Photoluminescence quantum yield of pure and molecularly doped organic solid films

We present measurements of the absolute photoluminescence (PL) quantum yield, phi(PL), for a wide variety of organic compounds in solid films, pure and molecularly doped with strongly fluorescent materials. The procedure, which uses an integrating sphere, does not entail comparison to other standards, and provides accurate measure of the photoluminescence efficiency for submicron thick films, prepared by high vacuum vapor deposition. Host materials include N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4-4'-diamine (TPD), a common hole transport material for light emitting diodes, tris (8-hydroxyquinolinolato) aluminum (III) (Alq(3)) and its methyl derivative, Almq(3), two aluminum chelates used as electron transport and/or green emitting materials. Dopants include tetraphenylnapthacene (rubrene) and N,N'-diethyl quinacridone (DEQ). Doping results in a substantial increase (similar to a factor 2-4) of phi(PL) in comparison with that of the pure host. For instance, measured phi(PL) increases from 0.25 and 0.42 for pure Alq(3) and Almq(3), respectively, to near unity upon doping with rubrene at a concentration of similar to 1 mol %. The above data are discussed within the framework of Forster energy transfer from host to guest. (C) 1999 American Institute of Physics. [S0021-8979(99)03017-0].