Singlet Annihilation in Films of Regioregular Poly(3-hexylthiophene): Estimates for Singlet Diffusion Lengths and the Correlation between Singlet Annihilation Rates and Spectral Relaxation

For a film of regioregular poly(3-hexylthiophene) (P3HT), we calculate annihilation rates and diffusion lengths for P3HT singlet states via time-resolved emission measurements. We find that the annihilation reaction has a time-dependent annihilation constant whose value decreases by over an order of magnitude between 100 and 1000 ps and observe a correlation between the decreasing annihilation rate constant and the red shift in the P3HT emission spectrum. The onset of the annihilation regime in P3HT-where annihilation is the majority decay pathway for the P3HT singlet-occurs in P3HT at singlet densities above similar to 2 x 10(17) incident photons/cm(3), and we estimate an exciton diffusion length for P3HT singlets of 27 +/- 12 nm.