Morphological and molecular control of chemical vapor deposition (CVD) polymerized polythiophene thin films

Abstract

Oxidative chemical vapor deposition (oCVD) has emerged as one of the most promising techniques for conjugated polymer deposition, especially for unsubstituted polythiophene thin films. oCVD overcomes the insolubility challenge that unsubstituted polythiophene (PT) presents and adds the ability to control morphological and molecular structure. This control is important for enhancing the performance of devices which incorporate organic conductors. In this work, Raman spectroscopy, UV-vis spectroscopy, and AFM reveal that the relative amount of distortion in the polymer chains, the conjugation length and the film roughness are all affected by the CVD deposition conditions, in particular the reactor pressure. PT films deposited at 150 mT and 300 mT are found to have lower chain distortion, longer conjugation lengths and lower surface roughness compared to other deposition pressures. The oCVD PT film is also directly grafted to the trichloro(phenylethyl)silane (PTS) treated substrates, where the effect of PTS grafting is observed to significantly affect film roughness. In addition, we report the first study of the effect of oCVD PT films on the performance of lithium-ion battery electrodes. These oCVD PT films are used to engineer a LiCoO₂ cathode in lithium-ion batteries. The observed improvements are a 52% increase in the discharge capacity (67 mA h g⁻¹ to 102 mA h g⁻¹) at 10C and a 500% improvement in cycling stability tested at 5C within the voltage range of 3.0–4.5 V (capacity fading rate is reduced from 1.92%/cycle to 0.32%/cycle).