Improvement in Al-Alloyed emitter of rear-junction n-type mc-si cells using a stack of pure Al and screen-printed Al paste

Significant improvements in the fill factor and open circuit voltage of large area (>100 cm²) n-type mc-Si rear-junction cells have been obtained using a stack of pure Al and screen-printed Al paste. The Al-alloyed emitter of the cells was improved by using a thin 1-2 micron sputtered or evaporated Al layer under screen-printed Al paste. The process sequence that was used involved minor modifications to the industrial process for p-type cells which uses Al-alloying as back surface field. Adding a pure Al layer under screen-printed Al-paste led to large increase in shunt resistance of the cells, especially when a low quantity of screen-printed paste was used. Significant improvement in FF (from 46 % to 76 %) and Voc (from 480 mV to 610 mV) were obtained by using stack of 2 μm of pure Al under ~ 5 mg/cm² screen-printed Al paste. Furthermore, very high fill factors up to 82.2 % were obtained using a 1-μm sputtered Al layer under ~ 10 mg/cm² screen-printed Al layer. The improvements were made possible by reduction in discontinuities (shunts) in Al-alloyed junction.