DOPING EFFECT OF BUCKMINSTERFULLERENE IN POLY(2,5-DIALKOXY-P-PHENYLE VINYLENE)

Photoluminescence has been markedly quenched and photoconductivity has been enhanced by more than one order of magnitude upon introduction of several mol% of buckminsterfullerene (C60) to poly(2,5-dialkoxy-p-phenylene vinylene) (RO-PPV), especially at excitations about 2.2 eV, corresponding to the band gap energy of RO-PPV and also in bands at 1.8 and 3.5 eV, which correspond to optical excitation of C60 molecules, suggesting that photo-induced charge transfer occurs between RO-PPV and C60. On the other hand, absorption spectrum and electrical conductivity of RO-PPV have been scarcely influenced by doping of small amount of C60, suggesting that the ground state charge transfer between C60 and RO-PPV is not effective, contrary to the case of poly(3-hexylthiophene). These results are discussed by taking relative electronic energy states of RO-PPV and C60 into consideration. The photo-excited exciton-polaron (Ex-P) in RO-PPV is interpreted to migrate along about 100 monomer units along a polymer main chain in its lifetime and dissociates when encountered with C60. These unique doping characteristics of C60 in RO-PPV are not dependent on the alkyl chain length, contrary to the case of poly(3-alkylthiophene).