Fabrication and characterization of 18.6% efficient multicrystalline silicon solar cells

Solar cell efficiencies as high as 18.6% (1 cm(2) area) have been achieved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 Omega-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM), Contactless photoconductance decay (PCD) analysis revealed that the bulk lifetime (tau(b)) in HEM samples after phosphorus gettering can exceed 100 mu s. At these tau(b) levels, the back surface recombination velocity (S-b) resulting from unoptimized hack surface field (BSF) design becomes a major limitation to solar cell performance. By implementing an improved aluminum back Surface field (Al-BSF); S-b values in this study were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM me-Si devices. This combination of high tau(b) and moderately low S-b resulted in the 18.6% device efficiency. Detailed model calculations indicate that lowering S-b, further can raise the efficiency of similar HEM me-Si devices above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and me-Si solar cells. For less efficient devices formed on the same material, the presence of electrically active extended defects have been found to be the main cause for the performance degradation. A combination of light beam induced current (LBIC) scans as well as forward-biased current measurements have been used to analyze the effects of these extended defects on cell performance.