Investigation Of Preparation And Spectral-Conversion Mechanism Of Rare Earth Ion-Doped Glasses For Solar Cells

The energy crisis is one major problem that all countries in the world need to solve.Solar energy as a clean energy source can be utilized by new technologies to be converted into electricity.Currently,one main problem limiting solar cell efficiency is the mismatch between the solar spectrum and the optimal spectral response of the photovoltaic cell.In view of this,with the help of energy transfer in different rare-earth ions,much sunlight can be converted into a spectral band with a relatively high spectral response in the photovoltaic device,thereby improving the utilization of sunlight.In this thesis,the rare-earth ions single-doped,co-doped and tri-doped oxyfluoride glasses or glass-ceramic glasses were used as spectral conversion devices to improve the photoelectric conversion efficiency of solar cells.There are five parts as follows:1.Ce³⁺ions doped oxyfluoride glass ceramics?50Si O₂-15Al₂O₃-5Ba CO₃-30Ca F₂-0.5Ce O₂?were prepared and investigated with different melting times?0.5-2 h?.Under the excitation of 427 nm light(4f ²F_5/2?5d₁),a wide emission band is obtained,which is located at 480-600 nm,corresponding to the partial parity-allowed electronic transition of Ce³⁺:5d₁?4f(²F_5/2).We can achieve the strongest emission intensity at the optimum melting time of 1.5 h.It can be seen that the photoluminescence?PL?performance is affected by the volatility and dispersion of Ca F₂ nanocrystals.In addition,we assembled silicon cell devices covered with the transparent Ce³⁺doped glass-ceramic glass sheets,which the values increased by 7.42%of the relative short-circuit current,and 5.5%of the relative photoelectric conversion efficiency.The highest efficiency belongs to the sample with a melting time of 1.5 h.2.Ce³⁺-Yb³⁺ions codoped oxyfluoride glass-ceramics?50Si O₂-15Al₂O₃-5Ba CO₃-30Ca F₂-0.5Ce O₂-0.5Yb₂O₃?were prepared.We investigated the effects of different annealing temperatures?610-640^oC?on the material structure,PL properties,and actual photoelectric conversion efficiency of silicon cell.At a 430 nm excitation,a broadband peak appears in the near-infrared region at 900-1100 nm,derived from the energy level transition of Yb³⁺:²F_5/2?²F_7/2.By further analysis,the results indicate that the energy transfer mechanism of Ce³⁺-Yb³⁺is multi-phonon-assisted and non-radiative cross-relaxation processes.In addition,we tested the I-V characteristic curves of the silicon solar cells covered with Ce³⁺-Yb³⁺spectral conversion sheets,which the relative short-circuit current and photoelectric conversion efficiency were increased by 8.9%and6.35%,respectively.3.Er³⁺-Yb³⁺,Tb³⁺-Yb³⁺and Pr³⁺-Yb³⁺ions co-doped oxyfluoride glasses were prepared,respectively.The structure,PL properties and spectral conversion mechanism of the materials were investigated in detail.For Er³⁺-Yb³⁺,under 378 nm(Er³⁺:⁴I_15/2?⁴G_11/2),450 nm(Er³⁺:⁴I_15/2?⁴F_3/2,_5/2)or 488 nm(Er³⁺:⁴I_15/2?⁴F_7/2)excitation,it shows the strong emission bands at 540 nm(Er³⁺:²H_11/2,⁴S_3/2?⁴I_15/2),660nm(Er³⁺:⁴F_9/2?⁴I_15/2)and 980 nm(Yb³⁺:²F_5/2?