Structural Characterization and Impedance Spectroscopy of Substituted, Fused-Ring Organic Semiconductors.

Abstract

Organic materials present a number of advantages over silicon that make them ideal candidates for modest performance devices like active matrix backplanes and RFID tags. The work detailed here describes both structural characterization of promising new materials, as well as the adaptation of impedance spectroscopy techniques to the study of organic transistors. Unit cells and solution casting behavior for dioctyl- and didodecyl-pentathienoacene are presented. Dioctyl pentathienoacene has an orthorhombic lattice with parameters a = 1.15 nm, b = 0.43 nm and c = 3.05 nm. Didodecyl pentathienoacene has an monoclinic lattice with parameters γ = 92.2º, a = 1.10 nm, b = 0.42 nm and c = 3.89 nm. Additionally, thermotropic phase behavior is detailed. Both materials exhibit a “side chain melting” transition—characterized by a dramatic unit cell contraction of more than 20%—and smectic C liquid crystal phases. The side chain melting transition shows similarity to phase transitions elicited by exposing these materials to high energy electron flux. In both cases, disorder in the substitutions results in new phases for these materials. Dioctyl-pentathienoacene also exhibits a unique phase, which is intermediately ordered and shows a threefold increase in critical dose over the as-cast phase. Impedance spectroscopy of triisopropylsilyl pentacene transistors suggests these devices are well fit by a Voigt model equivalent circuit. The gate bias dependent resistor represents the channel conductance and the capacitor represents the drain-gate and source-gate capacitances. This in turn suggests that conduction occurs through delocalized states available in ordered regions, with disordered regions contributing localized, immobile states. Impedance spectroscopy of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2- b]thiophene) shows similar behavior. The use of variable temperature impedance spectroscopy is also demonstrated. This technique is used to measure the reduction in trap energy—from 200 meV to 140 meV—produced by annealing the material in its liquid crystal phase.