In Situ Wilhelmy Balance Surface Energy Determination of Poly(3

Solvent-Dependent Structure Formation in Drying P3HT:PCBM Films Studied by Molecular Dynamics Simulations. Vikas Negi , Alexey Lyulin , Peter Bobbert...
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Langmuir 2006, 22, 9287-9294

9287

In Situ Wilhelmy Balance Surface Energy Determination of Poly(3-hexylthiophene) and Poly(3,4-ethylenedioxythiophene) during Electrochemical Doping-Dedoping Xiangjun Wang,*,†,‡ Thomas Ederth,§ and Olle Ingana¨s†,‡ Biomolecular and Organic Electronics, Center of Organic Electronics, and Sensor Science and Molecular Physics, Department of Physics, Chemistry and Biology (IFM), Linko¨ping UniVersity, SE-581 83 Linko¨ping, Sweden ReceiVed June 4, 2006. In Final Form: August 4, 2006 Changes in the contact angle between conjugated polymers surface poly(3-hexylthiophene) [P3HT] and poly(3,4-ethylenedioxythiophene) (PEDOT) upon electrochemical doping-dedoping in aqueous electrolyte were determined in situ using a Wilhelmy plate tensiometer in an electrochemical cell. The hydrophobic P3HT was less hydrophobic in the oxidized state than in the neutral state; the more hydrophilic PEDOT was less hydrophilic in the oxidized state than when neutral. The tensiometry results were in good agreement with those measured by contact angle goniometry, and further corroborated by the capillary rise upon doping in a fluid cell with two parallel polymer coated plates, another in situ dynamic determination method. The contact angle changes depend on doping potential, electrolyte type, and concentration. We also deconvoluted the surface energy into components of van der Waals and acid-base interactions, using three probe liquids on the polymer surfaces, ex situ the electrochemical cell. The methods and the obtained results are relevant for the science and technology areas of printed electronics and electrochemical devices and for the understanding of surface energy modification by electrochemical doping.

1. Introduction Electrochemical doping of conjugated polymers by oxidation or reduction, with concomitant electron and ion transfer, cause the appearance of new electronic states (polaron and bipolarons) in the band gap and change of chemical composition of the polymer solid. These effects can result in changes of polymer volume,1-2 color,3-4 and conductivity.5-6 These properties have led to many applications, such as actuators,1,2 smart windows,3,4 and electrochemical transistors.7 Many of these devices use, or are constructed by, polymer films in contact with liquids; therefore, surface and interfacial energies are of importance. Studies of polymer surface energies are widely available, and theories relating the surface energy to the van der Waals attraction acting between polymer segments in the solid phase are adequately developed.8 Electronic polymers are different, however, and we expect major changes to the surface energy of a polymer during doping. This is because a major change of polarizability is expected, due to the semiconductor/metal transition occurring; also changes of surface chemical composition due to ingress/ egress of anions/cations and associated solvent molecules should contribute. * To whom correspondence should be addressed. E-mail: [email protected]. † Biomolecular and Organic Electronics. ‡ Center of Organic Electronics. § Sensor Science and Molecular Physics. (1) Jager, W. H. E.; Smela, E.; Ingana¨s, O. Science 2000, 290, 1540. (2) Lu, W.; Fadeev, A. G.; Qi, B.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewcz, J.; Zhou, D.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297, 983. (3) Gustafsson C. J.; Ingana¨s O.; Andersson M. R. Electrochem. Acta 1995, 40, 2233. (4) Mecerreyes, D.; Marcilla, R.; Ochoteco, E.; Grande, H.; Pomposo, J. A.; Vergaz, R.; Pena, J. M. S. Electrochem. Acta 2004, 49, 3555. (5) Garcia-Belmonte, G.; Bisquert, J.; Popkirov, G. S. Appl. Phys. Lett. 2003, 83, 2178. (6) Hulea, I. N.; Brom, H. B.; Mukherjee, A. K.; Menon, R. Phys. ReV. B 2005, 72, 54208. (7) Nilsson, D.; Kugler, T.; Svensson, P.-O.; Berggren, M. Sens. Actuators B, Chem. 2002, 86, 193. (8) Jones, R. A. L.; Richards, R. W. Polymers at surfaces and interfaces; Cambridge University Press: Cambridge, U.K., 1999.

Studies of surface energy related phenomena in conjugated polymer porous materials for separation9-10 are available but rarely with the advantage of in situ electrochemical control. Pioneering studies of surface energy changes of polypyrrole films and graphite fibers,11-14 during electrochemical doping and dedoping, were reported. A short report on the in situ measurement of relative surface tension of polypyrrole at various electrochemical states is found in refs 11 and 12. There is a need for understanding the variation of surface energy with doping in conjugated polymers, when these materials are being used for electronics. As organic electronics is turning to production issues, the possibility to deposit and pattern electronic polymers from liquid phases is becoming important. Whether inkjet or more classical printing methods are used, the role of the surface energy in wetting and guiding liquids is critical in making devices.14,15 This issue is interlinked with that of the swelling of polymers in good solvents, which may convert a polymer surface to a solution of anchored polymer chains. Solvent swelling can be a detriment in the construction of organic electronic devices but may also be used with a purpose. For electronic polymers, there is the added complexity of coupled ion and solvent motion, demonstrated in several studies with impedance and other methods.16,17 The presence of immobilized dopants in a conjugated polymer film in the doped state has a major influence (9) Cehimi, M. M.; Abel, M. L.; Perruchot, C.; Delamar, M.; Lascelles S. F.; Armes, S. P. Synth. Met. 1999, 104, 51. (10) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. J. Colloid Interface Sci. 1997, 193, 190. (11) Lee, M.-H. Mol. Cryst. Liquid Cryst.1998, 316, 329. (12) Lee, M.-H.; Kim S. S.; Kang Y. Synth. Met. 1999, 101, 442. (13) Kocharova, N.; Lukkari, J.; Viinikanoja, A.; Aaritalo, T.; Kankare, J. J Mol. Struct. 2003, 651, 75. (14) Lapkowski, M.; Zak, J.; Kolodziej-Sadlok, M.; Guillerez, S.; Bidan, G. Synth. Met. 2001, 119, 417. (15) Wang, J. Z.; Zhang, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus, H. Nat. Mater. 2004, 3, 171. (16) Bay, L.; Jacobsen, T.; Skarup, S.; West, K. J. Phys. Chem. B 2001, 105, 8492. (17) Hillman, A. R.; Efimov, I.; Skompska, M. Discuss. Faraday 2002, 121, 423.

