In-situ measurements of the optical absorption of dioxythiophene-based conjugated polymers

TL;DR

This study develops an electrochemical cell to measure the optical absorption changes in conjugated polymers during doping, revealing sub-bandgap features and electrochromic behavior across a broad spectral range.

Contribution

Introduces a novel in-situ measurement setup for analyzing doping-induced optical changes in conjugated polymers with minimized electrolyte interference.

Findings

Reversible doping alters sub-bandgap absorption bands.

Polymer thickness affects spectral features.

Strategies to reduce electrolyte and water effects were identified.

Abstract

Conjugated polymers can be reversibly doped by electrochemical means. This doping introduces new sub-bandgap optical absorption bands in the polymer while decreasing the bandgap absorption. To study this behavior, we have prepared an electrochemical cell allowing measurements of the optical properties of the polymer. The cell consists of a thin polymer film deposited on gold-coated Mylar behind which is another polymer that serves as a counterelectrode. An infrared transparent window protects the upper polymer from ambient air. By adding a gel electrolyte and making electrical connections to the polymer-on-gold films, one may study electrochromism in a wide spectral range. As the cell voltage (the potential difference between the two electrodes) changes, the doping level of the conjugated polymer films is changed reversibly. Our experiments address electrochromism in poly(3,4-ethylene-dioxy-thiophene) (PEDOT) and poly(3,4-dimethyl-propylene-dioxy-thiophene) (PProDOT-Me$_2$). This closed electrochemical cell allows the study of the doping induced sub-bandgap features (polaronic and bipolaronic modes) in these easily oxidized and highly redox switchable polymers. We also study the changes in cell spectra as a function of polymer thickness and investigate strategies to obtain cleaner spectra, minimizing the contributions of water and gel electrolyte features.