Design, Synthesis, and Characterization of Ladder-Type Molecules and Polymers. Air-Stable, Solution-Processable n-Channel and Ambipolar Semiconductors for Thin-Film Transistors via Experiment and Theory

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Abstract

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). While over the past two decades there have been many reports on n-channel materials, solution-processability and air-stability still remain as major challenges. 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 their corresponding homo- and copolymers (P1-P14), and examine in detail the effects of core size, thiophene vs core regiochemistry, carbonyl vs dicyanovinylene functionality, and alkyl chain orientation on the physicochemical properties, thin film microstructures, and OFET device performance. New compounds are characterized by DSC, TGA, melting point, single-crystal X-ray diffraction (XRD), solution/thin film optical, PL, and cyclic voltammetry measurements to evaluate frontier molecular orbital energetics and intermolecular cohesive forces. Thin films are grown by vacuum deposition and spin-coating, and investigated by X-ray diffraction (XRD) and AFM. By tuning the HOMO/LUMO energetics of the present materials over a 1.1 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 x s and an I(on)/I(off) ratio of 10(7)-10(8), one of the highest to date for a solution-cast air-stable n-channel semiconductor. Here we also report solution-processed ambipolar films of thiophene-based molecule 12 and copolymers P13 and P14 which exhibit electron and hole mobilities of 1 x 10(-3)-2 x 10(-4) and I(on)/I(off) ratios of approximately 10(4), representing the first examples of molecular 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 to -4.1 eV, indicating an overpotential of 0.9-1.0 eV. Density functional theory calculations provide detailed insight into molecule/polymer physicochemical and charge transport characteristics.

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