Structural stability and improved properties of poly(3-alkylthiophenes) synthesized in an acid medium

Abstract

Poly(3-methylthiophene) (P3MT), poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT), with better chemical stability, efficiency and charge transfer properties for use in organic solar cells were synthesized on indium tin oxide (ITO) in LiClO4–acetonitrile (ACN) and in the presence of dyphenyl phosphate (DPHP), used as a Bronsted–Lowry acid. By means of cyclic voltammetry in the DPHP–LiClO4–ACN, we observed the displacement of two oxidation peaks, as well as the widening and definition of the second reduction peak in comparison to P3ATs synthesized in a neutral medium. Using electrochemical impedance spectroscopy, the Nyquist diagrams for the ITO/P3MT/DPHP–LiClO4–ACN system revealed a reduction in charge transfer resistance as the amount of DPHP was increased. However, for ITO/P3HT/DPHP–LiClO4–ACN and ITO/P3OT/DPHP–LiClO4–ACN this dependence was not observed. In the Bode-phase diagrams, the charge transfer processes were related to polaronic conduction in the ITO/P3MT/DPHP–LiClO4–ACN, and bipolaronic conduction in the ITO/P3HT/DPHP–LiClO4–ACN and ITO/P3OT/DPHP–LiClO4–ACN. These results were complemented by ex situ Raman spectroscopy, in which the charge transfer process in the ITO/P3MT/DPHP–LiClO4–ACN system was due to radical cation species stabilization in the P3MT as prepared, and in the ITO/P3HT/DPHP–LiClO4–ACN and ITO/P3OT/DPHP–LiClO4–ACN systems it occurred because of the stabilization of the dication on the P3HT and P3OT films obtained. The stability of these various species in the polymer matrix varied as time elapsed, leading to a disproportionate quantity of radical cation segments in aromatic and dication segments of the P3MT matrix, and the formation of a charge transfer complex with oxygen in the P3HT and P3OT. The radical cation segments stabilized 48 h after preparation.