Enhancing the thermoelectric performance of Bi₂S₃: A promising earth-abundant thermoelectric material

Recently, bismuth sulfide (Bi₂S₃) has attracted much attention in the thermoelectric community owing to its abundance, low cost, and advanced properties. However, its poor electrical transport properties have prevented Bi₂S₃ devices from realizing high thermoelectric performance. In this work, our motivation is to decrease the large electrical resistivity, which is recognized as the origin of the low ZT value in undoped Bi₂S₃. We combined melting and spark plasma sintering (SPS) in a continuous fabrication process to produce Bi₂S_3-xSe_x (x = 0, 0.09, 0.15, 0.21) and Bi₂S_2.85-ySe_0.15Cl_y (y = 0.0015, 0.0045, 0.0075, 0.015, 0.03) samples. Our results show that Se alloying at S sites can narrow the band gap and activate intrinsic electron conduction, leading to a high power factor of ~ 2.0 μW·cm^-1·K^-2 cat room temperature in Bi₂S_2.85S_0.15, about 100 times higher than that of undoped Bi₂S₃. Moreover, our further introduction of Cl atoms into the S sites resulted in a second-stage optimization of carrier concentration and simultaneously reduced the lattice thermal conductivity, which contributed to a high ZT value of ~ 0.6 at 723 K for Bi₂S_2.835Se_0.15Cl_0.015. Our results indicate that high thermoelectric performance could be realized in Bi₂S₃ with earth-abundant and low-cost elements.