Modeling of exciton diffusion in amorphous organic thin films

Time-resolved photoluminescence spectroscopy of amorphous organic thin films of aluminum tris-(8-hydroxyquinoline) show emission spectra that redshift with time following excitation by ultrafast laser pulses. Based on reports of similar phenomena in other materials, we attribute this effect to the exciton diffusion between energetically dissimilar molecules by means of Forster transfer. In analyzing results at 295, 180, 75, and 35 K, we show that existing theoretical treatments of exciton diffusion require two modifications to self-consistently fit our data: one must include spatial disorder in the model, and the energy dependence of Forster transfer must be calculated using the donor-acceptor spectral overlap, instead of a Boltzman distribution. Monte Carlo simulations utilizing these changes yield results that are self-consistent with the observed spectral shifts.