Easily processable phenylene-thiophene-based organic field-effect transistors and solution-fabricated nonvolatile transistor memory elements

The synthesis of a new series of mixed phenylene - thiophene oligomers is reported; 2,5-bis(4-n-hexylphenyl)thiophene (dH-PTP, 1), 5,5′-bis(4-n- hexylphenyl)-2,2′-bithiophene (dH-PTTP, 2), 5,5″-bis-(4-n-hexylphenyl)-2,2′:5′,2″-terthiophene (dH-PT₃P, 3), 5,5‴-bis(4-n-hexylphenyl)-2,2′:5′, 2″:5′,2‴-quater-thiophene (dH-PT₄P, 4), 1,4-bis[5-(4-n-hexylphenyl)-2-thienyl]benzene (dH-PTPTP, 5), and 2,5-bis[4(4′-n-hexylphenyl)phenyl]thiophene (dH-PPTPP, 6) were characterized by ¹H NMR, elemental analysis, UV-visible spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Vacuum-evaporated and solution-cast films were characterized by X-ray diffraction and scanning electron microscopy. All compounds display high p-type carrier mobilities as evaporated (up to 0.09 cm²/Vs) and as solution-cast (up to 0.03 cm²/Vs) films on both Si/SiO₂ and ITO/GR (glass resin) substrates. The straightforwardly synthesized dH-PTTP (2) displays an unprecedented combination of mobility, on/off ratio, stability, and processability. Both dH-PTTP (2) and dH-PPTPP (6) display a reversible, tunable, and stable memory effect even as solution-cast devices, with turn-on characteristics shifting from accumulation mode to zero or depletion mode after a writing voltage V_w is applied. The charge storage is distributed over the gate dielectric structure and is concentrated near the dielectric-semiconductor interface, as evidenced by the response of "floating gate" configuration devices. Simple nonvolatile elements have been fabricated by solution-only techniques on ITO substrates using spin-coated glass resin, solution-cast oligomeric semiconductors, and painted graphite paste electrodes.