Band structure engineering and near-infrared optical properties of Cu-Ag-Ge-S quantum dots for photoelectrochemical solar energy conversion.

I-IV-VI semiconductor quantum dots (QDs) have attracted increasing attention owing to their tunable optoelectronic properties and low toxicity. Herein, Cu-Ag-Ge-S QDs were synthesized via a facile solution-phase method, enabling systematic tuning of both the bandgap energy and localized surface plasmon resonance (LSPR) through compositional control. Ag incorporation into the Cu-based lattice induced lattice expansion and suppressed Cu-related defects, resulting in significant modulation of the electronic structure. With increasing Ag content, the optical absorption edge and incident photon-to-current efficiency (IPCE) spectra exhibited pronounced red shifts, extending the photoresponse to approximately 970 nm. Photoelectrochemical measurements further revealed a transition from p-type to ambipolar behavior accompanied by shifts in photocurrent onset potential, consistent with the evolution of the electronic band structure. These results demonstrated that Ag incorporation effectively tailored the charge transport and photoresponse characteristics of the QDs. Furthermore, quantum dot-sensitized solar cells (QDSSCs) fabricated with Cu-Ag-Ge-S QDs of higher Cu content exhibited improved performance, in which the QDs prepared with Cu/(Cu + Ag) = 1.0 exhibited an optimal short-circuit current density (Jsc) of 4.9 mA cm-2 and an optimal power conversion efficiency (PCE) of 1.06%. This enhancement was attributed to the change in the electronic energy structure of the QDs with the Cu content. This work demonstrates the potential of composition-controlled Cu-Ag-Ge-S QDs as low-toxicity, Earth-abundant materials for near-infrared optoelectronic and solar energy conversion applications.