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

5 years ago

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

$Revealing Ordered Polymer Packing during Freeze-Drying Fabrication of a Bulk Heterojunction Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric Acid Methyl Ester Layer: In Situ Optical Spectroscopy, Molecular Dynamics Simulation, and X-ray Diffraction$

Yu-Ching Huang, Cheng-Si Tsao, Yu-Bing Lan, Cheng-Kuang Lee, Chih-I Wu, Juen-Kai Wang, Chun-Wei Pao, Pin-Hao Sher, Ping-Tsung Huang

Formation of ordered poly(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^–1) 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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b01679

DOI: 10.1021/acs.jpcc.7b01679