Annealing-Induced Changes in the Molecular Orientation
of Poly-3-hexylthiophene at Buried Interfaces
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Abstract
The molecular organization at interfaces
of organic semiconducting
materials plays a crucial role in the performance of organic photovoltaics
and field effect transistors. Vibrational sum-frequency generation
(VSFG) was used to characterize the molecular orientation at interfaces
of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected
VSFG spectra of the CC stretch of the thiophene ring yield
the orientation of the conjugated backbone of P3HT, which is directly
relevant to the electronic properties at the interface. The molecular
orientation at buried polymer–substrate interfaces was compared
for films spin-cast on SiO<sub>2</sub> and AlO<sub>X</sub> substrates,
before and after thermal annealing at 145 °C. On SiO<sub>2</sub>, annealing results in the thiophene rings adopting a more edge-on
orientation, tilting away from the surface plane by Δθ
= +(3–10)°. In contrast, an opposite change is observed
for films deposited on AlO<sub><i>x</i></sub>, Δθ
= −(3–26)°, where annealing leads to a more face-on
orientation of the thiophene rings of the polymer. Although subtle,
such orientational changes may significantly affect the electron transfer
rates across interfaces and hence the overall photovoltaic efficiency