An optimized rapid aluminum back surface field technique for silicon solar cells

Screen-printing and rapid thermal annealing have been combined to achieve an aluminum-alloyed back surface field (Al-BSF) that lowers the effective back surface recombination velocity (S_eff) to approximately 200 cm/s for solar cells formed on 2.3 Ω-cm Si. Analysis and characterization of the BSF structures show that this formation process satisfies the two main requirements for achieving low S_eff: (1) deep p⁺ regions and (2) uniform junctions. Screen-printing is ideally suited for fast deposition of thick Al films which, upon alloying, result in deep BSF regions. Use of a rapid alloying treatment is shown to significantly improve the BSF junction uniformity and reduce S_eff. The Al-BSFs formed by screen-printing and rapid alloying have been integrated into both laboratory and industrial-type fabrication sequences to achieve solar cell efficiencies in excess of 19.0 and 17.0%, respectively, on planar 2.3 Ω-cm float zone Si. For both process sequences, these cell efficiencies are 1-2% (absolute) higher than analogous cells made with unoptimized Al-BSFs or highly recombinative rear surfaces