Deposition & characterisation of silicon and conductive layers on woven polyester
Lind, Anna Helena Nilsdotter
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Textile and semiconductor processes were combined to produce a flexible solar panel by depositing silicon thin film onto woven polyester. Semiconductor processes such as evaporation, Plasma Enhanced Chemical Vapour Deposition (PECVD) and RFsputtering were done under vacuum conditions. Microwave PECVD proved difficult for textile substrates as the weave was damaged using parameter settings commonly used for conventional substrates such as silicon wafers and glass. PECVD parameters such as temperature, gas flow-rate, mixture, pressure and power were adjusted to allow the textile to be processed and a good quality silicon thin film to be deposited. An extra conductive layer was introduced between the textile and metal back-contact to support the cell. The silicon film structure changed from amorphous to mixed crystal growth in an amorphous matrix, as revealed by Raman spectroscopy and light transmission. The silicon Raman spectrum often had three peaks with the middle one, a fingerprint for nanocrystal growth with a hexagonal wurtzite structure in between the amorphous and crystalline peaks. Process conditions for pure amorphous and microcrystalline structures were also established, requiring two peaks to fit the Raman spectrum. Different structures have different band-gap energies and these were determined by measuring the variation in light transmission. An amorphous structure has a band-gap energy of 1.8eV while a crystalline silicon structure has a band-gap of 1eV and a mixed nanocrystalline content has an intermediate value which depends on the crystal size. A microcrystalline structure has a band-gap of 1.6eV.