Flexible Solar Cells on Textiles
laddering. These problems are exacerbated if the shape of the fabric changes continuously, as in apparel usage. So-called nonwoven fabrics have also been explored. These are fabrics that are neither woven nor knitted, but are created by bonding yarns through the use of mechanical, thermal and/or chemical means. Felts are good examples. As yet, nonwoven fabrics do not, for the most part, possess the strength and dimensional stability of woven fabrics, but there is currently commercial interest in achieving these properties.
Solar Cell Structure
Our solar cell comprises thin layers of n-type, undoped and p-type silicon-deposited sequentially onto an electrically conducting surface. We presently use a metallic layer conductor on the polyester and a second, upper, transparent conducting oxide (TCO) that enables sunlight to pass into the Si21
(see Figure 2). By patterning the
electrodes, it is possible to interconnect cells on a single substrate to provide integrated series or parallel configurations.22
Conducting
polymers, such as PEDOT:PSS or polyaniline, are insufficiently conducting to act as the principal charge collector for solar cells, and common TCOs, such as indium tin oxide, may be too brittle for flexible cells. However, replacements for these materials are being developed, and include such novel options as metal nanowires and graphene. In future, it may be possible to use polymers as the active photovoltaic medium, but at present they are rather sensitive to oxygen and moisture and demand a high degree of integrity in hermetic sealing.
At this stage in the development there is no final decision on how the arrays will be encapsulated, but we have assessed a number of options
from conventional laminating to liquid-resin coating. We can also use PECVD to provide a first permeation barrier of plasma-deposited polymer, from a precursor such as hexamethyldisiloxane. This method has been used to deposit silica-like films and hydrophobic polymer coatings onto metals.23
The performance of solar cells on fabric is compromised by the complex shape of the surface, which makes great demands on the uniformity and conformity of thin-film coating (see Figure 3), but in contrast
Figure 1: Microwave Plasma-enhanced Chemical Vapour Deposition System
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