14 July 2009

Solar cell recipe: Aluminum foil, nano pillars

Beginning with low-cost, aluminum foil substrates, there is now a way to fabricate efficient solar cells from low-cost and flexible materials.

These substrates make it possible to grow dense arrays of single-crystal, negative-type semiconductors arranged as nano scale pillars. When these nano pillars combine with a transparent, positive-type semiconductor that serves as a window, the end result is a 3-D photovoltaic that could mean efficient, cheap, flexible solar cells.

An aluminum substrate forms a template for a forest of cadmium sulfide nano pillars and also serves as a bottom electrode.

The new design grows optically active semiconductors in arrays of nano scale pillars, each a single crystal, with dimensions measured in billionths of a meter, said researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley.

“To take advantage of abundant solar energy, we have to find ways to mass-produce efficient photovoltaics,” said 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 nano pillar array offers much more surface for collecting light. Computer simulations have indicated, compared to flat surfaces, nano pillar 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 1 to 2%,” Javey said. “Epitaxial growth on single crystalline substrates was often used, which is costly. The nano pillar 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 of single-crystal nano pillars. 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.

In the same furnace, they submerged the nano pillars, once grown, in a thin layer of hole-rich cadmium telluride, which acted as a window to collect the light. The two materials in contact with each other form a solar cell in which the electrons flow through the nano pillars to the aluminum contact below, and the holes connect to thin copper-gold electrodes placed on the surface of the window above.

The efficiency of the test device came in at 6%, which while less than the 10 to 18% range of mass-produced commercial cells is higher than most photovoltaic devices based on nanostructured materials, even though the nontransparent copper-gold electrodes on top of the Javey group’s test device cut its efficiency by 50%. In future, they can improve top contact transparency.

Other factors that greatly affect the efficiency of a 3-D nano pillar-array solar cell include its density and the exposed length of the pillars in contact with the window material. They can optimize the dimensions in future generations.

Concerned with practical applications as well as theoretical performance, the researchers made a flexible solar cell of the same design by etching away the aluminum substrate and substituting a thin layer of indium for the bottom electrode. They sheathed the whole solar cell in clear plastic (polydimethylsiloxane) to make a bendable device, which could flex with only marginal effect on performance and show no degradation of performance after repeated bending.

“There are lots of ways to improve 3-D nano pillar 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,” Javey said. “There’s the ability to grow single-crystalline structures directly on large aluminum sheets. And the 3-D configuration means the requirements for quality and purity of the input materials are less stringent and less costly. Nano pillar arrays are a new path to versatile solar modules.”

For related information, go to www.isa.org/manufacturing_automation.