| |
LW1DSE > TECH 13.02.08 20:01l 102 Lines 5982 Bytes #999 (0) @ WW
BID : 212-LW1DSE
Read: GUEST
Subj: More on Solar Cells
Path: IZ3LSV<IW2OHX<OE6XPE<DB0RES<ON0BEL<CX2ACB<LU8DBJ<LW1DRJ<LW8DJW
Sent: 080212/2238Z 49461@LW8DJW.#1824.BA.ARG.SA [Lanus Oeste] FBB7.00e $:212-LW
From: LW1DSE@LW8DJW.#1824.BA.ARG.SA
To : TECH@WW
The invention of conductive polymers (for which Alan Heeger, Alan G.
MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the
development of much cheaper cells that are based on inexpensive plastics.
However, all organic solar cells made to date suffer from degradation upon
exposure to UV light, and hence have lifetimes which are far too short to be
viable. The conjugated double bond systems in the polymers, which carry the
charge, are always susceptible to breaking up when radiated with shorter
wavelengths. Additionally, most conductive polymers, being highly unsaturated
and reactive, are highly sensitive to atmospheric moisture and oxidation,
making commercial applications difficult.
Nanoparticle processing
Experimental non-silicon solar panels can be made of quantum hetero-
structures, eg. carbon nanotubes or quantum dots, embedded in conductive
polymers or mesoporous metal oxides. In addition, thin films of many of these
materials on conventional silicon solar cells can increase the optical
coupling efficiency into the silicon cell, thus boosting the overall
efficiency. By varying the size of the quantum dots, the cells can be tuned
to absorb different wavelengths. Although the research is still in its
infancy, quantum dot-modified photovoltaics may be able to achieve up to 42%
energy conversion efficiency due to multiple exciton generation (MEG).
Transparent conductors
Many new solar cells use transparent thin films that are also conductors
of electrical charge. The dominant conductive thin films used in research now
are transparent conductive oxides (abbreviated "TCO"), and include
fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al),
and indium tin oxide (abbreviated "ITO"). These conductive films are also
used in the LCD industry for flat panel displays. The dual function of a TCO
allows light to pass through a substrate window to the active light absorbing
material beneath, and also serves as an ohmic contact to transport
photogenerated charge carriers away from that light absorbing material. The
present TCO materials are effective for research, but perhaps aren't yet
optimized for large-scale photovoltaic production. They require very special
deposition conditions at high vacuum, they can sometimes suffer from poor
mechanical strength, and most have poor transmittance in the infrared portion
of the spectrum (e.g.: ITO thin films can also be used as infrared filters in
airplane windows). These factors make large-scale manufacturing more costly.
A relatively new area has emerged using carbon nanotube networks as a
transparent conductor for organic solar cells. Nanotube networks are flexible
and can be deposited on surfaces a variety of ways. With some treatment,
nanotube films can be highly transparent in the infrared, possibly enabling
efficient low bandgap solar cells. Nanotube networks are p-type conductors,
whereas traditional transparent conductors are exclusively n-type. The
availability of a p-type transparent conductor could lead to new cell designs
that simplify manufacturing and improve efficiency.
Silicon wafer based solar cells
Despite the numerous attempts at making better solar cells by using new
and exotic materials, the reality is that the photovoltaics market is still
dominated by silicon wafer-based solar cells (first-generation solar cells).
This means that most solar cell manufacturers are equipped to produce these
type of solar cells. Therefore, a large body of research is currently being
done all over the world to create silicon wafer-based solar cells that can
achieve higher conversion efficiency without an exorbitant increase in
production cost. The aim of the research is to achieve the lowest $/watt
solar cell design that is suitable for commercial production.
Sliver cells
Professor Andrew Blakers and Dr Klaus Weber, working at Australian
National University and Origin Energy have developed a technique for slicing
a single silicon wafer, which allows a significantly larger collector surface
area from each wafer, compared to usual solar cells. The technique involves
taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of
parallel, transverse slices across the wafer, creating a large number of
slivers that have a thickness of 50 micrometres and a width equal to the
thickness of the original wafer. These slices are rotated 90 degrees, so that
the surfaces corresponding to the faces of the original wafer become the edges
of the slivers. The result is to convert, for example, a 150 mm diameter, 2
mm-thick wafer having an exposed silicon surface area of about 175 cmý per
side into about 1000 slivers having dimensions of 100 mm x 2 mm x 0.1 mm,
yielding a total exposed silicon surface area of about 2000 cmý per side. As
a result of this rotation, the electrical doping and contacts that were on
the face of the wafer are located the edges of the sliver, rather than the
front and rear as is the case with conventional wafer cells. This has the
interesting effect of making the cell sensitive from both the front and rear
of the cell (a property known as bifaciality).
Makers
Solar cells are manufactured primarily in Japan, Germany, USA, and China,
though numerous other nations have or are acquiring significant solar cell
production capacity. While technologies are constantly evolving toward higher
efficiencies, the most effective cells for low cost electrical production
aren't necessarily those with the highest efficiency, but those with a
balance between low-cost production and efficiency high enough to minimize
area-related balance of systems cost. Those companies with large scale
manufacturing technology for coating inexpensive substrates may, in fact,
ultimately be the lowest cost net electricity producers, even with cell
efficiencies that are lower than those of single-crystal technologies.
Read previous mail | Read next mail
| |
