Investigation of the influence of sintering process on silicon solar cells

The efficiency of industrial screen printed solar cells depends critically on the sintering process. Although the formation of Ag-Si contacts during sintering process and the current transport mechanism has not been fully understood, there´re basically two assumptions: (a) “isolated Ag crystallites” model proposed by Ballif and (b) “Ag colloids assisted tunneling” model by Li. In this paper, it´s found that our results are more in agreement with the Ag colloids assisted tunneling model under the optimal firing condition. This is based on the observation that, by varying the sintering peak temperature from 915°C to 875°C, the V_oc increases slightly from 631.3 mV to 633.8 mV while the series resistance decreases from 0.99 ohm.cm² to 0.61 ohm. cm². This results in the average cell efficiency increasing from 18.19% to 18.73% on a homogeneous emitter of 75Ω/□ on Cz silicon wafers. It is concluded that higher sintering temperature enhances the etching reaction of the glass frit, which dissolves more Ag to produce larger Ag crystallites along with a thicker glass layer between the Ag crystallites and Ag bulk. The larger Ag crystallites mean deeper penetration into the emitter, which causes a reduction in V_oc of the cell. In the over-fired condition, the thicker glass layer increases the resistance of current transport via tunneling, thus increases the series resistance of solar cells.