Re-designing Solar Cells by Using Black Silicon

published: 2011-08-08 10:30 | editor: | category: Knowledge

When one considers all the renewable energy technologies available right now, one will find that solar energy is one of the best and offers one of the most exciting prospects. As it is one of the cleanest and most abundant resources – available both as direct sunshine and as diffuse light on cloudy days - our focus should be on consuming this renewable resource instead of consuming fossil fuels that are finite and polluting. However, there are several constraints with this idealized picture of a world fuelled by solar power.

The Need to Improve Current Solar Energy Technology

Even though more sunlight reaches the earth in one hour than what is required to sustain our energy-intensive societies for one year, only a fraction of this incoming insolation is utilized. This restraint is due primarily to having technology unable to fully handle all this incoming light, by having technology unable to make full use of the entire electromagnetic (EM) spectrum that the light from the sun covers; in a broad sense, light from the sun covers the entire range of frequencies found within the EM spectrum while modern solar energy technology is only able to focus on a small portion. With current solar energy technologies only harvesting the visible light section (between 300 nanometers (nm) to 780 nm), there is considerable room to develop new technology to better capture the available light.

One such example of this progress in developing better solar energy technology came quite by accident in 1998 with the discovery of black silicon. This discovery in the 1990s was unwanted (at the time) and unexpected but now has the potential to alter the face of the solar energy industry. 

The Use of Silicon in Designing Solar Cells

Silicon, a common metalloid (a material that can behave like a metal and a like a non-metal), has been the material of choice for the solar industry due to its unrivalled ability to conduct electrons. Accordingly, silicon has been the backbone of the solar industry, even though alternatives do exist such as gallium arsenide (GaAs) and cadmium telluride (CdTe) thin film solar cells and alternatives are being developed (such as carbon nanotube-based solar cells and quantum dot-based solar cells). However, two constraints that affect all generations of solar cells (from first-generation monocrystalline silicon to second-generation thin film solar cells) are energy conversion efficiency rates and manufacturing costs. Advancements to the solar industry have led to incremental improvements in both areas but there has been no game-changing event yet. With the accidental discovery of black silicon, the solar industry might be a step closer to such a game-changing event, redesigning the solar cell and the use of silicon.

The Discovery of Black Silicon 

Black silicon derives its name from the color of the silicon; traditionally, silicon is silvery-gray in color and possesses a metallic silver luster. Black silicon was originally discovered in the early 1980s as a by-product of the reactive ion etching process, an etching process that is used in the microfabrication industry to produce components such as those found in flat-panel displays. Then, in 1998, Eric Mazur from Harvard University developed a technique allowing for the production of black silicon, and again, quite by accident.

Eric Mazur, a professor of applied physics at Harvard University, was investigating the effect of shining laser light on a variety of metals. On introducing gray silicon to the experimental apparatus (in the presence of a halogen gas and sulphur hexafluoride) and by directing ultrashort and ultra-intense laser pulses onto the material, Professor Mazur noted that the silicon component turned black.

Each laser pulse lasted for a femtosecond (approximately one quadrillionth of a second, which is the same as one-millionth of a billionth of a second) but each femtosecond pulse produced the equivalent energy output of sunlight striking the earth. After five hundred femtosecond laser pulses, the material turned color but the material did not appear to be charred.

What Mazur and his team saw was completely unexpected: the surface of the material was etched and transformed into a spectacular array of silicon needles. Through shining light on this etching, Mazur noticed that the silicon needles provided a structure where light would just bounce between the spikes for a longer period of time, with very little escaping. This realization that light could be better retained led to the belief that black silicon would make for a better solar cell.

The Properties of Black Silicon that Benefit the Solar Industry

In physics, a black body is one that theoretically absorbs all incoming light. In reality, very few (if any at all) can achieve even remotely high retention rates that a theoretical black body can. Black silicon is one of the few materials available that does retain the majority of incoming light, and with increased retention rates, black silicon becomes significantly attractive for use in solar cells.

With the introduction of black silicon, the absorption band widens and allows for absorption in the infrared region of the EM spectrum. This is achieved chiefly due to the presence of sulphur: the production of black silicon uses sulfur hexafluoride gas and the sulfur atoms are forced to mix with the silicon atoms on the surface, causing the bandgap to be lowered.  A lower bandgap results in the ability to absorb longer wavelengths of the EM spectrum. This enables more light to be absorbed and less light wasted. In fact, compared to the silicon used in solar cells, black silicon is between 100 – 500 times more sensitive to insolation (due to its rough surfaces that catches light at all angles) and covers both the visible and infrared regions.    

This increase in absorption has given rise to another feature: size. Silicon is expensive to purchase and process, and silicon is a material that is in relatively short supply. Having a material that increases absorption rates by up to 500 times allows for thinner solar cells to be manufactured. Furthermore, as the manufacturing process for black silicon is almost the same as for the manufacture of silicon, no change to the manufacturing process is required; in fact, the difference in cost is marginal and can be used with a conventional silicon foundry.

