Luminescence Down Shifting Effect On UV Spectral Response Of Solar Cells

Energy loss of short-wavelength photons is a major problem limiting the efficiency of crystalline silicon solar cells.Luminescence down shifting effect of rare earth phosphors,which converts a short wavelength photon into one or more long wavelength photons,is an effective solution to this limitation.The CaAlSiN₃:Eu²⁺phosphors can absorb short-wavelength photons in the range of200-600 nm and emit red light with a peak wavelength of 605 nm,which has been proved to be a suitable luminescence down shifting phosphors for solar cells.However,the emission efficiency of CaAlSiN₃:Eu²⁺phosphors is low the in wavelength below 280 nm,which limits the further efficiency improvement of solar cells.The Y₂O₃:Eu³⁺phosphors has strong emission efficiency in the wavelength below 280 nm,which might be a solution to this problem.In this thesis,CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors are mixed with ethyl vinyl acetate（EVA）to fabricate luminescence down shifting films for crystalline silicon solar cells.The details of the researches are as followings:（1）The luminescence characteristics of CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors has been studied.The excitation spectra of CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors can cover the low response band of silicon solar cells and maintain a high emission efficiency.The CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors both emit red light,so there is no absorption loss,which is helpful for the application of the mixed phosphors on the solar cells.By comparing the quantum yield of CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors,it can be found that,in the wavelength below 280 nm,the quantum yield of CaAlSiN₃:Eu²⁺phosphors is lower than 50%and the quantum yield of Y₂O₃:Eu³⁺phosphors is nearly 100%.Therefore,Y₂O₃:Eu³⁺phosphors might offset the low emission efficiency of CaAlSiN₃:Eu²⁺phosphors in the wavelengths below 280 nm.（2）Ethyl vinyl acetate（EVA）particles and phosphor particles are mixed and pressed into luminescence down shifting EVA films,and the optical properties of films are also studied.The results show that the EVA films with CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors（CASN+YO+EVA films）can absorb the photons in the wavelength of 200-600 nm,have a higher emission efficiency in the excitation range.The introduction of the phosphors raises the reflectivity of the films and the transmittance of the EVA film with phosphors is lower than the EVA film without phosphors.The absorption of CASN+YO+EVA films is higher than the EVA films with CaAlSiN₃:Eu²⁺phosphors,which indicates that CASN+YO+EVA film can absorb more UV photons.Compared with the UV resistance of different kinds of EVA films,the luminescence down shifting EVA films have stronger UV resistance than the EVA films without phosphors.（3）The prepared luminescence down shifting films are packaged with crystalline silicon solar cells and glasses.The measuring results show that the conversion efficiency of crystalline silicon solar cells with CaAlSiN₃:Eu²⁺phosphors increases from 19.60%to19.86%,and the sample with CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors increases from19.60%to 19.98%.The relative growth in the efficiency of the sample only with CaAlSiN₃:Eu²⁺phosphors is 1.33%,due to the luminescence down shifting effect of CaAlSiN₃:Eu²⁺phosphors,which improve the spectral response of the crystalline silicon solar cells.The efficiency of the sample with CaAlSiN₃:Eu²⁺and Y₂O₃:Eu³⁺phosphors has a better relative improvement 1.94%.This is because Y₂O₃:Eu³⁺phosphors enhance the luminescence down shifting effect of CaAlSiN₃:Eu²⁺phosphors in the packaged cells through two ways:one is that Y₂O₃:Eu³⁺phosphors improves low quantum yield of CaAlSiN₃:Eu²⁺phosphors in wavelength below 280 nm;other is that Y₂O₃:Eu³⁺phosphors can increase the absorption of CaAlSiN₃:Eu²⁺phosphors by scattering the incident light in the wavelength range above 280nm,which strengthens the luminescence down shifting effect of CaAlSiN₃:Eu²⁺phosphors.