Significantly Increasing the Ductility of High Performance Polymer Semiconductors through Polymer Blending

Abstract

Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, developing methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor-acceptor polymer poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiopen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT) with ductile poly(3-hexylthiophene). We find that the ductility of the blend film is significantly improved compared to neat PCDTPT films, and when employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is achieved due to intermixing of the polymers throughout the film thickness. Importantly, applying large strains to the ductile films is shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.

Plastic Deformation of Polymer Blends as a Means to Achieve Stretchable Organic Transistors

“…Our previous report investigating PCDTPT:P3HT blend films showed that the blend ratio had a significant impact on the segregation characteristics and ductility of the film. [25] A 1:1 PCDTPT:P3HT blend ratio by weight had a charge mobility similar to the neat PCDTPT film, but in addition to vertical segregation, large lateral segregation of the PCDTPT was observed that limited the crack onset strain to under 50%. When reducing the ratio to 1:4 the lateral segregation was suppressed and ductility increased but there was a decrease in charge mobility.…”

“…Previously it has been shown that P3HT and PCDTPT crystallites are present in the blend film, and that due to the packing differences of the crystals, their diffraction peaks in the π-stacking and alkyl-stacking directions are separated in reciprocal space. [25] We observe that as the film was strained the diffraction pattern becomes anisotropic with an increase in in-plane diffraction intensity of (100) and (010) peaks when the X-ray beam is parallel to the strain direction (i.e. scattering vector nominally perpendicular to the strain direction).…”

“…Previously, blending polymer semiconductors has been shown to modify the mechanical response of the film while maintaining charge transport behavior. [25, 27] While the ductility of films were shown to increase, the ability to cyclically stretch while maintaining electrical properties has not been shown. For a semiconductor that will be used in stretchable devices, not only is it important to be able to survive large tensile strain, but to also function after the reduction of the applied strain.…”

“…PCDTPT has been shown to be a high performance polymer semiconductor when applied in an OTFT, but is relatively brittle and cracks at a few percent strain. [25] Blending P3HT with PCDTPT significantly increases the ductility of the film over neat PCDTPT; while also maintaining the charge mobility associated with the higher performing PCDTPT when applied in an OTFT. This behavior was attributed to vertical segregation of the PCDTPT toward the gate dielectric interface, while also having significant intermixing of both polymers throughout the films thickness, as illustrated in Figure 1.…”

“…This behavior was attributed to vertical segregation of the PCDTPT toward the gate dielectric interface, while also having significant intermixing of both polymers throughout the films thickness, as illustrated in Figure 1. [25] In this work, we explore the ability of P3HT:PCDTPT blend films to function well under large cyclic strain. The morphology and charge transport is characterized for the film strained by 75% and released back to 10% strain, relative to its original length under a specified number of cycles.…”