Vapor Phase Infiltration of Conjugated Polymers Using TiCl4 and VOCl3 and Their Resulting Optical and Electrical Properties

Abstract

Chemical doping alters the electrical and optical properties of conductive, conjugated polymers. Although liquid-phase doping is effective, the solvents can deleteriously alter a polymer’s microstructure and are often environmentally-hazardous. In this work, we investigate the use of vapor phase infiltration (VPI) to dope conjugated polymers using both TiCl4 and VOCl3 as vapor-phase dopants. VPI results in not only oxidative doping of the polymer via adducts with the metal halide precursor but also acid doping via byproduct generation upon metal halide reaction with water vapor. The resulting electrical properties of the new hybrid materials are characterized with both in situ and ex situ electrical measurements to determine how exposure time to the VPI precursor affects doping. Initially electrical conductivity increases with metal halide exposure time due to increasing the number of charge carriers. However, upon oxidation of the metal halide precursor, conductivity begins to decrease due to the formation of metal oxide clusters that presumably scatter charge carriers. The effects on optical properties is found to differ significantly between VOCl3 and TiCl4 dopants. VOCl3 leads to a greater absorbance in the visible spectrum. To understand the structural changes that lead to these variations in optical behavior, UV-Vis spectroscopy and x-ray photoelectron spectroscopy are undertaken. In particular, we observed that the polaron peak initially increases but eventually saturates while the oxide peak seemingly continues to increase with metal halide exposure time. These effects are further understood by changes to the inorganic’s chemical environment studied with XPS.