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

Abstract

A light-emitting element which includes a plurality of light-emitting layers between a pair of electrodes and has low driving voltage and high emission efficiency is provided. A light-emitting element including first to third light-emitting layers between a cathode and an anode is provided. The first light-emitting layer includes a first phosphorescent material and a first electron-transport material; the second light-emitting layer includes a second phosphorescent material and a second electron-transport material; the third light-emitting layer includes a fluorescent material and a third electron-transport material; the first to third light-emitting elements are provided in contact with an electron-transport layer positioned on a cathode side; and a triplet excitation energy level of a material included in the electron-transport layer is lower than triplet excitation energy levels of the first electron-transport material and the second electron-transport material.

What is claimed is: 
     
         1 . A light-emitting element comprising:
 a cathode;   an anode; and   a first and a second light-emitting layer between the cathode and the anode,   wherein the first light-emitting layer comprises a first phosphorescent material and a first electron-transport material,   wherein the second light-emitting layer comprises a fluorescent material and a second electron-transport material,   wherein the first and the second light-emitting layers are each in contact with an electron-transport layer positioned on a cathode side,   wherein the electron-transport layer comprises the fluorescent material and the second electron-transport material.   
     
     
         2 . The light-emitting element according to  claim 1 ,
 wherein a triplet excitation energy of the second electron-transport material is lower than a triplet excitation energy of the first electron-transport material.   
     
     
         3 . The light-emitting element according to  claim 1 , further comprising a third light-emitting layer between the cathode and the anode,
 wherein the third light-emitting layer comprises a second phosphorescent material and a third electron-transport material,   wherein the third light-emitting layer is in contact with the electron-transport layer positioned on the cathode side, and   wherein the triplet excitation energy of the second electron-transport material included in the electron-transport layer is lower than a triplet excitation energy of the third electron-transport material.   
     
     
         4 . The light-emitting element according to  claim 1 ,
 wherein the first light-emitting layer further comprises a first hole-transport material, and   wherein the first electron-transport material and the first hole-transport material form an exciplex.   
     
     
         5 . The light-emitting element according to  claim 3 ,
 wherein the third light-emitting layer further comprises a second hole-transport material, and   wherein the third electron-transport material and the second hole-transport material form an exciplex.   
     
     
         6 . The light-emitting element according to  claim 1 , wherein the first phosphorescent material is an organometallic complex containing iridium. 
     
     
         7 . The light-emitting element according to  claim 3 , wherein the second phosphorescent material is an organometallic complex containing iridium. 
     
     
         8 . The light-emitting element according to  claim 1 ,
 wherein the first light-emitting layer emits red or green light, and   wherein the second light-emitting layer emits blue light.   
     
     
         9 . The light-emitting element according to  claim 1 , further comprising:
 a first hole-transport layer between the first light-emitting layer and the anode; and   a second hole-transport layer between the second light-emitting layer and the anode,   wherein a thickness of the first hole-transport layer is greater than a thickness of the second hole-transport layer.   
     
     
         10 . The light-emitting element according to  claim 9 ,
 wherein the first hole-transport layer in contact with the first light-emitting layer, and   wherein the second hole-transport layer in contact with the second light-emitting layer.   
     
     
         11 . The light-emitting element according to  claim 9 ,
 wherein the first hole-transport layer is over the second hole-transport layer.   
     
     
         12 . The light-emitting element according to  claim 1   wherein an electron- transport property of the first electron-transport material is higher than an electron-transport property of the third electron-transport material.   
     
     
         13 . A light-emitting device comprising the light-emitting element according to  claim 1 . 
     
     
         14 . An electronic appliance comprising the light-emitting device according to  claim 13 . 
     
     
         15 . A lighting device comprising the light-emitting device according to  claim 13 .