Fast Holes, Slow Electrons, and Medium Control of Polaron Size and Mobility in the DA Polymer F8BT

The nature of electron and hole polarons on poly(9,9-di-n, hexylfluorenyl-2,7-diyl) (pF) and a copolymer poly[(9,9-di-n-octylfluoreny1-2,7-diy1)-alt-(benzo[2,1,3]thiadiazol-4,8-diy1)] (F8BT) has been studied by chemical doping, pulse radiolysis, charge modulation spectroscopy, quantum chemical calculations, and microwave conductivity. While pF exhibits very similar behavior in all respects for the electron and the hole, this paper explores the hypothesis that the donor acceptor (push-pull) nature of F8BT will tend to localize, charges. Optical spectra and quantum chemical calculations point to an electron localized on the thiadiazole unit in polar liquids but becoming more delocalized as the solvent polarity decreases. Indeed, in the nonpolar solvent benzene, the electron mobility is only 2.7 times lower than that of the hole, which conversely is shown to be delocalized in all environments and has a similar mobility to polarons on the homopolymer polyfluorene. Advantageous modifications to the optoelectronic properties of conjugated polymers that come about by using alternating donor acceptor repeat units have thus been shown to not significantly hinder charge transport despite the corrugated energy landscape along the backbone.