There is a growing demand for “tailor-made“ new materials for special applications, in particular when the reservoir of the presently used raw materials is limited. Conjugated polymers, especially alternating copolymers meet these requirement and can be “tailor-made“ for particular application or processing issues. In this thesis, several conjugated, alternating copolymers with fused ring systems for optoelectronical applications are presented. The mostly used building blocks are benzothiadiazole, benzobis(thiadiazole), cyclopentadithiophene, and indoloindole. Alternating copolymers based on cyclopentadithiophene and benzothiadiazole- or benzobis(thiadiazole)-moieties (with and without additional thiophene “spacers“) are described in Chapter 2 and 3, respectively. Due to the smaller HOMO/LUMO energy gap of benzobis(thiadiazole) compared to benzothiadiazole, the absorption maxima of the corresponding copolymers are bathochromically shifted to wavelengths above 1 000 nm. The benzothiadiazole-based copolymers were tested with different acceptor materials (fullerenes, nanoparticles) in bulk-heterojunction-type organic solar cells. The best power conversion efficiency of ca. 4.5 % was reached with poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b‘]-dithiophene)-alt-4,7-[2,1,3]-benzothiadiazole] and C70-PCBM as acceptor in the active layer. In Chapter 4, several indoloindole-based homo- and copolymers are presented. However, the properties of the polymers (regarding molecular weight, solubility, absorption, emission, HOMO/LUMO energy gap) vary considerably. Dual fluorescence, which is observed for the benzothiadiazole containing copolymers, and the oxidation behaviour of the indoloindole homopolymer are studied in detail.