i 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 3,7,11-trimethyldodecyl-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 cm 2 . 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.ii Indenfor organisk solceller fokuseres i denne tid på polymere med lavt båndgap, dvs. polymerer som absorberer lys med bølgelaengder laengere end 620 nm. Disse lavbåndgabs polymere har vist sig at kunne øge effiktiviteten af organiske solceller ved et bedre overlap mellem polymerens absorptionsspektrum og solens spektrum. I denne afhandling beskrives syntesen af 16 nye lavbåndgabs copolymere som er baseret på thiophen, benzothiadiazol og benzo-bis(thiadiazol). Der blev benyttet to syntese strategier: én med oxydativ p...

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A series of low-band-gap copolymers of thiophene, benzothiadiazole, and benzobis(thiadiazole) were synthesized. The polymers were synthesized by Stille cross-coupling polymerization of distannylalkylthiophenes and dithiophenes with dibromo derivatives of benzothiadiazoles and benzobis(thiadiazole)s. The polymers were characterized using NMR, UV−vis, and size exclusion chromatography (SEC). The molecular weight, solubility, and film-forming ability were highly dependent on the choice of side chains. 3,7,11-Trimethyldodecyl side chains were found to give polymer products with high molecular weight, good film-forming ability, and good solubility. Band gaps were estimated from UV−vis to be 2.1−1.7 eV for polymers based on benzothiadiazole and ∼0.7 eV for polymers based on benzobis(thiadiazole). The band gap and electronic structure of the polymers were determined by a combination of UV−vis spectroscopy and ultraviolet photoelectron spectroscopy (UPS).

Aqueous nanoparticle dispersions of a series of three low-band-gap polymers poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b′)dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5′-diyl] (P1), poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P2), and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (P3) were prepared using ultrasonic treatment of a chloroform solution of the polymer and [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) mixed with an aqueous solution of sodium dodecylsulphate (SDS). The size of the nanoparticles was established using small-angle X-ray scattering (SAXS) of the aqueous dispersions and by both atomic force microscopy (AFM) and using both grazing incidence SAXS (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) in the solid state as coated films. The aqueous dispersions were dialyzed to remove excess detergent and concentrated to a solid content of approximately 60 mg mL–1. The formation of films for solar cells using the aqueous dispersion required the addition of the nonionic detergent FSO-100 at a concentration of 5 mg mL–1. This enabled slot-die coating of high quality films with a dry thickness of 126 ± 19, 500 ± 25, and 612 ± 22 nm P1, P2, and P3, respectively for polymer solar cells. Large area inverted polymer solar cells were thus prepared based on the aqueous inks. The power conversion efficiency (PCE) reached for each of the materials was 0.07, 0.55, and 0.15% for P1, P2, and P3, respectively. The devices were prepared using coating and printing of all layers including the metal back electrodes. All steps were carried out using roll-to-roll (R2R) slot-die and screen printing methods on flexible substrates. All five layers were processed using environmentally friendly methods and solvents. Two of the layers were processed entirely from water (the electron transport layer and the active layer).