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 Ge0 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 (κlat) of ∼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 ∼121 cm2 V-1 s-1 for 3 × 1019 cm-3 carrier density and a ZT of 1.32 at 923 K. This is ∼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 (ZTavg) of ∼0.76 from 400 to 923 K is the highest for PbS-based systems. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This study was supported primarily by the Department of Energy, Office of Science Basic Energy Sciences under grant DE-SC0014520, the DOE Office of Science (sample preparation, synthesis, XRD, TE measurements, TEM measurements, DFT calculations), and the Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ127). The authors acknowledge the Minjiang Scholar Professorship (GXRC-21004), the National Natural Science Foundation of China (52102218 and 61728401), the National Key Research and Development Program/Key Scientific Issues of Transformative Technology (2020YFA0710303), the EPIC facility of Northwestern University’s NUANCE Center, which received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the International Institute for Nanotechnology (IIN), Keck Foundation, State of Illinois, through IIN, and the Office of Science of the U.S. Department of Energy under Contract Nos. DE-AC02-06CH11357 and DE-AC02-05CH11231. The authors further acknowledge access to facilities for high performance computational resources at Northwestern University and Singapore MOE AcRF Tier 2 under Grant No. 2018-T2-1-010, Singapore A*STAR project A19D9a0096, and the support from FACTs of Nanyang Technological University for sample analysis.