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. Regio-regular poly [2,2'-(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.