Constructing Concurrent Upconversion/downconversion NaYF₄:Yb,Er/NaYF₄:Eu Core/shell Architecture For Enhanced Dye Sensitized Solar Cell Performances

In recent decades,extensive efforts have been devoted to the development of new photovoltaic devices or enhancement of the photoelectric conversion efficiency of existing solar cells in order to alleviate energy pressures arising from the excessive consumption of fossil fuels.Among these various photovoltaic technologies,Dye-sensitized solar cells?DSSCs?have been regarded as highly promising next-generation devices,which have the potential advantages of their low cost,ease of processing and relatively high power conversion efficiencies?PCEs?.The maximum DSSCs conversion efficiency can theoretically reach³0%.However,the current conversion efficiency record is still limited to¹2%.This problem is principally arose from the inability to absorb and utilize extra photons involving near-infrared?NIR?or ultra violet?UV?lights that constitute almost half of the radiant energy from the sun.One strategy that increases the conversion efficiency of DSSC devices is to convert photons with energies below the bandgap of photovoltaic devices into visible photons?<750 nm?that lie in the absorption region of the N719 dye:through upconversion process,the photons in NIR range can be transferred to the absorption range of N719,in addition,downconversion can convert one high energy photon into two lower energy phtotons,which can enhance the absorption of UV energy.Yet,concurrent and efficient upconversion?UC?and downconversion?DC?processes in the same nanoparticles incorporated in the photoanode for DSSCs remain unexplored.To address this problem,in this thesis,we designed a novel core/multi-shell architecture for integrating concurrent upconversion and downconversion processes within one nanoparticle,achieving their synergy effect at the maximum extent.Basing on the matching degree between the upconversion emission spectrum form lanthanides ion and absorption spectrum of N719 dye,we firstly exploited the optimized selection of lanthanides ions or ion pairs.For upconversion process,we compared the upconversion luminescence in Yb³⁺/Er³⁺,Yb³⁺/Ho³⁺and Yb³⁺/Tm³⁺doped NaYF₄ hosts under 980 nm laser radiated.In order to obtain the highest-efficiency downconversion process for matching the N719 dye absoption,the Eu³⁺,Pr³⁺,and Tm³⁺ions were tested under the excitation of 395 nm,374 nm,and 360 nm,respectively.The Yb³⁺/Er³⁺ions pair and Eu³⁺ion were demonstrated to be the optimal upconversion and downconversion doping systems.The core-shell architecture was subsequently constructed for bearing concrunt upconversion and downconversion processes.We exploited the precise doping location effect of activator ions in core or shell layers.The Eu³⁺doping concentration was also found to play important role on the luminescence intensity.The optimal Eu³⁺doping concentration was determined to 7%.However,the existence of severe cross-relaxation between Er³⁺and Eu³⁺activatiors at the core-shell interface results in unavoidable fluorescence energy loss.To solve this problem,an isolation layer was designed between the two active layers to suppress the cross-relaxation process.The designed NaYF₄:Yb³⁺,Er³⁺@Na LuF₄@NaYF₄:Eu³⁺three-layer core-shell structure is able to activate synergy effect between upconversion(Er³⁺)and downconversion(Eu³⁺).It is worth to note that this novel structure can further elevate the Eu³⁺doping concentration to 15%and achieve the strongest integrated luminescence output in visible region.To verify the performance advantages of the designed spectral conversion materials,those comparative samples were applied to the dye-sensitized solar cells.Compared with the DSSC without the spectral conversion materials,the NaYF4:20%Yb³⁺,2%Er³⁺@NaYF4 could increase the efficiency from 6.726%to 7.154%.Incorporating separatelyNaYF₄:20%Yb³⁺,2%Er³⁺@NaYF₄:7%Eu³⁺and NaYF₄:20%Yb³⁺,2%Er³⁺@NaLuF₄@NaYF₄:15%Eu³⁺core-shell nanoparticles into DSSCs,the efficiency were increased to 7.664%and 7.869%,yielding 13.9%and 17%efficiency enhancement over the regular DSSC,respectively.The incident photon-to-electron conversion efficiency?IPCE?measurement of modified DSSCs further confirms that the doped core-shell nanoparticles can effectively extent the response region of the DSSCs to infrared and ultraviolet light regions,and the electrochemical impedance spectroscopy?EIS?test results showed that the 5.033 ms lifetime of the electrons in the DSSCs treated by core-shell nanoparticles remain unchanged,suggesting the incorporated spectral conversion materials have no effect on the photons or electrons transport in the TiO₂ film.