Exciton annihilation and diffusion in semiconducting polymers

We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate γ, which was measured to be γ = (2.8±0.5)×10 -8 cm 3 s -1 in MEH-PPV and γ = (5.2±1)×10 -10 cm 3 s -1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1±0.2)×10 -9 cm 3 s -1 and the diffusion length to 4.7 nm.