Macromolecules 1989,22, 2917-2926 (6) Matgjka, L.; DuHek, K.; Dobig, I. Polym. Bull. 1985, 14, 309. (7) Reyx, D.; Costes, B.; Matgjka, L.; Ddek, K. Polym. Bull. 1988, 19, 269. (8) Attias, A. J.; Ancelle, J.; Bloch, B. European Symposium on
Polymeric Materials, Chemical Aspects in Processing Operations and use of Polymeric Materials; Lyon, 1987; p CPA 08.
2917
(9) Rozenberg, V. A. Epoxy Resins and Composites. Adv. Polym. Sci. 1986, 75, 113. (10) Costes, B.; Reyx, D.; Platzer, N. Makromol. Chem. 1989,190, 349. (11) Attias, A. J.; Ancelle, J.; Bloch, B.; Laupretre, F. Polym. Bull. 1987, 18, 217.
Charge Storage on a Conducting Polymer in Solution M. J. Nowak,ti*D. Spiegel,t S. Hotta,g A. J. Heeger,*?land P. A. Pincud Institute for Polymers and Organic Solids, University of California, Santa Barbara, California 93106. Received October 24, 1988; Revised Manuscript Received January 10, 1989
ABSTRACT Magnetic and optical studies of poly(3-hexylthienylene) (P3HT), doped in solution with NO'PFL in both chloroform and in the more polar solvent methylene chloride (CH2C12),indicate a relatively small difference in the energy of polarons versus that of bipolarons and imply that the weak effective electron-electron Coulomb repulsion can be controlled by varying the solvent and/or the polymer concentration. As a result, we have observed unambiguous magnetic and optical signatures characteristic of polarons in P3HT in CH2Cl2 at the lowest doping levels. The relative fraction of polarons and bipolarons has been analyzed in the context of various proposed statistical models. A theoretical model is developed to include the effect (in such solutions) of ionic screening on the effective Coulomb interaction. This model is tested by adding salt to a dilute solution of doped polymer.
results were obtained from measurements on the polymer I. Introduction in the form of solid-state films and powders and on the In a nondegenerate ground state conjugated polymer polymer in sol~tion."~ such as polythiophene or its soluble alkyl derivates (Figure There are reports by various groups,1o however, that la), the fundamental nonlinear excitations and the dompolarons are generated as well as bipolarons. This suggests inant charge storage configurations are polarons and bia near degeneracy in the energy of two polarons versus a polarons.14 The relative stability of these species depends bipolaron, implying an equilibrium for the reaction on the relative strength of the electron-phonon and electron-electron interactions in the polymer. The doubly p+ + p+ + B2+ charged bipolaron is a bound state (resulting from the Even in the case where bipolarons have the lowest creation electron-phonon interaction) between two polarons. energy, the entropy favors polarons so that a finite conHowever, the repulsive Coulomb interaction favors the centration of polarons can be expected. In this paper, we existence of two separated polarons. Hence, the energy address this question with visible-near-IR spectroscopy of creation of two polarons may be comparable to that of and electron spin resonance data on the poly(3-hexyla bipolaron. thienylene) ~ y s t e m . * ~ ~ In Figure lb,c, we show the band diagrams with gap The experimental results which we report here were states and allowed optical transitions appropriate for a carried out with poly(3-hexylthienylene) (P3HT) in a self-localized bipolaron and polaron, respecti~ely.l*~?~ methylene chloride solution. By conducting the experiClearly, the two have distinguishing optical and magnetic ments in this more polar solvent than that used in previous signatures. A bipolaron with its two empty states in the workg (in chloroform),we find distinctly different behavior gap carries a double-positive charge and is spinless. Two for the doped polymer in solution. For example, in the broad electronic transitions from the valence band to the most dilute doping regime, our data indicate that charge gap states are expected with energies less than the interstorage is predominantly in polarons, in marked contrast band transition. On the other hand, a polaron has one with the low doping regime results of P3HT in chloroform. unpaired electron in its lower gap state, and