Optimized aluminum back surface field techniques for silicon solar cells

Screen-printed Al and rapid thermal alloying have been combined in order to achieve an Al back surface field (Al-BSF) which lowers the effective back surface recombination velocity to 200 cm/s on 2.3 /spl Omega/cm Si. This Al-BSF process has been integrated into a high-efficiency, laboratory fabrication sequence as well as a high-throughput, industrial-type process in order to achieve solar cell efficiencies in excess of 19.0% and 17.0%, respectively, on 2.3 /spl Omega/cm FZ Si. For both process sequences, the efficiency values are 1-2 absolute percentage points higher than cells made with unoptimized Al-BSFs. The critical process requirements for optimal Al-BSF formation are: (1) the use of a fast ramp rate to reach the alloying temperature; and (2) thick film Al deposition prior to alloying.