Photoluminescence quantum yield of pure and molecularly doped organic solid films

We present measurements of the absolute photoluminescence (PL) quantum yield, ${\phi }_{\mathrm{PL}},$ 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 ${\mathrm{N,N}}^{\prime }-\mathrm{diphenyl}{\mathrm{-N,N}}^{\prime }-\mathrm{bis(3-methylphenyl)}{\text{-1,1}}^{\prime }-\mathrm{biphenyl}{\text{-4-4}}^{\prime }-\mathrm{diamine}$ (TPD), a common hole transport material for light emitting diodes, tris (8-hydroxyquinolinolato) aluminum (III) $({\mathrm{Alq}}_{\mathrm{3}})$ and its methyl derivative, ${\mathrm{Almq}}_{\mathrm{3}},$ two aluminum chelates used as electron transport and/or green emitting materials. Dopants include tetraphenylnapthacene (rubrene) and ${\mathrm{N,N}}^{\prime }$ -diethyl quinacridone (DEQ). Doping results in a substantial increase (∼a factor 2–4) of ${\phi }_{\mathrm{PL}}$ in comparison with that of the pure host. For instance, measured ${\phi }_{\mathrm{PL}}$ increases from 0.25 and 0.42 for pure ${\mathrm{Alq}}_{\mathrm{3}}$ and ${\mathrm{Almq}}_{\mathrm{3}},$ respectively, to near unity upon doping with rubrene at a concentration of ∼1 mol %. The above data are discussed within the framework of Förster energy transfer from host to guest.