Emergence of Copper(I/II) Complexes as Third-Generation Redox Shuttles for Dye-Sensitized Solar Cells

Redox shuttles play a crucial role in the photovoltaic performance of dye-sensitized solar cells (DSSCs). A traditional iodide/ triiodide (I-/I3(-)) redox shuttle has been widely utilized in the first generation, but it suffers from intrinsic disadvantages such as a limited open-circuit voltage (V-oc). On the other hand, cobalt (II/III) complexes with polypyridyl ligands have been established as second-generation redox shuttles and have achieved a power conversion efficiency (eta) of up to 13%. Since 2016, copper(I/II) complexes have appeared as third-generation redox shuttles owing to their more positive redox potentials. The eta values of the copper-based DSSCs were low in the early stages, but to date, they have reached up to 13% as a result of modulating the molecular structures of the copper complexes and sensitizers. In this Perspective, recent developments in and prospects for copper-based DSSCs are highlighted, toward over 15% power conversion efficiency.