Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface

We have fabricated thick, nanostructured composite films of titania (aerogel)–silica (aerogel) on transparent electrodes for use as composite photoelectrodes. Aerogels are excellent candidate materials for photovoltaic electrodes because the high specific surface area of titania aerogel (∼150 m2/g) significantly amplifies photoactive area, while the continuous pore network permits rapid mass transport of diffusing electron donors through the film. The high surface-to-volume ratio of titania aerogel films can also be a cause for concern because the amplified surface character begets a large number of surface defects that can act as energetically deep traps and hinder percolative transport of electrons through the film. Surface hydroxyls (Ti–OH, titanols) act as charge traps, whereby electrons localize at the semiconductor surface and promote recombination of electrons with solution-phase acceptors, resulting in diminished measured photocurrents. Titania (aerogel)–silica (aerogel) composite films can be modified by electrochemically generated organic radical anions, which graft to the titanols at the electrode surface. This organic modification improves photocurrent yields under bandgap-excitation by a factor of >2 relative to unmodified films, and shifts the onset of photoanodic current negatively by ∼45 mV. The improved photocurrent yields are likely due to more facile electronic transport through the modified mesoporous network, caused by changing the chemical nature of the surface species on the titania electrode. The negative shift of the onset potential for the flow of cathodic current is further evidence that the enhanced photocurrent arises from a change in electron mobility, as opposed to a lowering of the conduction band edge, which would improve photoelectron injection efficiency, but would involve a positive shift in the cathodic onset potential.