Moreover, as illustrated in FIG. 3(b), an energy difference (a bandgap) between a lower end of a conduction band and an upper end of a valence band in the oxide layer 124b is larger than an energy difference (a bandgap) between a lower end of an HTL conduction band′ and an upper end of an HTL valence band′ in the oxide layer 124a. Hence, the oxide layer 124b is lower in carrier density and higher in insulation than the oxide layer 124a. Hence, in the oxide layer 124b, the holes are transported by tunneling. As can be seen, the density of the holes in the oxide layer 124a serving as a hole-transport layer is higher than the density of the holes in the oxide layer 124b. The holes are injected into the light-emitting layer 24c of the first wavelength range by tunneling through the oxide layer 124b.
Note that, in FIG. 3(b), described as an example is only the light-emitting element 5R including the light-emitting layer 24c of the first wavelength range. When the oxide layer 124b is formed also in the light-emitting element 5G including the light-emitting layer 24c′ of the second wavelength and in the light-emitting element 5B including the light-emitting layer 24c″ of the third wavelength range, the holes can be injected efficiently as seen in the light-emitting element 5R including the light-emitting layer 24c of the first wavelength range.
