First Solar ( FSLR) announced late Monday that it agreed to purchase the solar power assets of the start-up firm OptiSolar in an all-stock transaction worth $400 million. The deal will give First Solar control over a 550-megawatt photovoltaic facility that OptiSolar was building for PG&E ( PCG). When complete, the facility will be the world's largest photovoltaic solar farm. Also included in the deal is a project pipeline worth 1,300 additional megawatts. The projects are mostly aimed at providing power to utilities in the western U.S. Operating rights to 136,000 acres worth 19 gigawatts of power capacity are also included in the deal. First Solar shares advanced 6.5% to $110.44 in Tuesday's trading session.