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 ∼2 × 10 17 incident photons/cm 3, and we estimate an exciton diffusion length for P3HT singlets of 27 ± 12 nm.