spectroscopies. Absorbance spectra of the DMS/PEDOT:PSS films were obtained at different weight ratios; they show a high optical

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Figure 2. Absorbance spectra for DMS/PEDOT:PSS thin films with weight ratios of 1:50, 1:25, and 1:15. The inset shows the absorbance band around 850 nm for a pristine film. For doped films, this band disappears.

transparency (around 90%) in the visible spectral range (Figure 2). From the inset of the figure, when the amount of DMS in the PEDOT:PSS blends increases, we note that the absorbance band around 850 nm for the doped film decreases, whereas the band around 2000 nm increases. These observed bands have been associated with polarons.20,21 The disappearing and/or decreasing of the band at 850 nm in PEDOT:PSS films is associated with an increase of the doping and the formation of bipolarons.22,23 Therefore, a possible effect of the DMS in the aqueous PEDOT:PSS dispersions is to increase the relative population of bipolarons with respect to polarons. Thus, we need to use other characterization techniques to clarify this issue. Raman spectra of the pristine PEDOT:PSS and the DMS/ PEDOT:PSS films are shown in Figure 3. The pristine films exhibit the main peak at 1442 cm-1, corresponding to the CdC symmetrical stretching vibration,24 whereas the DMS/PEDOT: PSS films exhibit a peak slightly shifted to the right (1451 cm-1). From detailed analyses of these spectra (Figure 3b,c), it is clearly observed that the alteration in the symmetry of these intense peaks results from the combination of two separate bands, which correspond to two present structures in the PEDOT chains, that is, the neutral and oxidized structures; this finding has been suggested previously.25,26 For the case of pristine films, the band near 1370 cm-1 is associated with the C-C stretching,24 whereas vibrations centered at 1440 and 1456 cm-1 can be assigned to the neutral (reduced) and oxidized structures, respectively. The peaks centered at 1508 and 1570 cm-1 have been associated with the CdC asymmetric stretching vibrations that correspond to thiophene rings in the middle and at the end of the chains, respectively.27 We also observe another band located at 1540 cm-1, which has been related to the splitting of these asymmetrical stretching vibrations.25,26 On the other hand, the peak position of the band from the neutral structure and the ratio intensities of the two bands (oxidized:neutral) vary as a function of DMS doping. The most remarkable shift, with respect to the pristine film, is observed for the sample with the highest conductivity (weight ratio of 1:25), which has the neutral structure peak centered at 1437 cm-1, whereas the oxidized structure shows the peak centered at 1455 cm-1. Another effect, due to the addition of DMS to the aqueous dispersion of PEDOT:PSS, is the decrease of the peak intensity of the neutral structure as compared to the peak intensity of the oxidized structure when the DMS concentration is increased. However, after the optimal concentration of DMS is reached, the ratio intensity decreases. Thus, our results indicate that there exists an increase in the level/charge carrier concentration, or a change

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Figure 4. FTIR spectra for PEDOT:PSS and DMS/PEDOT:PSS (optimal concentration) films. Note the two additional bands at 1105 and 615 cm-1 for the optimal DMS loading; these have been related to free SO4-2 ions.

Figure 3. Raman spectra of PEDOT:PSS films obtained at different DMS loading. The fitted curves for the spectra are also shown.

in the relative population of polarons and bipolarons,25,26,28 due to the addition of DMS to the aqueous dispersion of PEDOT: PSS. This increase/change is consistent with data observed in our absorbance spectra. It was also found that the pH of all the blend solutions diminished with the addition of DMS, becoming more acidic than the pristine solution. In particular, the pH of the pristine solution was 1.8, whereas the pH for the optimal blend solution was 0.8. It is known that a decrease of the pH is related to an increase of the oxidation in the PEDOT:PSS films and/or to a change in the relative population between polarons and bipolarons.25,26,28 Moreover, it has been reported by De Kok et al.28 that for a pH e 1.5, the PEDOT:PSS shows a metallic character that is not related to doping, that is, not related to an increase in the carrier concentrations, but rather to a change in

