π-conjugated polymers have reached an important role in semiconductor technologies. In particular so called donor/acceptor copolymers, which hold several advantages in applications like organic field effect transistors (OFETs) or solar cells compared to common conjugated polymers have emerged great attention. Low cost production, flexibility, large area devices with minimum weight and the preparation from solution are a few of the advantages. By exhibiting a low band gap these copolymers owen absorption which can cover the whole visible spectral region or go beyond. These materials posses also increased charge carrier mobilities. Especially cyclopentadithiophene (CPDT) based copolymers are presented as potential candidates for the application in OFETs and solar cells.
The synthesis of the CPDT monomer as well of the acceptor monomers is discussed in chapter 2. The acceptor monomers include building blocks like benzo thiadiazole, benzo selenadiazole, quinoxaline, tetrafluoro benzene, silole, thieno thiadiazole and thieno pyrazine. Some of the building blocks were also extended by two thiophene spacers. The syntheses of the alternating copolymers were carried out by using stille cross coupling conditions catalyzed by palladium. The polymers have been characterized by NMR, GPC and UV/Vis analysis.
The optical properties of the copolymers were described in chapter 3. The properties of the polymers were measured in chloroform solutions and film layers prepared by spin coating from concentrated chloroform solutions. The spectra of the polymers show absorption maxima in the range from 500 to 900 nm, partially caused by the intramolecular charge transfer effect (ICT). The measuring of the absorption spectrum allows for determining the optical band gap.
The investigation of the charge carrier mobilities of the copolymers was carried out in a top contact and bottom gate configuration by measuring the thin film transistor characteristics. Highly n-doped silicium wafer was used as a gate electrode with a 300 nm thick thermal grown up silicium dioxide layer as gate isolator. The semiconducting layer was prepared by spin coating from chloroform or chloro benzene solution. In this manner layer thicknesses of 50 nm can be reached. The copolymers show particularly high mobilities and on/off ratios.
In chapter 5 the applications in organic solar cells are discussed. For this investigation the bulk hetero junction configuration is favoured, where the donor and acceptor materials are coated as a blend from solution. In bulk hetero junction solar cells molecular weights of more than 10.000 g/mol are required to guarantee an optimized package of the polymer chains in the film layer. Likewise the hybrid solar cell configuration was chosen to test the polymer as donor material with CdSe is the acceptor component.