Low band gap polymers for organic photovoltaics

Several research groups within the area of organic photovoltaics are focusing on low band gap polymers, a type of polymer which absorbs light with wavelengths longer than 620 nm. These systems are believed to increase the efficiency of organic photovoltaics due to a better overlap of the absorption spectrum of the polymer with the solar spectrum. In this dissertation the synthesis of 16 new low band gap copolymers based on thiophene, benzothiadiazole and benzo-bis(thiadiazole) are described. The polymers have been prepared by two strategies; one using oxidative ferric chloride polymerization and one using Stille cross coupling polymerization. The polymers were purified chemically and by Size Exclusion Chromatography (SEC). The polymers were characterized by UV-vis and Ultraviolet Photoelectron Spectroscopy (UPS), and the optical band gap and the electronic structure of the polymer were determined. The copolymers show optical band gaps from 1.65 – 2.0 eV for the copolymers of thiophene and benzothiadiazole, where a decrease in the band gap was observed with an increase in the number of thiophene units in the repeating unit (n = 1 – 4). A band gap down to 0.65 eV was observed for the copolymers of thiophene and benzo-bis(thiadiazole). The film forming ability of the polymers was studied by attaching different alkyl side chains on the polymer back-bone, i.e. hexyl-, 2-ethylhexyl-, dodecyl- and 3,7,11trimethyldodecyl-groups. The 3,7,11-trimethyldodecyl-group was found to give the best film forming ability and highest absorbance, when the polymer was spin coated from solvents like THF, chloroform and 1,2-dichlorobenzene. The copolymer of thiophene and benzothiadiazole with four thiophenes in the repeating unit and 3,7,11-trimethyldodecyl-group as side chains with a band gap of 1.65 eV was applied in organic photovoltaic devices with active areas of 0.1, 3 and 10 cm2. The morphology of the active layer was studied, and it was found that the morphology and the photovoltaic performance of the device was affected by the choice of solvent, the spin coating conditions, the concentration of polymer, the ratio between polymer and PCBM and the annealing temperature. The highest efficiency of 1 % was achieved when the ratio between the polymer and PCBM was 1:2 and the device was annealed at 110 ºC. Lifetime and incident photon to current efficiency (IPCE) of the devices are also described. Finally, the polymer was applied in hybrid PV devices based on ZnO nano-fibers and the results of these studies are given.