2, University of California, Los Angeles, Los Angeles, California, United States
Conductive polymers are attractive materials for use in supercapacitors due to their high specific capacitances, high electronic conductivities, and ease of processing. However, the poor cycle-life of these materials due to volume changes associated with the doping/de-doping process has prohibited their adoption in commercial devices. Research efforts have focused on improving the cycle life of conductive polymers through nanostructuring and compositing but have not yet investigated the importance of polymer chain orientation on the electrochemical properties of these materials.
Here we present a study of the electrochemical properties of PEDOT thin-films formed via oxidative chemical vapor deposition (oCVD). The oCVD process allows for simultaneous synthesis, doping, and thin-film formation of PEDOT via a surface, oxidative polymerization reaction. Initial comparisons to PEDOT:PSS thin-films reveal that oCVD PEDOT films are highly crystalline with electronic conductivities 20 times greater than amorphous PEDOT:PSS films. oCVD PEDOT films are also electrochemically active from 2.5 to 4.3 V vs. Li whereas PEDOT:PSS shows no redox behavior in this region. Many of these differences can be attributed to the doping counter-ion: oCVD PEDOT is doped with a highly-mobile chlorine anion as opposed to a bulky polyanion like polystyrene sulfonate (PSS). This enables ion exchange or “secondary doping” of oCVD PEDOT and results in the high specific capacitances observed in the material.
The deposition of PEDOT in vacuum and at moderate substrate temperatures facilitates crystal growth of the polymer. The orientation of the polymer film is dependent on the substrate temperature as oCVD PEDOT changes from predominantly [h00] to predominantly [0k0] at higher temperatures. The electrochemical performance of the [0k0]-oriented films are significantly better than their [h00] counterparts with higher capacities and the evolution of a smaller charge-transfer resistance after long-term cycling. These differences are attributed to an increased ease of doping/de-doping of the [0k0]-oriented films that lead to minimal volume changes and structural changes. These results suggest that the orientation of conductive polymers plays an important role in their electrochemical properties and that manipulation of deposition may finally enable the use of conductive polymers in commercial devices.