Origin of electrophosphorescence from a doped polymer light emitting diode- art. no. 235206

The origin of electrophosphorescence from a doped polymer light emitting di ode (LED) has been investigated. A luminescent polymer host, poly(9,9-dioct ylfluorene) (PFO), was doped with a red phosphorescent dye, 2,3,7,8,12,13,1 7,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP). The maximum external quantum efficiency of 3.5% was obtained at a concentration of 4% PtOEP by weight. Energy transfer mechanisms between PFO and PtOEP were studied by ab sorption, photoluminescence, and photoinduced absorption spectroscopy. Even though electroluminescence spectra were dominated by PtOEP at a concentrat ion of only 0.2 wt % PtOEP, Forster transfer of singlet excitons was weak a nd there was no evidence for Dexter transfer of tripler excitons. We conclu de that the dominant emission mechanism in doped LED's is charge trapping f ollowed by recombination on PtOEP molecules.