  The kinetics of the charge carrier recombination in dye molecule-doped
multilayer organic light-emitting diodes (OLEDs) was quantified by transient
electroluminescence (EL). Three sets of dye molecules, such as derivatives of
naphthalimide and stilbene, were used as dopants in light-emission layer.
Although the devices show almost the same EL spectra for each set of molecules,
they show very different EL efficiency. The difference in EL efficiency was
attributed to the difference in charge carrier recombination, as revealed by
transient EL. The recombination coefficient ({\gamma}) was determined from the
long-time component of the temporal decay of the EL intensity after a
rectangular voltage pulse was turned off. It was found that {\gamma} and EL
efficiency were both strongly dependent on the molecular structures of the
dopants, and the donor groups and {\pi}-conjugated structure guaranteed high
{\gamma} and EL efficiency in OLEDs.
