A team of researchers from the Berkeley Lab (LBNL) and the University of California (UC) at Berkeley have recently announced their achievements in the field of nanopillar-based solar cells, with the impressing results of trapping 99 percent of the incoming light, without needing antireflex treatments.
They used highly ordered nanopillar arrays of germanium or cadmium sulfide, which they tuned and drastically enhanced their optical absorption properties. The research has been led by Ali Javey, who, along with his colleagues had demonstrated a method to cheaply produce cadmium sulfide nanopillars in high quantities and flexible shapes. They used half the semiconductor material required to make thin film solar cells and 1% of the quantity needed to make bulk solar cells.
“To enhance the broad-band optical absorption efficiency of our nanopillars we used a novel dual-diameter structure that features a small (60 nanometers) diameter tip with minimal reflectance to allow more light in, and a large (130 nanometers) diameter base for maximal absorbtion to enable more light to be converted into electricity,” Javey says. “This dual-diameter structure absorbed 99-percent of incident visible light, compared to the 85 percent absorbtion by our earlier nanopillars, which had the same diameter along their entire length.”
Javey and his team made their 3D arrays of semiconductor nanopillars by using molds they made in 2.5 millimeter-thick aluminum foil. The pores have been created to be thick at the bottom and thin at the top.
“This process enables fine control over geometry and shape of the single-crystalline nanopillar arrays, without the use of complex epitaxial and/or lithographic processes,” Javey says. “At a height of only two microns, our nanopillar arrays were able to absorb 99-percent of all photons ranging in wavelengths between 300 to 900 nanometers, without having to rely on any anti-reflective coatings.”
The newly-developed nanopillar material made with germanium can be used in ultra-sensitive light detectors and the cadmium sulfide/telluride in solar cells. Javey also says that the shape of the pillars can be tinkered with, and the cross-sectional portion of the nanopillar arrays can thus be molded into squares, rectangles or circles. This can lead to a better understanding of how the nanopillars can perform at their best.