Electrodeposition of insulating Poly(phenylene oxide) films with variable thickness

Electrochemical polymerization is a useful method for polymer coating formation that allows room temperature processing, selectivity in electrode modification and conformal deposition on high aspect ratio structures. This work is focused on the electropolymerization of phenol to prepare pinhole free poly(phenylene oxide) (PPO) films. PPO films have been shown promising for various applications including protective coatings of the battery electrode from electrochemical dissolution [1], blocking layers for reducing the leakage current in supercapacitors [2] or gate dielectric in field effect transistors [3]. Our interest in this study is to examine the electrodeposition of PPO on various conductive surfaces by cyclic voltammetry leading to its rapid passivation. We show that a thin PPO film that conformally coats porous MnO₂ electrode can act as a lithium ion conductor with electronically insulating properties, interesting for battery applications. The thickness of the PPO film was altered by controlling the potential applied between the working and reference electrodes of an electrochemical cell (see Figure). Electronically insulating PPO films with the thickness of 10-50 nm were deposited on TiN substrates. Thermal stability of the PPO films of various thickness was investigated in various environments by in-situ spectroscopic ellipsometry in the temperature range of 0-400^oC. The ellipsometric results of the PPO thickness lay in the same range as the AFM measurements. It was found that thicker films withstand the annealing temperature up to 250^oC prior to the onset of the PPO thickness decrease. This insight is important for the optimization of the  PPO anneal conditions in order to remove the residual solvent and potentially improve the polymer chains stacking.

[1] Rhodes C.P., et al, J. Non-Cryst. Solids, 350 (2004)

[2] Tevi T., et al., J. Power Sources, 241 (2013)

[3] Lipatov A., et al., Chem. Mater., 27 (2015)