An integrated circuit device (200, 300) comprising: a transistor on a substrate (210, 310); and a capacitor structure electrically connected to the transistor, wherein the capacitor structure comprises: a first electrode including a first conductive material having a first work function; a dielectric layer (120, 263) on the first electrode, the dielectric layer (120, 263) including a first metal; a second electrode on the first electrode with the dielectric layer (120, 263) therebetween, the second electrode including a second conductive material having a second work function that is less than the first work function; and an interfacial layer (140-143, 150, 160, 267) between the dielectric layer (120, 263) and the second electrode, wherein the interfacial layer (140-143, 150, 160, 267) increases an electrical energy barrier between the second electrode and the dielectric layer (120, 263) relative to that of a direct interface therebetween, wherein the interfacial layer (140-143, 150, 160, 267) comprises an insulating interfacial layer including a second metal, and a valence of the second metal of the insulating interfacial layer is less than a valence of the first metal of the dielectric layer (120, 263), wherein a thickness of the insulating interfacial layer is about 5 Å or less in a vertical direction perpendicular to a surface of the second electrode facing the dielectric layer (120, 263), and the electrical energy barrier between the second electrode and the dielectric layer (120, 263) is substantially the same as an electrical energy barrier between the first electrode and the dielectric layer (120, 263). a transistor on a substrate (210, 310); and a capacitor structure electrically connected to the transistor, wherein the capacitor structure comprises: a first electrode including a first conductive material having a first work function; a dielectric layer (120, 263) on the first electrode, the dielectric layer (120, 263) including a first metal; a second electrode on the first electrode with the dielectric layer (120, 263) therebetween, the second electrode including a second conductive material having a second work function that is less than the first work function; and an interfacial layer (140-143, 150, 160, 267) between the dielectric layer (120, 263) and the second electrode, wherein the interfacial layer (140-143, 150, 160, 267) increases an electrical energy barrier between the second electrode and the dielectric layer (120, 263) relative to that of a direct interface therebetween, wherein the interfacial layer (140-143, 150, 160, 267) comprises an insulating interfacial layer including a second metal, and a valence of the second metal of the insulating interfacial layer is less than a valence of the first metal of the dielectric layer (120, 263), a first electrode including a first conductive material having a first work function; a dielectric layer (120, 263) on the first electrode, the dielectric layer (120, 263) including a first metal; a second electrode on the first electrode with the dielectric layer (120, 263) therebetween, the second electrode including a second conductive material having a second work function that is less than the first work function; and an interfacial layer (140-143, 150, 160, 267) between the dielectric layer (120, 263) and the second electrode, wherein the interfacial layer (140-143, 150, 160, 267) increases an electrical energy barrier between the second electrode and the dielectric layer (120, 263) relative to that of a direct interface therebetween, wherein the interfacial layer (140-143, 150, 160, 267) comprises an insulating interfacial layer including a second metal, and a valence of the second metal of the insulating interfacial layer is less than a valence of the first metal of the dielectric layer (120, 263),