In business for a bare four years, thin film solar module manufacturer First Solar, Inc., just announced the achievement of a somewhat arbitrary yet still resonant goal in the solar world: to manufacture solar for under one dollar per watt. Thin film still isn’t easily deployable for residential solar panel installations, but the cost difference between thin film and traditional silicon modules is tantalizing. And the time scale is pretty amazing–in more than thirty years of commercial production, the cost traditonal silicon-based solar has fallen perhaps 30%. In only a handful of years, thin-film has dropped about twice that. First Solar’s announcement marks an interesting point for thin-film…and perhaps cries a challenge to silicon solar manufacturers.