Exciton diffusion in polyfluorene copolymer thin films : kinetics, energy disorder and thermally assisted hopping.

A series of {(9,9-dioctylfluorene)0.7?x-(dibenzothiophene-S,S-di-oxide)0.3-[4,7-bis(2-thienyl)-2,1,3-benzothiadiazole]x}(PFS30-TB Tx),where x represents the minor percentage of the red emitter4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) randomly incor-porated into the copolymer backbon e, is investigated in orderto follow the energy transfer from PFS30to TBT moieties. Theemission of the donor poly[(9 ,9-dioctylfluorene)0.7-(dibenzothio-phene-S,S-dioxide)0.3identified by PFS30and peaking at450 nm, is clearly quenched by the presence of the red TBTchromophore emitting at 612 nm, with an isoemissive pointobserved when the spectra are collected as a function of tem-perature. A plot of the ratio between the TBT and PFS30emis-sions as a function of the reciprocal of temperature gives aclear linear trend between 290 and 200 K, with an activationenergy of 20 meV and showing a turn over to a non-activated egime below 200 K. Picosecond time-resolved fluorescencedecays collected at the PFS30and TBT emission wavelengths,show a decay of the PFS30emission and a fast build-in, fol-lowed by a decay, of the TBT emission, confirming that thepopulation of the TBT excited state occurs during the PFS30lifetime(~600 ps). The population of the TBT excited stateoccurs on a time regime around 150 ps at 290 K, showing anenergy barrier of 20 meV that turns over to a non-activatedregime below 200 K in clear agreement with the steady-statedata. The origin of the activation barrier is attributed to thepresence of physical and energetic disorder, affected by fastthermal fluctuations that dynamically change the energy land-scape and control the exciton migration through the polymerdensity of states