Vapor Phase Infiltration Doping of the Semiconducting Polymer Poly(aniline) with TiCl₄ + H₂O: Mechanisms, Reaction Kinetics, and Electrical and Optical Properties

Abstract

This study reports on the optical and electronic properties of organic–inorganic hybrid semiconductors synthesized via vapor phase infiltration (VPI) of poly(aniline) (PAni) with TiCl4 and H2O. This process results in (1) oxidative doping of PAni with the TiCl4 precursor, (2) acid doping of PAni with the HCl byproduct to form polaronic charge carriers, and (3) infiltration and deposition of a UV absorbing material (TiOx) within the final hybrid film. The electrical conductivity initially increases with increasing TiCl4 exposure time up to 0.2 S/cm, but eventually (above ca. 400 min of TiCl4 exposure), the electrical conductivity decreases due to increased TiOx deposition. Increasing the TiCl4 exposure time also decreases visible absorption while increasing UV and IR absorptions to a saturation point. This narrow window for visible light transmission is uncommon in these materials. The changes in optical absorption as a function of precursor exposure time are also fit to a diffusion kinetics model to ascertain effective diffusion constants of ca. 1 × 10–15 cm2/s for the TiCl4 + H2O VPI process in PAni at 80 °C. In summary, this work demonstrates that VPI precursors, products, and byproducts can be used to synthesize hybrid organic–inorganic semiconductors with distinctive optical and electronic properties.