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

• We provided the long-sought characteristic absorption spectra of radical anionic P3HT.
• We found the novel role of polymer–fullerene interface in breaking the electron–hole symmetry of the crystalline P3HT phase.
• We revealed the intrinsic anisotropic hole and electron conductivities of the crystalline P3HT phases.

To investigate the effect of film morphology on the electronic structures of anionic and cationic polarons of poly(3-hexylthiophene), P3HT
• − and P3HT
• +, 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
• − resemble closely to those of RRa-P3HT
• +. For the solvent vapor annealed neat film of regioregular (RR) P3HT, RR-P3HT
• − shows distinctly different absorption spectra from RR-P3HT
• +, 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/PC61BM 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.

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