Defect-induced optical and thermoelectric properties of cobalt doped ZnO nanostructures prepared through hydrothermal route

In this paper, we studied the thermoelectric properties of hydrothermally produced cobalt (Co) doped zinc oxide (ZnO) nanoparticles with varying Co concentrations by correlating with optical defects. X-ray diffraction results verify the generation of ZnO with a well-defined hexagonal structure. whereas the existence of Zn, Co, and O elements is verified through energy-dispersive X-ray spectroscopy for all under-inspected samples. Photoluminescence spectroscopy reveals a strong emission peak around 380 nm in the ultraviolet region and a second peak at ~ 506 nm which exhibits a broad emission in the green band ascribed to oxygen vacancies. As the doping concentration increases, the UV near-band emissions are observed to be red-shifted with decreased PL intensity in comparison to pure ZnO. We found that electric conductivity and Seebeck coefficient are increased by increasing the cobalt concentration in ZnO. Our findings reveal that incorporating Co into ZnO enhances the Seebeck coefficient to an extreme value of ~ 160 µV/K and ZT of 0.1, which is directly associated with thermoelectric proficiency and oxygen vacancies are found to be primary factor for this enhancement consistent with photoluminescence analysis. The study, on the one hand, depicted that the incorporation of defects represents a good opportunity for improving thermoelectric properties and on the other hand could be useful in the context of atomic scale defect engineering for improving the performance of thermoelectric materials.