Effects of Surface Passivation on Trap States, Band Bending, and Photoinduced Charge Transfer in P3HT/TiO₂ Hybrid Inverse Opals

We investigate the effect of a common TiO₂ passivation reagent, TiCl₄, on the photoinduced charge transfer of poly­(3-hexylthiophene) (P3HT) to TiO₂ in the inverse opal structure. Treating the inorganic oxide framework with TiCl₄ leads to an increase in the size of the TiO₂ nanoparticles, a thickening of the inverse opal framework, and a decrease in the trap-state photoluminescence. These changes lead to different energy alignments at the interface. In comparison to the unpassivated P3HT/TiO₂ inverse opal, we measured a larger polaron yield, by as high as ninefold, and significantly shorter and more uniformly distributed polaron lifetimes in TiCl₄-treated samples. We show that downward band bending in the polymer can be circumvented by tuning the trap states on the metal oxide using TiCl₄, thereby eliminating the energetic barrier for photoelectron injection from the polymer to the metal oxide. The findings suggest a way to overcome a potential factor that has plagued the performance of metal oxide–polymer hybrid photovoltaics.