Electrophosphoresence from a doped polymer light emitting diode

The photophysics and device physics of a phosphorescent polymer light emitting diode (LED) have been investigated. The emissive host, poly(9,9-dioctylfluorene) (PFO), was doped to a variety of concentrations between 0.2 and 8% with a red emissive phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP). The energy transfer mechanisms between the host and the dopant were studied using both photoluminescence (PL) and photo-induced absorption techniques. Fِrster transfer was observed from the host to the dopant, but no evidence of Dexter transfer was obtained. Electroluminescent (EL) devices prepared from PFO/PtOEP blends showed a maximum external quantum efficiency (QE) of 3.5% and a peak brightness >200 cd/m2. We attribute this high efficiency to the capturing of both singlet and triplet excitons by direct charge trapping on the PtOEP molecules. Finally, we report that the EL and PL quantum efficiencies show notably different dopant concentration dependencies. This questions the linkage between the PL and EL quantum efficiencies in doped organic systems.