The Down-conversion Of Rare Earth Couples In NaYF₄for GaAs Solar Cells

Solar energy is inexhaustible.The enormous gap between the potential of solarenergy and our applications is attributed to the high cost and low conversionefficiency. The European Joint Research Centre (JRC) predicted that solar PV willoccupy an important seat of the world energy consumption in the21st century, themain part of the world's energy supply.The process of solar energy Convert into electrical energy,the main causes ofenergy loss are related to the spectral mismatch. So far, the common characteristicsof the improved spectral mismatch to improve the the solar cell efficiency are: forthe narrowband c-Si (Eg=1.1ev) solar cells in AM1,which is close to the Yb³⁺2F7/2^-2F_7/2,980nm. The GaAs (Eg=1.43ev,⁸67nm) solar cell has the higherconversion efficiency up to32%and the tolerance to cosmic ray, supplying powerfor applications in space, such as satellites and space vehicles. In this thesis,firstproposed by adjust the mismatch of the GaAs-based solar cells in AM0ultravioletsolar spectrum by Quantum cutting. we have chosen the new donor-acce ptorcouples and Observation of quantum cutting process. In the extraterrestrial space thesolar radiation has stronger ultraviolet (UV) radiation compared to the earth surface.Cutting a high energy UV photon into two lower energy photons, both of which canbe efficiently absorbed by GaAs solar cells. Quantum efficiency can reach200%theoretically,will greatly improve the conversion efficiency of photovoltaic applications. Main contents is:1The quantum cutting process in NaYF₄: Tm³⁺, Er3+for GaAs-based solarcells were observed at first time, the maximum quantum efficiency is141%.Crystalline powder samples NaYF₄: Tm³⁺,Er³⁺was prepared by the hydrothermalmethod.Under UV excitation of Tm³⁺:³I₆(291nm) through resonant energy transferTm³⁺(³I₆,→¹G₄)→Er³⁺(4I15/2→4I9/2) This results creating Tm³⁺(¹G₄→3F6,476nm),Er³⁺(4I15/2→4I9/2,796nm) two photons. From the luminescence decay curvesof the Tm³⁺emission¹G₄→³H₆at476nm, With Er3+-dopedconcentrationincreased, the concentration quenching lead to light-emitting weakened. Proved thatby improve the spectral mismatch to improve GaAs-based solar cell efficiency is thetheoretical feasibility.2Cut the vacuum ultraviolet into a Blue and a near-infrared photons in theNaYF₄: Tb3+, Er3+were observed at the first time.The quantum efficiency is188%close to the limit of200%. The hexagonal microcrystal NaYF₄: Tb³⁺, Er³⁺wasprepared by the solid state method. Under UV excitation of Tb³⁺:5L1(294nm), thehigh-energy ultraviolet photon is quantumly cut into two lower energy photons: onein the near-infrared region (Er³⁺:⁴I_9/2→⁴I_15/2,800nm) and the other in the blueregion (Tb³⁺:5D4→7F6470nm), both of which can be efficiently absorbed by GaAssolar cells. The cross-relaxation (Tb³⁺:5L1, Er³⁺:4I15/2)→(Tb³⁺:5D4, Er³⁺:4I9/2)required for down conversion occurs. The energy migration dynamics of the donorTb³⁺approaches to the diffusion model. In the nearly resonant energy transfer theinitial process of the interaction between the Tb³⁺-Er³⁺couple is governed by thedipole-dipole interaction.3Synthesized hexagonal phase particles of different sizes.Used thehydrothermal method synthesis hexagonal structure of NaYF₄Tb³⁺, Er³⁺microcrystals with sodium citrate as a chelating agent. The effects of the amount ofFˉions,citrate, temperature and hydrothermal time on the nano-crystallinemorphology, size and structure were analyzed in detail.It is found that excessivelevels of Fˉion can get nanocrystals with better crystallization at a relatively low hydrothermal temperature, the strong chelating ability between rare earth cation andcitric acid molecules can be able to effectively control the growth rate, phase-changetime of NaYF₄nanocrystals; Hexagonal nano-crystals phase NaYF₄: Tb³⁺, Er³⁺wasprepared by Hot solvent method in oleylamine of ligand/solvent. Synthesis of purehexagonal phase samples, the particle dispersion, uniform size, particle size12nmLigand/solvent and reaction temperature are critical factors in the synthesis ofhexagonal-phase nanoparticles. Oleylamine appears to mediate the transfer from thecubic to the hexagonal phase. The nucleation and growth of the hexagonal-phasenanoparticles was studied.4We studied the crystal phase effect on the down-conversion for Tb³⁺-Er³⁺couples in NaYF₄micro-and nano-crystals. The thermodynamically stable andordered hexagonal phase is favorable for the quantumcutting processes,however, thedowconversion can not be observed in our cubic phase samples. Using differentexcitation wavelengths294nm and355nm,indicating that the cross-relaxationrequired for down conversion occurs. Analysis of the reasons for the hexagonalphase is conducive to the light-emitting.