the relative population between polarons and bipolarons, which induces a change in the morphology of the PEDOT chains. Thus, all these results indicate that the increase in the conductivity in our doped films is due to an increase in the doping level, a decrease in the pH value, or both. To elucidate the possible factors that produce this enhancement, we also carried out studies with FTIR spectroscopy. As shown in Figure 4, the FTIR spectra of the pristine film and the blend film with optimal ratio differ significantly. The spectrum obtained for the pristine PEDOT:PSS film is similar to those reported in the literature,29 while the FTIR spectrum of blend film, with optimal ratio, has two additional bands: a very strong one at 1105 cm-1 and another less intense one centered at 615 cm-1. When we compare the spectra of blend films with the spectrum of DMS, it is observed that none of the characteristic bands of this compound coincide with the new bands that appear. It is, therefore, presumed that these new bands are associated with the presence of some other components present in the films as a result of hydrolysis by the DMS. The new bands in the spectrum correspond to free sulfate ions30 (SO4-2); thus we consider that the process results in a complete DMS hydrolysis. Therefore, on the basis of these results, we can suppose that the PEDOT chains undergo a doping because the free SO4-2 ions are replacing some PSS segments in some parts of the PEDOT backbone. A possible mechanism for the doping process is the following: (1) a slow mix of DMS in the aqueous dispersion of PEDOT:PSS, without hydrolysis, could be occurring (resulting in a uniform solution); and (2) a rapid hydrolysis during the baking of the films. In this way, the DMS hydrolyzes to sulfuric acid and methanol and then this acid reacts, as a result of the hydrolysis, with water present in the films, finally obtaining SO4-2 ions, whereas the subproducts are evacuated. Because of the fact that the SO3- ions of the PSS segment only carry one negative charge each, it is more probable that polaronic states are created. Similarly, the SO4-2 ions are double-charged, increasing the possibility of creating bipolaron carriers in the PEDOT structure. In this way, an ion exchange process increases the population of bipolarons, with respect to polarons, and, as a consequence, the doping level is increased. In addition, it is known that PEDOT does not match very well with PSS because the PSS chains are longer than PEDOT chains; these excess PSS units allow for the solvation of these polymers in water. Furthermore, PSS adopts a coiled conforma-

Electrical Conductivity in PEDOT:PSS Films

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20223 Conclusions We report a new doping scheme by the use of dimethyl sulfate in a PEDOT:PSS dispersion. The electrical conductivity of deposited films can be enhanced 1880 times greater than that of pristine PEDOT:PSS films after the addition of DMS to the PEDOT:PSS aqueous dispersion. The DMS produces an enhancement in the doping level in the PEDOT chains, as is shown in the absorbance, Raman, and FTIR spectra. This enhancement is produced, principally, by a decrease in the pH of the solution and the replacement of some PSS- units by SO4-2 ions in the thin films. Our FTIR spectra clearly show that this increase in the doping level, and therefore in the electrical conductivity, is produced by the SO4-2 ions. Moreover, from AFM images, we found that the morphology, the size of the conductive domains, and the partial PSS phase separation are other key factors affecting the increase of the films’ electrical conductivity. Acknowledgment. This work was supported at UASLP and IPICYT by SEP-PROMEP through grant no. 103.5/07/2574 (M.R.-R.) and by CONACYT through grant nos. J48897-Y (M.R.-R.), S-3148 (R.L.-S.), and a scholarship (I.C.-C.). We also wish to thank Professor David L. Carroll for very valuable discussions and a critical reading of this manuscript. References and Notes

Figure 5. Topographic AFM images (2 µm × 2 µm) of (a) pristine PEDOT:PSS films and DMS/PEDOT:PSS films with a weight ratio of (b) 1:25 and (c) 1:15. The rms roughness is shown.

tion in water, and PEDOT follows this conformation. The replacement of some PSS- units for SO4-2 ions can then induce a conformational change from a coiled to a linear configuration; this change can also contribute to the increase in the conductivity. On the other hand, we observed, from AFM images, that the rms roughness of the doped PEDOT:PSS films increases as a function of the DMS concentration (Figure 5). The rms roughness was obtained by averaging, at least, eight measurements per sample. Additionally, we have obtained AFM images with length greater than 2 µm, for example, 5, 10, and 50 µm, and their rms roughness is similar to that shown in Figure 5. The observed roughness indicates that the size of the conductive domains grows due to a partial phase separation.18,19,31 Note that for DMS concentrations bigger than the optimal one (1:25), the AFM images continue to show an increase in both the rms roughness and the average size of the conductive domains; however, the conductivity begins to decrease. We can, therefore, deduce that there is an optimal morphology of the conductive PEDOT domains for the highest conductivity.31

