Light-Emitting Device

A light-emitting device comprising:

• a first electrode over an insulating surface;
• a second electrode facing the first electrode;
• and
• an EL layer between the first electrode and the second electrode,
• wherein the EL layer includes a light-emitting layer, a hole-transport layer, a first electron-transport layer, and a second electron-transport layer,
• wherein the first electron-transport layer is between the first electrode and the second electron-transport layer,
• wherein the light-emitting layer is between the hole-transport layer and the first and the second electron-transport layers,
• wherein the light-emitting layer includes a first light-emitting substance and a second light-emitting substance,
• wherein the first light-emitting substance is a substance capable of converting triplet excitation energy into light emission,
• wherein the second light-emitting substance is a fluorescent substance,
• wherein a peak wavelength in an emission spectrum of the first light-emitting substance is shorter than a peak wavelength in an emission spectrum of the second light-emitting substance,
• wherein the first electron-transport layer includes a first organic compound,
• wherein the second electron-transport layer includes a second organic compound,
• wherein each of the first organic compound and the second organic compound independently has a π-electron deficient heteroaromatic ring,
• wherein a GSP_Slope (mV/nm) of a vapor deposited film of the second organic compound is larger than a GSP_Slope (mV/nm) of a vapor deposited film of the first organic compound, and
• wherein the GSP_Slope (mV/nm) is represented by ΔV/Δd, where ΔV (mV) is an amount of change in surface potential and Δd (nm) is an amount of change in thickness.

A light-emitting device comprising:

• a first electrode over an insulating surface;
• a second electrode facing the first electrode;
• and
• an EL layer between the first electrode and the second electrode,
• wherein the EL layer includes a light-emitting layer, a hole-transport layer, a first electron-transport layer, and a second electron-transport layer,
• wherein the first electron-transport layer is between the light-emitting layer and the second electron-transport layer,
• wherein the second electron-transport layer is between the first electron-transport layer and the second electrode,
• wherein the light-emitting layer is between the hole-transport layer and the first electron-transport layer,
• wherein the light-emitting layer includes a first light-emitting substance and a second light-emitting substance,
• wherein the first light-emitting substance is a substance capable of converting triplet excitation energy into light emission,
• wherein the second light-emitting substance is a fluorescent substance,
• wherein a peak wavelength in an emission spectrum of the first light-emitting substance is shorter than a peak wavelength in an emission spectrum of the second light-emitting substance,
• wherein the first electron-transport layer includes a first organic compound,
• wherein the second electron-transport layer includes a second organic compound and a first substance,
• wherein each of the first organic compound and the second organic compound independently has a π-electron deficient heteroaromatic ring,
• wherein a GSP_Slope (mV/nm) of a vapor deposited film of the second organic compound is larger than a GSP_Slope (mV/nm) of a vapor deposited film of the first organic compound, and
• wherein the GSP_Slope (mV/nm) is represented by ΔV/Δd, where ΔV (mV) is an amount of change in surface potential and Δd (nm) is an amount of change in thickness.