Basic Research On The Application Of Rare Earth Doped Quantum Tailoring Luminescent Materials In Crystalline Silicon Solar Cells

At present, fossil energy has limited the global economic development because it will be exhausted and has caused serious environment pollution. Therefore, the renewable clean energy represented by solar energy has led the development trend of new generation energy system. It is well known, the crystalline silicon （c-Si） solar cells have been extensively employed in the daily lives as a kind of free, clean, and abundant new energy. However, the actual conversion efficiency of c-Si solar cells is only 19.0%that is much lower than theoretical value of 31% because of spectrum mismatch. In the solar spectrum arrived at ground surface （λ=300-2500 nm）, the photons with energy near the c-Si band gap （Eg=1.12 eV, λ=900-1100 nm） can be absorbed by the solar cells sufficiently. A large part of the solar energy is dissipated during the photovoltaic process owing to the thermalization of charge carriers whose energy exceeds the c-Si energy gap, and another part of energy is lost due to the transmission loss whose energy less than the c-Si energy gap （λ>1100 nm）. These two part of energy loss occupy 65% proportion of the total solar energy, thus significantly limiting the conversion efficiency of solar cells. Based on the Trupke’s down-conversion （DC） model to improve the conversion efficiency for solar cells, adapting the solar spectrum through DC is an optimal scheme for reducing the energy loss caused by the thermalization, which involves the conversion of one ultraviolet （UV）-visible photon into two near-infrared （NIR） photons in the range of 900-1100 nm.In this thesis we focus on researching optimal rare-earth ion couples in order to acquire the efficient NIR quantum cutting and deeply investigate the energy transfer mechanism, which can provide theoretical support for the feasibility of experiment. Meanwhile, the glass ceramics （GCs） have been synthesized and acted as matrixes, which have the advantages of low phonon energy, strong stability and high light transmittance. The rare earth co-doped quantum cutting luminescence materials have potential to act as the DC layer and improve the conversion efficiency of c-Si solar cells. The detailed research contents are as follows:Eu²⁺-Yb³⁺ codoped α-SrAl₂O₄ fluorescent powders have been prepared by conventional solid state reaction. The photoluminescence properties of samples in visible and NIR regions have been measured to verify the energy transfer （ET） from Eu²⁺ to Yb³⁺, and the results demonstrate that the cooperative energy transfer （CET） process dominates the ET process. During. the CET process, the Eu²⁺-Yb³⁺ion couple can convert one UV photon in the range of 250～450 nm to two NIR ones in the range of 900-1100 nm, and the quantum yield （QY） approaches 173.68%. Therefore, this DC material has potential application in c-Si solar cells to improve conversion efficiency.The oxyfluoride glass ceramic （GC） has been prepared by the conventional melt-quenching method using high-purity reagent powders and served as the matrix. A two-step ET including cross relaxation and directly energy transfer has been achieved in Er³⁺-Yb³⁺ co-doped transparent GC, which involves down-conversion of an absorbed visible photon to two emitted NIR photons. Compared to the CET mechanism, the two-step process has the advantages of low doping concentration and high QY, and then the Yb³⁺ emission centered at 980 nm has been efficiently enhanced in response to the strongest absorption of c-Si solar cells. However, the absorption cross-section of Er³⁺is very narrow due to the nature of 4f→4f transition and limits the actual QY between Er³⁺-Yb³⁺. It is discovered that Ce³⁺ion can act as efficient sensitizer with 350-600 nm broadband emission spectrum because of its allowed 4f→5d transition. Therefore, we have tried to increase the DC luminescence of Er+-Yb+ codoped oxyfluoride GC according to adopting Ce³⁺as sensitizer. In Ce³⁺-Er³⁺-Yb³⁺ tridoped GC, the NIR intensity of Yb³⁺has been enhanced dramatically with low Yb³⁺concentration. As a result, NIR quantum cutting transparent oxyfluoride GC will open a route to enhance the energy efficiency of c-Si solar cells.The NIR quantum cutting has been achieved in Pr³⁺-Yb³⁺ codoped GCs and emitted two NIR photons in the range of 900◇1100 nm. The ET mechanism has been investigated according to the photoluminescence properties of samples in visible and NIR regions, which has been indicated as two-step cross relaxations. Due to the advantages of two-step ET mechanism, the QY between Pr³⁺-Yb³⁺ is as high as 185.0%. Therefore, the Pr³⁺-Yb³⁺ co-doped transparent nanostructured GCs make it possible to enhance the efficiency of c-Si solar cells as a DC layer.The GC containing Y₃Al₅O₁₂（YAG） nanocrystal has been synthesized by the conventional melt-quenching method. The YAG GC has acted as matrix because the excellent luminescence properties of Nd³⁺ in YAG substrate. A two-step cross relaxations has been acquired in Nd³⁺-Yb³⁺ codoped transparent YAG GC, and the maximal QY value has been estimated to be as high as 179.34%. According to the DC process, one visible photon can be converted into two NIR photons, which can be efficiently absorbed by c-Si solar cells. Furthermore, YAG GCs are advantageous owing to their higher thermal stability, transparency and rare earth ions doped uniformly, which can adequately utilize the solar spectrum. Therefore, the Nd³⁺-Yb³⁺ co-doped GCs have potential applications as a DC layer to improve the conversion efficiency of c-Si solar cells. However, the absorption cross-section of Nd³⁺ is very narrow due to it’s 4f→4f transition and limits the actual QY between Nd³⁺-Yb³⁺. According to the previously reported, the Ce³⁺ ion can sensitize Nd³⁺ efficiently in YAG substrate. Therefore, Ce³⁺ ion is chosen as sensitizer in Ce³⁺-Nd³⁺-Yb³⁺ tridoped GC, the NIR intensity of Yb³⁺ has been enhanced dramatically with low Yb³⁺ concentration. As a result, our research may offer an effective way for the efficient utilization of solar spectrum and improve conversion efficiency of c-Si solar cells greatly.