Directing the Aggregation of Native Polythiophene during in Situ Polymerization

Abstract

The performance of semiconducting polymers strongly depends on their intra- and intermolecular electronic interactions. Therefore, the morphology and particularly crystallinity and crystal structure play a crucial role in enabling a sufficient overlap between the orbitals of neighboring polymers. A new solution-based in situ polymerization for the fabrication of native polythiophene thin films is presented, which exploits the film formation process to influence the polymer crystal structure in the resulting thin films. The synthesis of the insoluble polythiophene is based on an oxidative reaction in which the oxidizing agent, iron(III) p -toluenesulfonate (FeTos), initially oxidizes the monomers to enable the polymer chain growth and secondly the final polymers, thereby chemically doping the polythiophene. To exploit the fact that the doped polythiophene has a different crystal packing structure compared to the undoped polythiophene, we investigate the structural effect of this inherent doping process by varying the amounts of FeTos in the reaction mixture, creating polythiophene thin films with different degrees of doping. The structural investigation performed by means of grazing incidence wide-angle X-ray scattering (GIWAXS) suggests that the strongly doped polymer chains aggregate in a π-stacked manner in the film formation process. Moreover, this π-stacking can be maintained after the removal of the dopant molecules. GIWAXS measurements, molecular dynamics simulations, and spectroscopic analysis suggest the presence of polythiophene in a novel and stable crystal structure with an enhanced intermolecular interaction.

Solution‐Processed, Insoluble Thin Films through In Situ Chemical Polymerization of Semiconducting Native Polythiophene

“…Also, for this oxidizing agent we can obtain both conducting and semiconducting thin films. [16] Here, we demonstrate the successful application of the ethanol-rinsed semiconducting films for use in solar cells.…”

“…Subsequently, we rinsed the films in ethanol and thermally annealed them. Rinsing in ethanol is known to have two effects, namely, the removal of undesired reaction by‐products from the films and additionally the dedoping of the polymer to its neutral, semiconducting state . However, these semiconducting films (Figure S1, Supporting Information) show large inhomogeneities on the micrometer scale as shown by means of optical microscopy (Figure a,b).…”

“…Rinsing in ethanol is known to have two effects, namely, the removal of undesired reaction by-products from the films and additionally the dedoping of the polymer to its neutral, semiconducting state. [12,16] However, these semiconducting films (Figure S1, Supporting Information) show large inhomogeneities on the micrometer scale as shown by means of optical microscopy (Figure 1a,b). At low magnification (Figure 1a), the microscopy images reveal that the used in situ polymerization generates polymer films without the formation of any large grains or aggregates.…”

“…Furthermore, we demonstrate the adaptability of the presented synthesis to a thin film fabrication by different types of solution deposition methods. We have shown elsewhere that the intrinsic reaction mechanism of such oxidative in situ polymerization of PT can be exploited for improving the intermolecular electronic interactions between the PT polymer chains by altering their crystal structure . We believe that integrating the material synthesis into the deposition process will eventually allow to apply a wide range of control mechanisms to tailor the material properties on the nanoscale for a large variety of materials.…”

“…We have shown elsewhere that the intrinsic reaction mechanism of such oxidative in situ polymerization of PT can be exploited for improving the intermolecular electronic interactions between the PT polymer chains by altering their crystal structure. [16] We believe that integrating the material synthesis into the deposition process will eventually allow to apply a wide range of control DOI: 10.1002/pssa.201900895…”