Use of in situ electrical conductance measurements to understand the chemical mechanisms and chamber wall effects during vapor phase infiltration doping of poly(aniline) with TiCl4 + H2O

Abstract

Vapor doping is commonly used to alter the electronic and optical properties of semiconducting polymers. Despite its ubiquity, the effects of vapor doping process conditions (e.g., time, temperature, and pressure) and the effects of reactor chamber walls on the resulting optical and electrical properties are seldomly considered. Herein, we use in situ electrical conductance measurements to understand the vapor phase infiltration (VPI) doping of poly(aniline) thin films with TiCl4 + H2O at low pressures and high temperatures (ca. 1 Torr and 100 °C). VPI doping is performed in a large vacuum chamber (28 l) with uncontrolled wall chemistries. Initial in situ electrical conductance measurements reveal conductance values higher than expected for purely oxidative doping from the TiCl4 precursor alone. To understand whether side reactions of the TiCl4 vapors with surface bound hydroxyls or water molecules to form HCl by-products that could act as acid-dopants is influencing the doping process, two processing schemes were explored: (1) a “standard process” that does not alter the uncontrolled wall chemistry and (2) a “passivated process” that reduces surface hydroxyls and/or adsorbed water by reacting these moieties with trimethylaluminum vapors. In situ conductance measurements reveal a significant change in the doping kinetics between the “standard” and “passivated” processes. Further in situ observed differences in conductance decrease during VPI purging steps provide additional evidence that the standard process generates more acid doping than the “passivated process.” Ex situ analysis using electrical conductivity, UV-vis-NIR spectroscopy, and x-ray photoelectron spectroscopy serves to further confirm these differences in doping chemistry. Ultimately, this work demonstrates the utility of in situ electrical conductance measurements for monitoring and quantifying vapor phase infiltration doping mechanisms and kinetics and demonstrates that reaction chamber walls can have significant impacts on the polymer doping mechanism.