An optimized rapid aluminum back surface field technique for silicon solarcells

Screen-printing and rapid thermal annealing have been combined to achieve a n aluminum-alloyed back surface field (Al-BSF) that lowers the effective ba ck surface recombination velocity (S-eff) to approximately 200 cm/s for sol ar cells formed on 2.3 Ohm-cm Si. Analysis and characterization of the BSF structures show that this formation process satisfies the two main requirem ents for achieving ion S-eff: 1) deep p(+) regions and 2) uniform junctions , Screen-printing is ideally suited for fast deposition of thick Al films w hich, upon alloying, result in deep RSF regions. Use of a rapid alloying tr eatment is shown to significantly improve the BSF junction uniformity and r educe S-eff. The Al-BSF's formed by screen-printing and rapid alloying have been integrated into both laboratory and industrial-type fabrication seque nces to achieve solar cell efficiencies in excess of 19.0 and 17.0%, respec tively, on planar 2.3 Ohm-cm float zone Si. For both process sequences, the se cell efficiencies are 1-2% (absolute) higher than analogous cells made w ith unoptimized Al-BSF's or highly recombinative rear surfaces.