10.1021/la061606p CCC: $33.50 © 2006 American Chemical Society Published on Web 09/29/2006

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Figure 1. Upper row: the geometries of sample SS: P3HT/ITO/ PET, DS: P3HT/ITO/PET/PU/PET/ITO/P3HT and C: PET/ITO/ P3HT/spacer/P3HT/ITO/PET. Lower rows: the chemical structure of P3HT, PEDOT, OTS, and PSS.

Wang et al.

spread use as a buffer layer or electrode in polymer electronics;29-30 it is also available as a thin film on flexible support. There are several methods to prepare high conductivity PEDOT,31-33 and this vapor-phase polymerized PEDOT [VPP-PEDOT] is therefore of interest. We have chosen these two polymers for our exploratory study of the variation of wetting with doping in conjugated polymers. An added advantage are the extensive studies of the electrochemistry of P3HT and related polyalkylthiophenes,18 the structural studies of PEDOT films,34-35 and surface spectroscopy of both polymers. Among the various surface energy analysis methods,36-39 the sessile drop goniometry method is the most conventional. It is versatile but ex situ and, therefore, normally not able to monitor surface energy changes when electrochemical doping of a conjugated film is undertaken in an electrolyte. In this communication, the dynamics of surface energy of P3HT and PEDOT were determined using the Wilhelmy balance during in situ electrochemical doping. This method gives more definite control of the polymer condition during evaluation of surface energy. We also demonstrated the capillary rise at a polymer/liquid interface upon the electrochemical doping and the possibility to use this as a method for in situ surface energy measurement. Results will be helpful for understanding the interaction of the neutral and doped electronic polymers with liquids and may be relevant to printing of electronic polymers. 2. Experimental Details

on the relative importance of anion/cation transport.18,19 Reorganization of amphiphilic dopants at surfaces have been observed as part of the electrochemical reduction and oxidation.20 Given this plethora of contributing effects to the surface energy of polymer surfaces, and therefore to the dynamics of liquids at polymer/liquid interfaces, it is of interest to better understand the character of surface energy in some representative electronic polymers. Poly(3-hexylthiophene) (P3HT; Figure 1) is a semiconducting polymer with extensive use in field-effect transistors20-21 and photovoltaics,23-24 because of the high hole mobility due to the highly ordered structure, in its turn related to the high regioregularity. Poly(alkylthiophenes) play a major role in the development of polymer field effect transistors.25-26 Another representative polymer for organic electronics is the poly(3,4ethylenedioxythiophene) [PEDOT], another polythiophene (Figure 1) with high electrical conductivity in a stable doped state.27-28 The commercial poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), namely PEDOT-PSS, has found wide-

Preparation of Samples. P3HT (Aldrich Chem. Co.), ∼100 nm thick, was deposited on an indium tin oxide [ITO] coated plastic poly(ethylene terephthalate) [PET] foil (175 µm thick and ∼2 × 2 cm2 in size), by spin-coating. These samples have the structure of P3HT/ITO/PET and are called single side samples, labeled “SS” (Figure 1). VPP-PEDOT films (100-400 nm thick) were deposited on the PET foil using vapor phase polymerization. A layer of Fe(III) tosylate (Fe(OTS)3; Figure 1), mixed with pyridine, is used as oxidizing agent. The monomer 3,4-ethylenedioxythiophene (Baytron M) was polymerized as its vapor meet the oxidizing agent on the substrate. The details of the VPP-PEDOT preparation procedure can be found in ref 31. We used a commercial formulation of PEDOT, known as Orgacon and available from Agfa-Gevaert. This is a material including PEDOT (polystyrenesulfonate) [PEDOT-PSS], which is also available as a water based dispersion. The Orgacon (Figure 1) on polystyrene foil was cut to ∼2 × 2 cm2 size and used for contact angle measurement during the electrochemical doping and dedoping. The initial state of P3HT is at neutral state and VPP-PEDOT, Orgacon is at the oxidized state, respectively.

