Molecular Orientation of Poly-3-hexylthiophene at the Buried Interface with Fullerene

Molecular orientation at the donor-acceptor interface plays a crucial role in determining the efficiency of organic semiconductor materials. We have used vibrational sum frequency generation spectroscopy to determine the orientation of poly-3-hexylthiophene (P3HT) at the planar buried interface with fullerene (C₆₀). The thiophene rings of P3HT have been found to tilt significantly toward C₆₀, making an average angle θ ≈ 49° ± 10° between the plane of the ring and the interface. Such tilt may be attributed to π-π stacking interactions between P3HT and C₆₀ and may facilitate efficient charge transfer between donor and acceptor. Upon annealing, the thiophene rings tilt away from the interface by Δθ = 12-19°. This may be attributed to higher crystallinity of annealed P3HT that propagates all the way to the interface, resulting in more "edge-on" orientation, which is consistent with the observed red-shift by ∼6 cm^-1 and spectral narrowing of the C=C stretch bands.