The present invention relates to organic light-emitting devices comprising (a) an anode, (i) a cathode, and (e) an emitting layer between the anode and cathode, comprising 40.01 to 99.95% by weight of a luminescent organometallic complex X having a difference of the singlet energy (ES1(X)) and the triplet energy (ET1(X)) of smaller than 0.2 eV [Δ(ES1(X))−(ET1(X)) ES1(Y)]. By doping, for example, an emitting layer containing a luminescent organometallic complex having a small S1−T1 splitting, with a fluorescent emitter the emission decay time can significantly be shortened without sacrificing external quantum efficiency (EQE) because of very efficient energy transfer.