Illumination-induced deep trap state activation at the nanocluster/TiO2 interface: the origin of intrinsic photocapacitance in sensitized solar cells

Noble metal nanoclusters (NCs) in photoelectrochemical systems reveal novel functionalities. This study unveils that metal nanocluster-sensitized solar cells (MCSSCs) can intrinsically function as photocapacitors, storing charge in the dark. Illumination of Au₂₂(SG)₁₈ NC-sensitized TiO₂ activates deep surface trap states, forming an interfacial capacitance responsible for a persistent dark built-in potential. Open-circuit voltage decay experiments demonstrate a stable dark voltage post-illumination, a phenomenon absent in applied-bias voltage decay experiments, highlighting the crucial role of photogenerated holes within NCs in charging this capacitor. A proposed model features TiO₂ deep traps as the negative electrode, glutathione ligands as the dielectric, and the NC core (hosting holes) as the positive electrode. This intrinsic photocapacitive behavior, achieved without external storage components, is unprecedented in sensitized solar cells. These findings offer profound insights into NC/TiO₂ interfacial dynamics and suggest MCSSCs as candidates for integrated solar energy conversion and storage, paving the way for novel photocapacitor designs.