To enhance the properties of bulk thermoelectric (TE) materials, preparation methods have been intensively developed with advanced techniques, e.g., zone-melting, Bridgman or Czochralski unidirectional methods, hot pressing or spark plasma sintering, that follow various complicated synthesis steps. However, these innovative methods remain less competitive than conventional techniques for large-scale production and application. Here, we report that, by manipulating the liquid state of the Bi2Te2.7Se0.3 alloy doped with KI, n-type bulk specimens with desired TE performance can be simply solidified by air cooling and without costly equipments. The specimens solidified from the parent melt experienced temperature-induced liquid-liquid structural transition, which is indicated by the resistivity behavior of the liquid, the presence of a refined matrix and eutectics, and the observation of more nanoparticles and a higher density of lattice defects. It was confirmed these refined multiscale refined structures lead to a "phonon glass electron crystal" effect, i.e., a much lower lattice thermal conductivity but a much higher power factor (PF). The present method, which synergistically obtains higher PF and lower thermal conductivity, could also be applied to other TE compounds.