8.2.3 Summary to: 8.2 Making Bulk Si Solar Cells

	Bulk Si solar cells are made from (cheap) single crystalline wafers (cut squarish) or from square multicrystalline (mc) wafers. They account for about 85 % of the installed solar power at present (2008).			Processing Time 1s / solar cell Cost Decrease 5 % / a Efficiency Increase 20 % in 2012 ? Key Material Supply 30 % /a more Si Industry Growth Rates > 30 % for many years Supply capital and people
		A yearly production of 1 GWpeak means about 107 m2=10 km2 pn-junction of good quality and much more
		Consider Þ
		A big problem is cranking up world wide Si production by 30 % - 40 % per year.
	mc wafers are produced by Si casting. Problems are
		Expansion upon crystallization.
		Reaction with walls of mold
		Columnar grain growth required
		300 kg ingots are routinely cast in 2007; liquid encapsulation and precise temperature control are essential
	Sawing the ingot into mc-Si wafers with as little losses as possible and with wafer thicknesses of < 300 µm, while straight-forward, is "high-tech".
		Saw damage is removed by a chemical etch.
	Processing, simple in principle, has to meet the conditions above and is highly specialized. Essential processes are:
		Diffusion, edge isolation, passivation, screen printing contacts and sintering contacts.
		Essential device features are back surface field, gettering of impurity atoms, H-passivation of grain boundaries and other defects.
		Exercise 8.2-1 Summing up Class Exercises

© H. Föll (Semiconductor Technology - Script)