Ligand sensitized strong luminescence from Eu3+-doped LiYF4 nanocrystals: a photon down-shifting strategy to increase solar-to-current conversion efficiency.

Lanthanide (Ln)-doped nanocrystals generally display low luminescence quantum efficiency due to forbidden nature of the 4f-4f transition besides possessing low absorption cross sections (∼10 M-1 cm-1). Considering the demand for these materials, particularly for light emission and bioimaging applications, it is very important to improve their quantum efficiency. This work demonstrates a strategy to enhance Si solar cell efficiency via sensitization of Eu3+ ions luminescence from colloidal nanocrystals. We have for the first time developed a simple ligand exchange approach to attach 4,4,4-trifluoro-1-phenyl-1,3 butanedione (TPB) to the surface of Eu3+-doped LiYF4 nanocrystals (NCs). Owing to the good overlap between the emission of the TPB ligands and the energy levels of Eu3+ ions, an efficient energy transfer takes place from the ligand to Eu3+ ions upon ultraviolet (UV) excitation of the ligand, leading to intense red emission. The sensitization of Eu3+ ions greatly enhanced the quantum yield of Eu3+ ions (∼31%) compared to the ∼5% obtained via direct excitation of Eu3+ ions (λexi = 394 nm) in Eu3+-doped LiYF4 NCs. A device was fabricated by embedding the nanocrystals on a Si solar cell to capture the UV photons and convert them into visible ones, which subsequently creates charge carriers inside the cell. Upon exposure to UV light, the nanocrystal embedded Si solar cell shows overall enhancement in the photocurrent upon excitation under UV radiation.