How can you trap the most light through ultra-thin film solar cells — but keep costs down?
A question that’s kept many a researcher up at night. In the renewable energy industry, especially at such a fledgling stage, it is not only maximizing power output that is important. Unless new techniques ensure the resulting products are commercially viable and not so expensive that investors shy away, then projects are doomed to failure from the outset.
In solar cell arrays, it is the highly purified silicon elements that costs the most — in some cases, expenditure can be up to 40 percent of overall production.
So, how can you maximize the power output but keep silicon use — and cost — down to a minimum? MIT researchers in the Department of Mechanical Engineering have been working on a solution, and have created a new approach which may be able to reduce silicon usage by up to 90 percent, without detrimental effects for power output.
Instead of using thick, conventional layers of silicon, the team — Anastassios Mavrokefalos, professor Gang Chen, and three other assisting students — etched small inverted pyramids into the surface of the silicon. By creating this patterned texture, the team found that every indentation was able to trap light as effectively as thick silicon.
Each pyramid is less than a millionth of a meter across, and yet maintained the same capture rate of light as silicon surfaced that were up to 30 times thicker.
“We see our method as enhancing the performance of thin-film solar cells. It would enhance the efficiency, no matter what the thickness.”
If the technique is developed further and becomes commercial, it wouldn’t only be the silicon which would reduce cost, but due to less weight, there would also be savings in the installation process.
The team used equipment and materials that are already standard in silicon-chip processing, and report that the indented pyramid texture is very easy to fabricate. To create the dents, two sets of overlapping laser beams were used to punch tiny holes in a layer of material — called a photoresist — which is then integrated into the silicon using lithography techniques.
A chemical, potassium hydroxide, is then used to burn away parts of the surface not covered by the photoresist. According to Mavrokefalos, it is the crystal structure of the silicon which sets the pyramid shape.
At this stage, the team have produced the surface on silicon wafer and tested its rates of trapping light. The next step will be to produce a photovoltaic cell and prove its efficiency is comparable to conventional, commercial solar cells. The scientists expect the energy-conversion rate to peg at approximately 20 percent efficiency — compared to the average of 24 percent in the most sophisticated cells currently available.
Yi Cui, an associate professor of materials science and engineering at Stanford University, said that the developments have produced “very exciting results [..] The potential practical impact of this work could be significant, since it provides an effective structure for photon management for enabling thin cells”.
The work was supported by the U.S. Department of Energy and the National Science Foundation.
Image credit: Anastassios Mavrokefalos