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

Abstract

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.

Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography

“…Finally, the resist residues are removed by dissolution in acetone. For the solar cell fabrication, a mesa heterojunction-emitter cell structure is used [24].…”

“…This "Epifree" (epitaxy-free) technique and its specific use in solar cells are described extensively in ref. [24].…”

Film transfer enabled by nanosheet seed layers on arbitrary sacrificial substrates

“…Several available methods for the transfer of perovskite films are the laser lift-off process, 2,3 ion cutting, 4 the use of water-soluble substrates, 5,6 and the epifree process. 7,8 In this letter, an approach for film transfer is demonstrated that makes use of seed layers of nanosheets on arbitrary sacrificial substrates. Nanosheets are essentially two-dimensional single crystals; they have a constant thickness of a few nanometers at most and their lateral size is mostly in the micrometer range, 9 while their oriented crystal structure enables them to direct epitaxial growth of thin films.…”

Sunlight-thin nanophotonic monocrystalline silicon solar cells

“…Here, the nanophotonic ultra-thin monocrystalline-silicon solar cell efficiently combines a kerf-less thin film method [18], a higher-absorption self-assembled nanotexture with amorphous order [19], with a robust surface passivation [20] and an optimal nanotexture integration [21]. This allows the scaling of the silicon thickness (830 nm) down to the sunlight wavelength, while boosting the optical performance (Jsc, short-circuit current) both for light in-coupling and light trapping without degrading the electrical performance (Voc, opencircuit voltage and FF, fill factor).…”

“…The front surface of the initially micron-thin solar cell is textured with rounded inverse nanopyramids, the 'nanocups', that self-assemble with short-range order and are the result of a carefully engineered trade-off between photon and charge-carrier collection [19,21] ( figure 1(a); see the methods section for the fabrication details of the kerf-less monocrystalline film [18] and the nanocups [19]). The nanocups are covered with standard hydrogenated amorphous-silicon (a-Si:H) serving as the passivating layer and heterojunction emitter.…”