Abstract
Recent investigations of the front side metallization of high-efficiency mono-crystalline Si solar cells proved that the glass layer formed at the Si/metallization interface during the screen printing plays an essential role for the charge transport, both in n- and p-type cells. High-efficiency cells (~ 18.0% for p-type and ~ 20% for n-type cells) show similar microstructure of the glass layer and similar temperature dependence of the series resistance. From this it is concluded that the microstructure of the glass layer determines the series and contact resistance of the front side metallization.The glass layers of high-efficiency cells contain a high density of nano-Ag colloids and other precipitates which reduce the contact resistance. A percolation model was proposed and is best suited to describe the charge transport in a dirty semiconductor containing metallic precipitates.Quantitative chemical composition of the glass layer of p- and n-type cells was investigated by Energy Dispersive X-ray (EDX) microanalysis in SEM and TEM. The chemical composition of the glass layer showed (SiOx)Pb, as main constitutes and Zn, Ti, Al, Ag, P and B as minor constituents with mole fractions above the detection limit of EDX. The glass layer is therefore considered to be a dirty semiconductor rather than a perfect insulator. The mole fractions of Zn (~ 1at%) and Al (~ 1at%) were quantitatively analyzed. Such analyses are important to correlate microstructural features with electrical properties of the front side metallization.We could prove that in p-type cells the efficiency of the cells correlated with the chemical composition of the glass layer. In n-type cells, with Al-containing pastes EDX spectroscopy yielded Al beyond the detection limit in the glass layer, whereas for Al free pastes the Al mole fraction was below the detection limit of EDX and yielded reduced efficiencies. In p-type cells pastes with enhanced Zn mole fraction yielding zinc oxide phases in the bulk Ag finger and Zn mole fractions up to 5at% in the glass layer.
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