Fabrication and characterization of 18.6% efficient multicrystalline silicon solar cells

Abstract

Solar cell efficiencies as high as 18.6%(1 cm/sup 2/ area) have been achieved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 /spl Omega/-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). Contactless photoconductance decay (PCD) analysis revealed that the bulk lifetime (/spl tau//sub b/) in HEM samples after phosphorus gettering can exceed 100 /spl mu/s. At these /spl tau//sub b/ levels, the back surface recombination velocity (S/sub b/) resulting from unoptimized back surface field (BSF) design becomes a major limitation to solar cell performance. By implementing an improved aluminum back surface field (Al-BSF), S/sub b/ values in this study were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM mc-Si devices. This combination of high /spl tau//sub b/ and moderately low S/sub b/ resulted in the 18.6% device efficiency. Detailed model calculations indicate that lowering S/sub b/ further can raise the efficiency of similar HEM mc-Si devices above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and mc-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.