(18) Pei, Q. B.; Ingana¨s, O. J. Phys. Chem. 1992, 96, 10507. (19) Pei, Q. B.; Ingana¨s, O. J. Phys. Chem. 1993, 97, 6034. (20) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. AdV. Mater. 2004, 16, 316. (21) Yang, H.; Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z. AdV. Funct. Mater. 2005, 15, 671. (22) Wang, G.; Swensen, J.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2003, 93, 6137. (23) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (24) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Func. Mater. 2003, 13, 85. (25) Arias, A. C.; Ready, S. E.; Lujan, R.; Wong, W. S.; Paul, K. E.; Salleo, A.; Chabinyc, M. L.; Apte, R.; Street, R. A.; Wu, Y.; Liu, P.; Ong, B. Appl. Phys. Lett. 2004, 85, 3304. (26) Chabinyc, M. L.; Wong, W. S.; Arias, A. C.; Ready, S.; Lujan, R. A.; Daniel, J. H.; Krusor, B.; Apte, R. B.; Salleo, A.; Street, R. A. Proc. IEEE 2005, 93, 1491. (27) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Ingana¨s, O. Polymer 1994, 35, 1347. (28) Groenendaal, B. L.; Jonas F.; Freitag D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481.

(29) Zhang, F.; Gadisa, A.; Ingana¨s, O.; Svensson, M.; Andersson, M. R. Appl. Phys. Lett. 2004, 84, 3906. (30) Jo¨nsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; van der Gon, A. W. D.; Salaneck, W. R.; Fahlman, M. Synth. Met. 2003, 139, 1. (31) Admassie, S.; Zhang, F.; Manoj, A. G.; Svensson, M.; Andersson, M. R.; Ingana¨s, O. Solar Energy Mater. Solar Cells 2006, 90, 133. (32) Lai, S. L.; Fung, M. K.; Lee, C. S.; Lee, S. T. Mater. Sci. Eng. B 2003, 104, 26. (33) Pettersson, L. A. A.; Carlsson, F.; Ingana¨s, O.; Arwin, H. Synth. Met. 1998, 313-314, 356. (34) Aasmundtveit, K. E.; Samuelsen, E. J.; Inganas, O.; Pettersson, L. A. A.; Johansson, T.; Ferrer, S. Synth. Met. 2000, 113, 93. (35) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Inganas, O.; Johansson, T.; Feidenhans, R. Synth. Met. 1999, 101, 561. (36) Barraza, H. J.; Kunapuli, S.; O’rear, E. A. J. Phys. Chem. B 2002, 106, 4979. (37) Abe, K.; Takiguchi, H.; Tamada, K. Langmuir 2000, 16, 2394. (38) Mohamed, M. C.; Abel, M.-L. A.; Perruchot, C.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Synth. Met. 1999, 104, 51. (39) Hirsa, A. H.; Lopez, C. A.; Laytin, M. A. Appl. Phys. Lett. 2005, 86, 014106.

Surface Energy Determination of P3HT and PEDOT

Langmuir, Vol. 22, No. 22, 2006 9289 Contact angles of P3HT (SS and DS) and VPP-PEDOT (SS) and Orgacon during in situ electrochemical doping-dedoping were determined using a Wilhelmy balance (Sigma 70, KSV Instruments Ltd.), controlling the reservoir movement up and down (50 µm/s). The instrument control program calculates the wetting force and a contact angle θexp during the instrument operation, assuming the two faces of the sample are the same, i.e., θ ) θm(see Figure 2). Recalculation of experimental data from asymmetric samples (SS) is necessary in order to obtain the correct contact angle at the polymer side. From eqs 2 and 3, it follows that cos θexp ) (cos θ + cos θm)/2, and rearranging this gives cos θ ) 2 cos θexp - cos θm

Figure 2. Schematic diagram of the simultaneous electrochemical and contact angle measurements using a Wilhelmy balance. The sample consists of a polymer layer coated onto a supporting substrate medium, forming contact angles θ and θm with the electrolyte, respectively, and in general θ * θm. To make the sample symmetric in layer structure, two SS samples were glued together along their common plastic side, using polyurethane (PU) UV cured for 90 s. Consequently, samples with the structure P3HT/ITO/PET/PU/PET/ITO/P3HT were prepared and labeled “DS”. Samples, “C”, that consisted of two parallel SS samples with a distance (PDMS spacer, 500 µm ∼ 1 mm) between them and with P3HT facing each other were also prepared (Figure 1.). Electrochemical Cyclic Voltammogram (CV) and Electrochemical Doping of Polymer Films. The CV was carried out in a three electrodes one-compartment electrochemical cell [see Figure 2]: (1) a platinum foil as a counter electrode, (2) Ag/Ag+ reference electrode, +0.22 V relative to the normal hydrogen electrode, and (3) the prepared sample as working electrode. The CV was operated by an Auto lab, General Purpose Electrochemical System 10 (EchoChemie, Netherlands), and 0.05 M LiClO4 was used as the aqueous electrolyte. The potential scan rate was set at 50-100 mV/ s. Electrochemical doping of sample P3HT was undertaken by applying a positive potential (from 0.1 to 1 V with 0.1 V step) on the working electrode (i.e., the samples) when samples were immersed into or emersed from the electrolyte reservoir. Dedoping of P3HT was done at a negative potential of -0.1 V to the samples, after the samples had been doped at a certain level. Similar doping-dedoping processes were also applied for VPP-PEDOT and Orgacon samples. Absorption Spectra. Absorption spectra of films at neutral and doped states were measured with a Perkin-Elmer Lambda-9 UV-vis-NIR spectrophotometer. In Situ Wilhelmy Balance Contact Angle Tensiometry. A Wilhelmy balance (Figure 2) records the total force (F) acting on a sample plate in the vertical direction as it is moved upward or downward, relative to the electrolyte reservoir. F is the sum of the gravitational force (G), buoyancy (f), and the wetting force (s) and is given by eq 1 at equilibrium (where positive direction is downward). F)G-f+s

(1)

The wetting force is the weight contribution from the meniscus, and for a homogeneous sample it is given by s ) PγLV cos θ

