Photochemical upconversion is a process whereby two lower-energy photons are converted into a higher-energy photon by sensitized triplet-triplet annihilation. While recent interest in this process has been motivated by improving the efficiencies of solar cells, many applications are being explored. In this review, we address the underlying physicochemical phenomena that are responsible for photochemical upconversion. We review their kinetics, and the requirements for annihilators and sensitizers to design efficient upconversion systems. We discuss the spin physics of the bi-excitonic interactions and how the spin character of the triplet pairs can fundamentally limit the upconversion efficiency and give rise to the magnetic field effect on delayed photoluminescence. Finally, we address light-matter coupling phenomena that could be employed to enhance photochemical upconversion.
Keywords: kinetics; magnetic field effect; sensitizer-chromophore pair; spin statistical limit; triplet–triplet annihilation; upconversion quantum yield.