Nanopillars form cheap, flexible solar cells

Berkeley, CA – Researchers at the 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.

Computer simulations have indicated that, compared to flat surfaces, nanopillar semiconductor arrays should be far more sensitiveand efficient.

“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 Berkeley’s Professor Ali Javey, adding: “Epitaxial growth on single crystalline substrates was often used, which is costly.”

Javey devised a new 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 the same furnace they submerged the nanopillars in a layer of hole-rich cadmium telluride, which acted as a window to collect the light. This formed a solar cell in which the electrons flow through the nanopillars to the aluminum contact below, and the holes are conducted to thin copper-gold electrodes placed on the surface of the window above.

The efficiency of the device was measured at six percent – less than the 10 to 18 percent of mass-produced commercial cells, but higher than most photovoltaic devices based on nanostructured materials.

A flexible solar cell was achieved by removing the aluminum substrate, substituting an indium bottom electrode, and embedding the 3D array in clear plastic.

“There are lots of ways to improve 3-D nanopillar photovoltaics for higher performance, and ways to simplify the fabrication process as well, but the method is already hugely promising as a way to lower the cost of efficient solar cells,” said Javey.

Details are published in Nature.