For solar firms, 2011 brought dark clouds. First Solar cut its profit forecast and shifted its focus to large power plants. Once worth $25 billion, the Arizona company was recently valued as low as $2.64 billion. The world’s largest maker of thin-film solar panels may now attract takeover interest on the potential that demand will rise over the next decade as prices become more competitive with fossil fuels and other forms of electricity. Marlborough, Mass.-based Evergreen Solar Inc. was delisted in August, after filing bankruptcy.