Enhanced Thermoelectric Performance in Earth‐Abundant Bi₂S₃ by Band Structure Modification Using CuCl Doping

Abstract Bismuth sulfide has garnered considerable attention in recent years for thermoelectric applications because it comprises of earth‐abundant, low‐cost sulfur. However, it has a large bandgap causing low electrical conductivity compared to other chalcogenides, limiting its thermoelectric performance. In the present work, using a small concentration of CuCl doping, 9‐times ZT‐enhancement is demonstrated in Bi₂S₃ attaining a maximum ZT≈1.02 at 723 K. It is achieved primarily by improving electron transport behavior in Bi₂S₃ as evident from unprecedented 29‐times increase in electrical conductivity attained in CuCl doped Bi₂S₃. Using density funtional theory (DFT) calculation, it is shown that Cu occupying the interstitials in Bi₂S₃ indeed creates a mid‐gap state, and modifies its band structure by generating multiple valleys in conduction band minima. Hence, a one‐order of magnitude increase in electron concentration is observed in CuCl‐doped Bi₂S₃. Moreover, the presence of nano‐scale Cu‐rich region along with nano‐size grains in doped Bi₂S₃ as detected by high‐resolution transmission electron microscopy (HRTEM) facilitates enhanced phonon scattering leading to suppressed lattice thermal conductivity. A prototype of a 4‐legged thermoelectric power generator (TEG) has been fabricated demonstrating a 3 mW power output with a power density of 7500 mW m⁻², which potentially opens up a new avenue of making high‐performance TEG made of non‐toxic, low‐cost, earth‐abundant elements.