Solar cells that convert sunlight to electric power traditionally have been dominated by solid state junction devices, often made of silicon wafers. Thanks to nanotechnology, this silicon-based production technology has been challenged by the development of a new generation of solar cells based on thin film materials, nanocrystalline materials and conducting polymeric films. These offer the prospects of cheaper materials, higher efficiency and flexible features. Thanks to a highly efficient polymer solar cell fabrication method by a novel coating process - roller painting - even the mass production of polymer solar cells is now within reach. A particular advantage of roller painting compared to other coating processes is ease of control of the film thickness and uniformity.