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 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.
In the same furnace they submerged the nanopillars, 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 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 test device was measured at six percent, which while
less than the 10 to 18 percent 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 percent. In future, top contact transparency
can easily be improved.
Other factors that greatly affect the efficiency of a 3-D nanopillar-array
solar cell include its density and the exposed length of the pillars in contact
with the window material. These dimensions are easily optimized in future generations
of the device.
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 be flexed with only marginal effect on
performance – and no degradation of performance after repeated bending.
"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,"
says Javey. "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.
Nanopillar arrays are a new path to versatile solar modules."
"Three-dimensional nanopillar-array photovoltaics on low-cost and flexible
substrates," by Zhiyong Fan, Haleh Razavi, Jae-won Do, Aimee Moriwaki,
Onur Ergen, Yu-Lun Chueh, Paul W. Leu, Johnny C. Ho, Toshitake Takahashi, Lothar
A. Reichertz, Steven Neale, Kyoungsik Yu, Ming Wu, Joel W. Ager, and Ali Javey,
appears in the August issue of Nature Materials and is available in advance
online publication at http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2493.html.
This work was supported in part by the Helios Solar Energy Research Center,
which is supported by the U.S. Department of Energy's Office of Science, Office
of Basic Energy Sciences.