Black solar cells have lowest reflectance for silicon solar cells

May 14, 2012 // By Julien Happich
Scientists at Natcore Technology using simple liquid bath processes, have created a black surface on a silicon wafer with an average reflectance in the visible and near-infrared region of the solar spectrum of 0.3%.

This makes it the "blackest" silicon solar cell surface ever recorded. Compared with standard production cells now available, this represents a tenfold reduction in reflectance over that portion of the spectrum, which is the source of about 80% of the usable power that can be drawn from sunlight.

The black colour of black silicon results from the near-total absence of reflected light from the porous wafer surface. With solar cells, "blackness" is highly desirable because it indicates that incident light is being absorbed for conversion to energy rather than being reflected and thus wasted. Quantitatively, reflectance is the proportion of light striking a surface that is reflected from it. Thus a reflectance of 0.3% means that only 0.3% of incident light is reflected from the solar cell's surface, while 99.7% of incident light is absorbed by the cell and is available for conversion into electrical energy. A tenfold reduction in reflectance would mean that up to 3% more usable light would get into the cell, effectively increasing the cell efficiency by that amount.

Uncoated silicon wafer (left), an “absolute black” silicon wafer (middle) and a wafer with a standard antireflective coating.

 

But there are additional benefits to be derived from black silicon. A panel made from black silicon solar cells will produce significantly more energy on a daily basis than will a panel made from cells using the industry standard antireflective coating. First, because it reflects less light. Second, because it performs better during the morning and afternoon hours when the sun hits at an angle. (It also outperforms standard cell panels on cloudy days.) Its higher energy output, combined with a lower cost using Natcore's patented process, could quickly make black silicon the global solar technology of choice.

Natcore's process began with an uncoated, textured silicon wafer that had an average reflectance of approximately 8%, giving it a mottled gray appearance. First, nanoscale pores were etched into the