Characterization of the interaction between P3ATs with PCBM on ITO using in situ Raman spectroscopy and electrochemical impedance spectroscopy

Abstract

In this study, phenyl-C 61 -butyric acid methyl ester (PCBM) was deposited onto indium tin oxide (ITO) using the Langmuir-Schaefer technique, and poly(3-methylthiophene) (P3MT) and poly(3-hexylthiophene) (P3HT) were then electrochemically synthesized onto the ITO/PCBM, producing PCBM/P3MT and PCBM/P3HT films respectively. These films are widely used to create heterojunctions in organic photovoltaic cells, and they were produced in this study in order to monitor the charge transfer process between the P3ATs and PCBM. Cyclic voltammetry (CV) of the ITO/PCBM/P3AT systems demonstrates dislocation of oxidation and reduction peaks and a variation in the current of these peaks in relation to those observed in ITO/P3ATs. Charge transfer resistance (R CT ) were determined using electrochemical impedance spectroscopy (EIS) to produce Nyquist plots at open circuit potential and at different applied potentials. ITO/PCBM/ P3HT had a lower R CT value than ITO/P3HT and ITO/ PCBM/P3MT. Bode phase plots produced using EIS indicated that the charge transfer process in ITO/PCBM/P3HT is associated with polaron conduction (radical cation) and in ITO/PCBM/P3MT with bipolar conduction (dication). This study of the behaviour observed in these systems using CV and EIS techniques was supplemented with ex situ and in situ Raman spectroscopy. Structural variations were observed in segments of the P3HT polymer chain when interacting with PCBM in the ITO/PCBM/P3HT system, but not for ITO/P3MT in the ITO/PCBM/P3MT system. The charge transfer observed between PCBM and P3HT was associated with the stabilization of the radical cation species found in P3HT in this material.

Influence of solvents on the morphology of Langmuir and Langmuir–Schaefer films of PCBM and PCBM-based oligomers and polymers

“…The strong photoluminescence signal characteristic of the fullerene-based materials is maintained for all samples. The presence of P3HT shows two peaks at 1446 and 1390 cm À1 that are associated with the stretching modes of CQC and C-C, 56 respectively, and a third one at 727 cm À1 , associated with the deformation of the C-S-C thiophene ring, 79,80 seen in films solubilized with chloroform. The high intensity at 1446 cm À1 indicates an increase in the crystallinity of the P3HT polymer and the extension of the effective length of the conjugation along the chain, 81 which is often used to indicate the ordering of the film.…”