Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films

Abstract

Domain alignment in conjugated polymer thin films can significantly enhance charge carrier mobility. However, the alignment mechanism during meniscus-guided solution coating remains unclear. Furthermore, interfacial alignment has been rarely studied despite its direct relevance and critical importance to charge transport. In this study, we uncover a significantly higher degree of alignment at the top interface of solution coated thin films, using a donor-acceptor conjugated polymer, poly(diketopyrrolopyrrole-co-thiophene-co-thieno[3,2-b]thiophene-co-thiophene) (DPP2T-TT), as the model system. At the molecular level, we observe in-plane π-π stacking anisotropy of up to 4.8 near the top interface with the polymer backbone aligned parallel to the coating direction. The bulk of the film is only weakly aligned with the backbone oriented transverse to coating. At the mesoscale, we observe a well-defined fibril-like morphology at the top interface with the fibril long axis pointing toward the coating direction. Significantly smaller fibrils with poor orientational order are found on the bottom interface, weakly aligned orthogonal to the fibrils on the top interface. The high degree of alignment at the top interface leads to a charge transport anisotropy of up to 5.4 compared to an anisotropy close to 1 on the bottom interface. We attribute the formation of distinct interfacial morphology to the skin-layer formation associated with high Peclet number, which promotes crystallization on the top interface while suppressing it in the bulk. We further infer that the interfacial fibril alignment is driven by the extensional flow on the top interface arisen from increasing solvent evaporation rate closer to the meniscus front.

Enhancing Molecular Alignment and Charge Transport of Solution‐Sheared Semiconducting Polymer Films by the Electrical‐Blade Effect

“…This initial unfavorable alignment would be difficult to reverse with the E‐blading effect. We note that polymer backbones aligned perpendicular to the coating direction have also been previously observed in meniscus‐guided coating …”

Quantitative Image Analysis of Fractal‐Like Thin Films of Organic Semiconductors

“…The C 8 ‐BTBT thin films were deposited via a meniscus‐guided coating method on silicon wafers with 300 nm thermally grown SiO 2 with no further surface treatments used. The details of the deposition method were described in previous publications . C 8 ‐BTBT thin films were coated from 0.5 mg/mL solution prepared in four different solvents (hexane, chlorobenzene, chloroform, and toluene).…”

“…We propose a possible explanation to account for the trend of D P as a function of speed due to transition from free surface crystallization to substrate crystallization. At low speed, the high evaporation rate at the surface of the meniscus results a concentration gradient and a local supersaturation profile that promote free‐surface crystallization, whereas at intermediate speed, the thickness of meniscus decreases; therefore, heterogeneous nucleation is favored at the surface of the substrate . As the crystalizing environment differs, 2D diffusion coefficient on the substrate is much lower than on a free surface.…”

“…C 8 ‐BTBT thin films were coated on highly doped silicon wafers with a 300 nm thick layer of thermally grown SiO 2 by a meniscus‐guided coating method reported in our previous works . Substrates were rinsed with isopropanol, acetone, and toluene and blow‐drying with compressed nitrogen gas before coating.…”

“…Meniscus-Guided Coating C 8 -BTBT thin films were coated on highly doped silicon wafers with a 300 nm thick layer of thermally grown SiO 2 by a meniscus-guided coating method reported in our previous works. 11,43,50 Substrates were rinsed with isopropanol, acetone, and toluene and blow-drying with compressed nitrogen gas before coating. An OTS-functionalized 300 nm SiO 2 blade was used with 7 tilting angle, and ink solution was sandwiched between the blade and a stationary substrate with 100 μm gap between the two.…”