Versatile Ultrathin Films of Conducting Polymers by Vapor Phase Polymerization at Atmospheric Pressure: Synthesis, Process Optimization, and Characterization

The limitations such as toxicity, brittleness, scarcity, and high cost of currently available materials for energy storage and transparent electrode applications in bendable electronics create a need for green, sustainable, and cost-effective alternatives. Organic conducting polymers (CPs) can transport charge across conjugated sp2 carbon network. Their low-tuneable band gap, flexibility, and transparency could make them viable alternatives. However, the challenges such as processability, stability, biocompatibility, and production cost remain milestones to achieve. Therefore, this thesis work is an effort toward a green and sustainable future. Poly(3, 4-ethylenedioxythiophene) (PEDOT), known for its conductivity, robustness, and biocompatibility and polyazulene (PAz), known for its high capacitance are selected for studies due to their unique abilities. Fabrication of highly electrically conducting ultra-thin films of PEDOT is done by developing and optimizing a cost-efficient vapor phase polymerization method at atmospheric pressure (AP-VPP) and combining a layer-by-layer (L-b-L) synthesis approach. As a result, AP-VPP PEDOT films showed comparative sheet resistance, transmittance, and conductivity with commercially available ITO-coated materials. In contrast, the flexible, green organic nature and high capacitance of PEDOT thin films overcome the competition with ITO-coated materials. Similarly, well-organized high-capacitance PAz films are L-b-L synthesized using an optimized AP-VPP process. The influence of factors such as substrate surface cleaning, oxidant solution, oxidant spin coating rate and time, cell and substrate temperature, polymerization time, drying and annealing temperature and time, air vs nitrogen atmosphere, and washingsolvents on film properties were studied during the optimization process for both the CPs. Properties like optical bandgap, sheet resistance, surface roughness, conductivity, capacitance, and % transmittance provided a route for process optimization. Furthermore, FTIR, Raman, and UV–Vis spectroscopy were utilized to analyse the extended conjugation along with the type and trend in the charge carriers generated upon doping the resulting films. Microscope imaging, AFM, and SEM were utilized to analyse surface morphologies and microstructures. The capacitance properties, transport of counterions across multiple layers, and charge transport resistance were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. In addition, the characterization techniques complement each other in data interpretation. PEDOT and PAz films produced in the present work and their properties are compared with available reports in the literature

Effect of Oxidants on Properties of Electroactive Ultrathin Polyazulene Films Synthesized by Vapor Phase Polymerization at Atmospheric Pressure

A non-benzenoid aromatic hydrocarbon azulene, naturally found in plants and mushrooms, is known for its derivatives applications in medicines. However, the processability of its chemically synthesized high-capacitance polymer is constrained by the sparingly soluble nature of its polymeric form. Oxidative chemical synthesis on a desirable substrate overcomes this difficulty. In this report, polyazulene (PAz) thin films are synthesized by vapor phase polymerization at atmospheric pressure using oxidants, such as CuCl2, CuBr2, FeCl3, and FeTOS. The effect of oxidants on morphologies of PAz films is studied using atomic force microscopy and microscope imaging. Each oxidant produced distinct microstructures in the films. The films synthesized using Cu(II) salts showed organized and knitted structures, whereas Fe(III) salts formed casted sheet-like disordered arrangements. The films synthesized using CuCl2 created uniform porous film assemblies. The pre-peak formations and their splitting observed in the cyclic voltammograms revealed phase segregations in the films. Oxidant-dependent structural and chemical differences such as charge carrier formation, doping levels, and polymer chain length in the PAz films are studied by using UV-Vis and FTIR spectroscopy. The results indicated that 240 and 180 mM are the optimum concentration of CuCl2 to produce high capacitance and well-organized single-and triple-layered PAz films, respectively.</p

Fabrication of electroactive multi-layered polyazulene thin films by atmospheric pressure-vapor phase polymerization

