Unveiling the Electronic Landscape in Lead-Free Silver (Copper)–Bismuth–Sulfide (Halide) Semiconductors: Insights into Charge Carrier Transport and Trapping Characteristics

Ag/Cu–Bi–S-based materials have recently garnered significant attention due to their good stability, low toxicity, and excellent potential optoelectronic properties. In this study, we systematically investigate the crystal structures, photophysical properties, and charge transport behaviors of four bismuth-based sulfochlorides (AgBiSCl2, CuBiSCl2, AgBiS2, and BiSCl) for their high-quality bulk polycrystals. Time-resolved microwave conductivity (TRMC) reveals the outstanding photoconductive characteristics of BiSCl and AgBiS2, which significantly outperform typical Bi-based halides such as Cs3Bi2I9 and Cs2AgBiBr6. In contrast, AgBiSCl2 and CuBiSCl2 exhibit low photoconductivity despite their dispersive band structures. Temperature-dependent TRMC reveals that BiSCl has distinct band conduction characteristics governed by phonon scattering, whereas AgBiS2 has trivial temperature dependence associated with impurity scattering. Furthermore, a complex photoconductivity analysis elucidates that both BiSCl and AgBiS2 include a larger contribution of free carriers than those of trapped carriers, which is beneficial for charge transport. Among the four compositions, BiSCl exhibits the highest photoconductivity of 2.01 × 10–1 cm2 V–1 s–1 followed by 2.75 × 10–2 cm2 V–1 s–1 for AgBiS2. This work presents the first comprehensive photoconductivity measurements and analyses of the different bismuth-based sulfur–chlorine semiconductors, providing mechanistic insight into their potential optoelectronic applications.