Direct Uniaxial Alignment of a Donor–Acceptor Semiconducting Polymer Using Single-Step Solution Shearing

Abstract

The alignment of organic semiconductors (OSCs) in the active layers of electronic devices can confer desirable properties, such as enhanced charge transport properties due to better ordering, charge transport anisotropy for reduced device cross-talk, and polarized light emission or absorption. The solution-based deposition of highly aligned small molecule OSCs has been widely demonstrated, but the alignment of polymeric OSCs in thin films deposited directly from solution has typically required surface templating or complex pre- or postdeposition processing. Therefore, single-step solution processing and the charge transport enhancement afforded by alignment continue to be attractive. We report here the use of solution shearing to tune the degree of alignment in poly(diketopyrrolopyrrole-terthiophene) thin films by controlling the coating speed. A maximum dichroic ratio of ∼7 was achieved on unpatterned substrates without any additional pre- or postdeposition processing. The degree of polymer alignment was found to be a competition between the shear alignment of polymer chains in solution and the complex thin film drying process. Contrary to previous reports, no charge transport anisotropy was observed because of the small crystallite size relative to the channel length, a meshlike morphology, and the likelihood of increased grain boundaries in the direction transverse to coating. In fact, the lack of aligned morphological structures, coupled with observed anisotropy in X-ray diffraction data, suggests the alignment of polymer molecules in both the crystalline and the amorphous regions of the films. The shear speed at which maximum dichroism is achieved can be controlled by altering deposition parameters such as temperature and substrate treatment. Modest changes in molecular weight showed negligible effects on alignment, while longer polymer alkyl side chains were found to reduce the degree of alignment. This work demonstrates that solution shearing can be used to tune polymer alignment in a one-step deposition process not requiring substrate patterning or any postdeposition treatment.

Sequential Blade‐Coated Acceptor and Donor Enables Simultaneous Enhancement of Efficiency, Stability, and Mechanical Properties for Organic Solar Cells

“…Besides, the film processing condition also has a crucial effect on the morphology of the active layer. It is demonstrated that blade‐coating can change the degree of alignment in neat polymer film . In our previous work, we demonstrated through blade‐coating to induce higher degree of molecular packing for both donor and acceptor.…”

Nanosheets of an Organic Molecular Assembly from Aqueous Medium Exhibit High Solid‐State Emission and Anisotropic Charge‐Carrier Mobility

“…[52] Interestingly, very less φΣμ ⊥ was observed for the water processed film when compared to the chloroform or MCH processed films. [53] For determining the maximum charge-carrier mobility of the 2D sheets processed from water, φ was estimated by a direct current mode photocurrent measurement by placing the sample over a comb-type interdigitated gold electrode on a glass substrate, using TPC. [53] For determining the maximum charge-carrier mobility of the 2D sheets processed from water, φ was estimated by a direct current mode photocurrent measurement by placing the sample over a comb-type interdigitated gold electrode on a glass substrate, using TPC.…”

Molecular Orientation of Conjugated Polymer Chains in Nanostructures and Thin Films: Review of Processes and Application to Optoelectronics

“…Not only do these roll-to-roll compatible processes allow fully exploiting the potential of conjugated polymers to produce extremely low-cost electronics, but also the deposition is also performed unidirectionally and involves shear forces which may lead to peculiar orientation of the polymer fiber in the deposited thin films. In 2016, Shaw et al demonstrated the correlation between deposition speed and molecular orientation in thin films deposited using a process they refer to as solution shearing [47]. The process is very similar to blade-coating with the blade tilted at a very low angle (8 ∘ ) with respect to the substrate.…”