²F_7/2),confirming that there are two energy transfer pathways for Er³⁺-Yb³⁺.Furthermore,the upconversion luminescence properties were investigated.We demonstrate that it is a two-photon emission process between Yb³⁺and Er³⁺.For Tb³⁺-Yb³⁺,strong peaks appeared at 489,545,585,622,669 and 980 nm,under an excitation of 355 and 482 nm,respectively.In addition,it is concluded that the Tb³⁺-Yb³⁺system contains two energy transfer pathways:the single photon process of energy transfer with the help of the virtual level(Tb³⁺:⁵D₄?Yb³⁺:²F_5/2)and the two-photon process of cooperative energy transfer(Tb³⁺:⁵D₄?2Yb³⁺:²F_5/2).For Pr³⁺-Yb³⁺,by measuring the intensity of the upconversion emission spectrum at different excitation light powers,it is determined that the Pr³⁺-Yb³⁺system contains two energy transfer pathways:two-step energy transfer(Pr³⁺:³P₀?Yb³⁺:²F_5/2;Pr³⁺:¹G₄?Yb³⁺:²F_5/2)and cooperative down-conversion(Pr³⁺:³P₀?2Yb³⁺:²F_5/2).4.Ce³⁺-Tm³⁺,Ce³⁺-Tb³⁺and Ce³⁺-Er³⁺ions co-doped oxyfluoride glasses were prepared,respectively.In these samples,the energy transfer mechanisms were analyzed by measuring the fluorescence spectra.The energy transfer processes of Ce³⁺?Tm³⁺consist of two paths:the resonant non-radiative transition(Ce³⁺:5d?Tm³⁺:¹D₂)and the cross-relaxation(Ce³⁺:⁵D₁+Tm³⁺:³H₆?Ce³⁺:²F_5/2+Tm³⁺:¹D₂).For the Ce³⁺-Tb³⁺sample,when excited by ultraviolet light,part of Ce³⁺energy is relaxed to ²F_j level by a radiation transition,and one photon at 354 nm is released.On the other hand,the energy is transferred to the ⁵H_j level of Tb³⁺by part of the phonon-assisted and electronic polarization interaction,followed by a fast transition,releasing three photons at 545,588 and 621 nm(⁵D₄?⁷F_5-0).For the Ce³⁺-Er³⁺,the emission of Ce³⁺occurs in the range of 500-700 nm with an excitation at 467 nm.The energy transfer of Ce³⁺-Er³⁺includes a multi-phonon-assisted process(Ce³⁺:5d?Er³⁺:²H_11/2),followed by a two-step cross-relaxation process of Er³⁺,realizing the three-photon emission(²H_11/2+⁴I_15/2?⁴I9/2+⁴I13/2;⁴I9/2+⁴I15/2?⁴I13/2+⁴I13/2).5.Ce³⁺-Er³⁺-Yb³⁺,Ce³⁺-Tb³⁺-Yb³⁺and Ce³⁺-Pr³⁺-Yb³⁺ions tri-doped oxyfluoride glasses were prepared,respectively.For the Ce³⁺-Er³⁺-Yb³⁺sample,under an excitation of 445 nm(Ce³⁺:4f?5d₁,Er³⁺:⁴I_15/2?⁴F_5/2),there is a wide emission band in the range of 950-1060 nm.The energy transfer of the Ce³⁺-Er³⁺-Yb³⁺contains three paths?a?Er³⁺-Yb³⁺two-photon emission process;?b?phonon-assisted energy transfer for Ce³⁺-Yb³⁺;?c?Ce³⁺as a medium for Er³⁺-Yb³⁺.For the Ce³⁺-Tb³⁺-Yb³⁺,with a 308 nm(Ce³⁺:4f?5d₂,Tb³⁺:⁷F₆?⁵D₂)as excitation wavelength,we find a strong fluorescence emission in the range of 530-640 nm.Upon the excitation at 488 nm(Ce³⁺:4f?5d₁,Tb³⁺:⁷F₆?⁵D₄),the emission peak of the sample occurs in the near-infrared region.The energy transfer of Ce³⁺-Tb³⁺-Yb³⁺is phonon assisted process.For the Ce³⁺-Pr³⁺-Yb³⁺,the energy transfer process depends on the excited Ce³⁺.On one hand,the energy of Ce³⁺transfers to Yb³⁺(Ce³⁺:5d₁?2Yb³⁺:²F_5/2).On the other hand,the energy transfers to Pr³⁺(Ce³⁺:5d₁?Pr³⁺:³P₀).Subsequently,with non-radiative relaxation and multi-phonon assistance,it promotes the near Infrared light emission of Yb³⁺(²F_5/2?²F_7/2).For the silicon cell devices covered with Ce³⁺-Pr³⁺-Yb³⁺triple-doped glass sheets,the relative short-circuit current and relative photoelectric conversion efficiency have been improved(?J_sc=3.97%,??=2.73%).It indicates that the rare-earth-doped glasses as spectrum conversion sheets are effective in improving photoelectric conversion efficiency of solar cells.