Abstract
Copper and cobalt coordination complexes have attracted growing attention as sustainable alternatives to two key molecular components of dye-sensitized solar cells (DSSCs), namely the photosensitizer and the redox mediator. However, no review has critically examined both metals within a unified mechanistic framework that consistently distinguishes their distinct device roles. Although ruthenium-based dyes and iodide/triiodide electrolytes once dominated the field, their high cost, scarcity and corrosive nature have motivated the exploration of first-row transition metals. In this context, copper and cobalt complexes have emerged as the most promising alternatives, as sensitizers and as redox mediators. Copper complexes demonstrate how ligand design directly governs performance through steric and geometric control. Homoleptic copper(I) complexes often suffer from flattening distortions, short excited-state lifetimes and narrow absorption ranges, which restrict their power conversion efficiencies to below 2%. In contrast, heteroleptic copper(I), incorporating anchoring and ancillary ligands, enables steric protection of the copper(I) coordination geometry, suppresses recombination and optimizes HOMO−LUMO alignment for efficient electron injection and dye regeneration. As sensitizers, heteroleptic copper(I) complexes achieve power conversion efficiencies up to ∼4.7% [1], while as redox mediators copper (I/II) phenanthroline couples have enabled device efficiencies exceeding 7% when paired with organic dyes [2,3], positioning heteroleptic frameworks as the most competitive copper-based components in DSSC architectures. In parallel, cobalt complexes have become the leading redox mediators, where polypyridyl frameworks provide tunable redox potentials, reduced corrosives and open-circuit voltages above 1.0 V when paired with fast-push-pull organic sensitizers [4]. This review examines how steric effects, geometry, and ligand asymmetry influence the success of copper and cobalt complexes, and explains why heteroleptic systems are favoured over homoleptic ones. Future prospects include stabilizing copper(II) motifs for dual roles, improving cobalt ligand frameworks to reduce recombination, and combining computational design with device engineering.
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