future, but are a long way to viability.
http://www.ecnmag.com/News/2010/10/Single-crystal-films-could-advance-solar-
cells/
Single-crystal films could advance solar cells Share: E-mail:
Print: Bookmark: RSS: [-] Text [+] Featured In: Academic Research
Cornell University | Thursday, October 7, 2010 Double-click any
word to search Highlight any phrase & click HotSearch By Bill
Steele Cornell researchers have developed a new method to create a patterned
single-crystal thin film of semiconductor material that could lead to more
efficient photovoltaic cells and batteries. The "holy grail" for such
applications has been to create on a silicon base, or substrate, a film with
a 3-D structure at the nanoscale, with the crystal lattice of the film
aligned in the same direction (epitaxially) as in the substrate. Doing so is
the culmination of years of research by Uli Wiesner, professor of materials
science and engineering, into using polymer chemistry to create nanoscale
self-assembling structures. He and his colleagues report the breakthrough in
the Oct. 8 issue of the journal Science. They used the new method to create
a film with a raised texture, made up of tiny pillars just a few nanometers
across. "Just the ability to make a single-crystal nanostructure has a lot
of promise," Wiesner said. "We combine that with the ability of organic
polymer materials to self-assemble at the nanoscale into various structures
that can be templated into the crystalline material." (A nanometer -- nm --
is a billionth of a meter, about three atoms wide.) Wiesner's research group
previously used self-assembly techniques to create Gräetzel solar cells,
which use an organic dye sandwiched between two conductors. Arranging the
conductors in a complex 3-D pattern creates more surface area to collect
light and allows more efficient charge transport, Wiesner said. Performance
improves the most when the conducting materials are single crystals, Wiesner
said. Most techniques for creating such films produce polycrystalline
material -- a collection of "grains" or small crystals bunched together at
random -- and grain boundaries retard the movement of electric charges, he
explained. Wiesner's method uses block co-polymers to create porous
templates into which a new material can flow and crystallize. A polymer
consists of organic molecules that link into long chains to form a solid. A
block co-polymer is made by joining two different molecules at their ends.
When they chain together and are mixed with metal oxides, one forms a
nanoscale pattern of repeating geometric shapes, while the other fills the
space in between. Burning the polymer away leaves a porous metal oxide
nanostructure that can act as a template. Wiesner's team created a template
with hexagonal pores on a silicon single-crystal substrate and deposited
films of amorphous silicon or nickel silicide over it. In collaboration with
Mike Thompson, associate professor of materials science and engineering,
they then heated the silicon surface with very short (nanosecond) laser
pulses. This melts the newly deposited layer and the top few microns
(millionths of a meter) of the silicon substrate. After only a few tens of
nanoseconds the molten silicon recrystallizes with the single crystal
silicon substrate acting as a seed crystal to trigger crystallization in the
deposited material above it, causing that crystal to line up epitaxially
with the seed. The template is dissolved away, leaving an array of
hexagonal pillars about 30 nm across. The team has made porous
nanostructured films up to 100 nm thick with other complex shapes. In
previous work Wiesner created lattices of cylinders, planes, spheres and
complex "gyroids" by varying the composition of co-polymers. Other materials
could be deposited, the researchers said. The goal here, they said, was to
demonstrate the formation of film with the same material as the substrate
(officially known as homoepitaxy) and with a different material
(heteroepitaxy). In a further proof-of-concept experiment, the researchers
showed that the structured thin film could be arranged in micron-scale
patterns, as might be necessary in designing an electronic circuit, by
laying a mask over the surface before applying laser heating. "We have
essentially gotten to the holy grail," Wiesner said. "It is not only a
nanostructured single crystal, but it has an epitaxial relation to the
substrate. There is no better control." The research was supported by the
National Science Foundation, Department of Homeland Security and Cornell's
Energy Materials Center, funded by the U.S. Department of Energy. SOURCE
Join the Discussion Rate Article: Average 0 out of 5 register or log in to
comment on this article! 0 Comments Add Comment Text Only 2000
character limit Page 1 of 1 • Latest News • News
From Europe • News From Asia DC-DC Power Converter Can Be Mounted
Vertically or Horizontally 2 hours ago Louisiana Tech researchers
design, fabricate innovative energy harvesting device 2 hours ago
SRA International gets $17.9 million task order 4 hours ago
Officials launch two advanced cyberspace courses 4 hours ago
Slim-Block LED Backlighting Tech Enhances LCD Performance 5 hours ago
More Latest News >> Phihong Introduces ENERGY STAR-compliant USB
Adapter for Portable & Mobile Applications 20 hours ago | News EIT
- IPTS Study Oct 6 | News Statistics : Investment rebound supports
OECD GDP growth in t
Sent via BlackBerry from T-Mobile
No comments:
Post a Comment