A new antireflective coating has overcome two major hurdles facing solar energy – boosting the amount of sunlight captured by solar panels and allowing those panels to absorb the entire solar spectrum from nearly any angle. The discovery by researchers at Rensselaer Polytechnic Institute brings the industry closer to realizing high-efficiency, costeffective solar power.
“To get maximum efficiency when converting solar power into electricity, you want a solar panel that can absorb nearly every single photon of light, regardless of the sun’s position in the sky,” says Shawn-Yu Lin, the Wellfleet Constellation Chair professor of physics at Rensselaer. “Our new antireflective coating makes this possible.”
An untreated silicon solar cell absorbs only 67.4% of the sunlight that hits the panel – meaning that nearly one-third of that sunlight is reflected away and cannot be harvested. From an efficiency and economic perspective, this unharvested light is wasted potential and a major barrier hampering the proliferation and widespread adoption of solar power.
After a silicon surface was treated with the new nanoengineered antireflective coating, the material absorbed 96.2% of sunlight – with only 3 .79% of the sunlight reflected and unharvested. This huge gain in absorption was consistent across the entire spectrum of sunlight, from ultraviolet to visible to infrared.
“At the beginning of the project, we asked ‘would it be possible to create a single antireflective structure that can work from all angles?’ Then we attacked the problem from a fundamental perspective, tested and finetuned our theory, and created a working device,” Lin says.
Typical antireflective coatings are engineered to transmit light of one particular wavelength. The new coating stacks seven of these layers in such a way that each layer enhances the antireflective properties of the layer below it. These additional layers also help to bend the flow of sunlight to an angle that augments the coating’s antireflective properties, so each layer not only transmits sunlight, it also helps to capture any light that may have otherwise been reflected by the layers below it.
The seven layers, each with a height of 50 nm to 100 nm, are made up of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle – each layer looks and functions similar to a dense forest where sunlight is “captured” between the trees. The nanorods are attached to a silicon substrate via physical vapor disposition. Lin says the new coating can be affixed to nearly any photovoltaic materials for use in solar cells, including III-V multi-junction and cadmium telluride.
This layered design successfully tackles the challenge of angles. Most surfaces and coatings are designed to absorb light (i.e., be antireflective) and transmit light (i.e., allow the light to pass through it) from a specific range of angles. Eyeglass lenses, for example, absorb and transmit quite a bit of light from a light source directly in front of them, but those same lenses absorb and transmit considerably less light if the light source is off to the side or in the wearer’s periphery.
The same is true of conventional solar panels, which is why some industrial solar arrays are mechanized to slowly move throughout the day so their panels are perfectly aligned with the sun’s position in the sky. Without this automated movement, the panels would not be optimally positioned and would therefore absorb less sunlight. The tradeoff for this increased efficiency, however, is the energy needed to power the automation system, the cost of maintaining the system, and the possibility of misalignment errors.
Lin’s discovery couìd antiquate these automated solar arrays, as the antireflective coating absorbs sunlight evenly and equally from all angles. This means that a stationary solar panel treated with the coating could absorb more than 95% of sunlight regardless of the position of the sun.