THALHEIM, Germany This sad stretch of eastern Germany, with its deserted coal mines and corroded factories, epitomizes post-industrial gloom. It is a place where even the clouds rarely seem to part. Yet the sun was shining here the other day and nowhere more brightly than at Q-Cells, a German company that surpassed Sharp last year to become the worlds largest maker of photovoltaic solar cells. Q-Cells is the main tenant among a flowering cluster of solar start-ups here in an area known as Solar Valley.