Order-Disorder Transition in Kesterite Cu2ZnSnS4: Thermopower Enhancement via Electronic Band Structure Modification

The order-disorder transition of kesterite (CZTS, Cu2ZnSnS4) from I4¯ to I4¯ 2m crystal structures has a marked effect on Seebeck coefficient, which displays a sharp enhancement at the transition temperature, around 533 K. Considered to be detrimental for the performance of photovoltaic kesterite, the order-disorder transition appears to be beneficial for thermopower. Experimental data and ab initio calculations explain the origin of this enhancement: the increase of crystal symmetry in the disordered polymorph leads to a favorable electronic band structure characterized by flat and converged bands. At the transition, a sharp drop in mobility and increase in carrier concentration experimentally prove this mechanism of Seebeck enhancement. This, other than providing a new understanding of the material, can cast light on some profitable mechanisms to enhance the thermoelectric performance. Additionally, the increase in Seebeck provides an efficient tool to observe the transition and possibly to quantify disorder.