Germany seems to be currrently leading the world in the devevlopment and deployment of photovoltaic system cost effectively. This is largely due to the feed-in tarrif which guarantees a good price for electricity generated by photovoltaics.
This report Experience of the German-based performance programme makes interesting reading. Its shows a market growth rate of 25% per year with significant cost-reductions each year.
Here are links to German companies supplying photovoltaics.