Black silicon was discovered almost by accident when a group of Harvard graduate students working at the university laboratory tried to find a way to make silicon more responsive.
Black silicon is a new type of semiconductor material that can absorb photons and release electrons in a much better manner as it forms billions of small, nano-scale pits during the process. These pits enable the material to capture a higher percentage of photons by preventing light from being reflected. Its distinctive black appearance, as opposed to the usual dull silver, is a result of very little light bouncing off its surface.
This discovery was brought to light by SiOnyx, a Massachusetts-based venture-funded startup company a few years ago, which is commercializing the patented semiconductor process.
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High-resolution SEM images of black silicon. Image credit: http://microdevices.jpl.nasa.gov
Properties of Black Silicon
The following are the key properties of black silicon:
- Needle-shaped surface structure with each needle above 10 µm in height and less than 1 µm in diameter
- Very low reflectivity
- Simple to process using just femtosecond laser pulses and chemical treatment
- Absorbs infrared light
- Reduces Fresnel reflection
- Unusual optical characteristics
- Semiconducting properties
- Excellent sensitivity to incoming photons
- Inexpensive, and easy to incorporate into existing technology
- Ability to detect wavelengths that silicon-based equipment cannot detect.
Production of Black Silicon
There are now several competing methods of producing black silicon. The Harvard graduate student team under the guidance of Eric Mazur found that when the surface of a silicon wafer was irradiated with a brief pulse of laser energy in a ‘sulfur hexafluoride gas mixed with dopants’ environment, the resultant material was what they termed “black silicon”.
The laser pulses enable large numbers of dopant atoms to be forced into the thin silicon layer on the surface of the cones. The use of laser helps a million times more sulfur to be put into the silicon surface than when combined and heated.
The new silicon layer has a smaller difference in energy between the valence band and the conduction band, which in turn reduces the amount of energy required to place electrons into the conduction band, thereby allowing the infrared photons to perform their job.
An alternative production method was developed in Munich have tried to better this process by boring holes into silicon wafers with gold. This process was further explored by a team of NREL researchers, who wanted to find a cheaper method. They used chloroauric acid instead of gold to produce black silicon without the need for an expensive vacuum manufacturing environment.
The NREL-produced black silicon is capable of reducing unwanted reflections to less than 2%, more efficient than the 3-7% achieved by the Munich researchers.
Use of Black Silicon in Photovoltaics
The properties of black silicon are well suited for use in photovoltaics. SiOnyx plans to use the advantages of black silicon to format new types of solar cells that will be able to capture solar energy across a wider range, thereby realizing greater efficiency in future photovoltaic cells.
The method of production discovered by NREL has brought the use of black silicon in photovoltaic arrays closer to commercial realisation, as it will reduce the cost of black silicon production.
Further Applications of Black Silicon
Black silicon is a revolutionary material that is likely to have multiple uses in numerous fields. Some of the areas identified are listed below:
- Roof-mounted solar panels
- Smaller, inexpensive, higher performance photonic devices
- Sensor applications
- Thermal imaging cameras
- Mechanical contacts and interfaces
- Terahertz applications
- Photodetector with high-efficiency
- Solar cells
- Night vision and surveillance equipment
- Light sensing devices for use in commercial and defence related applications
- Highly sensitive photovoltaic cells used in digital photography, solar energy generation, and medical imaging.
With advancements being made all the time, the future looks bright for this dark material.
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