Time-resolved infrared-absorption study of photoinduced charge transfer in a polythiophene-methanofullerene composite film

Abstract

Photoinduced charge separation and recombination in poly(3-hexylthiophene)-fullerene derivative composite films is investigated at low temperature (80 K) by measuring the time-resolved photoinduced absorption due to the infrared active vibrational (IRAV) modes of the polymer in the region 1125--1300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The charge separation under 532 nm excitation (repetition rate 5 Hz) is found to be faster than the time resolution of the setup (200 ps). The decay of charge carriers can be described with power-law kinetics ${\mathrm{At}}^{\ensuremath{-}\ensuremath{\alpha}}$ with a single exponent $\ensuremath{\alpha}=0.21$ from the subnanosecond to microsecond time scale. Hence, a large percentage of the charges created (80%) recombine within 30 ns, the remaining long-lived ones recombine on a millisecond time scale. On the millisecond time scale the decay can be approximated by a $1/t$ dependence. On the nanosecond time scale, the IRAV modes show only a small change in frequency upon varying the pump-probe delay, indicating that the polaronic charge carriers on the polymer do not undergo major relaxation processes in this time window.