Upconversion materials: a new frontier in solar water-splitting

Limited utilization of the solar spectrum is a major bottleneck in photocatalytic water-splitting, as most semiconductor photocatalysts only harness UV or visible light, leaving a large fraction of infrared photons unused. Upconversion materials have emerged as a promising solution by converting two or more low-energy photons into a single higher-energy photon, thereby extending the photoresponse of water-splitting systems. This review provides a technical overview of the two leading upconversion strategies for solar hydrogen generation: lanthanide (Ln)-based upconversion phosphors and triplet-triplet annihilation (TTA) upconversion systems, including purely organic and metal-organic approaches. We discuss how Ln-doped upconverters can enable near-infrared-driven photocatalysis, while highlighting their efficiency limitations under 1-sun illumination. We then examine TTA-based upconversion, which leverages molecular sensitizer-emitter pairs to achieve efficient upconversion under solar light intensities, and summarize recent demonstrations of TTA systems boosting H₂ production and even enabling overall water splitting under visible light. A comparative analysis of Ln-based vs. TTA-based systems is presented, underscoring their respective advantages (spectral range, stability, efficiency) and constraints. Finally, we outline future research directions and integration strategies aimed at combining the strengths of both upconversion approaches to maximize solar-to-hydrogen efficiency. The insights from this review suggest that upconversion materials can play a complementary and transformative role in next-generation solar water-splitting technologies.