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

Inverse-photoemission spectroscopy has been used to investigate dopant-dependent electronic properties of polypyrrole (PPy) films doped with BF₄^- (tetrafluoroborate) or (PSS)^- [poly(4-styrenesulfonate)]. The results show that the bonding and antibonding bipolaron bands appear at {~}1.0 and {~}2.6 eV above the valence-band maximum for heavily doped PPy films ({~}1 dopant per 3 pyrrole units). The bandwidth of each of these bipolaron bands is {~}0.4 eV and the band gap for heavily doped films is {~}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 ^∘{}C leads to dopant loss from the surface. This effect is less pronounced for the larger polymeric anions such as (PSS)^- than for the smaller counterions such as BF₄^-, suggesting that the former is more stable or has a greater thermal activation energy for bipolaron dissociation into single polarons.