INVESTIGATIONS ON THE MECHANISM BEHIND THE BENEFICIAL EFFECT OF A FORMING GAS ANNEAL ON SOLAR CELLS WITH SILVER THICK FILM CONTACTS

摘要

In this contribution the beneficial effect of the forming gas anneal (FGA) on solar cells with silver thick film1 contacts is investigated regarding its application for contacting emitters with phosphorous surface concentrations below N_(D.surface) = 1×10~(20) cm~(-3). Measurements and simulations of silver - silicon contacts show that for emitters with N_(D.surface) > 2×10~(19) cm~(-3) the limiting factor for contacting is the current path from the silver crystals, grown onto the emitter, into the silver finger. The beneficial effect of the FGA on the fill factor (FF) of Si solar cells with screen-printed Ag contacts is found to be thermally activated at temperatures above 300℃ and to be irreversibly stable in time. It is shown that the anneal affects the current path between silver crystals and finger. The formation of metal precipitates in the glass layer leading to an increase of the tunnelling probability and therefore lower contact resistance is suggested as the relevant mechanism behind the FF improvement. The results were successfully applied to fabricate solar cells with an emitter surface doping concentration of N_(D.suface) < 5×10~(19) cm~(-3).