T. Pingault, N. Zayyoun, P.S.P. Kuisseu, E. Ntsoenzok, J-P. Blondeau, P. Bellanger, S. Roques, A. Slaoui, A.G. Ulyashin, B. Belhorma
Keywords: kerfless, monocrystalline silicon, thin films, solar cells, hydrogen implantation
Summary:In first generation solar cells, the obvious way to reduce the cost is through the reduction of both silicon consumption and loss. Indeed, the cost of silicon, which is used in excess, represents up to 60% of the total solar panel cost. Because of the wafer sawing process, 180µm-thick wafers are usually used for solar cell processing, when 50µm would be enough to absorb most incident photons. Moreover, this sawing process leads to up to 50% of kerf losses in the slurry. In this work, an innovative combination of hydrogen implantation and stress-induced spalling is used in order to exfoliate 40 to 60µm-thick silicon foils, which are then used for solar cell processing. Our process can be described as follows: after a low energy hydrogen implantation (80 keV, for a projected range around 710 nm deep), the implanted silicon is thermally annealed in order to grow extended defects, then glued on a metallic substrate using polymeric glue. A dipping in liquid nitrogen then induces an important stress, leading to the exfoliation of an ultra-thin silicon seed layer. Depending on the thermal annealing preceding the gluing, a way to modulate the exfoliated thickness has been observed, and layers of thicknesses between 40 and 120µm have been produced. These layers have then been used in a classical homojunction solar cell process. Such a process is ideal in order to test the quality of our thin silicon layers by measuring the efficiencies of these solar cells. In order to avoid the drawback effect of implantation-induced defects at Rp (projected ion range), the first micrometer of the implanted side of the cells was removed. The rest of the process is then classical, with the front side doping obtained by diffusion of a phosphorus solution, and BSF and electrode deposition is made using electron beam deposition. Solar cells with efficiencies up to 8% have been produced using exfoliated thin silicon foils. This value is already very remarkable since it is only 5% less than the reference sample (with a thickness higher than 200µm) which provided an efficiency of 13% with this technology. Keeping in mind that (111) oriented foils are not ideal in such classical structures because of the impossibility to texture their surface, we are confident that by using more optimized structures and by optimizing the surface of these layers, we can improve the efficiency of solar cells elaborated by our approach. Citations:  A. Luque and S. Hegedus, Handbook of Photovoltaics, 1st edition, Wiley, 2003.  M. Green, Silicon Solar Cells: advanced principle & practice, chapter 7, 1995.  South Bay Technology Inc., “Kerf Loss Comparisons”. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 608593.