Infrared Quantum-cutting Materials For Solar Cell Application

In recent years,energy shortage and environmental problems have been being global issues and the development of new energy sources becomes very urgent.So Photovoltaic（PV）technology has become more attractive and a lot of efforts have been paid to increase the conversion efficiency of solar cells.For the c-Si solar cells,the conversion efficiency is predicted to be lower than the Shockley-Queisser limit mainly due to two types of energy loss:one is the inability to absorb the low energy photons whose energy is lower than the bandgap of Si;the other is the loss of excess energy of“hot”carriers generated by high energy photons.This problem is called as spectral mismatch between the solar spectrum and the energy gap of c-Si（1.12ev）.To improve the efficiency of c-Si solar cells,quantum cutting materials has been proposed to reduce the energy loss of short wavelength photons.The rare earth phosphors as a type of the quantum cutting materials,which can convert short-wavelength photons into long wavelength photons,have been widely investigatted.Y₃Al₅O₁₂（YAG）:Ce³⁺,Yb³⁺phosphors have become attractive as its broad absorption band due to 4f→5d transitions of Ce³⁺and NIR emission of Yb³⁺around 1000 nm(Yb³⁺:²F_5/2→²F_7/2),which matches the spectral response of the c-Si solar cells.Another advantage of YAG:Ce³⁺,Yb³⁺phosphors is that,because YAG:Ce³⁺phosphors has been used widely in semiconductor lighting for many years,their excellent properties have been proved.In this thesis,YAG:Ce³⁺,Yb³⁺phosphors are prepared as the quantum cutting materials and the energy transfer mechanism inside the phosphors is investigated.The reasons why the quantum cutting phosphors improve the solar cells are also discussed.After describing the basic theory of c-Si solar cells and rare earth luminescence,the effects of rare earth ions’concentration on the luminescence of YAG:Ce³⁺,Yb³⁺phosphors are investigated.YAG:1%Ce³⁺,x%Yb³⁺（x=5,10,15,20,25）phosphors with different Yb³⁺concentrations are synthesized via the high temperature solid state method.Optical properties of the phosphors are characterized by photoluminescence（PL）.Under excitation of 450 nm,the visible broadband emission from Ce³⁺:5d→4f with the central wavelength of 550 nm is observed.The NIR emission around 1030 nm from Yb³⁺:²F_5/2→²F_7/2 is also observed under the same excitation.The variation of emission intensity with the concentration of Yb³⁺shows that the energy transfer exists between Ce³⁺and Yb³⁺and the quenching concentration of Yb³⁺is 15%.Based on concentration characteristics of quantum cutting luminescence,it is concluded that CET（cooperative energy transfer）from Ce³⁺to Yb³⁺play a role in the quantum cutting luminescence.By testing the luminuscence of YAG:Ce³⁺,Yb³⁺phosphors under different temperature,the thesis investigates the another energy transfer mechanism of the phosphors.YAG:1%Ce³⁺,15%Yb³⁺,YAG:1%Ce³⁺and YAG:15%Yb³⁺phosphors are prepared,and their photoluminescence spectra,photoluminescence excitation spectra and fluorescence decay curves are measured in the temperature range from 8 K to 300 K.Experimental results can be explained by a photo-induced Ce-O-Yb CTS（charge transfer state）model,which plays an importnat role in the the energy transfer.The coating of polymethyl methacrylate（PMMA）mixed with the phosphors is deposited on the surface of concentrator c-Si solar cells by spin-coating method.It is found that the solar cells with the phosphor coating always have higher conversion efficiency than the bare solar cells under different illumination intensities.This is attributed to the quantum cutting effect of the phosphors and the reduced reflection due to the graded-index（especially in the wavelength range 350 nm-550 nm）.The reflection of the light emitted from the phosphor’s particles at the air/PMMA interface also contributes to the improvement.The relative growth in the conversion efficiency of the solar cells with the phosphor coatings increases with the illumination intensity from 4.86%under 100 mW cm^-22 to 6.04%under 400 mW cm^-2 because the increase of the emission from the phosphors is faster than that of the illumination intensity when the illumination intensity increases.