Sensitized triplet–triplet annihilation based photon upconversion in full organic and hybrid multicomponent systems

In the last 15 years, the attention dedicated to organic conjugated systems experienced outstanding growth because of the renewed interest in mechanisms involving triplet states such as singlet fission, thermally activated delayed fluorescence, and intersystem crossing enhanced phosphorescence. Photon upconversion via sensitized triplet–triplet annihilation (sTTA) enables the conversion of low-energy photons into high-energy ones, and it has been proposed in multicomponent systems as an efficient managing strategy of non-coherent photons. This mechanism exploits the annihilation of two optically dark triplet states of emitter moieties to produce high-energy photons. The annihilating triplets are sensitized through Dexter energy transfer by a light-harvester, typically a conjugated molecule or a nanocrystal, so sTTA upconversion is usually performed in bi-component systems. The high yield observed at low excitation intensities stimulated thriving research in the field, leading to the development of a large family of fully organic and hybrid sTTA multicomponent upconverters. Here, we compare the evolution of these two families of systems with respect to the sTTA upconversion main figures of merit, highlighting the strengths and weaknesses of both approaches, according to the results reported in the literature. The data presented are also discussed in the perspective of future developments in the field, pointing out the challenges that are still to be faced for the technological use of the sTTA upconversion process.