(2)

where P is the perimeter of the sample plate, γLV is the liquid surface tension, and θ is the contact angle. For asymmetric samples, such as the SS type [Figure 1], where the contact angles for the polymer (θ) and the supporting medium (θm) are different, the wetting force is instead given by eq 3 s ) PγLV (cos θ + cos θm)/2

(3)

(4)

The contact angles θm of the supporting PET and polystyrene films were also determined using the Wilhelmy balance, and advancing and receding contact angles were measured to be ∼76° and 43°, respectively, for PET, and ∼54° and 24° for polystyrene. Surface Tension of Aqueous Electrolytes. The Wilhelmy balance was used to measure surface tension in plate mode with a platinum (Pt) plate, 19.6 mm wide and 0.1 mm thick. A total of 10 immersion cycles were made for each surface tension data point. Contact Angle Goniometry. Contact angles of the P3HT at neutral, doped, and dedoped state were measured using goniometry method (CAM 200, KSV Instrument Ltd.) with the 0.05 M LiClO4 electrolyte as the probing liquid. The contact angle of the SS geometry P3HT sample, at the neutral state, was measured when a newly prepared sample surface was dispensed with the probing droplet. After this, the sample was immersed into the electrochemical cell (see Figure 2), operated as the working electrode, and voltages of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 V were applied for 3 min, resulting in a correspondingly doped state. Immediately after the doping processes, the sample was rinsed by ethanol and deionized water for 3 times and then dried by nitrogen flow. The contact angles of samples at different doping level were then measured using the goniometry at same condition as when measuring contact angle of the sample at neutral state. The contact angles of P3HT as well as VPP-PEDOT and Orgacon at the initial state were also measured with the goniometer, using aqueous electrolytes, deionized water (L1), ethylene glycol (L2), and n-hexadecane (L3) as probing liquids. All of the measurements were carried out at room temperature (T ∼ 21 °C) and ambient humidity, and contact angles were determined using the supplied image processing software. Capillary Rise. Sample C was designed for monitoring capillary rise during electrochemical doping, and was monitored using a CCD color camera (Sanyo VCC 65772P) and a video recorder.

3. Results 3.1. Electrochemical Behavior and Optical Absorption in P3HT and PEDOT Films. To determine proper conditions for doping of P3HT and PEDOT, cyclic voltammograms (CV) were examined (Figure 3). P3HT can be easily oxidized and is fully oxidized at ∼0.84 V (Figure 3a). On the other hand, the PEDOTs are prepared in the doped form, but can be reduced to the neutral state, at ∼ -0.5 V (Figure 3b). In the neutral state, the color of a P3HT film is reddish purple. The color changed to light blue rapidly upon electrochemical doping (V ) 0.3, 0.5, to 0.8 V), indicating that electrochemical oxidation was taking place. With higher potential applied to the samples, a lighter blue color of P3HT films were observed, which was consistent with absorption spectra (Figure 4a). Dedoping (at -0.1 V) of the doped P3HT brought the color of the doped films back to the reddish purple, as seen in the absorption spectrum. Upon reduction of VPP-PEDOT to the neutral state (at applied negative potential, -0.5 V to the sample), the PEDOT film goes dark blue. Further reduction to -1.0 and -1.5 V does not change the color appreciably. On the other hand, applying a voltage of

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Figure 3. Cyclic voltammogram of (a) P3HT film on ITO-coated PET foil, 0.05 M LiClO4, 50 mV/s. (b) VPP-PEDOT (solid line) on PET foil and Orgacon (dashed line) on PS foil, 0.1M LiClO4, 50 mV/s.

0.5 V makes the film more transparent as compared to the initial state (see Figure 4b). Reduction of Orgacon, at negative potential (-0.3 to -0.8), turns the films darker blue, due to the higher absorption in the visible and infrared regions (Figure 4c). Further reduction to -1.0 and -1.5 V does not change that color at all. Only very small changes in color were observed when a positive potential was applied (see Figure 4c). The absorption at short wavelength range for these three polymers is attributed to the transition between highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital LUMO, while absorption in the longer wavelength is assigned to optical transitions of the polaron/bipolaron states.40 3.2. Contact Angle Changes upon Doping-Dedoping. 3.2.1. Surface Tension of the Aqueous Electrolytes. Three aqueous electrolytes (LiClO4, C3H7NaSO3, and C8H17NaSO3) were used for the surface energy determination, and surface tensions of these electrolytes at different concentrations were determined with the Wilhelmy plate method. The results are listed in Table 1. The amphiphilic natures of C3H7NaSO3 and C8H17NaSO3 cause a significant reduction in surface tension at concentrations approaching the critical micelle concentration (CMC), at which point surface adsorption is saturated and the surface tension is minimized. We selected LiClO4, C3H7NaSO3, and C8H17NaSO3 at 0.05 M as the aqueous electrolytes for the subsequent contact angle measurement. As seen in Table 1, C3H7NaSO3 is far from the CMC at a concentration of 0.05 M, whereas for C8H17NaSO3, the surface tension has been reduced significantly at the same concentration. The C8H17NaSO3 solution is in micellar phase, and was not used for in situ Wilhelmy studies, as it could not be used to oxidize P3HT. Organic solvents were purposely avoided in this study, to reduce the impact of swelling of the polyalkylthiophene. 3.2.2. Contact Angle Tensiometry and Goniometry. Contact angles of the asymmetric SS and symmetric DS samples during

