High-efficiency cells from layer transfer: A first step towards Si thin-film/wafer hybrid technologies

The literature describes various techniques for fabricating thin monocrystalline Si films without the need of sawing. Layer transfer using epitaxy on porous Si (PSI) and subsequent layer separation from the growth substrate is one particular attractive option. A 40 μm thick epitaxial Si cell from this so-called PSI process is capable of saving about 80 % of the Si that is consumed by a 180 μm-thick Si wafer plus 100 μm Si kerf loss. In addition, the two crystallizations of growing the poly-feedstock and of growing the ingot are replaced by a single crystallization: the epitaxial growth of a thin Si layer. Applying the PSI process, we demonstrate an independently confirmed aperture efficiency of 19.1 % for a 4 cm²-sized layer transfer cell that has a thickness of only 43 μm. This is the highest efficiency ever reported for a thin-film (<;50 μm) crystalline Si layer transfer cell. We achieve this record with a passivated emitter and rear contact (PERC) structure. The passivation layer is Al₂O₃ from atomic layer deposition (ALD). The contacts on the rear side are formed by laser ablation using ultrashort ps pulses (LCO). The ISFH roadmap that our PSI process development is a part of is also discussed in this contribution. Our program aims at developing thin-film/wafer hybrid technologies that shall combine the high efficiency potential of thin monocrystalline Si films with the low cost per area of the Si-thin-film world. 19%-efficient and less then 50 μm-thick layer transfer cells mark a large step forward towards the realization of such thin-film/wafer hybrid technologies.