Exciton–Exciton Annihilation in Mixed‐Phase Polyfluorene Films

Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β ‐phase chains using time‐resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time‐independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time‐dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β ‐phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low‐energy sites. The annihilation rate in the mixed‐phase films increases with increasing fraction of β ‐phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersed β ‐phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line‐dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy and β ‐phase chromophores are intimately mixed rather than clustered or phase‐separated.