Influence of Molecular Orientation and Exciton Transfer Rates on Solid-State Triplet–Triplet Annihilation Photon Upconversion

Photon upconversion via triplet–triplet annihilation (TTA-UC) has garnered significant attention due to its potential to enhance the performance of optoelectronic devices. For practical applications, achieving high TTA-UC efficiency in solid-state systems is particularly crucial yet remains a significant challenge due to complex spin-dependent processes. A central step in TTA-UC is the fusion of two triplet excitons into a singlet exciton, which requires the pair to transiently adopt a singlet spin configuration. Crucially, this spin configuration is not static but evolves dynamically during exciton migration. Accordingly, both the mutual molecular orientation and the exciton transfer rate emerge as key parameters that can be strategically tuned to optimize spin-state interconversion and, consequently, the TTA-UC efficiency. In this study, we take this dynamic perspective as a starting point and develop a theoretical framework to systematically explore how molecular orientation and exciton transfer rate together modulate TTA-UC efficiency across different exchange-coupling regimes. Our results demonstrate that parallel molecular alignment facilitates fusion under weak exchange coupling, while slower exciton transfer enhances the upconversion efficiency by providing sufficient time for spin-state interconversion. These insights offer a new direction for the rational design of solid-state upconversion materials, shifting the focus from static molecular packing alone to dynamically accessible transport and spin-interaction pathways.