Preparation And Application Of Bifunctional NaYF₄:Yb³⁺/Er³⁺@BIOA?Cl,Br,I? Microcrystal For Dye-sensitized Solar Cells

As the third-generation solar cell,the dye-sensitized solar cells?DSSCs?have been determined to be the vital technique for utilizing clean solar energy duo to their ease of processing,low-cost fabrication and relatively high efficiency.The N719light-absorbing dye in DSSCs only harvest visible photons in 400-700 nm,which leads to the inability to utilize the near-infrared?NIR?photons,holding ?50%-60% of DSSCs.The photon-upconversion processes have the potential to break through this limitation through converting two NIR photons into one visible light that can be absorbed by N719 dye.Up to date,Yb³⁺/Er³⁺co-doped NaYF₄ is considered as the most efficiently upconversion materials and has been widely applied in DSSC device for improving the NIR photons response.However,their insulation suppress severely the photo-electrons transfer in the TiO₂ photoanode.To address this important scientific problem,the bifunctional NaYF₄:Yb³⁺/Er³⁺@BiOA?Cl,Br,I?microcrystals,for the first time,are designed for enhancing DSSCs power conversion efficiency.?1?The NaYF₄:Yb³⁺/Er³⁺core for photons conversion from NIR to visible.?2?The BiOA?Cl,Br,I?shell layer is employed to promote photo-electrons transfer in the TiO₂ photoanode.The specific research contains 3 parts as follows:?1?Constructing the dual-functional NaYF₄:Yb³⁺/Er³⁺@BiOA?Cl,Br,I?core-shell-structured microcrystals:the optimal NaYF₄:Yb³⁺/Er³⁺nucleus were obtained in the sodium citrate as complexing agent hydrothermal reaction with the optimized reaction conditions of 180? for 24 h.The as-prepared nucleus shows a hexagonal morphology with avergea diameter of ?2.5?m and ?0.25?m thickness.Subsequently,the NaYF₄:Yb³⁺/Er³⁺core was added into the ethylene glycol-dissolved BiCl₃solutionforsynthetizingbifunctionalcore/shell-structured NaYF₄:Yb³⁺/Er³⁺@BiOCl microcrystals.The optimal bifunctional sample can be obtained at the optimized reaction conditions of pH=8.0,heating at 160? for 2 h.?2?Dual-functional NaYF₄:Yb³⁺/Er³⁺@BiOA?Cl,Br,I?microcrystals UC luminenscence and conductivity measurements:Upon 980 nm excitation,the NaYF₄:Yb³⁺/Er³⁺@BiOCl indicates an excellent photon converting property of teansforming NIR photons into visible photons.Comparing with NaYF₄:Yb³⁺/Er³⁺core microcrystal,NaYF₄:Yb³⁺/Er³⁺@BiOA?Cl,Br,I?shows a slight decrease in luminescence output,implying that coated BiOCl layer has no adverse influence on the UC emission of core microcrystals.The tested conduction of NaYF₄:Yb³⁺/Er³⁺@BiOA?Cl,Br,I?are 1.13×10⁷??9.98×10⁷? and 1.12×10⁸?,which are lower than that of NaYF₄:Yb³⁺/Er³⁺respectively,showing its advantage for the improvement of photo-electrons transfer performance.?3?The application of dual-functional NaYF₄:Yb³⁺/Er³⁺@BiOCl?Br,I?microcrystals in DSSCs:The optimized thickness of photoanode layer and the doping amount of upconversion microcrystals in DSSCs are achieved.The optimal photoanode layer and doping amount were determined to be 6 layers and 1%,respectively.The NaYF₄:Yb³⁺/Er³⁺core and NaYF₄:Yb³⁺/Er³⁺@BiOCl core-shell microcrystals were separately incorporated into the photoanode for J-V test.The regular cell has a efficiency of 5.30%,when the NaYF₄:Yb³⁺/Er³⁺core and NaYF₄:Yb³⁺/Er³⁺@BiOCl core-shell microcrystals are introduced into the DSSCs,the efficiency are increased to 5.93%and 6.88%,showing a 29.81% efficiency enhancement.The EIS test of these three cells indicates the resistance of 8.56?,12.53?,9.70?,which confirms that the designed bifunctional NaYF₄:Yb³⁺/Er³⁺@BiOCl can modify effectively the condition of UC marticles.This conclusion is further supported by 13.69%and 13.28%enhanced cell efficiency through using NaYF₄:Yb³⁺/Er³⁺@BiOBr and NaYF₄:Yb³⁺/Er³⁺@BiOI as optical converters in DSSC devices.