Excitonic singlet-triplet ratio in a semiconducting organic thin film

A technique is presented to determine the spin statistics of excitons formed by electrical injection in a semiconducting organic thin film. With the aid of selective addition of luminescent dyes, we generate either fluorescence or phosphorescence from the archetype organic host material aluminum tris (8-hydroxyquinoline) $({\mathrm{Alq}}_{3}).$ Spin statistics are calculated from the ratio of fluorescence to phosphorescence in the films under electrical excitation. After accounting for varying photoluminescent efficiencies, we find a singlet fraction of excitons in ${\mathrm{Alq}}_{3}$ of $(22\ifmmode\pm\else\textpm\fi{}3)%.$