Figure 4. Absorption spectra of (a) P3HT films at neutral state (V ) 0, solid line), and doped states (V ) 0.3, 0.5, and 0.8V, corresponding to open triangles, open squares and open diamonds) as well as dedoped state (V ) -0.1V, closed circles); (b) VPPPEDOT at V ) -1.5 V (solid line), V ) -1.0 V (dashed line), V ) 0 V (gray solid line), and V ) 0.5 V (dot-dashed line). (c) Orgacon at reduced states (V ) -0.8 V gray solid line and V ) -0.5 V gray dashed line), initial state V ) 0 V black solid line and oxidized state (V ) 0.5 V black dashed line and V ) 0.8 V black dot line). All potentials were relative to the Ag/Ag+ reference electrode. Table 1. Surface Tensions (mN/m) of Aqueous Electrolytes at Varying Concentrationa concentration (M)

LiClO4

C3H7NaSO3

C8H17NaSO3

0 0.005 0.01 0.05 0.1

71.72 ( 0.09 71.65 ( 0.13

71.72 ( 0.09 71.65 ( 0.12 71.54 ( 0.13 71.11 ( 0.12 57.43 ( 0.35

71.72 ( 0.09 70.01 ( 0.11 68.47 ( 0.15 56.97 ( 0.10 46.60 ( 0.12

71.63 ( 0.13 71.63 ( 0.09

a A total of10 immersion cycles were made for each measurement. Tabulated data are mean valued and standard deviations.

doping-dedoping of P3HT, VPP-PEDOT, and Orgacon were measured with the Wilhelmy balance, in aqueous 0.05 M LiClO4 (see Figure 5 and the Supporting Information). Contact angles of SS sample were also measured by goniometry, using the same aqueous electrolyte as the probing liquid for comparison. The data obtained by the two methods were similar, with deviations less than 5° (Supporting Information).

Surface Energy Determination of P3HT and PEDOT

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Figure 5. Contact angles of P3HT (upper curve), VPP-PEDOT (middle curve) and Orgacon (lower curve), versus applied potential. The lines in the boxes are the mean values and the lines below and above the boxes represent the highest and lowest values obtained. The lines of each curve are for guiding eyes. LiClO4 0.05 M was used as the aqueous electrolyte.

The contact angle of P3HT decreased from 106° to 89° with increasing potential from 0.3, 0.5, to 0.8 V. Dedoping of P3HT from the doped state (V ) 0.8 V) by applying a -0.1 V voltage to the sample makes the contact angle return to that of the neutral state. Tensiometrically determined contact angles of VPP-PEDOT show a different trend of contact angle changes (Figure 5 and the Supporting Information). Upon increasing the applied positive potential from 0 to 0.6 V, the contact angle increases sharply from ∼65° to ∼80°. The VPP-PEDOT was less hydrophilic in the oxidized state than in its initial state. There was no change in contact angle with further increase of applied potential. Applying negative potentials to the film caused reduction, the contact angle increased to 70°, at -0.1 V, and then decreased to 50° with applied voltage from -0.2 to -0.8 V. At a more cathodic potential, when lower than -0.4 V, the film was more hydrophilic than at zero potential. The maximum difference in contact angle between the oxidized (V ) 0.6 V) and reduced (V ) -0.8 V) state was ≈30°. For Orgacon films, a somewhat different behavior was observed as compared to VPP-PEDOT (Figure 5 and the Supporting Information). Similarly, increased contact angle from ∼55° to ∼67° was obtained when positive potentials from 0.1 to 0.5 V were applied. Slightly increased contact angles relative to the initial state were also obtained when negative potentials (from -0.1 to -0.5V) were applied. However, the increase, from ∼55° to ∼60°, was not as high as at anodic potentials. A slightly lower contact angle was obtained for VPP-PEDOT and Orgacon when the applied potential was reset at 0 V, after oxidation processes, as compared to their initial states (Figure 5 and the Supporting Information). The changes are ∼1°, much less than the changes due to the electrochemical oxidation or reduction. These phenomena are also demonstrated by the Wilhelmy contact angle hysteresis loops of P3HT in the neutral, highly doped (V ) 0.8 V) and dedoped (V ) -0.1 V) states (Figure 6, panels a and b), when two different aqueous electrolytes were used (0.05 M LiClO4 and 0.05 M C3H7NaSO3). Dedoping of P3HT from the doped state (V ) 0.8 V) by applying a -0.1 V voltage on the sample in the 0.05 M C3H7NaSO3 was almost complete as compared to that in 0.05 M LiClO4. On the other hand, the absolute contact angle values under the same doping condition showed a 4-10° difference when different electrolytes were used (see Figure 6c). In addition, it was difficult to achieve oxidation of P3HT in 0.05 M C8H17NaSO3, as suggested by the

Figure 6. Wilhelmy contact angle hysteresis loops of P3HT in neutral, doped (V ) 0.8 V) and dedoped states as measured using (a) 0.05 M LiClO4 and (b) 0.05 M C3H7NaSO3 aqueous. (c) Comparison of contact angles of P3HT at neutral, doped (V ) 0.8 V) and dedoped states using 0.05 M LiClO4 (open squares) and 0.05 M C3H7NaSO3 (crosses) aqueous electrolytes.

unchanged film color and unchanged contact angle at V ) 0.8 V. These observations indicate that the modification of surface energy with electrochemical reduction and oxidation are also somewhat electrolyte type dependent. 3.2.3. Dynamic in Situ Contact Angle Measurements. Dynamic in situ contact angle measurements were carried out with the Wilhelmy balance (Figure 7) and clearly show the contact angle changes upon electrochemical doping-dedoping, consistent with the trends observed by the in situ Wilhelmy balance and the ex situ goniometry measurements. The contact angle of P3HT shows a clear response when doped/or dedoped from V ) 0, 0.5, to 0.7 and to -0.1 V during immersion into aqueous electrolyte (Figure 7a). A much larger response of VPP-PEDOT films was observed, where the contact angle switches from ∼55° to ∼80° between the reduced (-0.8V) and oxidized (0.8V) form (Figure 7b). Contact angle switches (7∼8° at applied potential of (0.5 V) of Orgacon were obtained (not shown here). All these responses to the applied potential pulses are consistent with the previous determination (see Figure 5 and the Supporting Information).

