We obtained efficient sensitized triplet-triplet annihilation based photon up-conversion (sTTA-UC) in a solid-state system consisting in a low viscosity transparent host matrix doped with molecular sensitizer dyes and loaded with annihilating/emitting MOF nanocrystals with size of tenths of nanometers. The maximum up-conversion yield of 5% has been achieved well below the solar irradiance, thanks to the peculiar properties of the nanocrystals embedded. Each one consists in a framework of identical and non-interacting optical active emitters that preserve the ground state electronic properties of the isolated molecules allowing for both the fast diffusion and the annihilation of the sensitized triplet excitons that generate higher energy fluorescent singlets. Specifically, each MOF nanocrystal can be considered as a giant supramolecular energy acceptor that works as triplet excitons collector. Thanks to its size, larger than that one of its molecular components, the collisional probability with the excited light harvesters is strongly enhanced, resulting in an effective sensitization of the nanocrystal triplets even with an ultra-low loading of the polymer matrix. These findings can be crucial for the development up-converting nanocomposite materials loaded with low concentrations of optically active species, with a remarkable simplification of the synthetic protocols without detrimental consequences for the material structural and optical properties.