Disclosed is a thermoelectric generator including a heat source contact, a heat sink contact, and a plurality of co-axial fibers. Each of the co-axial fibers include a core and a cladding disposed about the core. The plurality of co-axial fibers extend from the heat source contact to the heat sink contact. Thermoelectric generators are disclosed including hollow core doped silicon carbide fibers and doubly clad PIN junction fibers. Methods for forming direct PN junctions between oppositely doped fibers are additionally disclosed.