Photophysical properties of tris(bipyridyl)ruthenium(II) thin films and devices

Absorption and luminescence spectra as well as luminescence lifetimes have been measured for Ru(bpy)2þ 3 in solution and in thin films of varying thicknesses, and these properties have been correlated with the efficiency of organic light emitting devices (OLEDs) made of the films. The lifetimes decrease for films below about 50 nm in thickness but are relatively constant for thicknesses above about 100 nm. This behavior is consistent with a model in which quenching is caused both by intrinsic properties of the molecules and by Förster energy transfer between chromophores that carries the excitation to surface layers, where the excitation is more efficiently quenched. The external quantum efficiency of the OLEDs is also found to increase with thickness, approaching 1% for thicknesses near 200 nm.