Charge carrier photogeneration and recombination in ladder-type poly(para-phenylene): Interplay between impurities and external electric field

Abstract

Charge carrier generation and decay in $m$-LPPP polymer films were examined by means of femtosecond transient absorption spectroscopy in the time window of $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}--15\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. Two modes of polaron formation with distinct behavior were identified, impurity induced in the absence of an external electric field and electric field induced in pristine film. While field induced charge generation is relatively slow, occurring throughout the excited state lifetime, the rate of impurity induced charge generation is much faster and depends on excitation wavelength; it occurs on the several hundred femtosecond time scale under excitation within the main absorption band, but excitation into the red wing of the absorption band results in charge generation within less than $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. Polaron decay through geminate electron-hole recombination occurs with widely distributed lifetimes, from $\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ to microseconds; the polarons characterized by the shortest decay time have a redshifted absorption spectrum (as compared to more long-lived polarons) and are attributed to tightly bound polaron pairs.