Visible-Pump Terahertz Probe Measurements of Embedded Polymer Conductivity in Organic Matrices

We report measurements of ultrafast photoinduced charge separation and recombination processes in the conjugated donor-acceptor (D-A) polymer PSBTBT, both as pure film and blended in various polymer matrices. Using time-resolved terahertz spectroscopy (TRTS), time-dependent photoconductivity is measured for samples with PSBTBT weight fractions (W_PSBTBT:W_PE/PEG/PS) of 2.0% dispersed in high-density polyethylene (HDPE), polyethylene glycol (PEG), and polystyrene (PS). Charge carrier generation is an intrinsic feature of conductive polymers that occurs on sub-picosecond and longer timescales and is attributed to initially generated dissociation of bound polaron pairs into free carriers that reside on polymer chains, or to adjacent interchain charge transfer and migration. Both interchain and interfacial charge transfer contribute to the measured photoconductivity of the samples, which is found to increase as a function of increasing local polarity and an increasingly hydrogen-bonded environment. Pure-PSBTBT polymer film, PSBTBT dispersed in PS, and PSBTST dispersed in HDPE were all found to exhibit shorter photoconductive free-carrier long-time signal decay than PSBTBT in a hydrogen-bonded, semi-crystalline PEG environment.