Synthesis and up-conversion properties of Ho 3+ -Yb 3+ -F − tri-doped TiO 2 nanoparticles and their application in dye-sensitized solar cells

Abstract A series of Ho 3+ -Yb 3+ -F − tri-doped TiO 2 nanoparticles (abbreviated as UP-F-TiO 2 NPs) were prepared using a hydrosol−hydrothermal method, and their phase structure, morphology and luminescent properties were investigated. These nanoparticles were utilized to prepare a photoanode of dye-sensitized solar cells (DSSCs). Based on the up-conversion spectra of UP-F-TiO 2 NPs, an up-conversion mechanism is proposed. The results indicate that the UP-F-TiO 2 NPs were a mixture of anatase and rutile nanoparticles, and the mass fraction of the anatase phase first increases and then decreases with increasing incorporation of F − ions into UC-F-TiO 2 NPs, while the un-doped TiO 2 nanoparticles are in the rutile phase. The average sizes of the UP-F-TiO 2 NPs are estimated to be 10 nm. The HRTEM shows clear lattice fringes with spacings of 0.347 nm and 0.323 nm that correspond to the d spacings of the (101) facets in anatase TiO 2 and the (110) facets in rutile TiO 2 , respectively. Under the excitation of 980 nm NIR light, the UP-F-TiO 2 NPs exhibit a stronger emission band centered at 543 nm and two relatively weaker bands located at 667 nm and 751 nm. The DSSC assembled with a bilayer-structured UP-F-TiO 2 NP photoanode has a 35.2% enhancement in conversion efficiency compared with DSSCs assembled with un-doped TiO 2 photoanodes.