Donor–acceptor copolymers have recently been recognized as excellent materials for organic photovoltaic applications. Because of complex film formation properties, however, direct correlations between morphology and optical and electrical properties have yet to be established. Within our collaborative project within the German Science Foundation–funded program “Elementary Processes in Photovoltaics,” we focused on the morphology control and molecular doping of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-bʹ]dithiophene)-alt-4, 7(2,1,3-benzothiadiazole)] to control the optoelectronic properties. The main results are highlighted in this chapter.