Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C60 with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C60 compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C60− has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P+−P− ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C60− related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C60 molecules, has not been observed in the ps time domain.