The optimization and fabrication of high efficiency HEM multicrystalline silicon solar cells

Solar cell efficiencies as high as 18.2% (1 cm² area) have been achieved using a process sequence which involves impurity gettering on 0.65 Ω-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). This is the first time mc-Si device efficiencies have surpassed the 18% mark. Photoconductance decay (PCD) analysis revealed that process induced gettering raised the bulk lifetime (τ_b) of HEM device material from an as-grown level of 10 μs to 135 μs. Detailed cell analysis shows that for such high τ_b values, the back surface recombination velocity (S_b) of 10,000 cm/s or higher presents the major efficiency limiting factor for uniform, defect-free devices. It is shown that lowering S_b further can raise the efficiency of similar HEM solar cells above 19.0%. For less efficient devices formed on the same material, the presence of electrically active extended defects are found to be the main cause for the performance degradation. Enhanced gettering or passivation techniques are required to improve these regions of the material