Ethylene vinyl acetate films filled with ytterbium containing rare earth particles (Y 2 SiO 5 : Ce 3+,Yb 3+ ) which have optical down-conversion capabilities and useful for encapsulating solar cells

In order to improve conversion efficiency of crystalline silicon solar cells, optically down-converting encapsulate films were prepared by introducing rare earth particles Y₂SiO₅: Ce³⁺, Yb³⁺ into ethylene vinyl acetate. First, ethylene vinyl acetate and the filler particles (up to 5 wt%) were melt compounded. Then, 0.4-mm thick film samples were made from the compounds with a laminator. Light conversion properties of the ethylene vinyl acetate films were investigated by UV-Vis-NIR spectra and fluorescence spectra. The relationships between the light conversion and the film transmittance were analyzed. The results show that the Y₂SiO₅: Ce³⁺, Yb³⁺-filled films were able to optically down-convert the 457 nm blue-violet light to the 980 nm near-infrared light. Peel strength increased to 31% (from 14.2 N/mm to 18.6 N/mm) and thermal conductivity increased to 31.3% (from 0.220 W/mK to 0.289 W/mK) in the 1 wt% Y₂SiO₅: Ce³⁺, Yb³⁺ film sample along with a slight reduction in transmittance (from 91.1% to 89.0%), as compared to the unfilled ethylene vinyl acetate films. Bulk electrical resistivity at 1 wt% filler decreased slightly to 1.45 × 10¹⁵ ohm·cm, as compared to the unfilled ethylene vinyl acetate film (1.03 × 10¹⁶ ohm·cm). The results suggested that the prepared films with varying filler contents could be useful both as front encapsulate films and back sheets for solar cell panels.