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chemical doping can be used as a driving force to move a liquid level up, in competition with gravity. 3.3. Surface Energy. We followed the treatment of van Oss42 and of Good.43 Three probing liquids, including deionized water (L1), ethylene glycol (L2), and n-hexadecane (L3), their surface energies were listed in Table 2, were used for determination of contact angles on P3HT, VPP-PEDOT and Orgacon films. The surface energy of a solid (γs) can be calculated from contact angles measured using three different known probe liquids, of which one is nonpolar, as suggested by van Oss and Good and given as LW 1/2 - 1/2 + 1/2 γl(cos θ + 1) ) 2[(γLW + (γ+ + (γs γl ) s γl ) s γl ) ] (5)

Figure 7. In situ dynamic contact angle determination with the Wilhelmy balance: (a) SS P3HT was doped and dedoped from 0 f 0.5 f 0.7 f -0 and (b) VVP-PEDOT film to which voltage was applied in the sequence 0 (from start) f 0.8 f -0.8 f 0.8V f -0.8 (in withdrawal) for electrochemical reduction and oxidation during immersion into aqueous electrolyte. Insert figure shows the alternation applied potential pulse. LiClO4 (0.05 M) was used as aqueous electrolyte for both cases.

- 1/2 + 2(γ+ γs ) γLW s s γs )

(6)

+ - 1/2 γAB s ) 2(γs γs )

(7)

where γl and γs are the liquid (vapor) and solid (vapor) tension. + γ+ l , γl , γs , and γs are the acid and base components of the AB surface tension of the liquid and solid. γLW s and γs are attributed to Lifshitz-van der Waals interactions and acid-base interactions (such as hydrogen bonding, mainly electrostatic in nature). These polymers were characterized in the neutral state before doping (for P3HT), and in the virgin state as prepared from synthesis (for VPP-PEDOT and Orgacon). These two polymers were in the oxidized state; their resting potentials are estimated to be 0.49 and 0.54 V vs Ag/Ag+ reference electrode. From the experimental contact angle date, we calculated surface energies of the three surfaces using eq 5-7. All of the data are summarized in Table 3.

4. Discussion The oxidation is faster than the dedoping process, as can be observed from the slope of the response curve for P3HT from the in situ determination (Figure 7a). The reduction process is faster than the oxidation process for VPP-PEDOT (Figure 7b). The different kinetics for oxidation and reduction observed in P3HT compared to VPP-PEDOT should be due to the different mechanism of polarization of films when ions are doped, the interaction between conjugated polymers on the films with the electrolyte ions. Detail discussion can be found in the Discussion (section 4). 3.2.4. Capillary Rise during Electrochemical Doping. Capillary rise of liquids along vertical single plates and in parallel plate have been used for dynamic contact angle determination.36-41 In our experiment, sample C was used. Figure 8 shows a front view of the sample at different electrochemical doping levels. At the neutral state (V ) 0 V), there was a downward meniscus interface formed between air and the liquid at the inner part of the sample, induced by the difference of capillary rise at the outer surface PET of the sample and the capillary fall at the inner P3HT surface. The contact angles of 0.05 M LiClO4 aqueous were 76° and 106° on PET and P3HT surfaces, respectively. The depth of the downward meniscus decreased with increasing applied voltage, as a result of a decreasing contact angle on the P3HT surface upon electrochemical doping, in good agreement with the in situ dynamic contact angle determination using the Wilhelmy balance. A quantitative estimation of contact angles is possible, when the distances of the capillary rise and fall are measured precisely. The observation also implies that electro(40) Chen, X.; Ingana¨s, O. J. Phys. Chem. 1996, 100, 15202. (41) Chen, J. H.; Hsieh, W. H. J. Colloid Interface Sci. 2006, 296, 276.

Electrochemical doping/dedoping of P3HT in aqueous LiClO4 electrolyte is represented by the nonstoichiometric reaction

ClO4- + Li+ + P3HT(0)1/x T P3HT+1/x (ClO4-) + e- + Li+ (4.1) where 1/x specifies the varying doping level in terms of dopant per 1/x monomers of the polymer. The initial state of VPP-PEDOT after synthesis is the doped state, and reduction of the doped PEDOT is represented by the reaction

e- + Li+ + PEDOT+1/x (TS-) T PEDOT(0)1/x (Li+TS-) (4.2) We cannot exclude an anion loss in this system

e- + Li+ + PEDOT+1/x (TS-) T PEDOT(0)1/x + Li+ + TS(4.3) with the possibility of a later reaction based on anion exchange

ClO4- + Li+ + PEDOT01/x T PEDOT(+)1/x (ClO4-) + e- + Li+ (4.4) The initial state of Orgacon is also a mixture of doped PEDOT (42) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. AdV. Colloid Interface Sci. 1987, 28, 35. (43) Good, R. J.; Chaudhury, M. K.; van Oss, C. J. Fundamentals of Adhesion; Plenum Press: New York, 1991; p 153.

