Flexible Solar Cells on Textiles


Figure 2: The Component Layers of our Flexible Solar Cell on Textile


Sunlight


Power Textiles Limited


Performance prospects


The best-confirmed solar-cell efficiencies from various technologies have been presented at six-monthly intervals since 1993 by a group of experts.24


Currently, the best amorphous and nanocrystalline thin-film Cover film


+ -


Transparent conducting oxide


Silicon film Metal film


Organic conductor Fabric


Figure 3: Optical Micrograph of a Typical Woven Textile with Added Thin-Film Coatings


silicon cells of ~1 cm2 area have efficiencies in standard test conditions (global Air mass (AM) 1.5 spectrum, 1,000 W/m2, at 25 °C) of just over 10 %. These values come from cells on glass, with open circuit voltage = 0.886 V, short circuit current density = 16.75 mA/cm2 and fill factor = 67.0 % for the amorphous cell and corresponding values of 0.539 V, 24.4 mA/cm2 and 76.6 % for the nanocrystalline cell. It is to be expected that amorphous materials will generally have a worse performance than crystalline ones, even if only partially crystallised, but may have higher open-circuit voltages if their bandgaps are wider, as noted above. With the added disadvantage of an uneven substrate, we anticipate lower current density and fill factor for our cells on fabric, perhaps limiting efficiency to ~6 %. While this cannot compete with better cells in the same situation, we are targeting different applications, as noted above.


Whereas we have optimised the PECVD process for silicon on polyester and have functioning contact materials, this is work in progress and our devices show diode rectification characteristics, but with poor photocurrents. The remaining challenges to developing useful textile-based solar cells are centred on the fabrication of a reliable and continuous conducting coating, and on the fabrication of conformal thin-film silicon layers that do not have such defects as pinholes, which provide short-circuit paths between the two electrodes. We believe that both the underlying science and the technology are acceptable and can be extended to roll-to-roll coating of fabric strip, enabling a production process for flexible solar cells. n


The left-hand image shows the uncoated fabric, the next image has a conducting coating, with a metal coating added in the third image and the final image shows these with the addition of a silicon coating. The scale has 10 μm fine divisions.


there is the possibility of an enhanced optical absorption by forward scattering of weakly absorbed light at the rear face of the cells. Some form of such ‘optical engineering’ is advantageous for thin-film cells on


1. 2.


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Konarka, Travellers Choice Solar Bag. Available at: www.konarka.com/media/pdf/konarka_casestudy_ travelerschoice.pdf (accessed 23 January 2012).


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The authors would like to thank the UK Engineering and Physical Sciences Research Council and the Libyan Government for PhD studentships, the Scottish Government for SMART and SUPA START funding, and the Scottish Optoelectronics Association for Technology Transfer Opportunity Mechanism funding. Robyn Adams is acknowledged for her assistance with optical microscopy.


18. Lind H, Wilson J, Mather R, Raman spectroscopy of thin-film silicon on woven polyester, Phys Stat Sol A, 2011;208:2765–71.


19. Abdelfattah MS, Formation of textile structures for giant-area applications. In: Shur MS, Wilson PM, Urban D (eds), Electronics on Unconventional Substrates – Electrotextiles and Giant-Area Flexible Circuits, MRS Symposium Proceedings, Warrendale: Materials Research Society, 2003;736:25–36.


20. Bonderover E, Wagner S, Suo Z, Amorphous silicon thin transistors on Kapton fibers. In: Shur MS, Wilson PM, Urban D (eds), Electronics on Unconventional Substrates – Electrotextiles and Giant-Area Flexible Circuits, MRS Symposium Proceedings, Warrendale: Materials Research Society, 2003;736:109–14.


21. Patent applications GB 0808038.4 (2/5/08) and PCT/GB2009/000842, Power Textiles Limited.


22. Schubert M, Merz R, Flexible solar cells and modules, Phil Mag, 2009;89:2623–44.


23. Bonnar MP, Burnside BM, Little A, et al., Plasma polymer films for dropwise condensation of steam, Adv Mater CVD, 1997;3:201–7.


24. Green MA, Emery K, Hishikawa, et al., Solar cell efficiency tables (Version 38), Prog Photovolt: Res Appl, 2011;19:565–72.


MODERN ENERGY REVIEW – VOLUME 4 ISSUE 1


smooth, reflecting surfaces, for which texturing is considered desirable to increase low optical absorption. Textile production offers a wide variety of weaves and finishes that have yet to be explored for solar-cell optimisation.


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