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

Abstract

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 1,2dichlorobenzene 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 shortrange 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.

Deciphering Anomalous Raman Features of Regioregular Poly(3-hexylthiophene) in Ordered Aggregation Form

“…Despite our previous success 34 in revealing the ordered aggregation of P3HT with in situ Raman spectroscopy, the assignment of the acquired Raman signatures and the interpretation of modification during cooling, however, are indirect. The recent vast development of first-principles calculations can alter this scenario.…”

“…26 Quite recently, we have monitored the progression of P3HT molecular stacking in 1,2-dichlorobenzene (o-DCB) during the freeze-drying process with in situ spectroscopy of Raman scattering, absorption, and photoluminescence. 34 Upon being lower than 0 °C, P3HT undergoes ordered aggregation as evidenced by significantly red-shifting in absorption and photoluminescence spectra (see Figure S1 in the Supporting Information) and, more importantly, emergent narrow Raman peaks at lower frequencies. These results provided convincing evidence of intermolecular coupling engendered by molecular stacking of P3HT.…”

“…The assignment of the two emergent sharp peaks was based on the concurrent observations in absorption and photoluminescence spectrared shifting and pronounced vibronic featuresas well as coarse-grained molecular dynamic simulation. A more detailed description of this work can be found in ref 34.…”

“…This study investigated the anomalous Raman spectra of the ordered aggregation of P3HT in o-DCB enabled in the cooling process. 34 Calculation approaches were developed to simulate Raman spectra of different packing configurations. The comparison between the simulated and experimental findings deciphered the intricate spectroscopic details, thus yielding ultimate packing geometry of specific crystalline P3HT configuration which was never clarified by other means.…”

Nanostructure of organic semiconductor thin films: Molecular dynamics modeling with solvent evaporation

“…This reduces the predictive power of the simulations. Coarse-grained models of various resolution, i.e., describing a few atoms or up to whole monomers by effective particles or "beads," have successfully been employed to simulate bulk, blend morphologies and their scattering signatures [25][26][27], but extensive fitting of the coarse-grained force fields to either atomistic simulations or experiments is needed for new materials. Additionally, a too coarse resolution can greatly limit the chemical specificity of the model, in turn hampering further property calculations.…”