TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: In the device, an emission layer contains the composition, a hole transport layer containing hole transport material which directly adjacent to the emission layer, where absolute values of (highest occupied molecular orbital) HOMO energies of the bipolar host in the emission layer and the hole transport material in the hole transport layer is less than 0.3 eV, and an electron transport layer contains an electron transport material which directly adjacent to the emission layer, where absolute values of HOMO energies of the bipolar host in the emission layer and the electron transport material in the electron transport layer is less than 0.3 eV.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The composition further comprises organic functional materials including hole injection materials, hole transport materials, hole-blocking materials, host materials, emitter materials, electron blocking materials, electron transport materials or electron injection materials, a host material, a second light emitting dopant, which is a phosphorescent emitter, a third light emitting dopant, which is a phosphorescent emitter, and organic or inorganic constituents. Preferred Components: The dopant is a phosphorescent emitter. The bipolar host is pyridine, pyrimidine, triazine, benzimidazole, carbazole, indenocarbazole, indolocarbazole, 1,10-phenanthroline, 1,3,4-oxadiazole, phosphine, phenylsulfonyle, ketone, lactam or triarylamine. The composition satisfies the following conditions: difference of highest occupied molecular orbital (HOMO) energy of the neutral co-host (HOMO (C)) and smaller of the two values HOMO energy of dopant (HOMO (D)) and HOMO energy of bipolar host (HOMO (B)) is greater than 0.3 eV, preferably 0.6 eV; difference of (HOMO (B)) and (HOMO (D)) is less than 0.15 eV, preferably less than 0.1 eV and difference of (HOMO (B)) and (HOMO (D)) is greater than -0.2 eV, preferably greater than -0.1 eV; difference of lowest occupied molecular orbital (LUMO) energy of the bipolar host (LUMO (B)) and lowest occupied molecular orbital (LUMO) energy of the co-host (LUMO (C)) is greater than 0.3 eV, preferably greater than 0.6 eV; difference of (LUMO (B)) and LUMO energy of the dopant (LUMO (D)) is greater than 0; difference of larger of the two values (LUMO (D)) and (LUMO (B)), and (LUMO (C)) is greater than 0.3 eV, preferably greater than 0.6 eV; difference of (LUMO (D)) and (LUMO (B)) is less than 0.15 eV, preferably 0.1 eV and difference of (LUMO (D)) and (LUMO (B)) is greater than -0.2 eV, preferably -0.1 eV; difference of (HOMO (C)) and (HOMO (B)) is greater than 0.3 eV, preferably greater than 0.6 eV; and difference of (HOMO (D)) and (HOMO (B)) is greater than 0. EXAMPLE - Typical composition comprised of: 20 wt.% 3-(9-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazol-3-yl)-1,1-dimethyl-1,3-dihydroindeno(2,1-b)carbazole; 60 9,9-bis(3,5-bis(3,5-diphenylphenyl)phenyl)fluorene; and 20 wt.% tris(2-(3-(3-(3,5-diphenylphenyl)phenyl)phenyl)pyridine)iridium complex.