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Photon upconversion, the process of converting low-energy photons to higher energy ones, shows promise for applications in solar energy, photocatalysis, biomedicine, and additive manufacturing. In triplet-triplet annihilation (TTA), incident low-energy photons populate metastable spin-triplet states that annihilate to generate high-energy emissive spin-singlet states. Thus, TTA-based photon upconversion (TTA-UC) can operate efficiently under incoherent and low-intensity excitation, such as sunlight. In this Review, we discuss the recent emergence of halide perovskite-based materials as potent triplet sensitizers for a variety of applications. Due to their strong and tunable absorption and high defect tolerance, perovskite materials ranging from nanocrystalline to bulk semiconductors enable efficient TTA-UC in both solution and solid-state systems. After introducing the TTA-UC process and giving a brief overview of its beginnings, we first consider TTA-UC systems based on perovskite nanocrystals and low-dimensional perovskite-inspired materials and the achievements that have been made in those areas. We then focus on the mechanism of bulk perovskite-sensitized TTA-UC, the impact the underlying structure holds, and review the current challenges in perovskite-sensitized solid-state UC and outline future research directions to unlock the full potential of TTA-UC in practical applications.
VanOrman et al. (Tue,) studied this question.