Influence Of The Size Of YAG:Ce Particles On Mono-Crystalline Silicon Solar Cells

One of the most challenging issues facing photovoltaic（PV）researchers today is how to improve the conversion efficiency of PV cells by making better use of the solar spectrum.The power conversion efficiency of the silicon solar cells can be enhanced by improving their spectral response to short-wavelength photons.One potential solution is to introduce luminescent down-shifting（LDS）materials onto the solar cells to transform short-wavelength photons to long-wavelength photons.Yttrium aluminium garnet（YAG）doped with Ce³⁺is a luminescent down-shifting material with a rich history and a wide variety of applications.YAG:Ce can absorb the photons in the wavelength range of 300 nm to 500 nm and re-emit them at a longer wavelength where solar cell shows a significantly better spectral response.Because Ce-doped YAG phosphors have broadband absorption,high luminescent efficiency,and chemical stability,they are one of the down-shifting materials suitable to silicon solar cells.However,the introduction of YAG:Ce brings the scattering of light,which increases the reflection and negatively influence the performance of the solar cells.To solve the problem,this thesis investigates how the phosphor’s scatter increases the reflection,especially the effects of the phosphor particles on the reflection.Firstly,LDS particles have been simulated to clarify the effects of the particle’s size and index on the scattered power in ethylene-vinyl acetate（EVA）.The simulated results showed that larger particles have a higher ratio of the forward scattering energy to the total scattering energy because there is more constructive interference between the forward emissions from the dipoles inside the larger particles.When the refractive index of the particles is close to that of EVA,the forward scattering energy is high,and the backward scattering energy is low.When the refractive index of the particles is far away from that of EVA,the forward scattering becomes weakened,and the backward scattering becomes stronger.Secondly,the influence of the size of LDS particles inside EVA films on reflection is investigated theoretically.The results showed that the reflection of EVA films rises from the scattering by LDS particles,which depends on the particle’s size.The backscattered power increases with the particle’s size and decreases with incident wavelength,which can be explained by the dipole model.The total reflection caused by LDS particles can be calculated according to the size distribution of LDS particles.The thesis also investigates experimentally the relationship between the diameter of YAG:Ce particles and luminescent intensity.It is found that the particles with 15-20μm have the highest luminescent intensity.The higher concentration of YAG:Ce particles in EVA films leads to the higher reflection and lower transmission of EVA films.When the particle range is 15-20μm,the luminescent DS films with the mass ratio of phosphors to EVA0.03:10 have the highest transmission.The 15-20μm particles with the mass ratio of phosphors to EVA 10:0.08 increase the conversion efficiency from 17.57%of the bare cells to18.08%of the modules.The results show that the size and concentration of the phosphor’s particles have great effects on their luminescence and the performance of Si solar cells.These findings might be useful in the selection of down-shifting phosphors for photovoltaic cells.