Critical Role of Surface Energy in Guiding Crystallization of Solution-Coated Conjugated Polymer Thin Films

Abstract

It is well-known that substrate surface properties have a profound impact on the morphology of thin films solution coated atop and the resulting solid-state properties. However, design rules for guiding the substrate selection have not yet been established. Such design rules are particularly important for solution-coated semiconducting polymers, as the substrate-directed thin film morphology can impact charge transport properties by orders of magnitude. We hypothesize that substrate surface energies dictate the thin film morphology by modulating the free energy barrier to heterogeneous nucleation. To test this hypothesis, we systematically vary the substrate surface energy via surface functionalization techniques. We perform in-depth morphology and device characterizations to establish the relationship between substrate surface energy, thin film morphology and charge transport properties, employing donor-acceptor (D-A) conjugated polymers. We find that decreasing the substrate surface energy progressively increases thin film crystallinity, degree of molecular ordering, and extent of domain alignment. Notably, the enhanced morphology on the lowest surface energy substrate leads to a 10-fold increase in the charge carrier mobility. We further develop a free energy model relating the substrate surface energy to the penalty of heterogeneous nucleation from solution in the thin film geometry. The model correctly predicts the experimental trend, thereby validating our hypothesis. This work is a significant step toward establishing design rules and understanding the critical role of substrates in determining morphology of solution-coated thin films.

Understanding the Meniscus‐Guided Coating Parameters in Organic Field‐Effect‐Transistor Fabrications

“…Recent advances on the MGC crystallization mechanisms mainly depend on the surface energy (γ) analysis of the system. Zhang et al observed a better molecular packaging in the poly(diketopyrrolopyrrole‐thiophene‐thieno[3,2,b]thiophene‐thiophene) (DPP2T‐TTT) thin film when the system surface energy decreased . Similar results of surface energy regulation have been perceived by Vladimirov et al They observed n‐type crystals could start to grow at the solvent–air interface if the surface energy of solvents was higher than that of crystals .…”

Printing 2D Conjugated Polymer Monolayers and Their Distinct Electronic Properties

“…We noticed that monolayer films could not be fabricated on surfaces with low surface energies (γ) such as OTS (γ = 20.5 × 10 −3 n m −1 ) because of dewetting of ink solution. Therefore, we chose substrates with higher surface energies as reference static substrates—plasma treated (γ = 67.2 × 10 −3 m m −1 )13 and phenyltrichlorosilane (PTS) (γ = 36.0 × 10 −3 n m −1 )13 functionalized SiO 2 (Figure S1, Supporting Information). We note that the surface energy of the IL used is comparable (γ = 36.2 × 10 −3 n m −1 )9 to that of PTS and thus rule out the effect of surface energy when comparing polymer films coated on IL versus PTS.…”

“…In fact, 2D monolayer could not be obtained at any printing speed attempted (0.1–100 mm s −1 at 0.5 mg mL −1 ) (Figure S6, Supporting Information). While the low surface energy of the PTS might have lowered the Gibbs' free energy for nucleation on PTS substrates compared to plasma treated SiO 2 ,13 the monolayer film was still riddled with voids, giving a film coverage of ≈86%. Quantitatively, we determined using an image analysis protocol we developed18 that the fractal dimension of the monolayer films on IL is 2, which significantly decreases to 1.33 ± 0.06 and 1.48 ± 0.05 for those on plasma SiO 2 and PTS, respectively.…”

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

“…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.…”

“…Morphology control is crucial to achieving high‐performance devices and to elucidating the complex structure–charge transport property relationship . Morphology is shown to be highly sensitive to processing conditions, such as printing/coating speed, solvent choices, substrate chemistry and topology, and so on. It has been widely shown that morphological parameters can modulate charge carrier mobility by orders of magnitude.…”