High performance semiconducting polymers containing bis(bithiophenyl dithienothiophene)-based repeating groups for organic thin film transistors

New dithienothiophene-containing conjugated polymers, such as poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-yl)dithieno[3,2-b;2′, 3′-d]thiophene, 4 and poly(2,6-bis (2-thiophenyl-4-dodecylthiophene-2-yl) dithieno[3,2-b;2′,3′-d]thiophene, 8 have been successfully synthesized via Stille coupling reactions using dodecyl-substituted thiophene-based monomers, bistributyltin dithienothiophene, and bistributyltin bithiophene; these polymers have been fully characterized. The main difference between the two polymers is the substitution position of the dodecyl side chains in the repeating group. Grazing-incidence X-ray diffraction (GI-XRD) gave clear evidence of edge-on orientation of polycrystallites to the substrate. The semiconducting properties of the two polymers have been evaluated in organic thin film transistors (OTFTs). The two conjugated polymers 4 and 8 exhibit fairly high hole carrier mobilities as high as μ_ave = 0.05 cm ²/Vs (I_ON/OFF = 3.42 × 10⁴) and μ_ave = 0.01 cm²/Vs, (I_ON/OFF = 1.3 × 10⁵), respectively, after thermal annealing process. The solvent annealed films underwent reorganization of the molecules to induce higher crystallinity. Well-defined atomic force microscopy (AFM) topography supported a significant improvement in TFT device performance. The hole carrier mobilities of the solvent annealed films are comparable to those obtained for a thermally annealed sample, and were one-order higher than those obtained with a pristine sample.