Revealing the Dual Role of Iodine Dopant in Cu-Based Argyrodites via Defect Chemistry

Driven by their ultralow lattice thermal conductivity and the prospect of a cost-effective, environmentally friendly design, argyrodites have emerged as highly promising candidates for thermoelectric energy conversion. While Ag-based argyrodites can exhibit both <i>n</i>- and <i>p</i>-type conductivity, Cu-based analogues are typically dominated by <i>p</i>-type charge carriers. Moreover, despite the crucial role of defect engineering in enhancing thermoelectric performance, there is still limited knowledge of effective doping strategies for these materials. In this work, we investigate aliovalent iodine substitution at the chalcogen sites in Cu-based argyrodites. Two doping scenarios were explored: a charge-balanced series Cu_8-<i>x</i>Si(S_0.5Se_0.5)_6-<i>x</i>I_<i>x</i> and a charge-nonbalanced series Cu₈Si(S_0.5Se_0.5)_6-<i>x</i>I_<i>x</i>. In both cases, iodine substitution increases the lattice parameters and promotes the formation of Cu₂Se-based precipitates. Rietveld refinement and theoretical calculations confirm that iodine preferentially occupies the <i>Q</i>3 (4<i>a</i>) anion site. In the charge-nonbalanced samples, doping inefficiencies result in the presence of both electron and hole carriers, leading to complex transport behavior. Conversely, in the charge-balanced samples, iodine substitution increases the hole concentration by creating Cu⁺ vacancies, which also modifies the Seebeck coefficient and enhances the power factor at elevated temperatures. As a result, iodine-doped Cu_7.9SiS_2.95Se_2.95I_0.1 achieves a high thermoelectric figure of merit (<i>ZT</i> ≈ 0.9 at 773 K), demonstrating strong potential for midtemperature thermoelectric power generation.