An organic light-emitting device comprising (a) an anode, (i) a cathode, and (e) an emitting layer between the anode and cathode, comprising 2 to 40 % by weight of a luminescent organometallic complex X having a difference of the singlet energy (E s1) and the triplet energy (E T1) of smaller than 0.2 eV, wherein experimental measurements involving temperature dependent determination of excited state lifetimes and quantum chemical calculations are useable to calculate the singlet energy and triplet energy difference, and wherein the difference of the singlet energy (E s1) and the triplet energy (E T1) is determined as the energy difference of the lowest T 1 -sublevel to the first spin-orbit corrected S 1 -state, 0.05 to 5.0 % by weight of a fluorescent emitter Y and 55 to 97.95 % by weight of a host compound(s), wherein the amount of the organometallic complex X, the fluorescent emitter Y and the host compound(s) adds up to a total of 100% by weight and the singlet energy of the luminescent organometallic complex X (E s1) is greater than the singlet energy of the fluorescent emitter Y (E s1); wherein the emissive lifetime τo, which is calculated by τo=τv/QY, of thin films consisting of the luminescent organometallic complex X (2 to 40 % by weight), fluorescent emitter Y (0.05 to 5.0 % by weight) and host compound(s) (55 to 97.95 % by weight) is in the range 0.1 to 80 ns, wherein the decay time of the emission is the emissive lifetime τo, the excited state lifetime of the emitting layer is τv, and QY is the quantum yield of the emitting layer; and wherein the luminescent organometallic complex X comprises a luminescent iridium complex or a luminescent copper complex. (a) an anode, (i) a cathode, and (e) an emitting layer between the anode and cathode, comprising 2 to 40 % by weight of a luminescent organometallic complex X having a difference of the singlet energy (E s1) and the triplet energy (E T1) of smaller than 0.2 eV, wherein experimental measurements involving temperature dependent determination of excited state lifetimes and quantum chemical calculations are useable to calculate the singlet energy and triplet energy difference, and wherein the difference of the singlet energy (E s1) and the triplet energy (E T1) is determined as the energy difference of the lowest T 1 -sublevel to the first spin-orbit corrected S 1 -state, 0.05 to 5.0 % by weight of a fluorescent emitter Y and 55 to 97.95 % by weight of a host compound(s), wherein the amount of the organometallic complex X, the fluorescent emitter Y and the host compound(s) adds up to a total of 100% by weight and the singlet energy of the luminescent organometallic complex X (E s1) is greater than the singlet energy of the fluorescent emitter Y (E s1); wherein the emissive lifetime τo, which is calculated by τo=τv/QY, of thin films consisting of the luminescent organometallic complex X (2 to 40 % by weight), fluorescent emitter Y (0.05 to 5.0 % by weight) and host compound(s) (55 to 97.95 % by weight) is in the range 0.1 to 80 ns, wherein the decay time of the emission is the emissive lifetime τo, the excited state lifetime of the emitting layer is τv, and QY is the quantum yield of the emitting layer; and wherein the luminescent organometallic complex X comprises a luminescent iridium complex or a luminescent copper complex. 2 to 40 % by weight of a luminescent organometallic complex X having a difference of the singlet energy (E s1) and the triplet energy (E T1) of smaller than 0.2 eV, wherein experimental measurements involving temperature dependent determination of excited state lifetimes and quantum chemical calculations are useable to calculate the singlet energy and triplet energy difference, and wherein the difference of the singlet energy (E s1) and the triplet energy (E T1) is determined as the energy difference of the lowest T 1 -sublevel to the first spin-orbit corrected S 1 -state, 0.05 to 5.0 % by weight of a fluorescent emitter Y and 55 to 97.95 % by weight of a host compound(s), wherein the amount of the organometallic complex X, the fluorescent emitter Y and the host compound(s) adds up to a total of 100% by weight and the singlet energy of the luminescent organometallic complex X (E s1) is greater than the singlet energy of the fluorescent emitter Y (E s1); wherein the emissive lifetime τo, which is calculated by τo=τv/QY, of thin films consisting of the luminescent organometallic complex X (2 to 40 % by weight), fluorescent emitter Y (0.05 to 5.0 % by weight) and host compound(s) (55 to 97.95 % by weight) is in the range 0.1 to 80 ns, wherein the decay time of the emission is the emissive lifetime τo, the excited state lifetime of the emitting layer is τv, and QY is the quantum yield of the emitting layer; and wherein the luminescent organometallic complex X comprises a luminescent iridium complex or a luminescent copper complex. wherein the emissive lifetime τo, which is calculated by τo=τv/QY, of thin films consisting of the luminescent organometallic complex X (2 to 40 % by weight), fluorescent emitter Y (0.05 to 5.0 % by weight) and host compound(s) (55 to 97.95 % by weight) is in the range 0.1 to 80 ns, wherein the decay time of the emission is the emissive lifetime τo, the excited state lifetime of the emitting layer is τv, and QY is the quantum yield of the emitting layer; and wherein the luminescent organometallic complex X comprises a luminescent iridium complex or a luminescent copper complex.