Light-emitting element, light-emitting device, display device, electronic device, and lighting device

摘要

An object is to provide a light-emitting element which uses a plurality of kinds of light-emitting dopants and has high emission efficiency. In one embodiment of the present invention, a light-emitting device, a light-emitting module, a light-emitting display device, an electronic device, and a lighting device each having reduced power consumption are provided by using the above light-emitting element. Focus is placed on Förster mechanism, which is one of mechanisms of intermolecular energy transfer. Efficient energy transfer by Förster mechanism is achieved by employing a combination of molecules which makes it possible to obtain an overlap between an emission spectrum band of the molecule which donates energy and the longest-wavelength-side peak of a characteristic curve obtained by multiplying an absorption spectrum of the molecule which receives energy by a wavelength raised to the fourth power.

主权项

1. A light-emitting element comprising:
a first electrode; a first light-emitting layer over the first electrode comprising:
a first phosphorescent compound; anda first host material; a second light-emitting layer over the first light-emitting layer comprising:
a second phosphorescent compound; anda second host material; a third light-emitting layer over the second light-emitting layer comprising:
a third phosphorescent compound; anda third host material; and a second electrode over the third light-emitting layer, wherein light emitted from the second phosphorescent compound has a longer wavelength than light emitted from the first phosphorescent compound, wherein a longest-wavelength-side peak of a function ε(λ)λ4 of the second phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the first phosphorescent compound, wherein light emitted from the third phosphorescent compound has a longer wavelength than the light emitted from the second phosphorescent compound, wherein a longest-wavelength-side peak of a function ε(λ)λ4 of the third phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the second phosphorescent compound, wherein the second light-emitting layer has a thickness of less than 5 nm and greater than or equal to 1 nm, wherein λ denotes a wavelength, and wherein ε(λ) denotes a molar absorption coefficient at the wavelength λ.