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-dioxide)(0.3)-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole} (PFS(30)-TBTx), where x represents the minor percentage of the red emitter 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) randomly incorporated into the copolymer backbone, is investigated in order to follow the energy transfer from PFS(30) to TBT moieties. The emission of the donor poly[(9,9-dioctylfluorene)(0.7)-(dibenzothiophene-S,S-dioxide)(0.3) identified by PFS(30) and peaking at 450 nm, is clearly quenched by the presence of the red TBT chromophore emitting at 612 nm, with an isoemissive point observed when the spectra are collected as a function of temperature. A plot of the ratio between the TBT and PFS(30) emissions as a function of the reciprocal of temperature gives a clear linear trend between 290 and 200 K, with an activation energy of 20 meV and showing a turn over to a non-activated regime below 200 K. Picosecond time-resolved fluorescence decays collected at the PFS(30) and TBT emission wavelengths, show a decay of the PFS(30) emission and a fast build-in, followed by a decay, of the TBT emission, confirming that the population of the TBT excited state occurs during the PFS(30) lifetime (approximately 600 ps). The population of the TBT excited state occurs on a time regime around 150 ps at 290 K, showing an energy barrier of 20 meV that turns over to a non-activated regime below 200 K in clear agreement with the steady-state data. The origin of the activation barrier is attributed to the presence of physical and energetic disorder, affected by fast thermal fluctuations that dynamically change the energy landscape and control the exciton migration through the polymer density of states.