Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes

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Abstract
The photoluminescence and electroluminescence(EL)spectralcharacteristics of two phosphorescent dyes, green fac-tris(2-phenylpyridine) iridium(III) [ Ir ( ppy ) 3 ] and red tris-(4,7-diphenyl-1,10-phenanthroline)rhenium dications [ Ru (4,7- Ph 2 - phen ) 3 ] 2+ , doubly doped poly(N-vinylcarbazol) (PVK) films were investigated. The efficient energy transfers from PVK host to both Ir ( ppy ) 3 and [ Ru (4,7- Ph 2 - phen ) 3 ] 2+ dopants were observed in photoexcitation and EL devices. The intensity ratio (R i ) of red emission versus green emission as function of doping ratio (R c ) of [ Ru (4,7- Ph 2 - phen ) 3 ] 2+ and Ir ( ppy ) 3 in PVK host is found according to a power-law function with the slopes of 2.28±0.2 for PL which obey the power-law with R i –R c 2 predicated by Förster energy transfer model. ELspectra show a driving voltage depentent power law of R i –R c 1.53±0.14 at low voltage and R i –R c 1.16±0.08 at high voltage, respectively. The shift in the power law of Förster energy transfer is likely due to additional emission mechanisms such as carrier trapping and triplet exciton saturation at high current density.