Abstract: An energy conversion device is described which efficiently converts thermal energy to electrical energy or electrical energy to thermal energy through the mechanism of a structure which includes a first element (cathode), usually a metal, a second material such as an insulator or a semiconductor providing a voltage barrier and a conduction band, and a third material (anode), usually a metal, where the voltage barrier between the first element and the second element is greater than the voltage barrier between the second element and the third element. In some metals, work function is a relative indicator of voltage barrier heights and may be used as a criterion for selection of metals for the first and third elements, once the second element has been chosen.