Light-Emitting Element, Display Device, Electronic Device, and Lighting Device

Abstract

To provide a light-emitting element with high emission efficiency and low driving voltage. The light-emitting element includes a guest material and a host material. A HOMO level of the guest material is higher than a HOMO level of the host material. An energy difference between the LUMO level and a HOMO level of the guest material is larger than an energy difference between the LUMO level and a HOMO level of the host material. The guest material has a function of converting triplet excitation energy into light emission. An energy difference between the LUMO level of the host material and the HOMO level of the guest material is larger than or equal to energy of light emission of the guest material.

1 . A light-emitting element comprising:
 a pair of electrodes; and   a layer between the pair of electrodes, the layer comprising a guest material and a host material,   wherein the guest material is capable of converting triplet excitation energy into light emission,   wherein a HOMO level of the guest material is higher than a HOMO level of the host material,   wherein the guest material has an emission peak in a red wavelength range in an emission spectrum, and   wherein an energy difference between a LUMO level of the host material and the HOMO level of the guest material is larger than or equal to transition energy calculated from an absorption edge of an absorption spectrum of the guest material.   
     
     
         2 . The light-emitting element according to  claim 1 ,
 wherein an energy difference between a LUMO level of the guest material and the HOMO level of the guest material is larger than the transition energy calculated from the absorption edge of the absorption spectrum of the guest material by 0.3 eV to 0.8 eV.   
     
     
         3 . The light-emitting element according to  claim 1 , wherein the host material has a difference between a singlet excitation energy level and a triplet excitation energy level of larger than 0 eV and smaller than or equal to 0.2 eV. 
     
     
         4 . The light-emitting element according to  claim 1 , wherein the host material is capable of exhibiting thermally activated delayed fluorescence at room temperature. 
     
     
         5 . The light-emitting element according to  claim 1 , wherein the host material is capable of supplying excitation energy to the guest material. 
     
     
         6 . The light-emitting element according to  claim 1 , wherein an emission spectrum of the host material comprises a wavelength region overlapping with an absorption band on lowest energy side in the absorption spectrum of the guest material. 
     
     
         7 . The light-emitting element according to  claim 1 , wherein the guest material comprises iridium. 
     
     
         8 . The light-emitting element according to  claim 1 , wherein the guest material is an organometallic complex having an isoquinoline ligand. 
     
     
         9 . The light-emitting element according to  claim 1 , wherein the host material is capable of transporting an electron and a hole. 
     
     
         10 . The light-emitting element according to  claim 1 ,
 wherein the host material comprises a diazine skeleton such as a pyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton and a pyrrole skeleton such as an indole skeleton, a carbazole skeleton, or a 9-phenyl-3,3′-bi-9H-carbazole skeleton.   
     
     
         11 . The light-emitting element according to  claim 10 ,
 wherein the diazine skeleton is directly bonded to the pyrrole skeleton.   
     
     
         12 . The light-emitting element according to  claim 1 ,
 wherein the red wavelength range is greater than or equal to 580 nm and less than or equal to 680 nm, and   wherein the guest material has at least one peak in the emission spectrum.   
     
     
         13 . The light-emitting element according to  claim 1 , wherein an energy difference between the HOMO level of the guest material and the HOMO level of the host material is greater than or equal to 0.05 eV and less than or equal to 0.4 eV. 
     
     
         14 . The light-emitting element according to  claim 1 , wherein an energy difference between the LUMO level of the guest material and the LUMO level of the host material is greater than or equal to 0.2 eV. 
     
     
         15 . A light-emitting element comprising:
 a pair of electrodes; and   a layer between the pair of electrodes, the layer comprising a guest material and a host material,   wherein the guest material is capable of converting triplet excitation energy into light emission,   wherein a HOMO level of the guest material is higher than a HOMO level of the host material,   wherein the guest material has an emission peak in a red wavelength range in an emission spectrum, and   wherein an energy difference between a LUMO level of the host material and the HOMO level of the guest material is larger than or equal to light emission energy of the guest material.   
     
     
         16 . The light-emitting element according to  claim 15 , wherein an energy difference between a LUMO level of the guest material and the HOMO level of the guest material is larger than a transition energy calculated from an absorption edge of an absorption spectrum of the guest material by 0.3 eV to 0.8 eV. 
     
     
         17 . The light-emitting element according to  claim 15 , wherein an energy difference between a LUMO level of the guest material and the HOMO level of the guest material is larger than the light emission energy of the guest material by 0.3 eV to 0.8 eV. 
     
     
         18 . The light-emitting element according to  claim 15 , wherein the host material has a difference between a singlet excitation energy level and a triplet excitation energy level of larger than 0 eV and smaller than or equal to 0.2 eV. 
     
     
         19 . The light-emitting element according to  claim 15 , wherein the host material is capable of exhibiting thermally activated delayed fluorescence at room temperature. 
     
     
         20 . The light-emitting element according to  claim 15 , wherein the host material is capable of supplying excitation energy to the guest material. 
     
     
         21 . The light-emitting element according to  claim 15 , wherein an emission spectrum of the host material comprises a wavelength region overlapping with an absorption band on lowest energy side in the absorption spectrum of the guest material. 
     
     
         22 . The light-emitting element according to  claim 15 , wherein the guest material comprises iridium. 
     
     
         23 . The light-emitting element according to  claim 15 , wherein the guest material is an organometallic complex having an isoquinoline ligand. 
     
     
         24 . The light-emitting element according to  claim 15 , wherein the host material is capable of transporting an electron and a hole. 
     
     
         25 . The light-emitting element according to  claim 15 ,
 wherein the host material comprises a diazine skeleton such as a pyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton and a pyrrole skeleton such as an indole skeleton, a carbazole skeleton, or a 9-phenyl-3,3′-bi-9H-carbazole skeleton.   
     
     
         26 . The light-emitting element according to  claim 25 ,
 wherein the diazine skeleton is directly bonded to the pyrrole skeleton.   
     
     
         27 . The light-emitting element according to  claim 15 ,
 wherein the red wavelength range is greater than or equal to 580 nm and less than or equal to 680 nm, and   wherein the guest material has at least one peak in the emission spectrum.   
     
     
         28 . The light-emitting element according to  claim 15 , wherein an energy difference between the HOMO level of the guest material and the HOMO level of the host material is greater than or equal to 0.05 eV and less than or equal to 0.4 eV. 
     
     
         29 . The light-emitting element according to  claim 15 , wherein an energy difference between the LUMO level of the guest material and the LUMO level of the host material is greater than or equal to 0.2 eV.