An integrated circuit device comprising: a transistor on a substrate ; and a capacitor structure (100-105) electrically connected to the transistor, wherein the capacitor structure (100-105) comprises: a first electrode including a first conductive material having a first work function; a dielectric layer on the first electrode , the dielectric layer including a first metal; a second electrode on the first electrode with the dielectric layer 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 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 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 , 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 , and the electrical energy barrier between the second electrode and the dielectric layer is substantially the same as an electrical energy barrier between the first electrode and the dielectric layer . a transistor on a substrate ; and a capacitor structure (100-105) electrically connected to the transistor, wherein the capacitor structure (100-105) comprises: a first electrode including a first conductive material having a first work function; a dielectric layer on the first electrode , the dielectric layer including a first metal; a second electrode on the first electrode with the dielectric layer 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 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 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 , a first electrode including a first conductive material having a first work function; a dielectric layer on the first electrode , the dielectric layer including a first metal; a second electrode on the first electrode with the dielectric layer 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 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 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 , <EOS>