We study the effects
of tailored light–matter interactions
on charge transfer in conjugated polymer films. We use inverse opal
structures of titania as the electron acceptor and the model polymer
poly(3-hexylthiophene) (P3HT) as the electron donor. We systematically
tune the periodicity of the inverse opal to study how the photophysical
properties of the conformally coated P3HT are affected by the photonic
stop band and band-edge light localization to observe both suppression
and enhancement of absorption. Surprisingly, we observe changes in
the vibronic coupling in the P3HT absorption spectra in the inverse
opal structures as compared to control films. We determine that the
polymer in the inverse opals shows more J-aggregate-like behavior
with exciton bandwidths of 18 meV, compared to that of 124 meV for
P3HT on a planar mesoporous TiO2 film. We also study charge
transfer at the polymer/inorganic interface by photoinduced absorption
spectroscopy. We find that the polaron signal depends on the excitation
wavelength, the periodicity of the inverse opal, and the interfacial
area. The inverse opal structures exhibit significantly increased
charge generation compared to the control films, and we determine
that photonic effects of the lattice, while observable, play a secondary
role in this enhancement relative to the increased surface area.
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.
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