Nanopillars promise cheap, efficient, flexible s...( BERKELEY CA Researchers at the U.S....)

BERKELEY, CA Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley have demonstrated a way to fabricate efficient solar cells from low-cost and flexible materials. The new design grows optically active semiconductors in arrays of nanoscale pillars, each a single crystal, with dimensions measured in billionths of a meter.

"To take advantage of abundant solar energy we have to find ways to mass-produce efficient photovoltaics," says Ali Javey, a faculty scientist in Berkeley Lab's Materials Sciences Division and a professor of electrical engineering and computer science at UC Berkeley. "Single-crystalline semiconductors offer a lot of promise, but standard ways of making them aren't economical."

A solar cell's basic job is to convert light energy into charge-carrying electrons and "holes" (the absence of an electron), which flow to electrodes to produce a current. Unlike a typical two-dimensional solar cell, a nanopillar array offers much more surface for collecting light. Computer simulations have indicated that, compared to flat surfaces, nanopillar semiconductor arrays should be more sensitive to light, have a greatly enhanced ability to separate electrons from holes, and be a more efficient collector of these charge carriers.

"Unfortunately, early attempts to make photovoltaic cells based on pillar-shaped semiconductors grown from the bottom-up yielded disappointing results. Light-to-electricity efficiencies were less than one to two percent," says Javey. "Epitaxial growth on single crystalline substrates was often used, which is costly. The nanopillar dimensions weren't well controlled, pillar density and alignment was poor, and the quality of the interface between the semiconductors was poor."

Javey devised a new, controlled way to use a method called the "vapor-liquid-solid" process to make large-scale modules of dense, highly ordered arrays
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