Engineering of the optochemical properties of (ultra-)thin plasma polymerized polythiophene films

The control of optical and chemical properties in conjugated polymers is crucial for their application in optoelectronic and energy storage devices. Plasma polymerization offers a dry processing approach that enables precise tuning of material properties while maintaining compatibility with thin-film device fabrication. This study presents a comprehensive optochemical characterization of low-pressure plasma polymerized polythiophene (PPTh) films, focusing on the impact of iodine doping on their optical properties. Films were deposited using thiophene, with or without carrier gases (argon or helium), and synergistically analyzed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), UV-vis spectroscopy, spectroscopic ellipsometry (SE), and I-V measurements. Monomer fragmentation is shown to be significantly reduced when using thiophene alone or with helium, leading to higher conjugation length and improved conductivity. The optical bandgap was tuneable based on film thickness and doping duration, while UV-vis and SE confirmed that films deposited under low-fragmentation conditions exhibited the highest responsiveness to iodine doping. A dedicated case study on 20-nm-thick layers further demonstrated the tunability of optical properties upon doping, highlighting their potential for optoelectronic applications and device implementation.