Structural stability of poly(3-methylthiophene) and poly(3,4-ethylenedioxythiophene) in the ITO/PEDOT:PSS/P3MT system

Abstract

With the aim of understanding the factors that enhance the efficiency of Organic photovoltaic cells (OPVs), poly(3-methylthiophene) (P3MT) was anodically deposited (+1.65 V vs Ag/AgCl) on ITO coated with poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (ITO/PEDOT:PSS) in LiClO4-ACN. This active layer was studied by ex situ Raman spectroscopy as a function of time, and led us to the conclusion that the radical cation species in the P3MT thiophene ring was very stable compared the ITO/P3MT system. Bearing in mind that P3MT synthesis on ITO/PEDOT:PSS occurs at high anodic potential, the behavior of the PEDOT at the applied potential was verified by electrochemical impedance spectroscopy and in situ Raman spectroscopy of the PEDOT:PSS on Pt. The Bode phase plots at potentials of −0.60, 0.00, 0.40, 0.60 and 0.80 V for the Pt/PEDOT:PSS/LiClO4-ACN system showed the contributions of the polaron and bipolaron conduction of the PEDOT thiophene ring. To back up this study, in situ Raman spectroscopy was used at the same potentials to verify the characteristic frequencies of the radical cation and dication species of the PEDOT thiophene ring. These results provided a basis for determining that the radical cation form of the PEDOT thiophene ring was the same stable form at the PEDOT:PSS/P3MT interface, which could enhance charge injection processes in OPVs.