Ultrafast conductivity in a low-band-gap polyphenylene and fullerene blend studied by terahertz spectroscopy

Time-resolved terahertz spectroscopy and Monte Carlo simulations of charge-carrier motion are used to investigate photoinduced transient conductivity in a blend of a low-band-gap polyphenylene copolymer and fullerene derivative. The optical excitation pulse generates free holes delocalized on polymer chains. We show that these holes exhibit a very high initial mobility as their initial excess energy facilitates their transport over defects (potential barriers) on polymer chains. The conductivity then drops down rapidly within 1 ps, and we demonstrate that this decrease occurs essentially by two mechanisms. First, the carriers loose their excess energy and they thus become progressively localized between the on-chain potential barriers—this results in a mobility decay with a rate of ${\left(180\text{fs}\right)}^{-1}$. Second, carriers are trapped at defects (potential wells) with a capture rate of ${\left(860\text{fs}\right)}^{-1}$. At longer time scales, populations of mobile and trapped holes reach a quasiequilibrium state and further conductivity decrease becomes very slow.