FABRICATION AND CHARACTERIZATION OF 18.6-PERCENT EFFICIENT MULTICRYSTALLINE SILICON SOLAR-CELLS

Solar cell efficiencies as high as 18.6% (1 cm(2) area) have been achi eved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 Omega-cm multicrysta lline silicon (mc-Si) grown by the heat exchanger method (HEM), Contac tless photoconductance decay (PCD) analysis revealed that the bulk lif etime (tau(b)) in HEM samples after phosphorus gettering can exceed 10 0 mu s. At these tau(b) levels, the back surface recombination velocit y (S-b) resulting from unoptimized hack surface field (BSF) design bec omes a major limitation to solar cell performance. By implementing an improved aluminum back Surface field (Al-BSF); S-b values in this stud y were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM me-Si d evices. This combination of high tau(b) and moderately low S-b resulte d 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 eff icient devices formed on the same material, the presence of electrical ly active extended defects have been found to be the main cause for th e 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 perform ance.