Control of Thickness, Morphology and Band Gap of Poly-(3.4-ethylenedioxythiophene) Electrochemically Growth (e-PEDOT) to Use It as Anodic Buffer Layer in Organic Photovoltaic Cells

This contribution presents the electrochemical control of thickness, morphology and band gap of the polymer film poly[3,4-ethylenedioxythiophene] (PEDOT), which was obtained by electropolymerization of monomer 3,4-ethylenedioxythiophene (EDOT) in the transparent conductive indium-tin oxide (ITO). This study was carried out using mixtures of dry organic solvents such as acetonitrile, toluene and dichloromethane to determine the effect of the polarity solvent in the polymerization kinetics of the monomer EDOT. For this, cyclic voltammetry was used. It was observed that pure acetonitrile and the acetonitrile-dichloromethane mixture (2:1, v/v), the polymerization kinetics is very fast and the obtained films showed similar responses in both cases, however, the kinetics obtained for the mixture of toluene-acetonitrile (4:1, v/v) was slower, indicating that in a medium of higher polarity, the polymerization kinetics is much faster than in a medium of lower polarity. This fact coupled with low concentrations of monomer EDOT, let us to control the thickness and morphology of the electropolymerized PEDOT films (e-PEDOT). An optimization of the desired film properties was achieved through the implementation of the chronoamperometric technique (CA) for the electropolymerization. Here, the time of deposit and a low concentration of the EDOT monomer gave the desired thickness of the polymer film with a uniformly rough morphology. Furthermore, the study of the response UV -Vis of the films e-PEDOT was performed and found that optical band gap depends on the experimental conditions used to obtain the e-PEDOT films. Finally, these e-PEDOT films were used as buffer layers in organic photovoltaic cells (OPVs) with bulk heterojunction architecture and their performance was compared with those constructed with the classical spin cast deposited PEDOT-PSS buffer layer.

Acknowledgments: Funding for this research was obtained by CONACYT-CB project number 179356. The technical AFM support of Ing. Mario Monroy (Instituto de Física UNAM)