|Abstract:||Interfacial properties play a significant role in solution coating of conjugated polymers. On the one hand, interfacial molecular orientation and alignment critically influence the electronic performance in organic semiconductor thin films. On the other hand, the air-liquid meniscus instability lead to various coating morphology and drives morphology transition during conjugated polymer deposition.
We investigated the out-of-plane molecular orientation influenced by liquid crystalline mesophase and the in-plane crystalline polymer fiber alignment determined by fluid flow from solution coating. For out-of-plane molecular orientation study, we observe distinct edge-on orientation at the top interface compared to primarily face-on orientation at the bottom and the bulk film in solution coated poly[[2,5-bis(2-octadecyl)-2,3,5,6-tetrahydro-3,6-diketopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-(2-octylnonyl)-2,1,3-benzotriazole] (DPP-BTz) thin films from grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Interestingly, we also observe smectic-like lyotropic liquid crystal mesophase of DPP-BTz appearing during solution coating and adopting edge-on orientation near the air-liquid interface with face-on orientation in the bulk liquid layer, characterized by solution state small angle X-ray scattering (SAXS) and in situ GIWAXS measurements. We attribute the edge-on LC orientation at the top interface to surface energy minimization of alkyl side chains, and the face-on LC orientation in the bulk liquid and bottom interface to symmetry braking effect near the substrate. The out-of-plane molecular orientation is preserved in the LC mesophase and is carried to the solid-state thin film, creating the distinct edge-on interfacial alignment at the thin film top surface. In the in-plane crystalline polymer fiber alignment study, we uncover 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. At the mesoscale, we observe well-defined fibril-like morphology at the top interface with the fibril long-axis pointing towards the coating direction. 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.
For meniscus instability, we first constructed a surface free energy model for speed-dependent film-to-stripe morphology transition, and generated a dimensionless group, morphology number, to describe film-to stripe morphology transition at various coating conditions. We observe a film-to-stripe morphology transition caused by stick-and-slip meniscus instability during solution coating seen in multiple donor-acceptor polymer systems. There is coexistence of film and stripe morphologies at the critical coating speed. Surprisingly, higher charge carrier mobility is measured in transistors fabricated from stripes despite their same deposition condition as the films at the critical speed. To understand the origin of the morphology transition, we further construct a generalizable surface free energy model to validate the hypothesis that the morphology transition occurs to minimize the system surface free energy. As the system surface free energy varies during a stick-and-slip cycle, we focus on evaluating the maximum surface free energy at a given condition, which corresponds to the sticking state right before slipping. Indeed, we observe increase of the maximum system surface free energy with increase in coating speed prior to film-to-stripe morphology transition and abrupt drop in the maximum system surface free energy post-transition when the coating speed is further increased, which is associated with reduced meniscus length during stripe deposition. Such energetic change originates from the competition between pinning and depinning forces on a partial wetting substrate which underpins the film-to-stripe transition. To move a step further, we utilize meniscus guided solution coating to deposit conjugated polymers with various coating condition to study the meniscus instability driven morphology transition. We solution coated conjugated polymer DPP-TT on various substrate with various coating speed. We observe film-to-strip morphology transition on low surface energy substrates, while coating undergoes transition from evaporation to Landau-Levich regime on medium to high surface energy substrates. We constructed the dimensionless morphology number by multiplying evaporative Peclet number with modified capillary number to quantitatively describe the film-to-strip morphology transition. We observe a distinct decrease in the value of morphology number when film-to-stripe transition occurs. We validate with other coating condition that morphology number is capable of describing film-to-stripe transition, which may help with understanding morphology control in general conjugated polymer coating systems.