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

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.