GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of $2.1 at 723 K in In and Bi codoped GeTe along with an extremely high TE conversion efficiency of $12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness ($200 H v , H v = kgf/mm 2 ) reveals excellent mechanical stability.

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Mixed ion–electron conductor Ag2Se is an n-type semiconductor owing to the intrinsic Se vacancies. By reducing Se vacancies, Ag2Se has been demonstrated as a potential environment-friendly thermoelectric material for near room temperature application. In the present work, Al addition was found to be highly beneficial for improving the thermoelectric properties of Ag2Se. In Ag1.95Al0.05Se, a large enhancement in the charge carrier mobility (1127 cm2/V s) is witnessed due to the formation of Se-rich Ag2Se1.02 phase with improved grain connectivity through in situ formed AgAl phase. The synergetic effect of low carrier concentration and enhanced mobility in Al doped samples resulted in a high Seebeck coefficient and high electrical conductivity, leading to a high power factor of 3039  μW/m K2 at 300 K. The figure-of-merit of Ag1.95Al0.05Se was found to be 1.1 at 300 K i.e., 57% higher than for pure Ag2Se. The uni-leg device fabricated using Ag1.95Al0.05Se with electroplated Ni/Ag contacts showed a conversion efficiency of ∼3.2% for a temperature difference of 93 K, i.e., comparable to the best reported value for conventional bismuth telluride.

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