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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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The “Epifree” process involves the lift‐off of a high‐quality monocrystalline film formed by reorganization upon annealing of cylindrical macropore arrays in silicon, and can thus provide high‐quality silicon films without resorting to costly epitaxy. The challenge of this new process lies in etching controlled and regular pores in silicon in a cost‐efficient way, and in developing a process compatible with the difficulty of handling a micron‐thin material. Proof‐of‐concept cells have previously been achieved and this paper presents the latest progress, with a first development of thicker films and the inclusion of rear‐side passivation. The energy‐conversion efficiency of 1‐µm‐thin Epifree cells was improved from 2.6 to 4.1% by depositing a stack of amorphous silicon (a‐Si) layers as rear‐side passivation. The increase in Voc was, however, limited and bound to a drop in Jsc. The choice of a‐Si was revealed to be unsuitable because of the thinness of the film and the presence of a full aluminum rear contact. The thinness of the film leads to a decrease in rear‐side reflectivity by the a‐Si absorption, and the aluminum, although not leading to crystallization, partly migrates inside the a‐Si stack upon anodic bonding as shown by TEM. These factors indicate that an alternative surface passivation should be developed. In parallel to process developments, the material was thickened by modifying the macropore array dimensions, leading to a 2.4‐µm‐thick material over 1 cm × 1 cm areas. The efficiency of the next cells is expected to increase with this thicker material. Copyright © 2010 John Wiley & Sons, Ltd.

A concept that could provide a thin monocrystalline-silicon absorber layer without resorting to the expensive step of epitaxy would be very appealing for reducing the cost of solar cells. The empty-space-in-silicon technique by which thin films of silicon can be formed by reorganization of regular arrays of cylindrical voids at high temperature may be such a concept if the high quality of the thin film could be ensured on centimeter-large areas. While previous works mainly investigated the influence of the porous array on the final structure, this work focuses on the practical aspects of the high-temperature step and its application to large areas. An insight into the defects that may form is given and the origin of these defects is discussed, providing recommendations on how to avoid them. Surface roughening, pitting, formation of holes, and silicon pillars could be attributed to the nonuniform reactions between Si, SiO2, and SiO. Hydrogen atmospheres are therefore preferred for reorganization of macroporous arrays. Argon atmospheres, however, may provide high-quality silicon thin films as well, possibly even more easily transferable, as long as annealing is performed in controlled, clean, and oxygen-free conditions. Our experiments on large areas also highlight the importance of kinetics, which had not been considered up to now and which will require further understanding to ensure a complete reorganization over any wafer area.