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 Förster 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 Förster 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.

Figure 1

PL and absorption spectra for 100 nm films of AlQ3 at temperatures of 295, 180, 75, and 35 K, where the PL spectra are time integrated over the entire emission pulse and normalized to integrate over energy to unity. The bottom panel shows the spectra themselves, while the top panel shows the deviation of the cooled spectra from the 295 K spectra, to better illustrate the differences between the temperatures. To within the measurement error, the absorption spectra at 35 and 75 K are identical. Inset: chemical structure of AlQ3.

Figure 2

Time evolution of mean PL energy (gray points) for thin films of AlQ3 at 295, 180, 75, and 35 K, showing fits to the mean PL energy using model I (solid black lines) and model II (dashed black lines), using the parameters listed in Table . Inset: Integrated total PL intensity time decay data (gray dots) for the 295 K measurement with fit (black line).

Figure 3

Molecular AlQ3 absorption (${\sigma }_A$) and PL ($S_D$) spectra, calculated from 295 K data for the model II fit. Also shown are the bulk absorption (${\sigma }_{\mathrm{bulk}}$) and PL ($S_{\mathrm{bulk}}$) spectra, as well as the DOS [$g(E)$] for absorption and emission, and the time integrated energy distribution of emitted excitons ($n_{\mathrm{ex}}$).