Porous, aligned polymer films for improved organic electrochemical transistors

We study the role of porosity and molecular order on the performance of poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic electrochemical transistors (OECTs). We utilize the small molecule 1,3,5 trichlorobenzene (TCB) as a crystallizable solvent to induce directional crystallization during blade coating. Removal of the TCB results in a porous layer of semiconducting polymer. We find that films cast with TCB show a greater than 5x improvement in device performance compared to neat P3HT films. Additionally, at high TCB loading ratios, we observe significant alignment of the polymer resulting in polarization dependent absorption. In these aligned films, we find significant anisotropy in OECT performance depending on coating direction. When the coating direction is parallel to the current direction in the transistor, the current is more than 10x greater than when the coating direction is perpendicular to the current direction. Overall, these results illustrate the role of porosity in improving OECT performance, even for hydrophobic polymers. Furthermore, this work demonstrates a means of fabricating highly anisotropic OECTs.