Tetrabenzoporphyrin Organic Semiconductors for Flexible Organic Thin Film Transistors and Circuits

Organic semiconductors present several technological advantages as the active materials in the development of new, flexible electronics. Here we provide an overview of current developments in organic thin-film field-effect transistor (OFET)[1] and circuit technologies. Discussion and analysis are presented in the context of tetrabenzoporphyrin (TBP), a solution-processable organic semiconductor that forms polycrystalline thin-films[2], can be modified via well-known synthetic chemistry methods[3, 4], and has demonstrated high electronic performance[5-7]. OFETs are fabricated typically in either coplanar (or bottom-contact) and staggered (or top-contact) device structures, with highly doped crystalline silicon as the substrate and gate electrode, and SiO2 as the gate dielectric. For TBP, thin-films were fabricated by dissolving a precursor molecule, tetrabicycloporphyrin (CP), in common organic solvents. Spuncast films of the precursor exhibit amorphous, insulating behavior; following a thermal anneal in either vacuum or N2, the amorphous film converts to polycrystalline TBP, with crystal sizes exceeding 1 μm. TBP OFETs typically display field-effect mobilities, μFE, on the order of 10 cm/V-s, in the saturation regime, ON-/OFF-current ratios exceeding 10, OFF-state currents around 100 pA, and subthreshold slopes (S) around 1 V/decade. Many organic semiconductors, including TBP, demonstrate considerable hysteresis in their transfer characterstics; TBP OFETs exhibit threshold voltages, VT, of -20 – -15 V transferring from the ON-state to the OFF-state, and 0 – +5 V transferring from the OFF-state to the ON-state. TPB thin-films exhibit a HOMO and LUMO level of 5.1 and 2.9 eV, respectively; Au forms ohmic source-drain electrodes[6]. TBP OFET transfer characteristics also display a high degree of linearity as compared to other small molecule and polymer OFETs, which combined with the low subthreshold slope, indicates that trap states are likely confined to the grain boundaries and do not dominate device performance[7]. Many organic semiconductors, TBP included, exhibit environmental drawbacks, including sensitivity to light and ambient gases. For example, exposure to ambient laboratory conditions increases the overall thin-film conductivity in TBP OFETs, increasing μFE and S, lowering the ON-/OFF-current ratio, and positivelt shifting VT. Preliminary experimental results indicate that air exposure produces shallow band tail trap states and increases the overall density of states. Encapsulation of has been performed with different materials and found not only to improve OFET air stability, but to alter the electrical performance of the TBP OFETs. Successful commercialization of organic electronics for flexible electronics will rely upon the development of flexible organic insulators. Several groups have successfully fabricated Fig. 1: The TBP single-molecule.