Epitaxy-free monocrystalline silicon thin film: first steps beyond proof-of-concept solar cells

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 amicron-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-mu m-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 V-oc was, however, limited and bound to a drop in J(sc). 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-mu m-thick material over 1 cm x 1 cm areas. The efficiency of the next cells is expected to increase with this thicker material. Copyright (C) 2010 John Wiley & Sons, Ltd.