Structure-Property Relations in Nanostructured Materials: From Solar Cells to Gecko Adhesion

This thesis explores the structure-property relations in different nanostructured materials. Nanostructured polymer blends with interpenetrating network morphology of donor and acceptor materials have been considered ideal for organic bulk heterojunction photovoltaics. In this work we mainly investigate the self-organization of polymer blends via crystallization to generate functional nanostructures for organic electronics. Controlling morphology of organic photovoltaic thin films is crucial for the optimization of the device performance as there is a fine balance of exciton generation and separation as well as charge transport. To better understand the bulk heterojunction morphology, we first investigated the structure formation of poly (3-hexylthiophene) (P3HT)/ phenyl-C61butyric-acid methyl ester (PCBM) system. Crystallization-induced phase separation has been found to drive the formation of a nanostructure in the blends, the size of which is determined by the intrinsic 10 nm length scale of semicrystalline P3HT. The mixing of PCBM within amorphous P3HT interlayers does not disrupt the crystallinity of the P3HT. P3HT crystallization expels PCBM into the spherulitic interlamellar amorphous layers, where it enriches to its miscibility limit. Above the solubility limit, PCBM aggregates start to form. The results suggest that the crystallization of P3HT and the enrichment of PCBM in interlamellar regions give rise to interconnected donor and acceptor phases those are close to the optimal bulk heterojunction structure. This structure formation mechanism is manifested by the good photovoltaic performance of spherulitic P3HT/PCBM films. Structural studies of P3HT/poly[(9,9-dioctyfluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5yl)-2,1,3-benzothiadiazole)-2’, 2”-diyl] (F8TBT) are presented. P3HT/F8TBT system exhibits a crystallization-driven structure formation similar to the P3HT/PCBM system despite v the existence of a miscibility gap. The lamellar crystallization of P3HT is not perturbed by the addition of F8TBT. X-ray scattering studies indicate that F8TBT is mixed in the interlamellar amorphous phase up to a solubility limit, while a bulk heterojunction framework is established by the crystalline lamellae of P3HT. The excess F8TBT is accommodated at amorphous grain boundaries as well as the film/substrate interface. The structural studies are correlated with the photovoltaic device performance of P3HT/F8TBT films which exhibit spherulitic morphology. Devices based on spherulitic films show moderate efficiencies with improved fill factors but decreased photocurrents in comparison to that of thermal annealing condition. The results suggest that the nanostructure formation in P3HT/F8TBT blends is determined by the crystallization of P3HT, resulting in a structural size that are beneficial for exciton dissociation, while the F8TBT segregation at the substrate interface impair the device performance. The phase separation behavior in crystalline/crystalline blends consisting of P3HT and polyethylene oxide (PEO) is investigated. The self-assembly of P3HT in solution induces vertical segregation in blend films, in comparison to typical lateral polymer phase separation structures from non-aggregated solutions. Thin film transistors based on P3HT/PEO blends show show a nearly undegraded charge carrier mobility at low P3HT content due to the formation of a layered structure with P3HT nanowire networks segregated at the dielectric surface. Finally, a diversion from morphology studies of polymer blends involves the biomimetic fabrication of hierarchical fibrillar structures to achieve gecko adhesion. The hierarchical structures were fabricated based on polymer pillars tipping with carbon nanotube forests. The adhesion performance of the polymer-carbon nanotube hierarchical structures was tested by shear force measurements.