A general approach for non-coherently excited annihilation up-conversion: transforming the solar-spectrum

We demonstrate up-conversion with an external quantum yield of 3.2% realized at ultra-low excitation intensities (similar to 0.1 W cm(-2)). The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has substantially different photophysical characteristics when compared with the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc). Thus, we state the general requirements for the materials' parameters of the couple sensitizer/emitter for obtaining triplet-triplet annihilation up-conversion. One of the applications we suggest for active materials compositions is the achievement of energetically stepwise up-conversion of the terrestrial solar spectrum towards blue wavelengths: as such they could be used for improving the efficiency of the solar cells. Up-conversion with non-coherent excitation by sunlight with wavelengths between 550 and 700 nm is demonstrated.