Environmental Engineering Reference
In-Depth Information
copper deposited using a ' wet ' electroless deposition process. Both approaches have
the advantage of yielding high conductivity tracks but are not suited to short manu-
facturing runs or where interconnects between two elements on a surface need to
be applied in-line.
Inkjet printing of novel materials is now becoming a viable digital manufacturing
technique for the additive and/or subtractive printing of conductive elements. One
approach attracting much attention is the direct printing of low temperature sinter-
ing metal nanoparticles, so removing the need for subtractive or wet electroless
deposition processing steps. Copper nanoparticles have to date not been success-
fully developed since the associated copper oxide is non-conducting. Therefore,
most attention has focused upon silver. The use of nanoparticle suspensions allows
many of these factors to be overcome. The small particle size alleviates the issues
of blockages and therefore the ink may be readily applied using an inkjet printer.
The use of nanoparticles has a further advantage in that the particles themselves
may be readily sintered to form conductive tracks by the application of heat or
microwave irradiation. By careful selection of particle size and coating material the
temperatures required to attain a suitable track are low enough for printing to be
conducted on cheap plastic fi lms such as polyethylene terephthalate. The ultimate
challenge is to attain the appropriate balance of resolution, reproducibility and
costs required for mass implementation of disposable radio frequency identifi cation
tagging (RFID tags). This technology could revolutionise tracking of goods and
mass scanning of multiple items without the need for barcodes.
Solar c ells v ia s pin c oating
The major technology used in solar cells is currently based on thin fi lm semiconduc-
tor methods. Whilst the so-called dye sensitised solar cell (DSSC) exists its technol-
ogy is still under development and relies on the use of nanoparticles to function. The
development of the dye sensitised solar cell is discussed in more detail later.
Conventional solar cells rely on a junction between to materials with different band
gaps to fi rstly absorb photons and then separate the associated charge carriers so
that they may be harnessed to do useful work. One issue with the production of solar
cells of this type is that they rely on vapour deposition technologies to prepare the
thin fi lms. This requires batch processing, high temperatures (
400 °C) and closed
and carefully controlled atmospheres. These factors combine to make solar cells
expensive, rigid devices with limited production volumes. These restrictions can be
traced to the simple requirement of having to prepare thin fi lms of the various com-
ponents. Other methods exist which allow the preparation of thin fi lms, for example
spin coating is routinely used to prepare thin fi lms of polymers. However, the insolu-
bility of common semiconducting materials used in the preparation of solar cells
makes this approach unsuitable. The advent of nanotechnology allows the prepara-
tion of stable suspensions of nanoparticles that may be processed in a similar
manner to a solution of the bulk material. This means that spin coating technology
can be used to prepare solar cells directly from nanoparticles in an on-line fashion
using fl exible materials as the support. The application of this technology has been
repeatedly demonstrated for solar cells based both solely on nanoparticles, such as
cadmium telluride and cadmium selenide, as well as cells based on semiconducting
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