Relationship between the density of states effective mass and carrier concentration of thermoelectric phosphide Ag6Ge10P12 with strong mechanical robustness

A ternary phosphide Ag6Ge10P12 containing no toxic elements has attracted much attention as an ecofriendly thermoelectric material. This study reveals the relationship between the density of states effective mass m(D)(OS)*, and carrier concentration n for achieving higher thermoelectric performance in Ag6Ge10P12. The Seebeck coefficient S of Ag6Ge10-xGaxP12 (0.0 <= x <= 0.25) with various carrier concentrations is unexpectedly improved by increasing n. Scrutinizing electrical transport properties, including the S and electrical conductivity sigma, and electronic structure indicated that the improved S is owing to the enhanced m(D)(OS)*, which originated from tuning the Fermi level in a valence band with multi-valley and pudding-mold bands. The power factor S-sigma(2) is enhanced by both improved S and sigma. The total thermal conductivity K-tot monotonically decreases with increasing x because of the decrease in the lattice thermal conductivity K-L. Combining the improved S-sigma(2) and reduced K-tot, the maximum ZT value of Ag6Ge9.875-Ga0.125P12 at 390 K reaches similar to 0.33 with the optimal carrier concentration n similar to 7.0 x 10(20) cm(-3). The present results demonstrate a guideline for enhancing the thermoelectric performance of Ag6Ge10P12 by breaking the trade-off relationship between the S and sigma through n. Moreover, Young's modulus E and nano-indentation hardness H of Ag6Ge10-xGaxP12 are greater than 125 GPa and 9 GPa, respectively, comparable to those of thermoelectric Si-Ge alloy. These findings and insights in the present study will serve as a basis for enhancing the thermoelectric performance and fabricating the thermoelectric module for ecofriendly phosphide Ag6Ge10P12. (C) 2022 The Authors. Published by Elsevier Ltd.

Automated summary

This study reveals the relationship between carrier concentration and thermoelectric performance in Ag6Ge10P12. The improvement of Seebeck coefficient is attributed to the enhanced effective mass and the tuning of Fermi level. The power factor is enhanced by both improved Seebeck coefficient and electrical conductivity. By optimizing carrier concentration, the maximum ZT value of Ag6Ge10P12 is achieved. This study provides a guideline for enhancing the thermoelectric performance of Ag6Ge10P12.