抄録
The electrochemical, optical and magnetic properties in poly(p-phenylene vinylene), PPV, during electrochemical p-type doping have been investigated by cyclic voltammetry, optical absorption spectrum and electron spin resonance (ESR) measurements. The band gap of PPV was evaluated to be about 2.7eV. The evolution of localized states is evidenced by the spectral change with electrochemical doping. The spin susceptibility (χ) increased by about three orders of magnitude from 2.6×10-8 to 2.5×10-5 emu/mol up to a dopant concentration of about 2.4mol%. With further increasing dopant concentration, χ decreases slightly. The spin density evaluated from χ was consistent with the polaron density assumed to be formed by dopants of 2.4mol%. The ESR linewidth decreases by doping from about 9.5 to 0.4G and then again increases slightly with increasing dopant concentration. The g value of 2.0038 at the neutral state shifts upon doping to the smaller value of 2.0030. These results are discussed in terms of polaron and/or bi-polaron models. Thermally stimulated current (TSC) measurements have been performed on undoped stretched PPV. Two TSC peaks P1 and P2 are observed and become smaller as the stretching ratio (l/l0) was gradually increased. Especially, peak. P2 is markedly suppressed on stretching. The trap depths are evaluated from the initial increasing part of each separated peak by the thermal cleaning method to be-about 0.36 and 0.55eV for peaks P1 and P2, respectively, regardless of l/l0. Peak P1 is interpreted as originating from electron traps formed by impurities such as Br2 or SO3 which were not eliminated by heat treatment. In contrast, peak P2 was confirmed to be based on defects already existing in the amorphous parts of PPV since the crystallinity in PPV increases and χ evaluated by ESR spectra in stretched PPV decreases on increasing l/l0.
