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CRYSTALLINE SILICON SOLAR CELLS 261<br />

Ti/Ag finger<br />

Inverted<br />

pyramids<br />

SiO x +<br />

double ARC<br />

n ++ p substrate n +<br />

p ++<br />

Contact<br />

windows<br />

SiO x<br />

Al electrode<br />

(a)<br />

ARC<br />

Metal<br />

finger<br />

Upright, random<br />

pyramids<br />

n ++<br />

p substrate<br />

Back contact<br />

(b)<br />

Figure 7.2 (a) Passivated emitter and rear locally diffused (PERL) cell; and (b) industrial cell<br />

with screen-printed contacts. (Not to scale)<br />

metallic and light (O, C, N) impurities. This translates into the longest post-processing<br />

SRH lifetimes in the millisecond range, but still shorter than the Auger limit. Magnetic<br />

Czochralski (MCz) material contains much less oxygen than conventional Czochralski<br />

and also allows very high efficiencies to be obtained [36].<br />

Industrial cells use Czochralski (Cz-Si) wafers because of their availability. Cz<br />

wafers are also perfect crystals but they contain a high concentration of oxygen that affects<br />

lifetime in several ways [37]. Some commercial devices are made on multicrystalline<br />

(mc-Si) substrates grown in blocks or ribbons with procedures specially developed for<br />

photovoltaics. In addition to crystal defects, such as grain boundaries and dislocations, the<br />

potential content of metallics is higher because of lower segregation to the melt during<br />

the faster solidification process. As a result, the lifetime of mc-Si is lower.

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