Reversible P3HT/Oxygen Charge Transfer Complex Identification in Thin Films Exposed to Direct Contact with Water

Combined systems of semiconducting polymers and aqueous electrolytes are emerging as a new frontier of organic electronics, with many promising applications in neuroscience, biomedicine, and photoelectrochemical cells. A detailed characterization of the effect of direct, prolonged contact with water in working conditions, typically upon visible light illumination, is thus urgently needed. Here, we report a comprehensive study of processes occurring in thin films of regioregular poly(3-hexylthiophene) (rr-P3HT), the election material for such applications, exposed to different environmental conditions. We demonstrate that the contact with saline solutions is not worse than contact with open air: in both situations the reversible formation of a charge transfer complex between polymer and molecular oxygen is the main phenomenon, enhanced by visible light illumination. Experimental data and theoretical modeling provide an insightful picture of the complex formation, as a precursor of photoactivated doping, and first unambiguously identify its spectral signature by means of vibrational spectroscopy techniques. In perspective, this work validates use of semiconducting polymers in contact with electrolytes and paves the way to new, rapidly emerging trends in organic electronics.