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 μ_H ∼ 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 Pb_0.9875Ga_0.0125S, 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 Pb_0.9865Ga_0.0125In_0.001S, the maximum ZT rises to ∼1.0 at 923 K and achieves a record-high average ZT (ZT_avg) of ∼0.74 in the temperature range of 400-923 K.