S. Pokam, T. Pingault, E. Ntsoenzok, G. Regula, F. Mazen, P. Bellanger, S. Roques, A. Slaoui, T. Pingault
CNRS (National Center of Scientific Research),
Keywords: ultra-thin substrates, silicon, hydrogen implantation, delamination, c-Si solar cells, homojunction, HIT
Summary:Hydrogen implantation in the MeV energy range in silicon followed by a thermal annealing is known as a kerf-less cutting approach that can be used to slice ultra-thin silicon substrates with thicknesses compatible to PV applications. Such a process has two major advantages: it results in no losses of raw material and it allows a very sensitive reduction of silicon substrate thicknesses (or reduction of Si consumption). The implementation of the process might therefore help decreasing down solar cells prices. First results reported with this approach were performed by Assaf et al . These authors reported the delamination of ultra-thin (111)Si substrates with thicknesses in the range of 10 µm to 50 µm. Solar cell efficiencies of 13% were reported on such ultra-thin substrate [2, 3]. Currently, the feasibility of this process has been only reported on (111)Si, while almost no result was reported on (100)Si, although the latter is the main silicon used for solar cells. In this work, our effort focused on the realization solar cells with (100)Si ultra-thin substrates by using hydrogen implantation for layer delamination. By using different hydrogen doses and energies, we were able to produce (100)Si with thicknesses higher or equal to 50 µm. Production of thinner substrates remain unsuccessful yet, while in (111)Si successful delamination with large area was reported for thicknesses lower than 20 µm . Conventional homojunction solar cell prototypes were built and test for validation. Solar cells built using delaminated (100)Si ultra-thin substrates of 70 µm thick provide efficiencies only 3% lower than that obtained with a standard reference substrate. These promising efficiencies show the good crystalline quality of ultra-thin substrates produced by means of hydrogen implantation. However, the VOC of foils is a little bit weak (around 500 mV), showing a real impact from serial resistance and showing that there is still a large window for improvement. HIT technology is another potential way to optimize the efficiency of these layers since it’s more appropriated for these ultra-thin layers. The cells with HIT technology are in process.  H. Assaf et al, Nucl. Instrum. and Methods Phys. Res. B 240, 183 (2005)  G. Ryding et al, Twin Creeks White Paper, (2012)  F. Henley et al, 34th IEEE PVSC, June (2009). The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 608593.