Photoluminescence quenching of tris-(8-hydroxyquinoline) aluminum thin films at interfaces with metal oxide films of different conductivities

We report a comprehensive study of photoluminescence (PL) quenching of tris-(8-hydroxyquinoline) aluminum (Alq₃) at interfaces with thin films of tin oxide (SnO₂) using both steady-state and time-resolved measurements. Quenching of excitons generated in the Alq₃ layer increased with increased conductivity of the SnO₂ films, which we relate with the presence of nonradiative energy transfer from excitons in Alq₃ to transitions in SnO₂. In addition, due to the semitransparency of SnO₂, the effects of optical interference on the steady-state PL quenching of Alq₃ are determined. We demonstrate that without accounting for the interference effects in the excitation, the extracted exciton diffusion length (Ld) in Alq₃ is in the range of 10--20 nm. However, when using a numerical model to account for the optical interference effects, we find that Ld is in the range of 5--9 nm, which agrees with Ld extracted from time-resolved measurements (4--6 nm). We conclude that time-resolved measurements are least affected by optical interference, yielding the most accurate measurement of Ld.