(1) Heywang, G.; Jonas, F. AdV. Mater. 1992, 4, 116–118. (2) Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481–494. (3) Hsiao, Y. S.; Whang, W. T.; Chen, C. P.; Chen, Y. C. J. Mater. Chem. 2008, 18, 5948–5955. (4) Brown, T. M.; Kim, J. S.; Friend, R. H.; Cacialli, F.; Daik, R.; Feast, W. J. Appl. Phys. Lett. 1999, 75, 1679–1681. (5) Brewer, P. J.; Huang, J.; Lane, P. A.; de Mello, A. J.; Bradley, D. D. C.; de Mello, J. C. Phys. ReV. B 2006, 74, 115202–115207. (6) Winther-Jensen, B.; West, K. Macromolecules 2004, 37, 4538– 4543. (7) Lock, J. P.; Im, S. G.; Gleason, K. K. Macromolecules 2006, 39, 5326–5329. (8) Hulvat, J. F.; Stupp, S. I. AdV. Mater. 2004, 16, 589–592. (9) Han, M. G.; Foulger, S. H. Chem. Commun. 2005, 24, 3092–3094. (10) Kim, W. H.; Kushto, G. P.; Kim, H.; Kafafi, Z. H. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2522–2528. (11) Dobbelin, M.; Marcilla, R.; Salsamendi, M.; Pozo-Gonzalo, C.; Carrasco, P. M.; Pomposo, J. A.; Mecerreyes, D. Chem. Mater. 2007, 19, 2147–2149. (12) Fan, B. H.; Mei, X. G.; Ouyang, J. Y. Macromolecules 2008, 41, 5971–5973. (13) Ouyang, J.; Xu, Q. F.; Chu, C. W.; Yang, Y.; Li, G.; Shinar, J. Polymer 2004, 45, 8443–8450. (14) Huang, J. S.; Miller, P. F.; Wilson, J. S.; de Mello, A. J.; de Mello, J. C.; Bradley, D. D. C. AdV. Funct. Mater. 2005, 15, 290–296. (15) Pingree, L. S. C.; MacLeod, B. A.; Ginger, D. S. J. Phys. Chem. C 2008, 112, 7922–7927. (16) Robertson, R. E.; Sugamori, S. E. Can. J. Chem. 1966, 44, 1728– 1730. (17) Bertoluzza, A.; Fagnano, C.; Morelli, M. A.; Tosi, R. J. Raman Spectrosc. 1987, 18, 77–81. (18) Crispin, X.; Marciniak, S.; Osikowicz, W.; Zotti, G.; Van der Gon, A. W. D.; Louwet, F.; Fahlman, M.; Groenendaal, L.; De Schryver, F.; Salaneck, W. R. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2561– 2583. (19) Nardes, A. M.; Janssen, R. A. J.; Kemerink, M. AdV. Funct. Mater. 2008, 18, 865–8871. (20) Fesser, K.; Bishop, A. R.; Campbell, D. K. Phys. ReV. B 1983, 27, 4804–4825. (21) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309–315. (22) Havinga, E. E.; Mutsaers, C. M. J.; Jenneskens, L. W. Chem. Mater. 1996, 8, 769–776. (23) Hwang, J.; Tanner, D. B.; Schwendeman, I.; Reynolds, J. R. Phys. ReV. B 2003, 67, 115205. (24) Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. Macromolecules 1999, 32, 6807–6812.

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(25) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A. Synth. Met. 2005, 155, 80–88. (26) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A. J. Raman Spectrosc. 2006, 37, 1354–1361. (27) Tran-Van, F.; Garreau, S.; Louarn, G.; Froyer, G.; Chevrot, C. J. Mater. Chem. 2001, 11, 1378–1382. (28) de Kok, M. M.; Buechel, M.; Vulto, S. I. E.; van de Weijer, P.; Meulenkamp, E. A.; de Winter, S.; Mank, A. J. G.; Vorstenbosch, H. J. M.; Weijtens, C. H. L.; van Elsbergen, V. Phys. Status Solidi A 2004, 201, 1342–1359.

Reyes-Reyes et al. (29) Kvarnstrom, C.; Neugebauer, H.; Ivaska, A.; Sariciftci, N. S. J. Mol. Struct. 2000, 521, 271–277. (30) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds [Part B, Applications in coordination, organometallic, and bioinorganic chemistry], 6th ed.; Wiley-Interscience: Hoboken, NJ, 2009. (31) Cruz-Cruz, I.; Reyes-Reyes, M.; Aguilar-Frutis, M. A.; Rodriguez, A. G.; Lo´pez-Sandoval, R. Synth. Met. 2010, 160, 1501–1506.

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