Triplet–triplet annihilation photon upconversion (TTA-UC) has emerged as a promising approach to enhance the photoconversion efficiency of solar-driven devices, such as solar cells and photocatalysts, by efficiently operating even under weak, incoherent incident light. Metal cluster-based sensitizers, owing to their molecule-like electronic states involving energetically split singlet and triplet states resulting from single-electron transitions, offer tunability of excited state (especially, triplet state) properties, making them highly attractive for TTA-UC applications. This review highlights recent advances in the utilization of ligand-protected metal clusters (LMCs) as triplet sensitizers, with a particular focus on key strategies such as linking of superatomic units, heteroatom substitution, and ligand engineering. Furthermore, we discuss how these design strategies influence intersystem crossing, triplet energy transfer, and overall TTA-UC performance. A more profound understanding of these factors is anticipated to facilitate the development of LMC-based TTA-UC systems, enabling their utilization across a wide range of applications.
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