The synthesis of conjugated low band-gap copolymers based on thiophene and benzothiadiazole is described. The synthesis was carried out by oxidative ferric chloride polymerization or Stille cross coupling polymerization. The solubility of the polymer based on quarterthiophene and benzothiadiazole was tested with hexyl, 2-ethyl-hexyl and dodecyl as side chains on the thiophene. It was found that 2-ethyl-hexyl substituents gave high molecular weight polymer products with good film forming ability and good solubility. The polymers based on di-thiophene and benzothiadiazole were applied in photovoltaic devices and the coupling of the alkylthiophene showed no effect on the maximum photovoltaic performance. Band-gaps were estimated to be 2 eV for polymers based on di-thiophene and benzothiadiazole and 1.8 eV for polymers based on quarterthiophene and benzothiadiazole. Attempts to synthesize the polymers with a benzo-bis-thiadiazole unit are also described.