Nanostructuring of titania thin films by a combination of microfluidics and block-copolymer-based sol-gel templating.

Sol-gel templating of titania thin films with the amphiphilic diblock copolymer poly(dimethyl siloxane)-block-methyl methacrylate poly(ethylene oxide) is combined with microfluidic technology to control the structure formation. Due to the laminar flow conditions in the microfluidic cell, a better control of the local composition of the reactive fluid is achieved. The resulting titania films exhibit mesopores and macropores, as determined with scanning electron microscopy, X-ray reflectivity, and grazing incidence small angle X-ray scattering. The titania morphology has three features that are beneficial for application in photovoltaics: 1) a large surface-to-volume ratio important for charge generation with disordered hexagonally arranged mesopores of 25 nm size and a film porosity of up to 0.79, 2) enhanced light scattering that enables the absorption of more light, and 3) a dense titania layer with a thickness of about 6 nm at the substrate (bottom electrode) to prevent short circuits. An optical characterization complements the structural investigation.

Influence of annealing and blending of photoactive polymers on their crystalline structure.

Thin photoactive polymer films of poly(3-octylthiophene-2,5-diyl) (P3OT) and poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV) are investigated. With X-ray reflectivity measurements, a linear concentration-thickness dependence is found for both polymers and the molecular weight of CN-PPV is determined from this concentration-thickness dependence. Based on the molecular weights, the critical blending ratio is determined. Grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the crystallinity of thin films and to determine characteristic length scales of the crystalline structure. Moreover, the orientation of the crystalline parts regarding the substrate of both the homopolymer and the blended films is probed with GIWAXS. Temperature annealing is found to improve the crystallization for both homopolymers. In addition, reorientation of the predominant crystalline structures takes place. Blending both polymers reduces or even suppresses the crystallization during spin coating as well as temperature annealing. Absorption measurements complement the structural investigations.