Developing solid-state photon upconverters based on sensitized triplet-triplet annihilation

Photon upconversion assisted by sensitized triplet-triplet annihilation (sTTA-UC) is a wavelength-shifting technique where high-energy photons are emitted from the radiative recombination of high-energy singlets populated through the annihilation of the metastable triplets of two annihilator/emitter molecules. The emitter triplets are previously populated via energy transfer from a light-harvester/sensitizer moiety that absorbs the incident low-energy photons. In solutions, this process is efficient even at low excitation powers, whereas the limited molecular mobility and short exciton lifetimes typically observed in solid matrices hinder the bi-molecular interactions making the sTTA-UC process rather ineffective. We show here that controlling the confinement of the upconverting dye pairs in nanostructured or nanosized materials results in an increased effective local density of the excitation energy. This also activates a specific sTTA-UC kinetics independent of the triplet excitons' mobility that improves the material performance at low powers. We provide a complete modeling of the sTTA-UC process in confined systems. The results obtained afford useful guidelines for the future development of upconverting photonic devices operating at subsolar irradiances suitable for technological implementation.