Energy Flow in Hybrid Organic/Inorganic Systems for Triplet-Triplet Annihilation Upconversion

Photon upconversion via triplet-triplet annihilation (TTAUC) converts two low-energy triplet excitons into high-energy photon emission from a singlet. Hybrid TTAUC systems integrating inorganic sensitizers and molecular annihilators provide an effective architecture for NIR-to-visible upconversion with low input flux. Size or chemical-tunable band alignment and rapid flip of spins make them versatile for device engineering. However, thus far, the mechanisms of fundamental energy-transfer steps have been mostly investigated in solutionphase systems. There still lacks thorough understanding of energy upconversion in solid-state architectures, where additional challenges arise from the need to transport excitons within or across the heteromaterial layers. In this Review, we first present an overview of the theoretical basis of the essential energy flow processes in hybrid organic/inorganic TTAUC systems and comment on the bottleneck processes. We then discuss the mechanistic details of fundamental steps (including spin flip, triplet energy transfer, and triplet annihilation process) in various scenarios. We further focus on the competition, interplay, and synergistic effects of singlet fission, singlet energy back transfer, annihilation, and exciton transport on the overall performance. We highlight the fundamental challenge to be addressed for solid-state systems, which is the complicated picture of energy flow dynamics proceeding at multiple time and length scales. Based on these discussions, guidance is provided for designing TTAUC energy flow when interfacing hybrid materials with organic annihilators.