Optical studies of the charge localization and delocalization in conducting polymers
A systematic charge transport study on the thermochromism of polyaniline (PAN) doped with a plasticizing dopant, and on a field effect device using conducting poly (3,4-ethylenedioxythiophene) (PEDOT) as its active material, was made at optical (20 — 45,000 cm -1 ) frequencies to probe the charge localization and delocalization phenomena and the insulator to metal transition (IMT) in the inhomogeneous conducting polymer system. Temperature dependent reflectance [20 — 8000 cm-1 (2.5 meV — 1eV)] of the PAN sample, together with absorbance and dc transport study done by Dr. Pron at the Laboratoire de Physique des Métaux Synthétiques in Grenoble, France, shows spectral weight loss in the infrared region but the reflectance in the very low frequency (below 100 cm -1 ) remains unaffected. There are two localization transitions. The origin of the 200 K localization transition that affect >~ 15% of the electrons is the glass transition emanating from the dopants. The transition principally affects the IR response in the range of 200 — 8000 cm -1 . The low temperature (<75K) localization transition affects the few electrons that provide the high conductivity. It is suggested that these electrons are localized by disorder at the lowest temperature and become delocalized through phonon induced delocalization as the temperature increases to 75K. It is noted that this temperature is typical of a Debye temperature in many organic materials. The thermocromism is attributed to the weak localization to strong localization transition through the glass transition temperature. Below the glass transition temperature (Tg), the lattice is “frozen” in configuration that reduces the charge delocalization and lead to cause increase of strongly localized polarons. Time variation of source-drain current, real-time IR reflectance [20 — 8000 cm -1 (2.5 meV — 1eV)] modulation, and real-time UV/VIS/NIR absorbance [380 — 2400 nm (0.5 — 3.3 eV)] modulation were measured to investigate the field induced charge localization of PEDOT field effect device. Layer by layer thin film analysis showed strong localization of free carriers. The temperature dependence of the dc conductivity changes with application of the gate voltage demonstrating that the electric field effect has changed bulk charge transport in the active channel despite the expected screening due to mobile charge carriers. Mid IR (500 — 8000 cm -1 ) reflectance showed little change in the vibrational modes, which distinguish this phenomenon from the doping-dedoping induced electrochemical MIT. UV/Vis/NIR absorbance modulation clearly showed that the increase of the strong localization of charges with the ? - ?* bandgap transition unchanged. It is proposed that conducting polymer is near the metal to insulator transition and that the applied gate voltage leads to this transition through field induced ion motion.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:polyaniline plastdopant reflectance thermochromism glass transition pedot pss field effect drude lorentz model absorbance irav mode
Date of Publication:01/01/2005