US Patent 10270041 Light-emitting element, light-emitting device, electronic device, and lighting device

A light-emitting element including a first electrode, a second electrode, and an EL layer positioned therebetween is provided. The EL layer includes a stack of a hole-injection layer, a first layer, a second layer, a third layer, and a fourth layer; the hole-injection layer includes an organic acceptor; the first layer, the second layer and the third layer include first, second and third hole-transport materials, respectively; and the fourth layer includes a host material and a light-emitting material; the HOMO level of the second hole-transport material is deeper than that of the first hole-transport material; the HOMO level of the host material is deeper than that of the second hole-transport material; the HOMO level of the third hole-transport material is deeper than or equal to that of the host material; and a HOMO level difference between the second and third hole-transport materials is less than or equal to 0.3 eV.