We fabricated two types of electroluminescent cells, sample 1
(glass/indium-tin oxide (ITO), 300 nm/Au, 600 nm/N, N'-diphenyl-N,
N'-(3-methyphenyl)-1,1'-biphenyl-4,4'-diamine
(TPD):poly(N-vinylcarbazole) (PVK), 50 nm/tris(8-hydroxyquinoline) aluminum
(Alq3), 300 nm/Mg:Ag, 700 nm/Ag cap, 100 nm), and sample 2
(glass/ITO, 300 nm/TPD:PVK, 60 nm/Alq3, 300 nm/Mg:Ag, 700 nm/Ag cap,
100 nm). The full-width at half-maximum of emission narrowing with increasing
applied external electric field was observed in sample 1, and the threshold of
the gain narrowing was extrapolated as 2.0×103 A cm-2.