Difference in Doping Effects of C60 and C70 in Poly(3-hexylthiophene)

C60 (Buckminsterfullerene) can be an acceptor dopant for poly(3-hexylthiophene), inducing marked changes in optical absorption and photoluminescence due to charge transfer from polymer to C60 upon doping. Electrical conductivity also changes to some extent upon C60 doping. In contrast, C70 is not effective as a strong dopant in poly(3-hexylthiophene). That is, the absorption spectrum does not change markedly upon C70 doping and the change in electrical conductivity is also found to be much smaller than in the case of C60 doping. However, photo-luminescence is markedly quenched upon C70 doping, just as in the case of C60 doping. These results are discussed by accounting for the difference in electronic energy states of both C60 and C70 relative to that of poly(3-hexylthiophene). The important role of photoinduced charge transfer for quenching of photoluminescence is discussed and C70 is proposed to be a good sensitizer for photoconductivity of poly(3-alkylthiophene).