It looks like black is the new black when it comes to solar cell efficiency. A team of researchers at Lawrence Livermore National Laboratory (LLNL) has come up with a way to increase solar cell efficiency, by creating “black metals” etched with nanoscale structures that harvest energy from wavelengths all along the solar spectrum. The idea is to take advantage of the plasmonic effect, and if the research develops apace it could enable the prime affordability target of the solar market to expand from high-sunlight locations to just about anywhere.
Along similar lines, we’ve been following “black silicon” research that uses nanoscale etching to increase solar absorption, so let’s take a closer took at both and see what’s going on.
Black Metals For Solar Cell Efficiency
The LLNL solar cell efficiency project, loosely speaking, involves “roughening up” metals at the nanoscale level, which is where the concept crosses paths with its silicon-based cousin. The random, nanoscale irregularities increase the number of reflections, trapping more light.
The result is a black surface that has lower reflectivity and higher absorption than the original material, covering both the visible and infrared sections of the spectrum.
According to writer Kenneth K Ma at LLNL, existing work in gold and silver black metals has come up against a fabrication obstacle, in which it has been difficult to replicate the full solar absorption rate.
The research team worked on that angle and developed a “nanopillar” structure that can be manipulated with more predictability, enabling the team to create metals “as black as they want.”
LLNL has some competition out there, by the way. The research made the cover of Applied Physics Letters back in May, but a team from China also published the results of its work on light harvesting nanopillar structures in the publication’s online edition in April.
Also involved in the nanopillar horserace is UC-Santa Barbara, which has been developing a high efficiency solar cell based on a “forest of gold nanorods.”
Black Metals And The Plasmonic Effect
That leads us into the next question, which is how metals can generate an electrical charge from sunlight. Basically, it’s the same idea as semiconductors such as silicon, in which sunlight causes electrons to shift positions, leaving positively charged “holes.”
In metals, this shift creates free electrons and electromagnetic pulses similar to sound waves, called plasmons.
The nanorod approach provides one way to control and manipulate the plasmonic effect, but there are others. Over at Stanford University, for example, a team is working on a “waffle iron” plasmonic concept that involves creating nanoscale dimples in a layer of the semi-porous metal titania.
Meanwhile, a team at the University of Buffalo is testing out a low-efficiency but low-cost solar cell that incorporates the plasmonic effect into thin film organic solar cells. The idea is to create an affordable solar “paint” that could be applied to building surfaces.
On the meta-level, Duke University has been hot on the trail of an atomic-level explanation of the plasmonic effect, by studying the optical scattering that occurs when gold nanoparticles interact with a thin film of gold.