Infiltration of Regioregular Poly[2,2′‐(3‐hexylthiopene)] into Random Nanocrystalline TiO2 Networks

Polymer infiltration into random nanocrystalline TiO₂ networks is examined using a combination of imaging, surface analysis, and depth‐profiling techniques. Nanocrystalline TiO₂ network substrates were fabricated by established methods; the resulting networks were examined using scanning electron microscopy and found to be typical of those reported in the literature. Regioregular poly2,2′‐(3‐hexylthiopene)] (rrP3HT) was drop‐cast from solution onto the TiO₂‐network substrates. Infiltration of the polymer into the nanoporous TiO₂ network was determined by monitoring the ratio of carbon‐ion signal—by means of secondary‐ion mass spectrometry from a top overlayer of rrP3HT—to the carbon signal from the same polymer within the TiO₂ network. A very low incorporation of polymer was found (0.5 %), even for highly porous (≈ 65 %) networks. Several strategies were used to increase the degree of polymer infiltration, including heat treatment, surface derivatization, and the use of low‐molecular‐weight fractions. A high of 22 % rrP3HT as a percentage of the total volume of a random nanocrystalline film is reported. Previous results for hybrid rrP3HT/random nanocrystalline TiO₂ network devices are examined and analyzed in the context of these findings.