Optical Properties

The encapsulation affects the optical properties of the cells in several ways. The optical properties of the cells must be optimized, attending to cost and performance after encapsulation.

Some effects of encapsulation are [195]:

• The refractive index of glass and EVA is similar, around 1.5, between those of air and Si. Encapsulation acts then as a thick AR.

• The design of the ARC coating must account for the fact that the cell is illuminated from a medium with this index. The optimum ARC refractive index is slightly larger than in air.

• Glass and EVA absorb some light in the short-wavelength range.

• Typically, 4% reflection occurs at the air-glass interface (Figure 7.23(1)). ARC coatings and texturing can be applied to decrease this loss and to increase energy output [196].

• The light reflected by the metal fingers and the cell surface, if the reflected rays are tilted with respect to the normal to the glass surface, can be partly recovered by total internal reflection at the glass-air interface (Figure 7.23(2)). This effect could be enhanced by texturing the cell surface with tilted pyramids, instead of the upright pyramids obtained by alkaline etching of (1 0 0) surfaces [197].

Figure 7.23 Optical effects of encapsulation: (1) glass reflection; (2) trapping of cell reflectance;

(3) trapping of cell transmittance; (4) collection of peripheral light

• Although the trapping capabilities of the cell, due to the lower difference in refractive index, appear to worsen with encapsulation, the escaped rays are trapped in the glass so that the absorption enhancement in the ideal case is not affected.

• The white backsheet, since it reflects diffusively, allows some of the light incident between the cells to be collected (Figure 7.23(4)).

Updated: August 23, 2015 — 11:06 pm