(153c) Tuning the Structure of Titania Films with Orthogonally Aligned Cylindrical Nanopores for Bulk Heterojunction Inorganic-Organic Solar Cells

The objective of this work is to synthesize 2D hexagonal close packed (HCP) titania thin films with 10 nm orthogonally aligned cylindrical nanopores for photovoltaic materials. HCP structures cast onto hydrophilic oxide surfaces (such as those of titania or indium tin oxide that might be present in bulk heterojuction materials) are usually found to align parallel to the surface. To provide vertical alignment of the pores, we use surface modification of substrates with crosslinked Pluronic PEO-(b)-PPO-(b)-PEO triblock copolymer surfactant F127, which also is used as the pore template to provide a large-pore HCP structure. Here we show that this approach leads to vertically aligned HCP structures in titania with relatively large nanopores, as determined by XRD, SEM and TEM characterization. For the binary system of F127 and water, a hexagonal lyotropic liquid crystal phase is obtained at weight ratio of F127: water of 1.7 to 3.  By replacing the volume of water with an equivalent volume of hydrolyzed titanium precursors in the final film, this corresponds to a F127:Titania precursor ratio from 0.75 to 1.31, and this range is used as a target for initial synthesis studies.  Also, the degree of orthogonal orientation is expected to increase as film thickness decreases and the critical film thickness for alignment of F127-based films will be established. A hole carrying conducting polymer (such as P3HT), when loaded into such titania film with HCP cylindrical nanopores, is hypothesized to give better photovoltaic performance as compared to a disordered or bicontinuous cubic nanopore arrangement. Confinement in cylindrical nanopores provides isolated, regioregular “wires” of conjugated polymers with tunable optoelectronic properties, which has been shown to improve hole conductivity over that in bicontinuous cubic structure.  We plan to use the synthesized thin films to test this hypothesis by loading P3HT and comparing its photovoltaic performance to films with interconnected globular pores.