N-I-P Micromorph Solar Cells on Aluminium Substrates
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by our group (IMT). These so-called 'micromorph' cells are 'real' tandem cells, employing two materials of a different optical gap for the top and the bottom cell (1.7eV and leV, respectively [2]). The enhanced infrared absorption and the lack of light-induced degradation [3] of the microcrystalline silicon are successfully combined with the high Voc of the amorphous silicon solar cell. This lead sofar to solar cells of the p-i-n-p-i-n superstrate structure on classical TCO on glass of a confirmed stabilised efficiency of 10.5% [4]. Conceptionally, cells of the p-i-n deposition order are limited to transparent substrates like glass or some (rare!) highly transparent and temperature resistant polymers, as the light always enters the cell through the substrate in this configuration. To be able to use a wider choice of substrates, including opaque metal sheets, the inverse n-i-p deposition order has to be applied. In this case, the light enters the cell from the 'top' side, i.e. the side opposite to the substrate. One advantage of the n-i-p configuration is the better control of the delicate
/-interface, as it is deposited last, on the opposite side of the nucleation base [5]. Microcrystalline window layers are used more successfully in the n-i-p structure than in p-i-n cells, leading to high Voc-values even without any buffer layer. In this study aluminium sheets are used as substrates. The long term goal is the direct integration of silicon thin film solar cells into a type of fagade construction elements, which consists of an aluminium-polymer compound. This type of faqade construction elements- but without the incorporated solar cell- is a standard commercial product of which millions of square meters have been sold [6]. Aluminium has also been chosen for its ductility which offers the (not yet employed) possibility of controlled geometrical texturing for 877 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
improved light-trapping, allowing to deposit thinner and therefore more stable a-Si:H solar cells. EXPERIMENT AND RESULTS Amorphous silicon n-i-p single junction solar cells To be able to deposit a-Si:H solar cells on commercial aluminium sheets, as was our aim, several substrate-related problems had to be solved. Metal diffusion from the substrate or from the back contact had to be suppressed by an antidiffusion layer, as is reported elsewhere [7]. Surface topography of the aluminium sheets turned out to be a hard problem. The surface of the metal sheet has to be treated in an appropriate way to avoid 'pinholes' in the solar cells or in general to get high values of parallel resistance. In the n-i-p configuration the top contact is deposited last. A thin film of Indium Tin Oxide (ITO) can serve as electrical contact and antireflection (AR) coating at the same time. An antireflection coating is always optimised for a certain wavelength and has a certain 'spectral width', i.e. a range of wavelength, where the reflectivity is actually reduced. In our first case, the flat cell of amorphous
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