Photoexcitation dynamics in an alternating polyfluorene copolymer

Abstract

We have used transient photoinduced absorption on femtosecond to nanosecond time scales as well as delayed fluorescence up to microseconds to study the photogeneration and recombination of charges in thin films of the alternating polyfluorene copolymer poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(${4}^{\ensuremath{'}},{7}^{\ensuremath{'}}$-di-2-thienyl-${2}^{\ensuremath{'}},{1}^{\ensuremath{'}},{3}^{\ensuremath{'}}$-benzothiadiazole)]. We interpret the results using a coupled rate equation model and find that we can fit all our experimental results with a single set of parameters. The model includes prompt $(<0.1\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$ as well as slower $(\ensuremath{\sim}0.1\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ charge-pair formation, which we attribute to Coulombically bound intra- and interchain polaron pairs, respectively. The intrachain polaron pairs are promptly generated from vibronically excited (hot) primary singlet excitons ${S}_{1}^{*}$ and recombine geminately back to the lowest singlet exciton state ${S}_{1}$ with a lifetime distribution having a mean lifetime of $\ensuremath{\sim}2.4\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The interchain polaron pairs, which can be seen as precursors to free charges, are formed via two channels: via singlet excitons being dissociated with a linear rate constant of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ as well as via a time-dependent bimolecular exciton-exciton annihilation process generating higher-energy exciton states ${S}_{n}^{*}$ of which a fraction subsequently dissociates into interchain polaron pairs. We observe a total yield of 12%--23% interchain polaron pairs (a precursor to free polarons), depending on the excitation intensity used. This also defines the upper limit of the free polaron yield at zero electric field in this material. The long-lived interchain polaron pairs recombine geminately back to the ground state or to singlet excitons with a broad distribution of lifetimes having a mean lifetime of $\ensuremath{\sim}0.27\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. The fraction of interchain polaron pairs recombining back to singlet excitons, with subsequent radiative decay back to the ground state, gives rise to delayed fluorescence extending to microsecond time scales.