Highly efficient exciton-exciton annihilation in single conjugated polymer chains

The number of excitons that conjugated polymers can support at any one time underpins their optoelectronic performance in light-emitting diodes and as laser gain media, as it sets a natural limit on exciton density. Here we have measured the time-resolved photon statistics of single, isolated chains of polyfluorene to extract the absolute number of independent emitting sites present and its time dependence by studying the intramolecular exciton-exciton annihilation. We find that after 100 ps each chain can only support 1 or 2 independent excitons, and that even at the earliest times this number rises only to 4, suggesting a high degree of electronic coupling between chromophores that facilitates efficient exciton-exciton annihilation. In circumstances where a low density of low-energy sites is present, annihilation between them still dominates. The results indicate that achieving high exciton densities in conjugated polymers is difficult, and in applications where it is desirable new strategies should be devised to control exciton-exciton annihilation.