Abstract
Bulk samples of kesterite (Cu2ZnSnS4, CZTS) were produced by cold-pressing and sintering of CZTS powders obtained via reactive ball-milling. An increase in the Seebeck coefficient of more than 100 μV/K, almost doubling the expected value, is noticed around a temperature of 260 °C. As pointed out by thermal analyses, this is due to a second order transition of kesterite from an ordered I-4 to a disordered I-42m crystal structure. Conversely to what happens for solar cell materials, where the transition is considered to be detrimental for the performance, it appears to be beneficial for the thermoelectric Seebeck coefficient, suggesting that higher crystal symmetry and cation-disorder due to the transition lead to thermopower enhancement.
Highlights
Thermoelectric materials can operate the conversion of a heat gradient into a voltage drop through the Seebeck effect [1,2]
To what happens for solar cell materials, where the transition is considered to be detrimental for the performance, it appears to be beneficial for the thermoelectric Seebeck coefficient, suggesting that higher crystal symmetry and cation-disorder due to the transition lead to thermopower enhancement
In this work we show how to effectively observe and study the order-disorder transition with the measurement of Seebeck coefficient, and we confirm the results with the aid of accurate thermal analyses
Summary
Thermoelectric materials can operate the conversion of a heat gradient into a voltage drop through the Seebeck effect [1,2]. The reported values of figure of merit zT for kesterite remain quite low, ranging from 4.6 × 10−5 at 300 K to 0.026 at 700 K [7] For this reason several studies have focused on doping to increase the electrical conductivity [6,7], without considering, until now, the importance of structural aspects and the role of phase transformations. Cu and Zn are randomly distributed in the planes, leading to a transition to a I-42m crystal structure where both cations occupy 4d Wyckoff positions. This order-disorder transition has been linked to the low formation energy of the Cu-Zn antisite defects [16]. In this work we show how to effectively observe and study the order-disorder transition with the measurement of Seebeck coefficient, and we confirm the results with the aid of accurate thermal analyses
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