Valence Disproportionation of GeS in the PbS Matrix Forms Pb5Ge5S12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe > PbSe > PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge2+ substitution in Pb2+ and discordant off-center behavior; formation of Pb5Ge5S12 as stable second-phase inclusions through valence disproportionation of Ge2+ to Ge-0 and Ge4+. PbS and Pb5Ge5S12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb5Ge5S12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (kappa(lat)) of similar to 0.61 W m(-1) K-1. The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room-temperature electron mobility of similar to 121 cm(2) V-1 s(-1) for 3 x 10(19) cm(-3) carrier density and a ZT of 1.32 at 923 K. This is similar to 55% greater than the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZT(avg)) of similar to 0.76 from 400 to 923 K is the highest for PbS-based systems.

Automated summary

This study optimized the electrical and thermal transport properties of PbS-based compounds by alloying with GeS, leading to improved performance in converting waste heat into electricity. The addition of GeS triggered a complex cascade of beneficial events, resulting in increased power factor and electron mobility, as well as decreased lattice thermal conductivity. The highest performance was achieved with 14% GeS-alloyed samples, demonstrating a potential for significant global energy savings.