Order Parameter from the Seebeck Coefficient in Thermoelectric Kesterite Cu2ZnSnS4

Kesterite (CZTS, Cu2ZnSnS4) is a quaternary chalcogenide which structural complexity leads to polymorphism and numerous kinds of disorder in cation sites, with interesting effects on thermoelectric properties. Tetragonal kesterite faces an order-disorder phase transition from I-4 to I-42m crystal structures around 533 K, which causes a sharp increase in the Seebeck coefficient. The degree of order in the crystal structure determines the entity of this enhancement, locally influencing the steepness of the trend of thermopower in the transition region. The presence of secondary phases reduces the effect of the transition as well as causing a progressive reduction of the Seebeck coefficient. The present results show that a measurement of Seebeck coefficient provides direct and distinctive information on the degree of order and phase purity of kesterite samples, integrating results of Raman spectroscopy and X-ray diffraction.