A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline‐silicon (pc‐Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc‐Si layers obtained by aluminum‐induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc‐Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk‐Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc‐Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc‐Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin‐on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a‐Si layers that yield much higher Voc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc‐Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc‐Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.

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Crystalline Si (c-Si) technology is dominating the photovoltaics market. These modules are nonetheless still relatively expensive, in particular because of the costly silicon wafers, which require large thickness mostly to ease handling. Thin-film technologies, on the other hand, use much less active material, exhibit a much lower production cost per unit area, but achieve an efficiency still limited on module level, which increases the total system costs. A meet-in-the-middle is possible and is the object of this paper. The development of c-Si thin-foil modules is presented: first, the fabrication of the active material on a glass module and then the processing of the Si foils into solar cells, directly on module level. The activity of IMEC in this area is put into perspective with regard to worldwide research results. It appears that great opportunities are offered to this cell concept, although some challenges still need to be tackled before cost-effective and reliable industrial production can be launched.

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Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason for the grain size independent device performance. Besides dislocations and stacking faults, also Σ3 boundaries were electrically active as shown by combining electron backscattered diffraction with EBIC measurements. The electrical activity of the defects is probably triggered by impurity decoration. Plasma hydrogenation changed the electrical behavior of the defects, as seen by photoluminescence, but the defects were not completely passivated as shown by EBIC measurements. In order to reveal the origin of the defects, cross section transmission electron microscopy measurements were done showing that the intragrain defects are already present in the AIC seed layer and get copied into the epitaxial layer during epitaxial growth. The same types of intragrain defects were found in layers made on different substrates (alumina ceramic, glass ceramic, and oxidized silicon wafer) from which we conclude that intragrain defects are not related to the relatively rough alumina ceramic substrates often used in combination with high temperature epitaxy. Further improvement of the material quality, and hence device performance, is therefore not simply achieved by increasing the grain size, but the intragrain quality of the material also needs to be taken into account. For pc-Si layers based on AIC and epitaxial growth, the seed layer has a crucial impact on the intragrain defect formation.