Gettering and Passivation of High Efficiency Multicrystalline Silicon Solar Cells

A detailed study was conducted on aluminum and phosphorus gettering in HEM mc-Si and defect passivation by PECVD SiN in EFG mc-Si to achieve high efficiency solar cells on these promising photovoltaic materials. Solar cells with efficiencies as high as 18.6% (1 cm2 area) were achieved on multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM) by a process which implementsimpurity gettering, an effective back surface field, front surface passivation, and forming gas annealing. This represents the highest report 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 .mu.s. By incorportating a deeper aluminum back surface field (AL-BSF), the back surfacerecombination velocity (Sb) for 0.65 .OMEGA.-cm HEM mc-Si solar cells was lowered from 10,000 cm/s to 2,000 cm/s resulting in the 18.6% efficient device. It was also observed that a screen-printed/RTP alloyed Al-BSF process could raise the efficiency of both float zone and relatively defect-free mc-Si solar cells by lowering Sb. However, this process was found to increase the electrical activityof extended defects so that mc-Si devices with a significant defect density showed an overall degradation in performance. In the case of EFG mc-Si, neural network modeling in conjunction with a study of post deposition annealing was used to provide guidelines for effective defect passivation by PECVD SiN films. Appropriate deposition and annealing conditions resulted in a 45% increase in cellefficiency due to AR coating and another 25-30% increase due to defect passsivation by atomic hydrogen.