Enhancement of Thermoelectric Performance for n‑Type PbS through Synergy of Gap State and Fermi Level Pinning

We report that Ga-doped and Ga–In-codoped n-type PbS samples show excellent thermoelectric performance in the intermediate temperature range. First-principles electronic structure calculations reveal that Ga doping can cause Fermi level pinning in PbS by introducing a gap state between the conduction and valence bands. Furthermore, Ga–In codoping introduces an extra conduction band. These added electronic features lead to high electron mobilities up to μH ∼ 630 cm2 V–1 s–1 for n of 1.67 × 1019 cm–3 and significantly enhanced Seebeck coefficients in PbS. Consequently, we obtained a maximum power factor of ∼32 μW cm–1 K–2 at 300 K for Pb0.9875Ga0.0125S, which is the highest reported for PbS-based systems giving a room-temperature figure of merit, ZT, of ∼0.35 and ∼0.82 at 923 K. For the codoped Pb0.9865Ga0.0125In0.001S, the maximum ZT rises to ∼1.0 at 923 K and achieves a record-high average ZT (ZTavg) of ∼0.74 in the temperature range of 400–923 K.