Yerbolat Magazov, Asset Aliyev, Nursaya Zhumabay, Zhamilya Taubaldiyeva, Guldana Zhigerbayeva, Nurxat Nuraje
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.