hence, along Only at higher doping levels in methylene chloride do we with one positive charge, it has spin one-half. From Figure again observe the bipolaron as the dominant charge storage IC,a polaron should display an additional subgap tranconfiguration. This implies an important solvent depensition between two localized energy levels. dence to the polaron-bipolaron equilibrium described From these magnetic and spectroscopic signatures, above. studies of polythiophene and the poly(3-alkylthiophenes) By performing the experiments on a doped conducting have provided evidence that upon doping1v6or photoexpolymer while it is in solution, it becomes possible to excitation' charge is stored in spinless bipolarons. These plore the effect of counterion screening on the interactions in the polymer. The effective on-chain Coulomb interaction between two polarons, although always repulsive, is Department of Physics. strongly dependent on the extent to which the "bare" Current address: Bell Communications Research, Red Bank, NJ 07701-7020. positive charge of the polaron is screened. To first order, 5 Permanent address: Central Research Laboratories, Matsushita this is determined by the proximity of its counterion. Electric Industrial Co., Ltd., Moriguchi, Osaka 570, Japan. Second-order effects include the direct screening of the A Department of Physics and Engineering Materials Department. polarons by the solvent itself. This can be considered weak 11 Engineering Materials Department. f
0024-929718912222-2917$01.50/0
0 1989 American Chemical Society
Macromolecules, Vol. 22, No. 7, 1989
2918 Nowak et al.
(C1 CONDUCT1ON
Figure 1. (a) Chemical structure of poly(3-hexylthienylene) (P3HT). (b) Band diagram showing the gap states and allowed transitions for a self-localized bipolaron. (c) Band diagram showing the gap states and allowed transitions for a self-localized polaron.
COUNTER IONS CLOSELY BOUND
COUNTERIONS LOOSELY BOUND
Figure 2. Schematic drawing of counterion screening of charged states on a conducting polymer in solution. since even our “polar” solvent, methylene chloride, is only weakly polar (compared, for example, t o water). Illustrated in Figure 2 are two possible limits. If, as in a nonpolar solvent, t h e counterions a r e closely bound t o t h e polarons on a chain, t h e net polaron-polaron interaction d u e t o t h e electron-phonon interaction can be attractive. In a more ionic environment, such as a polar solvent, t h e counterions are more loosely bound t o t h e chains. W i t h a larger separation between charges in t h e a-system a n d counterion, t h e net polaron-polaron interaction becomes more repulsive. T o some extent, then, i t is possible t o control t h e relative energies of t h e doubly charged bipolaron i n comparison t o two singly charged polarons. I n this study of P3HT in solution, t h e strength of t h e counterion screening has been altered by varying t h e solvent, t h e polymer concentration, or t h e salt concentration of a conducting polymer solution.
11. Experimental Section Details of the preparation and characterization of the polymer can be found e l s e ~ h e r e . *The ~ ~ soluble P3ATs have a relatively high molecular weight (=300 monomer units) and high purity. Infrared studies have characterized these materials as 2,5-linked linear polythienylene chains. Solutions were prepared by dissolving P3HT in methylene chloride; all samples were prepared under a dry nitrogen atmosphere. The dopant (NOPF6) was
purified by sublimation at ca. 125 O C under a reduced pressure of =2 X Torr and then dissolved in acetonitrile. Both the dichloromethane and acetonitrile were dried and distilled over phosphorus pentoxide under dry nitrogen. Doped polymer solutions were prepared by adding the appropriate amount of the dopant solution (4 mg of NOPF6 in 1 mL of acetonitrile) to previously prepared dichloromethane solutions of P3HT. The quoted y values are nominal and assume that the doping reaction has gone to completion with each NO’ ion, yielding a transfer of one electron from a P3HT macromolecule. In chloroform, the doping of P3HT was strongly dependent on the polymer concentration and, consequently, all samples were doped at high