### 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)