Development of energy upconversion based on triplet-triplet annihilation

Energy upconversion based on triplet-triplet annihilation (TTA) has attracted much attention due to its potential applications in different fields, such as photovoltaics, photosynthesis, photocatalysis, Imaging, etc It exhibits many merits, such as excitation with non-coherent light sources, low excitation power density, and tunable excitation and emission wavelength simply by independent selection of the sensitizer and the acceptor during the upconversion process. Hence, TTA-based energy upconversion is now an emerging technique which has a promising future. Herein, the principle of TTA-based upconversion and the requirements of the sensitizer and its corresponding acceptor have been discussed. Besides, the examples of the sensitizer molecules, including indium complexes, palladium complexes, platinum complexes, ruthenium complexes as well as organic molecules, and their corresponding acceptor molecules are summarized. Meanwhile, the existing problems and the direction of future development of TTA-based upconversion have also been outlined in this review.