Spectroscopic, Morphological and Charge Carrier Storage Between a Conjugated Polymer and Its Incorporated Synthetic Monomer in Solution Processed Thin Film Field-Effect Transistors and Capacitors

This study investigates 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). The research explores how this incorporation influences the film's morphology, charge storage capabilities, and dielectric properties. Spectroscopy and X-ray diffraction analysis reveal that BTTT addition compromises the crystal structure of PBTTT, with higher concentrations leading to more significant disruption. In tri-layer organic field-effect transistor (OFET) devices with pentacene as semiconductor layer, the addition of BTTT to the PBTTT-polystyrene dielectric layer results in increased drain currents and unique threshold voltage shift behaviors, suggesting enhanced charge storage capabilities. Based on the logarithmic increasing behavior of threshold voltages of the devices with an increasing concentration of BTTT in the PBTTT dielectric layer, a hypothetical “half-order” charge storage mechanism is proposed. 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. 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.