LIGHT TRAPPING IN TEXTURED SOLAR-CELLS

The light trapping properties of textured optical sheets have attracted interest lately because of their potential in reducing the thickness of solar cell material. This provides a way to lower unit costs as well as reduce bulk recombination, thus raising open-circuit voltage. The strategy can be applied to any non-imaging system with limited absorbance. Two types of texture have been proposed to achieve light trapping in crystalline solar cells. One idealised system generates a random (Lambertian) distribution of light in the cell either by refraction from a diffuse top surface or reflection from a diffuse back surface texture. The other comprises top and/or back surface textures of realisable, "regular" facets belonging to a small class of orientations, with specific layout, to minimise escape patterns and so enhance overall beam pathlength in the cell. The latter facet class may be periodic or randomly sized. Concentrating solar cells that receive light over a limited range of angles of incidence provide an environment for further refining a light trapping technique. When the top surface of a crystalline silicon randomising cell is "tuned" to accept only this incident cone, its Auger-limited efficiency is almost independent of concentration. Studies have shown that some regular textures formed on crystalline silicon by an anisotropic etch yield a higher absorbance than a diffuse texture, over a limited range of angles of incidence. The most promising class of these uses a pre-etch crystallographic orientation other than the usual <100> to form asymmetric structures from intersecting <111> planes. These require only a top surface texture, which simplifies back surface fabrication and demands less from the quality of its reflector than a doubly textured design. They also nearly eliminate top surface reflection over the full solar spectrum without depending on an antireflection coating.