Electron-transporting materials for printed electronics

The design, synthesis, and characterization of new high performance n-channel molecular/polymeric semiconductors that are solution-processable and air-stable is of great interest for the development of p-n junctions, bipolar transistors, and organic complementary circuitry (CMOS). We report here the synthesis and detailed characterization of a highly electron-deficient class of indeno[1,2-b]fluorene-6,12-dione, 2,2′-(indeno[1,2-b]fluorene-6,12- diylidene) dimalononitrile, bisindenofluorene-12,15-dione, and 2,2′-(bisindenofluorene-12,15-diylidene) dimalononitrile-based ladder-type building blocks (1 - 12) and some corresponding copolymers (P1 and P2). The correlations between molecular structures, physicochemical properties, thin film microstructures, and OFET device performance are examined in detail by DSC, TGA, melting point, single-crystal/thin-film X-ray diffraction (XRD), AFM, solution/thin film optical, PL, and cyclic voltammetry measurements. By tuning the HOMO/LUMO energetics of the present materials over a 1.0 eV range, p-type, n-type, or ambipolar charge transport characteristics can be observed, thus identifying the MO energetic windows governing majority carrier polarity and air stability. One of these systems, thiophene-terminated indenofluorenedicyanovinylene 10 exhibits an electron mobility of 0.16 cm ²/V·s and an I_on/I_off ratio of 10 ⁷ - 10⁸, one of the highest to date for a solution-cast air-stable n-channel semiconductor. Here we also report solution-processed ambipolar films of a thiophene-based molecule and two new copolymers which exhibit electron and hole mobilities of 1×10^-3 - 2×10^-4 and I_on/I_off ratios of ∼10⁴, representing the first examples of oligomeric and polymeric ambipolar semiconductors to function in air. Analysis of the operational air-stabilities of a series of thin-films having different crystallinities, orientations, and morphologies suggests that operational air-stability for thermodynamically-predicted (i.e., no kinetic barrier contribution) air-stable semiconductors is principally governed by LUMO energetics with minimal contribution from thin-film microstructure. The onset LUMO energy for carrier electron stabilization is estimated as -4.0 - -4.1 eV, indicating an overpotential of 0.9 - 1.0 eV.