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 approximately 1.0 and approximately 2.6 eV above the valence-band maximum for heavily doped PPy films (approximately 1 dopant per 3 pyrrole units). The bandwidth of each of these bipolaron bands is approximately 0.4 eV and the band gap for heavily doped films is approximately 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-degrees-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.