2 research outputs found
Deciphering Anomalous Raman Features of Regioregular Poly(3-hexylthiophene) in Ordered Aggregation Form
PolyÂ(3-hexylthiophene)
(P3HT), being a prototypic conjugated polymer,
bears a high charge mobility that is sensitive to its packing configuration
in the condensed phase. Despite its extensive experimental study with
X-ray diffraction, its specified packing structure still remains stymied.
This study searched for possible structures of crystalline P3HT and
identified the one that holds a simulated Raman spectrum most approximate
to the experimental one of ordered P3HT aggregates in the frozen solvent.
The spectral correspondence shows that the Raman-active C–C
stretch peak exhibits a red shift in frequency, while the Cî—»C
stretch peak displays a blue shift as the layer planarity of P3HT
is relaxed. Moreover, the Cî—»C peak splits into two when adjacent
thiophene rings in the P3HT chain hold a dihedral angle of 22°
with respect to each other. This study demonstrates that Raman spectroscopy
plus first-principles simulations can serve as a powerful tool to
resolve fine structures of molecular crystals
Revealing Ordered Polymer Packing during Freeze-Drying Fabrication of a Bulk Heterojunction Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric Acid Methyl Ester Layer: In Situ Optical Spectroscopy, Molecular Dynamics Simulation, and X‑ray Diffraction
Formation of ordered
polyÂ(3-hexylthiophene-2,5-diyl) (P3HT) molecular
stacking during the freeze-drying process is tracked with in situ
spectroscopy of Raman scattering, absorption, and photoluminescence.
Raman spectra of pristine P3HT dissolved in 1,2-dichlorobenzene show
that P3HT polymers undergo drastic ordered aggregation upon being
lower than 0 °C, at which the solubility of P3HT is reached,
as evidenced by the emergence of pronounced red-shifted, narrow Raman
peaks (1422 and 1435 cm<sup>–1</sup>) caused by intermolecular
coupling. The absorption and photoluminescence spectra bear similar
temperature dependence as the results of Raman. Aggregation of P3HT
is further confirmed by coarse-grained molecular dynamics simulation
showing the enhanced order parameters of distance and orientation
between P3HT chains upon cooling. The incorporation of [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) does not significantly alter the P3HT packing
configuration, as verified by nearly identical Raman features observed
in P3HT:PCBM mixing solution upon cooling. While optical spectroscopy
and MD simulation portrayed the short-range order of P3HT aggregates,
grazing-incident X-ray diffraction exposed the long-range order by
the pronounced diffraction spots corresponding to the lamellar stacking
of P3HT. This study demonstrates the ability of Raman spectroscopy
to reveal the short-range order of polymer packing, while the in situ
monitoring illustrates that the ability of freeze-drying to separate
molecular aggregation from solvent removal thus is advantageous for
photovoltaic device fabrication without resorting to trial and error