Reducing the reflectance on the sun-facing surface of silicon photovoltaic cells is an area of great interest to researchers and industry. Reflectance is one of the biggest barriers to increasing efficiency, thus reducing reflectance will lead to more efficient solar cells without an increase in materials or major changes to solar cell technology.
SiOnyx, a semiconductor company that specializes in optics for security and defense and in semiconductor manufacture has developed an ultra-fast laser surface texturing technique that greatly reduces the reflectance of silicon semiconductor material. The SiOnyx technique called black silicon is used in the manufacture of black silicon photovoltaic cells, and has produced 156mm solar cells with absolute efficiencies over 17%. The average black silicon cell produced by SiOnyx’s technology boasts 16.9% efficiency on semiconductor wafers that are 20% thinner than industry standard multicrystalline cells at a cost reduction of 10-15% over conventional chemical isotropic texturing methods. The reflectance of these cells is below 5%. Additionally, SiOnyx fabrication produces cells with much tighter process binning, resulting in less manufacturing variance in cell efficiency.
The process works by using a high power laser with an ultra-fast pulse width to etch the surface of the multicrystalline silicon material. The result is a rough surface comprised of spires that are tens to hundreds of nanometers wide and about a micron tall. These “hills and valleys” trap light, causing light waves to bounce, thus reducing reflectance. Previous methods that produced reflectance this low utilized anisotropic etching in a very controlled environment. Anisotropic etching is not suitable for multicrystalline cells since it is dependent on grain direction. The SiOnyx Black Silicon Ultrafast laser surface texturing technique works independently of grain direction. This process can produce 1200 cells per hour in open air environments with fewer chemical inputs over previous etching methods.
The increase in efficiency via reductions in reflectance will reduce total system size, thus reducing PV system hardware costs. This increase in efficiency coupled with a reduction in cell cost can potentially lead to reductions in balance of systems costs, something critical to meeting the current Administrations initiative of reducing the installed cost of solar to $1.00 per watt. Additionally, the reduced cell thickness and dark black color improves the aesthetics of the solar panels, helping them blend into structures better. Lastly, the reduction in costs and system size may make solar PV systems more marketable to other types of properties besides single family homes as well as those with less disposable income.
This process enables higher cell efficiency and cost effective production of thin film silicon solar cells, conserving resources, reducing the system size demands, and contributing directly to the reduction in greenhouse gas emissions. Additionally, technology enhancements in incorporating silicon black and novel fabrication techniques makes thin film silicon solar cells more competitive with cadmium telluride thin film cells that use rare earth elements in their manufacture. All of these enhancements can be done using existing solar cell Fabrication techniques preventing the need for manufacturers to retool their factories.
Uncertain due to the proprietary nature, but some risks are inherent of solar cell production as they relate to semiconductor manufacturing. Semiconductor manufacturing uses carcinogenic and toxic chemicals. The storage of these chemicals and solvents as well as the exhaust produced during the process can lead to environmental contamination. Additionally, semiconductor manufacturing is energy intensive, and may pose additional secondary risks to environmental and human health depending on the energy source used during manufacture.