Flexible Solar Cells on Textiles a report by John Wilson,1,2 Robert Mather,1,3 Helena Lind4 and Adel Diyaf4 1. Director, Power Textiles Limited, Selkirk; 2. Professor of Materials Processing; 3. Honorary Senior Lecturer; 4. PhD Research Student; Heriot-Watt University
Photovoltaic solar cells are becoming commonplace as more and more domestic arrays are installed, but these mostly use crystalline silicon cells contained within heavy casings having a glass cover plate. Several alternative types reduce the manufacturing cost of solar cells, many based on thin-film versions of the active semiconductor that converts sunlight into electricity. Another advantage of thin-film cells is their reduced materials and embedded energy requirements, which lead to a faster energy pay-back time,1
efficiency is not compromised too much by the poorer quality of thin films. Furthermore, there are specific applications in which flexible arrays would be especially appropriate, for instance on tents and emergency shelters and on specialist clothing. In these uses, it would be beneficial to integrate the solar cells directly onto a woven fabric rather than to attach cells that were separately formed on a plastic sheet.
Meanwhile, there have been different approaches to combining solar cells and fabrics,2–10
including organic polymer solar cells put onto
fibres for subsequent weaving and ink-jet printed nanoparticles for dye-sensitised solar cells, as well as the easier option of depositing organic or the more common inorganic solar cell films onto a polymer sheet. Some of these ideas have progressed as far as concepts, but few are yet in commercial production.11–13
Thin-film Silicon on Polyester
The route that we have taken is to deposit thin-film silicon (Si) layers directly onto woven polyester, a commodity fabric that is resistant to ultraviolet (UV) damage and thermomechanically stable up to ~240 °C.14
Alternative fibre-based materials are UV sensitive or degrade
Our silicon-coating process is at a relatively low temperature for an inorganic material, but still requires substrates to
below 200 °C (e.g., natural fibres and acrylics), or are more expensive (e.g., polyimides).15
John Wilson is a Director of Power Textiles Limited and has been Professor of Materials Processing at Heriot-Watt University since 1996. His interests in the applications of solar energy have spanned over 30 years of research, pioneering amorphous silicon solar cell research in the UK and more recently applying the technology to enhancing the efficiency of crystalline silicon solar cells. He was a founder secretary of the Scottish Solar Energy Group.
E:
j.i.b.wilson@hw.ac.uk
Robert Mather is a Director of Power Textiles Limited and Director of Mather Technology Solutions, having previously been a Senior Lecturer in the School of Textiles and Design at Heriot-Watt University. At Heriot-Watt University, he was Director of its Technical Textiles and Polymer Innovation Unit, which successfully undertook the development projects for a variety of companies. He was also the Director of the Biomedical Textiles Research Centre at Heriot-Watt University and is now an honorary Senior Lecturer.
be heated to ~200 °C and to withstand attack by atomic hydrogen. The method used is plasma-enhanced chemical vapour deposition (PECVD)
in which low-pressure mixtures of gases that include silane (SiH4) are activated by an electrical discharge, thus producing a thin-film coating of silicon on adjacent heated surfaces. Atomic hydrogen is a byproduct of the decomposition of silane.
as long as their energy conversion
The active photovoltaic part of the cell is similar to the hydrogenated amorphous silicon (a-Si:H) used in small-scale photovoltaic arrays on glass. This originated from research using radiofrequency (RF) excitation of silane and dopant gases at Dundee University in the 1970s.16
This
technique circumvented the usual limitation of thin-film silicon, that of having too many defects to behave as a well-controlled semiconductor. It was gradually appreciated that hydrogen was a key feature in the synthesis process, then known as glow-discharge deposition, and it has remained an important element of the structure of thin-film silicon. Here it takes up dangling bonds in the random lattice of silicon atoms, thus removing these charge-carrier trapping centres and at the same time relieving stress in the distorted lattice. The enhanced material a-Si:H could then be doped either p-type or n-type by adding small amounts of boron or phosphorus, so enabling diodes to be constructed. In addition, it was realised that a more crystalline form of a-Si:H could be synthesised by adding large amounts of hydrogen to silane, where it etched away some of the less resilient amorphous material to leave crystallites in an amorphous matrix. Not surprisingly, this reduces the deposition rate,17
which may be increased to some
extent by using microwave PECVD instead of RF energy (shown in Figure 1). Indeed, either hydrogen or argon may be added to provide ‘nanocrystalline silicon’ rather than an amorphous layer.18
Textile Fabric Construction
In addition to the nature of the fibre comprising the textile fabric, the type of fabric construction is also important for the performance of the solar cell. The construction not only affects the physical and mechanical properties of a fabric, but can also influence the effectiveness of its conversion into an electrical conductor. Rendering a fabric electrically conductive can be achieved by a variety of methods15
but, unless the
fabric already contains electrically conducting yarns, they all require the deposition of electrically conducting materials. Conductivity is consequently more difficult to achieve in more loosely constructed fabrics, because they contain clear voids between the yarns.
Woven fabrics, such as our woven polyester, are generally preferred:19,20 they possess the greatest dimensional stability and can be constructed to give a vast range of desired flexibilities and conformations. Moreover, in most types of woven fabric, the yarns are arranged very close to one another, so that there is little void space between them. In contrast, knitted fabrics generally consist of much looser structures and retain their shapes less well. Also, the rupture of a constituent yarn may cause
66 © TOUCH BRIEFINGS 2012
Solar
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