The optical properties of solar cells before and after encapsulation

Abstract

In this work we investigate how the optical properties of monocrystalline silicon and polycrystalline silicon wafers are affected by texturing techniques and encapsulation. For monocrystalline wafers, the KOH etching is better than acid etching while reactive ion etching (RIE) is proven to be preferred compared to acid etching for polycrystalline wafers. The differences in reflectance (R) between two textures are apparent before encapsulation, but when the textured wafers are encapsulated with glass especially antireflectance coated (ARC) glass, the difference can be reduced from about five percentage points to below a percentage point. More important, the optical losses caused by reflectance (R) losses and parasitic absorption losses (A) for four types of commercial monocrystalline and polycrystalline silicon module are quantified and compared. Analyses are carried out for eight configurations using a stratified model consisting of solar cell wafers as well as cover glass and ethylene vinyl acetate (EVA). The model is to first obtain the external reflectance (R) and transmittance (T) in each layer, and the spectrally parasitic absorption loss associated with the cover glass and EVA is calculated with the aid of R and T measurements, which allows us to do a complete optical loss analysis for the solar module. The results show that the KOH texture with ARC glass encapsulation may be better choice for monocrystalline wafers, which gives a 94.04% of effective light collected by silicon. The RIE technique with ARC glass is suitable for polycrystalline substrates with excellent light trapping of 94.14%. After all, reflectance from glass surface and silicon surface accounts for over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss.