Solution-processed zinc tetrabenzoporphyrin thin-films and transistors

a r t i c l e i n f o Thin-films and organic field-effect transistors fabricated from a solution-processable precursor of zinc tetra-benzoporphyrin (ZnTBP) are reported. Amorphous, insulating precursor films were deposited by spin-casting and thermally converted into polycrystalline, semiconducting thin-films comprising grains on the order of 5 μm in diameter. Thin-film X-ray diffraction indicates a monoclinic unit cell with molecules arranged in a herringbone pattern, which in conjunction with optical and atomic force microscopy indicate a thin-film with grains comprised of randomly oriented ZnTBP aggregates. Optical absorption measurements display broad absorption with bands characteristic of a D 4h symmetric porphyrin molecule. Organic field-effect transistors displayed field-effect mobilities on the order of 10 − 2 cm 2 /V s and ON-/OFF-current ratios exceeding 10 2. Transition-metal porphyrins and phthalocyanines constitute vital components of ubiquitous biochemical systems [1]. Due in part to their high electrical performance and chemical stability, vapor deposited metallophthalocyanines (MPCs) have been staple organic field-effect transistor (OFET) materials during the last decade [2,3], and are increasingly appearing as both donor and acceptor layers in bulk heterojunction organic photovoltaics (OPVs) [4,5]. Zinc porphyrins in particular, due to their unique optical properties, have been utilized as an effective antenna for photosynthetic processes [6,7] and in chemically and optically sensitive molecular wires [8,9]. Zinc porphyrins are also potential materials for optoelectronic molecular devices exploiting exciton–polaron coupling for nonlinear optics and polariton lasers [10]; a zinc porphyrin was utilized in the initial demonstration of polaritons in an organic microcavity [11]. Precursor-route small molecule organic semiconductors are also the focus of intense research because they potentially harness the ease of processing allowed by solution-processable organic semiconductors while producing crystalline thin-films with high charge mobility [12–14]. The semiconductor is functionalized with a bulky side molecule that renders the small molecule soluble, which can then be removed by a thermal or optical process [15], or left in place to modify molecular ordering [16]. Precursor-route metallotetrabenzoporphyrins (MTBPs) have been demonstrated in solution-processed OFETs with field-effect mobilities in the range of 10 − 2 –1 cm 2 /V s, with metallation found to induce variations in single-crystal size, polycrystallinity, and aggre-gation [17–20]. Precursor-route MTBPs allow solution processable device fabrication, polycrystalline and semiconducting thin-films, and thermal and chemical stability following precursor thermal conversion by a retro-Diels–Alder reaction, enabling further processing without deleterious effects on the thin-film [21]. In addition to OFETs, precursor-route TBP has been demonstrated as …