energy returned on energy invested (EROEI) for photovoltaics

[pala]

Since some days I have been thinking about the EROEI (energy returned on energy investe) for photovoltaics.

In wikipedia stands that there are 3 types of silicn.
(silicon is the basic product to produce photovoltaic cells)

single crystal silicon
polycrystalline silicon
amorphous silicon

each kind of silicion is used for a different type of cells with others efficiency.

From my point of view I know that there is a fourth type,the Organic photovoltaic cells ,but this type is under way (under development).

The reason why i think about this topic is that one of my teachers told the whole class that every photovoltaic cell is inefficient.

So my questions are:

Is the extraction of silicon really such a energy-consuming process?
How much energy is needed for the production of cells?
How long does it take to return more energy than the investment needed?

The location is also a reason for the efficiency. South countries has more sunnydays than in countries in the North.