Spiegel: As oil prices have plunged, solar has become less cost-competitive. At the same time, the credit squeeze has made it harder to finance solar projects. If the recent five-year boom in solar energy marked the birth of a global industry, the next half-decade should be its coming of age. But like most adolescents, solar is experiencing growing pains. The economic crisis has weakened demand for everything from polysilicon to rooftop panels, just as manufacturers have spent billions …