Spectroscopic, morphological, and dielectric distinctions between a conjugated polymer and its incorporated monomer in field-effect transistors and capacitors

Abstract

We investigate the effects of incorporating the monomer 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into thin films of its corresponding polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We examine how this incorporation influences the film's morphology, charge storage capabilities, and dielectric properties. In tri-layer dielectric organic field-effect transistor devices with pentacene as the semiconductor layer, the addition of BTTT to the PBTTT-polystyrene dielectric layer results in increased drain currents and unique threshold voltage shift behaviors, indicating enhanced charge storage capabilities. The key step in this mechanism is that a constant portion of charge-stabilizing entities is generated continuously with the presence of applied voltage. Capacitance measurements show a peak in charge storage at low BTTT concentrations, followed by a decrease at higher concentrations. Notably, dielectric strength analysis using Weibull statistics indicates that films with 20% BTTT content exhibit higher voltage tolerance compared to pure PBTTT or polystyrene films. Spectroscopy and x-ray diffraction analysis reveal that BTTT addition compromises the original ordering of the PBTTT, with higher concentrations leading to more significant disruption, even though distinguishable BTTT domains are formed. We propose a mechanism where BTTT/PBTTT clusters form charge-stabilizing entities, leading to improved charge storage capability and dielectric strength. These findings provide insights into the distinct contributions of monomers in conjugated polymer films and their potential applications in organic electronic devices.