Emission mechanism in organic light-emitting devices comprising a europium complex as emitter and an electron transporting material as host

Abstract

The emission mechanism in organic light-emitting devices, where the emission layer is composed of $\mathrm{Eu}{\left(\mathrm{DBM}\right)}_3\text{pyzphen}$ ($\mathrm{DBM}=\text{Dibenzoylmethane}$ , $\text{pyzphen}=\text{pyrazino}$ -[2,3-f][1,10]-phenanthroline) doped into electron transporting/hole blocking material BPhen (4,7-diphenyl-1, 10-phenanthroline), is investigated. Energy transfer and carrier trapping simultaneously exist in the luminescence process, and carrier trapping is a main process. Direct carrier trapping by $\mathrm{Eu}{\left(\mathrm{DBM}\right)}_3\text{pyzphen}$ molecules is confirmed by the difference of electroluminescence and photoluminescence spectra as well as $J\text{-}V$ characteristics. Efficient Föster and Dexter energy transfer from BPhen to $\mathrm{Eu}{\left(\mathrm{DBM}\right)}_3\text{pyzphen}$ molecules were speculated in terms of analysis of photoluminescence spectra of fixed solutions, triplet energies, and phosphorescent lifetimes. Based on these mechanisms, the overall performances of these devices were improved. High efficiencies were obtained under carrier trapping by $\mathrm{Eu}{\left(\mathrm{DBM}\right)}_3\text{pyzphen}$ molecules, and the emission of BPhen was eliminated by efficient energy transfer from the BPhen to $\mathrm{Eu}{\left(\mathrm{DBM}\right)}_3\text{pyzphen}$ molecules.