Electrical, optical, and magnetic properties of poly(2,5-diethoxy-<i>p</i>-phenylene vinylene)

Abstract

The electrochemical, optical, and magnetic properties in poly (2,5-diethoxy-p-phenylene vinylene), ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$O-PPV, during electrochemical p-type doping have been investigated by cyclic-voltammetry, optical-absorption-spectrum, and electron-spin-resonance (ESR) measurements. The evolution of localized states is evidenced by the spectral change with electrochemical doping. The band gap of ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$O-PPV was evaluated to be about 2.2 eV and smaller than that of poly (p-phenylene vinylene), PPV, by about 0.5 eV. The spin susceptibility increased by about three orders of magnitude from 2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ to 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ emu/mol up to a dopant concentration of about 2.2 mol %. With a further increasing dopant concentration, the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was consistent with the polaron density assumed to be formed by dopants of 2.2 mol %. The ESR linewidth decreases by doping from about 3.5 G to about 0.4 G and then again increases slightly with increasing dopant concentration. The g factor of 2.0036 at the neutral state shifts upon doping to the smaller value 2.0026, which corresponds to the nearly-free-electron g factor. These results are discussed in terms of polaron and/or bipolaron models.