Triplet-triplet annihilation based photon up-conversion in hybrid molecule-semiconductor nanocrystal systems

Photon up-conversion based on triplet-triplet annihilation (TTA) exploits the annihilation of optically dark triplets of an organic emitter to produce high-energy singlets that generate high energy emission. In recently proposed hybrid systems, the annihilating triplets are indirectly sensitized by light-harvesting semiconductor colloidal nanocrystals via energy transfer from their capping ligands (h-sTTA). Here, we discuss quantitatively the performance of the h-sTTA up-conversion mechanism in a reference nanocrystal/organic emitter pair, by introducing a kinetic model that points out the relationship between the up-conversion yield and the excitation intensity. This model highlights the fundamental properties of the employed moieties that mostly affect the conversion efficiency. We derive a new expression for the excitation threshold specific for h-sTTA up-conversion, which allows us to estimate a priori the material performances from a few key parameters and to point out the most severe bottlenecks. The obtained results demonstrate that the up-conversion yield is mainly limited by ultrafast non-radiative recombinations of the optical excitons created on nanocrystals, which are competitive to the sensitization channel for emitter triplets in solution. Our results suggest that the quenching partially arises from charge transfer interactions between nanocrystals and surface ligands. Improved ligand design and optimized surface functionalization strategies are required to avoid energy losses and enhance the up-conversion performance, thus promoting the application of h-sTTA up-conversion materials in solar technologies.