Thiol-Free Synthesis of Bright Near-Infrared-Emitting Ag₂S Nanocrystals through Heterovalent-Metal Decoration for Ecofriendly Solar Cells

Environmentally friendly Ag 2 S nanocrystals (NCs) show potential applications as light-absorbing semiconductors for thin-film photovoltaics or biological taggants. However, thiol ligands are generally involved during the synthesis of Ag 2 S NCs to maintain colloidal stability, hindering postexchange of these crystal-bound thiolate ligands. Here, using common non-thiol ligands (oleic acid and oleylamine), we develop a general strategy of surface heterovalent-metal (Bi 3+, Ga 3+, In 3+, Sb 3+, and Zn 2+ ) decoration to synthesize colloidal cation-rich Ag 2 S NCs achieving a high photoluminescence quantum yield up to 22% at ∼1230 nm. Density functional theory calculations reveal the heterovalent-metal cations exhibit stronger orbital coupling to the ligands than Ag atoms, and thus stronger binding affinity, resulting in reduced dynamic surface ligands to produce compact cation-rich Ag 2 S NCs with high solubility in nonpolar solvents. We further show that surface-decorated Bi atoms can accumulate the electron charge density, leading to surface-related band-edge emissions with increased electron–hole separation and prolonged exciton lifetimes of the Bi-decorated Ag 2 S NCs, which are desirable for many optoelectronic conversion applications. Using a 40 nm-thick light-absorbing layer of Bi-decorated Ag 2 S NCs, the optimized solar cells show a short-circuit current density of 18.6 mA cm –2 and a power conversion efficiency of over 4%.