Surface Energy Determination of P3HT and PEDOT

Langmuir, Vol. 22, No. 22, 2006 9293

Figure 8. A front view of type C sample consisting of two parallel P3HT coated ITO/PET plates with P3HT facing each other and separated by a 0.5-1 mm spacer, at neutral and varied doping levels (V ) 0.3, 0.5, and 0.8 V). It was immerged in a 0.05 M LiClO4 aqueous electrolyte. Ag+/Ag was as reference electrode. Labels in the figure are as follows: 1. The part of the sample above aqueous electrolyte; 2. The surface of aqueous electrolyte, i.e., air/aqueous electrolyte interface out side the; 3. The curved interface of air/aqueous electrolyte at the inner part the sample; and 4. The part of the sample in aqueous electrolyte. Table 2. Surface Energy Components of Three Different Probing Liquids, Used for the Surface Energy Determination (mN/m) L1 L2 L3

γ

γLW

γAB

γ+

γ-

72.8 48 27.1

21.8 29 27.1

51 19 0

25.5 1.92 0

25.5 47.0 0

Table 3. Advancing Contact Angle of Three Probing Liquids on the Surfaces of P3HT, VPP-PEDOT, and Orgacon, at Initial State, and Calculated Surface Energy (mN/m) P3HT

VPP-PEDOT

Orgacon

advancing contact angle data

L1 L2 L3

97 ( 2° 66.7 ( 0.5° 25 ( 2.5°

33.6 ( 3° 10.6 ( 1.3° 5.4 ( 2°

25 ( 2.5° 19.3 ( 2° 26 ( 6°

calculated surface energy component

γ γLW γAB γ+ γ-

26.9 ( 0.5 24.6 ( 0.5 2.3 ( 0.2 0.37 ( 0.1 3.57 ( 0.3

47.3 ( 0.3 27 ( 0.1 20.3 ( 0.1 2.13 ( 0.1 48.4 ( 0.1

47.5 ( 1.4 24.4 ( 1.1 21.6 ( 0.3 1.66 ( 0.2 69.8 ( 0.4

and PSS, and reduction of the doped PEDOT is represented by the reaction

e- + Li+ + PEDOT+1/x (PSS-) T PEDOT(0)1/x (Li+PSS-) (4.5) Here we do not expect any anion movement, as this ion is large enough to prevent motion. The solvent shells associated with ions are mostly transported with the ions, but we do not have information on the contribution of solvent transport in the different materials. The different electrolytes might lead to a difference in the degree of swelling of the polymer film. Swelling has been observed previously for P3HT in propylene carbonate and aqueous LiClO4.44 Swelling of the P3HT film by aqueous LiClO4 was observed by the AFM imaging, and show roughness enhancement from 3-5 to 5-7 nm. This might be a partial reason the contact angle for the doped state was difficult to reverse with dedoping, even if the electronic structure was completely returned to the neutral state. P3HT. P3HT is hydrophobic at neutral state (∼110° in contact angle), which is attributed to the alkyl side group. Previous experiments have shown that longer (or more) alkyl chain(s) attached on the backbone of polythiophenes induce higher hydrophobicity; likewise dilution of side chains reduces hydrophobicity. When P3HT was oxidized, positive charges are injected on the polymer chain and counterions (anion)/solvent move into the polymer matrix. This process makes the polymer more polar (hydrophilic). Dedoping of P3HT is the reversed process of doping and therefore takes the film back to the hydrophobic state again. (44) Skompska, M.; Szkurlate, A.; Kowal, A.; Szklarczyk, M. Langmuir 2003, 19, 2318.

We note that withdrawal of C3H7 SO3- ions from doped P3HT polymer film was easier than that of ClO4-, as suggested by the completely returned contact angle during dedoping of P3HT in C3H7 SO3- aqueous electrolyte (Figure 6). On the other hand, the doping process is also dependent on the chain length of the counterion group. As the hydrocarbon chain length of the dopant is increased, it is rendered “more hydrophobic”.45 PEDOT. The more hydrophilic PEDOT samples, for both VPP-PEDOT and Orgacon, show contributions giving higher surface energies. These can be due to the presence of protic exchange at the polymer/liquid interface, in addition to the van der Waals contribution. The ethylenedioxy link of the thiophene mer is possibly contributing to the polarizability and could contribute to ion solvation.46 In the doped state with TS- or PSS- counterions, in the films of VPP-PEDOT or Orgacon, solvated ions interacting with water could be also a reason for their hydrophilicity. Reduction of VPP-PEDOT to the neutral state in the aqueous electrolyte led to neutral PEDOT including the counterion TSand the cation of the electrolyte. The increasing hydrophilicity of this surface could possibly be related to reorganization of the ion distribution at the interface, by making the SO3- group of TS- facing out of the surface and therefore the film more hydrophilic. When Orgacon is reduced at a negative potential, the number of charge carriers in PEDOT is reduced; it is therefore less polar, and an increased contact angle was observed. Electrochemical oxidization of either VPP-PEDOT or Orgacon led to increased contact angles, parallel to the enhanced polarizability. This result is opposite to that of P3HT and points to the complexity of the interplay between dopants, solvent, and polarizability. Further investigations of these processes by means of photoelectron spectroscopy are underway and are necessary to resolve the different contributions. The trend in contact angle changes of the semiconducting P3HT and conducting PEDOTs versus applied potential (within (1 V; Figure 6) show that they are mainly caused by electrochemical reduction and oxidation processes. It was noted that the contact angle under a doped state would remain until a dedoping process had taken place. These features are hard to connect with electrowetting, which is applicable to dielectric materials, where the contact angle value changes are obtained at a certain voltage, normally at a few hundred volts, and are not stable in the absence of the applied potential.46 Electrowetting is therefore a minor contribution, if any, to the effects reported here. From previous studies of P3HT,48 we have concluded that solvent is transported into the polymer film upon contact, a process (45) Nilsson, J. O.; Ingana¨s, O. Synth. Met. 1989, 31, 359. (46) Gustafsson, J. C.; Liedberg, B.; Ingana¨s, O. Solid State Ionics 1994, 69 (2), 145. (47) Frieder, M.; Baret, J.-C. J. Phys. Condensed Mater. 2005, 17, R705.

