Direct measurements of bipolaron-band development in doped polypyrrole with inverse photoemission.

Abstract

Inverse-photoemission spectroscopy has been used to investigate dopant-dependent electronic properties of polypyrrole (PPy) films doped with ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$ (tetrafluoroborate) or (PSS${)}^{\mathrm{\ensuremath{-}}}$ [poly(4-styrenesulfonate)]. The results show that the bonding and antibonding bipolaron bands appear at \ensuremath{\sim}1.0 and \ensuremath{\sim}2.6 eV above the valence-band maximum for heavily doped PPy films (\ensuremath{\sim}1 dopant per 3 pyrrole units). The bandwidth of each of these bipolaron bands is \ensuremath{\sim}0.4 eV and the band gap for heavily doped films is \ensuremath{\sim}3.6 eV. The bipolaron bands move toward the band edges and the gap shrinks with decreasing dopant concentration. Annealing of heavily doped films at 300 \ifmmode^\circ\else\textdegree\fi{}C leads to dopant loss from the surface. This effect is less pronounced for the larger polymeric anions such as (PSS${)}^{\mathrm{\ensuremath{-}}}$ than for the smaller counterions such as ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$, suggesting that the former is more stable or has a greater thermal activation energy for bipolaron dissociation into single polarons.