Sunlight represents the cleanest, greenest and far and away the most abundant of all energy sources, and yet its potential remains woefully underutilized. High costs have been a major deterrent to the large-scale applications of silicon-based solar cells. Nanopillars — densely packed nanoscale arrays of optically active semiconductors — have shown potential for providing a next generation of relatively cheap and scalable solar cells but have been hampered by efficiency issues. The nanopillar story, however, has taken a new twist, and the future for these materials now looks brighter than ever. “By tuning the shape and geometry of highly ordered nanopillar arrays of germanium or cadmium sulfide, we have been able to drastically enhance the optical absorption properties of our nanopillars,” said Ali Javey, a chemist who holds joint appointments with Lawrence Berkeley National Laboratory and the University of California, Berkeley. At left is a schematic of a germanium nanopillar array embedded in alumina membrane; at right are cross-sectional SEM images of a blank alumina membrane with dual-diameter pores. Inset shows germanium nanopillars after growth. (Image: Ali Javey) Javey, a faculty scientist with Berkeley Lab’s Materials Sciences Div. and a professor of electrical engineering and computer science, has been at the forefront of nanopillar research. He and his group first demonstrated a technique by which cadmium sulfide nanopillars could be mass-produced in large-scale flexible modules. In this latest work, they produced nanopillars that absorb light as well or even better than commercial thin-film solar cells, using far less semiconductor material and without the need for antireflective coating. Ali Javey, a Berkeley Lab-University of California, Berkeley, chemist, has been at the forefront of nanopillar research. (Image: Roy Kaltschmidt, Berkeley Lab Public Affairs) “To enhance the broadband optical absorption efficiency of our nanopillars, we used a novel dual-diameter structure that features a small (60-nm diameter) tip with minimal reflectance to allow more light in, and a large (130-nm diameter) base for maximal absorption to enable more light to be converted into electricity," Javey said. “This dual-diameter structure absorbed 99 percent of incident visible light, compared to the 85 percent absorption by our earlier nanopillars, which had the same diameter along their entire length." Theoretical and experimental works have shown that 3-D arrays of semiconductor nanopillars - with well-defined diameter, length and pitch - excel at trapping light while using less than half the semiconductor material required for thin-film solar cells made of compound semiconductors, such as cadmium telluride, and about 1 percent of the material used in solar cells made from bulk silicon. But until the work of Javey and his research group, fabricating such nanopillars was complex and cumbersome. The researchers fashioned their dual-diameter nanopillars from molds they made in 2.5-mm-thick alumina foil. Their two-step anodization process created an array of 1-?m-deep pores in the mold with dual diameters – narrow at the top and broad at the bottom. Gold particles were deposited into the pores to catalyze the growth of the semiconductor nanopillars. “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 said. “At a height of only 2 ?m, our nanopillar arrays were able to absorb 99 percent of all photons ranging in wavelengths between 300 to 900 nm, without having to rely on any antireflective coatings." The germanium nanopillars can be tuned to absorb IR photons for highly sensitive detectors, and the cadmium sulfide/telluride nanopillars are ideal for solar cells. Javic said that because the fabrication technique is so highly generic, it could be used with numerous other semiconductor materials as well as for specific applications. Recently, the group demonstrated that the cross-sectional portion of the nanopillar arrays can be tuned to assume specific shapes - square, rectangle or circle – simply by changing the shape of the template. “This presents yet another degree of control in the optical absorption properties of nanopillars,” Javey said. The research was partially funded through the National Science Foundation's Center of Integrated Nanomechanical Systems. The researchers reported their work, “Ordered Arrays of Dual-Diameter Nanopillars for Maximized Optical Absorption,” online in the journal NANO Letters. Co-authors were Zhiyong Fan, Rehan Kapadia, Paul Leu, Xiaobo Zhang, Yu-Lun Chueh, Kuniharu Takei, Kyoungsik Yu, Arash Jamshidi, Asghar Rathore, Daniel Ruebusch and Ming Wu. For more information, visit: www.lbl.gov