Construction And Application Of NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺ Core/shell Structure For Dye-sensitized Solar Cells

Dye-sensitized solar cells?DSSCs?have been considered to be ideal substitutes for silicon-based solar cells due to their potential advantages of low manufacturing cost,environment-friendly and relatively high power conversion efficiencies?PCEs?.The maximum energy conversion efficiency of DSSC can theoretically reach?30%,however,the current record experimental efficiency is about 12%.One factor that limits the energy conversion efficiency is the inability of utilizing infrared?IR?photons,leading to severe IR photons transmission losses approaching to 55%of the solar energy reaching the Earth surface.To overcome this limitation,extensive efforts have been focused on the development of upconversion phosphors that can convert unutilized sub-bandgap IR photons into useful above-bandgap visible photons.Herein,for the first time,we present a facile method to craft a heterogeneous YOF:Yb³⁺/Er³⁺shell on the NaYF₄:Yb³⁺/Er³⁺core with high chemical stability through high-temperature annealing-induced reaction between O₂ and NaYF₄:Yb³⁺/Er³⁺.This route enables the integration of lattice-mismatch NaYF₄ and YOF hosts into stable core/shell architecture,resulting in the increase of UC emission efficiency of NaYF₄:Yb³⁺/Er³⁺.The shielding of all surface activator ions from the environmental quenching factors and efficient coupling between photoelectrons and modulated phonons from YOF:Yb³⁺/Er³⁺shell.The main contents are as follows:?1?The high-quality hexagonal NaYF₄:Yb³⁺/Er³⁺was synthesized by the hydrothermal method at 180?for 24 h with CA/Ln³⁺=1:1 and NaF/Ln³⁺=12:1?CA=citric acid,Ln=Y+Yb+Er?,these primitive NaYF₄:Yb³⁺/Er³⁺are uniform and have a hexagonal shape with an average size of 2.08?m in diameter and 2.10?m in height.?2?Constructing heterogeneous fluoride and oxyfluoride core/shell structure.The SEM,XRD and Energy dispersive spectrometer?EDS?elemental mapping results demonstrate the successful construction of heterogeneous fluoride and oxyfluoride core/shell structure.Raman spectra indicates that the introduced YOF shell layer is able to modulate the phonon energy of host materials.The FTIR spectra confirms the elimination of citrate moieties.The overall UC luminescence intensity of the NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺system indicates that YOF shell can manipulate efficiently the R/G ratio.?3?To further verify the advantages of core-shell structure,we firstly changed the thickness of TiO₂ film,core@shell/TiO₂ film and the mass fraction of core@shell.Compared to regular device with the PCE of 7.02%,the DSSC incorporated with NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺microcystals exhibited an optimal PCE of 8.39%,corresponding to 19.52%performance enhancement,indicating core@shell microcystals can efficiently improve DSSCs efficiency.Incident photon-to-current conversion efficiency?IPCE?measurements show that the DSSC incorporated with core@shell microcystals exhibited IPCE of 62.5%,corresponding to 5.75%enhancement,verifying that the designed core@shell is beneficial for harvesting NIR sunlight and operates to DSSC.Compared with the pure TiO₂ photoanode,NaYF₄:Yb³⁺/Er³⁺@YOF:Yb³⁺/Er³⁺-based cell shows the lower electron transfer resistance,indicating that electron trapping center decrease efficiently.