9294 Langmuir, Vol. 22, No. 22, 2006

that precedes electrochemistry and which may be a necessary condition for electrochemistry to occur at all. If solvent is available throughout the polymer film, it must also be present at the supporting substrate/electrode, here ITO/PET, PET, or PS. Although ITO is very hydrophobic, PET and PS are hydrophilic. The thickness of polymer films (100-400 nm) is small, but the contribution of the substrate/electrode surface energy to the measured contact angles is assumed to be negligible. A caveat may be necessary for the case of Orgacon. This layer is prepared from a dispersion of water swollen particles and may be expected to show swelling on contact to water. The film is mechanically stable under characterization, however. The swelling may therefore be a prelude to the electrochemical reactions converting the chemical composition of the polymers; also solvent is transported in to and out of the films during the doping reactions. When comparing results from the different methods of contact angle data of P3HT obtained from SS samples, calibrated data of SS and DS samples by Wilhelmy balance, and the data obtained using goniometry, we find good agreement (see numerical data in the Supporting Information), which verifies the reliability of these methods. The magnitude of the surface energy on polymer surfaces can be attributed to the contributing interactions operating at the solid/liquid interface. In the classical treatment of surface energy on polymers,8 this will include the van der Waals interactions, but also more chemical and specific items, such as acid-base interactions and donor-acceptor interactions at the interface, contribute in an additive manner. More precise theories require knowledge of the distribution of matter at the solid/liquid interface, in the gradient of matter distribution. Quantitative theories correlate the van der Waals contributions to the Hamaker constant, which makes a connection to the dielectric properties of the material. With conjugated polymers, the presence of semiconductorto-metal transition during the doping processes cause major changes of the dielectric function of the polymer and addition of novel interactions because of the increased polarizability is expected. A more complex treatment will therefore be necessary. That treatment will also have to include the possibility of acidbase interactions at the interface, as two of the polymers in this study incorporate strong acids as counterions. Interactions with proton acceptors in the liquid phase would therefore have to be included in a larger study, which has not yet been performed. Our choice of electrolytes is dictated more by compatibility and electrochemical aspects; if electrochemistry cannot be done in the medium, measurements would be impossible. On the other hand, our determination of surface energy composition when using three different probing liquids for contact angle measurements allows us to discuss the different contributions at a particular potential but does not allow following the variation of this (48) Johansson, T.; Mammo, W.; Svensson, M.; Andersson, M. R.; Ingana¨s, O. J. Mater. Chem. 2003, 13 1316.

Wang et al.

composition at different potential. For determined surface energy data (Table 3), we note that the major contribution to the surface energy of P3HT is nonpolar, and it is plausible that alkyl chains are segregated to the surface. We know that various forms of PEDOT(PSS) may show segregation of PSS polymer to the surface, under certain conditions.49 This is probably also the case for Orgacon and contributes to the acid-base component of this surface, which is also observed with VPP-PEDOT. For VPP-PEDOT, this is not expected, and structural studies of a very closely related material show a paracrystalline structure.34,35 We see no reason other than to assume that both PEDOT chains and TS ions are exposed at the surface. Clearly, both the VPPPEDOT and Orgacon surface are dominated by the acid-base interaction.

4. Summary Wilhelmy balance tensiometry, sessile drop goniometry, and capillary rise methods were used to investigate the variation of surface properties of P3HT and PEDOT upon electrochemical doping-dedoping. We resolved the surface energy of the three surfaces, at one potential, into the van der Waals and the acidbase components; the acid-base component dominates the surface energy of PEDOT samples, presumably because of the contribution of acid sites at PSS and TS, whereas this contribution is small for P3HT. The hydrophobic P3HT was less hydrophobic in the oxidized state than in the neutral state; the more hydrophilic PEDOT was less hydrophilic in the oxidized state than when neutral. Data from three types of contact angle measurements were in good agreement. Results showed that the electrochemical doping-dedoping of polymer films can modify the surface energy in a reversible manner. In situ dynamic contact angle determination can be carried out with both Wilhelmy balance and capillary rise methods. The methods and the obtained results are relevant for the science and technology areas of electrochemical devices and also for the general development of printed electronics. Acknowledgment. X.W. thanks the Swedish Ministry of Education for financial support and the Center of Organic Electronics by SFF. Discussions on VPP-PEDOT preparation with S. Admassie and K. Tvingstedt at IFM and communication with Prof. M. Berggren group, Organic Electronics, ITN, Linko¨ping University are valuable. Supporting Information Available: Table depicting contact angles of P3HT, VPP-PEDOT, and Orgacon samples at neutral, doped, and dedoped states. This material is available free of charge via the Internet at http://pubs.acs.org. LA061606P (49) 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.