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 Bi 2 S 3 attaining a maximum ZT ≈1.02 at 723 K. It is achieved primarily by improving electron transport behavior in Bi 2 S 3 as evident from unprecedented 29‐times increase in electrical conductivity attained in CuCl doped Bi 2 S 3 . Using density funtional theory (DFT) calculation, it is shown that Cu occupying the interstitials in Bi 2 S 3 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 Bi 2 S 3 . Moreover, the presence of nano‐scale Cu‐rich region along with nano‐size grains in doped Bi 2 S 3 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.