Surface and bulk passivation of multicrystalline silicon solar cells by ...

CHAPTER 2
SILICON NITRIDE LAYER FOR SOLAR CELL APPLICATIONS: AN
OVERVIEW
2.1 Antireflection Coating
Until recently, the objective of using an antireflection coating (AR) on a solar cell has
been to minimize the reflectance losses due to the cell and maximize the light trapping
ability to obtain highest photocurrent for the incident solar photons.
Silicon is a shiny gray material and, therefore, acts as a mirror, reflecting more
than 30% of the light that is incident on it. In order to improve the conversion efficiency
of a solar cell, it is necessary to minimize the amount of light reflected so that the
semiconductor material can capture as much light as possible to generate charge carriers.
Maximum absorbance of the incident light is preferred to achieve ideal conversion
efficiency.
A technique to reduce the reflectance loss is to texture the top surface. 'Chemical
etching creates a pattern of cones and pyramids, which capture and trap light rays that
might otherwise be deflected away from the cell. Reflected light is redirected down into
the cell, where it has another chance to be absorbed.
In Si solar cells, a pseudoperiodic roughness is generated by chemical etching in a
NaOH or KOH-based solution. Figure 2.1 shows the surface morphology of a typical
texturized silicon surface using conventional NaOH texturization bath [39]. The etching
also serves to remove the surface damage produced when ingots are sawn into wafers. A
rough or textured surface typically has a much lower reflectance than that of a planar
surface—a feature particularly useful in the case of high refractive-index semiconductors.
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