Helium versus hydrogen dilution in the optimization of polymorphous silicon solar cells

We have studied how the increase of pressure in the deposition of polymorphous silicon can reduce the ion energy and the damage of the p-layer in p-i-n solar cells. By increasing the pressure during the deposition of the i-layer to 3 Torr, we have been able to increase the fill factor and the solar cell efficiency up to 8.4%. In an attempt to reduce the optical gap and increase further the deposition rate we have studied the use of He dilution. Changing the dilution from hydrogen to He leads to a higher deposition rate at lower rf power while keeping similar transport properties. Moreover, the lower hydrogen content of He diluted films results in a smaller band gap and a higher short circuit current in solar cells. Thus, silane-He mixtures allow to reduce the ion energy for a constant deposition rate and to minimize the damage at the p-layer and p/i interface. The cells show a fill factor of 0.69, V_oc of 0.91 V, an initial efficiency of 9.3% on textured SnO₂.