An aluminum substrate forms a template for a forest of cadmium sulfide nanopillars and also serves as a bottom electrode. (Berkeley Lab)
There's a new 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.

The process was demonstrated by researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley.

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 more surface for collecting light. Computer simulations have indicated that nanopillar semiconductor arrays should be more sensitive to light, have an 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 Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of Electrical Engineering and Computer Science at UC Berkeley.

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 of single-crystal nanopillars. Inside a quartz furnace, his group grew pillars of electron-rich cadmium sulfide on aluminum foil, in which geometrically distributed pores made by anodization served as a template.

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