Time-Resolved Study of Fullerene Doping Effects on Photoluminescence and Photoconductivity of Poly(3-alkylthiophene)

Abstract

We have carried out time-resolved measurements of photoluminescence and photocurrent in poly(3-octylthiophene) (PAT8) doped with fullerene (C60). For photocurrent study, we applied a laser two-correlated-pulse technique, which permitted the selection of electronic processes connected with recombination of intermediate short-living species responsible for the formation of free charge carriers. The wavelength and pulse width of the laser pulses were 400 nm and 150 fs, respectively, and delay time (Δt) between correlated pulses was controlled within the ps time domain. It was shown that the average photocurrent in undoped samples has a prominent component originating from interpair recombination of polaron pairs. This component, however, changed with doping: with low doping (C60 less than 0.1 mol%) it increased, but with higher doping (C60 up to 0.5 mol%) it disappeared. It was shown that the electron transfer from π-conjugated segments of PAT8 to C60 takes about 10 ps, and leads to the formation of polaron pairs which dissociate into free charge carriers much faster (lifetime is much shorter than 3 ps) than the pairs formed without doping.