Absolute photoluminescence quantum yield of molecular organic thin films: Effects of doping with the strongly fluorescent rubrene

We present data on the absolute photoluminescence quantum yield, phi(PL), for a set of pure and molecularly doped organic solid films. The procedure uses an integrating sphere to provide accurate measure of the photoluminescence efficiency for solid, sub-micron thickness, films. Host materials include a common hole transport compound, N,N,-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine, TPD, and two metal chelates used as electron transport and/or light emitting materials, tris (8-hydroxyquinolinolato) aluminum (III), Alq(3), and one of its methyl derivatives, tris (8-trimethylhydroxyquinolinolato) aluminum (III), Almq(3). Tetraphenylnapthacene, or rubrene, is used as the dopant. A substantial increase (a factor of 2-4) in phi(PL) is measured with respect to the pure host. For example, the measured phi(PL) increases from 0.25 and 0.40 for pure Alq(3) and Almq(3), respectively, to near unity upon doping with rubrene at similar to 1 mol%. The data are discussed within the framework of Forster energy transfer.