One difference is that the black silicon process requires a femtosecond laser to perform the etching, but the laser treatment actually can help reduce the manufacturing costs. The manufacturing costs are further reduced by the removal of the chemical processes typically used in the manufacture of silicon-based solar cells.

The Utilization of Black Silicon in Improving Current Solar Energy Technology

So, with only minor and cost-reductive changes to the manufacturing process, and with the prospect of using less material for a greater electrical output (i.e. higher efficiency), the use of black silicon will reduce the cost of the solar cell and improve the efficiency conversion rate.

At present, black silicon is rated at between 30% to 40%, a clear ten per cent increase on the best monocrystalline silicon solar cells available. This ‘lower cost – higher efficiency’ relationship is what solar cell manufacturers such as Motech and Trina Solar strive for, and with black silicon, this ideal balance could be attained.

Bringing Black Silicon to the Market      

In 2006, Mazur co-founded SiOnyx with the explicit aim of producing etched silicon surfaces for use in solar cells and applications that work on detecting low levels of light, such as night-vision apparatus and medical imaging applications.

By placing a silicon chip into a conventional vacuum chamber in the presence of a halogen gas (sulfur hexafluoride) and then by directing short bursts of the femtosecond laser onto the material, black silicon can be produced.  Figure 1 shows an electron micrograph of the surface of black silicon after laser etching.

  

Figure 1: Photograph of the surface of a black silicon chip, showing the needle-like structures, caused by the etching process using a femtosecond laser, that capture more of the incoming sunlight. Credit: SiOnyx

As shown in Figure 1, the femtosecond laser has the effect of ‘roughing up’ the surface and these needle-like cones are better light-traps, absorbing more of the insolation and thereby improving the electrical conversion efficiency rate.

Even though SiOnyx was spun-out of Harvard University to commercialize this new discovery, other research laboratories have also tried to produce variants of black silicon. One notable success story features the production of low-cost black silicon.

An Alternative Manufacturing Process to Produce Black Silicon

In 2010, researchers at the National Renewable Energy Laboratory (NREL) in the United States of America, led by principal investigator Howard Branz, described a new low-cost etching process for the production of black silicon.

Their inspiration came from the University of Munich, where researchers experimented with silicon containing a thin layer of gold on a surface. The gold would evaporate and in so doing, made boreholes on the surface. The researchers from NREL wanted to replicate the work but make it cost-effective, as the Munich researchers used vacuum pumps and evaporative equipment for their experiment, making it significantly expensive and not commercially viable.

Taking a typical solar wafer, the researchers at NREL were able to bore trillions of holes into the surface using a cost-effective methodology; in fact, this technique led to R&D Magazine awarding the NREL team an R&D 100 award for producing a major scientific breakthrough with their Black Silicon Nanocatalytic Wet-Chemical Etch technique.

The technique used by the NREL team involved spraying the gold layer onto the surface of the silicon. The gold, already infused with chloroauric acid, had hydrochloric acid and hydrogen peroxide added and after a few minutes, the process of etching was complete (three minutes at room temperature, less than one minute at 37.7 degrees Centigrade / 100 degrees Fahrenheit). The silicon produced was blacker than other versions of black silicon and contained boreholes of variable depths and size, allowing for even greater absorption.

Applying NREL Black Silicon to Solar Cell Industry

As with any solar cell, the aim of industry is to either reduce manufacturing costs or to try and improve the electrical conversion efficiency rates, with an ideal situation being both features were realized. With this environmentally friendly one-step etching process from NREL, the processing costs were reduced by up to eight per cent (8%), and the overall savings in the manufacturing process up to three per cent (3%).

This was achieved due to the changes to the manufacturing process, such as the removal of toxic and dangerous gases such as silane gas and nitrogen trifluoride. In addition to being more environmentally friendly, the technique also allows for a reduction in capital costs saving around ten per cent (10%) on the capital costs of a manufacturing factory line, achieved by replacing expensive vacuum deposition tools for a simple and inexpensive wet-etch bath.

With savings such as those described, and by producing a black silicon solar cell with an efficiency improvement of over one per cent (1%) on the best commercially available solar cells, the applications are aimed primarily at rooftop and conventional solar cell array systems. However, with the thickness of the solar cell being reduced, applications where black silicon integrated into building materials could also be considered either building-integrated photovoltaic systems (BIPV) or building-applied photovoltaic systems (BAPV).

Solar Industry Prospects for Black Silicon

This wide range of industrial applications will be of immense interest to solar cell manufacturers on the whole, so industry players such as Trina Solar, AUO, and Motech might benefit from licensing such technology from either NREL or SiOnyx to improve their margins on their solar products. Even newcomers to the solar field, such as manufacturing giant Taiwan Semiconducting Manufacturing Company (TSMC), will benefit tremendously from this cost-effective manufacturing process.

Improvements to the solar industry have traditionally been fractional in nature, but with the introduction of black silicon and improvements to the manufacturing cost considerations and electrical energy conversion efficiency rates, a game-changing step may have finally arrived.

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