Photon up-conversion (UC) enables the generation of high energy light from a lower energy excitation, thus it is a process of great interest for solar energy applications. UC allows for large anti-Stokes emission, recovering a fraction of photons not absorbed by devices and converting them to an energy range that is adequate for absorption, thus increasing solar cell efficiency. Among all up-conversion processes, sensitized triplet-triplet annihilation based UC (sTTA-UC) is the most promising for real-world applications, thanks to its high efficiency (~30%) that can be reached at excitation intensities comparable to the solar irradiance. In sTTA-UC, high energy light is generated after the annihilation of metastable triplet excitons on emitter molecules that generates a high-energy emissive singlet. These triplets are optically dark and thus they are populated via Dexter energy transfer from a sensitizer moiety employed as light harvester. Conventional sTTA-UC systems use metalated porphyrins as sensitizers and aromatic polyacenes as emitters. However, solid state systems for sTTA-UC are generally characterized by low diffusivity of the dyes, which strongly limits the UC yield at low powers, and have to face serious problems due to the aggregation of the optically active molecules at the high concentrations employed.
Here we introduce a novel approach towards the development of solid-state sTTA-UC by using optically active porous aromatic frameworks nanoparticels (PAFs), organic structures composed of sTTA-UC emitters covalently connected by optically inert cross-linking units. The high density of emitters in the PAF structure allows excitons to migrate and annihilate even in disordered networks. We synthesized a series of PAF by linking 9,10-diphenylanthracene (DPA) molecules with tetraphenylmethane units in different proportions. By using Pt(II)-octaethylporphyrin, we achieved green to blue UC, demonstrating for the first time sTTA-UC with a 10% quantum yield in porous, organic and ultra-stable covalent structures. Remarkably, we were able to achieve the maximum UC efficiency with an irradiance as low as 10 suns, highlighting the potential of these advanced materials for future applications in photonics.