Electron Paramagnetic Resonance Observation of Critical Behaviors in a Disordered Conducting Polymer System

Abstract

While most polymers are good insulators, some of them known as conducting polymers show remarkable electrical conductivities upon doping with electron donors or acceptors. The electrical conductivity is generally known to be governed by the one-dimensional diffusion of the chargecarrying solitons or polarons, and the main charge transfer mechanism in conjugated conducting polymers is known to be the intrachain diffusion and interchain hopping of polarons and/or bipolarons. Electron paramagnetic resonance (EPR) is a powerful tool for spin dynamics, and have been employed to study various conducting polymers as they can sensitively reflect the spin carrier motions and interactions. In conjugated conducting polymers like poly[2buthoxy-5-methoxy-1,4-phenylenevinylene] (PBMPV) where the charge carriers are polarons and bipolarons, the EPR intensity is proportional to the density of the spincarrying polarons (spin 1=2, charge e). A pair of polarons can form spinless singlet bipolarons (spin 0, charge 2e), which are energetically more stable. The spin-carrying polarons can give rise to EPR signals, whereas the spinless singlet bipolarons cannot make EPR transitions. Thus EPR can serve as a sensitive probe of spin carriers in the conducting polymers. PBMPV is a derivative of the conjugated PPV (polyphenylenevinylene) polymer. It can be easily doped to controllable degrees of doping and provides a unique opportunity to systematically study the doping as well as conductivity dependence of various properties including the dopant kinetics and spin/charge dynamics. Recently, we have reported a critical behavior associated with the delocalization of the charge-carrier wave function in a series of I2-doped PBMPV conducting polymers by means of nuclear magnetic resonance (NMR) measurements. In this work, we have investigated the charge/spin dynamics in the same series of samples by means of EPR measurements. While there have been some indirect indications of bipolaron formation in some electrochemically synthesized conducting polymers through the EPR and susceptibility measurements, this quantitative and systematic work is believed to represent a comprehensive observation of the collective critical behavior of bipolaron formation and spin–charge carrier delocalization in an iodine-doped conjugated polymer system. The PBMPV conducting polymer samples were prepared by thermal elimination of polyelectrolyte precursor polymer films and iodine-doping, and the iodine concentration was determined from the weight gain after doping. While the doped PBMPV samples are known to have inherent inhomogeneities, relatively thick good quality samples have been obtained according to visual inspection. The electrical conductivity was measured by the standard four-in-lineprobe method. The room temperature EPR measurements were made at 9.4 GHz using an X-band spectrometer, and the magnetic susceptibilities were obtained by doubleintegrating the differential lineshapes. The room temperature electrical conductivity was measured as a function of the degree of I2-doping, and the conductivity displayed a characteristic increase with the doping. A rapid increase of the electrical conductivity was noticed around the dopings of pc1 1⁄4 10 3 I3 /RU (repeating unit) and pc2 1⁄4 10 1 I3 /RU, which can be attributed to the dynamics governing the charge conduction mechanism. Figure 1 shows the doping degree dependence of the spinto-charge ratio obtained from the spin susceptibility, assuming that the charge density is the same as the dopant density per repeating unit of the polymer. Figure 1 displays an initial decrease of the spin-to-charge ratio until it reaches a minimum of less than 10 2 around pc2, which can readily be attributed to the polaron–bipolaron recombination. It is also interesting to note the spin-to-charge ratio minimum, followed by an increase of the ratio, which may be explained by the dissociation of some of the spinless bipolarons to polarons. Figure 2 shows the EPR lineshapes for samples with various degrees of doping. It is also worth noting the abrupt increase of the peak-to-peak EPR linewidth, in Fig. 3, at the same doping degrees where the spin-to-charge ratio shows a minimum, which indicates that they arise from the same spin dynamics. The abrupt increase in the EPR linewidth at pc2 appears to be a lifetime broadening, i.e., line broadening due to the lifetime shortening, arising from the delocalization of the spin/charge-carrier wavefunctions at the percolation threshold. Thus, EPR is shown to reflect sensitively the spin dynamics in our systems. Figure 4 shows the temperature dependencies of the EPR spin susceptibility and the linewidth for a heavily doped sample with p 1⁄4 3 10 . The spin susceptibility undergoes a decrease with decreasing temperature before showing a typical Curie behavior below a critical temperature around which the linewidth shows a characteristic peak. This