Abstract
Polypyrrole (PPy) is one of the most attractive conducting polymers for thin film applications due to its good electrical conductivity, stability, optical properties, and biocompatibility. Among the technologies in which PPy has gained prominence are optoelectronics and solar energy conversion, where transparent electrodes such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) are frequently used. However, FTO substrates have the notable advantage that their components are widely available in nature, unlike those of ITO. Recognizing the importance that the FTO/polypyrrole system has gained in various applications, here, we studied for the first time the nucleation and growth mechanism of electro-synthesized PPy on FTO. Additionally, the effect of the synthesis potential (0.9, 1.0, 1.1, and 1.2 V vs. Ag/AgCl) on the homogeneity, adhesion, conductivity, and HOMO energy levels of PPy films was determined. From current–time transients and scanning electron microscopy, it was found that films synthesized at 0.9 and 1.0 V exhibit 3D growth with progressive nucleation (as well as lower homogeneity and higher adhesion to FTO). In contrast, films synthesized at 1.1 and 1.2 V follow 2D growth with instantaneous nucleation. It was also evident that increasing the polymerization potential leads to polymers with lower conductivity and more negative HOMO levels (versus vacuum). These findings are relevant to encourage the use of electro-synthesized PPy in thin film applications that require a high control of material properties.
Highlights
Since the discovery of intrinsically conductive polymers (ICPs) by Shirakawa et al.in the 1970s [1], significant efforts have been made to understand the properties of these materials that combine the characteristics of metals and plastics
It is worth mentioning that our results show that the nucleation and growth mechanism of polypyrrole on fluorine-doped tin oxide (FTO) may differ from that reported on indium tin oxide (ITO)
The mechanism of polymer nucleation and growth at different constant potentials was explored by comparing current–time transients with theoretical curves
Summary
Since the discovery of intrinsically conductive polymers (ICPs) by Shirakawa et al.in the 1970s [1], significant efforts have been made to understand the properties of these materials that combine the characteristics of metals and plastics. Electrochemical polymerization is known as a very versatile technique to obtain ICPs because it is the one that best allows fine tuning of polymer properties [26], with the advantage that coatings are obtained directly on conductive and semiconductive substrates [27]. In this sense, the understanding of the electro-polymerization process (nucleation and growth mechanism) is necessary for the use of these polymers in thin film applications, which require homogeneous coatings with high thickness control and appropriate electronic properties
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