Systematic study of the process parameters affecting hydrogen plasma passivation of polycrystalline silicon and polycrystalline silicon solar cells

Abstract

We report detailed studies of hydrogen passivation of polycrystalline silicon wafers by RF plasma and the effect of various process parameters on the extent of passivation as seen through electrical measurements. Similar studies made on polysilicon solar cells are also reported. The need for such studies is emphasized via some interesting results which show an excessive dependence of effective passivation on the process conditions such as processing time, substrate temperature, gas pressure and RF power which seems to have been overlooked by earlier workers. Our results reveal that there is an optimum for almost all the above parameters which yield the most effective passivation of the grain boundaries in polycrystalline silicon. Physically, therefore, it implies that optimum hydrogen incorporation in a particular bonding configuration is a necessary condition to achieve best passivation. Results of samples passivated for several hours indicate the presence of yet another process along with the normal hydrogen diffusion. Possibilities of bond breaking or hydrogen incorporation in different bonding configurations cannot be completely ruled out. The dependence of effective passivation on these process conditions is also revealed by the systematic studies of variation in average carrier concentration, charge carrier mobility and resistivity with substrate temperature, RF power and gas pressure. Polycrystalline silicon solar cells passivated with the optimum process conditions show significant improvement in the efficiency and fill factor. Presence of hydrogen was confirmed by quadrupole mass spectrometry studies by heating the passivated samples.