polymer concentrations where the doping was more efficient and then diluted.ga The doping of the dichloromethane solution was efficient and exhibited only a very weak polymer concentration dependence. Volumetrically accurate amounts of solutions were sealed in optical cells or in ESR tubes to ensure against loss of volatile solvents during the measurements. Although we observed no obvious signs of degradation or instability, all of our studies routinely utilized freshly prepared samples. The solutions of both the neutral polymer and the doped polymer were clear and stable over long times with no indication of cloudiness, aggregation, or precipitation. For example, doped CHzClzsolutions were extraordinarily stable for periods of more than a month. Therefore, P3HT exists as a macromolecular chain in solution both when neutral and when partially oxidized by doping. Quasi-elastic light scattering from neutral polymer solutions yields a hydrodynamic diameter of ~ 3 0 A.” 0 Since the mean molecular weights corresponds to approximately 300 monomer units, this implies the polymer is a wormlike chain in solution. The dilute regime (mean separation between chains is greater than the hydrodynamic diameter) is therefore restricted to polymer concentrations of c 5 co = 1 mg/mL, or approximately 6 X M. For c 2 co, interchain interactions become important. Doping experiments were carried out at a polymer concentration of about co, the best compromise for high signal intensity without significant interchain interactions. Magnetic resonance measurements were performed on an IBM Instruments Bruker 200D electron spin resonance spectrometer interfaced with an Apple IIe computer for data acquisition and analysis. The visible-IR absorption spectrum was obtained with a McPherson EU700 monochromator and either a photomultiplier or an IR Industries Si/PbS two-color detector using standard light chopping and lock-in amplifier techniques. The monochromator grating drive and detector electrons were interfaced to an IBM PCXT which collected and stored the data. In order to obtain the optical density of P3HT alone, the optical spectrum of the pure solvent was subtracted from the spectrum of the polymer solution.
111. Data 1II.A. Electron Spin Resonance Measurements. Previous studiesga of P3HT in a chloroform solution indicate a strong dependence of t h e magnetic susceptibility on t h e polymer concentration. I n a methylene chloride solution, on t h e other hand, t h e susceptibility is independent of t h e polymer concentration. I n “nonpolar” chloroform (dielectric constant = 4-73), ionic screening caused by neighboring polymer chains becomes important at concentrations near or greater t h a n t h e overlap concentration c,; in effect, t h e medium becomes more polar d u e t o t h e dissolved polyelectrolyte a n d anions. As discussed above, a polar solvent will lead t o a counterion separation and thereby to a shift in the equilibrium toward a greater fraction of polarons. With a more polar solvent such as methylene chloride (dielectric constant = 9.73), a n increase in polymer concentration would be expected t o have a small effect on t h e ionic strength of t h e doped polymer solution. Figure 3 shows t h e dependence of t h e magnetic susceptibility on the nominal doping level y at a fixed polymer concentration of c = c,, in methylene chloride. I n t h e pristine polymer, a susceptibility corresponding to one spin
Charge Storage on a Conducting Polymer 2919
Macromolecules, Vol. 22, No. 7, 1989
o . I
I
I
I I
I
0.0061m/
1
:-t
z
W
4 DOPING LEVEL (CHARGWRING)
0
I
0
I
I
I
I
I
I
0.08
0.32
0.16 0.24 DOPING LEVEL y (CHARGESIRING)
Figure 3. Magnetic susceptibilityversus dopant level at a fixed polymer concentration c = co in dichloromethane. The solid line is the best fit given by the bead model with 7 = 582 (A = 0.16 eV) and p = 2.94. Inset: The initial behavior is reproduced with 7 = 400 (A = 0.15 eV). 1.01
v, 0.8
I
I
I
I
I
1
-
W
3.5
I .o
I .5 ENERGY (eV)
2.0
2.5
Figure 5. Optical absorption spectra of P3HT in CHzClzat c = co at various doping levels: y = 0%, 0.3%, LO%, 3.0%, and 20%. The residual structure in the spectrum labeled as 0% indicates the presence of residual dopant (
