Front side metallization of crystalline silicon solar cells using selectively laser drilled contact openings
Selective removal of the silicon nitride dielectric layer has been demonstrated even using nanosecond range laser pulses, through carefully controlling the energy density of laser pulses. It has been found that, a laser pulse with a peak energy density of 4.3 J/cm2 or lower can remove the nitride layer without altering the underlying silicon while a pulse with a peak energy density of 4.8 J/cm2 or higher will substantially damage the silicon. With the laser energy density maintained below the threshold for silicon ablation, multiple laser pulses can be used to more completely remove the nitride layer. Also, using high quality blanket sputtered nickel film as metal contact layer and screen printed silver gridlines as an etching protection mask, a new method for front side metallization has been developed. The specific contact resistance can be reduced by about two orders of magnitude compared to the conventional screen printed and fired through silver contact, and the firing temperature can be lowered to about 500C.