Theoretical and experimental investigations of the thermoelectric properties of Bi2S3

Electronic and transport properties of Bi₂S₃ with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi₂S₃ structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S².σ) from around 50 up to 500 μW K^-2 m^-1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K^-2 m^-1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl₃. The obtained results are plotted on a semilogarithmic log (σ) versus S graph to demonstrate that a very strong linear trend that limits the power factor around 500 μW K^-2 m^-1 exists. Further improvement of Bi₂S₃ as thermoelectric material will require finding new doping modes that will break through the observed trend. The results of stability tests demonstrate that properties of optimally doped Bi₂S₃ are stable.