Ultrafast photogeneration of charged polarons on conjugated polymer chains in dilute solution

Abstract

Ultrafast photoinduced absorption by infrared-active vibrational modes is used to study the photogeneration of polarons on semiconducting polymer chains in dilute solutions and in solid films of a soluble derivative of poly(para-phenylene vinylene). In dilute solutions, polaron pairs are photogenerated on the conjugated polymer within less than $250\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ with quantum efficiencies ${\ensuremath{\phi}}_{\mathrm{ch}}\ensuremath{\sim}3%$, about one-third of that for solid films of the same polymer. The excitation spectra of ${\ensuremath{\phi}}_{\mathrm{ch}}$ for both solutions and films show that ${\ensuremath{\phi}}_{\mathrm{ch}}$ is weakly dependent on photon energy between $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (the onset of absorption) and $4.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The recombination dynamics of polarons is very fast and highly dependent on the excitation density for polymer films, but it is significantly slower and less sensitive to pump intensity for the semiconducting polymer in dilute solution. We conclude that the positive and negative polarons on a single chain in solution are typically separated by hundreds of monomer repeat units and that their one-dimensional diffusion along the chain is inhibited by the intervening excitons. This, together with the suppression of interchain recombination, explains the surprisingly slower polaron recombination in isolated chains.