Exploring the doping effects of Ag in p-type PbSe compounds with enhanced thermoelectric performance

In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching process combined with a subsequent spark plasma sintering process, and systematically investigated the doping effects of Ag on the thermoelectric properties. Ag substitution in the Pb site does not introduce resonant levels near the valence band edge or detectable change in the density of state in the vicinity of the Fermi level, but moves the Fermi level down and increases the carrier concentration to a maximum value of similar to 4.7 x 10(19) cm(-3) which is still insufficient for heavily doped PbSe compounds. Nonetheless, the non-monotonic variation in carrier concentration with increasing Ag content indicates that Ag doping reaches the solution limit at similar to 1.0% and the excessive Ag presumably acts as donors in the materials. Moreover, the large energy gap of the PbSe-based material wipes off significant 'roll-over' in the Seebeck coefficient at elevated temperatures which gives rise to high power factors, being comparable to p-type Te analogues. Consequently, the maximum ZT reaches similar to 1.0 for the 1.5% Ag-doped samples with optimized carrier density, which is similar to 70% improvement in comparison with an undoped sample and also superior to the commercialized p-type PbTe materials.