Thermoelectric devices are critical to multiple NASA missions for power conversion with radioisotope sources. At present, commercially available TE devices typically offer limited heat-to-electricity conversion efficiencies, well below the fundamental thermodynamic limit, calling for the development of higher efficiency materials. The team of MicroXact Inc. and Virginia Tech is proposing to develop a revolutionary high efficiency thermoelectric material fabricated on completely new fabrication principles. The proposed material and device will provide NASA with much needed highly efficient (ZT>1.6), macroscopically thick (from 100s of micrometers to over a millimeter) thermoelectric material that will permit >15% conversion efficiency of thermoelectric generation when using high grade space-qualified sources. The proposed material is comprised of PbTe/PbSe three-dimensional "wells" of PbTe/PbSe quantum dot superlattices (QDS) fabricated by a conformal coating of a structured silicon substrate with electrochemical Atomic Layer Deposition (eALD). In Phase I of the project the feasibility of the approach will be demonstrated by proving ZT>1.6. In Phase II the team will fabricate the thermoelectric generator, and will demonstrate conversion efficiencies exceeding 15%. After Phase II, MicroXact will commercialize the technology.