Thin films of polyazulene (PAz) are produced by using an optimized atmospheric pressure–vapor phase polymerization (AP-VPP) method. Method optimization is carried out by studying the effect of cell temperature, substrate temperature, polymerization time, and washing-solvent on film properties like optical bandgap, sheet resistance, surface roughness, and % transmittance (%T). Multi-layered PAz films were produced by layer-by-layer engineering. The effects of thin, electroactive multiple layers on film properties are investigated. UV–Vis, IR, and Raman analysis are utilized to understand the extended conjugation length and nature of the charge carriers. The spectroscopic data revealed the anomalous behaviour of PAz at a high level of doping. The proportion and amount of quinoid conformation is discussed. The addition of layers changes the transport of ions across the electroactive PAz films, which is studied using cyclic voltammetry at various scan rates. AFM and SEM images reveal a change in structural properties which is further correlated with a deviation of capacitance values at elevated scan rate. Comparison with earlier reported literature on electrochemically and chemically synthesized PAz is also provided. The conductivity, transparency and high capacitance show a promising application of AP-VPP PAz in various fields.</p

Effect of Oxidants on Properties of Electroactive Ultrathin Polyazulene Films Synthesized by Vapor Phase Polymerization at Atmospheric Pressure

A non-benzenoid aromatic hydrocarbon azulene, naturally found in plants and mushrooms, is known for its derivatives applications in medicines. However, the processability of its chemically synthesized high-capacitance polymer is constrained by the sparingly soluble nature of its polymeric form. Oxidative chemical synthesis on a desirable substrate overcomes this difficulty. In this report, polyazulene (PAz) thin films are synthesized by vapor phase polymerization at atmospheric pressure using oxidants, such as CuCl2, CuBr2, FeCl3, and FeTOS. The effect of oxidants on morphologies of PAz films is studied using atomic force microscopy and microscope imaging. Each oxidant produced distinct microstructures in the films. The films synthesized using Cu(II) salts showed organized and knitted structures, whereas Fe(III) salts formed casted sheet-like disordered arrangements. The films synthesized using CuCl2 created uniform porous film assemblies. The pre-peak formations and their splitting observed in the cyclic voltammograms revealed phase segregations in the films. Oxidant-dependent structural and chemical differences such as charge carrier formation, doping levels, and polymer chain length in the PAz films are studied by using UV–Vis and FTIR spectroscopy. The results indicated that 240 and 180 mM are the optimum concentration of CuCl2 to produce high capacitance and well-organized single- and triple-layered PAz films, respectively

Electropolymerized polyazulene as active material in flexible supercapacitors

We report the capacitive behavior of electrochemically polymerized polyazulene films in different ionic liquids. The ionic liquids in this study represent conventional imidazolium based ionic liquids with tetrafluoroborate and bis(trifluoromethylsulfonyl)imide anions as well as an unconventional choline based ionic liquid. The effect of different ionic liquids on the polymerization and capacitive performance of polyazulene films is demonstrated by cyclic voltammetry and electrochemical impedance spectroscopy in a 3-electrode cell configuration. The films exhibit the highest capacitances in the lowest viscosity ionic liquid (92 mF cm−2), while synthesis in high viscosity ionic liquid shortens the conjugation length and results in lower electroactivity (25 mF cm−2). The obtained films also show good cycling stabilities retaining over 90% of their initial capacitance over 1200 p-doping cycles. We also demonstrate, for the first time, flexible polyazulene supercapacitors of symmetric and asymmetric configurations using the choline based ionic liquid as electrolyte. In asymmetric configuration, capacitance of 55 mF (27 mF cm−2) with an equivalent series resistance of 19 Ω is obtained at operating voltage of 1.5 V. Upon increasing the operating voltage up to 2.4 V, the capacitance increases to 72 mF (36 mF cm−2).acceptedVersionacceptedVersionPeer reviewe