Iron gettering with aluminium and back-surface passivation of chrystalline silicon solar cells

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School of Electrical Engineering | Master's thesis
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The purpose of this Master's thesis was to study aluminium gettering of iron impurities in single crystalline silicon. The intention was to determinate the iron segregation coefficient of aluminium segregation gettering at temperatures lower than 850°C. Hence, 20 nm of pure aluminium was sputtered onto the backside of 400 µm thick, 2.87 OMEGA -cm p-doped Czochralski silicon wafers, which had been intentionally iron contaminated to a level of (3.36 ± 0.14) x 10<sup>13</sup>cm<sup>-3</sup>. The iron concentration of the silicon wafer was measured through Surface Photovoltage (SPV) diffusion length measurements. Iron gettering with aluminium at temperatures 800 - 840°C reduced the initial iron concentration to the range 10<sup>10</sup> - 10<sup>11</sup> cm<sup>-3</sup>. Therefore, the iron segregation coefficient ranged from (5.28± 3.38) x 10<sup>7</sup> at 800°C to (5.23±2.48) x 10<sup>5</sup> at 840°C. The resulting high iron segregation coefficient also led to a large iron segregation enthalpy of 21.73 eV at 820 - 840°C. High temperature annealing of an aluminized crystalline silicon wafer also creates an aluminium back-surface field at the backside of the wafer. This back-surface field reduces the back-surface recombination and improves the solar cell efficiency together with the iron gettering. Hence, the back-surface recombination velocity of the aluminium back-surface field was also studied at 800, 850 and 900°C. In these experiments, 1 and 3.5 µm of pure aluminium was sputtered onto the back surface of clean 200 µm thick, 2.87 OMEGA -cm p-doped crystalline silicon wafers. The back-surface fields were formed by 30 min anneals and the diffusion lengths of the wafers were measured with Surface Photovoltage. The measurements resulted in back-surface recom bination velocities of (3.1<sup>+1x10<sup>7</sup></sup><sub>-1.85</sub>) x 10<sup>4</sup> cm/s for 3.5 µm aluminium annealed at 850°C and (5.5<sup>+3.7</sup><sub>-1.6</sub>) x 10<sup>3</sup> cm/s for 3.5 µm aluminium at 900°C. The 1 µm aluminium layer and the 3.5 µm layer annealed at 800°C did not reduce the back-surface recombination from its initial ohmic value 10<sup>7</sup> cm/s. The results confirmed the theoretical assumption that the recombination decreases as a function of increasing aluminium thickness and increasing formation temperature.
Savin, Hele
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Yli-Koski, Marko
silicon, kisel, solar cell, solcell, iron, järn, aluminium, aluminium, segregation, segregation, gettering, gettering, recombination, rekombination, passivation, passivering, diffusion length, diffusionslängd
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