Thermally induced transient absorption of light by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) films:: A way to probe charge-carrier thermalization processes

Infrared induced transient absorptions in the millisecond and sub-picosecond time domains used to study the dynamics of charge carriers of a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonic acid) (PED-OTPSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed infrared light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of infrared-induced changes in the resistivity of the material. Near 1.5 eV, the infrared-induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalisation processes using short laser pulses. Pump-probe spectroscopy on a subpicosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time constant of less than 500 fs, i.e., faster for noble metals.