Spectroelectrochemical characterization of anionic and cationic polarons in poly(3-hexylthiophene)/fullerene blend. Effects of morphology and interface

To investigate the effect of film morphology on the electronic structures of anionic and cationic polarons of poly(3-hexylthiophene), P3HT(center dot-) and P3HT(center dot+), we have characterized the subgap optical absorption of these polaronic species by means of spectroelectrochemistry. For the pristine films of regiorandom (RRa) P3HT or its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), both the ground-state bleaching and the subgap absorption of RRa-P3HT(center dot-) resemble closely to those of RRa-P3HT(center dot+). For the solvent vapor annealed neat film of regioregular (RR) P3HT, RR-P3HT(center dot-) shows distinctly different absorption spectra from RR-P3HT(center dot+), and the difference is substantially enhanced by mixing up with PC61BM. The spectral differences are ascribed to the effects of the boundaries among the polymer phases and the polymer-fullerene interfaces. With respect to the neat RR-P3HT film, the RR-P3HT/PC61 BM blend exhibits a 0.05-0.06 eV downshift of the SOMO levels for both types of polarons, implying the anisotropic hole and electron conductivities of crystalline RR-P3HT phases, which may play important roles in the transport and photogeneration of free charge carriers. (C) 2013 Elsevier B.V. All rights reserved.