Abstract
Solar cells with efficiencies as high as 18.6% (1 cm 2 area) and 17.5% (4 cm 2 area) have been achieved by a process which involves impurity gettering and effective back-surface passivation on multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). The former efficiency mark represents the highest reported solar cell efficiency on mc-Si to date. PCD analysis revealed that the bulk lifetime in certain HEM samples after phosphorus gettering can be as high as 135 μs. This increases the impact of the back-surface recombination velocity ( S b) on the solar cell performance. By incorporating a deeper aluminum back-surface field (Al-BSF), the S b for solar cells in this study was lowered from 10,000 to 2000 cm/s (for 1 and 10 μm evaporated aluminum layers, respectively, alloyed in a conventional furnace) and finally to 200 cm/s (for screen printed Al-BSFs alloyed in an RTP furnace). It was observed that the screen-printed/RTP alloyed Al-BSF process raised the efficiency of both float zone and relatively defect-free mc-Si solar cells by lowering S b. However, this process increased the electrical activity of extended defects so that mc-Si cells with a significant defect density showed degradation in performance.
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