Thermoelectric Properties of the Chalcopyrite Solid Solutions ZnGe_1–xSn_xP₂

To investigate whether environmentally benign chalcopyrite phosphides may be used in the thermoelectric energy conversion, the solid solution ZnGe 1– x Sn x P 2 was prepared by ball-milling, followed by hot-pressing with x = 0, 0.25, 0.5, 0.75, and 1. Despite the comparably light constituent elements and the simple crystal structure, the room temperature thermal conductivity went below 3 W m –1 K –1 in the middle of the series ( x = 0.5), where the phonon scattering caused by the alloying effect was maximized. Band structure engineering can be employed to enhance the band degeneracies either at the top of the valence band or at the bottom of the conduction band by varying x (the Sn content). Boltztrap calculations revealed that high power factor values can be achieved on either side of the Fermi level. Combining these results assuming a constant relaxation time with the experimental thermal conductivity data confirmed that zT values in excess of unity could theoretically be obtained at 900 K for all members at different doping levels; for p-doping, the highest zT of 1.7 was predicted to for ZnGe 0.5 Sn 0.5 P 2, and for n-doping, a peak zT of 1.4 was predicted for ZnGeP 2 . It remains to be seen whether these doping levels can be experimentally obtained.