Probing ion movement in conjugated polymers

Abstract: Electronic conduction in conjugated polymers can be increased through electrochemical doping. This is utilised in light-emitting electrochemical cells and allows for a less demanding manufacturing process compared to the related organic light-emitting diode. The doping process requires ion transport into the conjugated polymer and has yet to be quantified for an electrolyte-free conjugated polymer. In this project the ion movement of lithium trifluoromethanesulfonate in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) has been studied with several electrochemical techniques: cyclic voltammetry, galvanostatic cycling, staircase potentiostatic electrochemical impedance spectroscopy and staircase potentiostatic chronoamperometry. The measurements have been performed on a dry system of spin-coated MEH-PPV films with lithium trifluoromethanesulfonate dissolved in a copolymer of poly(epsilon-caprolactone) and poly(trimethylene carbonate), PCL–PTMC, as a solid electrolyte. Infrared spectroscopy (FTIR) showed evidence of ion association of lithium trifluoromethanesulfonate dissolved in MEH-PPV. The electrochemical analyses confirmed the possibility to insert trifluoromethanesulfonate anions into electrolyte-free MEH-PPV; extraction of the ions was also possible, albeit with limited reversibility. Insertion of lithium ions was not observed. Adding the ion transporter trimethylolpropane ethoxylate (TMPE-OH) was seen to increase the reversibility of the process. The results also indicated different ion mobilities in doped and undoped MEH-PPV. Doping of an MEH-PPV film was shown to change the behaviour of the film from capacitive to faradaic behaviour. MEH-PPV did also show indications of two different levels of p-doping, one above 3.3 V vs. Li+/Li and the other above 3.7 V, with over-oxidation occurring at potentials over 4.1 V.