Band-Structure Modification Leads to Enhanced Thermoelectric Performance of Bi₂S₃ by CaCl₂ Doping

Among various chalcogenides, bismuth sulfide has garnered considerable attention in recent years for thermoelectric applications because it comprises earth-abundant, low-cost sulfur. However, it has low electrical conductivity compared with other chalcogenides, limiting its thermoelectric performance. Here, using a small concentration of CaCl2 doping, we have demonstrated ∼3 times enhancement of figure of merit (ZT) in Bi2S3. Such a surge in ZT is achieved primarily by improving the electron transport behavior in Bi2S3. Due to the incorporation of CaCl2 in Bi2S3, a 2 orders of magnitude increase in electron concentration is observed. Further, the electron mobility is also found to be increased in CaCl2-doped Bi2S3. As a result, an unprecedented ∼370 times increase in electrical conductivity and a 5 times rise in power factor are attained in CaCl2-doped Bi2S3. DFT calculation implies that CaCl2 incorporation indeed modifies the band structure of Bi2S3, creating multiple valley degeneracies in the conduction band and paving the way for the smoother electron transport in environmentally benign n-type doped Bi2S3.