The photoluminescence and electroluminescence (EL) spectral characteristics 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 Forster energy transfer model. EL spectra 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 Foerster energy transfer is likely due to additional emission mechanisms such as carrier trapping and triplet exciton saturation at high current density.
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